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A method of generating post-mission position and orientation data comprises generating position and orientation data representing positions and orientations of a mobile platform, based on global navigation satellite system (GNSS) data and inertial navigation system (INS) data acquired during a data acquisition period by the mobile platform, using a network real-time kinematic (RTK) subsystem to generate correction data associated with the data acquisition period, and correcting the position and orientation data based on the correction data. The RTK subsystem may implement a virtual reference station (VRS) technique to generate the correction data.
1. A method comprising,
after a data acquisition period during which GNSS data and INS data have been acquired on a mobile platform:
generating position data representing positions of the mobile platform, based on the GNSS data and INS data acquired during the data acquisition period;
using a network RTK subsystem which includes a VRS subsystem to generate correction data associated with the data acquisition period; and
correcting the position data based on the correction data.
2. A method as recited in
acquiring differential GNSS observables measurements; and
performing integer carrier phase ambiguity resolution and carrier phase measurements.
3. A method as recited in
integrating observables from a plurality of GNSS reference receivers arranged in a network to correct for atmospheric delays in GNSS observables acquired by the mobile platform, when a distance from the mobile platform to the nearest reference receiver exceeds a specified distance.
4. A post-mission position and orientation system comprising:
a GNSS-AINS subsystem to generate position data representing positions of a mobile platform, based on GNSS data and INS data acquired during a data acquisition period by the mobile platform; and
a network RTK subsystem including a VRS subsystem to generate correction data associated with the data acquisition period, wherein the GNSS-AINS subsystem further is to generate the position data based on the correction data;
wherein neither the GNSS-AINS subsystem nor the network RTK subsystem is located on the mobile platform.
5. A system as recited in
a network adjustment subsystem to evaluate and correct antenna positions for a plurality of GNSS reference receivers arranged in a network, wherein the network RTK subsystem generates the correction data based on the corrected antenna positions.
6. A system as recited in
7. A system as recited in
8. A system as recited in
acquire differential GNSS observables measurements; and
perform integer carrier phase ambiguity resolution and carrier phase measurements.
9. A system as recited in
integrate observables from a plurality of GNSS reference receivers arranged in a network to correct for atmospheric delays in GNSS observables acquired by the mobile platform, when a distance from the mobile platform to the nearest reference receiver exceeds a specified distance.
At least one embodiment of the present invention pertains to a global navigation satellite system (GNSS) aided inertial navigation system (INS) and, more particularly, to a GNSS-aided INS (GNSS-AINS) post-mission high accuracy position and orientation system.
A Global Navigation Satellite System (GNSS) is a navigation system that makes use of a constellation of satellites orbiting the earth to provide signals to a receiver on the earth that computes its position on the earth from those signals. Examples of such satellite systems are the NAVSTAR Global Positioning System (GPS) deployed and maintained by the United States, the GLONASS system deployed by the Soviet Union and maintained by the Russian Federation, and the GALILEO system currently being deployed by the European Union (EU).
Each GPS satellite transmits continuously using two radio frequencies in the L-band, referred to as L1 and L2, at respective frequencies of 1575.41 MHz and 1227.60 MHz. Two signals are transmitted on L1, one for civil users and the other for users authorized by the Unites States Department of Defense (DoD). One signal is transmitted on L2, intended only for DoD-authorized users. Each GPS signal has a carrier at the L1 and L2 frequencies, a pseudo-random number (PRN) code, and satellite navigation data. Two different PRN codes are transmitted by each satellite: a coarse acquisition (C/A) code and a precision (P/Y) code which is encrypted for use by authorized users. A GPS receiver designed for precision positioning contains multiple channels, each of which can track the signals on both L1 and L2 frequencies from a GPS satellite in view above the horizon at the receiver antenna, and from these computes the observables for that satellite comprising the L1 pseudorange, possibly the L2 pseudorange and the coherent L1 and L2 carrier phases. Coherent phase tracking implies that the carrier phases from two channels assigned to the same satellite and frequency will differ only by an integer number of cycles.
Each GLONASS satellite transmits continuously using two radio frequency bands in the L-band, also referred to as L1 and L2. Each satellite transmits on one of multiple frequencies within the L1 and L2 bands respectively centered at frequencies of 1602.0 MHz and 1246.0 MHz. The code and carrier signal structure is similar to that of NAVSTAR. A GNSS receiver designed for precision positioning contains multiple channels each of which can track the signals from both GPS and GLONASS satellites on their respective L1 and L2 frequencies, and generate pseudorange and carrier phase observables from these. Future generations of GNSS receivers will include the ability to track signals from all deployed GNSSs.
The purpose of an AINS is to compute navigation data comprising vehicle position, velocity, acceleration, orientation (e.g., roll, pitch, heading) and angular rate via the combination of an inertial navigation system (INS) and aiding navigation sensors. A GNSS-aided INS uses one or more receivers capable of receiving and processing signals from one or more GNSS's as an aiding sensor. GNSS-AINS has been successfully demonstrated as an accurate source of position and orientation information for various survey applications from a moving platform. One of the most significant achievements in recent years is the successful demonstration and subsequent deployment of a GNSS-AINS for direct georeferencing of aerial photogrammetry images. Other applications include mobile mapping/survey from a land vehicle and sea floor bathymetry from a survey vessel.
To achieve accurate positioning of a mobile platform with a GNSS-AINS, relative or differential positioning methods are commonly employed. These methods use a reference GNSS receiver located at a known position, in addition to the data from the INS and the rover GNSS receiver (both on the mobile platform), to compute the position of the mobile platform relative to the reference receiver. The most accurate known method uses relative GNSS carrier phase interferometry between the rover and reference GNSS antennas plus resolution of integer wavelength ambiguities in the differential phases to achieve centimeter-level positioning accuracies. These differential GNSS methods are predicated on the near exact correlation of several common errors in the rover and reference observables. They include ionospheric and tropospheric signal delay errors, satellite orbit and clock errors, and receiver clock errors.
When the baseline length between the mobile platform and the reference receiver does not exceed 10 kilometers, which is normally considered a short baseline condition, the ionospheric and tropospheric signal delay errors in the observables from the rover and reference receivers are almost exactly the same. These atmospheric delay errors therefore cancel in the rover-reference differential GNSS observables, and the carrier phase ambiguity resolution process required for achieving centimeter-level relative positioning accuracy is not perturbed by them. If the baseline length increases beyond 10 kilometers (considered a long baseline condition), these errors at the rover and reference receiver antennas become increasingly different, so that their presence in the rover-reference differential GNSS observables and their influence on the ambiguity resolution process increases. Ambiguity resolution on single rover-reference receiver baselines beyond 10 kilometers becomes increasingly unreliable. This attribute limits the mobility of a GNSS-AINS with respect to a single reference receiver, and essentially makes it unusable on a mobile mapping platform that covers large distances as part of its mission, such as an aircraft.
A network GNSS method computes the position of a rover receiver using reference observables from three or more reference receivers that approximately surround the rover receiver trajectory. This implies that the rover receiver trajectory is mostly contained by a closed polygon whose vertices are the reference receiver antennas. The rover receiver can move a few kilometers outside this polygon without significant loss of positioning accuracy. A network GNSS algorithm calibrates the ionospheric and tropospheric signal delays at each reference receiver position and then interpolates and possibly extrapolates these to the rover position to achieve better signal delay cancellation on long baselines that could be had with a single reference receiver. Various methods of signal processing can be used, however they all yield essentially the same performance improvement on long baselines. As with single baseline GNSS, known GNSS solutions are still inadequate for a mobile mapping platform that covers large distances as part of its mission, such as an aircraft.
Another problem associated with mobile mapping/survey applications is an insufficiently fast recovery of positioning accuracy after a loss of the rover GNSS signal. The typical time to recovery of reliable precise positioning accuracy is 15-60 seconds, depending on the number of observables and their geometry used in the position solution. Such signal outages tend to occur on an aircraft engaged in a survey mission when the aircraft executes rapid high bank-angle turns ("sharp turns") from one survey line to the next. Sharp turns between survey lines provide the most economical execution of a survey mission. Typical survey trajectories include many parallel survey lines joined by 180-degree turns. Consequently these sharp turns and resulting signal outages can occur frequently. Previous GNSS-AINS implementations for this application required the pilot to fly low bank angle turns ("flat turns") to maintain the rover GNSS antenna orientation toward the sky and thereby avoid GNSS signal loss. Such flat turns required significantly longer times to execute than sharp turns, resulting in additional aircraft operation expenses.
The aiding sensors 5 are any sensors that provide navigation information that is statistically independent of the inertial navigation solution that the INS generates. Examples of aiding sensors are one or more GNSS receivers, an odometer or distance measuring indicator (DMI), and a Doppler radar velocity detector.
The purpose of the Kalman filter 4 in the AINS configuration is to estimate the errors in the inertial navigator mechanization and the inertial sensor errors. The Kalman filter 5 does this by comparing the INS navigation data with comparable data from the aiding sensors 5. The closed-loop error controller 3 then corrects the inertial navigator 2 to achieve a navigation accuracy improvement over what an unaided inertial navigator would be capable of achieving.
