Effectiveness of GPS/INS simulators for current and future range safety applications

In this paper, the effectiveness of Global Positioning System/Inertial Navigation System (GPS/INS) simulators is evaluated for current and future range safety applications. Range safety applications generally require missile navigation systems to deliver precise position and velocity updates over extremely large distances and they must sometimes operate under harsh environmental conditions. As a result, in order to evaluate the performance of the system, it is necessary to exercise the range safety system over flight scenarios that include both nominal as well as worst-case scenarios. The capabilities of current and projected future simulators are compared with those for an ideal simulator. The ideal simulator would be capable of testing all physically realizable trajectories, and providing access to all relevant simulation data at rates sufficient to capture important modeled effects. To the extent that hardware or phenomenon need to be simulated, it would incorporate realistic models for both INS and GPS elements. The INS models would include parameters for modeling errors in alignment, drift, and scale factor, and the GPS-related models would include those effects specific to the constellation, to the transmission media, and to the antenna system. The impact of unmodeled or insufficiently modeled effects is evaluated and discussed. These include, for example, the ability to accurately simulate and evaluate the effect of high dynamic trajectories. Dynamics is not the only technology challenge that range safety operation presents. Many flights can take place near the poles or the equator. Because of combined telemetry and security issues, many range safety systems operate without keys (CA mode LI only) and rely on prestored ionosphere models. In these regions, where turbulent ionosphere conditions exist, nominal terrestrial models may not be appropriate, and GPS simulator limitations should be identified and supplemented by Copyright (c) 2001 by The Charles Stark Draper Laboratory, Inc. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. analysis. The ability of the simulator to assess the impact of range safety architectures and their treatment of the adequacy of the GPS message set, the design of the GPS/INS integration computers and the accuracy of the time tags they generate, and the effect of vehicle motion, including vibration and center of gravity (eg) offset, are discussed. Test procedures and test equipment needed to validate these systems are explored. The remaining discussion centers around the use of a GPS/INS simulator to stimulate all the various validity flags to fully exercise the system software. For example, it will be important to ensure that the GPS portion can monitor satellite vehicle (SV) faults, ephemeris cutovers, changes in the user range accuracy (URA), and all the important dynamics and ionosphere/troposphere variations. Test plans that combine GPS/INS-capable simulators with appropriate live tests are also discussed. It will be seen that in order to compare live data with simulated data, small-scale vibration must be accounted for when performing simulation and when instrumenting test equipment for the collection of reference truth data. Finally, future GPS/INS simulator needs are summarized. We will see that there are no GPS/INS-capable simulators currently available to meet the full spectrum of range safety needs. It is important to note that almost all GPS/INS, as well as recent GPS simulator development, has been funded with private money. We hope that this paper will help vendors and sponsors to work together to address these limitations; if necessary, government funding, government RFPs requesting new equipment to support higher performance GPS/INS test interfaces, or demonstrating that there is enough of a market to justify a vendors commitment to addressing range safety needs.