Title of Invention: APPARATUSES AND METHODS FOR TRACKING A NAVIGATION RECEIVER Abstract: A satellite positioning component calculates a first position associated with a navigation receiver at a first time. An inertial positioning component calculates a second position associated with the navigation receiver at the first time. A combination positioning component provides a reference position, combines the first position and the second position into a third position based on distances among the first position, the second position, and the reference position, and locates the navigation receiver according to the third position. Detailed Description: RELATED APPLICATION This application claims priority to Chinese Patent Application No. 201210333348.3, filed on Sep. 10, 2012, with the State Intellectual Property Office of the People's Republic of China, which is hereby incorporated by reference in its entirety. BACKGROUND A satellite navigation system, e.g., a global positioning system (GPS), allows a satellite navigation receiver, e.g., a GPS receiver, to determine its location and velocity based on satellite signals. The GPS may include a constellation of GPS satellites orbiting the Earth. There can be at least four GPS satellites visible at a given time and a given place on the Earth's surface. Each GPS satellite continuously broadcasts GPS signals at a predetermined frequency. The GPS signals contain time and orbital information for the satellites. A GPS receiver can receive the GPS signals transmitted from at least four GPS satellites synchronously. Based on the time and orbital information from at least four GPS satellites, geographical coordinates of the GPS receiver including latitude, longitude, and altitude can be calculated. In places such as parking garages, tunnels, urban canyons, and near trees, the satellite signals may be unavailable or degraded because lines of sight to the satellites are blocked. Consequently, the geographical coordinates calculated by the GPS receiver may be inaccurate. A device such as a vehicle, a personal digital assistant (PDA) or a cellular phone can be equipped with a navigation system that includes both a GPS and an inertial positioning system, e.g., a dead reckoning (DR) system. The DR system includes a speedometer and a gyroscope that sense the speed and orientation of the moving device. Based on a previously determined position, the speed and the orientation, the DR system estimates the device's current position. However, the accuracy of the DR system decreases over time. A conventional navigation system is capable of combining the results from the GPS and the DR system based on position dilution of precision (PDOP) of the GPS. The PDOP value represents the additional multiplicative effect of GPS satellite geometry, which further affects the precision of the GPS positioning. For example, if the PDOP value is greater than a threshold value, indicating that the position determined using the GPS has a relatively low precision, then the navigation system uses the position fix calculated by the DR system to locate the vehicle. If the PDOP value is less than the threshold value, indicating that the position determined using the GPS has a relatively high precision, then the navigation system uses the position fix calculated by the GPS to locate the vehicle. However, factors other than the PDOP value may reduce the precision of the GPS. In some circumstances, errors present in the pseudo-range associated with the GPS can increase but the PDOP value is still lower than the threshold value. In other words, the GPS positioning result has relatively low accuracy but the PDOP value indicates that the precision of the GPS is good. Thus, the navigation system still chooses the result from the GPS to locate the system, which degrades the accuracy of the navigation system. SUMMARY In one embodiment, a satellite positioning component calculates a first position associated with a navigation receiver at a first time; an inertial positioning component calculates a second position associated with the navigation receiver at the first time; and a combination positioning component provides a reference position, combines the first position and the second position into a third position based on distances among the first position, the second position, and the reference position, and locates the navigation receiver according to the third position. BRIEF DESCRIPTION OF THE DRAWINGS Features and advantages of embodiments of the claimed subject matter will become apparent as the following detailed description proceeds, and upon reference to the drawings, wherein like numerals depict like parts, and in which: FIG. 1 illustrates a block diagram of a navigation apparatus, in an embodiment according to the present invention. FIG. 2 illustrates a block diagram of a memory, in an embodiment according to the present invention. FIG. 3 illustrates an example of a travel track of a navigation apparatus, in an embodiment according to the present invention. FIG. 4 illustrates another example of a travel track of a navigation apparatus, in an embodiment according to the present invention. FIG. 5 illustrates a flowchart of operations performed by a weight unit, in an embodiment according to the present invention. FIG. 6 illustrates a flowchart of operations performed by a validity checking module, in an embodiment according to the present invention. FIG. 7 illustrates a flowchart of operations performed by a special condition module, in an embodiment according to the present invention. FIG. 8 illustrates a flowchart of operations performed by a distance module, in an embodiment according to the present invention. FIG. 9A illustrates a diagram of an example of a reference position, a position P1 and a position P2, in an embodiment according to the present invention. FIG. 9B illustrates a diagram of another example of a reference position, a position P1 