The Kalman filter 4 implements a recursive minimum-variance estimation algorithm that computes an estimate of a state vector based on constructed measurements. The measurements typically comprise computed differences between the inertial navigation solution elements and corresponding data elements from the aiding sensors. For example, an inertial-GNSS position measurement comprises the differences in the latitudes, longitudes and altitudes respectively computed by the inertial navigator and a GNSS receiver. The true positions cancel in the differences, so that the differences in the position errors remain. A Kalman filter designed for integration of an INS and aiding sensors will typically estimate the errors in the INS and aiding sensors. The INS errors typically comprise the following: inertial North, East and Down position errors; inertial North, East and Down velocity errors; inertial platform misalignment errors; accelerometer biases; and gyro biases. Aiding sensor errors can include the following: GNSS North, East and Down position errors; GNSS carrier phase ambiguities; and DMI scale factor error.
The error controller 3 computes a vector of resets from the INS error estimates generated by the Kalman filter and applies these to the inertial navigator integration processes, thereby regulating the inertial navigator errors in a closed-loop error control mechanization. This method of INS error control causes the inertial navigator errors to be continuously regulated and hence maintained at significantly smaller magnitudes than an uncontrolled or free-inertial navigator would be capable of achieving.
Kinematic ambiguity resolution (KAR) satellite navigation is a technique used in applications requiring high position accuracy such as land survey and construction and agriculture, based on the use of carrier phase measurements of satellite positioning system signals, where a single reference station provides the real-time corrections with high accuracy. KAR combines the L1 and L2 carrier phases from the rover and reference receivers so as to establish a relative phase interferometry position of the rover antenna with respect to the reference antenna. A coherent L1 or L2 carrier phase observable can be represented as a precise pseudorange scaled by the carrier wavelength and biased by an integer number of unknown cycles known as cycle ambiguities. Differential combinations of carrier phases from the rover and reference receivers result in the cancellation of all common mode range errors except the integer ambiguities. An ambiguity resolution algorithm uses redundant carrier phase observables from the rover and reference receivers, and the known reference antenna position, to estimate and thereby resolve these ambiguities.
Once the integer cycle ambiguities are known, the rover receiver can compute its antenna position with accuracies generally on the order of a few centimeters, provided that the rover and reference antennas are not separated by more than 10 kilometers. This method of precise positioning performed in real-time is commonly referred to as real-time kinematic (RTK) positioning.
The reason for the rover-reference separation constraint is that KAR positioning relies on near exact correlation of atmospheric signal delay errors between the rover and reference receiver observables, so that they cancel in the rover-reference observables combinations (for example, differences between rover and reference observables per satellite). The largest error in carrier-phase positioning solutions is introduced by the ionosphere, a layer of charged gases surrounding the earth. When the signals radiated from the satellites penetrate the ionosphere on their way to the ground-based receivers, they experience delays in their signal travel times and shifts in their carrier phases. A second significant source of error is the troposphere delay. When the signals radiated from the satellites penetrate the troposphere on their way to the ground-based receivers, they experience delays in their signal travel times that are dependent on the temperature, pressure and humidity of the atmosphere along the signal paths. Fast and reliable positioning requires good models of the spatio-temporal correlations of the ionosphere and troposphere to correct for these non-geometric influences.
When the rover-reference separation exceeds 10 kilometers, the atmospheric delay errors become decorrelated and do not cancel exactly. The residual errors can now interfere with the ambiguity resolution process and thereby make correct ambiguity resolution and precise positioning less reliable.
The rover-reference separation constraint has made KAR positioning with a single reference receiver unsuitable for certain mobile positioning applications such as aircraft positioning for conducting aerial surveys. An aircraft on a survey mission will typically exceed this constraint. One solution is to set up multiple reference receivers along the aircraft's intended flight path so that at least one reference receiver falls within a 10 km radius of the aircraft's position. This approach can become time-consuming and expensive if the survey mission covers a large project area.
Network GNSS methods using multiple reference stations of known location allow correction terms to be extracted from the signal measurements. Those corrections can be interpolated to all locations within the network. Network KAR is a technique that can achieve centimeter-level positioning accuracy on large project areas using a network of reference GNSS receivers. This technique operated in real-time is commonly referred to as network RTK. The network KAR algorithm combines the pseudorange and carrier phase observables from the reference receivers as well as their known positions to compute calibrated spatial and temporal models of the ionospheric and tropospheric signal delays over the project area. These calibrated models provide corrections to the observables from the rover receiver, so that the rover receiver can perform reliable ambiguity resolution on combinations of carrier phase observables from the rover and some or all reference receivers. The number of reference receivers required to instrument a large project area is significantly less than what would be required to compute reliable single baseline KAR solutions at any point in the project area. See, for example, U.S. Pat. No. 5,477,458, "Network for Carrier Phase Differential GPS Corrections," and U.S. Pat. No. 5,899,957, "Carrier Phase Differential GPS Corrections Network". See also Liwen Dai et al., "Comparison of Interpolation Algorithms in Network-Based GPS Techniques," Journal of the Institute of Navigation, Vol. 50, No. 4 (Winter 2003-2004) for a comparison of different network GNSS implementations and comparisons of their respective performances.
A virtual reference station (VRS) network method is a particular implementation of a network GNSS method that is characterized by the method by which it computes corrective data for the purpose of rover position accuracy improvement. A VRS network method comprises a VRS observables generator and a single-baseline differential GNSS position generator such as a GNSS receiver with differential GNSS capability. The VRS observables generator has as input data the pseudorange and carrier phase observables on two or more frequencies from N reference receivers, each tracking signals from M GNSS satellites. The VRS observables generator outputs a single set of M pseudorange and carrier phase observables that appear to originate from a virtual reference receiver at a specified position (hereafter called the VRS position) within the boundaries of the network defined by a polygon having all or some of the N reference receivers as vertices. The dominant observables errors comprising a receiver clock error, satellite clock errors, ionospheric and tropospheric signal delay errors and noise all appear to be consistent with the VRS position. The single-baseline differential GNSS position generator implements a single-baseline differential GNSS position algorithm, of which numerous examples have been described in the literature. B. Hofmann-Wellenhof et al., Global Positioning System: Theory and Practice, 5th Edition, 2001 (hereinafter "Hofmann-Wellenhof [2001]"), gives comprehensive descriptions of different methods of differential GNSS position computation, ranging in accuracies from one meter to a few centimeters. The single-baseline differential GNSS position algorithm typically computes differences between the rover and reference receiver observables to cancel atmospheric delay errors and other common mode errors such as orbital and satellite clock errors. The VRS position is usually specified to be close to the roving receiver position so that the actual atmospheric errors in the roving observables approximately cancel the estimated atmospheric errors in the VRS observables in the rover-reference observables differences.
The VRS observables generator computes the synthetic observables at each sampling epoch (typically once per second) from the geometric ranges between the VRS position and the M satellite positions as computed using well-known algorithms such as given in "Navstar GPS Space Segment/Navigation User Interface," ICD-GPS-200C-005R1, 14 Jan. 2003 (hereinafter "ICD-GPS-200"). It estimates the typical pseudorange and phase errors comprising receiver clock error, satellite clock errors, ionospheric and tropospheric signal delay errors and noise, applicable at the VRS position from the N sets of M observables generated by the reference receivers, and adds these to the synthetic observables.
A network RTK system operated in real time requires each receiver to transmit its observables to a network server computer that computes and transmits the corrections and other relevant data to the rover receiver. The reference receivers plus hardware to assemble and broadcast observables are typically designed for this purpose and are installed specifically for the purpose of implementing the network. Consequently, those receivers are called dedicated (network) reference receivers.
An example of a VRS network is designed and manufactured by Trimble Navigation Limited, of Sunnyvale, Calif. The VRS network as delivered by Trimble includes a number of dedicated reference stations, a VRS server, multiple server-reference receiver bi-directional communication channels, and multiple server-rover bi-directional data communication channels. Each server-rover bi-directional communication channel serves one rover. The reference stations provide their observables to the VRS server via the server-reference receiver bi-directional communication channels. These channels can be implemented by a public network such as the Internet. The bi-directional server-rover communication channels can be radio modems or cellular telephone links, depending on the location of the server with respect to the rover.
The VRS server combines the observables from the dedicated reference receivers to compute a set of synthetic observables at the VRS position and broadcasts these plus the VRS position in a standard differential GNSS (DGNSS) message format, such as RTCM, RTCA or CMR. The synthetic observables are the observables that a reference receiver located at the VRS position would measure. The VRS position is selected to be close to the rover position so that the rover-VRS separation is less than a maximum separation considered acceptable for the application. Consequently, the rover receiver must periodically transmit its approximate position to the VRS server. The main reason for this particular implementation of a real-time network RTK system is compatibility with RTK survey GNSS receivers that are designed to operate with a single reference receiver.
Descriptions of the VRS technique are provided in U.S. Pat. No. 6,324,473 of Eschenbach (hereinafter "Eschenbach") (see particularly col. 7, line 21 et seq.) and U.S. Patent application publication no. 2005/0064878, of B. O'Meagher (hereinafter "O'Meagher"), which are assigned to Trimble Navigation Limited; and in H. Landau et al., Virtual Reference Stations versus Broadcast Solutions in Network RTK, GNSS 2003 Proceedings, Graz, Austria (2003); each of which is incorporated herein by reference.
One or more embodiments of the present invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:
A Post-Mission High Accuracy Position and Orientation System (PM-HAPOS) is described below. Note that references in this document to "an embodiment", "one embodiment", or the like, mean that the particular feature, structure or characteristic being described is included in at least one embodiment of the present invention. Occurrences of such phrases in this specification do not necessarily all refer to the same embodiment.
