The cube shaped satellite has a width of 10 inches (Fig. 1.) and comprises a power system, two independent transmitters and receivers and a command and control unit. As auxiliary payload an Orion GPS receiver will be flown, which transmits the instantaneous position of the satellite to ground based radio stations.
The PCsat GPS receiver has been built up at DLR/GSOC based on the ORION receiver design of Mitel Semiconductor. It employs the GP2015/2020 front-end and 12 channel correlator chipset as well as an ARM60B 32-bit microprocessor. The original firmware has received numerous modifications to ensure accurate and reliable under the highly dynamical environment of a low Earth orbit. Through the use of an analytical orbit propagator the receiver is able to perform hot starts with a typical time to first fix of better than 20s. This allows intermitted operations of the receiver with activation times down to one minute to minimize the average power consumption.
The ORION receiver is designed for use with an active GPS antenna having an LNA gain of typically +28dB at the L1 frequency (1575.42 MHz). Since the dense coverage with solar cells does not permit the mounting of a standard patch antenna on the satellite, a quarter-wavelength monopole mounted in one corner of the satellite and connected to a stand-alone preamplifier will be employed.
The Kodiak Star mission has been launched from Kodiak Island, Alaska, on Sept. 30, 2001 (2:40 UTC) and PCsat was successfully released into a circular orbit of 800 km altitude and 67° inclination.
The Orion GPS receiver onboard the PCsat satellite was activated within less than 24 hours after launch. Within the subsequent days it has not been possible to achieve a 3D navigation solution since the receiver failed to acquire more than two GPS satellites at a time. The most likely cause for this behavior lies in a tumbling motion of the spacecraft with an assumed period on the order of 20-30 seconds. Aside from the acquisition problems, the receiver worked nominally. This conclusion is supported by repeated bit and frame locks on individual channels as well as the acquisition of a GPS almanac for week 1134. The observed signals level were uncomfortable but matched those encountered on ground with the given antenna system. The GPS satellite visibility and the expected Doppler shifts were accurately predicted once a first set of twoline elements had been loaded. This confirms the proper function of the hot start algorithms employed in the flight software. Also, the real-time clock and non-volatile memory have been working to full satisfaction despite a two month off-period before the launch.
A new effort to activate the GPS receiver was made after a on month hibernation during the morning pass over the USNA ground station on 31 October 2001. After uploading a current set of twoline elements the receiver started to track 11-12 GPS satellites simultaneously and to produce position-velocity data with an estimated accuracy of about 10 m and 1 m/s, respectively. Apparently PCsat has achieved a more favorable orientation by now, which offers proper visibility conditions for the GPS receiver.
During a period of full Sun orbits in January 2002, the receiver could finally be operated almost continuously for more than ten days with telemetry data collected by a world wide net of radio amateurs. Using its analytical orbit model for Doppler and visibility prediction, the receiver was able to acquire tracking autonomously within a few minutes after power up and hot starts were always performed within less than 60 s. On average the receiver tracked 10-11 out of 13-15 GPS satellites visible above the Earth's rim using the simple monopole antenna. Within the ground based postpropcessing an rms pseudo-range and Doppler tracking accuracy of ca. 2.5 m, 0.5 m/s has been demonstrated which is a factor of 2-3 worse than encountered in a more benign GPS signal simulator environment. Despite PDOP values near unity, the onboard navigation accuracy is further limited to roughly 20 m (3D rms) due to a large number of outliers. Both the increased data noise and the large percentage of outliers is attributed to pronounced signal level variations and frequent signal losses/re-acquisitions caused by the continuously changing spacecraft attitude. A detailed description of the achieved results is given in the report GPS Operations on the PCsat Microsatellite presented at the ION-GPS-2002 conference.
The PCsat mission marks the first flight of a German GPS receiver in orbit. Special thanks are due to the United States Naval Academy for granting this opportunity and for providing continued support during the integration, launch and routine mission phase. Pre-flight testing and validation of the receiver in a signal simulator testbed has kindly been supported by Kayser-Threde GmbH, Munich. We furthermore wish to express our sincere thanks to Donald Jacob (California), Phil Jenkins (KC8SRG), Richard Katsch (VK2EIK Australia), Johann Lochner (ZR1CBC South Af-rica, ZS7ANT Antarktika), Pat Snyder (K4PAT Philip-pines), Andy Russell (Great Britain), Robert Turlington (G8ATE), Mineo Wakita (JE9PEL Japan), Christ van der Weide (JO21SK The Netherlands) as well as numerous anonymous radio amateurs around the world who have spent their leisure time to collect and provide PCsat telemetry data during the January 2002 GPS campaign. The enthusiasm and professional support of each individual were vital for the successful completion of this project.
Leung S., Montenbruck O., Bruninga R.;GPS Tracking of Microsatellites - PCsat Flight Experience;5th International ESA Conference on Guidance, Navigation and Control Systems, 22-25 October 2002, Frascati, Italy (2002).
Montenbruck O., Leung S., Bruninga R.;GPS Operations on the PCsat Microsatellite;ION GPS 2002 Conference, 24-27 Sept. 2002, Portland, Oregon (2002).
Montenbruck O., Markgraf M., Leung S., Gill E.;A GPS Receiver for Space Applications;B1-Ax; ION GPS 2001 Conference, Salt Lake City, 12-14 Sept. 2001 (2001).
Leung S., Montenbruck O., Bruninga B.;Hot Start of GPS Receivers for LEO Microsatellites;NAVITECH'2001, 10-12 Dec. 2001, Noordwijk (2001).