Design Features of a Boresighted GPM Core Radiometer Christopher S. Ruf Dept. of Atmospheric, Oceanic & Space Sciences University of Michigan, Ann Arbor, MI (voice), (fax) ( ) College of Engineering Department of Atmospheric, Oceanic & Space Sciences NASA Workshop on GPM Core Satellite Radiometer Improvements NASA Goddard Space Flight Center August 2001

30 August 2001Ruf, GPM Core STAR2 of 11 19&37 GHz Cross-track Scanning Radiometer on GPM Core 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 Conical Radiometer Cross-track Radiometer PR1 PR2

30 August 2001Ruf, GPM Core STAR3 of 11 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

30 August 2001Ruf, GPM Core STAR4 of 11 1-D STAR Antenna Configurations Small Maximum Baselines

30 August 2001Ruf, GPM Core STAR5 of 11 Antenna Aperture Layout – Aperture thinning allows 19 & 37 GHz antennas to be nested between one another Slotted Waveguides 107 cm 118 cm

30 August 2001Ruf, GPM Core STAR6 of 11 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

30 August 2001Ruf, GPM Core STAR7 of 11 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

30 August 2001Ruf, GPM Core STAR8 of 11 Demonstration of STAR Calibration GSFC/UMass ESTAR aircraft sensor Reported in Le Vine et al. Proc. IGARSS2001, , 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)

30 August 2001Ruf, GPM Core STAR9 of 11 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

30 August 2001Ruf, GPM Core STAR10 of 11 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

30 August 2001Ruf, GPM Core STAR11 of 11 Vicarious cold reference TBs calibrate TOPEX Microwave Radiometer Successfully identified a 0.27K/yr drift in 18 GHz T B s 21 and 37 GHz TB cold references are stable, indicating proper calibration (ref. Ruf, IEEE TGRS, 38(1), 44-52, 2000)