Page:Advanced Automation for Space Missions.djvu/40

 and Pakhomov, 1978). Somewhat artibrarily, six specialized sensor bands have been allocated to these uses for purposes of the present study.

Sensors are configured into a wide-angle medium- resolution sensor array and additionally into at least two arrays of narrow-angle, high-resolution sensors capable of independent accurate aiming over at least the swath width of the wider-angle array. Narrow arrays allow independent coverage of terrestrial features needed to satisfy conflicting requests during a particular orbital pass. They also may be used to obtain stereoscopic imagery, say, of cloud tops, by setting one array to view forward and the other aft during a pass and later combining the two image streams appropriately. Table 2.8 summarizes one possible sensor set.

2.4.5 Data Rate Estimation

The bit rate generated by one optical or IR sensor is given approximately by 8SBVW/A, where S is the number of sensor sets, ? is bands per sensor set, V is orbital velocity, W is swath width, and A is pixel resolution in km2 for an 8-bit pixel. For two sets of narrow sensors and one set of wide sensors the total bit rate generated at a 7 km/sec orbital speed is 8(2)(10X7 km/sec)(l 10 km)/(5X 10'3 km)2 + 8(1X20)(7 km/secX330 km)/(15X10~3 km)2 = 6.6X109 bits/sec. Of the remaining sensors the SAR will produce the maximum data rate by far. Today's SAR apparatus generates data at 0.65X109 bits/sec (Nagler and Sherry, 1978). Representative Doppler LIDAR, Doppler radar, and laser altimeters return data at the rate of several tens of kilobits per second. Thus, onboard computing capability requirements must be sufficient to handle data rates near 7X109 bits/sec. This is roughly an order of magnitude higher than that used in present Landsat orbiters.

2.4.6 Satellite Requirements

A summary of required measurement rates has been provided by Nagler and Sherry (1978) for a wide range of environmental and resource assessments. The necessary frequency of observation generally is lowest for land-based features, higher for ocean observation, and highest for atmospheric and weather assessments, with considerable overlap in the requirements. Table 2.9 indicates the frequencies of Earth observation and attendant swath widths believed reasonable for the IESIS system.

IMAGE Figure 2.17.- A projected advanced massively parallel processing system.