Development of a dual-frequency GPS/GLONASS receiver for space application

Laboratory breadboards of a high precision dual-frequency Global Navigation Satellite System (GNSS) receiver have been developed at the Institute of Satellite Navigation at the University of Leeds (ISN) under a European Space Agency (ESA) programme. The main objective of these breadboards is to serve as development tools for receiver signal processing research in view of the scientific applications of GNSS. Some of these applications will be realised by means of receivers embarked on satellites in low Earth orbit (LEO) as part of on-going and future ESA programmes and will include precise orbit determination, radio occultation measurements and measurements of the parameters characterising GNSS signals reflected at the sea surface. During an occultation measurement the GNSS signal propagation encounters a challenging environment. Not only substantial Doppler dynamics occur during a measurement due to the effect of gradients of the refraction index, but also attenuation and deep fading caused by multipath propagation. The signal tracking requirements for space applications are analysed and applied to the hardware and software tasks. Resultant enhancements to the receiver digital processing are presented along with their impact on the intended application. The areas of concern are the acquisition and tracking of signals in high dynamics and at low carrier-to-noise ratios (CNR), and on improving the performance of tracking the GPS P code without full knowledge of the spreading code.