A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Feb 14, 14:44 -0500
I agree that high end military systems can defeat jamming. A directional antenna increases the signal to noise ratio. This assumes that you know where you are such that you *can* point them accurately.
In an all out confrontation with a space faring opponent, one of the primary targets will be the satellites themselves. No nation will allow itself to be picked apart with precision munitions while it has recourse to eliminate that GPS precision. China has demonstrated this capability, as has the US. I believe Russia has this capability, while other nations *could* develop it. Its not impossible, just very difficult.
But all this is neither here nor there. The thrust of the article is the concern about the affect on civilian GPS reception. I'm a civilian. My GPS devices have omni directional antennas. Most civilian GPS devices (and many military GPS receivers I have handled) are omni directional. A powerful transmitter on the same narrow band of frequencies will have a large noise component compared to a weak signal component. The GPS signal *is* very weak. Hence, the devices cannot receive the information from the satellites. Its standard RF engineering. Increase gain of signal whilst suppressing noise. Hard to do that if your receiver has an omni directional antenna.
The article stresses the time component received as the main concern. That is true if that was your sole source of accurate time. But accurate time is also available over the internet, over the cell network and other places. Most maintain a local clock which is sync'd periodically to a master clock. So the jamming would have to be persistent and extremely widespread (like satellite elimination) in combination with internet suppression to accommodate the concern of the author.
On 14/02/2014 02:06, Brad Morris wrote: > Widespread jamming requires a series of powerful transmitters (or just > one). No big secret there. Actually, it is not quite as simple as that. Large and expensive systems that depend on GPS for navigation can use highly directional antennae to track the satellites using the ephemerides that are constantly downloaded to the receiver. This makes GPS reasonably jamming-proof. As I reported earlier, trials were done on one of the larger UK warships and the most determined effort to jam the GPS moved the ship a virtual 8 feet... Geoffrey Kolbe -- Dr Geoffrey Kolbe, Riccarton Farm, Newcastleton, TD9 0SN Tel: 013873 76715 Mob: 07811 154621
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