The humidities should be determined from a nearby, essentially concurrent 

 radiosonde observation. In the absence of such humidity data, approximate corrections 

 can be estimated fromi synoptic -climatological considerations, or even from 

 climatological data. 



In addition, it has been noted that the temperatures measured by Channel 2 



and corrected for atmospheric absorption (and also degradation; see Section 4. 4) 



14 

 are frequently lower than the actual surface temperature. The discrepancy has 



been attributed to thin ice crystal clouds or layers of atmospheric particulates not 



visible from the ground or in the TIROS TV pictures. 



4. 4 Sensor Degradation 



It has been recognized that the TIROS sensors have been subject to degradation 

 which increases with time after launch. Corrections for this effect have been pro- 

 vided by NASA in the TIROS VII manuals. These corrections were based on changes 

 in the average signal output with increasing orbit number. For Channel 2, the 

 corrections are shown in Fig. 78 of the TIROS VII manual. For a sensor reading 



of 300°K, the NASA correction is 2°K at orbit 780 and 4°K at orbit 1500. 



An examination of sample printouts of outgoing radiation, as prepared from 

 Channel 2 data by the National Weather Satellite Center of the U. S. Weather Bureau 

 (now of ESSA), was made in order to determine the adequacy of the NASA corrections. 

 The analysis was restricted to clear areas of the tropical Pacific between 5 S and 

 15 S, where the ocean temperature was anticipated to remain reasonably constant 

 over the period of interest (June to September 1963). Any change could thus be 

 attributed to degradation. Although there were some discrepancies, it was concluded 

 that, by and large, the NASA corrections were approximately valid. 



A periodic degradation of Channel 2 was found by NASA to be related to the 

 orbit-sun phase geometry (Fig. 75 in the TIROS VII manual). The NASA charts 

 showed that, over a 76 day period, errors could be as much as 3 K greater or less 

 than those shown in the average degradation curve. A correction for the effect would 

 require determination of the magnitude of the error for individual days during the 

 76 day period. Corrections to this degree of precision have not as yet been published 

 (the scale in the previously mentioned Fig. 75 is far to gross for this application). 

 It is understood that GSFC is continuing its investigation of the periodic degradation, 

 and that more precise corrections may be available in the future. As a result, no 

 quantitative correction for periodic degradation was attempted in our analyses. The 



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