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A review of the transponder calibration activities in the frame of the GAVDOS project

A review of the transponder calibration activities in the frame of the GAVDOS project. Hausleitner , W. (1) , J. Weingrill (1) , F. Moser (1) , J.-D. Desjonqueres (2) , N. Picot (2) , S.P. Mertikas (3) (1) Austrian Academy of Sciences, Graz, Austria

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A review of the transponder calibration activities in the frame of the GAVDOS project

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  1. A review of the transponder calibration activities in the frame of the GAVDOS project Hausleitner, W.(1), J. Weingrill(1), F. Moser (1), J.-D. Desjonqueres(2), N. Picot (2), S.P. Mertikas(3) (1) Austrian Academy of Sciences, Graz, Austria (2) Centre National d’EtudesSpatiales (CNES), Toulouse, France (3) Technical University of Crete, Chania, Greece

  2. The GAVDOS Project • GAVDOS – Establishment of a European radar altimeter calibration and sea-level monitoring site for Jason, Envisat and EURO-GLOSS • Objectives • Establish an absolute sea-level monitoring and altimeter calibration permanent facility on the isle of Gavdos • Conduct tide-gauge measurements as well as direct altimeter transponder, Global Positioning System (GPS), Doppler Orbitography by Radio-positioning Integrated on Satellite (DORIS) and Satellite Laser Ranging (SLR) measurements for altimeter calibration • Deliverables • Jason absolute altimeter bias, Marine geoid, Sea level variations, local deformations / land displacements, etc. IWF/ÖAW GRAZ

  3. Gavdos Calibration Work • Calibration of satellite radar altimeters a Gavdos Cal/Val facility using three different methodologies (S.P. Mertikas, et.al. 2010) • Comparing sea level anomalies between the satellite and the in-situ observations • BIAS = SSH_altim – SSH_tide_gauge ≈18±5 cm • Relate measurements to the Mean Sea Level of CLS01_MSS • BIAS = SSH_altim – MSS_CLS01 – SLA_tide_gauge • Usage of a microwave transponder placed at the satellites ground-track • BIAS = ALT_altim – range_altim – H_TRP IWF/ÖAW GRAZ

  4. Microwave Altimeter Transponder • The principle of a microwave altimeter transponder is to receive, amplify and retransmit a satellite radar altimeter pulse. • The emitted pulse is received by the altimeter on-board the satellite again with the 2-way travel-time of the pulse giving the range between the satellite and the transponder • Technical specifications • Minimal distortion of the signal • High frequency stability • Frequency: 13.7 GHz • Bandwidth: 600 MHz • High amplification rate • Total gain: 77 dB • Stable instrument delay • Response delay: 13.24 ns IWF/ÖAW GRAZ

  5. TRP Location • Deployment on Gavdos at DIAS • Cross-over of Jason • 3 km apart of Envisat • ITRF2005 coordinates • GPS campaign by S. Mertikas (Nov. 2010) • Φ = 34°49‘17.2953“ • λ = 24°05‘27.6631“ • h = 251.5627 m IWF/ÖAW GRAZ

  6. TRP Site Setup • Housing • Aluminium frame enveloped with fabric • Cover plate from acrylic glass • Protection from wind, dust contamination, animals, etc. • Electrical Power Supply • 12 V car batteries • recharched by solar panels (2x50 W) • Modem Telecommanding • Watchdog timer (daily at 0.0h) • GSM/GPRS-modem for remote controlled TRP switch-on IWF/ÖAW GRAZ

  7. Ocean vs. Point Target Response IWF/ÖAW GRAZ IWF/ÖAW GRAZ

  8. The Transponder Principle cont‘d • OSTM Level-2 S-IGDR Data Files • 1 Hz / 20 Hz data • Ku / C-band waveforms • Latency: 2 days • Provided by CNES • (Envisat Picture) IWF/ÖAW GRAZ IWF/ÖAW GRAZ

  9. ENVISAT Calibration • Two years ENVISAT calibration campaign • TRP approx. 3km outside of footprint center • RA-2 set to Preset Loop Output (PLO) mode • RA2/MWR products analyzed • RA2_science_level-1b • RA2_average_waveforms • RA-2 instrument bias: 39.0±3.3cm (Cristea at.al., TGRS-20067-0032) IWF/ÖAW GRAZ

