Page:Advanced Automation for Space Missions.djvu/39



2.4.4 Sensor Requirements

IESIS sensor requirements are dictated by the specific terrestrial environment that must be scanned to fulfill an Earth resource mission. Full utilization of satellite capabilities and operating time demands useful sensing during daylight and nighttime passes of an orbit and during cloud cover.

Figure 2.16. - IESIS downlink processing.

Daylight operation involves observation by sunlight filtered through atmosphere twice before detection by satellite sensors. Most filtered sunlight lies in the visible region extending somewhat into the near-UV and further into the near-infrared. An optimum match between the daylight Earth environment and the satellite passive scanning system must include a visible, near-infrared, and some ultraviolet detection capability. A properly chosen 3-dimensional color space obtained using a red, green, and blue filter set yields color discrimination roughly comparable to that of the human eye. Detection at the chlorophyll absorption region near 650 nm gives useful discrimination for vegetation, while for water detection the low reflectivity region near 850 nm is useful (Schappell and Tietz, 1979). A pair of UV, five visible, and three near-IR bands should provide sufficiently broad color space (10 dimensions) to allow very widespread signature analysis of important terrestrial features such as crops, rivers, lakes, clouds, forests, and snow covers.

The nighttime environment may be scanned passively for thermal radiation at a temperature near 300 K. The black- body emissions of the cool Earth peak in the far-IR near 10 Aim. Four wideband far-IR sensors would allow accurate temperature and signature definition of nighttime features, although not to the same precision and resolution as with daytime sensing.

All-weather capability requires active microwave scanning of the Earth. The Synthetic Aperture Radar (SAR) operating at 1-10 GHz (Nagler and Sherry, 1978; OAST, 1980) is capable of essentially all-weather observation at good resolution. The SAR system also would augment nighttime passive measurement in the far-IR.

Altitude sensing provides useful information about terrestrial resources such as crop height, reservoir levels, or mountain snow cover. Height is recorded from differential altitude measurements performed at a boundary, e.g., by comparing the heights of crop tops to nearby level ground. A differential altitude measurement system is possible using rapid Q-switched LIDAR. Absolute altitude measurements can be obtained by LIDAR or microwave altimeter. Differential velocity measurements at a boundary (e.g., a river bank) can be taken by Doppler shift analysis.

Undoubtedly there will be requirements for additional specialized optical, infrared, and microwave sensor bands to detect important surface and atmospheric components such as ozone, water, water vapor, and carbon dioxide (Golovsko