# Presentation: 373890

-                                                     **Metop-A  AVHRR/3 A305  Cal/Val report**
**                                            ****Thermal channels 3B, 4, 5**
-                                               Jan. 11, 2007         Jerry Sullivan      { _[jerry.t.sullivan@noaa.gov](mailto:jerry.t.sullivan@noaa.gov)_ }
**AVHRR Space Clamp Stability**
- The space count is electronically reset (clamped) each scanline to ~ 990 counts.  This test checks the space clamp electronics.
- RESULT:  1-minute averages differ by  ~ 0.1  counts over an orbit.
**AVHRR Repeatability (NEdT) When Viewing Blackbody at 300K**
- When viewing the internal blackbody (BB) held at a constant temperature for 1 minute, the AVHRR count output should be constant to within ± 1 count of the average count.  This is equivalent to 0.12K at a 300K BB temperature.
- RESULT:  1-minute standard deviations  ~ 0.5 counts    { = 0.06K }.
**AVHRR Horizontal Striping Evaluation**
- When viewing the internal blackbody (BB) held at a constant temperature for 1 minute, the BB count within-line variance should be ~ 4 or 5 times the BB count line-to-line variance.
- RESULT:  For random orbits, the criterion was met.
**Examination of the “Stray Light” Problem**
- Monitor the intensity and duration of the stray light problem that occurs in morning satellite AVHRRs during certain parts of the orbit.
- RESULT:  When converted to equivalent temperature units, the count anomaly associated with “stray light” is  ~ 0.3K for channel 3b and  ~ 0.1K for channels 4 and 5.

**Notes:**
These 4 tests check the stability of the AVHRR thermal channels when the radiometer looks at either SPACE, a constant energy source, or the internal BLACKBODY, a constant source during 1-minute intervals.  The results of the first 3 tests are all positive, for random orbits over the late Nov., 2006  - early Jan., 2007 period.  The NEdT specification for the BLACKBODY look is met; actual performance is twice as good as the spec requires.  The SPACE count clamp test and horizontal striping test do not have specifications, but the Metop-A A305 AHVRR performance in these tests compares well to what NESDIS has measured in earlier AVHRRs.  Test #4, the stray light check, gave results comparable to what NEDSIS has found in the past from satellites in a morning orbit.  

**Notes:**
Late breaking news from the EUMETSAT Media site on the web !!
In late November, EUMETSAT started providing AVHRR Level 1b products to users over its EUMETCAst broadcast system.  
In mid-December, the AVHRR was declared ready for routine operations.  EUMETSAT found that the instrument performances were within specification.

**Notes:**
Every scanline, in each of the 3 thermal channels, the AVHRR records 10 count values when it looks at SPACE, 2,048 count values when it looks at the EARTH, and 10 count values when it looks at the internal BLACKBODY.  The energy (radiance) coming from the SPACE and BLACKBODY sources is known, and is used to calibrate the radiance emitted from the 2,048 EARTH pixels.  Almost all calibration tests are concerned with checking AVHRR performance when it looks at either the SPACE or BLACKBODY source.  

**Notes:**
NOTE:  Each of the remaining slides is for one random orbit only  BUT  the result shown on the slide is representative of all the other orbits I checked over a 50-day period.
When the AVHRR looks at SPACE every scanline, the count value of all channels is electronically clamped to a constant value, typically around 990 counts for the 3 thermal channels.  This graph shows that the clamp works.  A 1-minute average of count values, 3,600 counts, of 992.1 indicates that 90% of the values were 992 and 10% were 993.  

**Notes:**
In a 1-minute interval, the BLACKBODY temperature typically varies by 0.01 – 0.02K; we can consider this a “constant” temperature.  In this same 1-minute interval, the AVHRR should output the same count value, since the source is constant.  In 99.9% of the 1-minute intervals I checked, around 75% of the counts had the same value COUNT (~ 475) and the remaining 25% had values within ± 1 count of COUNT.  The standard deviation was ~ 0.5 counts.  In the 300K region,  a change of 0.5 counts =   a change of 0.06K.  So, when looking at a constant source, the AVHRR “thermal noise” is 0.06K which is twice as good as the specification of 0.12K. 

**Notes:**
The Striping Index =     (within-line VARIANCE) / (line-to-line VARIANCE) 

For 10 count values per scanline, an Index value of 9 is ideal but a value of 3-5 suffices in practice.  You want the count values in each scanline to jump randomly between 990 and 991; you don’t want all 990s in one line and then all 991s in the next line.  This can lead to “stripes” in imagery, especially SST imagery.  NESDIS was forced to change detectors on NOAA-15, the first of the KLM AVHRRs, because of this problem so we now check for it.

