Use of GPS Receivers On Board Shells in Autonomous And Translator Modes of Operation.

Gerald Whitworth

Abstract:	This paper describes two methods of using GPS to track shells during the trajectory. Both methods are currently in the feasibility stage but could be advanced to a practical conclusion in the near future. Method 1 uses an autonomous GPS receiver on board the shell. Prior to firing, the GPS receiver is activated and uploaded with current satellite ephemeris, time and details of the imminent trajectory, including firing and target locations, physical shell details and atmospheric parameters. Throughout the flight, the GPS position and velocity solution is passed to an embedded trajectory model that estimates the landing point and the error from the target location. The shell is deliberately fired long (over range) and the amount of overshoot from the target is established by the on-board trajectory model. The overshoot is compensated by the deployment of a drag device. The calculation of deployment time is also a function of the onboard trajectory model. Early results from simulated trajectories are discussed. Method 2 uses only a GPS translator system on board the shell. The translator relays, via an S band link, the satellite signals to a GPS receiver on the ground. This receiver is termed the TGPS receiver. The TGPS receiverThis paper describes two methods of using GPS to track shells during the trajectory. Both methods are currently in the feasibility stage but could be advanced to a practical conclusion in the near future. Method 1 uses an autonomous GPS receiver on board the shell. Prior to firing, the GPS receiver is activated and uploaded with current satellite ephemeris, time and details of the imminent trajectory, including firing and target locations, physical shell details and atmospheric parameters. Throughout the flight, the GPS position and velocity solution is passed to an embedded trajectory model that estimates the landing point and the error from the target location. The shell is deliberately fired long (over range) and the amount of overshoot from the target is established by the on-board trajectory model. The overshoot is compensated by the deployment of a drag device. The calculation of deployment time is also a function of the onboard trajectory model. Early results from simulated trajectories are discussed. Method 2 uses only a GPS translator system on board the shell. The translator relays, via an S band link, the satellite signals to a GPS receiver on the ground. This receiver is termed the TGPS receiver. The TGPS receiver has to be capable of receiving satellite signals at very low levels and overcome the phase noise and modulation introduced by the fast and slow spinning motions of the shell. The GPS unit passes pseudorange and delta range to a host computer that utilises a Kalman filter with an embedded trajectory model which can correct the position output by using single satellite measurements. The design of the TGPS unit is discussed including the transition from standard GPS mode to TGPS mode and the modifications made to the tracking loops to acquire at low signal levels with high degrees of phase noise and modulation on the carrier frequency. Early results simulating the trajectory and phase noise are discussed. A brief discussion and comparison of the two methods concludes the paper.
Published in:	Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997) September 16 - 19, 1997 Kansas City, MO
Pages:	1717 - 1724
Cite this article:	Whitworth, Gerald, "Use of GPS Receivers On Board Shells in Autonomous And Translator Modes of Operation.," Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997), Kansas City, MO, September 1997, pp. 1717-1724.
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