Adjustments of a Real-time Software GNSS Receiver for use on a Medium Earth Orbit

Ulrich Haak and Peter Hecker

Abstract:	Precise positioning on a medium earth orbit (MEO) seems to be less demanding than on earth due to the lack of atmospheric delays and the reduced signal shadowing by earth. Nevertheless there are several challenges for positioning on very high altitudes like high Doppler shift or the limited signal visibility due to out-of-lobe areas. While signal visibility, especially in higher altitudes, has improved and will further improve due to the increasing number of GNSS satellites, receiver adjustments are still needed for that environment beside considerations regarding tolerance of cosmic radiation. Off-the-shelf receivers do not provide navigation information at velocities exceeding 600 m/s due to dual-use reasons. Satellites on an earth orbit move with more than 7000 m/s on low altitudes and even still more than 3000 m/s on a geosynchronous orbit. On an eccentric orbit even higher maximum speeds can be reached, which excludes off-the-shelf receivers (even in a simulation environment) from orbit based applications. The Institute of Flight Guidance (IFF) has developed a software based GNSS receiver. It is implemented in C++, while the receiver tracking routines are based on NVIDIA CUDA and enable the receiver to track several dozens of signals in real-time, depending on the GPU processing power as shown at the ION GNSS 2012 ([1]). The digitized signals can be provided by different types of receiver front-ends and are sent to the receiver via USB. The receiver is able to process GPS, Galileo and SBAS signals. Since software based solution is fully adaptable to special applications, it can be adjusted for precise positioning on an earth orbit as well. The receiver has particularly been adapted with respect to signal availability and accuracy in order to fit the requirements on an earth orbit. The tracking has been changed from scalar control procedures to a vector based code and carrier frequency tracking routine, in which a Kalman based state estimator controls the tracking states. Since the movement on an orbit can be predicted very precisely, a dynamic model has been derived to describe the Kepler mechanics and therefore reduce the average measurement residuals in the correction step of the Kalman filter. In order to estimate the influence of the receiver dynamic on the measurements, maximum values for Doppler shift and Doppler rate were derived for worst-case scenarios. Further considerations in this paper are due to relativistic effects on the Doppler measurements.
Published in:	Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014) September 8 - 12, 2014 Tampa Convention Center Tampa, Florida
Pages:	1403 - 1409
Cite this article:	Haak, Ulrich, Hecker, Peter, "Adjustments of a Real-time Software GNSS Receiver for use on a Medium Earth Orbit," Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 1403-1409.
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