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its olny as acurate as the info put in from both the mapping system & the user, for example a Polish lorry recently drove to east Grimstead instead fo east Grinstead! one letter difference but about 175 miles apart! no doubt there are thousands more with simular spelling easy mistake but very costly!
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... and don't disengage your brain when entering a post-code. A driver once stopped me in Overstrand (NE Norfolk) and asked me where he was. It turned out that he wanted to be in Fakenham - about 25 miles away. Again, you only have to be one digit/letter out.
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I've had a couple of different PDAs with separate receivers, another 2 with built in receivers, Mrs M's TomTom Europe V3, and a handheld (Garmin GPSMAP75CSx). I think the newer chipsets are more accurate, or at least consistent - the Garmin will find the same spot within a yard or two.
Garmin say their devices are accurate to 10 metres 95% of the time, or 3 metres with WAAS/EGNOS enabled (not sure if this is standard on newer devices - I've had mine a while and it's switchable). Mine is fine with the EGNOS which I always leave off (it uses more battery)
www8.garmin.com/aboutGPS/waas.html
The inaccuracies you tend to notice as AE says are of the map rather than the device; or the map resolution. When you zoom in you can see that roads are just dots joined up with straight lines. When navigating via roads, sat-navs "snap to" the road to avoid the apparent oddity of driving through the field on your left or right. This can be mildly annoying when you're on an unmapped bit if it snaps to nearby roads and gives you duff instructions, but invariably you know when it's doing it.
House numbers on the TomTom must be interpolated - it can give the impression that it knows where every house is but it doesn't really, if the houses are all down one end rather than evenly distributed then it can be half a mile out.
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I find my TomTom substantially accurate but, whatever direction I am driving in (N/S/E/W) there is a lag or delay of about 20 yards. I pass under a motorway flyover, in the real world, and TT has me going under it just that bit later. Otherwise no real complaints!
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There's your answer then. 20 meters sounds good.
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except mine doesent lag like that.
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Nor mine come to think of it - and its an elderly unit.
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You know things like this bring me up short. Here we are discussing how accurate Sat Navs are.
My Dad passed away in 1981. It sometimes occurs to me that if I could speak to him now and show him that I have a thing in my car which knows how to take me to pretty much anywhere without having to consult a map. That I have a pocket phone which works almost anywhere in the world. That my house has several computers in it. That I can write instant letters to almost anyone in the world. That I can send photographs in a moment to the same people and that petrol is nearly £5 a gallon......
That's in less than 30 years for goodness sake.
What next?
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"Intelloshoe" with built in sat-nav !
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Your father (and mine) would not have been in the least bit suprised. It was all on Star Treck (the orginal) . The only two things we have to come are transporters and Warp drive. Both are now belived to be possible.
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"I could speak to him now and show him that I have a thing in my car which knows how to take me to pretty much anywhere without having to consult a map"
My Dad had a GPS unit in his ship (HMS Endurance) in 1979. They found out that the Falkland Islands were a bit out - about 70 yds in one axis I think, but not bad when you consider that the charts had been done using sun-shots. GPS was first deployed in 1978 by the USN, I think, and the Royal Navy was loaned some early in 1979.
I first used SMS-type messages in 1981 on HF single side band. We had to replace hand speed morse in my regiment as the Russians had developed direction finding kit that could locate a transmission in less than 2 seconds. The kit we had squirted 160 digits 0-9 in 0.7 of second. I was quite amused when I first used SMS about fifteen years later to see that it too had a 160 character limit.
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I believe the first Royal Navy ships fitted with GPS were the four Polaris submarines built in th late 60s. GPS was developed as part of the US Navys Polaris weapon system. The military system is more accurate than open access system which has "dither" added to reduce accuracy, can you imagine the problems if joe public had satnav accurate enough to drive through their front door?
Edited by Old Navy on 14/12/2008 at 19:34
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I had a thing about Lt Uhura.
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Ah right. She had a good sat nav. Inter-galactical mapping. Speed camera alerts must've driven her mad though....
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Commercial surveying equipment, using differential GPS, is accurate to a few millimetres.
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I read somewhere that there was a deliberate error built in to the overall service. This had something to do with the technology being originally for defence purposes and not wanting it to fall into the wrong hands. Maybe more likely though that they can sell more accurate servces (such as surveying one cited above) at a higher charge.
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As I understand the accuracy of GPS, without any whistles and bells, it will get you to an area about the size of a tennis court as your location.
