GNSS Radio Occultation: Introduction and Missions

Transcription

1 GNSS Radio Occultation: Introduction and Missions J. Wickert GFZ Potsdam, TU Berlin

2 GNSS propagation errors and remote sensing Temperature and water vapor Water, ice and land surfaces, temperature, water vapor Water vapor Electron density Slide 2

3 1 GNSS Radio occultation: How does it work?

4 Sun and satellite sets Novel by A. Saint-Exupery: The little prince

5 GNSS radio occultation Wickert, 2002 Key properties: global coverage, all-weather, calibration free, very precise, high vertical resolution Very attractive for weather forecast, Climate and atmospheric research

6

7 Some basic principles

8 GPS phase observations Radio occultation measurement are based on phase measurements on L 1/L2 GPS carrier frequencies onboard CHAMP. The carrier phase can be described by the expression : = + c(dt - dt) - d I + d ρ c dt dt di da dmp dq dq N ε A +d MP + dq + dq + N + [m] (1) true range between GPS satellite and receiver along ray path s velocity of light satellite clock error receiver clock error Ionospheric phase delay along s Atmospheric phase delay along s error due to multipath instrumental bias of the satellite instrumental bias of the receiver radio wave length phase ambigiuity number (integer) residual error on carrier phase Simplification of equation (1) mainly due to the fact, that bending angles will be derived from Doppler frequencies (time derivative of the phase ) gives: = + c(dt - dt) - d I + d A +

9 Some errors can be reduced or eliminated, e.g. by differencing schemes

10 Precise and rapid satellite orbit determination: a key task (see talk A. Jäggi) 7,5 cm Comparison of GPS with laser data show the high quality of the rapid orvit determination for the weather forecast (Near realtime orbits CHAMP) Michalak [GFZ] et al., 2010

11 The bending of the signal path is important

12 Atmospheric bending angle Can be derived from atmospheric doppler

13 n(r) sin n((ppp)) ' en2x(ppr 0 1) p p '1 p'2(d l)pnp(' )d dp' Abel inversion p22 Assumption : refractive index field is spherically symmetric Snell s law Abel transform : tangent point r0 p CHAMP Earth GPS p : impact parameter : bending angle

14 Some atmospheric physics... Occultation measurement N(h) Atmospheric refractivity (e.g., Smith and Weintraub, 1953) p p [ p] mbar N w2 T T [T ] K State law of ideal gas (h) N ( h) ~ ( h) dry air case: ( h) M p ( h) M N ( h) R T (h) R 77,6 Hydrostatic equilibrium: R=8314 J/K/kmol M= kg/kmol (h<80 km) dp (h) g (h) (h)dh p(h) p (h) g (h ) (h )dh h State law of ideal gas: T ( h) M p ( h) R ( h) T(h)

15 p p n 5 7 w N 7.6T T fe2 Ambiguity DryWet: Water vapor retrieval Dry Iono Wet For separation dry and wet additional information necessary. N refractivity T atmospheric temperature [K] p atmospheric pressure [hpa] pw water vapor partial pressure [hpa] ne electron density [m-3] f carrier frequency [Hz] p, T; e.g., from ECMWF Refractivity profile Humidity profile

16 2 Initial demonstration in space: GPS/MET, a real pioneer

17 GPS/MET: Satellite & project Satellite: Microlab-1 Launch: April 3, 1995 (Pegasus rocket) NASA(JPL), UCAR; ~75 kg Inclination: ~70, orbit between 743 and 747 km Build by Orbital Science Corp. NASA lightning mapper, GPS/MET GGI proposal 1988 (GPS Geoscience Instrument) (EOS NASA, call for new instrument concepts) UCAR 1991 mission concept and proposal to NSF

18 GPS/MET:Launch

19 Yes: It worked! (perfectly)

20 GPS/MET:First profile April 16, 1995, Equador 1s data used only LC Phase correction iono University of Arizona, April Thanks: B. Herman, D. Feng

21 GPS/MET:Statistical comparisons Early papers: e.g. Ware et al. (1996), Rocken et al. (1997), Kursinski et al. (1996, 1997)

22 3 Another pioneer: A real CHAMPion

23 CHAMP: A GFZ product (with a little help of our friends)

24 COST Summer GFZ Preparation of thej. Wickert, onboard-software

25 J. Wickert, COST Summer GFZ Occultation antenna(s) CHAMP

26 Father of CHAMP: Prof. Reigber

27 Naming cerenomy Nov 13, 1998

28 4 Big international GPS RO Missions

29 GPS Antennas Orbit, Occultations COSMIC (U.S.-Taiwan, Launch 2006)

30 MetOp (Launch 2006, 2012) Antenna array for GPS-RO

31 Successful German/U.S. gravity mission Operational occultations since May 22, 2006 GRACE

32 TerraSAR-X (Start 2007) TanDEM-X (vor 9 Tagen) TerraSAR-X and TanDEM-X GPS-Antennen Basislinie, Okkultationen GFZ, DLR Quelle: DLR

