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launch of NIMBUS-7 satellite until the above mentioned problems were detected. 
This study, therefore, is limited to the period during which lamp 1 was being 
used for inflight calibration. The calibration lamp was calibrated against a 
single laboratory source (14) during the NIMBUS-7 prelaunch investigation. The 
calibration lamp was expected to operate at the nominal temperature of 2000K 
(15). At this temperature, the radiances for the CZCS channels 1 to 5 detectors 
were established to be, respectively, 2.04, 1.55, 1.37, l.ll and 5.25 in units 
of mW-cm-2-sr-l-yum-l (16). These radiances are known as the standard radiances. 
The standard radiances, the active and voltage calibration data are sufficient 
for determining the reliability of the inflight calibration constants. 
We have studied the inflight calibration data for a number of dates. A 
way of analysing these data has been previously reported (17). Since this 
report we have studied the inflight calibration data for eight other dates. 
Because of the importance of the inflight calibration quality, we feel that we 
should recall the main points from the above report and from the work which has 
been done more recently. For this reason we have shown the active calibration 
data in Table 1 which have been analysed to date. The data within the parenth- 
eses are taken from reference (14) and represent the average active calibration 
counts over the first six months for which the NIMBUS-7 satellite was in orbit. 
The data in this table show that, for a given gain, the active calibration 
counts, in general, are decreasing with the aging of the satellite. At times 
there have been drastic changes in these counts. This non-repeatability of the 
active calibration counts may be attributed to (i) failure of the analogue to 
digital converter, (ii) loss in the sensitivity of the CZCS detectors, (iii) 
failure of the calibration lamp 1, and (iv) deterioration of the medium between 
the detectors and the calibration lamp. A detailed analysis (17) shows that the 
behaviour of the active calibration data in Table 1 can be attributed satisfact- 
orily due to (iii). If one assumed that the calibration lamp 1 was operating at 
its nominal temperature during the first six months of the NIMBUS-7 flight then 
it is possible to compute the temperature of the calibration lamp using the 
active calibration data and the preflight values of the standard radiances. 
Since there are five CZCS channel data which can be used for this purpose, for a 
given date one obtains five values of the calibration lamp temperature, one 
value from each channel. In fig.l we have summarized the results of this invest- 
igation where we have plotted the five channel mean temperature of the calibrat- 
ion lamp as a function of the age of the NIMBUS-7 satellite. 
  
  
  
pu es 
kis +3 uH 
(8 
1990 r~ = 4 
| & 
| 2 t 
1965 La h ; - 
| & + 
1900 | ] 
1 8 Figure 1. Five channel mean 
il v | temperature of the calibration lamp 
PE ow / as a function of the age of 
wo | 8 4 NIMBUS-7 satellite. 
1965 + i À 
= Lif! 
1960 + u. fl 
| f 
195 3 
AGE OF NIMBUS-7 SATELLITE IN DAYS 
= 4 L dues ing ge see 
0 200 400 600 800 1000 
699