The GRAS Instrument

1. The GRAS Instrument C. Marquardt, Y. Andres, A. von Engeln, F. Sancho

2. GRAS on Metop • Metop-A is the first of three satellites in the European Polar System (EPS) • Europe’s contribution to the Initial Joint Polar-Orbiting Operational Satellite System (IJPS) • GRAS (GNSS Receiver for Atmospheric Sounding) • 1st custom build GPS receiver for operational RO

3. GNSS Receiver for Atmospheric Sounding • Build by Saab Space (Sweden; instrument and antennas) and Austrian Aerospace (Austria; S/W, DSP) • Mass ~30kg, power ~40W • ~20MB per orbit / ~280MB per day • First GPS receiver specifically designed for operational radio occultations from space; H/W & S/W design until ~ 2003 (or so) Launched 19th Oct 2006; switched on 27th Oct 2006; worked out of the box.

4. GRAS Measurement Data • SF/DF and OL/RS tracking: • Code Code phase • Carrier Carrier phase & I’s/Q’s from correlators • Dual frequency channels: • Navigation 8 • Occultation 2 rising & 2 setting • Antenna: • Navigation 10° – 90° elevation • Occultation ± 55° azimuth, beam shaped • Sampling rates: • Navigation SF/DF 3 Hz (1, 3, 10, 25, 50 Hz) • Occultation SF/DF 50 Hz (1, 3, 10, 25, 50 Hz) • Occultation RS 1 kHz (250, 500, 1000 Hz) • Code phase 1 Hz • Onboard navigation 1 Hz

5. Measurement Modes GRAS measurement modes: • Dual Frequency Carrier Tracking: code and carrier for L1 and L2 are tracked; both (+ C/A) are reported @ 50 Hz • Single Frequency Carrier Tracking: C/A code and carrier phase are tracked; C/A code and carrier are reported @ 50 Hz • Single Frequency Raw Sampling: C/A code tracked, 1 kHz sampling of carrier • SF carrier tracking and raw sampling can occur simultaneously • Either L2 or RS • GRAS uses a geometrical doppler model when in raw sampling: • Implemented as lookup table in the receiver (~ 10 Hz) • Transparent to the user / in the measurement reconstruction • Tracking state information available

6. Tracking States

7. Tracking States (cont’d) • Settable parameters: with default values given in SLTA (and RTH) • Rising:SLTA_V = -140 km (0 km)(start C/A acquisition) • SLTA_L2 = -35 km (5 km)(start L2 acquisition) • SLTA_A = 0 km(delay L2 acquisition) • Setting:SLTA_AV = -140 km (0 km)(release SV) • (courtesy Saab)

8. Tracking States Sample (Setting) • C/A SF I’s and Q’s • Dual I-branch due to navigation message (is usually removed via sign(I)) DF SF RS

9. Tracking States Sample (Setting, cont’d) • C/A SF I’s and Q’s • Tracking states and actual data highly consistent! DF SF RS L1 carrier lock check

10. Tracking States Sample (Setting, cont’d) • C/A RS I’s and Q’s • Tracking states also consistent with raw sampling data DF SF RS L1 carrier lock check

11. Tracking States Sample (Rising) DF SF RS

12. Tracking States Sample (Rising, cont’d) DF SF RS

13. Tracking States (cont’d) • Tracking states • highly useful for pre-selecting data in both closed and raw sampling measurement modes • much better than heuristic algorithms based on, e.g., SNR thresholds used for other receivers’ data • Other useful information provided by the receiver: • digital and analogue gain changes • noise levels • Overall, we have a much better view on what the receiver is doing compared to pre-GRAS instruments, which is highly appreciated

14. Instrument Characterisation – Electronic Units • both delays and signal gains available – used for, e.g., estimation of inter-channel biases and (if desired) absolute amplitude calibration • for GRAS, only pseudo ranges are affected by instrument delays

15. Instrument Characterisation - Antennas • amplitude patterns for the rising antenna • rectangle denotes elevation and azimuth range relevant for occultation measurements

16. Instrument Characterisation - Antennas • phase patterns for the rising antenna • rectangle denotes elevation and azimuth range relevant for occultation measurements

17. Instrument Characterisation – Antennas (cont’d) • phase patterns for the zenith antenna • represents double patch antenna design

18. Instrument Characterisation (cont’d) • GRAS has been characterized on ground before launch • Some parameters are required in measurement reconstruction: • zenith antenna patterns (~ 20 cm along track error in POD if not used) • code delays (large inter-channel biases in pseudoranges if not used) • antenna positions (systematic biases in bending angles if not used) • Others provide additional refinement of reconstructed measurements: • occultation antenna patterns (for ‘absolute’ amplitudes) • temperature dependencies of the above (weak)

19. Operational Aspects / Anomalies • Buffer overflow on rare occasions (5 times since launch); a threshold parameter needed to be adapted via firmware patch • Single event of simultaneous tracking loss on the zenith channels; receiver recovered after a few seconds (SAA) • GPS SV selection for onboard POD based on GPS Almanachs instead of Ephemerides; • occasional degradation of onboard navigation solution • single event divergence of the onboard POD; required navigation solution reset from the ground

20. Conclusions • GRAS has been performing excellently since Metop’s launch • No major anomalies • Only a single firmware update so far (and none foreseen) to fix a parameter setting • We’re now beginning to explore the raw sampling data • Prototyping of processing and science algorithms over the next few months (w/ DMI & the GRAS SAF, University of Graz, RUAG/Saab) • Will become operational in <= 1 year from now (we hope) • Test data will be available earlier • Data reconstruction and processing algorithms to be OSS