Geosynchronous GPS?

Gary Coffman wrote in message . ..
On Wed, 13 Aug 2003 15:48:46 -0400, Robert Munck wrote:
On Mon, 11 Aug 2003 04:50:49 -0400, Gary R Coffman
wrote:

... Just put a couple of kilos of gravel in the same orbit but
going the other way, and pretty soon no comsats.

You'd have to be mighty lucky to hit even one comsat by doing that.
... A comsat doing good station keeping could be anywhere in a
cubic mile volume around the ideal GSO track. ... a comsat only
presents a few square meters of cross section along the orbital
track.

Not by my figures. If there are only 1,000 pieces of gravel and
a given comsat takes up 1/1,000,000th of the (one-mile cross-
section of the) orbital track, you've about one chance in 1,000 of
hitting it.

You're implicitly assuming that the gravel is randomly distributed.

Obviously, wouldn't be at first. But our experience with debris
patterns is that they become randomly distributed over long time
scales. I'm thinking here of Markov processes. All the sites that can
have positive probability eventually will with a decay process from
the overpopulated orbital states to the underpopulated states.

But the gravel is moving through the entire GSO track
twice a day (at 6 km/s relative) so it has a good chance of
hitting one comsat every day or two and all of them within a year
or two.

You're implicity assuming the gravel randomly changes squares
each orbit.

snip

That is a good assumption to make particularly with perturbations from
the Sun and Moon.


Karl Hallowell

"Ultimate Buu" wrote:
Wouldn't it be prudent for the U.S. to develop a GPS-like system which is
based in a geosynchronous orbit, as both the Chinese and Russians are
developing ASW (anti-satellite warfare) systems? At the very least such a
system should be used as a backup since most precision weapons rely on GPS.
It would be much harder for the Chinese to knock out a geosynchronous
satellite and it would be almost impossible to do so using lasers (their
preferred method at this time).

Any nation which can deliver satellites to GEO can also
deliver anti-satellite weapons to GEO. China and Russia
certainly can, even India can.

Also, as others have pointed out, the GPS constellation
is already in a pretty high orbit, which takes nearly as
much energy to reach as GEO, so they're about as safe
as can be orbit wise.

As nobody's pointed out yet so far as I can see, the GPS
constellation is already designed with ASAT capabilities in
mind. The electronics are hardened to a certain degree
from EMP. There are, classified, procedures for avoidance
of ASAT weapons. And there are orbital spares designed to
provide service in the case of lost satellites. Since the
system is designed with a fair degree of redundancy when
the satellite constellation is "full" (i.e. there are
usually more satellites in the sky at any given time for
any given location than are absolutely necessary for geo-
location) it would take a large number of lost satellites
to create a service loss.

Think about it a second, the GPS system was designed in an
era when the Soviets already had a demonstrably functional
ASAT system, and when the most likely major war larger than
a regional conflict was a global exchange of thermonuclear
warheads etc. GPS was designed from the get go to have the
maximum chance of surviving *that*. In comparison, some
middling state or even a big regional power (like China)
developing a functional ASAT weapon is small potatoes.



Finally, as for your comment that the Chinese prefer to
use lasers as an ASAT weapon, I think you ought to lay off
the crack pipe just a teensy bit. But that's just a
suggestion.

"Sander Vesik" wrote:
Sounds like this would merely lead to development of better sattelite
knockout weapons... This planet definately doesn't need yet another
dumb arms race.

Everyone says that, except there aren't usually a whole lot
of takers for being on the losing side of an arms race.

"Alex Terrell" wrote:
As mentioned, there half way to GEO (energetically, almost all the way
there).

Another defence would be to have secret, dormant GPS satellites (in a
stealth shroud?) which would activate if other ones were knocked out.

I wonder if they have that already?

The GPS constellation nominally contains 24 satellites, but
needs only 21 satellites to meet the minimum coverage
requirements (near global coverage, periods of degraded
accuracy beyond about 10m lasting only around an hour or
less and limited in extent). The current GPS constellation
has 27 functional satellites. The modern GPS satellite
configuration doesn't really use "spares" per se but
something more along the lines of "failure tolerant
configurations". The current configuration can handle 2
satellite losses without hardly a hiccup. And decent
service with only as few as 18 satellites is possible (but
would require repositioning, I think).


It is, of course, unknown whether the US military maintains
secret orbital spares for GPS. Though the amateur satellite
tracking community is good enough to where I'd think that
fairly unlikely (though not entirely out of the question).