GRAS. Första operationella instrumentet för temperaturmätning med GPS-signaler ger förbättrade väderprognoser och klimatmodeller.

Transcription

1 GRAS Första operationella instrumentet för temperaturmätning med GPS-signaler ger förbättrade väderprognoser och klimatmodeller. Magnus Bonnedal, RUAG Stefan Nilsson, SMHI RUAG Space AB 1

2 Radio Occultation (RO) Measurement Principle 1/2 Phase measurement Bending angle Refractive index Measurement data Products - Temperature - Pressure - Water vapor Qualities - Good vertical resolution t 1 t2 - High accuracy t 3 - Self calibrating small bias - All weather - Global coverage - Inexpensive GPS RUAG Space AB 2

3 RO Measurement Principle 2/2 Väderballonger GRAS-mätningar The carrier phase measurement provides information on: phase delay Doppler shift impact parameter a (~ R e +h) bending angle α refractivity temperature and pressure α(a) µ(a) T(a) Navigation/POD Overhead TEC Anti-Velocity Antenna (Shaped beam) Setting occultations L 1 L 2 L O IF GEU Electronic Unit Zenith Antenna RF Conditioning Unit Velocity Antenna (Shaped beam) Rising occultations Flight Direction RUAG Space AB 3

4 GRAS: GNSS Receiver for Atmospheric Sounding Receiver Electronics Deployment Mechanism Front ends (Filter, LNA, DC) Launched 19 th Oct 2006; switched on 27 th Oct 2006; Worked Out of the box (EUMETSAT). Operational bending angles April 2008, included in ECMWF May 2008 Zenith Antenna Data: Carrier phase and amplitude Code phase L1 and L2 noise for each chain Gain and histograms Navigation solution Navigation data Temperature and voltages More on command High Instrument visibility. Occultation Antenna Instrument: Mass ~30kg Power ~40W 650 occultations / per day Setting & Rising Occultations GPS dual freq, codeless tracking Main Characteristics: 650 Occultations/day rising and setting Ultra Stable Oscillator (USO) stab. <10-12 Wide coverage high gain antennas Excellent RFI rejection Closed Loop (CL) sampling 50 Hz Open Loop (OL) sampling Hz RUAG Space AB 4

5 GRAS Outstanding Performance Other RO missions: COSMIC (Taiwan/US) operational 6 mini-s/c constellation CHAMP (Germany) mini satellite GRACE (US/Germany) mini satellite and more GRAS outperforms other RO receivers due to: Wide coverage and high gain antennas Low noise front end with excellent RFI rejection Ultra Stable Oscillator (USO) stab. <10-12 Open Loop (OL) sampling at Hz GRAS built to space flight standard MetOp is a stable platform All data processed by EUMETSAT, identical settings, noise in estimates vs. CIRA / MSIS between 60 and 80 km GRAS Bending Angle requirement: σ < 0.64 µrad km Figure courtesy of Christian Marquardt, EUMETSAT RUAG Space AB 5

6 RO a Major Contributor in NWP Operational ECMWF system September to December Averaged over all model layers and entire global atmosphere. % contribution of different observations to reduction in forecast error. GPS RO has significant impact (ranked #5 among all observing systems) in reducing forecast errors, despite the small number of soundings. Forecast error contribution (%) Courtesy: Carla Cardinali and Sean Healy, ECMWF 22 Oct RUAG Space AB 6

7 SMHI Forecast Models MATCH Different variables HYPE Different versions ECMWF Dispersionmodel, Sea HIROMB 1D-HIRLAM Fire-risk model HARMONIE SCOBI HIRLAM Climate analysis Lake model Mesoscale analysis HBV-model SIMAIR UV-index STRONG SWAN RUAG Space AB 7 OBSERVATIONS BIOLA

8 GRAS Doppler Model In difficult tracking conditions, the instrument captures the signal in Open Loop guided by an internal Doppler model Average and 95% confidence interval Better than 10 Hz on-board Doppler model accuracy. ±25 Hz captures all significant power in the occultations OL sampling rate of 100 Hz is sufficient Closed loop carrier tracking not needed in future receivers SLTA [km] Doppler offset from model [Hz] # occultations RUAG Space AB 8

9 Examples of Additional Observations Ground Based Interference RO gain peaks on earth Noise level fluctuate several db GND Radar Saturates Rx High selectivity required GRAS: 80 db front-end, 180 db total Occ Latitude [deg] 90 o N 60 o N 30 o N 0 o 30 o S 60 o S Distrubution of L1 NPD increase of 2.0 db. North = 0 90 o S 180 o W 120 o W 60 o W 0 o 60 o E 120 o E 180 o W LEO Longitude [deg] 2.5 x L1 open loop I+jQ spectrum Amplitude (~V) Amplitude (db) Time (s) Frequency (Hz) RUAG Space AB 9

10 Examples of Additional Observations Co-channel interference Interf. GNSS 30 to 40 db above desired signal C/A orthogonality: db Discriminate on Doppler Re-confirm acquisitions No impact on retrieval RUAG Space AB 10

11 GRAS OL: Surface Reflections (SR) Surface Reflections are frequently observed SR typically 20 db below Direct Ray SR resolved by FSI (and similar inversions) 1500 Direct Ray Ocean Land DR Power 1000 Theoretical GO model # occultations db P_DR [dbw] Reflected Ray SR Power # occultations P_SR [dbw] RUAG Space AB 11

12 RUAG GNSS Receivers RUAG Space AB 12

13 GNSS Sensor on Future Missions Possible missions: MetOp Second Generation Iridium NEXT CLARREO COSMIC Second Generation Zenith Antenna Iridium NEXT MetOp Second Generation Occultation Antenna RO Electronics Occultation Antenna RUAG High Performance Navigation Receiver RUAG Space AB 13

14 Conclusions Radio occultation measurements already play an important role in NWP and climate models, and will be even more important with increased number of measurement GRAS on MetOp provides high quality measurements With modernised open signals, GALILEO and GPS, open loop carrier tracking is sufficient. Next generation RO instrument is under development at RUAG. New signals and a better knowledge of the atmosphere dynamics will enable measurements down to even lower altitudes. Constellations of satellites with RO instruments and emerging GNSS systems will provide measurements with major impact on NWP and climate research. Reference: M. Bonnedal, J. Christensen, A. Carlström, A. Berg, Metop-GRAS in-orbit instrument performance, GPS Solutions Jan 2009, Vol 14, p RUAG Space AB 14