Navigation employing GPS and inertial sensors is a synergistic relationship. The integration of these two types of sensors not only overcomes performance issues found in each individual sensor, but also produces a system whose performance exceeds that of the individual sensors. GPS provides bounded accuracy, while inertial system accuracy degrades with time. Not only does the GPS sensor bound the navigation errors, but the GPS sensor calibrates the inertial sensor. In navigation systems, vehicle gps tracking device performance issues include susceptibility to interference from external sources, time to first fix (i.e., first position solution), interruption of the satellite signal due to blockage, integrity, and signal reacquisition capability. The issues related to inertial sensors are their poor long-term accuracy without calibration and cost.
GPS is a line-of-sight radio navigation system, and therefore GPS measurements are subject to signal outages, interference, and jamming, whereas an IMU is a self-contained, nonjammable system that is completely independent of the surrounding environment, and hence virtually immune to external disturbances. Therefore, an IMU can continuously provide navigation information when GPS experiences short-term loss of its signals. Similarly, dead reckoning sensors are internal to the vehicle.
One primary concern with using personal gps tracking device as a stand-alone source for navigation is signal interruption. Signal interruption can be caused by shading of the GPS antenna by terrain or manmade structures (e.g., buildings, vehicle structure, and tunnels) or by interference from an external source. Each vertical line indicates a period of shading while driving in an urban environment. The periods of shading (i.e., less than three-satellite availability) are caused by buildings and are denoted by the black lines in the lower portion. (This experiment was conducted when five to six satellites above a 5º mask angle were available for ranging.) When only three usable satellite signals are available, most receivers revert to a two-dimensional navigation mode by utilizing either the last known height or a height obtained from an external source. If the number of usable satellites is less than three, some receivers have the option of not producing a solution or extrapolating the last position and velocity solution forward in what is called dead-reckoning (DR) navigation.
Inertial navigation systems (INSs) can be used as a flywheel to provide navigation during shading outages. The discrete-time nature of the GPS solution in some equipment is also of concern in real-time applications, especially those related to vehicle control. If a vehicle’s path changes between updates, the extrapolation of the last GPS measurement produces an error in the estimated and true position. This is particularly true for high-dynamic platforms, such as fighter aircraft. In applications where continuous precision navigation is required, inertial sensors can be employed. An alternative solution is the use of a GPS tracking device for cars that provides higher rate measurement outputs. In principle, rates on the order of 100 Hz are possible.
In addition to providing navigation continuity during short GPS shading outages and between GPS sensor position outputs, an INS, when calibrated using a Kalman filter, can be used to improve the GPS receiver performance in two other ways. First, the information that is maintained by the integration filter can be used to reduce the time to reacquire GPS signals that have been lost through interference or obscuration; second, the integration filter can be used to aid the receiver’s tracking loops, extending the thresholds for signal tracking. Both techniques have been used since the very first GPS sets were designed.