Re: GPS and Iridium

In article <1137474753.945612.82610@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, Charles
Talleyrand says...

>How dependent are satellite systems like GPS and Iridium upon ground
>based service?

>I understand that the GPS satellites require ground based corrections
>sent to them as their clocks and orbits drift. But suppose these
>corrections did not arrive? How long would the GPS system remain
>useable? Note I'm not asking about how long the GPS satellites remain
>in orbit. After all, a GPS satellite in orbit and broadcasting
>mis-information is worse than useless.

GPS is designed to remain usable, with some degradation of service,
for a month or so after complete loss of ground support. After that,
and I'm not sure whether "a month or so" is based on a strict clock
or MTBF estimates, the satellites will go into a "safe mode" where
their limited autonomous-ops capability is directed entirely to keeping
the satellites minimally functional and awaiting new instructions from
the ground. They might last years in safe mode, but they wouldn't be
transmitting navigational signals.


>Similarly, how long can Iridium and Inmaresat and so on remain useful
>without ground based support?

I'm thinking not at all. Those are fee-for-service commercial systems,
and the billing computers are all on the ground. It's possible that the
DoD's anchor tenancy agreement with Iridium includes a requirement for
GPS-like autonomous (and free) operation in case Al Qaeda manages to
blow up the Iridium headquarters, but I doubt it. Those satellites
will go into safe, and no-free-lunch, mode at the slightest provocation.


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.

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