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’Etudes Spatiales (CNES), Toulouse, France (3) Technical University of Crete, Chania, Greece

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

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

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

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

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

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

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

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

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

Cal-2 Calibration Data Calibration-2 Mode 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

Calibration Processing Concept IWF/ÖAW GRAZ

Cal-2 Calibration Configuration 5 sec calibration waveforms ±1s ±1s 11 sec data gap IWF/ÖAW GRAZ

Data Screening Pass 050 Cycle 018 IWF/ÖAW GRAZ

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

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

Individual Calibration Fitting IWF/ÖAW GRAZ

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

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

Geophysical Parameters Range Wet Troposph. Source Uncorr. effect Cycle 50 Corr. Wet Troposph 0-30 cm 15 cm Dry troposphere 230 cm 229.5 cm Iono troposphere 0.2-20 cm 1.6 cm Sea state bias 0-20 cm 8.8 cm Dry Troposph. Ionosph. Sea State Bias DOY IWF/ÖAW GRAZ

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

Sea Surface Height IWF/ÖAW GRAZ

Geophysical Correcitons IWF/ÖAW GRAZ

Cal-2 Ranging Budget / Residual Corrections Effect Magnitude (m) Calibration bin range Ambiguity Altimeter delay CoM Correction 33738.088 1310718.858 -5.173 0.471 r Dry troposphere Wet (water vapour) Ionosphere -2.297 -0.116 -0.016 TRP delay (electronic) TRP delay (geometric) Station marker eccentricity Slant range correction - 3.972 -1.477 -0.824 TRP ell.height Solid earth tides 251.169 -0.060 1344398.967 44.575 Geoid Ellipsoid Orbital height Residual IWF/ÖAW GRAZ

Outlook Analysis of full set of Cal-2 passes Limited number of passes available Usage of precise orbits Provided with a 60 days latency DIODE DEM coupling mode Usage of onboard digital elevation model for pre-defining the altimeter tracking window Transponder calibration with nominal tracking rate (51 ms) Pass Cal Status Data Date Cycle OK / fail S-IGDR SIMPACAL Cal. range Aux. Data 09 09 12 44 OK   YES 09 09 22 45 fail 09 10 01 46 09 10 11 47 09 10 21 48 no CAL 09 10 31 49 09 11 10 50 09 11 20 51 09 11 30 52 09 12 10 53 09 12 20 54 09 12 30 55 09 01 09 56 09 01 19 57 Pass Cal Status Data Date Cycle OK / fail S-IGDR SIMPACAL Calibration Aux. Data 10 01 28 58 fail YES   10 02 08 59 no CAL 10 02 17 60 10 02 27 61 10 03 09 62 OK 10 03 19 63 10 03 29 64 10 04 08 65 10 04 18 66 10 04 28 67 10 05 08 68 10 05 17 69 10 05 27 70 10 06 06 71 10 06 16 72 IWF/ÖAW GRAZ

Figure Pool IWF/ÖAW GRAZ

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

Orbit Slant Distance Range_ku TRP Altitude Sea Surface SSH Ellipsoid IWF/ÖAW GRAZ

Date 2009/11/10 Time 14:19:34 Date Exec. CAL2 Only known to integer seconds ? Ho 4604232 LSB Ho .0073191 m NSPEC 3 NCAL 32