Time to remember how to do it the old fashioned way..

[jdc]

And require just a tiny signal strength to jam them

The idea that GPS is easy to jam because the received signal strength at the earth's surface is so low is superficially attractive but actually true only for the most old-fashioned receivers (unfortunately quite a few yacht's ones are very old-fashioned). Modern GPS chips are designed to be mounted inside mobile phones, quite often sharing antennas with the GSM transmitter, and are built:
1. to have only passive band-pass filters between the antenna and LNA, so out-of-band signals are attenuated
2. to look for single tones and subtract these before the remaining signal is applied to the correlators
3. to look again at the signal strengths of all the detected GNSS signals, and regenerate the codes of the stronger ones and then subtract these from the residual signal before correlating again for the weaker ones.

This means that even quite strong in-band signals don't deafen the weaker ones from the satellites themselves.

Additionally, the Doppler, data timing and the carrier phase of each detected SV is looked at and compared to what it should be given the position estimate and the data in the almanack and ephemeris, and rejected if it looks bogus. So the position is derived from apparently valid signals from a number of different sources with approximately correct signal strengths, frequency offsets and timing. This means that even transmitting a GPS like interferer with specious information is unlikely to get accepted as genuine and cause disruption. Of course it doesn't mean that sohisitcated attack is impossible - far from it, navies do it all the time - but they do have pretty complex kit: I don't think you'd do it easily with bits from Maplins.

One thing which does strike me is that if Trinity House wanted to operate a near-harbour radio nav system they could do it now, with almost no infrastructure cost and no new radio towers, by using tri-lateration of the emissions from mobile phone towers, esp 3G. We (the company I worked for) did lots of trials of this, and 60m accracy was easily achieved even 10 years ago and the number of 3G base-stations has increased since then. But the hang-up of TH turned out not to be making a robust system, but making one they control (jobs for the boys, or girls, you see). So they carried on blindly with the expensive, and utimately unsuccessful white elephant of Loran.

 

[Bru]

I don't think you'd do it easily with bits from Maplins.

It's not my field of expertise but I sort've understand what you wrote

But, but but ... surely if I knocked up a wide band down and dirty fairly powerful transmitter and fed it white noise and swamped the relevant frequency band that would be crude bt effective (and very very easy to track down I admit)?

 

[grumpy_o_g]

It's not my field of expertise but I sort've understand what you wrote

But, but but ... surely if I knocked up a wide band down and dirty fairly powerful transmitter and fed it white noise and swamped the relevant frequency band that would be crude bt effective (and very very easy to track down I admit)?

True, but you're unlikely to annoy anyone except your neighbours. Exponential laws mean you need a hell a lot of power to disrupt something even a few miles away though - beyond a few miles and you start needing power stations directly attached. And, as you say, it takes a very small and cheap missile to take the target out, and said missile can be launched from a long way away. Some of them are actually designed to fly to an area and then loiter around for a few hours or so. If they detect a signature they've been programmed with they'll go for it (and they've got ways of staying on target even if the transmitter is switched off). The rule of thumb in modern warfare is "don't even reflect, never mind transmit".

In practice the far bigger threat is the satellites being taken out or EMP's taking out unhardened electronics I reckon.

 

[grumpy_o_g]

A lot of ships have gone over to completely electronic nav with no chart table at all.

It does make you wonder what would happen if tye GPS was jammed in the Solent on a foggy morning?

Using electronic charts isn't the same as relying on GPS though. Perfectly possible to use DR to calculate your position and put estimated waypoint/positions in and then update the plotter when you get a chance to take a bearing or cross a depth contour or whatever. It's a shame that plotters don't automatically ask for DR inputs when they lose the GPS for more than a minute or so. B&G stuff has all sorts of fancy data to let you get the best performance from the boat that could help tremendously to give some seriously accurate DR. I reckon that Inertial Nav will start to become available on the leisure market soon for a sensible price too - marine systems are incredibly expensive as they have to be accurate over weeks but most leisure users don't need that so you could probably knock a decent system for less than the price of a high-end plotter using off-the-shelf MEMS chips. - still a hell of a lot more than GPS admittedly.

 

[DJE]

https://www.google.co.uk/url?sa=t&s...ggcMAA&usg=AFQjCNF_QcstQB-vATI2XLXBPfisfzrijw

I am no expert but I went to a talk about this trial. They said they used a car-battery powered transmitter and it distrupted GPS reception out to 30 miles from the jammer.