GPS

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binch

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Don't want to worry yer but. . .
brush up the astro
Half current GPs satellites are operating in single point failure mode which means that one more thing down and the sat shuts down. The US Govt Accountability Office says that many sats have now reached the point in their life when things can start going wrong.
Brad Parkinson, the father of GPS says that if the constellation drops below 24 (it is now 31 and falling) there will be brown-out. Possible by 2018 unless someone out-fingers.
Now there's good news for you
Anyone want to buy a sextant?
 
I'm not the tiniest bit worried. It's not going to happen. Too many people, countries, governments, organisations with too much invested in it. Until the European, and before long after that the Chinese, system is up and running - the world needs Navstar and it will not be allowed to fail.

IMHO
 
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Brad Parkinson, the father of GPS says that if the constellation drops below 24 (it is now 31 and falling) there will be brown-out. Possible by 2018 unless someone out-fingers

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I'm hoping we get brown-out in June 2010, if not before /forums/images/graemlins/smile.gif
 
The US military will not accept a situation where the USA no longer controls satellite navigation.

One of the original ideas for the European system was that they would be European GPS satellites, but the US did not like the fact that that would result in GPS carrying on working without their permission.

What happened instead was that the European Galileo system is designed to operate on the same frequency but with different transmissions that were designed not to interfere with GPS.
In general, current GPS recievers cannot see Galileo.

So hoping for the EU system to rescue all our GPS recievers we have now is not going to be an option.

We will be in the same situation as with Decca, a lot of expensive kit that can not be upgraded cheaply to use Galileo.

And then because the EU wants to make a business of it to pay for the satellites we will probably be expected to cough up for license keys to get the kind of accuracy we have come to expect from 'free' DGPS.
 
I heard an interview on the radio with someone from the US department responsible for the satalites. She was saying that though it is possible that we may be down to 18 satalites this will mean that the accuracy of the system will suffer. That is very bad news for car satnavs that rely on accuracy to within a few meters but sea-going ones that only need accuracy to a couple of hundred meters (by then you'r on pilotage and Mk1 eyball takes over) will continue to function, albeit with less accuracy than at present.
 
Looking on the bright side, this may clobber government plans to introduce "pay-as-you-drive" systems for car tax based on sat-nav with differential costs depending on which type of road you are on. These would require a reliable system capable of accuracy within 3 metres in all three dimensions! They would also (based on information released) be much more expensive than the combination of taxes we pay just now.
 
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I heard an interview on the radio with someone from the US department responsible for the satalites. She was saying that though it is possible that we may be down to 18 satalites this will mean that the accuracy of the system will suffer. That is very bad news for car satnavs that rely on accuracy to within a few meters but sea-going ones that only need accuracy to a couple of hundred meters (by then you'r on pilotage and Mk1 eyball takes over) will continue to function, albeit with less accuracy than at present.

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In fact car mapping uses a trick to give impression of great accuracy - it is programmed to jump to nearest road / track. You can see this when you are on one road and pass over / under or run alongside another ... motorways are notorious for this - where your car icon jumps to wrong road etc.
There is no magic accuracy greater than you have for marine - its the above trick that appears to be.
 
SWMBOs has shown us in the middle of a field when we've been in the midle of a field.

I'm not saying car ones are more accurate, just that they can't be of any use with the margin of error that marine ones can.

Anyway, how would the Yanks be able to stand back and chuck lead at the wrong location with any accuracy if they let GPS fail?
 
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SWMBOs has shown us in the middle of a field when we've been in the midle of a field.

I'm not saying car ones are more accurate, just that they can't be of any use with the margin of error that marine ones can.

Anyway, how would the Yanks be able to stand back and chuck lead at the wrong location with any accuracy if they let GPS fail?

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My point is that there is no greater accuracy in a car GPS than a marine one - what makes you think there is ?
 
That's what I'm saying; they aren't more accurate. At sea I can live with an accuracy to within a couple of hundred yards. That kind of accuracy renders a car one useless. At sea I can see that distance, am at single figure speeds and have other means of checking like depth sounder, compass etc. Think how many roads and other features there can be in that distance in a town and going at 30mph.

As satalites drop out, the accuracy of the fix will lessen but at sea we can live with that loss.
 
Some of the car ones do indeed make cunning assumptions that you will be driving on a road. Most GPS's employ some averaging in various ways, to improve the indicated accuracy.
They assume that if you have been walking/sailing at a few knots, you will not suddenly skip 10m to one side, then instantaneously 20m back. Quite a lot of subtle sums being done! It can all come undone if you break the rules. When I switch my tomtom on at home, it will put me either in my own road or the adjacent one, never in between.
Military GPS also is fundamentally more accurate due to using the L2 signal to help correct for atmospheric effects (among other things) and access to another layer of coded info.
 
The GPS location in a car Sat-Nav has EXACTLY the same precision as a marine one. However, the car sat-nav uses optimizations based on things like "I've got to be on a road", "I am am probably proceeding at a predictable speed along a road, so I can use clever filtering to improve the computed position (i.e. Kalman filtering)." These things mean that that a car sat-nav, by making a few reasonable assumptions, can provide a more accurate position.
 
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The GPS location in a car Sat-Nav has EXACTLY the same precision as a marine one. However, the car sat-nav uses optimizations based on things like "I've got to be on a road", "I am am probably proceeding at a predictable speed along a road, so I can use clever filtering to improve the computed position (i.e. Kalman filtering)." These things mean that that a car sat-nav, by making a few reasonable assumptions, can provide a more accurate position.

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I think I would like to insert the words 'apparently more' in front of accurate position - as the GPS itself is not more accurate and it's software tricking the position. But thanks for post.
 
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Anyway, how would the Yanks be able to stand back and chuck lead at the wrong location with any accuracy if they let GPS fail?

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This is the salient point surely, I think if the accuracy of the base signal was to degrade markedly the effect on the US military who let's face it hardly have a reputation for brilliant geographic/ navigational ability would soon get the satellites fixed/ replaced. I'm a bit suspicious of the present scaremongering: too many people with their own angle to promote.
 
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The Garmin 17X NMEA 2000 GPS Antenna is supposed to be accurate to within 3 metres*

*but only in WAAS areas (Currently North America) Nevermind eh? /forums/images/graemlins/wink.gif

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Think you'll find that near all todays GPS of whatever manufacturer or useage designed will quote same. It's not a Garmin only thing !

Nevermind eh !! /forums/images/graemlins/wink.gif
 
Charlie chartplotter (Navionics Silver) put us firmly in Gavin Maxwell's* derelict basking-shark-processing shed when we were anchored at Soay, more or less in the middle of the available anchorage.

*Wikipedia picture of basking-shark-processing factory

Switching to crusty old salt mode, I proceeded to extol the virtues of good old paper charts. Alas, my smart alec crew plotted the GPS position on the Admiralty chart (which uses the OSGB 1936 datum, and yes, Smart Alec was using the correct datum). There we were, right on the jetty.

The Admiralty chart was of course compiled long before satellite position fixing was even dreamed of. It shouldn't be a surprise that the Navionics Silver chart showed more or less exactly the same error - it was presumably compiled using the same UKHO data.
 
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