and a position P2, in an embodiment according to the present invention. FIG. 9C illustrates a diagram of another example of a reference position, a position P1 and a position P2, in an embodiment according to the present invention. FIG. 10 illustrates a diagram of another example of a reference position, a position P1 and a position P2, in an embodiment according to the present invention. FIG. 11 illustrates a diagram of another example of a reference position, a position P1 and a position P2, in an embodiment according to the present invention. FIG. 12 illustrates a flowchart of operations performed by a filter, in an embodiment according to the present invention. FIG. 13 illustrates a flowchart of examples of operations performed by a navigation apparatus, in an embodiment according to the present invention. DETAILED DESCRIPTION Reference will now be made in detail to the embodiments of the present invention. While the invention will be described in conjunction with these embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Embodiments described herein may be discussed in the general context of computer-executable instructions residing on some form of computer-usable medium, such as program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or distributed as desired in various embodiments. Some portions of the detailed descriptions which follow are presented in terms of procedures, logic blocks, processing and other symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. In the present application, a procedure, logic block, process, or the like, is conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, although not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present application, discussions utilizing the terms such as “calculating,” “providing,” “combining,” “locating,” “weighting,” “comparing,” “determining” or the like, refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. Embodiments described herein may be discussed in the general context of machine- or computer-executable instructions residing on some form of machine- or computer-readable storage medium, such as program modules, executed by one or more computers or other devices. By way of example, and not limitation, machine/computer-readable storage media may comprise non-transitory machine/computer-readable storage media and communication media; non-transitory machine/computer-readable media include all computer-readable media except for a transitory, propagating signal. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or distributed as desired in various embodiments. Non-transitory machine/computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as machine/computer-readable instructions, data structures, program modules or other data. Machine/computer storage media includes, but is not limited to, random access memory (RAM), read only memory (ROM), electrically erasable programmable ROM (EEPROM), flash memory or other memory technology, compact disk ROM (CD-ROM), digital versatile disks (DVDs) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store the desired information and that can accessed to retrieve that information. Communication media can embody machine/computer-executable instructions, data structures, and program modules, and includes any information delivery media. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency (RF), infrared, and other wireless media. Combinations of any of the above can also be included within the scope of machine/computer-readable media. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. Embodiments in accordance with the present invention provide a navigation apparatus including a navigation receiver. The navigation receiver includes a processor that executes machine-executable components stored in a machine-readable medium to locate the navigation apparatus. The machine-executable components include a satellite positioning component that calculates a first position associated with the navigation receiver at a first time according to satellite signals, and include an inertial positioning component that calculates a second position associated with the navigation receiver at the first time according to motion signals indicating a speed and an orientation of the navigation receiver. Advantageously, the machine-executable components further include a combination positioning component that provides a reference position, combines the first position and the second position into a third position based on distances between the first position, the second position, and the reference position, and locates the navigation apparatus according to the third position. The distances between the reference position, the first position, and the second position indicate the precisions of the positioning results associated with the satellite positioning component and the inertial positioning component, no matter what may be the cause for the inaccuracy of the first position and/or the second position. For example, either a relatively poor position dilution of precision (PDOP) value of GPS or an error of the pseudo-range can cause the difference between the first position and the reference position to be greater than a threshold, which indicates that the first position is inaccurate. In this condition, the combination positioning component calculates the third position totally relying on the second position associated with the inertial positioning component. Therefore, compared to a conventional navigation system, the accuracy of the navigation apparatus is improved. FIG. 1 illustrates a block diagram of a navigation apparatus 100, in an embodiment according to the present invention. In one embodiment, the navigation apparatus 100 is utilized in a device such as a vehicle, a mobile phone or a portable computer. In the example of FIG. 1, the navigation apparatus 100 includes a receiver 102, an antenna 104, and a motion sensor 106. The