**Notes:**
“Stray-light” problem.  The red curve shows the BLACKBODY temperature times series for one orbit.  The blue curve shows channel 3B “delta-count”, the AVHRR response to radiance from the BLACKBODY.  In this small temperature range (1K) the two variables should be proportional.  This is true, except for near the end of the orbit near scanline 34,000, where the AVHRR output “jumps”.  The horizontal line at the bottom shows that near line 34,000 the satellite has just crossed from satellite night (blue) to satellite day (yellow).  Probably stray light entered the system, causing the channel 3B output to jump by about 0.3K.  The jump is closer to 0.1 - 0.2K for channels 4 and 5.   Operations software corrects the 3B jump, but not for 4 and 5.  This effect is stronger on morning satellites, like the Metop-A, and is seasonal.  

**Notes:**
A SNO (Simultaneous Nadir Overpass) event between the Metop-A AVHRR and the NOAA-18 AVHRR.  The event occurred in Antarctica, 15 degrees from the South Pole.  The values shown are AVHRR channel 4 brightness temperatures (add 241K to get real values).  The temperatures measured by the two AVHRRs are fairly close; area-averages differ by 0.3K.  The Metop-A FRAC data is 1x1 km resolution, the NOAA-18 GAC data is 3x5 km resolution.  NESDIS navigation for the METOP-A was still problematic during this period but should be good enough for these nadir events.  {Need to do more SNO work}

**Notes:**
Check of AVHRR geo-location accuracy using current NESDIS Operations data, near the Equator (Panama Canal).  This test uses the NDVI (Normalized Difference Vegetation Index), which is a very good discriminator between land and water.  
   NDVI =  (R2 – R1) / (R2 + R1)
Where R1, R2 are the channel 1, 2, reflectances.
The green (land) and blue (water) areas are obtained from the Metop-A AVHRR NDVI values for Jan. 5, 2007.  The white borders are from the World Vector Shoreline database, which claims an accuracy of ~ 0.5 km.  
Each scanline starts on the left of the image, Pixel #1, and ends on the right near pixel 2,000.  In the next 3 slides, the 3 areas circled in red will be enlarged and a more detailed NDVI map will be used to check geo-location.  

**Notes:**
This shows the NDVI data near the START of scanlines, the area under the left circle in the Panama Canal NDVI image.   Green is land, blue is water, and the black line is from the World Vector Shoreline database.  Geo-location (navigation) appears to be off by 1-2 pixels here.  { THANKS to John Sapper, for originally showing this kind of image. }

**Notes:**
This shows the NDVI data near the MIDDLE of the scanlines, the area under the middle circle in the Panama Canal NDVI image.   Green is land, blue is water, the black line is from the World Vector Shoreline database, and the red line is nadir.  Geo-location (navigation) appears to be off by 1-2 pixels here.  (Notice that the pixel centers are more densely packed near nadir than they are on the wings of the scanlines.)

**Notes:**
This shows the NDVI data near the END of scanlines, the area under the right circle in the Panama Canal NDVI image.   Green is land, blue is water, and the black line is from the World Vector Shoreline database.  Geo-location (navigation) appears to be off by 3-5 pixels here.

**Notes:**
This image compares NDVI (Normalized Difference Vegetation Index) from the Metop-A VAHRR A305, using PRELAUNCH calibration coefficients for channels 1 and 2, to NDVI values from the NOAA-17 AVHRR. 
     GREEN = high    YELLOW = medium     ORANGE/RED = low   { NDVI }
Both Metop-A and NOAA-17 are morning satellites.  The match of the two NDVI images is OK but not great.  The match is also complicated by the fact that one AVHRR receives backscattered radiation (check the red nadir lines) and the other gets forward scattered radiation.  This is known to produce different NDVI values from the same scene.  

**Notes:**
This image compares NDVI (Normalized Difference Vegetation Index) from the Metop-A VAHRR A305, using POSTLAUNCH calibration coefficients for channels 1 and 2 provided by Dr. Wu on 1/8/07, to NDVI values from the NOAA-17 AVHRR. 
    GREEN = high    YELLOW = medium     ORANGE/RED = low   { NDVI }
Both Metop-A and NOAA-17 are morning satellites.  The match of the two NDVI images is an improvement over the comparison using PRElaunch Metop-A coefficients but is still not exact.  

**Notes:**
An example of the surprises that arise from having 1-km resolution FRAC data over the whole globe.  The rectangle shown in brown is the sacred AVHRR calibration site, the Libyan Desert target at (28-29E, 21-23N).  NESDIS picked this site because it was assumed to be very homogeneous, and checks using previous AVHRR data confirmed this.  However, we never had 1-km data before, only 3x5 km GAC data (maybe 8x8 km ISSCP data originally).  The histogram (for 10/31/06) shows that 99% of the target pixels had channel 2 counts with values between 505 and 515.  Only 1% of the pixels had count values less than 500.  But the locations of these pixels, shown in black in the rectangle, have structure.  Magnifying this area with Google Earth showed what appeared to be irrigation circles (??) to Dan Tarpley and myself.  Truly a puzzle.  On the Good News side, the small area of the surprise pixels (1%) means changes to our previous results are well below the noise level.  {Thanks to Tom Kleespies for Google Earth instruction.}