Re: Bush to consider shutting down GPS in extreme emergency

From: Alan Browne (alan.browne_at_FreeLunchVideotron.ca)
Date: 12/17/04 

Date: Thu, 16 Dec 2004 20:14:28 -0500

Juergen Nieveler wrote:

>
>>DCF77 and similar long-wave radio systems produce time accurate in the
>>milliseconds, GPS in nanoseconds. Not comparable.
>
>
> They aren't comparable because GPS is designed to calculate positions,
> not to tell the time.

er, no. GPS provides 3 independant products in the receiver: Position, Velocity
and Time (PVT). They all come from the same signals of course, but they are
arrived at seperately. The cornerstone for all of them is time.

> As a source for accurate time, DCF77 is more than
> enough for just about any purpose I can imagine.

There are things that reuqire finer timing than a long wave system can deliver.
  More to the point, using a single outside references at a distance away means
that all receivers believe the time is different. You could correct each
receiver according to its distance from the xmtr, but unfortunatley diurnal,
geography and weather effects would skew this badly at these low freq's. GPS
receivers at widely different locations can be sync'd much more accurately... as
well as providing microsecond or better accuracy at each receiver.

Cheers,
Alan 

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