antenna 104 receives satellite signals 103 transmitted from multiple satellites, e.g., global positioning system (GPS) satellites. The motion sensor 106 senses motion of the navigation apparatus 100, and provides motion signals 105. In one embodiment, the motion sensor 106 includes a speedometer and a gyroscope that sense speed and orientation of the navigation apparatus 100, respectively. Thus, the motion signals 105 contain information about the speed and the orientation of the navigation apparatus 100. The receiver 102 receives the satellite signals 103 and the motion signals 105, and locates the navigation apparatus 100 accordingly. In one embodiment, the receiver 102 includes a satellite signal receiver 112, a timer 113, a motion signal receiver 114, a processor 116, and memory 118. The timer 113 provides a reference clock signal 136. The motion signal receiver 114 receives the motion signals 105, and provides digital motion data 132 indicating the speed and the orientation of the navigation apparatus 100. The satellite signal receiver 112 coupled to the antenna 104 is capable of analyzing the satellite signals 103 and providing acquisition and tracking data 130 accordingly. More specifically, in one embodiment, the satellite signal receiver 112 analyzes the satellite signals 103 to determine if one or more corresponding satellites are within view of (visible to) the receiver 102. If a satellite is acquired, the satellite signal receiver 112 tracks the satellite to provide the acquisition and tracking data 130. The acquisition and tracking data 130 contains information about the tracked satellites, such as a Coarse/Acquisition code, GPS date and time, ephemeris data, etc. The acquisition and tracking data 130 and the motion data 132 are stored in the memory 118 and are accessed by the processor 116 to position the navigation apparatus 100. The processor 116 can be a central processing unit (CPU), a microprocessor, a digital signal processor, or any other such device that can read and execute programming instructions. The memory 118 stores multiple machine-executable components that are executed by the processor 116. In one embodiment, the machine-executable components include a satellite positioning component 122, an inertial positioning component 124, a combination positioning component 126, and a data file 128. The data file 128 includes multiple machine-readable data sets such as the acquisition and tracking data 130 and the motion data 132, which can be read by the processor 116. When executed by the processor 116, the satellite positioning component 122 calculates a position P1 associated with the navigation apparatus 100 at a current time t1 according to the acquisition and tracking data 130, e.g., the position P1 can be a GPS positioning result. The inertial positioning component 124 calculates a position P2 associated with the navigation apparatus 100 at the current time t1 according to the motion data 132, e.g., the position P2 can be a dead reckoning (DR) positioning result. Advantageously, the combination positioning component 126 provides a reference position PREF, combines the position P1 and the position P2 into a position P3 based on distances between the reference position PREF, the position P1, and the position P2, and further determines the final location point LT1 of the navigation apparatus 100 at time t1 based on the position P3. As will be further described in relation to FIG. 2 and FIG. 3, the distances between the reference position PREF, the position P1, and the position P2 indicate the precisions of the positioning results generated by the satellite positioning component 122 and the inertial positioning component 124, no matter what may cause the inaccuracy of the position P1 and/or the position P2. For example, either a relatively poor position dilution of precision (PDOP) value of GPS or an error of the pseudo-range can cause a distance between the position P1 and the reference position PREF to be greater than a threshold DTH1, which indicates that the position P1 is inaccurate. In this condition, the combination position P3 can be generated according to the position P2 rather than the position P1. Therefore, compared to the conventional navigation system, the accuracy of the navigation apparatus is improved. FIG. 2 illustrates a block diagram of the memory 118, in an embodiment according to the present invention. Elements labeled the same as in FIG. 1 have similar functions. FIG. 2 is described in combination with FIG. 1. In one embodiment, the memory 118 stores machine-executable components including a satellite positioning component 122, an inertial positioning component 124, and a combination positioning component 126. The memory 118 further stores a data file 128 including machine-readable data sets. As shown in the example of FIG. 2, the data file 128 includes acquisition and tracking data 130, signal intensity data 232, motion data 132, GPS position data 222, DR position data 226, reference position data 224, a satellite flag 238, an inertial flag 236, a combination flag 234, weight data 262, combination position data 264, and location data 268. The data file 128 can include other data sets and is not limited to the example of FIG. 2. The processor 116 accesses the data sets in the data file 128, and executes the machine-executable components to locate the navigation apparatus 100. In one embodiment, the satellite positioning component 122 includes a GPS module 242 and a flag setting module 244. The GPS module 242, when executed by the processor 116, calculates the position P1 of the navigation apparatus 100 at the current time t1 according to the satellite signals 103. More specifically, in one embodiment, the GPS module 242 reads the acquisition and tracking data 130 from the data file 128, and receives the reference clock signal 136 generated by the timer 113. The GPS module 242 employs the reference