Page:Advanced Automation for Space Missions.djvu/34

 equatorial

Figure 2.12,- Ground-based world model data processing operations.

region is T, and the number of passes per day (generally 15-16) is/, then, taking the Earth's circumference roughly as 4X10" km, T = (4X104)//W. If n denotes the number of niches in one pass for a given swath width, then n = (4X104)W/niche area.

For example, if swath width is 330 km and the satellite executes 15 rev/day, then T is on the order of 8 days. If the area of a representative niche is roughly (for simplicity) 3.3 km2, then n = 4X106 niches and N = ni = (4X106) (2600)= 1.04X1010 bits.

Niche at each pixel. A gross theoretical estimation is obtained by assuming that each pixel element is associated with 8 bits of information of t different types. Thus, each pixel element plays the role of a niche. If A is the resolution of each pixel in km2 and E is the approximate surface area of Earth (5.15X10? km2), then the total number of bits required to describe pixel elements covering the entire planet is %tE/A. If pixel resolution is 10 m, then A - 10"4 km2/pixel which gives

(5.15X108 km2)/(10~4 km2/pixel) = 5.15X1012 pixels

for all of Earth. The total information required to store this gross world model is of the order

8tE/A = 8/(5.15X 10?)/10~4 = 4.12X 101 3 / bits.

It appears that a memory capacity onboard a satellite of the order 10'1 bits is sufficient to allow system operation on an orbital swath basis. Presumably between 1014-1015 bits are needed for ground memory. It is estimated that by the year 2000 roughly 10'4 bits of in-space memory will be available, so these requirements do not appear particularly stringent (Opportunity for Space Exploration to Year 2000. Address delivered by A. Adelman at Goddard Space Flight Center, 1980).

2.3.3 Ground-Based World Model Processing

Processing at the central ground facility can be broken into paths as shown in figure 2.12. Action items which the system should generate in response to observation of an anomaly are carried out, appropriate alarms sounded and individuals notified. Anomalies that the system cannot