12.5 jum region, can produce measurement errors of several degrees. 

 Temperature gradients, however, should be seen quite well because of the 

 extremely low moist equivalent temperature difference of this sensor. 

 Channel 6 senses equivalent blackbody temperature with the same spatial 

 resolution as the other channels and is co-registered with those channels. 

 All channels of the CZCS instrument operate simultaneously. During 

 daytime operations all six channels provide useful information. If the 

 sensor operates at night, only data from Channel 6 is usable. 



The CZCS has considerable flexibility built-in to accommodate a wide 

 range of conditions. The first four spectral bands, for instance, have four 

 separate gains that change, on command, to accommodate the range of 

 sun angles observed during the complete orbit and throughout the various 

 seasons. The gains are changed to utilize the best dynamic range possible 

 without saturating over-water targets. Normally, the gain used in the first 

 four channels is determined by the solar elevation angle of the target to be 

 acquired. When a special circumstance is expected, such as a particularly 

 bright material in the water, the gain can be changed to accommodate the 

 special circumstances. 



In addition to gain change, the CZCS scan mirror can be tilted from 

 nadir to look either forward or behind the spacecraft line of flight. It can 

 tilt in two degree increments, up to twenty degrees in either direction. This 

 feature was built into the instrument to avoid the glint caused by capillary 

 waves on the ocean that would obscure any scattering from below the 

 surface. The angle of tilt of the scan mirror is determined by the solar 

 elevation angle. It is normally tilted to avoid sunglint and would only be 

 commanded to look into the glint for a special sunglint study. 



Since Nimbus-7 orbits from south to north in daylight, the scan mirror 

 is positioned to look behind the satellite when the spacecraft is south of the 

 subsolar point and ahead of the spacecraft when it is north of thesubsolar 

 point. Tilt and gain setting information is transmitted with the CZCS data 

 and is part of the data product records. 



To improve the instrument response to ocean color, a DC offset can be 

 inserted into the on-board processing of the radiance measured in the first 

 four bands. In the DC offset mode, the entire digital capability of the on- 

 board digitizer is utilized to cover approximately the top 30 percent of the 

 signal which contains modulation due to change in ocean color. Since the 

 knowledge of the exact amount of the offset eliminated in the on-board 

 processing is always known, it can be reinserted where needed for 

 processing on the ground. 



The sensor is turned on in sufficient time prior to collection of data to 

 allow for instrument warm-up and for the sensor to stabilize. Since all 

 channels are calibrated continuously during flight, any effect of turn-on 

 transient should be noticed immediately. 



The most important aspect to be understood about the CZCS operation 

 is that data acquisition is limited, due to spacecraft power constraints, to 

 approximately two hours per day. Because of this operational limit, data 

 must be taken in carefully preselected locations. Minimum on-off data 

 taking time is a two-minute segment. Frequently, longer segments are 

 taken — up to a maximum of ten minutes of continuous data. 



The CZCS data is transmitted from the spacecraft to ground receiving 

 stations at a rate of 800 kb either in real time or in playback of the tape 

 recorder. Whenever possible the data is recorded in real time. However, 

 when a satellite is out of the range of tracking stations, the data is recorded 

 on an on-board tape recorder. These tapes contain both radiometric 

 information from the imagery and CZCS housekeeping information. 



XI 



