New improved GPS (1cm accuracy?!)

[TangoWhiskey]

BBC News is reporting that Europe is planning to build their own satellite-navigation network that will be backward and forward compatible. There's going to be 5 levels ranging from free (1m accuracy) to commercial (1cm accuracy)! Provision is also being made for a search and rescue mode where a signal can be sent to confirm that help is on the way. The system will supposedly even work with existing US network after upgrades to the network.

http://news.bbc.co.uk/1/hi/sci/tech/4555276.stm

 

[Areeda]

The other real cool feature is guaranteed availability. Unlike GPS (they have another name that escapes me at the momemt) the military cannot degrade it for their own purposes. It will be world wide but not compatible with our existing GPS receivers, I believe.

Joe

 

[RotaryWingBob]

The article is incorrect about the accuracy of GPS -- the current GPS system is actually capable of cm accuracy -- it's our receivers which are not. The GPS units used by the military and by surveyors use two channels from each satellite instead of one, and self-correct for atmospherics much better that WAAS. They have not been made available to the public for fear that they would provide a nice way for enemies to guide missles...

 

[fgcason]

1cm? OMG?!!

Ok. I can see that for construction, some survey applications (volcano's for one) and to see how much Galloping Gurdie is moving before having to evacuate the place.

Navigation though? That's serious overkill in a big way. Without support I can just barely hold a receiver to those limits when standing up in a steady very light breeze if I'm really careful... But I should be able to get to a location and when it says "arrived", slide my key into the slot in the door knob without a funnel or additional guidance.

Not getting lost, or getting unlost after getting lost, I can see that. But 1cm for navigation, while it's technically possible, is in the realm of excessive overkill in the real world.

They have not been made available to the public for fear that they would provide a nice way for enemies to guide missles...

I often wonder about that. We're already in the close enough that it really doesn't make any difference category until they shut down or seriously degrade the system for security reasons. WAAS is good for what? 3 meters give or take a meter? Off the shelf $100-$150 GPS WAAS systems commonly have accuracy to 20ft and 30ft is almost a given. They may not be able to put a missile through a small bathroom window with it, but 1000lbs of high explosive traveling at 500mph with 3-10m accuracy is still going to be able to wipe out unhardened targets even considering stacked tolerance errors. Same goes for someone walking up and setting a suitcase down at a given location and walking away.

 

[silver-eagle]

Galileo, like the Airbus and the Ariane rocket, is just another way of the European Union asserting their independence and proving they are technically capable. While it is redundant (a promise to be GPS compatable), it will be run by a private concern not military in nature. If it is still true that the US Military can disable GPS, then it is a valid undertaking.
But wouldn't it be better to get an agreement from the US Military to leave GPS alone? Our main concern back when was the Russians. Seems to me the BIG STICK theory our Military has is better than futzing GPS.

 

[RotaryWingBob]

If it is still true that the US Military can disable GPS, then it is a valid undertaking. But wouldn't it be better to get an agreement from the US Military to leave GPS alone? Our main concern back when was the Russians. Seems to me the BIG STICK theory our Military has is better than futzing GPS.

What they did was called Selective Availibility (SA) which applied a random error to the fix data to make it less accurate. My recollection is that SA was turned off by Clinton via executive order. It can be turned back on at any time. SA, of course, does not affect military receiver accuracy.

My guess is that WAAS is immune to SA because the ground station would correct the errors introduced into the fix data.

 

[gismo]

1cm? OMG?!!

Ok. I can see that for construction, some survey applications (volcano's for one) and to see how much Galloping Gurdie is moving before having to evacuate the place.

Navigation though? That's serious overkill in a big way. Without support I can just barely hold a receiver to those limits when standing up in a steady very light breeze if I'm really careful... But I should be able to get to a location and when it says "arrived", slide my key into the slot in the door knob without a funnel or additional guidance.

Not getting lost, or getting unlost after getting lost, I can see that. But 1cm for navigation, while it's technically possible, is in the realm of excessive overkill in the real world.



I often wonder about that. We're already in the close enough that it really doesn't make any difference category until they shut down or seriously degrade the system for security reasons. WAAS is good for what? 3 meters give or take a meter? Off the shelf $100-$150 GPS WAAS systems commonly have accuracy to 20ft and 30ft is almost a given. They may not be able to put a missile through a small bathroom window with it, but 1000lbs of high explosive traveling at 500mph with 3-10m accuracy is still going to be able to wipe out unhardened targets even considering stacked tolerance errors. Same goes for someone walking up and setting a suitcase down at a given location and walking away.

Off the shelf GPS receivers are designed to disable themselves if the speed is greater than something like 1000 Kt or the altitude is real high (like > 60,000 MSL). While it's quite possible for someone to build their own receiver without these limitations, that would require significantly more knowledge and sophistication than ordering one from Garmin.

