LOCATION VS. ACCURACY

[JohnyReb]

i LIVE IN A AREA CLOSE TO SEVERAL MILITARY BASES AND IT SEEMS THAT THE CLOSER I GET TO ONE OF THOSE BASES, THE "TIGHTER" MY EPE GETS. IS THIS A COINCEDENCE OR CAN THEY MANIPULATE THE ACCURACY OF THE SIGNALS IN ONLY CERTAIN AREAS?

[+Jamie Z]

They (the government) do have the ability to insert an error into the GPS system in certain areas. This is referred to as Selective Availability, and was global until just a few years ago.

 

The thing is, I highly doubt the government is currently doing this in any area, except for maybe short tests... and also, if they do insert an error, your GPS won't know. It will display a normal-looking EPE.

 

Basically, if Selective Availability is active, the orbiting satellites are sending manipulated data. Your GPS will never know it's being manipulated and will simply calculate its error based on the signal you are receiving.

 

So in short. Yes its possible for the goverment to "manipulate" the signal. No they probably aren't doing this. If they are, you won't know it.

 

Jamie

 

By the way, I think your Caps Lock button is stuck.

[+Jamie Z]

They (the government) do have the ability to insert an error into the GPS system in certain areas. This is referred to as Selective Availability, and was global until just a few years ago.

 

The thing is, I highly doubt the government is currently doing this in any area, except for maybe short tests... and also, if they do insert an error, your GPS won't know. It will display a normal-looking EPE.

 

Basically, if Selective Availability is active, the orbiting satellites are sending manipulated data. Your GPS will never know it's being manipulated and will simply calculate its error based on the signal you are receiving.

 

So in short. Yes its possible for the goverment to "manipulate" the signal. No they probably aren't doing this. If they are, you won't know it.

 

Jamie

 

By the way, I think your Caps Lock button is stuck.

[Kerry.]

Just to clarify a couple of things.

 

If for "certain" reasons there was a need to cover specific installations the the EPE most likely wouldn't get "tighter" it would get worse, much worse to the point of nothing once jammers were used, which is easier to do for small specific areas.

 

SA is technically not shutdown just set to zero (0) but Selective Deniability is something else.

 

Be assured that if the signals are manipulated then your GPS will show that clearly especially in the EPE. I don't know where the source of this comment is coming from that somehow they can put SA back into to the signal and nobody will notice My word you'd know about it your position would go ape and so would your EPE to match.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

Just to clarify a couple of things.

 

If for "certain" reasons there was a need to cover specific installations the the EPE most likely wouldn't get "tighter" it would get worse, much worse to the point of nothing once jammers were used, which is easier to do for small specific areas.

 

SA is technically not shutdown just set to zero (0) but Selective Deniability is something else.

 

Be assured that if the signals are manipulated then your GPS will show that clearly especially in the EPE. I don't know where the source of this comment is coming from that somehow they can put SA back into to the signal and nobody will notice My word you'd know about it your position would go ape and so would your EPE to match.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Jamie Z]

Obviously, I stuck my foot in my mouth...

 

Kerry, it is my understanding that during the days of SA... GPS's would not display crazy EPE's of hundreds of feet or yards, but would indicate a EPE of several feet, just as they do now.

 

It was explained to me that if the signal is degraded somehow, the GPS receiver does not know this, but simply calculates its position (and its EPE) based on the signal it receives.

 

An analogy: lets say I blindfold you and send you on a walk through the neighborhood, telling you which way to turn. We stop, and I tell you we are 10 feet from a building, when in fact we could be much further. If someone asks, you would tell them you are within 10 feet of a building because that's the information I've given you. Being blindfolded, you have know way of knowing how accurate the information is that I gave you.

 

Does the GPS system not work like that?

 

BTW, JohnyReb, Kerry is much more knowledgable on this subject than I am, so his answers should hold much more weight than mine.

 

Jamie

[Kerry.]

Jamie, EPE is basically a "managed" value based on some propriety software code.

 

When SA was terminated EPE's didn't change as much as the "actual" accuracy but change they did and one thing it really highlighted was that manufacturers were caught out a little in that the EPE routines reflected accuracy that didn't really exist anyway.

 

Howevever change they did and since SA was terminated most manufacturers have "re-managed" the EPE routines to "best" reflect really what they want it to reflect (an estimate).

 

NO, EPE's certainly didn't indicate a few feet with SA On and EPE's that consistently indicate a few feet now are also a worry.

 

EPE is only an Estimate but is an estimate of the relative position(s) (plus some other "unknowns") and if that position is wandering around then so does the Estimated Position Error. And it tries to all this without actually knowing where it is anyway.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

Jamie, EPE is basically a "managed" value based on some propriety software code.

 

When SA was terminated EPE's didn't change as much as the "actual" accuracy but change they did and one thing it really highlighted was that manufacturers were caught out a little in that the EPE routines reflected accuracy that didn't really exist anyway.

 

Howevever change they did and since SA was terminated most manufacturers have "re-managed" the EPE routines to "best" reflect really what they want it to reflect (an estimate).

 

NO, EPE's certainly didn't indicate a few feet with SA On and EPE's that consistently indicate a few feet now are also a worry.

 

EPE is only an Estimate but is an estimate of the relative position(s) (plus some other "unknowns") and if that position is wandering around then so does the Estimated Position Error. And it tries to all this without actually knowing where it is anyway.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Alan2]

Two questions:

1. What is the basis of SA? It can't be just a shift in the transmitter data of the satellites actual position. Then the military units would be less accurate as well. Is it a second transmitted signal? Dual signal including a coded signal that only the military units decode?

