The Need for Transfer Alignment in a GPS Jamming Environment and Optimisation for MEMs IMUs

Paul D. Groves, C. Aaron Littlefield and Daniel C. Long

Abstract:	Transfer alignment is the process of initialising and calibrating a guided weapon INS using data from the host aircraft’s navigation system. QinetiQ has previously developed a robust ‘rapid’ transfer alignment algorithm using attitude as well as velocity measurements that remains stable during rolls, enabling effective alignment to take place without requiring the pilot to change trajectory. The transfer alignment algorithms can also handle transients in the aircraft integrated navigation solution following GPS re-acquisition after an extensive outage and have been verified in real-time flight trials on a Tornado aircraft. Where GPS jamming is likely to be encountered, it is important to optimise the calibration of the INS in order to obtain the maximum navigation accuracy should GPS signals be lost. Some designers now advocate using GPS alone to align and calibrate a weapon INS prior to launch, or even relying on the GPS measurements after launch. However, transfer alignment, using the aircraft INS/GPS as the reference, provides better calibration of the INS. This is demonstrated with simulation results. Where an INS/GPS is relying on an anti-jam integration technique, such as adaptive tightly-coupled (ATC) or deep, to maintain track of the GPS signals, prior calibration of the INS is essential. These techniques rely on the inertial velocity solution to maintain the GPS receiver’s response to dynamics. Because the GPS bandwidth is reduced, the gains used to calibrate the INS must be set low to avoid problems with time-correlated measurement noise. Simulation results are presented which show that simply initialising the INS prior to launch, rather than performing a full transfer alignment, can degrade navigation accuracy by an order of magnitude when ATC is used at the maximum tolerable jamming level. Previous transfer alignment research has focused on tactical grade IMUs. However, the advent of MEMS silicon IMU technology brings the prospect of significantly reduced unit costs. A simulation study has been conducted using laboratory measurements of a BAE Systems SiIMU01, the first commercially available tactical grade MEMS silicon IMU. A series of simulations was conducted to determine the optimum transfer alignment algorithm configuration. Performance was found to be very good with position drifts of order 20 m per axis over two minutes.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	775 - 783
Cite this article:	Groves, Paul D., Littlefield, C. Aaron, Long, Daniel C., "The Need for Transfer Alignment in a GPS Jamming Environment and Optimisation for MEMs IMUs," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 775-783.
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