A New Way of Integrating GPS with INS

John Diesel, Rob Conley

Abstract:	Direct use of INS to determine GPS integrity will not meet the accuracy requirements for the non-precision approach phases of flight because gradual drift in the GPS signal cannot be distinguished from INS drift. Rather than directly integrating GPS with INS, the inertial system can instead be used to give the external Integrity Monitor (IM)system (such as SATZAP), more time to react. Inertial system errors grow very slowly for the first few minutes after initial alignment or calibration by an external reference such as GPS. After calibration, one can “coast” for several minutes using the INS by itself without GPS but with very little loss in accuracy from that obtainable with additional GPS updates. Consequently, GPS corrections may be stored and only used after an integrity-assurance delay time T, which is the maximum time required for the external IM system to assure the integrity of the GPS data. The GPS data are used first to determine an integrated GPS/INS solution at time t-T in the past. This solution, which includes optimally calibrated inertial error states, is then immediately propagated forward to present time, t. In effect, the navigation solution is always “coasting” from time t-T in the past to the present time t, because the propagated solution is based on a pure inertial solution without additional GPS updates. This type of integration is referred to as delayed, integrity-assured GPS/INS integration. With this approach, the integrity and redundancy requirements for sole means of navigation are achieved, while meeting both the accuracy requirements and goals for all phases of flight, including non-precision approach. It requires only the presently planned 21 or 24 satellite constellation, and a standard commercial 2 nmile/hr. INS, together with the existing GPS Operational Control system. Future systems, like GLONASS or the GPS integrity channel, are not required.
Published in:	Proceedings of the 4th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1991) September 11 - 13, 1991 Albuquerque, NM
Pages:	191 - 196
Cite this article:	Diesel, John, Conley, Rob, "A New Way of Integrating GPS with INS," Proceedings of the 4th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1991), Albuquerque, NM, September 1991, pp. 191-196.
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