The PM-HAPOS introduced here includes processing software and circuitry to implement a GNSS-AINS integrated with a network GNSS solution. A network of GNSS reference receivers that surround the mobile platform trajectory ("project area") is used to overcome the limitation on baseline length. The techniques introduced here use an implementation of inertially-aided RTK (IARTK) with GNSS. IARTK integrates the AINS Kalman filter and RTK engine, which has the benefit of significantly accelerating the ambiguity resolution process (e.g., from a typical 15-30 seconds to fix, to about 1 second) without compromising reliability. The PM-HAPOS operates on data recorded during one or more survey missions, and therefore includes a post-mission subsystem (e.g., a software package) that implements a GNSS-AINS integrated with a network GNSS solution. A particular embodiment described below implements a GNSS-AINS capable of processing reference observables from multiple reference receivers with a VRS algorithm.
The term "VRS", as used henceforth in this document, is used as shorthand to refer to any system or technique which has the characteristics and functionality of VRS described or referenced herein and is not necessarily limited to a system from Trimble Navigation Ltd. Hence, the term "VRS" is used in this document merely to facilitate description and is used without derogation to any trademark rights of Trimble Navigation Ltd. or any subsidiary thereof or other related entity.
Two possible applications of the PM-HAPOS introduced here are aerial photogrammetry and laser altimetry. Both applications require accurate position and orientation time histories of a camera or LIDAR to assign geographic position coordinates to the image pixels or laser ground spots, a process known as georeferencing. One advantage of using the PM-HAPOS introduced here in these applications is that a large project area can be instrumented with a few reference receivers that are located inside the project area or near its perimeter. For example, a project area with dimensions 100 km�100 km can be instrumented with as few as four reference receivers evenly distributed around the perimeter of the project area. These can be dedicated receivers or permanent receivers.
The PM-HAPOS introduced here can achieve several important performance attributes that previous GNSS-AINS implementations cannot achieve. These include:
1) Providing positioning with accuracy in the range of a few centimeters over large project areas in which the shortest baseline to a reference receiver is always long, i.e., greater than 20 kilometers.
2) Allowing for fast and reliable recovery of this positioning accuracy (e.g., within about 1 to 2 seconds) following a loss of rover GNSS signal outage. Such signal outages tend to occur on an aircraft engaged in a survey mission when the aircraft executes a high bank-angle turn from one survey line to the next. Typical survey trajectories include many parallel survey lines joined by 180-degree turns; consequently, these signal outages can occur frequently. Sharp turns provide the most economical execution of a survey mission. Previous GNSS-AINS implementations for this application required the pilot to fly flat turns with low bank angles to maintain the rover GNSS antenna orientation toward the sky and thereby no signal loss. Such flat turns required significantly longer times to execute than sharp turns, resulting in additional aircraft operation expenses.
It is to be noted that the techniques introduced here are not necessarily limited to these applications.
In this description, the terms "rover" and "mobile platform" are used interchangeably to refer to a vehicle that carries the survey sensors during a mobile survey/mapping mission. It is noted, however, that in other embodiments, the mobile platform or rover need not be a vehicle; for example, it could be a person.
The techniques introduced here use a network of GNSS reference receivers that surround the project area, to overcome the limitation on baseline length. These reference receivers can be a combination of dedicated reference receivers installed by the user and/or permanent receivers that are part of a network installed by some other agency, such as a local or national government for some other purpose such as earthquake detection or atmospheric research. Examples of such permanent receiver networks are the Continuously Operating Reference System (CORS) and the International GNSS System (IGS). Typically these permanent receivers provide access and data download via the Internet to the general public or to service subscribers.
In at least one embodiment, the PM-HAPOS includes a network adjustment subsystem 7 and a post-processing subsystem 36, as shown in
As described further below, the network adjustment subsystem 7 evaluates and corrects published antenna positions 6 for selected GNSS reference receivers. The post-processing subsystem 36 operates on data 20 acquired from the network of GNSS reference receivers as well as IMU data 15, rover GNSS data 16 and other aiding data 17 previously acquired and recorded during a mobile mapping/survey mission by a GNSS-AINS vehicle subsystem on the vehicle that carries the survey sensor. The output of the PM-HAPOS is a best estimate of trajectory (BET) 41, which is a highly accurate position and orientation solution for the mobile platform over the duration of the mapping/survey mission.
The vehicle subsystem 9 includes an IMU 10 mounted on or near the survey sensor so as to measure the sensor's accelerations and angular rates. The vehicle subsystem 9 further includes the rover GNSS receiver 11 and antenna and possibly other aiding sensors 12. The rover GNSS receiver 11 and antenna are an aiding sensor. The other aiding sensors 12 may include, for example, any one or more of the following: an odometer on a land vehicle that measures the distance traveled; a two-antenna GNSS compass that measures vehicle heading; a magnetic compass that measures vehicle heading; a laser distance meter that measures one or more distances to a fixed position; another position sensor such as a LORAN-C receiver; another velocity sensor such as a speed log on a ship or boat.
The vehicle subsystem 9 further includes a synchronization device 13 that generates a survey sensor synchronization signal. For example, on an aerial camera, the synchronization device 13 could be a mid-exposure pulse generator.
The vehicle subsystem 9 further includes a data acquisition computer 14 that receives the data streams from the IMU 10, GNSS receiver 11 and other aiding sensors 12, and records these to data files 15, 16 and 17, respectively, on one or more mass storage devices 37, such as one or more disk drives and/or flash memory cards. The data acquisition computer 14 can be part of a system with other functionality. For example, the data acquisition function can be part of an Applanix Position and Orientation System (POS), available from Trimble Navigation Limited, which also computes a real-time position and orientation solution.
As noted above, the rover GNSS receiver 11 is an aiding sensor. A reference GNSS receiver allows the Kalman filter in the IIN 19 to compute differential GNSS observables and thereby cancel the dominant errors in the rover GNSS observables. An AINS using differential GNSS observables generates a more accurate AINS navigation solution than it could by using uncorrected GNSS observables. In the technique introduced here, the VRS module 18 receives and uses reference GNSS observables from multiple fixed-location GNSS reference receivers distributed around the project area. The VRS module 18 may implement the VRS technique described by Eschenbach and O'Meagher (mentioned above). Note that VRS software which implements that technique can operate with any set of reference receiver observables, including permanent reference receiver observables. The rover GNSS data 16 and VRS GNSS data 24 are fed to the Kalman filter in the IIN 19 for the purpose of obtaining good control of the inertial navigation errors, to thereby generate an accurate navigation solution.
The VRS module 18 is essentially a network KAR subsystem. It receives as input the adjusted antenna positions 8 as well as the reference GNSS data 20 and rover GNSS data 16 that were recorded during the data acquisition period (i.e., during the survey mission), and uses them to compute and output a set of VRS GNSS data 24. The IIN module 19 inputs the VRS GNSS data 24 and data files 15, 16 and 17 (the IMU data, rover GNSS data, and data from other aiding sensors, respectively), and uses them to compute and output a set of smoother data 29. The smoother 40 inputs the smoother data 29 and uses that data to compute and output a final set of high-accuracy position and orientation data 41 for the rover for the data acquisition period; this set of position and orientation data is the BET, which is a highly accurate position and orientation solution and which is the final output of the PM-HAPOS.
The VRS module 18 includes VRS server software to compute a set of "synthetic" observables, i.e., observables for a virtual reference station (VRS). In certain embodiments, the position of the virtual reference station is taken as the geographic center of the project area. Note that the rover GNSS data 16 is used by the VRS module 18 to allow it to interpolate atmospheric delays to the recorded rover positions and apply those delays to the synthetic VRS observables.
The VRS module 18 is further illustrated in
The following is a description of the input data 8, 16 and 20 to the VRS estimation algorithm implemented by the VRS estimation module 21. The network comprises N reference receivers whose antennas are located at positions given by the Cartesian coordinates (xk, yk, zk) with respect to a terrestrial reference frame, such as WGS84 for k=1, 2, . . . , N. All subsequent Cartesian position coordinate specifications are given with respect to this coordinate frame, hereafter referred to as the "terrestrial reference frame". The transformation from these coordinates to any other is unique and well-defined, and therefore does not limit the generality of the algorithm.
Each receiver tracks L1 and L2 signals from M GNSS satellites. For the mth tracked satellite in m=1, 2, . . . , M, the nth reference receiver in n=1, 2, . . . , N generates the following observables on frequencies i=1 (L1) and 2 (L2): pseudorange observables βn,m i and carrier phase observables φn,m i.
All reference receivers generate the same broadcast ephemeris and satellite clock parameters for all satellites tracked by the receiver. These well-known parameters are specified in ICD-GPS-200 (referenced previously) and therefore not repeated here.
The precise ephemeris and clock parameters comprise periodic satellite positions in Cartesian coordinates with respect to the terrestrial reference frame and periodic satellite clock offset and drift parameters. These are available from various agencies that include NASA's Jet Propulsion Laboratory (JPL) and the International GNSS Service (IGS).
The VRS position is specified by its terrestrial reference frame coordinates (xVRS, yVRS, zVRS).
The geometry of the space segment (positions of orbiting satellites as viewed from each reference receiver) varies continuously, and the number of satellites M visible at each reference receiver changes with time t. The physical separation of any pair of reference receivers in the network is typically on the order of 10-100 km. The satellites are typically more widely dispersed, and therefore, their signals received at a given reference receiver probe largely different sections of the sky. A strong correlation between the ionospheric effects from receiver to receiver is therefore assumed, while the ionospheric effects from satellite to satellite are considered independent. Each satellite is (at this stage of processing) treated independently of the others for the entire period during which it is visible to the network. Differences between state estimates among different satellites are built later so that errors common to the satellites can be eliminated.
