An Integrated Diversity Switching GPS Receiver for Sounding Rocket Missions

Kurzfassung

GPS is widely used on sounding rocket missions for providing position, velocity and timing information for onboard control systems and experiment payloads. The spin stabilisation of many sounding rockets allows only one antenna fitted in the nosecone to give a reliable GPS performance. If the nosecone is not available for antenna placement due to experiment or vehicle restrictions, two different methods are currently used. Wrap-around antennas do have an almost spherical field of view and provide an uninterrupted view of the GPS satellites. These antennas have to be adapted to each launch vehicle which results in high non-recurring engineering costs and high unit prices. A common method for cost sensitive missions is the use of two or more antennas connected to a power combiner to produce an omni-directional field of view. This approach leads to destructive interference in certain directions and limits the maximum spin rate. The paper presents a GPS receiver which uses two antennas and a diversity algorithm to eliminate the problems encountered with RF power combiners. The signal power of each antenna is constantly evaluated and used as an input into the diversity algorithm. The digital intermediate-frequency data is then combined and then subsequently used to calculate the PVT solution. The limited performance of embedded systems poses restrictions on the complexity of the diversity algorithm. A selection combining approach is implemented in the Namuru V2 GNSS receiver (Mumford et al.). A specialised FPGA hardware has been developed to take care of signal correlation and power calculation for each antenna. Special care is taken to ensure a correct demodulation of the navigation data. Testing has been conducted using a two channel Spirent GSS7700 GPS simulator as well as on a 432mm diameter rocket structure mounted on a turn table. Spin rates of up to 4Hz have been successfully tested in simulation and 3Hz in field testing. The tests have shown the ability of the receiver to acquire satellites, decode navigation data and provide a PVT solution even at high spin rates. It has further been demonstrated that a continuous tracking of satellites is achieved during realistic spin rate profiles. The simulator and open sky tests are compared with a traditional dual antenna system with passive power combiner and discussed in this paper.