ATM calibration procedure

(as described by Andrew Wilson and dodgily transcribed by Mike, then tested and annotated further)

1. The ATM is placed in the light bench kept at Kidlington.
2. The ATM is run for 500-1000 scans at the 12.5, 25 and 50 scan speeds with the light off (gets dark current values).
3. The light is then switched on and left to warm up for 3-4hrs.
4. Another set of captures is then made with the light on (gets sensor value at calibrated light level).
5. These data are then processed into level 1 HDFs (without calibration being applied!) to get raw sensor values.
6. For each channel (1-10)
   1. Using some smoothing, find the brightest columns in the image (this will be near the centre point and will be where the ATM scan head is closest to perpendicular to the light source. Use this column number on the dark images too, provided the ATM and bench aren't moved.
   2. Take the average of the column to get the typical value (mode best for dark current, mean best for lit value)
7. For channel 11, do nothing?

For channels 9 and 10 (SW IR), there is a fudge factor - apparently the ATM instrument records lower values when in the air than on the ground. One should process a flight line and look at dark areas (no cloud, directly under instrument) such as water to get some idea of these, but the dark current is normally ~100-300 lower when flying. does this apply to the max value too?

• For 2007, these offsets were measured (based on averages over large patches of dark water) as:

Band	Speed	Measured dark value	Difference
9	12.5	min value 24360	offset -127
10	12.5	min value 24470	offset -128
9	25	min value 24310	offset -177
10	25	min value 24470	offset -128
9	50	min value 24200	offset -287
10	50	min value 24300	offset -298

• Used nearest multiple of 10 as the "true" dark value