Layers of ionised gas caused by solar radiation, located between 60 and 200 miles above the earth’s surface, and varies daily and with seasons and latitude.
Radio transmission energy that continues through the ionosphere (without reflection or fraction) into space. Space waves neither follow the curvature of the earth, nor bend around obstructions.
Two signals are transmitted from the ground station, the first is the non-directional reference phase, and the second is the rotating variable phase. Both signals are transmitted 30 times per second. The variable phase transmits cyclically around the 360 degrees of the azimuth approximately 30 times per second. The reference phase signal is transmitted every time the variable phase transmission sweeps past magnetic north. Radial information is derived from the difference in time between the two signals (which would be different for each radial); this computation is referred to as phase difference, and is displayed by the airborne equipment.
In the upper left quadrant, the CDI is deflecting right, indicating to the pilot that the aircraft is to the left of course. To intercept the selected 030° Radial, the aircraft must be flown in the direction of the needle, but this only works if the aircraft is oriented to the OBS selection. Note the FROM indication, showing that the aircraft is to the north east of the 90° off-setting radials—a line combining the 300° and 120° Radials. In the upper right quadrant, everything is the same as the upper-left except the needle is deflecting left. Here (upper right quadrant) the pilot must fly left to intercept the selected 030° Radial.
Rather than a traditional VOR display, the position of an aircraft relative to a VOR transmitter can also be displayed by way of a Radio Magnetic Indicator (RMI) display. With an RMI, a bearing indicator continually points to the VOR station. The bearing indicator, in turn, is superimposed on an azimuth display that is virtually identical to the aircraft’s heading indicator. The azimuth, driven by a remotely mounted slaved compass, continually adjusts itself to reflect the aircraft’s heading—rotating just as the heading indicator rotates. In the example below, the aircraft is flying a heading of 086°, the bearing to the VOR is 055°, and the aircraft is currently crossing the 235° Radial. As discussed below, an RMI display can also be used to display ADF navigation information.
ADF information is conventionally displayed by one of three means, the first of which is referred to as a fixed card bearing indicator, the second is referred to as an adjustable bearing indicator, and the third is referred to as a Radio Magnetic Indictor (RMI).
In the case of the fixed-card bearing indicator, the ADF needle displays the relative bearing of the transmitter station—i.e., the bearing of the transmitter station relative to the longitudinal axis of the aircraft (relative bearing is discussed beginning P. 68). The ADF display card remains fixed with the 360° bearing set at the top of the azimuth display.
ILS must have a final approach fix (FAF) which provides position confirmation; the FAF is either an NDB or a DME fix.4 The FAF is typically 4 NM from the runway,5 but vary between 3.5 and 6 NM.
The transmitter of a Localizer signal is located on the centre line at the far end of runway for which is serves (i.e., off the departure end).
A Localizer has specified areas of reliability. Outside this area, indications must be regarded as inaccurate. Approximately 18 NM from the transmitter, the Localizer is reliable 10° either side of the on-track indication, while 10 NM from the transmitter, reliability increases to 35° either side of the on-track indication (below).
DME signals are UHF and are therefore subject to line-of-sight limitations. DME frequencies are paired with VOR frequencies; therefore, by selecting the VOR VHF signal, the appropriate UHF frequency is automatically selected on the DME equipment—but you must pay close attention to depiction on navigation charts. DME is also paired with military TACAN signals—accordingly, while VOR equipment in civil aircraft cannot utilise TACAN signals (as TACAN signals are UHF), civil DME equipment can dial in TACAN frequency and obtain distance information. Note that VORTAC stations (combining military UHF and civil VHF) provide DME data.
The initial position of the aircraft is entered into the computer (usually the precise longitude and latitude of the aircraft on the airport ramp), and a system of two gyro-stabilised accelerometers measure acceleration of the aircraft relative to a north-south and east-west electronic grid.
The latest generation of INS systems—referred to as IRS for Inertial Reference System—uses laser sensors rather than gyros, and the date is directly inputted into an aircraft flight management system.
The term “RNAV” is currently used in two meanings, the first referring to all Area Navigation System—INS, IRS, GPS (also referred to as GNSS, meaning “Global Navigation Satellite System”), LORAN C, etc.—and the second sense denotes an airborne system that utilises conventional VORTAC and DME ground stations. In this section, RNAV denotes the latter conventional ground stations.
The system is based on the measurement of distances between the airborne receiver and satellites, thereby establishing position information based on triangulation. Precise calculation of distances is made possible by atomic clocks carried on each satellite, and distance is derived from measuring the time required for a satellite signal to reach the airborne receiver. The current accuracy of GPS with respect to horizontal and vertical positions can be 6 meters and 8 meters, respectively, 95% of the time.
Five measurements with good geometry concerning the satellite positions are needed to provide RAIM; since an IFR GPS utilises as a source of measurement the aircraft’s altimeter, the minimum number of satellites is four. If the measurements are inconsistent, the pilot is notified and RAIM does not exist.
There are numerous augmentation systems for GPS—systems that enhance the accuracy and reliability of GPS position determination. Perhaps the more prominent of these is the Wide Area Augmentation System (WAAS) has been developed by the U. S. FAA, and consists of a network of 29 ground stations (referred to as WRS—Wide-area Reference Stations---these are located the continental U.S., Hawaii, and Puerto Rico), and four geostationary satellites. GPS/WAAS receivers obtain position data from both the GPS satellite network, as well as ground stations (communicated via the WAAS stationary satellites) to civil position determination to within 1.0 meters laterally and 1.5 meters vertically. WASS systems exist in Japan, Europe, and India.
We’ll end this section on GPS with a bunch of acronyms: “TSO” stands for “Technical Standing Order”, and GPS systems certified for IFR use that do not use WAAS fall under TSO-C129; GPS systems that do utilize WASS fall under TSO-C146a (required for Commuter and Airline operations).
In contrast, SSR relies on airborne transponder and measure data based on SSR interrogation and transponder replies.
It is characterised as a short range, high definition PSR system that provides highly accurate information related to altitude, azimuth and range—sufficient accuracy to be classified as a “precision approach” comparable to an ILS system.
12 The GPS system is composed of three segments: the Space Segment, the Control Segment, and the User Segment. The Space Segment is composed of the 24 NAVSTAR satellites that orbit the earth. The satellite orbits are precisely planned so that the entire surface of the earth may use the GPS system 24 hours per day, every day. There are almost always more than six satellites in view from every location on Earth. The Control Segment consists of a network of ground-based monitoring and control stations, with the Master Control Station located in Colorado Springs. The satellite data is collected by other ground stations, but is assimilated and analysed in Colorado Springs. The Master Control Station has facility for the “uplinks” required, for example, to make system clock corrections. The User Segment includes pilots with GPS.
14 PAR facilities are far more common in the U.S.