The following is a description of the VRS estimation algorithm, according to an embodiment of the invention.
The pseudorange or code observable from satellite m at carrier frequency i generated by receiver n is modeled as follows:
The carrier phase observable from satellite m at carrier frequency i generated by receiver n is modeled as follows:
Equation (2) characterizes the carrier phase as the integrated Doppler frequency, so that carrier phase increases in the negative direction as the range increases. Currently Navstar GPS offers signals at two wavelengths λ1=0.19029 m and λ2=0.24421 m. There is a known physical relationship between the ionospheric group delay for different wavelengths, which relates the effect experienced for waves of different frequencies to a first order approximation as follows
This approximation is fully sufficient for purposes of the technique introduced here.
The troposphere delay Tn,m, the clock offsets δTn and δtm and the true range between station and satellite rn,m are all independent of signal frequency. This fact can be exploited by taking the difference of the phase measurements for the station—satellite pairs to eliminate the frequency-independent parameters. From equation (2) the following geometry-free phase combination of L1 and L2 phases is obtained:
Note that Nn,m gf is not an integer and has units of distance (meters). The purpose of constructing and then processing the measurements φn,m gf is to determine the parameters Nn,m gf, MPn,m gf, and In,m within a consistent framework and consistent error estimates.
The linear ionosphere delay model of equation (3) allows the construction of L1 and L2 code and carrier phase combinations without ionosphere delay errors as follows. The ionosphere-free pseudorange is
The ionosphere-free carrier phase is
Note that Nn,m if is not an integer and has units of cycles. The purpose of constructing and then processing the measurements φn,m if is to determine the parameters Nn,m if, MPn,m if, and Tn,m within a consistent framework and consistent error estimates.
The range-equivalent ionosphere-free carrier phase is given by
The ionosphere-free code minus carrier observables combination is constructed to cancel geometric terms as follows:
The wide-lane carrier phase is given by
The wide-lane carrier phase minus narrow-lane pseudorange is constructed to cancel geometric terms and atmosphere delay errors.
Ultraviolet radiation and a constant stream of particles from the sun ionize the gases of the earth's atmosphere to produce a layer of charged gases called the ionosphere. A charged gas is a dispersive medium for electromagnetic waves such as GNSS signals. To a very good approximation, the refractive index ε for an electromagnetic wave of frequency f (in units of 1/second) is given as
The electron density of the ionosphere is known to have a pronounced maximum at an altitude of approximately 350 kilometers above ground. D. Bilitza, International Reference Ionosphere 2000, Radio Science 2 (36) 2001, 261 (hereinafter Bilitza [2001]) provides a detailed description. For this reason, the commonly called "lumped two-dimensional (2D) model" assigns the complete ionospheric effect to a thin shell surrounding the earth at this altitude. This is described first herein as an introduction to the subsequent model used by the VRS algorithm.
For each satellite m in view equation (30) contains parameters (I0,m, am, bm) to characterize the ionosphere across the network area. These parameters together with the carrier-phase integer ambiguity and multipath states are to be estimated. The other terms (mn,m iono, Δλn, ΔLn) in equation (30) are deterministic quantities given by the geometry of the network and the position of satellite m. Knowledge of these parameters allows equation (30) to the slant ionospheric delay Ir,m to be predicted at any roving receiver position r in the network.
The linear model given by equation (30) can be improved by taking account of the ionosphere thickness as described in Bilitza [2001] (referenced previously).
There are several different methods by which the troposphere delay along a slant signal path can be modeled for the purpose of estimating the delay in a least-squares estimation process. Hofmann-Wellenhof [2001] (referenced previously) provides a description of the theory behind various predictive models such as the Hopfield model. Most of these models comprise a model for the zenith troposphere delay at a given position multiplied by a mapping function that is a function of the zenith angle 67 of the satellite 65 at the receiver position 65 as shown in
Hofmann-Wellenhof [2001] (referenced previously) provides examples of different mapping functions. This algorithm specification is not dependent on which troposphere model or mapping function is implemented.
The predicted slant troposphere delay is then assumed to differ from the actual delay at each reference receiver by a scale factor Sn that lumps the different sources of prediction error for all satellite signal paths, as follows:
Given a set of troposphere scale factors S1, . . . , SN applicable at the N reference receiver positions, the following linear spatial interpolation model is used to construct the troposphere delay error at any position in the network.
The parameters S0, c and d are determined from a least-squares adjustment of the over-determined set of linear equations using any statistical information on S1, . . . , SN that may be available to weight the adjustment.
The troposphere delay at any position r in the network is then computed as
A set of M ionosphere filters 103 in
The state vector containing the state variables to be estimated for each satellite m in 1, 2, . . . , M is given by
The state transition matrix is given by
The process noise covariance is given by
The measurement vector contains the geometry-free phases (4) as follows:
A set of M code filters 104 in
The state transition matrix is
The code filter measurement for satellite m and receiver n is
A geometry filter 105 in
The geometry filter constructs measurements from the range-equivalent ionosphere-free carrier phases given by (17) and the ionosphere-free code minus carrier observables combinations given by (18). The ionosphere-free carrier phase measurement from satellite m and receiver n at a given measurement epoch is given as follows:
Referring again to
The VRS observables generation algorithm will now be described, according to one embodiment, with reference to
Process 115 in
In one embodiment of the invention, process 115 in
The precise ionosphere filter state vector becomes
The precise ionosphere filters process the following geometry-free phase measurements:
The precise geometry filter processes the following ionosphere-free measurements:
The transition matrix (51),(51), process noise covariance (52) and measurement model matrices derived from (64) and (65) are truncated to reflected the reduced state dynamics model. The resulting estimated state elements {right arrow over (S)}=[S1 . . . SN]T provide troposphere scale factors at the N reference receiver positions at a level of accuracy consistent with a fixed integer ambiguity position solution. These are used to construct the troposphere delay error at any position in the network using a linear spatial interpolation model (36).
The VRS observables ("VRS GNSS data") 24 (
The VRS carrier phases are constructed as follows:
These VRS observables have the same receiver clock offset, satellite clock errors, multipath errors and observables noises as the master reference receiver. They have the approximately same troposphere and ionosphere delay errors as the roving receiver. Consequently, single differences between rover and VRS observables will result in the approximate cancellation of troposphere and ionosphere delay errors as well as exact cancellation of satellite clock errors.
In another embodiment of the invention, process 115 in
A spatial interpolation model similar to (30) for the double-differenced residuals for satellite m in 1, . . . , M is given by:
The parameters δ∇Δ{circumflex over (φ)}0,m, �m i and {circumflex over (b)}m i are computed as estimates from a least-squares adjustment using the model (71) with measurements (70). The estimated residuals at the rover position are then given by
The VRS observables ("VRS GNSS data") 24 are then computed as follows. A master reference receiver R is identified from among the N reference receivers, typically the reference receiver that is closest to the VRS position. The VRS pseudoranges are computed using (67) with {circumflex over (T)}r,m−{circumflex over (T)}R,m computed from the interpolation model (36) using the model parameters derived in (35) from the estimated troposphere scale factors in the estimated geometry filter state (50), and �r,m−�R,m from the interpolation model (30) using the model parameters in the ionosphere filter states (39). The VRS carrier phases are constructed as follows:
Referring again to
The IIN module 19 includes an inertial navigator module 25, a Kalman filter 26, an error controller 27 and an ambiguity resolution module (ARM) 28. The details of the functionality and implementation of these components will be well-understood and by those of ordinary skill in the art in view of this description, and need not be described herein. The functionality of these elements is described here at a high level to provide context.
The inertial navigator module 25 solves Newton's well-known equations of motion on the earth in a conventional manner to compute the position, velocity and orientation of the vehicle IMU 10 from the recorded IMU data 15.
The Kalman filter 26 imports the inertial navigation solution 30 output by the inertial navigator module 25 and aiding sensor data 16,17 and 24, and estimates the inertial navigator errors and inertial sensor errors in a conventional manner using a conventional INS error model. The Kalman filter 26 outputs the estimated INS errors 31 to the INS error controller 27 and to a smoother data file 29.
The error controller 27 translates the estimated errors 31 into resets 33 to the inertial navigator's integration processes. It also returns a correction 32 to the Kalman filter to account for the correction 31 to the inertial navigator 25. The control loop formed by the inertial navigator module 25, Kalman filter 26 and error controller 27 is called the INS error regulation loop.
The Kalman filter 26 also estimates the floated carrier phase ambiguities in the combinations of rover and VRS carrier phase observables that are part of the rover GNSS data 16 and VRS GNSS data 24, respectively. These carrier phase observables typically include two or more frequencies to obtain fast and reliable floated ambiguity estimation and subsequent ambiguity resolution. The Kalman filter 26 outputs the estimated floated ambiguities and their covariances 34 to the ARM 28.
The ARM 28 employs any of various well-known conventional algorithms to determine the integer ambiguities from the floated ambiguity data 34 once these have converged to sufficient estimation accuracy. Once the ARM 28 has fixed and verified the integer ambiguities 35, it returns the verified integer ambiguities 35 to the Kalman filter 26, which then constructs precise and unambiguous carrier phase measurements that result in precise position error estimation and INS error regulation. The resulting INS navigation solution 30 contains very accurate position and accurate orientation data.