clock signal 136 to extract a ranging code (e.g., a Coarse/Acquisition code) and navigation data from the acquisition and tracking data 130. The ranging code includes a pseudorandom noise code (PN or PRN code) that identifies a corresponding satellite. Each satellite has a unique pseudorandom noise code. Pseudoranges between the tracked GPS satellites and the GPS apparatus 100 can be obtained from the ranging code. The navigation data contains GPS date and time, ephemeris data indicating a position of a corresponding satellite, and almanac data indicating information and status concerning all the satellites. Geographical coordinates of the tracked GPS satellites can be obtained from the navigation data. As such, based on the obtained pseudoranges and the geographical coordinates associated with at least four GPS satellites, the GPS module 242 calculates the position P1 of the navigation apparatus 100, e.g., at a current time t1. For example, P1 can be represented by a set of coordinates (x1, y1, z1). In one embodiment, the GPS position data 222 in the data file 128 indicates position fixes calculated by the GPS module 242. Thus, the GPS module 242 updates the GPS position data 222 in the data file 128 with the coordinates (x1, y1, z1), such that the GPS position data 222 contains data indicating the position P1 calculated by the GPS module 242. In one embodiment, the flag setting module 244 is executed by the processor 116 to control the satellite flag 238 and the signal intensity data 232 contained in the data file 128. The satellite flag 238 indicates the validity of the position P1. The signal intensity data 232 indicates the intensity level of the satellite signals 103. More specifically, in one embodiment, the flag setting module 244 checks if there is any abnormal condition associated with the GPS module 242 and sets the satellite flag 238 accordingly. For example, the flag setting module 244 identifies the number of visible satellites that are acquired and tracked, e.g., at time t1, according to the acquisition and tracking data 130. If the number of visible satellites is less than a predetermined value, e.g., four, the flag setting module 244 sets the satellite flag 238 to a first value, e.g., digital 0, to indicate that the position P1 is invalid. Moreover, the flag setting module 244 identifies whether the position P1 is a wild point. A wild point represents a positioning point that apparently departs from the trace of the apparatus 100. For example, if the navigation apparatus 100 is on a mountain, and the position P1 indicates that the navigation apparatus 100 is in the ocean, then the calculated position P1 is a wild point. The flag setting module 244 sets the satellite flag 238 to the first value indicating that the position P1 is invalid if the position P1 is identified as a wild point. If, however, no abnormal conditions are discovered, the flag setting module 244 sets the satellite flag 238 to the second value (e.g., digital one) indicating that the position P1 is valid. In one embodiment, the flag setting module 244 identifies the intensity level of the satellite signals 103 according to the acquisition and tracking data 130 and sets the signal intensity data 232 according to the intensity level. By way of example, the signal intensity data 232 can be set to 1 and 2 indicating that the intensity of the satellite signals 103 is weak and strong, respectively. The intensity of the satellite signals 103 affects the precision of the GPS positioning result P1. For example, the position P1 can be more accurate when the signal intensity data 232 has the value 2 than when the signal intensity data 232 has the value 1. In one embodiment, the inertial positioning component 124 includes a DR module 252 and a flag setting module 254. The DR module 252, when executed by the processor 116, estimates the position P2 associated with the navigation apparatus 100 at the current time t1 according to the motion data 132 and a previous position fix, e.g., calculated at a previous time t0 prior to the current time t1. In one embodiment, the DR position data 226 stored in the data file 128 indicates the position fix calculated by the DR module 252, e.g., the position P2. In one embodiment, the DR module 252 selects the previous position according to the signal intensity data 232 and the satellite flag 238. More specifically, if the signal intensity data 232 indicates that the intensity of the satellite signals 103 is strong at time t0, and if the satellite flag 238 indicates that the position P1—T0 is valid at time t0, the DR module 252 reads the GPS position data 222 from the data file 128 to obtain the position fix P1—T0. Then, the DR module 252 reads the motion data 132 from the data file 128 to acquire the speed and the orientation of the navigation apparatus 100 at time t0. Based on the speed, the orientation, and the position fix P1—T0, the DR module 252 calculates a position P2—T1 at time t1. If the signal intensity data 232 indicates that the intensity of the satellite signals 103 is weak at time t0, or if the satellite flag 238 indicates that the position P1—T0 is invalid at time t0, the DR module 252 reads the DR position data 226 from the data file 128 to obtain the position fix P2—T0 that is calculated by the DR module 252 at time t0. Furthermore, the DR module 252 reads the motion data 132 from the data file 128 to acquire the speed and the orientation of the navigation apparatus 100 at time t0. Based on the speed, the orientation, and the position fix P2—T0, the DR module 252 calculates the position P2—T1 associated with the navigation apparatus 100 at time t1. Therefore, if the satellite signals 103 remain at a low intensity level or the position P1 remains invalid, then the DR module 252 continues to estimate the DR positioning result according to a previous DR positioning result as time passes. Because of errors associated with the