GPS Inter-Signal Corrections (ISCs) Study

W. Feess, J. Cox, E. Howard, K. Kovach

Abstract:	In the 20 years since GPS reached its Initial Operating Capability (IOC) milestone in 1993, tremendous progress has been made in modernizing the system to improve the performance delivered to users and bring new capabilities on-line. New generations of satellites have been launched with ever more precise clocks and expanding ability to also broadcast new signals. The ground control network has densified its terrestrial tracking network and enhanced its computational abilities to provide far more accurate satellite clock and satellite orbit predictions for the navigation (NAV) messages broadcast to users. GPS receiver manufacturers have developed new and better measurement methods to exploit the broadcast signal-in-space (SIS) characteristics for solving novel geospatial problems and making old solutions more robust. For civil users, one of the most exciting aspects of GPS modernization has been the addition of three new positioning signal services: L2C, L5, and L1C. (The L5 positioning signal service comprises two separate positioning signals on one carrier, as does the L1C positioning signal service.) Together with the legacy L1 C/A positioning signal service, the multiplicity of signals provided by the modernized satellites enables robust dual-frequency or triple-frequency elimination of the ranging errors induced by ionospheric path delays as well as providing three spectrally separate services that can be used independently of each other if localized radio frequency interference (RFI) should occur which renders one or perhaps two of the three frequencies unusable. For proper operation of the four civil positioning services, the ranging effect of the inter-signal biases (ISBs) between the transmit times of each signal as they leave the satellite broadcast antenna must be accounted for. To facilitate simple accounting within the GPS receivers regardless of which signal or combination of signals/services the receiver is using, inter-signal correction (ISC) parameters have been added to the civil navigation (CNAV) messages that will be broadcast as part of the new L2C, L5, and L1C positioning signal services. There is one ISC parameter for each civil signal and each of the ISC parameters defines the ISB for that civil signal relative to a common reference time. This paper describes the results of a recent study performed to determine how accurately the ISCs can be estimated using the modernized GPS Control Segment (“OCX”), how rapidly those accuracies can be obtained after a satellite is launched or after a satellite configuration is changed, and determine whether any additional signal monitoring measurements are needed. In addition, this paper will also provide a practical application study methodology by showing how the ISCs were generated for the on-orbit CNAV messaging demonstration conducted during the Summer of 2013.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	951 - 958
Cite this article:	Feess, W., Cox, J., Howard, E., Kovach, K., "GPS Inter-Signal Corrections (ISCs) Study," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 951-958.
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