Referring again to
The manner in which the PM-HAPOS can be used will now be described with reference to
The overall process has three phases: mission planning, mission execution and post-processing. The mission planning phase begins when a photogrammetry analyst (PA) reviews the project area, the CORS receivers in and around the project area, and the project GNSS satellite coverage during a planned flight. The PA selects a set of five or more CORS receivers and a time window during which their data are required (step 45). The PA then downloads observables files from each selected CORS receiver for the previous 24 hours (step 46). Typically, these data are downloaded via the Internet.
Next, the PA runs the 24-hour data through the network adjustment subsystem 7 (step 47) to compute the relative position of each CORS antenna with respect to one selected antenna. This network adjustment either verifies or corrects the published antenna positions or rejects the observables from one or more CORS receivers because of bad data quality. The required relative accuracy of each antenna position is typically 1-2 centimeters. The PA then decides whether or not to use the network based on the results of the network adjustment (step 48). The network is considered to be acceptable if a sufficient number of CORS receivers evenly surround the project area and generate reliable observables as determined by the network adjustment. A sufficient number is typically four or more reference receivers. If the network is not acceptable, the PA selects a new network (i.e., a new set of CORS receivers) and repeats the previous steps. Once the PA has configured an acceptable network, the mission planning is finished. The mission execution phase can then proceed.
In the mission execution phase, an aircraft equipment operator (AEO) responsible for operating the aircraft-mounted camera and supporting equipment typically starts the vehicle subsystem (VS) data acquisition just before the survey begins, e.g., a few minutes before the aircraft starts to taxi towards take-off (step 49). During flight, the vehicle subsystem 9 acquires data (i.e., IMU data 15, rover GNSS data 16 and other aiding sensor data 17) and stores it on one or more removable (disconnectable) mass storage devices 37 (e.g., disk drives) within the vehicle subsystem 9 (step 50). The AEO typically turns off the vehicle subsystem data acquisition after the aircraft has landed and taxied to a stationary position (step 51). The AEO then retrieves all of the recorded data, including the camera image files and recorded vehicle subsystem data (step 52), by removing the mass storage device(s) from the vehicle subsystem 9, to complete the mission execution phase.
In the post-processing phase, the PA loads the recorded vehicle subsystem data onto the post-processing subsystem 36 (step 53). This can be done by, for example, directly connecting the mass storage devices from the vehicle subsystem to the post-processing subsystem 36 and loading the data from those devices, or by loading the data from some other data storage facility to which the data may have been stored or transferred after the flight. Next, the PA downloads the CORS observables spanning the data acquisition period (the photogrammetry mission time) via the Internet, to the post-processing subsystem 36 (step 54).
The PA then activates the post-processing subsystem 36 to cause it to execute a sequence of operations to compute a precise position and orientation solution (the BET 41) for the recorded GNSS-AINS data, for the entire data acquisition period, based on the recorded GNSS-AINS data files. (The BET 41 will subsequently be associated with the corresponding images from the survey camera.) Note that the term "solution" in this context refers collectively to a large number of associated position and orientation data items, each associated with a different instant in time. Note that the following sequence of operations can be performed automatically in response to a single initiating user input by the PA, or each operation may be initiated individually by the PA.
First, the VRS module 18 in the post-processing subsystem 36 computes a VRS synthetic observables file (the VRS GNSS observables 24) based on the reference receiver observables (reference GNSS data 20) and history of real-time rover antenna positions (step 55). The IIN 19 then computes the position and orientation solution 30 based on the VRS synthetic observables file and the data recorded by the vehicle subsystem 9 (step 56). The smoother module 40 then computes the BET 41 (step 57), which is the end product of this process.
While the embodiments described above use IARTK with GNSS, it is possible to implement a GNSS-AINS that uses VRS but does not implement IARTK. Such a system can run a separate RTK module to resolve ambiguities and can then correct the rover-reference differential phase observables to obtain the precise differential phase ranges that characterize RTK positioning. These precise phase ranges then would be used as aiding data in the Kalman filter.
Thus, a PM-HAPOS has been described.
Software to implement the technique introduced here may be stored on a machine-readable medium. A "machine-accessible medium", as the term is used herein, includes any mechanism that provides (i.e., stores and/or transmits) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant (PDA), manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-accessible medium includes recordable/non-recordable media (e.g., read-only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; etc.), etc.
The term "logic", as used herein, can include, for example, hardwired circuitry, programmable circuitry, software, or any combination thereof.
Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense.
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Darren Bent took advantage of a rare start to put the visitors ahead but Emmerson Boyce levelled before an own goal from Nathan Baker just before half-time put the FA Cup winners ahead.
Wigan were pegged back on the hour mark by Ron Vlaar's brilliant volley, and that was the way it stayed.
Asked whether he will still be at the DW Stadium next season, he told Sky Sports: "No, it's not something is of the concern at the moment. We'll enjoy tomorrow and then we'll sit down with the chairman (Dave Whelan). The football club is in a fantastic position whatever happens.
"It's something to be looking forward to the future. Wigan Athletic is extremely bright and there will be great times ahead."
The Spaniard felt the draw epitomised his side's season - great going forward, but leaky at the back.
"It's not just going forward or defending, it's both boxes," he added. "For long spells we were dominant and our movement and attacking play was outstanding.
"The one shot they had on target they scored and that's why we are where we are.
"I'm just so proud and delighted that they've been able to show what they can show in the other tournament and winning the FA Cup is an incredible achievement.
"I think it was a reflection of why we haven't been able to get that consistency at the back.
"I thought we created enough chances to win the game comfortably, but clearly that was the sign of the season."

subset: commoncrawl


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Something's Fishy at the Science Museum…
On October 1st, students from Roosevelt School (Bridgeport, CT) came to the Museum to participate in the first shark dissection of the 2010-2011 school year. By the looks on their smiling faces it was clear this group of young scientists were ready for a rare treat!
Mr. Dave, a trained marine biologist, led the 7th and 8th grade class through an hour-long lesson about shark anatomy and biology. Students were eager – or hesitant in some cases! – to explore the shark's coiled, spiral-like stomach and its oil-filled liver. Mr. Dave warned the students of leaking liver oil, which sharks produce in great quantities to help control their buoyancy in the water.* The girls were squeamish to learn that the very same oil can be found in vitamins and cosmetic products they buy in stores!
The lesson was not all slicing and dicing, however, as Mr. Dave urged students to be aware of the conservation issues facing global shark populations. He noted that these specimens were raised in captivity rather than hunted in the wild, and that although sharks are often killed as an exotic food item or for other use by humans, in the United States we have set higher standards for shark conservation and do not practice shark hunting.
At the end of the day, all of the students left with a greater understanding of why and how sharks are studied by scientists at SMLI and beyond.
* H. David Baldridge, Jr., 1970. Sinking factors and average densities of Florida sharks as functions of liver buoyancy. Copeia 4 (Dec. 12, 1970), pp. 774-754.


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Scott Snyder - Bringing Batman Back
By Mistermorriss 2 Comments
After having read Batman comics for the last 25 years, I have to say that up until now, Scott Snyder is the most consistent comic writer I've read. In the past, the great Batman writers (Loeb, Miller, Moench, Wagner) have made fleeting appearances in graphic novels, one-shots and have had limited runs in the comics themselves. Now however, we finally have a writer who is at least as good as the aforementioned luminaries as the chief writer of Batman: The New 52. According to the man himself, he's here to stay and that's the best news any Batman fan could ask for as the balance of his Batman stories have so far been perfect, with equal helpings of the Dark Knight as a detective and Batman the brawler, not to mention Bruce Wayne as a believable businessman and patriarch.
My first glimpse of Snyder was in an interview on Batman: Year One Animated where he said that he saw Batman's rogues gallery as individual elements of Batman's psyche. Each villain represents a personal demon, one in which Bruce knows he must overcome, thus explaining his drive and doggedness when it comes to defeating his nemeses; he must constantly prove to himself that he's not like them, that he's better – he can keep the madness the he's so familiar with at bay. Comics legend Denny O'Neill was visibly surprised and interested in that comment as was I. It showed that the young Snyder had an intriguingly fresh take on the character that demanded attention.
The Black Mirror: Here Snyder deftly shows us how Grayson's Batman differs from Bruce.
His initial run in Detective Comics #871-#881 was the closing chapter before The New 52 reboot and it gave all of us a taste of the kind of hero we would be encountering in the future. In these 10 comics we experienced Snyder's writing style and his love for the Batman universe. I was particularly impressed at ow he took the mantle from Christopher Nolan and carried his investigation of Gotham City as a living, breathing entity, with its own personality into new territories.
The Black Mirror starts with Grayson wondering if he still knew Gotham, whether the city had changed, whther there was a darkness that he didn't know any more. There are many incidences of these kinds of questions in all of the opening issues but Dick, like Bruce, still believes that Gotham can house goodness as well as evil; thus further defining the role of Batman, just as in Nolan's trilogy. This not only gives the universe more depth but it also adds another layer to Batman's role within the universe: that of the healer, the believer.
