Re: How many satellites are in geosynchronous orbit by now?

In article <1134083295.186183.64360@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
<lou@xxxxxxxxxxx> wrote:
>Be careful here - these radars can steer fast, but do so by giving up
>sensitivity. The first problem is that if you slew quickly, you don't
>get nearly as much energy on a given target...
>Second, you need to steer the receiver, too...

Third, when you start talking about seriously long ranges, radar search
becomes a three-dimensional problem, not a two-dimensional one, because
you must point in each direction long enough for an echo to come back from
an uncertain distance. For example, if you want to search out to lunar
distances, you cannot change direction more often than about once every
three seconds, because it takes that long for an outbound pulse to travel
out to that distance and an echo to come back. Even for GSO, where the
round trip is about 1/4 second, this issue isn't trivial.

If you're looking for a target at a known range, then if your transmitter
and receiver can be steered independently, you can avoid this by having
the receiver scan following the transmitter scan, one round-trip delay
behind. But that's no good for search, where the range is uncertain.

In theory, I suppose, you could record the signal at each element of a
receiver array, and then do both the beam-forming and the search at all
distances simultaneously in signal processing afterward. Don't know of
any practical system that does that, though.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | henry@xxxxxxxxxxxxx
.

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