Message-ID: <485A7BC5.1080206@g3ysx.org.uk>
Date: Thu, 19 Jun 2008 16:31:17 +0100
From: Stewart Bryant <stewart@g3ysx.org.uk>
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Subject: Re: LF: Re: Loran C
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Andy Talbot wrote:
> The article has a very good point to make about GPS jamming though. I
> think its probably the first time I've seen it talked about in the
> open-press.  Our telecomms infrastructure now is so critically
> dependent on accurate timing that a few well deployed low power
> jammers could cause serious disruption. 

Although modern communications systems are dependent on sync,
the current systems in the UK need frequency not time.

Although the UK SDH networks use GPS for local time sources,
to drive reference oscillators, all the systems I know
use multiple Cesiums as a backup. Most other networks
(for example the mobile phone networks) connect to
an SDH network by some means or other and take their sync
from that source.

The big R&D problem in this space is how we manage the
migration to packet networks with Ethernet delivery, and
great lengths are being taken to avoid the need to rely
on GPS for critical sync functions. Indeed GPS-only sync
would be regarded as a show-stopper by all of the sync
network designers I have spoken to.

A lot of newer sync technology (the media access systems)
needs relatively course (1us class) time as well as frequency
to function, but there is work in hand to deliver that
over packet networks (IEEE1588, IETF TICTOC etc)

So I would not worry about GPS jamming disrupting comms.

Navigation is a different matter, and as Alan says Loran
is vulnerable, perhaps not quite as vulnerable as civilian
GPS, but certainly more vulnerable than seems to be claimed.

Stewart/G3YSX