A scene from Scott's Gordon story: Skeleton Cases – pencils by Francesco Francavilla
What was even more pleasing, in fact probably the most pleasing thing I've read in Batman since Miller's incredible Batman: Year One, was the amount of attention shown to Jim Gordon. In Skeleton Cases and Skeleton Key we learn about Gordon's estranged and seemingly psychotic child, James. In issue #874 we don't even Batman once and the issue is all the more stronger for it – Gordon takes centre stage. It was a brave move by both Snyder and DC but one that was a master stroke. Frank Miller's landmark Year One planted Gordon firmly in our hearts and Snyder being of our generation (he's a year younger than me), obviously wanted to show his appreciation and did so by creating an emotional Gordon-family thriller that keeps you guessing throughout.
As we discussed in our Court of Owls review, Snyder creates his own piece of Gotham history and adds an interesting element to Dick Grayson's back-story. Here Snyder creates a new villain, brings back Alfred's past, adds to Grayson's and makes Batman synonymous with the city – an amazing undertaking. In Snyder's new universe Batman is no longer Gotham's son, he is Gotham City. We could say that this narrative shift perhaps now makes Gotham a part of his rogues gallery, part of him: he must rid the city of it's darkness in order to heal himself.
One thing that stuck out whilst reading The Court of Owls crossovers was the considerable rise on quality when the timeline demanded I picked Snyder-penned Batman comic, rather than a Red Robin or a Nightwing. All due respect to the other writers, but the difference was dramatic.
So now we have Death of the Family, the return of the Joker. The title alone is Snyder yet again showing he understands and appreciates the history of the character and is recalling the dark images surrounding Jason Todd's brutal death (cleverly reminisced in The Black Mirror it has to be said).
Greg Capullo's enticing and poised picture is gracing the cover of Batman #17.
I have a golden rule that I don't read an arc until its finished and I now rue that rule. I really want to start reading but I know I'll be left hanging and the wait will be too much. The hype surrounding the series has been enormous, the comics are selling for three times the price of their original value. The premise is wickedly simple: The Joker thinks that Batman is more fun when he's on his own, and perhaps has become weaker by relying on his cronies, so he plans to kill them all so it's just him and Batman again; the good old days. Big promises have been made as to the fate of certain characters while the question of whether The Joker knows Bruce is Bats lingers.
After years of wandering through the wilderness, Snyder has put Batman back on track. The days of Batmite and time travel are hopefully buried forever. Scott Snyder's considered and respectful renaissance of the greatest super hero of all time has been laden with fresh twists but remains firmly rooted in the elements that made, and continue to make, the character enigmatic.
In 2012, Scott Snyder has won various Stan Lee Awards and the Eagle Award for Best Writer, these complement his Eisner and Harvey Awards for American Vampire. As a reader of of Batman comics for all of my adult life, I hope Snyder continues to maintain the superb quality of writing beyond the current Joker run. His passion for his craft resonates from the pages and if you haven't read one of his Batman stories yet, hopefully this article has convinced you to give them a try. You wont regret i


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Prenatal Exposure To Mercury And Childhood Risk For ADHD
Tuesday, November 27th, 2012
The role of prenatal influences on development can be profound. But it is still a very murky science, as was beautifully illustrated in Annie Murphy Paul's "Origins: How The Nine Months Before Birth Shape The Rest Of Our Lives". Case in point: a new study that looks at the role of prenatal exposure to mercury and risk for ADHD.
Forget about the study details – let's cut to the two bottom lines of the study. First, documented mercury exposure during pregnancy (validated using gold standard methods) was indeed predictive of risk for ADHD in the offspring. Second, eating fish during pregnancy, which is sometimes thought to be a risk factor for mercury exposure, was protective with respect to ADHD.
So…what does a pregnant woman do with these findings?
Well, right now, not too much. This study did not identify the sources of mercury exposure, so it's hard to say how to prevent it. And it's not entirely clear which type of fish to eat, and what type to avoid.
Now, this is kind of where the science is at these days. It's very important research, and not easy to conduct. But it's worth keeping in mind, if you are following it, that it will be a long and winding road before the studies sort through all the complexities and possible contradictions and yield empirically validated guidelines.
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Tags: ADHD, eating fish during pregnancy, mercury, Pregnancy, prenatal, prenatal exposure to mercury | Categories: Behavior, Health, Intervention, Must Read, Parenting, Pregnancy, Red-Hot Parenting


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Accommodations close to Western Wall are shown below sorted in the order of their distance (closest at the top). Browse our hotels to find accommodations near Western Wall in Jerusalem Israel for your upcoming travels.
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What do you think? Give us your opinion. Anonymous comments allowed.
#306 - aikochan (06/29/2010) [-]
im surprised you're still here. :P on this site. its even worse now than the last time i was here
#307 to #311 - Arsonist (06/29/2010) [-]
yea what can I say? I've been here too long to give it up, was anonymous since its beginning. And if you haven't noticed, the majority of the internet has turned to **** . Know whose fault it is? Myspace and Facebook. They made the internet easy for old people and young kids to access and now they are all over. I've got a feeling everything is going to go back though. Just like the stock market, it's following some kind of pattern.


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Candlewood Suites Kenosha
Candlewood Suites Kenosha
Conveniently located halfway between Milwaukee and Chicago, Kenosha is a thriving city on the shores of Lake Michigan. The close proximity to General Mitchell International Airport (MKE) and Chicago's O'hare International (ORD) make traveling to the area effortless.
As a corporate traveler, you'll appreciate the convenient location of our hotel. Home to Snap-On, Jockey, and Uline's world headquarters, Kenosha has a flourishing corporate community. Other major corporations include Amazon and Meijer distribution centers and the Lakeview Corporate Park and Kenosha Business Park- all within 5 miles of the hotel.
Visiting the Kenosha area for fun? There is no shortage to be had here! Explore Downtown Kenosha on the picturesque shores of Lake Michigan. With museums, restaurants, and Kenosha's famed Saturday Farmers Market, it's the perfect place to spend the day. Pleasant Prairie Premium Outlets boast over 90 stores including: Nike, Michael Kors and Restoration Hardware. Other must-see Kenosha attractions include Frank's Diner, The Jelly Belly Factory Store, and Uke's Harley Davidson.
Spend your day at work or play and then join us back at the hotel. You'll enjoy preparing dinner in your room or out on the grills located on our back patio. Get a good night sleep in one of our suites and wake up refreshed for another day in our great city!
Our Candlewood staff looks forward to welcoming you to your home away from home!
Quick Info
- Check In: 3:00 PM
- Check Out: 12:00 PM
- Minimum Check-In Age: 21
- Parking: Free
- Print Fact Sheet
- Chat With Us
- On-site Guest Self-Laundry Facilities
- Standard Wireless Internet service is free for all members
- On-site Fitness Center Available
It's hard to leave your best buddy at home when you're traveling for extended periods, so we provide a pet friendly experience. Restrictions and fees apply.
Whether you are booking suites for a family reunion or suites for your company's training needs, Candlewood Suites hotels can accomodate your group.
Workout whenever you want: Candlewood Gym always has an open door and lots of aerobic and cardio equipment to make it easier to stick to your exercise routine, no sweat.
At Candlewood Suites® hotels, we want you to experience comfort in every sense.
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Guest Reviews
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extended stalys
Byntsureof return
July 29, 2015
extended stays usually offer some kind of breakfast for free.
extended stay rooms should be cleaned twice a week
spacious suite, comfortable stay
July 23, 2015
i stayed during a time when rates seemed to be inflated due to increased demand for rooms. the value for the room was fine. we used the kitchen amenities and liked the convenience of them. everything was clean! we knew that the hotel did not have a pool when reserving a room, but our kids really missed that feature.
great service, sleep
July 20, 2015
I did not sleep but it had nothing to do with the bed. I have difficulties sleeping in general no matter where I am. The air conditioner was loud though, but not a reason for sleep dififuclties.
Like having your own apartment!
July 16, 2015
We always try to stay at this Candlewood Suites hotel when attending the Bristol Renaissance Faire. We have always found it to be clean, great kitchen with cooking utensils, pots, pans and dishes. Lending closet was great for our need of wine glasses. Staff is great. Highly recommend this place. (Only small complaint was the noisiness of the air conditioner in the living room when running.) Super convenient location just off the highway with restaurants, shopping, and Costco just across the street.


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Inspired by high-performance Formula 1 automotive technology, the TAG Heuer Formula 1 is the definitive casual watch - bold case, balanced colors and design, yet unpretentious, functional and super tough, with all the specific characteristics of a professional TAG Heuer sports watch. Launched in 1986 and entirely revamped in 2004, the TAG Heuer Formula 1 was the favorite watch of F1 legends like Ayrton Senna, Alain Prost, David Coulthard, Kimi Räikkönen and Fernando Alonso. It is one of the TAG Heuer's most successful lines to date, due to high-grade components and the versatility of the design. Beach, gym or on holiday, uptown or at home, the TAG Heuer Formula 1 looks great everywhere.
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text: Join us for a formal or casual dining experience in one of our nine exclusive spaces, including our Main Dining Room, Bar, or outdoor Patio. Our menus feature seasonal ingredients and impeccable presentations. In addition to our lunch buffet and evening a la carte menu, we have fresh seafood & beef features and full bar service available. We have a unique, historical venue available for weddings, rehearsal dinners, corporate sales meetings, birthday parties, retirement celebrations, and everything in between.


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Hundreds Attend Patten Memorial Service
More than four hundred people attended the memorial service of a Maine climbing ranger on Friday. 34-year-old Nick Hall fell to his death during a rescue operation a few weeks ago in Washington. News Source 8s Gina DiPietro has more on the service from Patten.
