ACC/GPSR Splinter Summary Christian Siemes 28/09/2016

Summary SLR tracking priority GPS instrument data from Sentinel missions GPS instrument Setting optimization for phase and code loops Antenna phase centre offset GPS antenna FoV Data release: POD-only and ACC products ACC instrument New type of scale factor calibration manoeuvre Monitoring of spikes

SLR tracking priority Priority list today Recommendation Suggest priorities to ILRS after analysis of GPSR setting optimization completed Sentinel 3 (highest)  Sentinel 3 being an altimetry critically depends Swarm A/C on position accuracy in radial direction Swarm B (lowest) Priorities only become of relevance when multiple (LEO) satellites have a common visibility over a tracking station. This is naturally the case for the SWARM-A/C pair and TerraSAR-X/TanDEM-X. General request to ILRS to critically review the LEO priority based on mission needs for SLR tracking and related scientific return

Data availability from Sentinel missions L1b data from Sentinel missions not publically available, which includes GPS instrument data. However, GPS instrument data, supplemented with satellite attitude data, is of scientific interest for POD and gravity field research Analysis of TEC  “TEC tomography” Geodetic reference frame research Etc. Recommendation Swarm and Sentinel DQWs community request public release of POD related L1b data (RINEX observation files, satellite attitude, etc.) from Sentinel missions Note Swarm mission manager agreed to forward this request to Sentinel mission management

GPS receiver setting optimization RMS of carrier phase residuals in polar areas has decreased due to wider L2 bandwidth Carrier phase residuals increases slightly at low elevation angles Less carrier phase observations are affected by high STEC gradients No final conclusions can be drawn at this point. More data and analysis is needed. Recommendation: Keep the current settings for now, i.e. L2 PLL bandwidth Swarm A 0.75 Hz Swarm B 0.5 Hz Swarm C 1.0 Hz Collect more data, in particular during northern hemisphere winter when carrier phase observation residuals show a larger RMS, and perform further analysis. Then decide how to proceed, considering also potential altitude lowering manoeuvres

GPS code loop bandwidth Code loop bandwidth was reset to original value on 3–4 May 2016. After the reset, Code range observations show decreased noise level: RMS 100 cm  80 cm Tracking of L2 carrier phase starts about 12 seconds later compare to before Preliminary assessment suggests that impact on POD is minor. No final conclusion can be drawn now. Recommendation: Keep current settings for now. Further review in view of POD and TEC determination.

GPS antenna field of view GPS satellites are tracked in aft direction down to zenith angles of 90o, whereas the antenna field of view is set to 88o. Now understood: The GPSR assumes that the antenna boresight is pointing into radial direction, i.e. along the vector from Earth’s centre to the satellite position Recommendation: No changes needed currently. If need arises in the future, the antenna field of view setting could be changed.

New POD-derived acceleration and thermosphere neutral density products Two new products are available for Swarm A/B/C for July 2014 – August 2016 : ACCxPOD_2_ (non-gravitational acceleration derived in POD) - improved product DNSxPOD_2_ (thermosphere neutral density derived in POD) - new product Recommendation: Release the products for Swarm A/C as soon as possible Swarm B products should not be release in present form Signal level too low compared to accuracy  negative densities Revisit processing scheme for Swarm B

New acceleration and thermosphere neutral density data sets ACCxCAL_2_, ACCx_AE_2_, DNSxWND_2_ Swarm C time series extended: 30 May - to 23 September 2015 First Swarm B data set for 11-14 December 2015 Recommendations Do not release Swarm B products  show no resemblance to force models Do not release Swarm C products from 16 July to 7 August 2015  unrealistic signals at eclipse transitions Release Swarm C products: 30 May – 15 July and 7 August – 3 September 2015  supplement ACCxCAL_2 products with spike index Replace “old” ACCxPOD products with “new” ones in the calibration of accelerometer data and reassess ACCxCAL products

ACC scale factor calibration manoeuvres Recommendation Investigate new type of calibration manoeuvre where a small change in velocity is created for a limited time  observe in POD and deduce real thrust force Repeat the above manoeuvre at different ACC temperatures  variations of estimated scale factors due to temperature dependency on or varying thrust force? Design the manoeuvre such that hysteresis of ACC data has minimal impact on the calibration results

ACC monitoring ACC reboot due to EDAC event Spikes stopped occurring after ACC reboot that was triggered by ACC EDAC failure. Recommendation Implement the spike index for operational monitoring and trigger ACC reboot upon degradation of data quality. ACC reboot due to EDAC event