GPS Failures due soon?

[Channel Ribs]

I just read on the YBW site that the US Airforce forgot to send up replacement satellites, anyone heard similar?

[chris123]

There is a bit more info here:

http://www.pocketgpsworld.com/GPS-Co...ouble-6997.php

Cheers

Chris

[Steve_B]

Cant comment on whether the US forgot to send up replacements, but certainly NO threat to GPS.

As the vast majority of both civil and military flights are using this as there primary navigation tool now, I cannot see this being allowed to happen. Would make for very interesting times trying to keep them apart! Mind you it would make the job more interesting whilst the traffic has dropped so much .........

[PeterM]

This story seems to pop up every few years - but as 250 says as the majority of military & civial flights ( let alone marine & land based users ) rely on this I dont think it will ever happen. The global impact of GPS dropping away is just not viable. The military will see to that - even then if we go back to the days of selective availablity & deliberate errors on the system to stop the civil users there are ways around it now.

[SeaSkills]

Probably not, but if you listen to some stories it's all about to come to an end. If you fancy a longish and not particularly interesting read, this is the study that has the detail http://www.gao.gov/new.items/d09325.pdf

If you want a more readable version - try this http://news.sky.com/skynews/Home/Tec...y_Office_Warns

[Bigmuz7]

Isn't navigation by traditional methods still a critical part of Navy/forces training ?

[m chappelow]

it should not make any difference to us seafarers , we can all fall back on to the basics paper charts and compasses ,dividers ,carn,t we .......lol.

[chewy]

Some of us can Mart.

[havener]

Originally Posted by 250kts

Cant comment on whether the US forgot to send up replacements, but certainly NO threat to GPS.

As the vast majority of both civil and military flights are using this as there primary navigation tool now, I cannot see this being allowed to happen. Would make for very interesting times trying to keep them apart! Mind you it would make the job more interesting whilst the traffic has dropped so much .........

As an aside, whilst GPS may play a part in aircraft navigation systems, it doesn't have role in keeping them apart - this is done by ATC using primary and secondary radars, Mode S and various other little gizmos. It doesn't rely at all on the aircraft being able to tell ATC where it is, ATC already knows that, especially as GPS is two dimensional.

[Poly]

Originally Posted by havener

especially as GPS is two dimensional.

are you sure? most gps will give altitude readings - and I think they all need to take it into account to accurately calculate long/lat.

[Bigmuz7]

Originally Posted by Polwart

are you sure? most gps will give altitude readings - and I think they all need to take it into account to accurately calculate long/lat.

But that segment doesnt seem to be accurate,. my last 2 boats' systems showed an unlikely figure when I was hauled up on the hard well above sea level (200ft or so) , could ofcourse just have been my gear but I suppose a hill climber could advise us on that one (If any exist on a RIB site )

[Poly]

Originally Posted by Bigmuz7

But that segment doesnt seem to be accurate,. my last 2 boats' systems showed an unlikely figure when I was hauled up on the hard well above sea level (200ft or so) , could ofcourse just have been my gear but I suppose a hill climber could advise us on that one

I would say its reasonably accurate in the "vertical" direction. All GPS signals are of course subject to error - I think the "vertical" error is supposed to be a bit bigger than the "horizontal" error - but still fairly accurate. Of course it does depend on visibility of the sky - and its generally more likely to be obscured on land than at sea.

{Edit see here for some techy info: http://gpsinformation.net/main/altitude.htm }

[SeaSkills]

The problem with altitude is that it works to an average around the world. The GPS model makes assumptions about the shape of the earth and WGS84 has defined that shape to be an ellipsoid, with a major and minor axis Those dimensions are in reality only an approximation to the real shape. Ideally, the GPS world-shape would correspond precisely to "sealevel" everywhere in the world but there are very few places where the WGS84 ellipsoid definition coincides with sealevel. The discrepancy is averaged out to be zero, but can be as large as 300ft in some isolated locations.

[Bigmuz7]

Originally Posted by SeaSkills

The problem with altitude is that it works to an average around the world. The GPS model makes assumptions about the shape of the earth and WGS84 has defined that shape to be an ellipsoid, with a major and minor axis Those dimensions are in reality only an approximation to the real shape. Ideally, the GPS world-shape would correspond precisely to "sealevel" everywhere in the world but there are very few places where the WGS84 ellipsoid definition coincides with sealevel. The discrepancy is averaged out to be zero, but can be as large as 300ft in some isolated locations.

That makes sense Ian .. I was further north when I observed this .. so begs the question,.. given a spherical planet shape, how do pilots flying over the poles cope with this .. or is it the old story that the military system is more accurate than the stuff we get access to ?

[PeterM]

To clarify a little on aircraft - above a certain altitude ( which I forget right now as I dont often get that high on one engine !) you fly at a given ' flight level' based on a pressure setting of 1013, there is also a prescribed rule dictating heading & level to ensure some degree of automatic seperation if all the transponders packup ! Beneath this & assuming you are talking to someone ( ie an airfiled) on the radio or even slightly sensible you take the local pressure setting ( before you take off , or as you move around the world ) - either QNH or QFE - one being height above a a certain place ( usually an airfield) or sea level, pressure is updated regularly to account for the weather ) . This will hopefully allow all aircraft to use the same pressure setting - so will all see the same altitude shown on the altimeter , and this also ensure they dont fly into 'tall things' -mountains,spinaker tower, buildings etc by accident as they will know the minimun safe altitude to fly at - this is shown on thier chart anyway - abit like water depth you see on your boating charts.

GPS as has been said is very good & is used , but backed up by trusted methods of navigation - in my case a line on chart & good bit of compass dead reckoning ............OH and you can always just follow the motorway network in the UK.

[MikeL]

I am a hill walker!

Well a climber anyway

I don't know why but the error on the vertical seem to be really bad. I use a garmin hand-held and I often get accuracy down to 5 or 10 meters. I have watched the altitude reading change by over 100 meters while I was walking along on the flat and level!

Mike

[PeterM]

The big problem is that everything you base altitude on changes - the earth , sea level , pressure - its always a comparison to a variable - thats why they were still arguing over the height of Everest till very recently - where do you measure from ?

[SeaSkills]

Originally Posted by Bigmuz7

how do pilots flying over the poles cope with this .. ?

I don't know, so I spoke to a pal who's a pilot, and his answer was that they don't rely too much on altitude readings unless it becomes a sensitive issue - ie they're getting near the ground! On airfield approach, they are guided by ground radar, and a sonar gadget which seems to do pretty much the same as a depth sounder on a boat. Over mountains they combine eyeball, altimeter, and GPS, and if vis is bad they increase their safety margin.

I have to say that he's wound me up about stuff before, but this sounds as though he may be telling the truth for once

[DGR]

The military wouldn't let the system go down, simply because most of its weapons can't use a compass and chart!! Most long-range air to ground weapons use Inertial Navigation (IN) updated by GPS to increase accuracy - but IN on it's own isn't good enough to get them to where they need to be.

The accuracy (in 2 dimensions) on my GPS is often within 3 metres, nearly always within 6 metres - and the military signal is more accurate still.

The altitude accuracy on a GPS depends on how many satellites you have locked in - 12 will give quite good results, 3 or 4 will only give you a 2D fix - some units will tell you that on a status bar.

I've also been told (but don't know if it's true) that WAAS only corrects/adjusts the 2D position, and not altitude.

D...

[jyasaki]

Originally Posted by Bigmuz7

or is it the old story that the military system is more accurate than the stuff we get access to ?

Used to be that consumer GPS was downgraded in performance; that, I believe, went away several years ago.

jky