speedometer and the gyroscope, the error of the positions calculated by the DR module 252 accumulates under those circumstances. In both circumstances, the position P2 can be represented by a set of coordinates (x2, y2, z2). In one embodiment, the DR module 252 updates the DR position data 226 with the coordinates (x2, y2, z2), such that the DR position data 226 includes data indicating the position P2 at time t1. The flag setting module 254 is executed by the processor 116 to control the inertial flag 236 contained in the data file 128, in one embodiment. Similar to the flag setting module 244, the flag setting module 254 checks whether an abnormal condition is associated with the motion sensor 106, and sets the inertial flag 236 accordingly. By way of example, the motion sensor 106 including the speedometer and the gyroscope perform self-examinations once the sensors 106 are powered up. The flag setting module 254 analyzes the motion data 132 to check whether the self-examinations have been finished. If either the speedometer or the gyroscope is running the self-examination, the flag setting module 254 sets the inertial flag 236 to one value (which may also be referred to herein as the third value), e.g., digital zero, indicating that the position P2 is invalid. If the self-examinations are finished and no other abnormal condition is present, then the flag setting module 254 sets the inertial flag 236 to a different value (which may also be referred to herein as the fourth value), e.g., digital one, indicating that the position P2 is valid. The combination positioning component 126 is operable for combining the position P1 and the position P2 into the position P3. In one embodiment, the combination positioning component 126 includes a coordinate system transformer 202. In one embodiment, the position P1 and the position P2 are generated in accordance with different coordinate systems. For example, the coordinates (x1, y1, z1) are calculated by the GPS module 242 in an Earth-Centered Earth-Fixed (ECEF) coordinate system. The coordinates (x2, y2, z2) are calculated by the DR module 252 in a North East Up (NEU) coordinate system. If the positioning results from the GPS module 242 and the DR module 252 are generated in different coordinate systems, the coordinate system transformer 202 transforms coordinates in one system to corresponding ones in the other. In one embodiment, the coordinate system transformer 202 transforms the coordinates (x2, y2, z2) in the NEU system to corresponding coordinates (x2′, y2′, z2′) in the ECEF system. Alternatively, the coordinate system transformer 202 transforms the coordinates (x1, y1, z1) in the ECEF system to corresponding coordinates (x1′, y1′, z1′) in the NEU system. As such, the position P1 and the position P2 are labeled in the same coordinate system, which makes the combination operations more convenient. In one embodiment, the combination positioning component 126 further includes a weight unit 204, a combination unit 206, a reference estimator 218, and a filter 220. The weight unit 204 provides weight data 262 indicating weights A1 and A2 for the positions P1 and P2. In one embodiment, the weight A1 or A2 has a value between 0 and 100% and a sum of the weights A1 and A2 is equal to 1. The weights A1 and A2 are used to represent the relative importance of the position P1 and P2 when generating the combination position P3. Upon obtaining the weights A1 and A2, the combination unit 206 combines the positions P1 and P2 into the position fix P3. In one embodiment, the combination unit 206 weights the positions P1 and P2 to provide weighted positions A1*P1 and A2*P2, and combines the weighted positions to obtain the position P3. Assuming the combination operation is performed in the ECEF coordinate system, the position P3 can be given according to equation (1): P3=A1*P1+A2*P2=(A1*x1+A2*x2′,A1*y1+A2*y2′,A1*z1+A2*z2′). (1) According to equation (1), if A1 is equal to 100% and A2 is 0%, the position P3 is generated fully relying on the position P1 generated by the GPS module 242. If A1 is equal to 0% and A2 is 100%, the position P3 is generated fully relying on the position P2 generated by the DR module 252. Moreover, if both A1 and A2 are equal to an amount greater than 0% and less than 100%, the position P3 is generated relying on both the position P1 and the position P2. In this condition, if A1 is greater than A2, the position P3 is generated relying on the position P1 more than the position P2, and vice versa. In one embodiment, the combination unit 206 updates the combination position data 264 with the coordinates (A1*x1+A2*x2′, A1*y1+A2*y2′, A1*z1+A2*z2′), such that the combination position data 264 includes data indicating the position P3 at time t1. In one embodiment, the weight unit 204 includes a validity checking module 212, a special condition module 214, and a distance module 216. The validity checking module 212 accesses the satellite flag 238 and the inertial flag 236 to check the validity of the position P1 and the position P2, and determines the weights A1 and A2 accordingly. The special condition module 214 checks whether the navigation apparatus 100 matches one or more predetermined conditions, and determines the weights A1 and A2 accordingly. The distance module 216 determines the weights A1 and A2 according to the distances between the position P1, the position P2, and the reference position PREF. In one embodiment, the weight unit 204 updates the combination flag 234 indicative of the validity of the combination position P3. In one embodiment, if the combination flag 234 has one value (which may also be referred to herein as the fifth value), e.g., digital one, it indicates that the position P3 is valid. If the combination flag 234 has a different value (which may also be referred to herein as the sixth value), e.g., digital zero, it indicates that the position P3 is invalid. The operations of