33 5 RO-Applications: Weather

34 Complex Infrastructure needed to provide rapid data

35 Operational use of GPS-RO for weather forecast 2006: Initial operational CHAMP/GRACE and COSMIC data use by MetOffice and ECMWF Currently used in addition by weather centers at Germany, U.S., France, Canada, Japan, Taiwan Relatively small number of observations has big impact: Why? *Superior vertical resolution compared to other satellite sounders *Assimiliation without bias correction

36 Example: Improvement of Hurricane forecast using GPS-RO: Ernesto (2006) with GPS-RO without GPS-RO Liu, NCAR

37 Example: Improvement of Hurricane forecast using GPS-RO: Ernesto (2006) with GPS-RO Observation (GOES) T. Schmitt, SSEC

38 RO-Applications: Climate

39 Climate: Global Temperature Change From CHAMP/GRACE GPS-RO data Preliminary results, Schmidt [GFZ] et al., 2009, subm. Schmidt et al., 2010

40 RO-Applications: Atmospheric Physics only one example: Atmospheric waves

41 Atmospheric waves Mean potential energy (CHAMP, GRACE, COSMIC) Waves in vertical temperature profiles visible

42 RO-Applications: Ionosphere monitoring

43 Ionosphere: Vertical electron density profiles and detection of disturbances Sporadic E-Layer Relevant for navigation, communication, Studies of atmospheric coupling processes Characterization of space weather

44 Global ionosphere monitoring mit GPS-RO Important phenomenon: ~50% occurrence probability Sporadic-E Results from CHAMP, GRACE, FormoSAT-3/COSMIC 2007/2008 PhD work C. Arras, GFZ

45 Global Assimilation Ionospheric Model (GAIM) (ground + space based measurements) Thanks: A. Komjathy, JPL

46 6 Synergistic Add-on for GNSS-RO: Reflections

47 Reflections & Occultations (e.g. CHAMP) Direct & reflected signal Occultation antenna (RHCP) Nadir antenna (LHCP)

48 Space based ocean and ice reflections (as already seen by CHAMP) (red with reflection; blue - without reflection) 5 months of CHAMP occultation data Beyerle et al., 2002

49 Remote Sensing with Reflected GNSS Direct signals LEO Reflected signals Earth Applications Ocean Altimetry (topography, circulation) Scatterometry (sea state, surface winds) Atmospheric and Ionospheric Imaging Thanks: T. Yunck

50 7 Future mission and Outlook

51 COSMIC-2 Mission (U.S./Taiwan) (NOAA), NASA, AirForce, NSPO (Taiwan) Operational RO mission Tracking GPS, GLONASS, and Galileo (Up to 6 occultations in parallel, Up to 24 satellites for POD) Preliminary design: 12 LEO; first launch 6 in low inclination (24 deg) at 520km (with Space Weather Payloads); second Launch 6 in high inclination (72 deg) at 800km > 8,000 RO soundings per day 45-min (TBR) Average Data Latency Expected first launch in 2017 (tbc), second in satellites; 2 inclinations: Data are distributed more homogeneously

52 MetOp -3 (launch 2018) Sucessors (EPS) with GPS-RO Antenna array for GPS-RO

53 GNSS atmosphere sounding with GRACE-FO GRACE-FO: Launch planned in 2017 (August 4) New and improved GNSS receiver compared to GRACE: TriG? Initial application of Galileo for operational GNSS radio occultation

54 New and small GNSS receivers, new missions Javad Triumpf Microsatellite Understanding of the GNSS receiver as powerful scientific sensor for Earth Observation

55 Small satellite missions: Phase A studies MicroGEM, 2009 (~130 kg) Pyxis NanoGEM, 2012 (~50 kg) Javad/Triumpf NanoX, 2012 (~50 kg) Javad/Triumpf

56 POD: GNSS upper ionosphere tomography Initial satellite based usage of Galileo for Earth Observation Small satellite (LEO) POD: SLR POD: VLBI Reflectometry Ocean/Ice with Galileo with GPS Occultation Atmosphere/Ionosphere Combination with Boden-GNSS

57 Vision: Small satellite constellation for Earth Observation with navigation satellites

58 GNSS reflectometry for Tsunami detection with satellites in Low Earth Orbit (LEO)

59 Summary and conclusions Radio occultation is a scientific key application of GNSS tremendous progress in the GNSS radio occultation technique and its applications with data from CHAMP/COSMIC/Metop (e.g. operational use by the weather centers started in 2006) coherent reflections are synergistic add-on for future RO missions, New RO missions will be realised and are in preparation (COSMIC-2, Metop/EPS GRACE-FO, small satellite constellations)