GPS/IMU Navigation and Simulation for Higher-Than-GPS Orbits

Kenn Gold and Alison Brown

Abstract:	NAVSYS Corporation has modified its software GPS receiver (SGR) and built an engineering development unit of the High-gain Advanced GPS Receiver (HAGR) optimized for GPS tracking in an orbital environment. Enhancements to the SGR include the ability to track both direct GPS signals, as well as weaker GPS sidelobes. This requires digital beam steering and beam nulling to increase gain towards the weaker signals, while partially nulling direct lobe signals to achieve signal strength balancing. The space based SGR approach also makes use of inertial aiding data to allow continuous navigation through high dynamic mission phases such as launch, or orbit transfer. Goddard Space Flight Center´s GEONS navigation software has also been integrated with the SGR to allow for real time onboard navigation. To test the engineering development unit, significant enhancements were made to the NAVSYS Advanced GPS Hybrid Simulator (AGHS) to allow simulation in an orbital environment. The AGHS is driven by Matlab and can receive trajectory information in real time from Satellite Tool Kit or other commercial-off-the-shelf or government-off-the-shelf profile generation software. The simulator also contains a Digital Storage Receiver (DSR) capable of recording live GPS data from complicated RF environments (such as urban canyon or indoor GPS) for later playback with an RF Modulator, or that can be played back directly into a digital GPS receiver. The capabilities offered by the AGHS substantially enhance the utility of GPS simulation. Additionally, a capability to simulate inertial measurement unit (IMU) data simultaneously with the GPS and from the same input trajectory has been added. The simulator is useful in testing and developing next generation receiver and navigation which will utilize coupled GPS/inertial navigation scenarios. The AGHS is capable of simulating 12 GPS satellites in 8 RF channels. The RF channels of the AGHS are fully coherent, allowing for GPS wavefront simulation. IMU measurement data coherent in time with the GPS data is generated at a rate of up to 400 Hz. Simulation and tracking results will be shown for various orbital scenarios. Orbital platform simulation and tracking results will be shown for highly eccentric orbits, in which GPS tracking of side lobes of satellites on the other side of the Earth are achieved.
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:	2709 - 2720
Cite this article:	Gold, Kenn, Brown, Alison, "GPS/IMU Navigation and Simulation for Higher-Than-GPS Orbits," 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. 2709-2720.
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