Flight Results of the NOX Dual-frequency GPS Receiver Payload On-board the TET Satellite

Kurzfassung

The German small satellite TET-1 has been launched into a low-Earth orbit on July 22nd, 2012, from the launch site in Baikonur, Kazakhstan. The satellite carries various technology demonstration payloads, among them the Navigation and Occultation Experiment NOX. This payload consists of a geodetic-grade GPS receiver (Septentrio PolaRx2), which is connected via an antenna selector to two Sensor Systems GPS L1/L2 patch antennas. One of the antennas is mounted on the satellite’s zenith panel and receives signals primarily used for precise orbit determination (POD) experiments. The second antenna is pointed towards the anti-flight direction of the satellite for collecting measurements of low elevation satellites for ionospheric or even tropospheric occultations. The antenna switch allows to select either the POD or the occultation antenna for signal reception. The PolaRx2 receiver is based on commercial-off-the-shelf (COTS) electronics and has been complemented with a rapid electronic fuse to protect against single-event latch-up effects. Furthermore, the unit has been qualified in vibration, thermal-vacuum and total-ionizing radiation tests to ensure a proper operation in a space environment. The receiver is operated with a dedicated firmware version without height and velocity limitations. Furthermore, the Doppler search window has been increased to 45 kHz to facilitate satellite acquisition under the high dynamics of the space-borne scenario. The receiver provides GPS pseudorange as well as C/N0 measurements for C/A-code on L1 and P(Y) code on L1 and L2 for up to 16 satellites. Carrier-phase and Doppler observations are provided for L1 C/A-code and L2 P(Y) code. The receiver settings can be changed via ground command, which gives full flexibility to adjust the configuration for the required operation mode for POD or occultation measurements. Being part of the satellite's experimental payload instead of the bus system, the NOX receiver is not operated continuously but instead during dedicated experimental slots, in general once per week for 12-24 hours. So far, no latch-up effects or other radiation-induced failures have occurred during the operation of the payload. The time-to-first-fix (TTFF) after receiver activation has been measured to range from 90-180 seconds. These results confirm previous results of a signal simulator test for an orbit scenario with an identical receiver and firmware [1]. A short TTFF is particularly relevant for the use of space-borne COTS-technology, since a radiation-induced receiver failure only leads to a short interruption of operation. The carrier-to-noise spectral density ratio in boresight direction is approximately 47 dB-Hz (+/- 2 dB-Hz) for L1 C/A and 32 dB-Hz (+/- 2 dB-Hz) for P(Y). These results are slightly lower than expected from pre-launch ground tests and can be attributed to the use of a simple patch-antenna without choke-ring. At a zero-degree elevation mask the receiver was able to track up to 14 GPS satellites simultaneously, which is significantly higher than other space-borne receivers with a typical limit of 8-12 satellites. The advantages are a higher robustness of the POD especially in kinematic processing as well as higher chances to detect and eliminate measurement outliers in the data screening. Using an empirically calibrated phase-center variation pattern for the POD antenna, the carrier-phase residuals for reduced-dynamic POD can be reduced to 9 mm, which is slightly higher than the noise typically obtained for other space-borne dual-frequency receivers. Pseudorange noise of the receiver is approximately 70 cm for the ionosphere-free dual-frequency combination. The receiver's navigation solution is computed from dual-frequency data and has an error of approximately 2 meter 3D rms compared to a highly-precise carrier-phase-based POD solution. The final paper includes a detailed description of the TET satellite and the NOX payload. The receiver's tracking performance and the accuracy of the navigation solution is analyzed. Finally, tracking results of the occultation antenna are presented and the suitability of COTS receiver technology for GNSS radio science investigations is assessed. [1] Montenbruck O., Garcia-Fernandez M., Williams J.; Performance Comparison of Semi-Codeless GPS Receivers for LEO Satellites; GPS Solutions 10, 249-261 (2006). DOI 10.1007/s10291 - 006 - 0025 - 9