# Presentation: 097616

## IR Calibration

**Steve Gaiser**
**Jet Propulsion Laboratory**

## IR Calibration Summary

**The IR calibration algorithms are performing extremely well.**
**Radiance validation has shown no problems.**
**Radiometric performance has been extremely stable.**
**All algorithm changes under consideration are small refinements.**

## Radiance Validation (SHIS)

**Analysis by H Revercomb and D Tobin**
**University of Wisconsin, SSEC**

**UW Validated AIRS Radiances to <0.1K in window channels**

## Radiometric Stability

**Analysis by H Aumann**

## Instrument Table Changes,Potential “Gotchas”

**Detectors may change properties after thermal cycling**
**“****A/B weight table” (redundant sensor weighing selection) uploaded to optimize performance.  Causes changes to:**
***Noise properties (NeN, Gaussian-ness)***
***Spectral Frequency Centroids:  up to ~0.4% delnu***
**Channel Properties File Update**
***	******Reflects Noise Property Changes***
***	******Assigns new “A/B State”***
**Noise limits re-calculated for high noise QA bit**

## Algorithm Refinement Options

| Description | Impact | Status | Max Bias
@ 250K | Bias Type |
| --- | --- | --- | --- | --- |
| Scan Angle Conversion | Low | Implemented in V3.5 | N/A | N/A |
| Scan Mirror Temperature Correction | Very Low | Not planned | <0.045K | Latitudinal, Spectral |
| Offset Smoothing | Low | Likely for V4.0 | 0 | N/A |
| Gain Smoothing | Very Low | Not planned | 0 | N/A |
| Offset Fitting | Very Low | Under consideration | <0.07K | Spectral |
| Polarization Coefficients | Low | Likely for V4.0 | ~0.1K | Scan Angle, Spectral |
| Cold Scene Noise | Very Low | Under consideration | N/A | N/A |
| Moon-in-view | Very Low | Not planned | <<0.1K | Spectral |
| Pop detection | Very Low | Not planned | 0 | N/A |

## Scan Angle Conversion (geolocation)

**Based on coastline crossing analysis by Dave Gregorich, we had a geolocation error in the cross-track direction of 1-2 km at nadir.  See ADFM-652 for details.  The observed errors at other scan angles are consistent with a constant offset error in scan angle.**
**Approach:  A 0.118 degree correction was determined by Jan Gohlke, within the uncertainty of the scan angle calibration.  The scan angle calibration was changed accordingly.**
**Result:  the change is a significant improvement.  We may have slightly overshot, but the remaining error seems to be less than the 0.5 km accuracy claimed for the coastline map.**
**Status:  implemented in V3.5.**

## Scan Angle Conversion (Cont.)

**Before**

**After**

**Analysis by D Gregorich**

## Scan Mirror Temperature Correction

**The temperature of the scan mirror enters the radiometric calibration because the scan mirror coating polarizes the thermal emission, causing the radiances seen by the detectors to vary as a function of scan angle. **
** ****By mechanical design, the temperature measurement of the scan mirror is made by radiometric coupling, rather than by thermal conduction.  This coupling acts as a low-pass filter, resulting in an observed orbital variation that is reduced in magnitude and shifted in phase from the actual temperature of the scan mirror.  Rudy Schindler has a detailed analysis in ADFM #662.**
**Tom Pagano performed a radiometric analysis of this effect, documented in ADFM #663.  The effects were no larger than 45 mK for a 255K scene.**
**Option:  use Schindler's analysis to add a time-dependent correction to the measured temperature.**
**Status:  not a significant change.  Not planned.**

## Scan Mirror Temperature (Cont.)

**Analysis by T Pagano**

## Offset Smoothing

**Currently we're calculating offsets once per scanline for each channel, based on the four space views immediately before and the four space views immediately after.  This makes radiances correlated within scanlines, resulting in what's been described as "striping".**
**Impact:  the radiances are as reported:  no bias is being introduced, and the uncertainties are as reported.  Thus the impact for climate studies is zero.  The problem is strictly with the distribution of the error.  With the current algorithm, the scanline radiance bias is as large as the channel's noise 1 scan in 60.  The scanline radiance bias is as large as twice the channel's noise roughly 1 scan in 1000000.**
**Option:  fit to more space views, over a wider window.**
**Benefit:  reduce scanline-correlated signals to below the detectability threshold.  Also, will reduce noise overall by up to 5% (see Thomas Hearty's ADFM #654).**
**Harms:  none.**
**Status:  planned for V4.0.**