  10. Jason Calibration • Poseidon-2/3 instrument characteristics • Dual-band (C/Ku) pulse compression radar • 128 waveform samples • Frequency / PRF / t-res. = 13.575 GHz / 2060 Hz / 3.125 ns • Operation modes • Acquisition mode: Detects ocean returns and init. Tracking loops • Tracking mode: Nominal mode for the altimeter • Problem:The TRP signature is not visible in the waveform data • Because: ▶ J-Tracking is more sensitive to coast/land transitions generating loss of tracking resulting to a 1.5 sec data gap (desc. passes) ▶ Gavdos is small enough to keep tracking the sea shifting the TRP outside of the altimeter ranging window (asc. passes) • Calibration 2: Measures the transfer function of the internal receiver channel IWF/ÖAW GRAZ

  11. Cal-2 Calibration Data 32 Individual Calibrations 128 bins 0.153 s • Calibration-2 Mode • NO absolute epoch time available! • Dating of individual calibrations is known with 1s accuracy only 3.125 ns IWF/ÖAW GRAZ

  12. Calibration Processing Concept IWF/ÖAW GRAZ

  13. Cal-2 Calibration Configuration 5 sec calibrationwaveforms ±1s ±1s 11 sec data gap IWF/ÖAW GRAZ

  14. Data Screening • Pass 050 Cycle 018 IWF/ÖAW GRAZ

  15. Data Interpolation • Densification to 20 Hz rate (cubic spline interpolation) • Precise computation of point of closest approach (φ,λ) 11s Gavdos Crete φ λ IWF/ÖAW GRAZ

  16. Individual Calibration Analysis • Definition/Estimation of maximum power reflection by • 1)Maximum value of received energy • 2)Max. val. of Gaussian fit to return power • 3)Half ma. power at leading edge of Gaussian fit • 4)Centroid of received energy IWF/ÖAW GRAZ

  17. Individual Calibration Fitting IWF/ÖAW GRAZ

  18. Optimal Parabola Fit 32 Individual Calibrations 128 bins IWF/ÖAW GRAZ

  19. Power Vertex Adjustment • Alignment of PCA of orbit vertex of fitted parabola • Absolute time of PCA fromorbit • Absolute datingof bin #1 ofindivid. cal. #1 • Altimeter range command • Set for each calibration • Provided by CNES on a pass-per-pass basis • Ranging ambiguity IWF/ÖAW GRAZ

  20. Geophysical Parameters Range Wet Troposph. Dry Troposph. Ionosph. Sea State Bias DOY IWF/ÖAW GRAZ

  21. Range Correction  -2.49 m [m] [m] DOY IWF/ÖAW GRAZ

  22. Sea Surface Height IWF/ÖAW GRAZ

  23. Geophysical Correcitons IWF/ÖAW GRAZ

  24. Cal-2 Ranging Budget / Residual Calibration bin rangeAmbiguityAltimeter delayCoM Correction 33738.0881310718.858 -5.173 0.471 r -2.297-0.116-0.016 Dry troposphereWet (water vapour)Ionosphere TRP delay (electronic)TRP delay (geometric)Station marker eccentricitySlant range correction - 3.972 -1.477 -0.824 TRP ell.heightSolid earth tides 251.169 -0.0601344398.967 44.575 Geoid Ellipsoid Orbital heightResidual IWF/ÖAW GRAZ

  25. Outlook • Analysis of full set of Cal-2 passes • Limitednumber ofpassesavailable • Usage of precise orbits • Provided with a 60 days latency • DIODE DEM coupling mode • Usage of onboard digital elevation model for pre-definingthe altimeter tracking window • Transponder calibration withnominal tracking rate (51 ms) IWF/ÖAW GRAZ

  26. Figure Pool IWF/ÖAW GRAZ

  27. Geophysical Corrections Mean sea surface Solid Earth tide Geocentr. Ocean tide Pole tide height Inv. barom. height eff. HF fluctuations of SST IWF/ÖAW GRAZ

  28. Orbit SlantDistance Range_ku TRP Altitude SeaSurface SSH Ellipsoid IWF/ÖAW GRAZ

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