Page:Advanced Automation for Space Missions.djvu/41



TABLE 2.8,- SENSOR CONFIGURATION OF AN EARTH OBSERVING SATELLITE.

Configuration and array of a possible set of active and passive sensors in an early mission. All sensor bands to be operated simultaneously if desirable.

Configuration

1 set of wide angle sensors comprising the full array of sensors to scan 330 km swath at 15 m resolution, or as limited by individual sensor.

2 sets of narrow angle UV, visible and near IR sensors, capable of accurate aiming, to cover 110 km swath at 5 m resolution; ~7X 10? bits/sec.

Sensor Array

10 bands - UV, visible and near IR (daylight) 4 bands - far IR (night) 6 specialized bands (atmospheric composition) SAR (all weather) LIDAR Differential height Differential velocity Altimeter

The IESIS satellite program is envisioned as developing in a long-term sequence carrying well into the next century. A detailed world model of land features already exists as contour maps covering a significant portion of the continents. Land features have sharp boundaries and vary only slowly over time. Oceans have wider geographic features that vary seasonally. The atmosphere requires three- dimensional modeling of rapidly varying phenomena. An obvious difference between land and ocean or atmosphere from a user standpoint is the large human population on land and its virtual absence elsewhere. Table 2.4 summarizes the characteristics of world models of the land, oceans, and atmosphere.

The logical deployment sequence of user-oriented resource satellites begins with a set of basic land-observing satellites whose world model already can be rather fully detailed. Since the satellites will spend about 75% of their time over the ocean it is natural to include ocean-sensing capability with as much ocean modeling as is feasible at the time of design and launch. Atmospheric sensing and rudimentary modeling should be included, both for understanding the state of the atmosphere and also as a necessary part of the interpretation process for sensor readings of land and ocean observations.

To assure long life for these sophisticated satellites, reasonably high orbits are required. Atmospheric path dis

TABLE 2.9,-POSTULATED OBSERVATION FREQUENCY AND SWATH WIDTHS.

Niche features observed	Observation frequency, per day	Maximum swath width, km Land 0.5	350 Ocean	3	700 Atmosphere	12	1400

tortion and sun angle introduce errors and complications into the interpretation process for imaging data. Path distortion causes reddening and other wavelength- dependent absorptions, and Rayleigh and Mie scattering are especially sensitive to particle size in the atmosphere and to sun angles.

The use of sun-synchronous satellites simplifies the situation considerably, a rational initial constraint which could be removed at some later time when more sophisticated modeling becomes available. It appears reasonable to have a set of sun-synchronous satellites operating continuously so that each Earth ground point is covered at the equator every 2 days by at least one satellite of the set. Sun- synchrony produces roughly the same sun-angle conditions over an observed land point for a particular satellite and helps to standardize image interpretation for that satellite at that point. An orbit near present-day Landsat altitudes (920 km, nominal) will support a long-lived satellite. If altitude is adjusted to a 14-1/8 rev/day rate, the ground trace of a particular satellite repeats every 8 days. Four such satellites could cover the Earth with the desired 2-day period.

The swath width required of a satellite for 8-day coverage at 14-1/8 rev/day is about 350 km. However, in order to take account of partial cloud cover the team proposes six satellites in sun-synchronous orbits. If these are placed substantially uniformly about the Earth's circumference the local viewing times for each satellite are spaced about 2 hr apart. Bunching may be desirable if there are reasons to pick a particular local viewing time. Polar conditions can be monitored by a seventh polar satellite, which may also act as a spare if one of the sun-synchronous satellites is disabled.

To relay data to the continental United States, two geostationary satellites are required. These satellites are also used to monitor global conditions, particularly cloud cover. Global cover information is compiled by IESIS to prepare each satellite for the tasks it can most usefully perform during its upcoming orbit, by enabling modifications in sensors and processing to optimize the information obtained from each series of observations.

Ocean coverage of a particular ocean point three times per day with a 700-km swath width requires 12 satellites,