Satgen 451  Earth Observation   by GM4IHJ  (BID SGEN451)    15 Nov 97

We are now far beyond the point where people believed " You can do
anything with satellites" ,(once you get them launched). But there is
still an element of "altitude = superiority" , in popular thinking, as
recently exemplified by the question " Why do they need U2 spy planes,
when they have got satellites". So perhaps this is a good time to talk
about some of the problems of pictures from space.

1. To take good pictures you need to have your imaging equipment on a
stable platform. Several amateur satellites have been less than 100%
effective because of platform motion.

2. If you want pictures in the visual part of the spectrum, your
target must be sunlit. Arctic observation is therefore markedly
seasonal and even at middle latitudes solar illumination across the
portion of the earths surface being viewed, can vary enormously.

3. Satellites are in fixed orbits . Unless you add a large orbital
engine and a very big tank of fuel. So satellites only cover a
specific target adequately, twice a day at most.

4. Satellites are predictable. So your target can be hidden or
obscured for the few minutes of a satellite pass, ie do not move your
missiles , when sats are in range.

5. Weather conditions and atmospheric transparency vary enormously.
Some places seem to be permanently clouded out. 

6. While satellites at high altitude see large tracts of the earth ,
they see it far less clearly than low earth orbit sats. Hi altitude
sats are useful for detection of radar, radio and missle plumes
because they stand out clearly from the background . But even there,
decoys can obscure the diagnosis.

7. To further complicate things, visual observation is far superior if
you have some shadow detail. So High noon, dawn or dusk, local time,
orbits are out. Because the former has no shadows, and the latter are
all shadows.

8. Many satellites use a number of separate sensors each for a
different part of the Ultra violet, Visual, Infra red or microwave
spectrum ( Eg Okean,Seastar, and the microwave Noaas). But these can
be jammed and you would need a lot of satellites to give continous
whole earth cover - as the 66 sat Iridium LEO constellation for mobile
phones shows. Contrast this, with the flexibility of the U2 spy plane
which can be set up specially for each mission, fly at just the right
time, and take pictures from a variety of viewing angles, and you may
see why it has not been replaced by satellites.

PS. Questions about Ariane 502 orbit. It reached Apogee 26631 kms,
Perigee 534 Kms 7.8deg Inc, ie well below geostationary transfer orbit
Error Satgen 450, correct American low VHF TV frequency is 55.25 Mhz
Not as reported Sgen450 which said 56.25. Tnx to CO2JA for correction