Implementation of vector tracking loop algorithm in modern GPS receiver

Nowadays, application of GPS is employed more and more in aviation field for purpose of positioning and navigation. GPS device is use to support other navigation device or it can act as secondary navigation instrument by itself. Very well-known problems of GPS signal are Doppler affect and very low level of signal, which are caused by long distance and high speed motion of satellite transmitters. Doppler affect causes signal frequency shift. GPS signal traveling to the ground is under the thermal noise level. These problems of the GPS signal challenge the processing in the receiver. The challenge even becomes more complicating in aviation because of aircraft velocity. The traditional receiver processes signal following one direction: from front-end module to tracking to acquisition module and then to navigation processor. There is no feedback from the navigation processor or any correcting signal to support the tracking and acquisition module. In other words, each module of the traditional receiver acts independently. By the signal is becoming degraded or antenna is hidden during maneuver, the tracking process is easily interrupted and the signal is lost. This paper presents implementation of Vector Tracking Loop (VTL) algorithm in modern GPS receiver. In contrast to the traditional receiver, VTL algorithm combines function of two modules: acquisition and tracking module and navigation processor. A feedback from navigation processor comes to acquisition and tracking module to improve tracking process. Delay Lock Loop, which is implemented for tracking, is also supported by the navigation processor. For purpose of calculating position and velocity, Kalman filter is employed in navigation processor. The VTL is implemented in Matlab and the results show better receiver immunity to degraded or interrupted signal.