Deep-fried GPS?

[Tucepi]

With the ever increasing offshore wind farms around the UK why dont someone put up signs on them telling us the way / direction to various locations and attractions. At least that way they would be of use to someone!

 

[AntarcticPilot]

+1

Unless by jamming they mean spoofing of positions. A bit unlikely IMHO. Who the hell is going to spend time following a leisure boat around beaming "plausible but incorrect" data at it!? (Obviously you'd have to pick a specific boat and stick with it or the calculated COG wouldn't ever be "plausible" and the position wouldn't remain plausible for long even if by luck it started that way!)

A cynic might say the "General Lighthouse Authority" are a bit worried that GPS is *so* effective that someone might suggest closing some light houses.

Personally I find light houses far more reassuring than a GPS fix, but no doubt a light house could be spoofed too if we're going to get that paranoid. (It would have the advantage that you wouldn't have to tailor your spoofing to one boat. You could just stick a light characteristic from a real lighthouse in the wrong place and with luck fool a load of boats.) [1]

A few years back we did a poll on YBW. I think over 95 per cent said they're fine with conventional nav & would have no problems whatsoever if GPS packed up.

It's a non-issue.

The 5pc of sailors who can't cope with conventional nav will have to devote an afternoon to picking up enough to sail about a bit, it's not difficult

[1] Why would anyone spoof the position of a leisure boat?

Going back to basics, remember that the accuracy of GPS is defined as within x.x metres xx% of the time. With Selective Availability that became x.x+n metres xx-y% of the time. Supposing the interference (however thats caused and remembering that GPS signals are extremely weak) was causing a signal variation giving a 10-15% shift in position its quite possible to imagine scenarios where believable but inaccurate data is presented to a navigator and transmitted on AIS.

In essence GPS does not give an absolute position fix, there will always be some error and that error is variable.

btw, beer is always acceptable but beer tokens are much better

Every commercial GPS unit ive used in aviation uses RAIM. They issue warnings if integrity is lost. you would need all signals to be in error by perfectly matching amounts to spoof a good receiver.

anyone interested in data rather than rumour can find all they need to know on the egnos website including accuracy levels.
http://egnos-user-support.essp-sas.eu/egnos_ops/service_performances/global/position_error#

GPS depends on accurate timing of radio signals from satellites. Each signal contains its time of origination, so looking at the time of arrival allows you to compute your position (it's a bit more complicated than that, but that gives the essentials for this situation). So, anything that messes up the time of flight of the signals can produce erroneous results.

Solar flares can cause exactly such changes in time of flight (they change the total electron content of the ionospheric part of the path, which effectively alters the speed of light in the ionosphere). Such changes may well be correlated so that the positions are silently in error - that is, the whole ionosphere may get faster or slower, or at least, all the bit through which the signals you're receiving pass does. A lot of the time the errors will not be correlated and automatic error computations WILL trap the error - but not always. However, it isn't a one-off situation - if the signals are bad, they're likely to remain bad for hours.

You could identify the error by looking at the carrier phase of the signal, as is done with high precision survey equipment. But this is unlikely to be available in consumer GPS systems. The DOP computation cannot always identify the type of errors that a solar flare can plausibly create.

 

[RAI]

WAAS / EGNO / MSAS / etc., reception enable a receiver to remove most of the ionospheric induced errors. That's why they were introduced. With GPS III two frequency reception will enable future receivers to remove the Iono effects themselves.