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I. Introduction to Viruses and HIV
II. Why Gene Therapy Might be a Reasonable Tool for Attacking HIV
III. The Grand Challenge: Engineering Immunity
Part III: The Grand Challenge: Engineering Immunity
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Lecture Overview
In this set of lectures, I describe the threat facing the world from the human immunodeficiency virus (HIV) and a bold proposal on how we might meet the challenge of eliminating this disease by engineering the immune system. In part 1, I provide a broad introduction to viruses, describing their basic properties and my own history of studying the replication RNA viruses which led to the discovery of reverse transcriptase. I also illustrate the distinguishing features of equilibrium viruses (e.g. the common cold) that have adapted to co-exist with their host and non-equilibrium viruses (e.g. HIV) that have recently jumped from another species, are not adapted to the new host, and which can lead to disastrous outcomes (e.g. loss of immune function with potential lethality in the case of HIV).
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Speaker Bio
After serving as President of the California Institute of Technology for nine years, in 2006 David Baltimore was appointed President Emeritus and the Robert Andrews Millikan Professor of Biology. Born in New York City, he received his B.A. in Chemistry from Swarthmore College in 1960 and a Ph.D. in 1964 from Rockefeller University, where he returned to serve as President from 1990-91 and faculty member until 1994.
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Related Talks
- Ari Helenius iBioSeminar: The Cell Biology of Virus Entry
- Don Ganem iBioSeminar: The Biology of Karposi's Sarcoma
- David Baltimore Short Clip: Introduction to Viruses
- Joseph DeRisi iBioEducation: Genome Sequencing for Pathogen Discovery


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Armed and dangerous
Samuel L. Jackson is in negotiations to star as a mysterious supervillain in the Frank Miller-helmed comic book adaptation "The Spirit" for Lionsgate and Odd Lot Entertainment.
The story centers on Denny Colt, an ambitious young cop murdered in the line of duty who is reborn as the masked mystery man known as the Spirit.
Jackson would play the Spirit's nemesis, the Octopus, a meek lab assistant who reinvents himself as a psychotic nightmare that kills anyone unfortunate enough to see his face. The Octopus' tentacles reach into every aspect of crime in fictitious Central City, a city he plans to wipe out.
Miller said during an introduction of the project at the Festival de Cannes that Jackson was his first choice to play the evil genius. He added that he is beginning to mull different actors to play the comic book hero.
'Righteous' men
Robert De Niro and Al Pacino are teaming to play New York police investigators hunting a serial killer in Jon Avnet's $60 million thriller "Righteous Kill" for Avi Lerner's Millennium Films and Emmett/Furla Films.
The feature written by Russell Gewirtz ("Inside Man") unites the longtime friends on-screen for some 90 percent of the film unlike their one shared scene in Michael Mann's "Heat" and no shared scenes in Francis Ford Coppola's "The Godfather Part II."
"You see those two icons on-screen together for virtually the whole film," Lerner said at his Festival de Cannes offices, "(something) never seen before in the history of cinema. It's one of the hardest deals we've ever done to put these two actors together."
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Robert Letcher – The right to keep and bear arms versus the right of five and six-year olds to live was probably not in the minds of the Framers of the Constitution
Pat Robertson: Words Matter
Free Riders: Why is Health Care Any Different from Food Stamps?
Cenk Uygur: Can Progressives Reclaim the Democratic Party?
Harsh Enforcement May Not Drive Unauthorized Immigrants Out
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Arizona Congressman Grijalva Challenges Obama and Mexico
Marijuana Initiative Challenges Costly, Bloody Drug War
Tom Hayden: I support the November ballot initiative because our country's long drug war is a disaster and there is an alternative that is better for our health, safety and democratic process.
Dude, Let Students Help Make Those Teacher Cuts
Nea Friberg-Price & Jed Von Dielingen: Because our school makes these cuts strictly on seniority, it's always the newer teachers who go, the ones who connect so much better with us students. Athough we like and respect our older, more experienced teachers, these younger ones are more flexible in their teaching plans and are closer to us in age, so they understand us so much better. So we're terribly sad to see their teaching careers end, at least for now.
Campaigning For and Being President As Two-Way Learning: A Benchmark
Robert Letcher: It's not that I expected Obama to communicate with me about his policy positions, although I certainly hoped he would – even though I don't wait around for him to call. Instead, it just seemed to me that his campaign – our campaign – would likely improve his prospects for winning if he were somehow to really communicate with this me and all the other me's whose support he needs in order to be elected and govern successfully.


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If I was a gambler, I would not go to any casino that allows smoking. I wonder if the new Ohio ones will be non-smoking? I would definitely go there if it was non-smoking or to Windsor where it is non-smoking as well. I won't support any Michigan casino because of the stupid smoking laws in this state.
Why yes!It would not bother me at all. I would not miss any electricity. A tree is a living thing and electricity causes air pollution and global warning. (coal is bad) if that means not being able to use the my computer...so be it. I am not teathered to my electrical machine like you are BBm...
BTW CINNA...I did not call Mr Riney a "ding dong".
He is a hero in my book...he stands up for the environment and his beliefs...HURRAY!!!
I am not all that surprised that this is going on in Manchester....There are alot of farms and it could hidden from view easily. (Manchester resident here)
But to state, "really isn't a very harmful substance"...well you must be a pothead.LOL!
Memory loss, lung cancer...I would call those harmful dude!
Calling this awesome man a "ding dong" makes you look stupid A'amlive".
Anyone who stands up for their beliefs is fantastic in my book!
Bill, you are the best..Hug and save all trees...
No stupid...(democratic voter here) I say the parents and grandparents are at fault for not explaining gun safety to their son or taking him to a gun safety class. They are at fault...if I would ban anything it would be dumbasses like you Norsehawk and bad parents.
You people make me laugh....Any Michigan driver could have done that &0 mph in a 50 mph zone. it happens all the time and it doesn't mater what nationality the driver is.
All of you are ugly racists that need to STFU!
I am sure the Art Fair (or is it NOT ART NOT FAIR)
brings in alot of money but to all the vistors...use the recycle containers that are all over and don't throw your trash on the ground. Be courteous drivers...don't use your horn unless it is a emergency (it's the law) Be patient with all pedestrians and people in wheelchairs at the fair.
Then please go home! lol!
Even if it was a person his own age, that does not excuse what he did to his parents and family. His family must have been sick with worry. My first comments were directed to the story about the Saline female teenager who ran off to Puerto Rico to meet up with an older man. I am not suggesting anything else...although bruceae is suggesting something.
I am wondering if there is something wrong with the teens in Saline? This is the second time this has happened. First it was a female and now this male. Parents need to keep an eye on their kids inline activities and not just the females!! There are female pedophiles out there too.
I would love to go to this festival every year but $15 is TOO steep to pay in this economy...why so expensive? This is much better festival than that stupid Elvisfest in Ypsitucky...oops Ypsilanti...lol!
right on Joezoo...why would any woman want to put a pedophile on their back? Wake me up when all this Michael Jackson stuff is over....There were better human beings that passed away on the same day that will be missed more than MJ. I am sure they contributed more to society than MJ did....druggiescum! :((((
I have no idea why Ypsilanti citizens would allow a representation of Elvis to attend a Fourth of July parade. He was a drug addict and a pedophile for God sakes! The Fourth is about America's independance. If I had my way, I would ban that stupid Elvis festival. What a waste of money and time.
well 1john and djm12652...ha ha...sarcasm is so funny coming from two stupid males. How would you like your female loved one to be attacked in one of these parking ramps? Would you laugh then? It has happened there! I am taxpayer and I pay for using these ramps but I won't anymore. I think they are unsafe and I guess the city council doesn't care about womens safety...just like these two bozos...1john and djm12652 :P
Rates going up again? Well if the City Council keeps doing this, why not put up security cameras? I have never seen any in the parking ramps. Are there any at all? There have been some really scary characters in the lots and I am very hesitant to park there. Security should be on the forefront of the CC always!!
I think that woman is horrible!! I have shopped in that store and the owner is a very sweet man who does not deserve this at all. How can someone do this? The morals of this country amaze me.
I feel so sorry for the Jimmy John's employees...I would quit after seeing that line. Customers are sooo aggressive when it comes to their food...like dogs really! LOL!!!
Oh how horrible! This part of US 23 is a speed death trap. You have all the drivers trying to get off and on Washtenaw from the cloverleaf and then the speedy drivers going north or south. Please slow down! Where are State Of Michigan police?? They need to issue more speeding tickets in this area.
My deepest condolences to the family of this driver!!
I am looking forward to the chocolate shop but no way will I step foot into Tio's. The old place was a dump and it was so dirty! How do two white owners know anything about true Authentic Mexican food?
I am so glad that this horrid man has been captured. My heart goes out to these poor girls. But that being said...where were their parents while they were communicating with this weirdo? Parents HAVE TO monitor their kids ( and I mean both boys and girls) when the kids are on facebook and myspace and the entire Internet!! I find it disturbing that these girls were chatting with a 32 yr old man. Did the parents warn them about online predators? i have to say I fault the parents the most here.
I remember when Jeff came into the Ann Arbor store where I sued to work...we were told a "celebrity" was inside shopping and not to make a fuss...when Jeff and his wife (?) came to the counter, I thought to myself.."what is the big deal?" (he was much cuter back then)
The funniest thing was that his credit card was declined!!!
This also happened to a former co-worker (another store) and his credit card was declined as well.This time he got really mad at my former co-worker. Bad form there Jeff!!


