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Message-ID: <D57991E6D6854891B9AEC9FDEFED589F@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <fc7eccec0907081436q46485213re11a9295fc05638b@mail.gmail.com> <PAEFLMFGBDKKCONLMLBIAECPDEAA.ganymede444@btopenworld.com> <8bf118410907100011s696aa712sbd88bd3a023e1c39@mail.gmail.com>
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Date: Sat, 11 Jul 2009 01:22:57 +0100
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Subject: Re: LF: Forthcoming GPS jamming exercises
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Dear Dave, LF Group,

G3YXM wrote:
>Perhaps we should encourage a Loran jamming excercise to prove that it
isn't the un-jammable marvel that its cracked up to be?
;-)

It's interesting to speculate about possible GPS and Loran jamming devices. 
One sees "hand-held GPS jammers" mentioned on the internet - the GPS signal 
strength is perhaps a few uV/m at ground level, and the signal is a 
noise-like pseudo-random sequence, so a low power, noise generating 
"barrage" jamming device that swamped the GPS signal would probably be 
feasible over a line-of-sight area. On the other hand, Loran produces signal 
strengths of millivolts/m, so something like a million times as much jamming 
ERP would be needed, compounded by the low efficiency of feasible 
clandestine 100kHz jammer antennas. Another antenna-related problem would be 
the 10kHz or so bandwidth of the Loran signal, which presents a problem even 
with full-sized Loran antennas. In current Loran transmitters, the antenna 
itself forms part of the 100kHz pulse generator, which uses thyristors to 
discharge capacitors into a pulse-forming network incorporating the 
antenna - I guess some similar technique would be needed for the jammer so 
that power consumption did not become too enormous.

Then there are "counter-countermeasures". It seems that anti-jamming GPS 
receivers for the military etc rely mostly on multiple RX antennas and front 
ends, and DSP-based beam-forming to reject signals that are not coming from 
the expected direction of a satellite. This probably isn't workable for 
Loran, due to the large spacing that would be needed between antennas at LF. 
But Loran has a precise time and phase relationship betweeen the pulse 
patterns making up the signal, and is designed to work with signals from 
multiple sources in the same channel, along with CW interference, so 
attempting to "spoof" a reasonably sophisticated Loran receiver would also 
need a sophisticated jammer.

I don't doubt it would be possible to construct an effective Loran jammer, 
but it would probably not be hand-held!

Cheers, Jim Moritz
73 de M0BMU