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Performance Assessment of GFO Water Vapor Radiometer

This document summarizes the methods of evaluating the performance of GFO Water Vapor Radiometer, including absolute calibration of brightness temperatures, precision of TBs, statistical analysis, and ground truth comparisons. Contact Chris Ruf at cruf@umich.edu for more information.

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Performance Assessment of GFO Water Vapor Radiometer

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  1. College of Engineering Department of Atmospheric, Oceanic & Space Sciences GFO Water Vapor RadiometerPerformance Assessment GFO Cal/Val Meeting June 12th, 2001 NOAA Laboratory for Satellite Altimetry Bldg. SSMC-3 Silver Spring, MD 20910 Chris Ruf University of Michigan cruf@umich.edu, 734-765-6561 (v), 734-765-5137 (f)

  2. Summary of Methods of Evaluation • Absolute Calibration of Brightness Temperatures • Histogram analysis of coldest TBs • Precision of TBs • Statistical analysis (Allan Variance) of TBs • Precision of Wet Tropospheric Path Delay Correction • Statistical analysis (Allan Variance) of PDs Ruf, GFO Cal/Val, WVR

  3. Absolute Calibration of Brightness Temperatures (pg 1 of 3) • Minimum TB is predictable from first principles given residual atmospheric water vapor burden • Residual Water Vapor Burden derived from SST at which the TB is a minimum Ruf, GFO Cal/Val, WVR

  4. Absolute Calibration of Brightness Temperatures (pg 2 of 3) • Minimum TB at GFO-WVR frequencies predicted by model • at 22.2 GHz: TBmin = 136.4 K @ SST = 16.5 C • at 37.0 GHz: TBmin = 152.7 K @ SST = 25.6 C • Perform histogram analysis of TBs extracted from Operational NDGR Data Products • Compute cumulative density function of TBs near minimum & extrapolate to point of 0% occurrence • Results: TBmin = 136.9K @ 22.2 GHz, 152.6K @ 37 GHz • Averaged over cycles 0-9 Ruf, GFO Cal/Val, WVR

  5. Absolute Calibration of Brightness Temperatures (pg 3 of 3) Ruf, GFO Cal/Val, WVR

  6. Precision of Brightness Temperatures • Estimate the standard deviation of individual TB samples extracted from the Operational NGDRs • Select time-series of triplets of TBs • Compute unbiased estimate of their standard deviation • This estimate of the statistic is very noisy given an individual triplet • Average many estimates of standard deviation • Results: NEDT = 0.202 K @ 22.2 GHz, 0.129 K @ 37.0 GHz • Averaged over cycles 0-9 • Consistent with pre-launch sensor performance Ruf, GFO Cal/Val, WVR

  7. Precision of Wet Tropospheric Path Delay Correction (PD) • Estimate the standard deviation of individual PD samples extracted from the Operational NGDRs • Use identical statistical analysis tool as for TB precision • Results: White Noise Standard Deviation of PD samples = 0.136 cm • Averaged over cycles 0-9 • Compare to contractual performance requirement of <= 0.15 cm Ruf, GFO Cal/Val, WVR

  8. Time History of GFO-WVR White Noise Standard Deviations Ruf, GFO Cal/Val, WVR

  9. Ground Truth Comparisons with Island-launched Radiosondes • Earlier comparison leveraged off of TOPEX comparison study • Operational phase comparisons pending start up of Jason-1 comparison study (probable Summer’01-Fall’01 start) GFO-WVR v. RaOb Comparison 13 May – 21 June 1998, N=263 Ruf, GFO Cal/Val, WVR

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