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Re: LF: Forthcoming GPS jamming exercises

To: <[email protected]>
Subject: Re: LF: Forthcoming GPS jamming exercises
From: "James Moritz" <[email protected]>
Date: Sat, 11 Jul 2009 01:22:57 +0100
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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

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