the weight unit 204 are further described in relation to FIG. 5-FIG. 8. The filter 220 can be, but is not limited to, a Kalman filter, and is capable of checking the combination flag 234, and filtering the position P3 to obtain the final location point PLOC—T1 at time t1 if the combination flag 234 indicates that the position P3 is valid. More specifically, in one embodiment, the location data 268 in the data file 128 indicates the final location point at each time point, e.g., at times tB, tA, and t0 that are prior to the current time t1. The filter 220 accesses the location data 268 to obtain the location points at previous times tB, tA, and t0, and filters the position P3 accordingly to provide the final location point PLOC—T1. Therefore, the track of the navigation apparatus 100 including the location points at times tB, tA, t0 and t1 can be smoothed. If the combination flag 234 indicates that the position P3 is invalid, then the filter 220 does not use the position P3. Instead, the filter 220 estimates the location point PLOC—T1 based on the previous position point PLOC—TO, as is further described in relation to FIG. 12. Furthermore, the filter 220 updates the location data 268 with the location point PLOC—T1, such that the location data 268 includes data indicating the location point PLOC—T1. In one embodiment, the reference position data 224 indicates the reference position PREF. The reference estimator 218 provides a reference position PREF associated with the navigation apparatus 100 at the current time t1, and updates the reference position data 224 accordingly. The operations of the reference estimator 218 are further described in relation to FIG. 3. FIG. 3 illustrates an example of a travel track 300 of the navigation apparatus 100, in an embodiment according to the present invention. FIG. 3 is described in combination with FIG. 2. FIG. 3 shows the location points PLOC—TB, PLOC—TA, and PLOC—TO indicated by the location data 268 at time tB, tA and t0, respectively, where tB is prior to tA, and tA is prior to t0. FIG. 3 describes how to calculate the reference position PREF—T1 at time t1. In one embodiment, the reference estimator 218 accesses the location data 268 to obtain the previous location point PLOC—TO, and accesses the motion data 132 to obtain the speed VTO and the orientation ORT0 measured by the motion sensor 106. Then, the reference estimator 218 estimates the reference position PREF—T1 based on the previous location point PLOC—TO, the speed VTO and the orientation ORTO. In an alternative embodiment, the speed VTO is a combination speed, and the orientation ORTO is a combination orientation. More specifically, the GPS module 242 calculates a GPS speed and a GPS orientation for the navigation apparatus 100 at time t0. The motion sensor 106 measures the DR speed and the DR orientation for the navigation apparatus 100 at time t0. The reference estimator 218 combines the GPS speed and the DR speed into the speed VTO, and combines the GPS orientation and the DR orientation into the orientation ORTO. Then, the reference estimator 218 calculates the reference position PREF—T1 based on the previous location point PLOC—TO, the speed VTO, and the orientation ORTO. Advantageously, since the reference position PREF—T1 is estimated according to the final location point, the speed, and the orientation at the previous time t0, an accurate GPS or DR positioning result is expected to be within a predetermined distance from the reference position PREF—T1. As shown in the example of FIG. 3, if a distance DPREF-P1 between the position fix P1 and the reference position PREF is greater than a threshold DTH1, e.g., the position fix P1 lies out of the range 302 when the intensity of the satellite signals 103 is weak, then the position fix P1 can be regarded as inaccurate. If the distance DPREF-P1 is less than the threshold DTH1, e.g., the position fix P1 lies within the range 302, then the position fix P1 can be regarded as accurate. Likewise, if a distance DPREF-P2 between the position fix P2 and the reference position PREF is greater than a threshold DTH1, then the position fix P2 can be regarded as inaccurate. If the distance DPREF-P2 is less than the threshold DTH1, then the position fix P2 can be regarded as accurate. FIG. 4 illustrates another example of a travel track 400 of the navigation apparatus 100, in an embodiment according to the present invention. FIG. 4 is described in combination with FIG. 2 and FIG. 3. FIG. 4 describes how to calculate the location point PLOC—T1 at the current time t1. Similar to the travel track 300 in FIG. 3, the reference estimator 218 provides the reference position PREF at time t1 based on the previously location point PLOC—T0, the velocity VTO, and the orientation ORTO. The GPS module 242 calculates the position P1 according to the acquisition and tracking data 130. The DR module 244 calculates the position P2 according to the motion data 132. The combination unit 206 combines the position P1 and the position P2 into the position P3. The filter 220 filters the position P3 based on the previous location points PLOC—TB, PLOC—TA and PLOC—TO to locate the navigation apparatus 100 at position PLOC—T1, such that the travel track 400 is smoothed. FIG. 5 illustrates a flowchart 500 of operations performed by the weight unit 204, in an embodiment according to the present invention. FIG. 5 is described in combination with FIG. 2. FIG. 5 describes how to determine the weights A1 and A2 of the positions P1 and P2. At step 502, the validity checking module 212 determines the weights A1 and A2 based on the satellite flag 238 and the inertial flag 236. At step 504, the special condition module 214 checks whether the navigation apparatus (e.g., apparatus 100 of FIG. 1) is in a predetermined condition and determines the weights A1 and A2 accordingly. At step 506, the distance module 216 determines the weights A1 and A2 