 

2. How can the military control the area they want a larger error? The satellites don't know where you are. They're just sending signals where they are. The GPS interpolates that information from your own position.

 

ALan

[JohnyReb]

Actually, I did'nt so much as notice the EPE getting smaller but on the map the whole time I was near base the plotter showed me directly on the roads, even with tight turns over short distances. Perhaps the map data my Mapsource is using is just more accurate for that area?

[Kerry.]

Alan2, SA certainly isn't (wasn't) simply a shift as SA was unique to each individual satellite. The overall effect was the less than 100 metres 95% of the time specification. It is believed that SA could throw a position by more than 2000m if fully wound up (of course nobody will officially confirm that one).

 

Basically SA created an unpredictable (but not random as some believe) continuous shift in displayed navigation from the real world.

 

SA was accomplished by the purposeful manipulation of the navigation message orbit data and/or the satellite timing.

 

Military units have the advantage of a specific military frequency (still with SA), using coded/keyed encrypted receivers (which eliminate the effects of SA) and things like Anti-spoofing to guard against fake transmissions.

 

The approval to discontinue SA was due to the DoD implementing other measures which suited their specific requirements. Selective Deniability is its name but just what type of functionality it has (coverage, accuracy degradation etc) I've not aware.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

Alan2, SA certainly isn't (wasn't) simply a shift as SA was unique to each individual satellite. The overall effect was the less than 100 metres 95% of the time specification. It is believed that SA could throw a position by more than 2000m if fully wound up (of course nobody will officially confirm that one).

 

Basically SA created an unpredictable (but not random as some believe) continuous shift in displayed navigation from the real world.

 

SA was accomplished by the purposeful manipulation of the navigation message orbit data and/or the satellite timing.

 

Military units have the advantage of a specific military frequency (still with SA), using coded/keyed encrypted receivers (which eliminate the effects of SA) and things like Anti-spoofing to guard against fake transmissions.

 

The approval to discontinue SA was due to the DoD implementing other measures which suited their specific requirements. Selective Deniability is its name but just what type of functionality it has (coverage, accuracy degradation etc) I've not aware.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+brdad]

quote:

---

Originally posted by JOHNYREB:

Perhaps the map data my Mapsource is using is just more accurate for that area?

---

 

That could be a contributing factor. I have made GPS tracks of many of the trails & roads around here, and have noticed accuracy seems to be much better on the highways, then state roads, then town roads, and back roads can be off quite a bit. I think it depends on who surveys the roads and how accurate their standards are.

 

Aerial photos seem to be just as accurate for all roads & trails, which would make sense.

 

Save our forests, wipe your *** with a tree-hugger.

[+david&diana]

Before SA was turned off, my GPS III would typically report an EPE of 45-60 feet. With no changes to the GPSr, it now typically reports an EPE of 15-25 feet.

 

Also before SA was turned off, I remember reading a notice somewhere, probably the U.S. Coast Guard web site, of a military test that would cause any GPSr within a defined area off the coase of North Carolina, to report an incorrect position. This notice said that the test would affect ships and aircraft. I assume this was a test of Selective Deniability.

[+Alan2]

Thanks Kerry. That's the answer to my first question and possibly my second one.

 

Can I assume then that even the military cannot limit it to a specific area (other than maybe one side of the world)? - my second question?

 

Alan

[Kerry.]

Alan, as I understand it DoD can basically localize an area under Selective Deniability but just what size that foot print can be is a bit of a mystery (to me anyway, sort of).

 

However I do get the impression that it can be "very specific". There were hints of this in some regions of recent times but there's no certainity if it was SD or something else. If it was SD then it's fairly controllable based on the surrunding data available.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

Alan, as I understand it DoD can basically localize an area under Selective Deniability but just what size that foot print can be is a bit of a mystery (to me anyway, sort of).

 

However I do get the impression that it can be "very specific". There were hints of this in some regions of recent times but there's no certainity if it was SD or something else. If it was SD then it's fairly controllable based on the surrunding data available.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[NeilG]

Hi,

 

Here is the official site detailing the "performance of GPS" from the DoD.

http://www.peterson.af.mil/usspace/gps_support/performance_reports.htm#Predictions

 

Accuracy of GPS varies over time in different parts of the world, these maps generated by the DoD show how this error could affect your GPS.

As far as I am aware they can alter the position of the satellites to change how the "performance" in different areas. Nothing to do with SA just satelite geometry, ionospheric interference and probably a few other factors.

If they adjust it so that the signal is better at a base or not I have no idea (sorry!)

The site contains some interesting infos in the FAQ too.

NG

[Kerry.]

The "performance" still has to meet the "specifications". The specifications are what they say they guarantee to provide, the performance is more actually happens or is supposed to happen.

 

The measured performance (accuracy and other things) still has to fall under the specification otherwise the system isn't meeting its specifications.

 

Also the specifications these days don't take into account things that the system can't predict such as atmospherics etc hence the specification are based on Signal-in-space characterics as there's no way anybody can control what a GPS user actually does.

 

Yes, they can (and do) change things at times for specific purposes (and not just military related either) but they still have to meet minimum specifications world wide.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

The "performance" still has to meet the "specifications". The specifications are what they say they guarantee to provide, the performance is more actually happens or is supposed to happen.

 

The measured performance (accuracy and other things) still has to fall under the specification otherwise the system isn't meeting its specifications.

 

Also the specifications these days don't take into account things that the system can't predict such as atmospherics etc hence the specification are based on Signal-in-space characterics as there's no way anybody can control what a GPS user actually does.

 

Yes, they can (and do) change things at times for specific purposes (and not just military related either) but they still have to meet minimum specifications world wide.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go