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N. Pierdicca Dept. Electronic Eng., Univ. La Sapienza of Rome

ENVISAT RA-2/MWR CCVT Fourth Plenary Meeting 25-27 March, 2003 – ESA/ESRIN. The RA-2 Passive calibration. N. Pierdicca Dept. Electronic Eng., Univ. La Sapienza of Rome. Introduction. ERS-1 RA Digital Counts vs T B at Ku band.

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N. Pierdicca Dept. Electronic Eng., Univ. La Sapienza of Rome

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  1. ENVISAT RA-2/MWR CCVT Fourth Plenary Meeting 25-27 March, 2003 – ESA/ESRIN The RA-2 Passive calibration N. PierdiccaDept. Electronic Eng., Univ. La Sapienza of Rome

  2. Introduction ERS-1 RA Digital Counts vs TB at Ku band • Passive calibration of s° is based on the assumption that the altimeter transmitted power Pp is known. • The receiver can be characterized by sensing the evironmental noise coming from known targets, as in the case of external calibration of microwave radiometers • Note: the altimeter is not a “good radiometer” (limited bandwidth, low radiometric resolution, etc.). Ice Land Sea

  3. Simulations of Brightness Temperature SSM/I, TMI, MWR data ATMO MW model ECMWF Atmo profiles ATMO Tmr, Opacity Simulation Q. C. SEA MW emissivity ECMWF Surface data Tb TOA S & Ku, MWR SSM/I, TMI LAND MW emissivity Static (histo) data

  4. Radiative Transfer models • MW Tb Model for LAND-forest • Detailed description of forest canopy as an ensamble of cylinders (trunks and brancjes) and disks (leaves) over a rough soil (Univ. of Tor Vergata) • MW Tb Model for sea • Two scale rough sea with foam cover, made of tilted facets with small scale waves modelled by a second order perturbation theory • Fully polarimetric model (Stokes vector) including emission and scattering of downwelling atmospheric Tb • MW Tb Model for LAND-desert • Integration of bistatic scattering coefficients based on Geometric Optics and/or IEM model (single scattering).

  5. RA-2 Passive acquisitions Boxes representavailable acquisitions over selected test area

  6. Overview of data presently available October – November 2002 Extremely critical for land targets, especially Sahara Delay time between ECMWF and RA-2 passes

  7. X(k) P(k) Pu*W(k)+Pn k=1,128 (Ku) k=1,128 (Ku) k=1,64 (S) k=1,64 (S) ’ A G /AGC MR sig G 4 e A F SPSA 3 Rx A A 2 1 SPSA FEE with AGC D R x and A/D A ’’ A Scientific data I&Q 4 cal_FEE Tracker HPA Pp The RA-2 power budget • W(k) is the normalised shape of the echo as function of time • GRX: gain receiver (from antenna to scientific data output at RAW level) • GMR, GSPSA: Analogical and digital gains • AR, AT: Attenuations within FEE and waveguides (see figure) • eF: Gain of the variable part of the receiver (computed by internal calibr.)

  8. Pn=kB(T’REC+T’A) GRX A3=l Receiver The antenna temperature k: costant of Boltzmann B: bandwidth of noise sample detector l: antenna efficiency (losses) ML: main lobe efficiency (angular discrimination) T0: Antenna physical temperature Nadir TB, from RT simulations

  9. X(k) P(k) Pn=kB(T’REC+T’A) k=1,128 (Ku) k=1,128 (Ku) k=1,64 (S) k=1,64 (S) ’ A G /AGC MR sig G 4 e A F SPSA 3 Rx A A 2 1 SPSA FEE with AGC D R x and A/D Scientific data I&Q Pn TBML The RA-2 power in “passive mode”

  10. The evaluation of gain • The assumptions • The information about l, ML is still not reliable and we do not monitor T0, therefore we assume: • ML1 (probably OK) • l=0.7-0.9 (preliminarly computed from antenna patterns) • T0 , T’REC constant • The method • Estimate by averaging on 1.114 sec • Read AGC setting (AGCpass) • Looking at different targets whose TBML is known

  11. Computing RA Antenna efficiencies S band From antenna patterns provided by ESA: 0.7 Ku band 0.7 Note: Antenna patterns available for a limited range of elevations only

  12. The results – Ku band (03/2003) AGC 1&2=0 & 5 AGCpass=4.595 B=50 kHz (FFT resolution) l=0.9 (0.7)

  13. The results – S band (03/2003) AGC 1&2 = 0&5 AGCpass=0.875 B=50 kHz l=0.9 (0.7)

  14. ATMO Attenuations Q. C. One-way attenuations from ECMWF+RTE compared to those from MWR (provided by C. L. S.) Discrepancies due to clouds from ECMWF analysis shifted wrt MWR FOV. Need for discarding cloudy pixels in GRX computation

  15. Q.C. over sea: comparing SSM/I &TMI Simulations over sea of TRMM-TMI 10GHz channels (H & V) Simulations over sea of TRMM-TMI & DMSP-SSM/I 19GHz channels (H & V) Cloudy pixels are filtered out (based on ECMWF and Tb’s)

  16. Q.C. over sea: comparing MWR Simulations over sea of ENVISAT/MWR channels ENVISAT/MWR data from C.L.S./Space Oceanography Division Cloudy pixels are filtered out (based on ECMWF and Tb’s)

  17. Q.C. over land: comparing SSM/I &TMI Simulations over Amazonian forest of TRMM-TMI and DMSP-SSM/I ECMWF~8:00 LT TMI ~ 12:00 LT ECMWF~20:00 LT TMI ~ 00:00 LT Time shift of ECMWF vs Tb’s is critical (especially for Sahara)

  18. G’ G /AGC MR sig SPSA Rx SPSA PPTR_REF FEE with AGC and A/D A cal_FEE HPA Pp The “nominal” receiver gain - 1 • According to our knowledge, the overall nominal receiver gain (including the AGC attenuation) is: GMR: analogic receiver GSPSA: digital receiver AR: waveguide attenuations • Some terms are determined • by:

  19. The “nominal” receiver gain - 2 • Therefore the nominal receiver gain: • Pp=60 W Pp =17.78 dBW • GSPSA=800G’SPSA (at Ku-band) 200G’SPSA (at S-band) • AR (Ku)=.14+.051+1.074=1.265 dB • AR (S)=.6524+.771=1.4234 dB • GTX_RX(Ku)=-10.16+98.99+54.851+23.770=167.46 • GTX_RX(S)=-9.003+99.17+52.801+17.74897=160.70 Ku band S band

  20. Comparison of results about gain ……….. by the support of ESA staff ………….

  21. Conclusions • The RA-2 output in “noise listen” mode confirms to be sensitive to the observed scenario • Passive data are being processed. More acquisitions are envisaged. • Brightness temperatures computed from model and on-line data (ECMWF, other radiometers) have shown acceptable accuracy in the range 10-22 GHz (TMI & SSM/I channels). • Magnitude of receiver gain seems to be in agreement (fractions of dB) with “official” Ku calibration. As for S-band a greater gain from passive cal. would imply lower backscattering values (??).

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