based on distances between the reference position PREF, the position P1 and the position P2. The steps 502, 504 and 506 correspond to the flowcharts 502, 504 and 506 as further described in relation to FIG. 6, FIG. 7 and FIG. 8, respectively. FIG. 6 illustrates a flowchart 502 of operations performed by the validity checking module 212, in an embodiment according to the present invention. FIG. 6 is described in combination with FIG. 2 and FIG. 5. The flowchart 502 describes how the validity checking module 212 determines the weights A1 and A2 as illustrated in relation to step 502 in FIG. 5. At step 602, the validity checking module 212 starts to determine weights A1 and A2 of the positions P1 and P2. At step 604, the validity checking module 212 accesses the satellite flag 238 and the inertial flag 236. At step 606, the validity checking module 212 checks whether the position P1 is valid according to the satellite flag 238. If the position P1 is valid, the flowchart 502 goes to step 608. If position P1 is invalid, the flowchart 502 goes to step 616. At step 608, the validity checking module 212 checks whether the position P2 is valid according to the inertial flag 236. If the position P2 is valid (both P1 and P2 are valid), the validity checking module 212 sets the weights A1 and A2 to default values, e.g., 50% and 50%, at step 610. Then, the flowchart 502 goes to the step 504 (in FIG. 5 or FIG. 7), such that the special condition module 214 and the distance module 216 can determine the weights A1 and A2. If the position P2 is invalid (P1 is valid and P2 is invalid), the validity checking module 212 sets the weight A1 to 100% and sets the weight A2 to 0% at step 612. In addition, the validity checking module 212 sets the combination flag 234 to the fifth value indicating that the position P3 is valid. At step 616, the validity checking module 212 checks whether the position P2 is valid according to the inertial flag 236. If the position P2 is valid (P1 is invalid and P2 is valid), the validity checking module 212 sets the weight A1 to 0% and sets the weight A2 to 100% at step 618. Then, the validity checking module 212 sets the combination flag 234 to the fifth value indicating that P3 is valid. If the position P2 is invalid (both P1 and P2 are invalid), the validity checking module 212 sets the combination flag 234 to the sixth value to indicate that the position P3 is invalid at step 620. As shown at steps 612 and 618, if one module of the GPS module 242 and the DR module 252 generates an invalid positioning result, the position P3 is generated totally relying on the result from the other module. In both circumstances, the combination flag 234 indicates that the position P3 is valid. FIG. 7 illustrates a flowchart 504 of operations performed by the special condition module 214, in an embodiment according to the present invention. FIG. 7 is described in combination with FIG. 2, FIG. 3, FIG. 5 and FIG. 6. The flowchart 504 describes how the special condition module 214 determines the weights A1 and A2 as mentioned at step 504 in FIG. 5. As described in relation to FIG. 5 and FIG. 6, the flowchart 500 enters the step 504 if the validity checking module 212 identifies that both the positions P1 and P2 are valid according to the satellite flag 238 and the inertial flag 236, in one embodiment. At step 702, the special condition module 214 starts to determine the weights A1 and A2 by checking whether the navigation apparatus 100 is in one of multiple predetermined conditions. In one embodiment, the predetermined conditions include a GPS recovering condition, a DR long-term condition, and a GPS strong intensity condition. At step 704, the special condition module 214 checks whether the GPS is in the recovering condition, as about to be described. When the navigation apparatus 100 is in inaccessible places such as parking garages, tunnels, urban canyons, and near trees, the satellite signals 103 may be unavailable or degraded. The satellite flag 238 is set to a value indicating that the position P1 is invalid. As discussed in relation to FIG. 2 and FIG. 6, the validity checking module 212 sets the weight A1 to 0% and sets the weight A2 to 100%, and the combination result is generated fully relying on the position P2 calculated by the DR module 252. If the navigation apparatus 100 remains in an inaccessible place, an error of position P2 calculated by the DR module 252 increases over time. When the satellite signals 103 become visible, the satellite flag 238 can be switched to the second value indicating that the position P1 becomes valid again. In order to correct the error of the DR module 252, the special condition module 214 is capable of setting the weights A1 and A2 to determine the position P3 fully based on the GPS result P1 for a predetermined time period TTH1. In one embodiment, the special condition module 214 initiates a first timer according to the reference clock signal 136 once the satellite flag 238 is switched from the first value to the second value, so as to measure the time period T1 during which the position P1 remains valid after becoming valid. The special condition module 214 further compares the time period T1 with the predetermined time threshold TTH1. If the time period T1 is less than the time threshold TTH1, it indicates that the navigation apparatus 100 is in the GPS recovering condition. Therefore, if the GPS recovering condition is indentified at step 704, then the flowchart 504 goes to step 706. Otherwise, the flowchart 504 goes to step 708. At step 706, the weight A1 is set to 100%, the weight A2 is set to 0%, and the combination flag 234 is set to the fifth value indicating that position P3 is valid. At step 708, the special condition module 214 checks whether the DR long-term condition is present. As discussed in relation to FIG. 2 and FIG. 3, weak satellite signals 103 may cause the position P1 to become inaccurate, e.g., the