# Presentation: 265590

## On-Orbit Calibration of WindSat

** **
**Frank J. Wentz and Marty Brewer**
**Remote Sensing Systems, Santa Rosa, CA**** **
**This research is support by NASA’s Earth Science Division**

- Joint AMSR Science Team Meeting
- Telluride, Colorado, July 14-16, 2008

##       Inter-Calibrating Satellite Sensors:

|  | Geolocation
Analysis | Attitude
Adjustment. | Along-Scan
Correction | Absolute
Calibration | Hot Load
Correction | Antenna
Emissivity | Resampling
Algorithm | Rain
Threshold | OOB
Q/C |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| SSM/I | NRL/RSS | No | Yes | TA Offsets | No
See note 1 | 0 | Opt. Intrep. | 0.18 mm | Ocean RTM |
| TMI | Goddard | Dynamic | Yes | TA Offsets | No | 3.5% | Opt. Intrep.
 | 0.18 mm
 | Ocean RTM |
| AMSR-E | RSS | Fixed | Yes | APC | Yes | 0 | Opt. Intrep.
 | 0.18 mm
 | Ocean RTM |
| AMSR-A | RSS | Dynamic | Yes | APC | Yes | 0 | Opt. Intrep.
 | 0.18 mm
 | Ocean RTM |
| WindSat | NRL/RSS | Fixed | Yes | APC | Yes | 0 | Opt. Intrep.
(Earth grid) | 0.18 mm
 | Ocean RTM |
| SSMIS | RSS | No | Yes | APC | Yes | Yes | Opt. Intrep.
 | 0.18 mm
 | Ocean RTM |

-       **Inter-Calibrating Satellite Sensors:**
**Consistency, Consistency, Consistency**

- STEP-2:  Produce a Calibrated Brightness Temperature

- STEP-1:  All Level-1 (NASA) or TDR (DMSP) datasets are reversed engineered back to radiometer counts

- Note-1:  Errors due to Hot Load are removed when doing the zonal TB intercalibration

## WindSat has a Hot Load Problem 

**WindSat has a Hot Load Problem **
**But Not as Bad as AMSR**
**Only 19 and 37 GHz Appear to be Affected**

- Error in Hot Load (C) for 19 GHz
- Y-axis is intra-orbit position

- orbit number (time) ->

- South Pole
- Equator
- North Pole
- Equator
- South Pole

## Hot Load Error Easily Modeled as a

- Hot Load Error for 19 GHz
- Y-axis is sun zenith angle  relative to S/C coordinates
- X-axis is sun azimuth angle relative to S/C coordinates

**Hot Load Error Easily Modeled as a**
** ****Function of the Sun Zenith Angle**

## WindSat Optimum Resampling Algorithm

- Each Frequency has different swath limits
- Conventional Optimum Interpolation Resampling does not work
- Complex resampling algorithm is used to map WindSat Observations
- directly to an Earth-Fixed Grid
- Rigorous Optimum Interpolation is employed
- About 1 Million Sets of Pre-Computed Resampling Weights are required
- (3 GB of Computer Memory, new 64-bit computer required)
- Results in utilizing maximum available swath
- (Example:  Aft 7-GHz observations are combined with forward 24 GHz obs.)

**WindSat Optimum Resampling Algorithm**
**                  ****(i.e.  L2A Processing)**

## WindSat Absolute Calibration

- Values for antenna spillover and cross-pol are derived so as to match WindSat TB to RTM Ocean Tb.  These are called “on-orbit” spillover and cross-pol.
- Except for 7 GHz, on-orbit spillover values lie between NRL original values and Chris Ruf rederived values (NRL < RSS < Ruf).  Both NRL and Ruf are based on prelaunch data.  (Ruf says NRL made mistake in doing numerical integration.)
- On-orbit spillover for 7 GHz is higher than both NRL and Ruf.
- (3% higher than NRL and 1% higher than Ruf.)
- On-orbit cross-pol values agree with NRL values to better than 0.5%.
- (No Ruf values to compare with)
- Conclusion:  WindSat appears to be well calibrated in an absolute sense except that the 7 GHz spillover is somewhat more than expected.
- Bottom Line:  Considering everything, WindSat is probably the best calibrated MW Imager so far with the possible exception of the original SSM/Is.

## V05

- V06

**Validation Using Wind Speed Histograms **

## Slide 8

## Slide 9

## Slide 10

## Slide 11

## Slide 12

## Thank You

** **
**Frank J. Wentz and Marty Brewer**
**Remote Sensing Systems, Santa Rosa, CA**** **
**This research is support by NASA’s Earth Science Division**
**Thank You**

- Joint AMSR Science Team Meeting
- Telluride, Colorado, July 14-16, 2008