GPS and Iridium

How dependent are various satellite systems like GPS and Iridium on
ground based support?

I understand that these systems, as different as they are, both require
ground based assitance to work. The GPS satellites need to be sent
periodic corrections and the Iridium system needs attention from the
ground. How long could they function without this ground based
support?

-Curious
-Charles Talleyrand

Note: I'm not sking how long these satellites will stay in orbit. A
GPS satellite in orbit but sending an uncalibrated signal is actually
worse than useless.

Note: I don't care why the ground based support stopped, just the
effect. Maybe because of a power outage or outer space aliens or legal
changes or .... It just doesn't matter.

"Charles Talleyrand" wrote:

How dependent are various satellite systems like
GPS and Iridium on ground based support?

I understand that these systems, as different as
they are, both require ground based assitance to
work. The GPS satellites need to be sent periodic
corrections and the Iridium system needs attention
from the ground. How long could they function
without this ground based support?

Note: I don't care why the ground based support
stopped, just the effect. Maybe because of a
power outage or outer space aliens or legal
changes or .... It just doesn't matter.

When I worked on Iridium, the last 18 months of its
main software development effort at Motorola, they
were contemplating a 24x7 ground support team of
around 20 individuals. I don't remember if that was
"per shift" or "total", though.

Iridium takes a _tremendous_ amount of tweaking,
since it depends on satellites knowing each others
ephemeris to high accuracy, so the satellite to
satellite laser communications can be kept aimed
accurately, knowledge of the satellites own
ephemeris and changes to it, so the solar panels can
be kept faced toward the sun, so results of
atmospheric drag can be overcome and pitch and yaw
away from the intended orbit due to half a dozen
un-"spherically symmetrical" forces that act on the
satellites can be countered by correcting with
gyroscopes or with steering gases.

Too, the satellites don't last forever, and must be
replaced on a continuing basis. I seem to recall
that the planning included a set of hot spares in
orbit, too.

There are many ground stations, to send control
signals to the satellites, and to provide
connections between the satellites and the local
"POTS" (plain old telephone system) and those, also,
need ongoing maintenance.

All of this means that Iridium would "stop working"
fairly promptly if the ground crew went on strike.

HTH

xanthian.

I haven't a clue about GPS, but some of the same
considerations may apply.



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