Don Tuite wrote in
:

On 16 Apr 2005 00:26:54 GMT, (Jay Masino)
wrote:

Newps wrote:


Suppose you are navigating solely by GPS. What are you going to do
in the event the military chooses to disable the GPS system while
you're airborne

Can't be done. There is no on/off switch.

I'm involved with supporting spacecraft operations of NASA's
constellation of EOS spacecraft. Somewhere, there's a satellite control
center that operates the constellation of GPS spacecraft. It may be
true that the military can't "throw a switch", but there's no doubt that
they can command each satellite to shut itself down and go into "safe
hold", effectively shutting down the GPS system.

I wrote some articles on GPS for Trimble in the early '80s. I don't
have my notes from then, but there are a few things I sort of remember
that sort of come down evenly on both sides of the debate.

One is that the Navstar system was a joint military/civilian effort,
implying a promise to keep it operational in most circumstances.

The other is that the satellites' orbits have to be tracked and
corrected ephemerides regularly updated with an uplink from the Naval
Observeratory, which I assume is how they can deliberately degrade C/A
coverage over specific geographic areas.

(The NO would have been a single point of failure. By now, there is
probably some redundancy.)

Feel free to bring me up to date.

Don

A small hobby of mine is satellite observing, so I asked about
this amongst some of those folks and got the following tidbits.

Individual NAVSTAR sats can surely be shut down, aka sleep mode.
This would be necessary in the event of a satellite sending
erroneous data. You need to be able to shut it off to keep the
system working. If they really wanted to, they could put them
all into sleep mode, but that is near nil probability as it
would also deny the military of their use.

However, it may take a few hours to accomplish for any given
sat as it must be within communications range which isn't
100% of the time.

Although President Clinton ordered the 'selective accuracy' mode
to be turned off, it surely can be turned back on again. This
would degrade the accuracy, but the system woudl still be useable.

Also, it was the military can use 'selective deniability' to
degrade or disabel to civilian signal in a local region.

There are GPS jammers, but their effectiveness is disputed.

Due to the tight control of GPS by the US gov, the Europeans
are develping their own system called GALILEO which will use
the same base frequency (L1) as the NAVSTAR which means most
consumer level GPS units will be compatible. Once this is
available, the idea of the US Gov 'turning off' GPS is moot.

The Russians have GLONASS but it's of limited use.

As for getting rid of VOR training/testing, I think it's silly,
and I don't even have a PPL!!! I know flight sims aren't 100%
accurate but I have no trouble using VOR's in MSFS. Besides,
I think arguing to get rid of VOR training/testing is like
arguing to get rid of parallel parking training/testing in
drivers ed. They've tried making drivers licenses easier to
get and look where it got us. A lot of bad drivers. Now they're
starting to make it tougher again. (too little too late if you
ask me)

Brian
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