rapiddescent

The german press are leaping up and down because the US Military (that owns the GPS system) will be switching on Selective Availability on thursday morning (anticipated time of attack). The SA is indeed region based but the press are saying that the USA will be switching it on for europe - no doubt a great big thankyou for all their support lately.

This means that any civilian GPS receiver will become inaccurate to whatever the US military choose (typically 100m).

You can tell if SA has been switched on by looking at your receiver whilst stationary. SA will show a random movement of 1-4mph whilst the receiver is stationary.

GPS based speed trap announcers will be less effective whilst SA is on.

god help you if you are trapped upside down in your scoob and the GPS equipped fire engine is on the wrong road.

rd

Cupraman

Really noticed this when I had the Leon and Satnav during the Afganistan war. It is VERY notcible how inacurate it becomes.

speedking

If you have your laser jammer on during "the conflict" could you become a legitimate target Visions of fast cars being taken out by even faster jets

rapiddescent

theres an urban myth around about a traffic cop on bodmin moor (A38) who pointed his radar gun at a passing Harrier "to see how fast it was going" - which promptly set off all the SAM (surface to air) alerts in the cockpit scaring the living daylights out of the pilot.

The pilot reportedly did a big swoop round and went VERY low over the cop car which resulted in an RAF inquiry...

rd

Puff The Magic Wagon!

iTrader: (2)

Well, people had better get the EuroSat version when it goes live in a few months

Pete Croney

My GPS is still working very accurately. It knew I was on a slip road and not the main carriageway in my way in this morning.

I suspect that the civil GPS is being jammed locally in Iraq. The signal is so weak, that this is very easy to do.

TopBanana

I was under the impression that they can switch this on regionally

Merc_Cosworth

According to my mate in the RAF, that Harrier incident did occur...

The Harrier was wanging around, minding his own business when all of a sudden his warning panel lights up like the proverbial festive tree - specifically on laser alert.

Assuming a potential SAM threat he is then authorised to take appropriate action - i.e. find and destroy whatever is trying to lock onto him.

The aggressor in this case was the copper

The Harrier follows procedure and hits the deck and, being only armed with his 50mm ADEN cannon, flys straight at the Laser source.

Copper, duly pointing at the Harrier, reads 348mph before it all goes Pfffffzzzzttt.

The Police tried to bill the RAF for £8000 for a new laser gun.

The RAF replied by pointing out that had the gun not gone Pfffzzzttt at that point, then the police would have been invoicing them for a new patrol car, radar gun, and policeman.....







[Edited by Merc_Cosworth - 3/21/2003 1:46:22 PM]

GU5

Does anyone know for sure that this is going to be switched off?

Adrian F

As far as i can remember GPS was paid for by the American military and they let people have free use of it!

I certainly don't pay for using it and i think it is ungrateful to complain about it being altered by the people who own it during time of a war if you are not happy go by a different system!

TopBanana

Authorised to destroy a laser source.. Yeah right! I smell b.s.!

john_s

iTrader: (1)

That story has been knocking about since before officers of the taxation law began to use lasers... used to be radar when the story first started kicking about.

[Extra_Pedantic_Bit]A harrier doesn't have a 50mm Aden cannon, but they do sometimes carry a pair of 30mm cannons. :P [/Extra_Pedantic_Bit]

John.

[Edited by john_s - 3/22/2003 8:23:50 PM]

carl

On the contrary -- because the signal is below the background noise it's quite difficult to jam. Noise and jamming are exactly that -- broadband noise. The pseudorandom code is ordered, so you know what you're looking for in the noise.

Simple anti-jamming -- cut off everything above the noise floor, then try to match up the pseudorandom code that the receiver knows with the pseudorandom variations in the noisy signal received. Once you're receiving a signal that goes up and down in the way you expect, you're locked on

In reality, they will have turned the SA back on. Still should be sufficient for civilian use -- it was possible to work out which side of the carriageway you were on on my mate's GPS in the old SA days.

In reality, the reason behind using SA to degrade the C/A code is to stop 'bad guys' with ordinary receivers using the SPS service using them to precision target missiles, etc. The US military is a an authorized PPS (Precise Positioning Service) user and its GPS untis have then necessary encryption/decryption keys to use the encrypted P-code (sometimes called Y-code) service which offers far greater accuracy. One of the reasons (aside from the longer pseudorandom code length) is that P(Y)-code is broadcast on the L1 and L2 frequencies (C/A is only broadcast on L1), and these two frequencies propagate differently through the ionosphere. Measuring the delay between the L1 and L2 P-code provides a better navigation solution than just using the one frequency, where you have to model the ionosphere.

All this fascinating stuff about how 'proper' GPS () works can be found at http://www.colorado.edu/geography/gc...gps/gps_f.html

[Edited by carl - 3/25/2003 3:08:29 PM]

Toerag

Was out in my boat at the weekend and my GPS worked fine, ie. same as usual, 16ft error normally.MY plotter draws a trace of where the boat has been, and moored on a pontoon in the marina for an hour or so it didn't move more than it's usual 20ft

boomer

Merc_Cosworth,

your mate seems to have embellished the original urban legend rather a lot

See http://www.snopes.com/horrors/techno/radar.asp

mb

Midmotorsteve

came back from sailing round the BVI last month, GPS sytem on the Yacht was approx 20metres out, in a boat with 8foot draw and shallow water quite dangerous. Enquired about it, told since sept 11, GPS has randomly been out in varying directions by this amount as it means it makes it not quite as easy to walk in a building with a handheld gps, store location, walk out program plane /missle with co-ordinates and leave be to deliver a precise hit.

GPS is run by the americans, they don't want people using it against them, fair enough i think. There are always charts for boats and eyes for speed camras.

Toerag

Charts are no use when you're trying to find a 200ft long wreck 12miles offshore. Unless you happen to go straight over the top of it and you pick it up on the fishfinder you'll never find it.