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’Etudes Spatiales (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: 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‘ “
λ = 24°05‘ “
h = 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 )
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. = GHz / 2060 Hz / 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 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

12. Calibration Processing Concept
IWF/ÖAW GRAZ

13. Cal-2 Calibration Configuration
5 sec calibration waveforms
±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)
Crete
11s Gavdos 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 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

20. 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 cm
1.6 cm
Sea state bias
0-20 cm
8.8 cm
Dry
Troposph.
Ionosph.
Sea
State Bias
DOY
IWF/ÖAW GRAZ

21. Range Correction
  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
Corrections
Effect
Magnitude (m)
Calibration bin range Ambiguity Altimeter delay CoM Correction r
Dry troposphere Wet (water vapour) Ionosphere
TRP delay (electronic) TRP delay (geometric) Station marker eccentricity Slant range correction
TRP ell.height Solid earth tides
Geoid
Ellipsoid
Orbital height Residual
IWF/ÖAW GRAZ

25. 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 44 OK
YES 45
fail 46 47 48
no CAL 49 50 51 52 53 54 55 56 57
Pass
Cal Status
Data
Date
Cycle
OK / fail
S-IGDR
SIMPACAL
Calibration
Aux. Data 58
fail
YES 59
no CAL 60 61 62 OK 63 64 65 66 67 68 69 70 71 72
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 Slant Distance Range_ku TRP Altitude Sea Surface SSH Ellipsoid
IWF/ÖAW GRAZ

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