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Blog Comments & Posts
5 Reasons You Should Link Out to Others From Your WebsiteBlog Post: February 24, 2009
We're Hiring an SEOBlog Post: February 17, 2009
Linkscape Index Update Doubles Size!Blog Post: December 08, 2008
Help! My Industry Is Boring And My Imagination Is HurtingBlog Post: October 22, 2008
Announcing SEOmoz's Index of the Web and the Launch of our Linkscape ToolBlog Post: October 06, 2008
Sigh... More Ignorance About SEOBlog Post: August 18, 2008
Unwritten Google Webmaster Guideline: Don't End URLs in .0Blog Post: June 13, 2008
Whiteboard Friday - Give It Up!Blog Post: June 13, 2008
Whiteboard Friday - Using Trust Rank to Guide Your Link BuildingBlog Post: March 28, 2008
Haha, brilliant! :)
I think perhaps we're at the sharp end of a culture difference here. Being a good ol' stiff upperlipped Brit, the music especially sets off alarms for the American shopping channels which have been invading our TV the last few years.
It does look professional though :)
Also, Rand does a great job of basically explaining this www.vldb.org/conf/2004/RS15P3.PDF (which is worth a read if you want a more tech view on TR/Spam).
Keep up the good work.
I Hate the Advertising Bait-and-Switch, But There's a Lesson HereBlog Post: March 27, 2008
Generation Google a General DisappointmentBlog Post: February 29, 2008
Sounds a bit disappointing! Being a "mere" 24 years myself, the answers provided didn't surprise me either. To be honest, understanding the mechanics of the web/search is pretty intrinsic to the "generation Google", so I don't find it amazingly impressive that 16yr olds are getting into SEM/affiliate stuff.
There's a split for me though, I like reading some of the, um "older" SEMers blogs because just their extra years give them a better context to place current techniques/trends in a bigger picture – so you get a better view of the current market. However, I hate it when people say "oh I've been doing SEO for 10 years", like it's some kind of demonstration of skill. I'd always ask if they're so good at SEO, why have they been doing it so long and not made some big bucks and retired? There's obviously exceptions to this and people that stay in the industry, but they are the rarity.
Some of the best SEO people I've found have been in the quietest corners of the net and they don't blog/go to conferences. They just learn their game and within a couple of years their a 10k a week person.
Job-wise, I think SEM may be approaching a saturation spike over the next 5 years :o)
Google Dominating Hollywood as Well as the Search DemographicBlog Post: October 01, 2007
I have actually seen a movie where they used Yahoo! search. However, it was an incredibly poor movie and resulted in (can you tell they're geeks) people jumping on the sofa shouting "omgz! he's using Yahoo! now, what a n00b!".
Haha :) Sinister if you think Google is placing products intentionally :)
My Personal Opinion - 90% of the Rankings Equation Lies in These 4 FactorsBlog Post: September 26, 2007
QR Codes And Physical Hyperlinking, No More Typing And No More Search Engines?Blog Post: September 23, 2007
Isn't Google in the middle of a project driving around in vans taking photos of just about everything? I thought they were aiming so you can snap a picture on your phone and using matching technology you can retrieve information on that CD, restaurant etc...
Maybe I just dreamt that, but it would be cool.
An Open Challenge: Applying Jim Collins' Good to Great - Asking the Tough QuestionsBlog Post: September 24, 2007
Whiteboard Friday - Link WarsBlog Post: September 21, 2007
The Wisdom & Folly of Directory Link BuildingBlog Post: July 24, 2007
Before I probably get beat down out of existance I'll say I have he highest respect for Rand & Co - I think SEOmoz is one of the very few "seo celebrity" blogs worth reading..
However, I don't see where you're coming from with this, why make directory submission so much more complicated than it is? My thought is, if you've built up a medium/strong link profile (i.e. your site isn't new) then go to town the directories - you can get 1200 submissions for $200.
Sure some links may be a bit crud - but who cares? Are you telling me those couple of hundred cruddy links will damage my rankings?? If so - brilliant, I've got lots of competitors I'm going to buy directory submissions for then! The crud ones don't even want email verification!
It's not as if you're going to get a link growth spike either. With hundreds of small directories, it sometimes takes months to get all the backlinks indexed - so it actually looks like organic link growth.
So directories - I can do the big boys like DMOZ myself, punt $200 at over 1,000 directories, some will stick, some will miss - who cares? It took 2 minutes of my time and $200 of my money. There are better things you can be spending your time on, rather than directory submissions.


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Dr. Dorval completed a community pharmacy residency in Albany, NY before moving with her family to St. Thomas, US Virgin Islands. It was during her time at Chelsea Drug Store, Inc., where she was able to use her residency training and knowledge to initiate medication management programs and precept pharmacy students. She was blessed to have the opportunity to move to Florida in 2009 and begin her academic career at the Gregory School of Pharmacy. Her skills as a community pharmacist are able to assist her not only in the classroom but also in the role of director of experiential programs.
Dr. Dorval is one of the advisors for the American Pharmacist Association (APhA-ASP). She also is a member of the prescription drug abuse task force with the Palm Beach County Substance Abuse Coalition. Her areas of interest include non-prescription medication, self-care and Alzheimer's disease.
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Life at PBA is about gaining perspective, and finding amazing opportunities to explore your faith, your potential and your world.


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That's all fine and dandy, but with the programs I use, ATi just isn't an option. We use NVIDIA cards at work, and the migration to ATi isn't a good idea right now. If I bring the work home and I can't work on the footage, then I'm screwed. I can always try an ATi card to see if it's compatible, but then I'll just end up losing money on restocking fees, return shipments, etc., if it's not. Trust me, the price/performance is something I've considered when I was deciding to get an ATi card. Sure, I may be able to play games with
"high detail levels AND good framerate"
like you said, but if that's all I can do, it's just not worth it to me. Like I said, it may be more of a hassle than it's worth at this point.
With GPU companies talking more with developers about implementation of OpenGL, CUDA, Stream, etc., it's a no-brainer in post-production. And right now, I'm not seeing a huge market share for
in post-processing houses. It's practically non-existent. Everything is CUDA. Until I see them implementing more of it, I don't think I'm willing to take that chance right now. OpenGL is fine, but if I need CUDA, I'm screwed. Unless I can offload it to my 9600GT that I'll use for PhysX, then I'll be okay I think.
Like I said, I don't want to spend more than $200 on a card. $200 without tax and shipping is really pushing it. It's just not in my budget. I'm not a high-end gamer, I'm more mid-entry/mainstream user. The 5850 is practically useless to
at this point. Guess we'll have to wait and see.
Ah fair enough. That is special needs indeed
On a sidenote I don't hope for (or foresee) any wider success for proprietary systems such as PhysX or CUDA (for gaming). I hope it goes away tbh and replaced by a platform where Nvidia, ATI and others can still complete in performance and quality, not exclusiveness!
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The New York Fed released their household credit and debt report yesterday, and they show increases in student loan debt relative to other forms (mortgage, credit card, auto loan and others). To keep this in perspective, mortgage debt represents 71% of all debt, and student debt represents about 8-9%. So there's still a wide gulf here, and we shouldn't jump to call this the "student loan bubble." The scales of the markets are in no way similar.
However, student loan debt is defaulting at higher and higher rates, and that should raise a lot of concern, since it has downstream effects for recent college graduates throughout the economy.
A recent New York Fed study found that 94% of recent graduates had borrowed to help pay for their education, and average debt levels among student borrowers is $23,000. Remember, that average includes seasoned borrowers, who presumably borrowed less and also in many cases reduced the principal amount of their loans, so the average amount borrowed by recent grads is certain to be higher. Student debt is senior to all other consumer debt; unlike, say, credit card balances, Social Security payments can be garnished to pay delinquencies. As a result, it has contributed to the fall in the homeownership rate, since many young people who want to buy a house can't because their level of student debt prevents them from getting a mortgage […]
Student loan delinquencies are getting into nosebleed territory. The Wall Street Journal, citing New York Fed data, tells us that student debt outstanding increased 4.6% in the last quarter. Repeat: in the last quarter. Annualized, that's a 19.7% rate of increase* during a period when other consumer borrowings were on the decline. And this growth is taking place while borrower distress is becoming acute. 11% of the loans were 90+ days delinquent, up from 8.9% at the close of last quarter. The underlying credit picture is certain to be worse, since many borrowers aren't even required to service loans (as in they are still in school or have gotten a postponement, which is available to the unemployed for a short period). And it was the only type of consumer debt to show rising delinquency rates.
This is the new subprime: escalating borrowing taking place as loan quality is lousy and getting worse. And in keeping with parallel to subprime, one of the big reasons is, to use a cliche from that product, anyone who can fog a mirror can get a loan.
The WSJ blames a push toward federal student lending, and I agree that loosened standards and an assumption toward student loans to pay for college is a bad trend (though the feds have also promoted 529 and Coverdell savings plan for families to get a head start on education funding). But the bigger problem here is the rising cost of higher education generally. You can argue that the accessibility of easy lending facilitates the cost rises – the market cannot adapt if there's no such thing as a breaking point for cost. But I do think this boards of trustees must come up for some blame here as well. Too much money in higher ed goes toward fat salaries for administrators and unnecessary building renovations. There doesn't seem to be any oversight over the process.
Student debt was a prime motivator for the Occupy movement. Some real oversight in higher education costs would go a long way toward driving that problem back down. But the easy accessibility to lending should raise concerns as well.