However, the car based satnavs presume you are on a road (it's what cars are expected to do, for the most part) and do their utmost to put you on a road it knows about, and also assume you will be progressing along that road.
It can get a bit interesting when they loose sight of sufficent satellites for an accurate fix, decide you are on another road, then direct you off the correct road for a tour of some mystery location before directing back onto the road you should have stayed on all the time.
Then there's the separate issue of map errors.
In answer to the post above:
A few years ago there was a deliberate error built into the system for civilian use, but that was switched off some years ago. Differential GPS would have been accurate then.
Edited by Another John H on 14/12/2008 at 19:54
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If the question was how accurate are the civilian channels on the GPS satellites (without help from Internet connections and other ground base signals) then I'd say 5-10m is what you're meant to get at best. I think some GPS receivers tracking reflected GPS signals and especially if using other signals like WAAS (Wide Area Augmentation System - USA only)
The thing to remember about GPS use for navigation in a car, because the software assumes you have to be on a road (fair assumption) then it will snap to the nearest road. Any one using a sat nav system on the M6 toll road without modern maps might get amused. For use for ordnance survey maps this "snapping" does not occur so you can spot inaccuracies at times more easily. I use both MemoryMap (PDA) and ViewRanger (Nokia smartphone).
BTW I think the first test GPS satellites were launched in the early 70's.
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2I believe the first Royal Navy ships fitted with GPS were the four Polaris submarines built in th late 60s. "
The Polaris boats used INS (inertial navigation, which computes the accelerations the vessel is subjected to) - GPS would then have been a) not developed and b) not very easy to receive that deep under water.
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The GPS was used to calibrate and reset the INS, a specific telescopic mast similar to a periscope was used to recieve the gps signal. You cannot hit a target with a missile if you dont know the launch point, try throwing a stone at a target after wearing a blindfold for a week.
Edited by Old Navy on 14/12/2008 at 20:37
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.....and the Vulcans that flew to Stanley used off the shelf INS "donated" by British Airways in the grand tradition of make do and mend of the British military.
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oldnotbold, I would add that GPS is unlikely to be received at depth now. The signal strength can be stopped by some car windscreens! And the deflection/effect of the water would render it pretty useless. They will use something else that is calibrated via GPS - my guess anyway.
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....yes Trident missiles, that's one of the main reasons the Sat net was built. "Turn left at Lenin Avenue"
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Thats cruise missiles, they use the "good" GPS to fly through windows, Trident goes up and comes straight down from space and ruins your whole day.
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Original cruise missiles as used in the first Gulf War did not use GPS. They were programmed with terrain info and had computers to compare images. They literally flew down streets, recognised an image and turned.
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I don't use car sat-nav but do have a nautical handheld. It happened to be on during our trips to the West coast of Scotland this year..... it will show a track over a grey background even if it is outside the nautical chart area. I was amazed after two weeks away it correctly laid a plot back down the other side of the dual carriageway we had travelled in the other direction on the way up. Just looking at the scale on the screen the tracks were the correct distance apart give or take a metre or so.
David
Edited by M.M on 14/12/2008 at 20:47
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>>it correctly laid a plot back down the other side of the dual carriageway we had travelled on the way up. Just looking at the scale on the screen the tracks were the correct distance apart give or take a metre or so.<<
That was what was behind my original question. Having just read tody about another fatal collision involving a sat nav user blindly following its instructions but driving on the wrong side of the road I was wondering whether they could tell which side of the road they are on and therefore they could warn the driver if they were travelling on the wrong side.
I wonder how much it would add to the cost to have this safety feature as standard.
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I wonder how much it would add to the cost to have this safety feature as standard.
Yes its possible where the split of the road is wide enough.
My sat nav has a warning by country feature. It knows what country you are in when powered up and can bleep a warning at you saying "drive on the left" (or right)
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Really? Do you mean it recognises that you're abroad and gives you a general warning when you turn the key or does it detect if you have strayed over the central line?
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Really? Do you mean it recognises that you're abroad and gives you a general warning when you turn the key
Yes.
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Just to add to my impression of accuracy.....
When we were boating on holiday the gps would lay a track on screen that we could follow back if needed say when fog descended... not sure if car sat nav users know of this type of facility.
Once we'd beached and secured the boat at the end of each trip we'd walk up the lawn to the holiday cottage and I'd only get the gps out of my lifejacket pocket to turn off when at the front room table.
