Does Lulworth practice GPS jamming?

[wonkywinch]

An odd thing happened when we were on passage from Torquay to Studland last week. The inner ranges were active so we routed to keep south of 50'34" as requested but on the leg, the SOG kept jumping up to more than 30 knots and my BTW info jumped all over the place. We were riding some pretty large swell but average speed was generally 8-9 knots.

The Marine Traffic trace looked smooth and resembled what we really did but the GPX file from the chart plotter shows the track it thought it did. My sailing isn't so bad that we lost control like this so I wonder if there was local jamming going on.

 

[PabloPicasso]

Interesting

 

[Boathook]

Local jamming would affect the AIS as well, seeing it uses GPS for your position.

I wonder whether marine traffic smooths out your or anyone elses position ?

 

[wonkywinch]

Local jamming would affect the AIS as well, seeing it uses GPS for your position.

I wonder whether marine traffic smooths out your or anyone elses position ?

I wondered that. My system is set up that the chart plotter uses it's own internal GPS as source and of course the AIS has it's own with separate antenna. I wonder if there was some low level jamming that affected only the chart plotter as the military wouldn't be able to jam larger ship GPS/AIS without a prominent notice.

It was weird that it all stopped around 5pm when the ranges shut down. I liked his closing announcement too, "see you in September".

 

[Boathook]

The range safety boats have there work cut out in July and then weekends in June. September can be bad for them as well depending upon the weather !

 

[lustyd]

Its a firing range so I cant see any purpose in them doing so.
Was someone sitting on the puck?

 

[wonkywinch]

Its a firing range so I cant see any purpose in them doing so.
Was someone sitting on the puck?

Chart plotter gets it's GPS internally, ie via the screen which is open to the elements. Never seen this behaviour before or since. Just that narrow "Lulworth" window.

 

[LittleSister]

I don't know what may have caused your GPS location to apparently jump about, but I imagine that Marine Traffic samples your AIS signal much less frequently than your GPS updates, and therefore smooths out, or misses entirely, those 'errors'.

 

[[70521]]

Unlike @lustyd I think there is every reason to mess with GPS signals in a military firing range.

Marine Traffic might not be using every AIS transmission to display on the web, what use would it serve? So you throttle back the number of 'AIS pings' displayed and the reduce the processing on the back end server.

 

[lustyd]

Unlike @lustyd I think there is every reason to mess with GPS signals in a military firing range

They’re firing at static targets, what would be the benefit?
All it achieves is making it harder to keep traffic out as you can’t track via AIS reliably. Even if they did, they’d be obliged to send an NTM out so we’d all already be aware of it.

Far more likely an issue on the boat interfering with the signal.

 

[Pye_End]

'GPS inaccuracies in Lulworth, or any location, can stem from several factors including signal interference, atmospheric conditions, multipath errors, and receiver quality.' AI.

Highly unlikely to be jamming or spoofing from an official source without going through correct procedures - the potential consequences are too high.

 

[sailorbenji]

Local jamming would affect the AIS as well, seeing it uses GPS for your position.

I wonder whether marine traffic smooths out your or anyone elses position ?

Presuming OP has a class B AIS transceiver, he will only be pinging a position out on AIS every 30 seconds, whereas his plotter-generated GPX is likely utiliising data at a higher frequency than that, which could alone explain some smoothing effect.

Specifics on MarineTraffic (I run a station). Terrestrial stations transmit every AIS ping they detect (so every 30secs for every class B contact in motion they can pick up), however MarineTraffic updates the vessels (in motion) once every 60 seconds (i.e. every other AIS Class B transmission, give or take).

 

[[70521]]

They’re firing at static targets, what would be the benefit?
All it achieves is making it harder to keep traffic out as you can’t track via AIS reliably. Even if they did, they’d be obliged to send an NTM out so we’d all already be aware of it.

Far more likely an issue on the boat interfering with the signal.

Are they? Do you know what they are currently testing and the guidance/defensive systems they are using?

They have at least one guard vessel, people onshore with binoculars, AIS and radar so keeping traffic out of the way is not really an issue. In my only experience of the range we were very politely asked to turn south (nothing to see here) not an issue I put in a 30° starboard 'course correction'. After all why waste all our tax £ by stopping play while we move off a three knots.

 

[wonkywinch]

Are they? Do you know what they are currently testing and the guidance/defensive systems they are using?

They have at least one guard vessel, people onshore with binoculars, AIS and radar so keeping traffic out of the way is not really an issue. In my only experience of the range we were very politely asked to turn south (nothing to see here) not an issue I put in a 30° starboard course correction. After all why waste all our tax £ by stopping play while we move off a three knots.

Assume you were heading east

On our outbound passage, one motor vessel was hailed a few times, politely asked to stay south of 50'34" and proceeded to track about 50-100m inside a line along that latitude I'd stuck in our chart plotter. Both their real AIS and my radar return showed him north of the line. They stopped firing whilst he was between Worbarrow and Lulworth.

 

[[70521]]

Assume you were heading east

On our outbound passage, one motor vessel was hailed a few times, politely asked to stay south of 50'34" and proceeded to track about 50-100m inside a line along that latitude I'd stuck in our chart plotter. Both their real AIS and my radar return showed him north of the line. They stopped firing whilst he was between Worbarrow and Lulworth.

We had spent a pleasant night in Lulworth Cove and were heading towards Swanage and one of the crew was 'keen' to check out his old climbing haunts thus we had come inshore.

Heading up and down the English Channel I am usually about 10 miles off.

 

[lustyd]

Are they? Do you know what they are currently testing and the guidance/defensive systems they are using?

Its a well known firing range where they either fire little guns or big guns into the bay at buoys. Its gunnery training 99% of the time and well documented so no need for any secret squirrel conspiracy theories. The fun stuff is usually Cardigan Bay or Scotland as they aren’t as busy. The Navy can disrupt the public but they usually choose not to where possible.

 

[[70521]]

Its a well known firing range where they either fire little guns or big guns into the bay at buoys. Its gunnery training 99% of the time and well documented so no need for any secret squirrel conspiracy theories. The fun stuff is usually Cardigan Bay or Scotland as they aren’t as busy. The Navy can disrupt the public but they usually choose not to where possible.

So you don't know

A fun, but old story, of a 'mistake' by one of two rather special torpedoes some 40 years ago off the west coast where instead of both racing towards each other, then with 100 metres separation turning to starboard, sinking to the bottom to be recovered one of them 'forgot or ignored' the command. What followed was a very big explosion, yes they were live, as one destroyed the other.

Lots of red faces and the occasional, 'oh, f&&k how am I going to explain that to the First Sea Lord'.