Manfred Schroeder 
  
6. CALIBRATION | oi 
Under the assumption that a linear relation between input (radiance L) and output (Digital Number DN) exists 
the absolute calibration factor A can be determined from: 
DN=A-L (5) 
  
  
  
  
  
  
  
  
  
  
  
  
  
The absolute calibration factor is the product of the absolute colibration coefficient A’ and a gain factor Gm: 8. 
Th 
— ' . 
A T A G (6) ; e 
9 
Gm = 1.379 m=1,2,3, ‚8 for SPOT-1 ° 
Gm z42 m=0,1,2, ,7 for MOMS-2P e 
Gm - 1.50? m=1,2,3, ,8 for SPOT-4 
The calibration factor for each spectral channel was derived from the computed upwelling radiances and the 9 
corresponding average Digital Number of the Calibration Area in the SPOT and MOMS-2P images. Table 2 : 
shows the values for MOMS-2P, SPOT-01 and SPOT-04 images. W 
e 
me: 
Upwelling Calibration factor 
Time Sensor Gain Channel Radiance L DN (sr m? nm)/mW 
um mW/(sr m? nm) A A 10. 
29 June 1998 3 0.50-0.59 74.2 82.79 1.116 0.744 [1] 
10:51 GMT SPOT 4 3 0.61-0.68 73.0 102.29 1.401 0.934 
Sun Zenith: 23.15? HRVIR-2 2 0.78-0.89 65.9 66.26 1.005 1.005 
3 1.58-1.75 20.1 172.48 8.581 5.721 
02 July 1998 SPOT 1 8 0.50-0.59 80.0 120.38 1.505 0.405 [2] 
10:50 GMT HRV-1 8 0.61-0.68 82.4 88.38 1.073 0.289 
Sun Zenith: 23.40? 7 0.78-0.89 72.2 104.73 1.451 0.508 [3] 
02 July 1998 2 0.449-0.511 89.1 116.02 1.302 0.651 
11:47 GMT MOMS-2P 2 0.532-0.571 84.4 116.67 1.382 0.691 
Sun Zenith: 20.60? 5 0.645-0.677 83.5 179.86 2.154 0.381 
1 0.772-0.8]5 82.3 125.20 1.521 1.076 [4] 
  
  
  
  
  
  
  
  
  
  
Table2: . Calibration factors for MOMS-2P, SPOT-01 and SPOT-04 derived from upwelling radiance 
computed with the 6S-program and corresponding average Digital Number (DN) of the calibration area. 
7. DISCUSSION 
For SPOT-01 calibration besides La Crau other test sites, e.g. White Sands, and also other methods (lamp 
calibration) are used. From all these measurements a ‘best estimate” for the calibration factor is derived. [6] 
The obtained calibration factors for SPOT-01 (Table 2) differ only slightly from the published best estimate 
4.73% (XS 1), 0.31% (XS 2) and 2.34% (XS 3). For SPOT-04 the agreement with official published calibration 
factors is in all channels better than 5%. The calibration accuracy of SPOT by various methods is estimated by 
A. Meygret et al. [6] to 4-896. [7] 
The good agreement between the actual and the previous calibration measurements for SPOT-01 shows that the 
applied methods are reasonable. As the measurements for MOMS-2P and SPOT-4 were carried out under the 
same environmental conditions as for SPOT-01 and with the same methods it can concluded that they also lead 
to reasonable results. 
With respect to MOMS-2P two remarks have to be made: 
e The weather situation for vicarious calibration was not optimal on 2 July 1998 because some clouds developed 
near the test site between SPOT-01 and MOMS-2P overflight. This may have affected the calibration of 
MOMS-2P at 11:47 GMT. 
  
286 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000.