Jason Microwave Radiometer performance and on-orbit - Remote ...

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208 S. Brown et al.TABLE 2 JMR–TMR Vicarious Cold Reference DifferencesAfter Adjusting the Calibration CoefficientsJMR–TMR vicarious cold reference TB differences (Cycles 3–20)JMR 18.7–TMR 18.0 1.60 K ± 0.28JMR 23.8–TMR 21.0 4.02 K ± 0.58JMR 34.0–TMR 37.0 −6.09 K ± 0.32Cold TB Calibration ResultsIt is essential that the JMR TBs are properly calibrated at the low end of the TB spectrumbecause roughly 50% of the open ocean TBs are within 15 K of the cold reference at18.7 <strong>and</strong> 34.0 GHz. The differences between the JMR <strong>and</strong> TMR cold reference TBs atadjacent channels are required to agree with the theoretical cold reference differences, asdiscussed previously. The observed cold reference differences after calibration are presentedin Table 2. The JMR–TMR cold reference differences in Table 2 represent the average <strong>and</strong>st<strong>and</strong>ard deviation of the differences for cycles 3 through 20. The observed cold referencedifferences agree with those predicted by theory within their respective uncertainties.Since a relative calibration reference is used for the cold TB calibration, it is prudent tocheck that the JMR vicarious cold TBs also agree with the absolute theoretical cold referencevalue. The uncertainty of the theoretical cold reference TB is given as the root-sum-squareof the repeatability of the value <strong>and</strong> the deviation of the cold reference TB determined usingthe Stogryn (1971) sea water dielectric model from those using the Ellison et al. (1998) <strong>and</strong>Klein <strong>and</strong> Swift (1976) dielectric models. The maximum deviation of the theoretical coldreference TB using the other models from the cold TB found using the Stogryn model wasabout 0.5 to 0.9 K for the three JMR channels.The JMR vicarious cold TBs were found for JMR cycles 3–21. This is presented inTable 3. The JMR vicarious cold TBs are all within the uncertainty of the absolute vicariouscold reference. The vicarious cold reference TBs can also be used to detect calibration driftsover time. Long term monitoring of the JMR cold TBs over time scales of years is advisablein order to assess the long term stability of the instrument calibration, in general, <strong>and</strong> thenoise diodes, in particular.Hot TB Calibration ResultsTwo regions in the Amazon were used as hot calibration references. Region 1 is 5 ◦ –10 ◦ S<strong>and</strong> 65 ◦ –74 ◦ W <strong>and</strong> region 2 is 1 ◦ S–4 ◦ N <strong>and</strong> 53 ◦ –59 ◦ W. Table 4 shows the magnitude ofthe SSM/I-measured vertical <strong>and</strong> horizontal polarization TB difference averaged over theTABLE 3 Comparison of the Absolute Vicarious Cold Reference CalculatedUsing the Stogryn Emissivity Model with JMR Vicarious cold TBs for Cycles3–21. (The Uncertainty is the RSS of the Deviation from the Other SurfaceEmissivity Models <strong>and</strong> the Repeatability of the Value.)Absolute theoretical cold JMR vicarious cold TBsreference TBs for cycles 3–2118.7 GHz 125.94 ± 1.00 K 125.50 ± 0.32 K23.8 GHz 135.78 ± 1.03 K 136.01 ± 0.65 K34.0 GHz 148.72 ± 1.13 K 147.51 ± 0.41 K
<strong>Jason</strong> <strong>Radiometer</strong> Performance <strong>and</strong> Calibration 209TABLE 4 Statistics of the V-pol–H-pol TB Difference from SSM/I Data for Jan.–Sep.2002 Over two Heavily Vegetated Regions of the Amazon Rainforest (Region 1 is5 ◦ –10 ◦ S <strong>and</strong> 65 ◦ –74 ◦ W <strong>and</strong> Region 2 is 1 ◦ S–4 ◦ N <strong>and</strong> 53 ◦ –59 ◦ WAbs(V-pol minus H-pol) TB Region 1 Region 219.35 GHz Mean: 0.9198 K Mean: 0.7636 KSt. dev.: 0.6717 K St. dev.: 0.5767 K37.0 GHz Mean: 1.0115 K Mean: 0.8011 KSt. dev.: 0.7258 K St. dev.: 0.5785 Kregions at 19.35 <strong>and</strong> 37.0 GHz. The small (∼1 K) V-pol minus H-pol TB differences indicatethat the chosen Amazon reference regions are emitting essentially as black bodies.The SSM/I observations from F13, F14, <strong>and</strong> F15 platforms were converted to JMRreference TBs using the methods described in the appendix. Table 5 shows the referenceTBs from all three platforms <strong>and</strong> the average values across the platforms. The averagesin Table 5 represent SSM/I overpass data from 4 February 2001 through 15 August 2001,which corresponds to the start of JMR cycle 3 through the end of JMR cycle 21. Eachplatform has morning <strong>and</strong> evening local overpass times. The average of all the platformsconsists of six local times (6, 8, 10, 18, 20, 22 LST). A large JMR time series, the cycle 3–21average, was used for comparison to get an adequate sampling of data <strong>and</strong> local overpasstimes. The average calibrated JMR TBs over the same regions <strong>and</strong> same time period arepresented in Table 6. The JMR TBs are within ∼0.5 K for the 23.8 <strong>and</strong> 34.0 GHz channels.The 18.7 GHz hot TBs are 1.6 K lower than the reference, but this has little affect on theretrieved path delays since most of the open ocean TBs are near the cold calibration point.JMR Wet Path Delay ValidationResults of Comparison with TMR PDsThe availability of TMR in an identical orbit, with only 70 seconds of separation from JMR,allows for unprecedented accuracy in the intercalibration of the JMR <strong>and</strong> TMR path delaymeasurements. TMR has been extensively calibrated against various sources of ground truth<strong>and</strong> has been demonstrated to have PD accuracy of 1.1 cm (Keihm et al. 2000). The initialcomparison with TMR showed that JMR was biased 8–12 mm, JMR low. There was alsoa large PD scale error. The path delay bias was approximately 15 mm, JMR low, for dryTABLE 5 JMR Hot TB Calibration References Derived from SSM/IPlatforms F13, F14, <strong>and</strong> F15 Over Region 1 <strong>and</strong> 2 (F13 is at Local Time6 <strong>and</strong> 18, F14 is at 8 <strong>and</strong> 20, <strong>and</strong> F15 is at 10 <strong>and</strong> 22. Data is fromFebruary Through August of 2002, JMR Cycles 3–21.)Nadir hot reference TB (Region 1/Region 2) (K)Local time 18.7 GHz 23.8 GHz 34.0 GHz6, 18 285.0/285.6 284.3/284.4 281.0/281.48, 20 286.6/287.1 285.5/285.6 282.8/283.310, 22 287.7/287.8 286.7/286.3 283.4/283.4Average over time 286.6 285.5 282.6<strong>and</strong> region
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