1. ASI, February 2009 The GRAS Instrument C. Marquardt, Y. Andres, A. von Engeln, F. Sancho

2. Slide: 2 ASI, February 2009 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)  1 st custom build GPS receiver for operational RO

3. Slide: 3 ASI, February 2009 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 19 th Oct 2006; switched on 27 th Oct 2006; worked out of the box.

4. Slide: 4 ASI, February 2009 GRAS Measurement Data SF/DF and OL/RS tracking:  CodeCode phase  CarrierCarrier phase & I’s/Q’s from correlators Dual frequency channels:  Navigation8  Occultation2 rising & 2 setting Antenna:  Navigation10° – 90° elevation  Occultation± 55° azimuth, beam shaped Sampling rates:  Navigation SF/DF3 Hz (1, 3, 10, 25, 50 Hz)  Occultation SF/DF50 Hz (1, 3, 10, 25, 50 Hz)  Occultation RS1 kHz (250, 500, 1000 Hz)  Code phase1 Hz  Onboard navigation1 Hz

5. Slide: 5 ASI, February 2009 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. Slide: 6 ASI, February 2009 Tracking States TSStatus / descriptionC/A code C/A, P1, P2 code RS occ SF carrier DF carrier 0Acquisition and tracking ended 1C/A-code aquisition 2C/A-code lock checkXX 3L1-carrier lock checkXXX 8SF carrier frequency tracking @ 1 msXXX 9SF carrier frequency tracking @ 10 msXX 13P-code aquisitionXX 14P-code trackingXX 15P-code and L2 carrier trackingXX

7. Slide: 7 ASI, February 2009 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. Slide: 8 ASI, February 2009 Tracking States Sample (Setting) DF SF RS  C/A SF I’s and Q’s  Dual I-branch due to navigation message (is usually removed via sign(I))

9. Slide: 9 ASI, February 2009 Tracking States Sample (Setting, cont’d) DF SF RS  C/A SF I’s and Q’s  Tracking states and actual data highly consistent! L1 carrier lock check

10. Slide: 10 ASI, February 2009 Tracking States Sample (Setting, cont’d) DF SF RS  C/A RS I’s and Q’s  Tracking states also consistent with raw sampling data L1 carrier lock check

11. Slide: 11 ASI, February 2009 Tracking States Sample (Rising) DF SF RS

12. Slide: 12 ASI, February 2009 Tracking States Sample (Rising, cont’d) DF SF RS

13. Slide: 13 ASI, February 2009 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. Slide: 14 ASI, February 2009 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. Slide: 15 ASI, February 2009 Instrument Characterisation - Antennas  amplitude patterns for the rising antenna  rectangle denotes elevation and azimuth range relevant for occultation measurements

16. Slide: 16 ASI, February 2009 Instrument Characterisation - Antennas  phase patterns for the rising antenna  rectangle denotes elevation and azimuth range relevant for occultation measurements

17. Slide: 17 ASI, February 2009 Instrument Characterisation – Antennas (cont’d)  phase patterns for the zenith antenna  represents double patch antenna design

18. Slide: 18 ASI, February 2009 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. Slide: 19 ASI, February 2009 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. Slide: 20 ASI, February 2009 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