[ntp:questions] GPS Weakness Could Sink Wireless

John Hasler jhasler at newsguy.com
Thu Dec 12 16:35:19 UTC 2013

I wrote:
> 1) Better front-end selectivity
> 2) Better front-end dynamic range

David Woolley writes:
> I don't really see how these help against all but the crudest jammers,
> which transmit pure CW carriers or are off frequency.

Better dynamic range means that the jammer must be closer or more
powerful.  Both it and better selectivity help against out-out-band
interference, which is one of the concerns (probably a more serious one
than jamming) (and most jammers *are* going to be of the crudest sort).

I wrote:
> 3) Directional antennas for fixed sites such as cell towers

David Woolley writes:
> That might help a bit, if you rejected signals near the horizon (which are
> good for navigation, but may be bad for time).

Signals near the horizon are weak and have gone through a lot of
atmosphere. You don't really want them for any purpose if you can find
better (and you can unless you are in the [ant]arctic).

> However, to get full benefit from directionality, you'd need to run
> large phased arrays and steer their beams to track the individual
> satellites.  As most military users want the navigation data, they
> would be better off with steerable beams.

99.9% of jammers and 99.99% of interference sources are going to be on
the ground.  A well-designed "fence" (shielding, really) with a cutoff
at, say, 30 degrees above the horizon can easily knock them down 100db
or more.

Due to the use of spread-spectrum jamming GPS has no effect as long as
it stays within the dynamic range of the receiver (spoofing is a
different issue entirely, and *much* more difficult to achieve).

My point is that this "weakness" is more of a design weakness in
commercial receivers than a fundamental weakness in GPS.

In any case designers of things like cell towers should no more assume
that GPS is always "just there" than they should assume that electric
power is always "just there".
John Hasler 
jhasler at newsguy.com
Dancing Horse Hill
Elmwood, WI USA

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