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Old 22 May 2009, 11:34   #21
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Civilian use is still limited to the Coarse Aquisition signal, even though Selective Availability was switched off a long time ago (Differential GPS made it redundant, anyway). The military do still have a greater degree of accuracy, but the signal I get is more than good enough for me

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Old 22 May 2009, 13:32   #22
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Originally Posted by DGR View Post
The military wouldn't let the system go down
If I was being a cynic .. .. I might be thinking the Yanks are putting the frighteners out so they can start to make some sort of charge for its use .. quite how that would be done (the charge) I'm not sure, but I know thats the reason Europe has been talking about launching their own satellites to have their own system (Galileo ) so the Yanks cant hold us to ransom ... presumably the Euro goons then would

Heres a quote from this link

"GPS was initiated by the US military; and although it is highly unlikely its signals would be degraded or switched off in Europe, the Americans as owners have ultimate control. Yes, the service is free, but its continuity and quality come with no guarantees - which means it cannot be relied upon, certainly not for safety-of-life applications such as landing planes and controlling trains (not without an augmentation service). "

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Old 22 May 2009, 15:18   #23
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Just to clarify my comment and on Havener's reply.

GPS is one of two, prime methods of long range navigation used for both Civil and Military flights (the other being INS, Inertial Navigation System). Aircraft do not use GPS for maintaining their vertical position, although that information is available as a cross check, they use very acurate altimeters. So much so, that we have in the last 5 years, reduced the vertical separation of aircraft above FL290 (29000ft) from 2000' to 1000', therefore at a stroke doubling our airspace capacity. (Below 300', approaching to land, Radio Altimeters are used. Very much like sonar).

However, the vast majority of the routes flown by aircraft (airways) are now marked by RNAV (GPS) fixes and not the old fashioned ground radio beacons. Therefore other than providing vertical separation, controllers can provide lateral separation knowing that if an aircraft is on the centre line of one route, it is separated from another aircraft on another route. I would estimate that over 60% of our routes and fixes are now specifically related to GPS, such that any failure of the system would bring chaos to aviation and would not be allowed to happen. To that end, Europe is well on track with its own independent GPS system.

We do still use radar for separation, particularly in the climb and descent phase, obviously always around our airports.

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Old 23 May 2009, 05:58   #24
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A bit more information - highlights of the original report (with link to the full report):

And a rebuttal from the US Air Force:



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