Also, as pointed out our GPS system is capable of cm accuracy, but only in the relative sense AFaIK. IOW a survey grade receiver can determine it's location relative to a reference receiver physically located at another surveyed point. This kind of measurement requires that the receiver be stationary for at least several seconds or even a few minutes, hardly something one would use for missile guidance. I suspect that the proposed European system wouldn't be capable of absolute accuracy in the cm range, at least not dynamically. I think that might require bending a few laws of physics.

 

[AirBaker]

What they did was called Selective Availibility (SA) which applied a random error to the fix data to make it less accurate. My recollection is that SA was turned off by Clinton via executive order. It can be turned back on at any time. SA, of course, does not affect military receiver accuracy.

My guess is that WAAS is immune to SA because the ground station would correct the errors introduced into the fix data.

I remember that being correct. Something having to do with being able to buy foreign-made systems that had better accuracy.

Surveyors use Differential GPS. Again a 2 system (recievers) that get around the error by holding one station fixed.

Don't hold your breath on getting a certified system for your airplane though...

 

[flyingcheesehead]

Not getting lost, or getting unlost after getting lost, I can see that. But 1cm for navigation, while it's technically possible, is in the realm of excessive overkill in the real world.

I disagree. I make extensive use of GPS navigation on the roads, and while my GPS will warn me "In one point four miles, exit right" or "In point three miles, keep left" it's often kind of vague at complex intersections. It'd certainly be nice to have that sort of accuracy so it knew what lane I was in as well as what road I was on. "Move to the right one lane" would be MUCH more helpful!

 

[gismo]

I disagree. I make extensive use of GPS navigation on the roads, and while my GPS will warn me "In one point four miles, exit right" or "In point three miles, keep left" it's often kind of vague at complex intersections. It'd certainly be nice to have that sort of accuracy so it knew what lane I was in as well as what road I was on. "Move to the right one lane" would be MUCH more helpful!

I think you'd find that one meter accuracy would be more than adequate. 1cm accuracy requires a huge number of samples at the frequency used by GPS since the wavelength at that frequency is over 20 cm.

 

[RotaryWingBob]

I think you'd find that one meter accuracy would be more than adequate. 1cm accuracy requires a huge number of samples at the frequency used by GPS since the wavelength at that frequency is over 20 cm.

Lance, with all due respect, I don't think that's true -- what it takes is a good way to correct for atmospherics -- DGPS and WAAS does that it a half assed way, but the two channel system the military uses can already do that. And we're not talking about 20 cm wavelengths -- that's VHf and the satellites are at much, much shorter wavelengths...

 

[Henning]

BBC News is reporting that Europe is planning to build their own satellite-navigation network that will be backward and forward compatible. There's going to be 5 levels ranging from free (1m accuracy) to commercial (1cm accuracy)! Provision is also being made for a search and rescue mode where a signal can be sent to confirm that help is on the way. The system will supposedly even work with existing US network after upgrades to the network.

http://news.bbc.co.uk/1/hi/sci/tech/4555276.stm

GPS with very high accuracy has been available for quite some time and I've been working with it offshore. When the boat moves even slightly, so do the readings (5 decimal places). Thing about it is, it's a $20,000 dollar system.

 

[gismo]

Lance, with all due respect, I don't think that's true -- what it takes is a good way to correct for atmospherics -- DGPS and WAAS does that it a half assed way, but the two channel system the military uses can already do that. And we're not talking about 20 cm wavelengths -- that's VHf and the satellites are at much, much shorter wavelengths...

The two GPS frequencies currently in use are 1575 Mhz and 1227.6 Mhz. Wavelength in meters (in a vacuum) is equal to 300 divided by the frequency in Mhz. For 1575 Mhz (the only frequency civilian receivers use) that's just over 0.19 m or 19 cm.

With our current system IIRC, absolute accuracy is expected to be in the one meter range most of the time using the P code and both frequencies all of which are encrypted so only our military can use them. Relative accuracy or repeatability (what Henning was referring to WRT detecting a change of position) can be much better using a technique called carrier phase monitoring but this requires many measurements over a significant period of time (many seconds) and can only be used for relative measurements like detecting a change in position or the difference between two locations with a receiver at each and some other means of establishing the initial difference.

 

[RotaryWingBob]

The two GPS frequencies currently in use are 1575 Mhz and 1227.6 Mhz. Wavelength in meters (in a vacuum) is equal to 300 divided by the frequency in Mhz. For 1575 Mhz (the only frequency civilian receivers use) that's just over 0.19 m or 19 cm.

With our current system IIRC, absolute accuracy is expected to be in the one meter range most of the time using the P code and both frequencies all of which are encrypted so only our military can use them. Relative accuracy or repeatability (what Henning was referring to WRT detecting a change of position) can be much better using a technique called carrier phase monitoring but this requires many measurements over a significant period of time (many seconds) and can only be used for relative measurements like detecting a change in position or the difference between two locations with a receiver at each and some other means of establishing the initial difference.

Sorry, brain didn't kick in on the wavelength