GPS accuracy

fisherman

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Last Friday several of us had problems with GPS positions, just sorted out today that the gear I shot was off by 0.31 nm and 345 deg. That's enough to get you in trouble. I wonder sometimes if the yanks might not introduce an error for their own military purposes.
 

jwilson

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Last Friday several of us had problems with GPS positions, just sorted out today that the gear I shot was off by 0.31 nm and 345 deg. That's enough to get you in trouble. I wonder sometimes if the yanks might not introduce an error for their own military purposes.

I've had total no-signal on three separate GPSs several times in the same area of the China Sea, but generally modern GPS is very very accurate, though many charts may not be. If you set a waypoint on the spot on your own GPS charting errors are not the issue. It's 15 years since Clinton ordered "selective availability" to be turned off although I'm sure there are still occasional and local tweaks/jamming for military purposes. Mostly my two current GPS antennas about 6 ft apart usually show 0.001 nautical mile differences - ie 6 ft.
 

Yngmar

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In addition to stuff just breaking (satellites and uplinks can have problems too, not just receivers), GPS jamming may also be part of naval exercises, and it is part of stealing cars and trucks now. I wouldn't be surprised if it was also used in pirate infested waters. All of these work by overwhelming the relatively weak satellite signals with much stronger local broadcasts that send falsified position data. Very useful in electronic warfare and in telling a stolen vehicle that it hasn't left the parking lot while in reality it is halfway to the chop shop. I think someone even stole a drone that way (too lazy to Google myself).

This and mutual distrust is why there is more than one global navigation satellite system (GNSS) now. The Russians built one called GLONASS, the Chinese BeiDou and the EU Galileo (because none of these trusts the other not to muck with theirs). Some of them aren't quite complete yet, but work for certain regions. For us civilians that means we can get multi-network location receivers now (e.g. http://www.navilock.de/produkte/G_62530/merkmale.html). Having one of those on your boat might be a good idea, so you can sanity check when your chart plotter shows you on the yellow bits of the chart. Of course if you're near the coast, take 3 bearings and draw a cocked hat on your paper chart for much less.
 
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lpdsn

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Last Friday several of us had problems with GPS positions, just sorted out today that the gear I shot was off by 0.31 nm and 345 deg. That's enough to get you in trouble. I wonder sometimes if the yanks might not introduce an error for their own military purposes.

I had quite a bit of random variation in SOG, which didn't tie up with the movement of the boat, in the vicinty of Portpatrick last weekend. Position seemed reasonably OK, but I was navigating by eye.

I believe they've been doing a bit of playing around with GPS in the vicinity of Luce Bay, so I assumed I was being affected by that.
 

fisherman

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My difficulty was identifying which bit of gear I'd hauled, just poor repeatability of my own recorded positions, plus thick fog. Didn't get sorted until a day of good vis. Had I lost any ends I would have been stymied and unable to find them with the creep. Of course in the old days we had witchcraft, runes..........and transits.
 

lw395

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My difficulty was identifying which bit of gear I'd hauled, just poor repeatability of my own recorded positions, plus thick fog. Didn't get sorted until a day of good vis. Had I lost any ends I would have been stymied and unable to find them with the creep. Of course in the old days we had witchcraft, runes..........and transits.

And Desmond, the Decca, which was repeatable in it's inaccuracy.
 

lw395

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I had quite a bit of random variation in SOG, which didn't tie up with the movement of the boat, in the vicinty of Portpatrick last weekend. Position seemed reasonably OK, but I was navigating by eye.

I believe they've been doing a bit of playing around with GPS in the vicinity of Luce Bay, so I assumed I was being affected by that.
Random SOG variation is usually a sign the signal is marginal, the receiver is losing lock between fixes.
Time to check the antenna.
 

lpdsn

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I think GPS is more accurate than charts - as I found going into Utklippan the night before last.

That's unfortunate, especially if you were just aiming for the Folly Inn for the night.

Is the toilet block there still pretty low tech (Utklippan, not Folly Inn)?
 

ip485

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In the avaition world there are some tools available to predict GPS outages.

If you really want to and are that concerned for example in Europe you can enter a route between two airports or waypoints.

Select the "right" waypoints or airports and you can establish if there are any predicted outages. Other sites give you the current health of the constellation.

I doubt any of us do it mind you and even in aviation where GPS approaches are now far more common place most accept the RAIM monitoring for the purposes of an approach or en route navigation assuming they even bother to go this far.

http://augur.ecacnav.com/augur/app/home
 

TonyBuckley

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I once lost all GPS in Gosport for three hours. Others did too. So assumed one of the Navy ships had pushed a button.

Also once completely lost it on the A34 between Newbury and the M3 - assumed that time it was the Army.
 

ip485

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NOTAMS to airmen publish GPS blocking trials so you could monitor the daily NOTAMS as well.

Is there a marine equivalent?
 
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My chart plotter regularly coincides with my depth sounder as I go over depth contours, I am amazed by its accuracy which can't be more than a few feet out.
 

fisherman

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On another tack, one day in the 90s off the Lizard in thick fog we saw a lot of what looked like insulation floating about in the air. At the same time the radar whited out. I called the CG who called C in C Plymouth who confirmed they had been playing with chaff and were roundly chastised for potentially causing a disaster. Of course they only did it on a foggy day to see what reaction they got.
 
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