On Fri, 6 Oct 2006 10:49:23 -0500, "Gig 601XL Builder" wrDOTgiaconaATcox.net
wrote:

So you think those pictures on Google Earth are real time. Satellites both
commercial and government are in orbits that are designed to cover areas of
interest. To task a satellite to cover a specific area that isn't under its
flight path requires fuel to be used. The chances of any given place on any
given week being covered is low. The exception to this is places where the
government has special interest. I'm sure the Mid East is covered pretty
well about now.

Not quite. There are few low-earth orbits where a given satellite can't see the
entire Earth in a week. The major exceptions are those cases where the
satellite inclination (orbit tilt with respect to the equatorial plane) low; in
these cases, the satellite won't pass over higher latitudes at all.

In any case, satellites that take imagery in natural light are generally in a
sun-synchronous orbit. In this kind of orbit, the satellite's orbit plane stays
in a fixed relationship with the sun. In this way, the satellite passes
overhead at the same approximate ground time each day (and 12 hours later at
night, too). The satellite's photos then always have the sun at the same local
angle to optimize the images taken.

The other factor is the altitude of the spacecraft. The lower the orbit, the
closer the vehicle is to the target and the higher the photo resolution (too
low, of course, and you quickly use up your propellant just keeping the thing in
orbit). But by flying low, you lessen your Field Of Regard... the satellite
can't see as much of the surface at given moment. A given target may fall
right between two adjacent satellite passes, for instance. But unless the orbit
meets some pretty specific criteria, it should pass over that target area within
a couple of days.

A satellite CAN change its orbit to catch that target, but the propellant cost
is pretty fierce. It's traveling at ~18,000 MPH; changing the orbit may require
the expenditure of enough propellant to change the velocity by ANOTHER 2,000
MPH. It's a lot cheaper just to put up a second satellite with complimentary
coverage.

As to why the news media don't have fresh S/C pictures every time something
happens, the answer is "tasking." These satellites are pretty busy, and
resources (including onboard storage capacity) have to be carefully planned.

Ron Wanttaja