Software Signal Simulators for GPS and Galileo Signals

Anthony R. Pratt

Abstract:	The several available GPS signal simulators are complex instruments able to provide representative rf signals at GPS and Galileo transmission frequencies. The paper address the design of a sub-class of simulators which avoid the complexities of the GNSS signal up-conversion and the related effects in the receiver of the rf processor, filters and AD conversion. This type of signal simulator provides the equivalent of the digitised output stream of the rf/if processor. The paper discusses the benefits of this type of signal simulator and the role it can play in GPS receiver development. There are specific signal characteristics which this class of simulator has which give greater flexibility than the ‘rf signal’ simulators which have hitherto been the norm. The output of these signal simulators are typically a data file. QinetiQ has developed a signal simulator of this class. The flexibility allows separate generation of each satellite signal as well as selected spreading codes and spreading symbol waveform. Through this process the new Galileo and GPS waveforms and codes can be generated. This can be extended to Compass and QZSS signals when required. A separate file provides generally for thermal noise excitation though noise spectra other than white (flat) could also be included. Carrier to noise ratios, satellite and host vehicle dynamics can set within a wide range including the effects of signal warping due to relative velocity and relativistic effects. One feature of the use of random noise files is that a specific experiment can be repeated with exactly the same noise waveform if required. Both satellite and local receiver antenna effects are also incorporated. Specific multi-path situations can be simulated though this has not been an immediate focus for the signal generator. By combining the signal and noise files the carrier to noise density ratios can be set at will from very strong signal levels (for example from pseudo-lites or for test purposes) or very weak levels. In the later case there is no feed-through of weak signals through the leakage of signals from the simulator, bypassing the signal attenuators. A further feature allows the width of the AD converter to be determined at signal playback time. This benefits applications to a wide variety of different receiver types.
Published in:	Proceedings of the 2008 National Technical Meeting of The Institute of Navigation January 28 - 30, 2008 The Catamaran Resort Hotel San Diego, CA
Pages:	248 - 256
Cite this article:	Pratt, Anthony R., "Software Signal Simulators for GPS and Galileo Signals," Proceedings of the 2008 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2008, pp. 248-256.
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