Jon Dough

I'm constantly amazed that my GPS receiver can receive signals directly from
satellites overhead, and translate that information into a display loaded
with location, speed, direction, and time data..... Most of the time we
accept technology as a given, but sometimes I'd like to know a little more
about it. For example:

1. How large (size and weight) are the satellites? I'm guessing that they
are solar powered, so there are probably large panels deployed once they
reach orbit, but what are the general dimensions? Larger than a
refrigerator? A car?

2. I've seen references to onboard clocks being "switched" from Cesium to
Rubidium... How many clocks do they have, what happens that makes switching
necessary, and is one type of clock significantly more accurate than the
other?

3. My understanding is that the satellites require constant monitoring and
ground-based support to maintain the accuracy of the GPS system...
Hypothetically, if support from ground stations were to cease, how long
would the GPS system continue to provide useful navagational information?
Are we talking hours, days, weeks, what?

4. Who pays for the system? What assurance do we have that it will not be
suddenly cut off without warning? I understand there is provision for
decreasing the accuracy, but what is the worst case I can expect to see?

5. When there is a reference that a given satellite is nearing the end of
its life, I'm guessing that means it's running out of propellant to adjust
it's orbit when necessary. Is it just "shut down" or do they "de-orbit" it
and let it burn up on re-entry? What do they cost anyway?


m9876c at yahoo dot com