North American Network Operators Group

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RE: NTP, possible solutions, and best implementation

  • From: David Schwartz
  • Date: Thu Oct 02 21:05:04 2003

> > It depends upon how low a probability failure you're willing to consider
> > and how paranoid you are. For one thing, the U.S. National
> > Command Authority
> > could decide that GPS represents a threat to national security
> > and disable
> > or derate GPS temporarily or indefinitely over a limited or
> > unlimited area.

> Derating GPS wouldn't affect the time reference functionality. Turning off
> GPS entirely would seriously affect military aviation operations.

	Not so:

"Selective Availability (SA) is the deliberate introduction of error by
either altering the precise timekeeping of GPS satellites or the position of
the satellites in space, through the on-board software, thereby reducing
both positioning and timing accuracy for civilian users."

	GPS accuracy is generally reduced by adding noise to the timing. Now you
would have to derate GPS pretty significantly before timing accuracy would
be significantly affected. But it's possible that some time references would
refuse to lock on at all with sufficient derating. The affects of more
extreme derating than SA are, at least to some extent, unknown.

> > Aviators try very, very hard not to trust their lives to GPS.

> As opposed to LORAN ?

	Generally, aviators don't like SPOFs. So they try very hard not to trust
their life to any one thing. GPS is used in conjunction with VORs, pilotage
(navigation by reference to fixed objects), and dead reckoning.

	GPS is used for instrument approaches, but only under extremely controlled
conditions by very experienced pilots. A significant fraction of instrument
training is how to cross-check instruments and detect failures. GPS
approaches are individually approved by the FAA and factors such as runway
lighting are critical. FAA approved GPS units must be used and one of the
things these GPS units must do is monitor signal integrity (RAIM).	From time
to time, you will read FAA accident reports of people who attempted to
perform GPS approaches with just a handheld GPS.

	DS