position P1 lies out of the range 302 as shown in FIG. 3. Then, the position P3 is calculated fully relying on the position P2, as is further described in relation to FIG. 8. The position P2 has an error which increases over time. Thus, if such status (in which the position P3 is still calculated totally relying on only the position P2) remains for a relatively long time, e.g., longer than TTH2, a check is performed to determine whether the intensity of the satellite signals 103 has strengthened and, thus, the accuracy of the GPS has improved. In one embodiment, the special condition module 214 initiates a second timer according to the reference clock signal 136 to measure the time period T2 when the signal intensity data 232 indicates that the intensity of the satellite signals 103 is weak. The special condition module 214 further compares the time period T2 with the predetermined time threshold TTH2. If the time period T2 is greater than TTH2, it indicates that the navigation apparatus 100 is in the DR long-term condition. In one embodiment, if the DR long-term condition is identified at step 708, the flowchart 504 enters step 710. Otherwise, the flowchart 504 enters step 716. At step 710, the special condition module 214 compares a distance DPREF-P1 between the reference position PREF and the position P1 with a threshold DTH1. If the distance DPREF-P1 is greater than the threshold DTH1, it indicates that the position P1 is still inaccurate. Thus, the special condition module 214 sets the weight A1 to 0%, sets the weight A2 to 100%, and sets the combination flag 234 to the fifth value indicating that the position P3 is valid, at step 712. If the distance DPREF-P1 is less than the threshold DTH1, it indicates that the position P1 is now accurate under the DR long-term condition. Thus, the special condition module 214 sets the weight A1 to 100%, sets the weight A2 to 0%, and sets the combination flag 234 to the fifth value indicating that the position P3 is valid, at step 714. At step 716, the special condition module 214 checks whether the GPS strong intensity condition occurs. In one embodiment, if the signal intensity data 232 indicates that the intensity of the satellite signals 103 is strong, then the special condition module 214 determines that the GPS strong intensity condition is present. Then, the special condition module 214 sets the weight A1 to 100%, sets the weight A2 to 0%, and sets the combination flag 234 to the fifth value indicating that the position P3 is valid at step 718. If no special condition is identified by the special condition module 214, the flowchart 504 goes to the step 506, e.g., flowchart 506, such that the distance module 216 is capable of determining the weights A1 and A2. FIG. 8 illustrates a flowchart 506 of operations performed by the distance module 216, in an embodiment according to the present invention. The flowchart 506 describes how the distance module 216 determines the weights A1 and A2 as mentioned at step 506 in FIG. 5. As described in relation to FIG. 5-FIG. 7, the flowchart 500 enters the flowchart 506 after the validity checking module 212 identifies that both the positions P1 and P2 are valid and the special condition module 214 discovers no predetermined condition, in one embodiment. FIG. 9A, FIG. 9B, FIG. 9C, FIG. 10 and FIG. 11 are diagrams 900, 902 and 904 illustrating examples of the reference position PREF, the position P1, and the position P2, in embodiments according to the present invention. FIG. 8 is described in combination with FIG. 2, FIG. 5-FIG. 7 and FIG. 9-FIG. 11. At step 802, the distance module 216 starts to determine the weights A1 and A2. The distance module 216 sets the weights A1 and A2 according to the distances between the position P1, the position P2, and the reference position PREF at the current time t1. In one embodiment, the distance module 216 compares a distance DPREF-P1 between the reference position PREF and the position P1 with a threshold DTH1, compares a distance DPREF-P2 between the reference position PREF and the position P2 with the threshold DTH1, and sets the weights A1 and A2 based on results of the comparisons. In one embodiment, the distance module 216 compares a distance DP1-P2 between the position P1 and the position P2 to the distance DPREF-P1, compares a distance DP1-P2 between the position P1 and the position P2 to the distance DPREF-P2, and sets the weights A1 and A2 based on results of the comparisons. In one embodiment, the distance module 216 compares the distance DPREF-P1 to the distance DPREF-P2, and sets the weights A1 and A2 based on a result of the comparison. At step 804, the distance module 216 compares the distance DPREF-P1 with the threshold DTH1. If the distance DPREF-P1 is greater than the threshold DTH1, the flowchart 506 goes to step 806. Otherwise, the flowchart 506 goes to step 812. At step 806, the distance module 216 compares the distance DPREF-P2 with the threshold DTH1. The flowchart 506 goes to the step 808 if the distance DPREF-P2 is greater than the threshold DTH1, and goes to the step 810 if the distance DPREF-P2 is less than the threshold DTH1. At step 812, the distance module 216 compares the distance DPREF-P2 with the threshold DTH1. The flowchart 506 goes to the step 814 if the distance DPREF-P2 is greater than the threshold DTH1, and goes to the step 816 if the distance DPREF-P2 is less than the threshold DTH1. Referring to the diagram 900 in FIG. 9A, both the positions P1 and P2 are out of the range 302; that is, the distance DPREF-P1 is greater than the threshold DTH1, and the distance DPREF-P2 is greater than the threshold DTH1. As such, both the positions P1 and P2 are inaccurate, as discussed in relation to FIG. 3. Therefore, at step 808, the distance module 216 assigns the sixth value to the combination flag 234 to indicate that the position P3
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