## Striping Example

**Image Courtesy of SY Lee**

## Gain Smoothing

**Currently detector response is calculated as the granule mean of the responsivities calculated from each view of the OBC.  This introduces discontinuities at granule boundaries.**
**Option:  perform fitting to gains across several granules.**
**Benefit:  remove slight granule-level "striping".**
**Harms:  none.**
**Status:  not planned.**

## Offset Fitting

**The current radiometric calibration algorithm was developed treating all four space views as if they were made at a scan angle of 90 degrees;  All four views are treated identically in determining offsets.  It is observed that the space view signals are different between the different space views.  This is an expected consequence of the variable scan mirror emission described above.**
**Consequences:  the method currently employed ignores the difference in signal between the different space views.  Those differences appear as biases, and can be over 50 mK in some bands for scene temperatures of 250K.**
**Option:  treat each space view as unique.  Apply a correction to each before determining the offset.**
**Benefit:  remove biases of up to 50 mK.**
**Harms:  this does attempt to correct a bias.  Validation of radiances may be required again.**
**Status:  under investigation.  Likely for V4.0.**

## Offset Fitting (Cont.)

## Polarization Coefficient Improvement

**The preflight measurements of instrument polarization are used in the radiometric calibration.  Not all parameters solved for are currently in the PGE tables.  Specifically, spectrometer phase is currently set to zero at all frequencies.**
**Option:  reanalyze preflight data to determine spectrometer phase.**
**Benefit:  possibly remove or reduce biases mostly less than 100 mK.**
**Harms:  this does attempt to correct a bias.  Validation of radiances would likely be required again.**
**Status:  under investigation.  Likely for V4.0.**

## Current Polarization Correction

## Cold Scene Noise Identification

**Some AIRS detectors have noise properties which are time-dependent and depend on the temperature of the scene.  Specifically, some channels sometimes have anomalously high noise when viewing cold scenes.  Granules in which this occurs usually (but not always) have the high noise QA flag set depending on the duration and magnitude of the noise within the granule.**
**Option:  perform noise analyses separately for the cold and warm calibration views, and use a QA bit to indicate this condition.**
**Benefit:  better identify misbehavior in ~40 channels.**
**Harms:  none.**
**Status:  under consideration.**

## Cold Scene Noise Example

**Analysis by T Hearty**

## Moon-in-view Improvement

**When the moon is present in space views, those views are no longer valid as radiometric references.  The current PGE algorithm excludes these views by thresholding potentially contaminated views against moon-free views.  When the threshold is exceeded, the view is excluded.  But before and after the threshold is met, the moon is contributing a small amount of contamination.**
**Option:  add a look-ahead and look-behind mechanism to the current thresholding approach.**
**Benefit:  the maximum radiometric error when the moon is present below the threshold is around 100 mK for a 250K scene (Thomas hearty, ADFM 643).  This occurs in roughly 20 granules per month (of 7200).**
**Status:  not planned.**

## Pop Detection/handling Improvement

**AIRS detectors sometimes change their baseline levels.  An algorithm is currently in place to detect this and report it in a QA field.**
**Options:  investigate methods of detecting smaller magnitude pops and shorter duration pops.  Try to correct for pops within a scanline.**
**Status:  not planned.**

## Algorithm Refinement Options

| Description | Impact | Status | Max Bias
@ 250K | Bias Type |
| --- | --- | --- | --- | --- |
| Scan Angle Conversion | Low | Implemented in V3.5 | N/A | N/A |
| Scan Mirror Temperature Correction | Very Low | Not planned | <0.045K | Latitudinal, Spectral |
| Offset Smoothing | Low | Likely for V4.0 | 0 | N/A |
| Gain Smoothing | Very Low | Not planned | 0 | N/A |
| Offset Fitting | Very Low | Under consideration | <0.07K | Spectral |
| Polarization Coefficients | Low | Likely for V4.0 | ~0.1K | Scan Angle, Spectral |
| Cold Scene Noise | Very Low | Under consideration | N/A | N/A |
| Moon-in-view | Very Low | Not planned | <<0.1K | Spectral |
| Pop detection | Very Low | Not planned | 0 | N/A |