Post Processing Algorithms for Translator-Type GPS Receivers

A.R. Pratt and D. J. Molyneux

Abstract:	There is one important category of GPS receivers that have to operate entirely autonomously and under high dynamic conditions such as greater than 50g acceleration in free flight host vehicles. Such receivers would normally have a high risk of losing the tracking process on the satellite signals received if a conventional GPS receiver architecture were to be employed. Several approaches have been developed to combat this problem. These are distinguished from conventional GPS receivers by having all or part of the tracking process installed in ground-based equipment, connected to the GPS receiver by a variable transmission delay data-link. The recovery of GPS data and the tracking of both carrier phase and code signal is more complicated for receivers having this type of architecture. The simplest form of such a receiver is known as a translator, frequency changing and amplifying the received GPS signals prior to transmission to the ground- based processor. A development of the analogue translator digitises the input signal before transmission to the ground processor. These architectures suffer from having large bandwidth requirements (greater than 2MHz). A further characteristic of the translator in a dynamic host vehicle is that there is local time or location solution. An alternative processing approach has been developed in which the code tracking loops are closed in the host vehicle. These are far more robust than the carrier trackers and can withstand dynamic stresses up to 200g with adequate signal to noise ratios. Measurements of the residual carrier waveform are made after filtering to improve the signal to noise ratio, and these are transmitted to a ground-based processor along with frequency corrections and pseudo-range measurements. The requirement for data-link bandwidth is significantly reduced by more than an order of magnitude for this type of sensor. The paper introduces the processing structures that are required in ground-based equipment to form tracking loops, derive the GPS satellite data message, form a location solution and transfer GPS time to the host vehicle without a data uplink. The time transfer function is important since the timing of events occurring in the host vehicle is required to a high degree of accuracy. Many performance benefits can be derived from post-processing the received GPS measurement including the removal of the latency of aiding data, processing the tracking loops with both forward and reverse time order and processing the tracking algorithms with the GPS data modulation removed. A revised form of Costas tracking loop has been developed using an analytic signal approach. It is able to operate to zero frequency in base- band translated GPS. An interesting property is that the filtering requirements in the Costas arms are greatly simplified. Results showing the reconstruction of translated GPS carrier wave signals with dynamic host vehicle motions of up to 80g are shown together with the Costas track equivalent waveforms. Results also include the location and time transfer performance of actual receivers showing the ability to locate timed events to within 30cm.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	695 - 702
Cite this article:	Pratt, A.R., Molyneux, D. J., "Post Processing Algorithms for Translator-Type GPS Receivers," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 695-702.
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