The Space GPS Reflectometry Experiment On the UK Disaster Monitoring Constellation Satellite

A spaceborne GNSS reflectometry experiment is to be undertaken by Surrey Satellite Technology Limited with support from the British National Space Centre (BNSC) to investigate the reception of reflected GPS signals off the ocean for oceanography applications. The GPS receiver on the UK-DMC satellite has been modified to accommodate a downward (nadir) pointing high gain antenna, to send sampled data to a solid-state data recorder and to process reflected signals in real time. This instrument will be used to search for signals reflected off the ocean and when detected, characterise their relationship with the sea state. If successful, this experiment could prove an enabling step towards a constellation of low cost small satellites that could make real-time measurements of the sea state around the globe. INTRODUCTION In 1993 the concept of using GPS and other signals reflected off the Earth’s surface as a means of remote sensing the environment was proposed by the European Space Agency [1]. Potentially reflected GNSS signals could be used to accurately determine sea height from space as a low cost alternative to satellite altimetry. As the ideas and the theory were subsequently developed, other potential GNSS-derived measurements emerged, such as sea state, wind speed, ice sensing and soil moisture sensing. Reflectometry experiments from a variety of platforms followed, including from aeroplanes, balloons and the Space Shuttle [2] [3] [4], and most recently from the SAC-C GPS receiver [5]. Nevertheless, it is recognised that there is still a shortage of data from LEO altitudes to validate the current models. SSTL has an interest in the potential of GNSS reflectometry as an enabling technology for low cost ocean sensing constellations such as that proposed in the GANDER concept [6]. As a GNSS reflectometry receiver is a passive instrument, the payload could fit on a very small satellite, perhaps even a sub-10 kg nanosatellite, significantly lowering the cost of any constellation. SSTL has expertise in GPS receiver design, and SSTL’s SGR has been flown on numerous LEO missions [7,8,10]. When an opportunity arose on a forthcoming SSTL satellite, it was a logical step to upgrade the capability of the SGR from just a navigation tool into a scientific instrument to demonstrate this new technique. THE UK-DMC SATELLITE The UK-DMC satellite is one of a small constellation of 700km altitude polar-orbiting satellites intended to image disaster areas and provide images promptly to relief agencies. Alsat-1 was the first DMC satellite to be launched, and has been in orbit since November 2002 [9]. Three more, including UK-DMC, are scheduled for launch in September 2003. A number of secondary experiments are carried on UK-DMC, including an Internet router, a miniature propulsion system and the GPS reflectometry experiment. The generic design of the DMC satellites includes both a dual antenna SSTL SGR -10 GPS receiver and a solidstate data recorder, fortuitously carried in the same module tray, and so the reflectometry instrument could be constructed from electronics mostly already present in the satellite.