GRAS In-Flight Performance Review for GRAS SAG 2011 22nd of June 2011
Onboard Events and Activities June 2010 to June 2011 • Availability of Occultation Mode:99.95%
GRAS RFCU & AntennaTemperatures Velocity Chain (Panel 2F) In the last 2 to 3 years the annual temperature evolution on Panel 2F has now a nominal shape, however the slight long term decrease is uncommon and different from the other panels, which increase their temperature over time. The temperature of the zenith antenna NTPLM23 acquired by the SVM-EIU is affected by the operation of the SARR heater lines S03 and S30. The effect of these heaters is not visible in the zenith antenna acquired by the GRAS. Anti-Velocity Chain (Panel 2B) Zenith Chain (Panel 1D)
GRAS Power Consumption The temporary change of the SLTA parameters in December 2009 and the permanent change in October 2010 caused a small increase (~0.1 W) of the average power consumption.
GRAS Voltages Voltages are stable and consistent between Housekeeping (blue) and Measurement Data (green) Telemetry. Digital 5V DBU Power Voltage Thermistor Supply Voltage
GRAS Measurement Data Buffer Free Words The free words of the measurement data buffer were occasional reaching 0 until the buffer handling was patched on 2008-02-19. The increase of the size of the measurement data buffer ensures sufficient margin against buffer overflow. The patch also prevents a switch-down in the unlikely case that the measurement data buffer gets full. • The increase of the measurement data rate due to the changed SLTA values from 2009-12-10 to 2009-12-17 and since 2010-10-05 reduced the free words slightly, however sufficient margin remains.
GRAS HKTM Assessment from Mid-Term Review 2011 All the observed parameters are well within specification range. This clearly shows that the functional performances status of the instrument does not raise any particular concern to date. Temperatures are behaving in line with those over the satellite as a whole showing seasonal, diurnal and orbital variations. Temperature evolution on panel 2F is returning to nominal behaviour and will be further monitored by PLM team. HKTM stability shows good behaviour and all observations are consistent with known events and behaviour, except for the power/temperature increase Events U1 and U2 and the repetitive spikes in the power consumption. The GRAS current was monitored at 3 Hz and ½ Hz in on 2010-02-26 and 2010-03-08, suggesting that the repetitive spikes are an artefact of a beating pattern between telemetry sampling frequency and GRAS processing cycle. However they are no concern for instrument health and no further formal investigation is planned, provided the behaviour remains as is. The GRAS operations guide has been updated to check power consumption and temperature after the next PL-SOL or GRAS anomaly to detect reoccurrence of a phase of increased power/temperature. Long term evolution of temperatures (warming/cooling) of electronic units and antennas will be monitored for inversion of long term trends.
USO Ground Voltage GRAS USO Performance USO temperatures and ground voltage are expected to remain nominal throughout the mission. USO Internal Temperature USO External Temperature
GRAS Receiver PerformanceTracking States The tracking counts for both zenith and occultation antennas do not change significantly with time. Changes can be caused by an increase or decrease of the number of active satellites in the NAVSTAR-GPS constellation. The average of the tracking states is above the thresholds of 5 for zenith, 0.2 for anti-velocity and 0.08 for velocity. The average of the velocity chain tracking state dropped noticeably during the temporary SLTA update in December 2009. Data gaps continue to cause problems for the processing of GRAS data from rising occultations in the tropics. This is very likely caused by gradients in vertical water vapour abundance (cloud tops) and affects about a third of rising occultations, ref. AR.11836 Zenith Antenna Tracking Distribution Zenith & Occultation Chain Tracking Average
GRAS Signal DynamicsGain Changes Zenith Gain Changes The periodic pattern of the gain changes continues throughout the reporting period. Velocity Gain Changes Anti-Velocity Gain Changes
GRAS Software Configuration History GRAS was launched with GRAS on-board software (GOBS) v1.7. • Early on in the mission GOBS v1.8, a patch to increase the size and the handling of the measurement data buffer was applied. GOBS v1.9 to resolve NCR.3593 was meant to be patched to Metop-A on 2010-05-20, however due to a mistake in the patch file generation process, the applied patch was GOBS v1.8 GOBS v1.9 was successfully uplinked on 2011-05-05.
Conclusion The GRAS instrument is fully operational. All functional and trending indicators are fine. The effects observed on Panel 2F temperature and the periodic power spikes are of no concern for instrument health and performance, but will be monitored further. Closed loop data gaps observed in rising occultations in the tropics pose a problem for the processing of GRAS data. This is very likely caused by gradients in vertical water vapour abundance (cloud tops) and affects about a third of rising occultations. Raw sampling data gaps caused by code phase loss of lock affect all occultations. They pose a problem for processing of lower tropospheric data and deep occultations. It is desirable to improve the code phase tracking to reduce the raw sampling data gaps and to extend the lower altitude limit of occultations, False acquisitions of L2 signals causing low SNR affect 6% of all occultations (10% of setting occultations). This causes a significant degradation of product quality of the derived geophysical parameters. RUAG presented a proposal to update carrier loss of lock detection, code loss of lock detection and L2 signal acquisition threshold parameters. It is recommended by all parties to take steps to implement fixes to the observed problems. The data buffer patch has successfully prevented the reoccurrence of the data buffer overflow anomaly. Monitoring is in place to identify reoccurrence of the on-board navigation solution anomaly, caused by inclusion of unhealthy satellites in the on-board navigation solution. GOBS has been patched to version 1.9. The instrument anomaly recovery procedures have been updated to permit a more robust and efficient recovery from the different states reached by onboard autonomous anomaly reconfigurations back to nominal operations. All GRAS units remain fully operational. There is no indication of coming degradations preventing reaching the extended lifetime of 2012.