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Design Features of a Boresighted GPM Core Radiometer

College of Engineering Department of Atmospheric, Oceanic & Space Sciences. Design Features of a Boresighted GPM Core Radiometer. NASA Workshop on GPM Core Satellite Radiometer Improvements NASA Goddard Space Flight Center 30-31 August 2001. Christopher S. Ruf

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Design Features of a Boresighted GPM Core Radiometer

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  1. College of Engineering Department of Atmospheric, Oceanic & Space Sciences Design Features of aBoresighted GPM Core Radiometer NASA Workshop on GPM Core Satellite Radiometer Improvements NASA Goddard Space Flight Center 30-31 August 2001 Christopher S. Ruf Dept. of Atmospheric, Oceanic & Space Sciences University of Michigan, Ann Arbor, MI 734-764-6561 (voice), 734-764-5137 (fax) cruf@umich.edu (email)

  2. Cross–track scanning radiometer fits beside 35 GHz PR Cross-track scanning geometry is similar to both PRs Electrical beam steering achieved by ground processing 19&37 GHz Cross-track Scanning Radiometer on GPM Core Conical Radiometer Cross-track Radiometer PR1 PR2 Ruf, GPM Core STAR

  3. Synthetic Thinned Aperture Radiometer (STAR) Principles and Heritage • Used in radio astronomy since the 1950’s. • First used in Earth Remote Sensing by NASA/UMass ESTAR aircraft sensor to measure sea surface salinity and soil moisture. • ESA SMOS Mission (launch ~2006) will be the first spaceborne mission that will use the technique Ruf, GPM Core STAR

  4. 1-D STAR Antenna Configurations Small Maximum Baselines Ruf, GPM Core STAR

  5. Antenna Aperture Layout – Aperture thinning allows 19 & 37 GHz antennas to be nested between one another 118 cm Slotted Waveguides 107 cm Ruf, GPM Core STAR

  6. GPM Core Boresighted STAR Precision and Spatial Resolution • STAR cross track spatial resolution is adjustable • Can be matched precisely to PR at each off-nadir angle via ground processing (image formation algorithm) • Along track resolution smeared by s/c motion • trade-off between NEDT and matching to PR footprint Ruf, GPM Core STAR

  7. Related Technology Enabling Developments Currently Underway • Lower TB noise floor of measurements • Low loss slotted waveguide antenna array elements • Low noise MMIC-based receiver modules • Lower power required by sensor • Low power MMIC-based receiver modules • Ultra low power digital quadrature demodulators & cross correlators • Lower calibration risk • Laboratory correlated noise calibration standard • Testbed aircraft sensor uses identical calibr. method Ruf, GPM Core STAR

  8. Demonstration of STAR CalibrationGSFC/UMass ESTAR aircraft sensor • Reported in Le Vine et al. Proc. IGARSS2001, 2537-2539, 2000. • Coincident ship in situ and aircraft radiometer surveys across Gulf Stream on 29 August 1999 • ESTAR salinity retrievals from calibrated brightness temperatures at 1.4 GHz • < 1 psu salinity differences correspond to < 0.5 K absolute calibration error in ESTAR TBs (from Le Vine et al., 2000) Ruf, GPM Core STAR

  9. Improvements to ESTAR Design Help Calibration Accuracy & Stability • Antenna design • Individual antenna array elements are slotted waveguides • lower insertion loss, less noise, less thermal instability • Use an aperture taper along linear array (was uniform taper) • Reduce edge coupling to support structure, reduce sidelobes • Extend ground plane behind radiating elements • Reduce coupling to structure and aircraft & increase pattern repeatability • Custom MMIC-based receiver modules • Improve repeatability between units • Simplify calibration fine-tuning • Reduce temperature dependence of calibration • Digital back-end • quadrature demodulation after digitization improves phase stability • integrated digitization counters monitor threshold levels Ruf, GPM Core STAR

  10. GPM STAR Calibration Approach • On board calibration provides ~2K absolute TB cal and ~0.5K stability • Inject correlated noise between each antenna and receiver for gains • Switch each channel to uncorrelated reference loads for offsets • Use stationary statistics of ocean radiobrightness for end-to-end absolute calibration • Demonstrated operationally with TOPEX Microwave Radiometer and GEOSAT Follow On Water Vapor Radiometer • ~1.0K accuracy and ~0.3K long term stability • Variable angle of incidence with cross track scanner improves accuracy Ruf, GPM Core STAR

  11. Vicarious cold reference TBs calibrate TOPEX Microwave Radiometer • Successfully identified a 0.27K/yr drift in 18 GHz TBs • 21 and 37 GHz TB cold references are stable, indicating proper calibration (ref. Ruf, IEEE TGRS, 38(1), 44-52, 2000) Ruf, GPM Core STAR

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