If we went out several times a day leaving the screen track to overlay each trip the line as it showed us entering the cottage front door was exactly the same place each time. This again made me think it must be tracking to a metre or so.
David
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b) not very easy to receive that deep under water.
Indeed and still a problem - modern subs still dont use GPS (they will raise a reciever bouy or use the multiband esm when near the surface for a "fix" now and again but thats risky) but mainly navigation is INS, charts, accurate clocks, and a skilled navigator.
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A chart and clock "dead reckoning" is not accurate enough to navigate under water for more than an hour or so in open water, no use for a fire control solution. The "reciever buoy" as you call it is only used for recieving communications and is rarely used as there are other systems.
Edited by Old Navy on 14/12/2008 at 20:53
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The UK's Polaris-carrying boats were launched in about 1968? That would be about 10 years before GPS but no doubt they used it later. Unless my dates are wrong the Polaris boats' INS would have to be calibrated using some other method - possibly the US Navy system called Transit which t'internet says became operational in 1964. Five satellites were needed for global coverage, but a fix could only be obtained when a satellite was above the horizon, which could mean several hours between fixes. Alternatively, or as well, they could have used one of the hyperbolic radio navigation systems (Decca type).
Edited by Manatee on 14/12/2008 at 21:11
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They used all of the above. INS did not require frequent updates.
Edited by Old Navy on 14/12/2008 at 21:15
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Sorry Manatee, you missed one.
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No, in the days before GPS they used a base port optical INS calibration system. Thanks mods I will shut up now.
Edited by Old Navy on 14/12/2008 at 22:01
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Well just one last post, It always amazes me that the cutting edge technology of the 60s/70s which consisted of a mechanical INS and electronics that would fill 40ft shipping container has been shrunk to fit in the palm of my hand.
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But it has not been shrunk. It is different. Try taking your handheld GPS unit into a tunnel and then plot a route.... okay your electronics example size will be shrunk but the huge computers of the 60s are less powerful than my mobile phone.
All should remember an inbuilt car sat nav unit also has inertia (gyroscope) sensors for when there is no signal. (The TomTom 900 had this).
Edited by rtj70 on 14/12/2008 at 23:47
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If you take your satnav to the prime meridian which runs through Greenwich you will find that zero degrees of longitude on your satnav is about 102 metres East of the prime meridian marked on the ground.
This is because the satnav uses a different baseline, the International Reference Meridian.
Not a lot of people know that!
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I didnt know that satnavs had gyros (at least 3 required) to operate as an INS. Doesnt the software just predict your position on its map when it loses signal? I believe modern INS systems use solid state ring laser gyros.
Edited by Dynamic Dave on 15/12/2008 at 10:22
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The ones in cars do - they also have a sensor to detect reverse gear. But most units like TomTom, Navman, etc do not. but a few of these have.
Once my TomTom 720 loses a signal, it will update the map for a few seconds and then stop working until the signal is acquired again. So if you went into a tunnel and had to take an exit at the other end it might not tell you quick enough.
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I didnt know that satnavs had gyros (at least 3 required) to operate as an INS. Doesnt the software just predict your position on its map when it loses signal? I believe modern INS systems use solid state ring laser gyros.
The Honda (Alpine) one in the CRV definitely has some sort of gyro/accelerometer/inertia system. It can be programmed to show a breadcrumb trail when off piste, and it tracks my twists and turns in our underground car park which has a five storey building on top. It also means it knows which way the car's pointing when it's stationary.
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The VDO 5000 range had a pickup from the speedo sensor, reverse switch input, gyro/accelerometer/inertia as well as GPS reciever.
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> That's in less than 30 years for goodness sake.
Sorry to digress, Patrick Tilley's "Fade Out" is a classic sci fi novel dealing with this very issue. Dated now, but the principle the same. Back to GPS ..
Isn't the accuracy of many "spoofed" in that it knows it's approximate position (to a few yards) but makes the assumption that it's a car device and therefore must be on the nearest road? I have a Garmin Colorado which runs OS maps as well and in the menu you can switch the "car" function on or off.
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>Isn't the accuracy of many "spoofed" in that it knows it's approximate position (to a few yards) but makes the assumption that it's a car device and therefore must be on the nearest road?
Yes its called the "snap to" function as described further up. It is a perfectly valid thing to do for a device that's designed for in car road map use.
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