Clock Stability Determination of Dual-Frequency Downlink Navigation Transponders

A. J. Van Dierendonck, Robert Coker, Randy DeBord, Oleg Razumovsky and George Kinal

Abstract:	The various Satellite Based Augmentation Systems (SBASs) currently under development utilize Geostationary Satellites (GEOs) to provide ranging signals to the end user that also contain integrity data on GPS and GLONASS satellites. This signal is generated on the ground and provided via a C (or K-Band) uplink to the GEO, whose navigation transponder frequency translates to it to Ll and C (or K-Band) downlink frequencies, then broadcasts the signals throughout the GEO’s edge-of-coverage footprint. The GEOs incorporate this dual-frequency downlink in order to provide ionospheric delay observability. An important aspect of the navigation transponder is its frequency stability. In the long term, the frequency of the propagated signal to maintain code/carrier coherency would be controlled via the uplink signal. This is accomplished using a closed-loop algorithm capable of keeping the code chipping rate and carrier frequency of the received downlink signal in a constant ratio of 1: 1540, as well as maintaining the correct Doppler and ionospheric divergence as observed by the user. This closed loop control must overcome the long term drift and instability of the satellite’s reference oscillator. In the short term, the frequency stability reduces to the short- term clock stability of the oscillator, and is not controlled via the uplink. The purpose of this paper is to describe a technique for characterizing these instabilities after the GE0 is on orbit using dual frequency carrier phase measurements. It provides the theoretical basis for this technique, its implementation in In-Orbit-Test (IOT) of two Inmarsat-3 satellites and the results of the testing. These results are compared to the signal-in-space requirements, showing that the stability of the reference oscillators easily meets those requirements.
Published in:	Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997) June 30 - 2, 1997 Albuquerque, NM
Pages:	451 - 460
Cite this article:	Van Dierendonck, A. J., Coker, Robert, DeBord, Randy, Razumovsky, Oleg, Kinal, George, "Clock Stability Determination of Dual-Frequency Downlink Navigation Transponders," Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997), Albuquerque, NM, June 1997, pp. 451-460.
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