# Presentation: 865779

## NOAA-18 AVHRR Calibration Overview

- Transmittance

**Notes:**
The red curves are the spectral response functions for thermal window channels 3B, 4, and 5 on the NOAA-18 AVHRR.  The black curve is a MODTRAN transmission curve, an Arctic atmosphere at 270K.  The radiation received in channels 4 and 5 is emitted from the Earth, while there is an additional reflected solar component in channel 3B.  

**Notes:**
NOAA-18 AVHRR thermal channels are now turned on.  This is the first check of the data, the Sanity Check.  All three channels correctly sense Earth thermal features such as deserts, ice, and clouds.  The added solar component of channel 3B is apparent when the Earth is in sunlight (marked by the yellow portions of the orbit).  

**Notes:**
Fundamental AVHRR calibration test.  In one orbit, the blackbody temperature changes by only 2K.  For this small range, the AVHRR output should be linear when it views the blackbody.  When both variables, blackbody temperature and AVHRR counts, are scaled the same way and plotted, the linear relation is evident.  Chs. 4 and 3B show the same behavior.    {Radiance is “linear” with temperature for a 2K range.} 

**Notes:**
SPACE:           The Space Count is electronically clamped and should remain constant over many orbits.
BLACKBODY:  The blackbody temperature changes during an orbit, so the Blackbody Count also changes.  However, for almost all 1-minute intervals, the temperature only varies by about 0.02K.  An average blackbody count for 1 minute is computed, then the RMS (root-mean-square) around this average.  The 1-minute RMS is plotted for many orbits:  this is a measure of AVHRR stability when viewing a constant-temperature blackbody.   {Channel 4}

**Notes:**
1 minute of NOAA-18 AVHRR count values when viewing the internal BLACKBODY.  During a 1-minute interval, the blackbody temperature typically changes by only 0.02K, so the AVHRR output should be stable.  It is !!  {  GAC data:  1,200 blackbody counts per minute}

**Notes:**
STRIPING INDEX:  One statistical test to ensure that the instrument noise is random.  The AVHRR views a constant-temperature (almost) blackbody for a  1-minute data block, 120 GAC scanlines.  The index is defined as:
           [within-line Count variance]/[line-to-line Count variance] .     
More is better !!    A larger Index value indicates that within-line noise (SALT/PEPPER) dominates line-to-line noise (STRIPING).  Striping was a major problem on NOAA-15, the first of the AVHRR/3 models, so we continue to monitor it.

**Notes:**
When the NOAA-18 satellite moves into the dark (black portion of Solar Zenith Angle curve) the temperatures measured by the 4 PRTs approach each other, indicating a more uniform temperature across the blackbody.   This is an important part of C. Cao’s assertion that the dark part of the orbit is the most accurate place to estimate the true AVHRR gain, a strategy we’ll use to re-process the 25-year AVHRR data set.

**Notes:**
A  SNO  (Simultaneous Nadir Observation) event over Greenland at location (43.78W. 71.24N) on June 19, 2005 at approximately 15:06 .  The NOAA-18 passed over the “target” and 52 seconds later, the NOAA-17 passed over the same location to within about 1 km accuracy { using 1b GAC (lon,lat) data }.  The interior of Greenland makes a good calibration target because of its surface uniformity in temperature and reflection.

**Notes:**
SNO (Simultaneous Nadir Observation) event over Greenland, using GAC data.  The data shown is AVHRR counts.  For easier interpretation (smaller numbers), I subtracted 598 from the N-18 counts and 595 from the N-17 counts.  The “relative” count  PATTERNs  are very similar.  When the count values are converted to radiances and then to blackbody temperature, the 150-pixel averages are about 0.05K apart.  

**Notes:**
This navigation uses the NDVI vegetation index as a land-sea tag.  NDVI < 0 is water (black) and NDVI >= 0 is land (green).  The reference geolocation lines (yellow) have 0.5 km accuracy 90% of the time.  This 1-km HRPT image of Lake Michigan, USA shows the AVHRR navigation to be off by about 3 pixels along-track and 2 pixels cross-track.  The red diagonal line is the nadir path.  A near-nadir image 2 months later showed the same results.

## Post-Launch Calibration of the AVHRR Solar Reflectance Channels

- No on-board calibration device
- Pre-launch calibration may not be valid even on Day 1
- Have been using Libyan Desert as vicarious calibration reference

## Overview of Method and limitations

- Annual variation of target reflectance (surface BRDF)
- Long latency (28 months) to start N16 calibration update
- 20 months for N17
- No latency for N18 – lessons learned from NOAA-16/17

## Results

|  | Channel 1 | Channel 2 | Channel 3A |
| --- | --- | --- | --- |
| Reference Reflection | 37.8 | 42.6 | 66.9 |
| Apparent Reflection | 41.00 | 41.80 | 63.58 |
| NOAA-18, Day 1 | 40.46 | 41.08 | 61.92 |
| NOAA-17, Day 1 | 39.66 | 37.67 | 59.84 |
| NOAA-16, Day 1 | 38.60 | 40.33 | 65.59 |
| Required adjustment | -6.6% | +3.7% | +8.0% |

## Slide 15

## Calibration Update

- Based on the initial assessment, the following calibration update is recommended for NOAA-18 AVHRR Ch_1_lo = 0.05045*count -  1.990, count<500.54 Ch_1_hi =  0.1504*count -  51.73, count>500.54 Ch_2_lo = 0.05485*count -  2.161, count<500.40 Ch_2_hi =  0.1646*count -  57.35, count>500.40 Ch_3_lo = 0.02828*count -  1.061, count<500.56 Ch_3_hi =  0.1998*count -  87.39, count>500.56

## SNO Comparison

**SNO Comparison**

## Slide 18

## Slide 19

## Slide 20

## Slide 21

## Slide 22

## Summary

- Initial post-launch calibration update for AVHRR solar channels was implemented
- Results seem reasonable but expected to improve with time
- Ready for operation
- No co-registration error between Channel 4 & 5
- Unclear with Channel 1 & 2
- Other channel pairs not checked yet