Implementation and Testing of Open-loop Tracking for Airborne GPS Occultation Measurements

B. Ventre, J.L. Garrison, M.H. Boehme, J.S. Haase

Abstract:	As the GPS signal passes through the atmosphere the ray path is refracted and a signal delay is induced compared to that of the geometric straight line path through a vacuum. This delay can be measured as the excess phase, and inverted to retrieve a profile of atmospheric properties. However, the rapid phase variations due to multipath in the troposphere can make traditional closed-loop (CL) tracking impossible at low elevations, particularly as the tangent point reaches below 10 km altitude. An open-loop (OL) tracking algorithm is much more robust for occultation sensing, and is relatively immune to signal fading, easily reestablishing contact with the signal when it reappears. This paper will describe the implementation and testing of an OL tracking algorithm for use on airborne occultation data. This algorithm requires a pre-computed model of a reference Doppler frequency and knowledge of the 50 Hz data message. The argument of a complex correlation between this local copy of the model GPS signal and the received signal will then be used as a measure of the phase error in the local reference signal. Once initialized, the OL algorithm can coherently integrate across multiple code cycles, as well as maintain sufficient power in the signal without relying upon feedback from a delay- or phase-locked loop. The code phase and subsequent subframe edges are updated based upon carrier-aiding of the code rate, where the model Doppler frequency is used instead of the results from a phase-locked loop. We also discuss a method for producing a predicted Doppler model to use in OL tracking. The implementation of the open-loop tracking algorithm is done in the Purdue Software Receiver (PSR) [7]. The OL algorithm has been tested to date on high elevation satellites, recorded at a fixed antenna. Comparison of the excess phase from the OL tracking with the phase-locked Doppler results from the CL tracking are favorable. The OL method is also consistently able to track the L1 C/A code cycle edge based purely on Doppler prediction. OL tracking is self-consistent in that its output accurately reflects any errors in the input. The GNSS Instrument System for Multi-static and Occultation Sensing (GISMOS) instrument is being developed to make GPS occultation and reflection measurements from the High-altitude Instrumented Airborne Platform for Environmental Research (HIAPER) aircraft. In addition to several commercial receivers, which can provide occultation measurements and precise estimates of the aircraft velocity, GISMOS will also employ a custom GPS recording system. This recorder captures three channels (direct, port and starboard occultation) at L1 and L2 with a 10 MHz sample rate. Post-processing of this raw, sampled data with a software receiver is intended to provide the ability to track GNSS satellites at lower elevations during occultation events.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	620 - 633
Cite this article:	Ventre, B., Garrison, J.L., Boehme, M.H., Haase, J.S., "Implementation and Testing of Open-loop Tracking for Airborne GPS Occultation Measurements," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 620-633.
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