War and GPS Degradation
Discussion
I believe it always used to be the case (it certainly was during the last gulf war) that military and civilian GPS had two different levels of accuracy, and the GEODS earth stations did deliberatly scramble the signal and put in deliberate off sets, this was published well in advance and certainly signals were broadcast as "general notices to mariners" that GPS should not be relied on. However since then I believe the NATO software has been removed from GPS and both civilian and military GPS sets work to the same accuracy, also it has become far more widespread in its use and a lot of people depend on it, I don't doubt that they have retained the ability to "wobble" the signal and could even switch it off but the bottom line is why bother ?? Most young squaddies now days have got £80 GPS hand helds that they have bought themselves and will be using, whereas Saddams conscrpts have barely got uniforms, OK Iraqi "special" units will have it but do you really think that it makes that much difference to "Bedouin" dessert troops fighting on their own turf ? Personally I think it would cause more problems than it solved to wobble the signal and I certainly haven't heard of any advance notice unlike last time.
I fink selective availability (bugger factor that kept civi GPS to +-100m) was actually switched off during the first gulf war to allow the military to get accurate fixes using civilan kit due to certain shortcomings with their own ... probably an urban myth, but I seem to remember a job lot of Sony yatch nav kit getting bought and shipped out sharpish.
There are two GPS signals C/A code and P code, the C/A code is used by non-authorised individuals (i.e. Joe Public) and the P code is used by the military and other authorised users (protected by encryption). By design the P code is capable of higher accuracy than C/A code, typical figures are about 7m for P code and 15m for C/A code, this is mainly due to a higher bit rate and the provision of the code on two frequencies which allows the receiver to correct for ionospheric delays.
However until a few years ago the C/A code had a dither applied to the signals at the sats which decreased the accuracy of the signal, this was called Selective Availability (SA) and reduced the accuracy down to about 50m typical.
During the last Gulf war the military did not have enough military (P Code) receivers and so most of the receivers used by the military were C/A code only and for this reason the SA was reduced to very low levels during the conflict with the result that ALL non military GPS receivers produced far higher accuracy results.
However the military now have many more P-Code receivers and the latest sats have the ability to vary accuracy (with progressive levels of SA) as they transit around the world. But as already pointed out many military personnel will have their own civilian sets and so who knows what the military will do with the GPS signals?
However until a few years ago the C/A code had a dither applied to the signals at the sats which decreased the accuracy of the signal, this was called Selective Availability (SA) and reduced the accuracy down to about 50m typical.
During the last Gulf war the military did not have enough military (P Code) receivers and so most of the receivers used by the military were C/A code only and for this reason the SA was reduced to very low levels during the conflict with the result that ALL non military GPS receivers produced far higher accuracy results.
However the military now have many more P-Code receivers and the latest sats have the ability to vary accuracy (with progressive levels of SA) as they transit around the world. But as already pointed out many military personnel will have their own civilian sets and so who knows what the military will do with the GPS signals?
I was under the impression that Selective Availability was no longer applied (ie. has been switched off permanently)?
There's not a lot of point anyway due to the widespread availability of Differential GPS (which is where a fixed base station knows its exact position and then publishes the offset between its known position and where GPS currently says it is, so that mobile stations can then apply the same offset to their own GPS position to get their true position).
And all this become moot if the European Community succeed in get Gallieo off the ground. Naturally the Americans are totally against it as it would break their stranglehold on GNSS by offerering an alternative to GPS. And it will be an order of magnitude more accurate too.
There's not a lot of point anyway due to the widespread availability of Differential GPS (which is where a fixed base station knows its exact position and then publishes the offset between its known position and where GPS currently says it is, so that mobile stations can then apply the same offset to their own GPS position to get their true position).
And all this become moot if the European Community succeed in get Gallieo off the ground. Naturally the Americans are totally against it as it would break their stranglehold on GNSS by offerering an alternative to GPS. And it will be an order of magnitude more accurate too.
JonRB said: I was under the impression that Selective Availability was no longer applied (ie. has been switched off permanently)?
There's not a lot of point anyway due to the widespread availability of Differential GPS (which is where a fixed base station knows its exact position and then publishes the offset between its known position and where GPS currently says it is, so that mobile stations can then apply the same offset to their own GPS position to get their true position).
And all this become moot if the European Community succeed in get Gallieo off the ground. Naturally the Americans are totally against it as it would break their stranglehold on GNSS by offerering an alternative to GPS. And it will be an order of magnitude more accurate too.
SA has been switched off, but the US DoD have reserved the right to switch it back on without notice. They also reserve the right to switch off GPS altogether without notice but have stated that this would only happen by the order of the President.
There are not many differential GPS base stations in Iraq and most hand helds do not use correction data as often a seperate receiver is required.
It would be nice to have Gallieo up and running but it may take the EU a long time to agree all the details. Don't forget the US are providing their own wide area differential system with the WASS on INMARSAT.
The USA do not have a monopoly on GNSS, GLONASS is still faltering along with a few sats still operational. If the EU wants an alternative so badly it could always fund a few more GLONASS sats!
GLONASS is the Russian equivalent, if I remember correctly?
I see what you're saying, but half the reason for GNSS-2 (Gallileo) is to get a quantum leap in accuracy, not just to break the USA's monopoly of global positioning (I exclude GLONASS from the term 'global positioning' because, as far as I am aware, it doesn't have global coverage. Do correct me if I'm wrong though).
Having said that, the USA's repeated statements that it's thier ball and they'll take it away whenever they want is probably a fairly big motivation for Gallileo too.
I see what you're saying, but half the reason for GNSS-2 (Gallileo) is to get a quantum leap in accuracy, not just to break the USA's monopoly of global positioning (I exclude GLONASS from the term 'global positioning' because, as far as I am aware, it doesn't have global coverage. Do correct me if I'm wrong though).
Having said that, the USA's repeated statements that it's thier ball and they'll take it away whenever they want is probably a fairly big motivation for Gallileo too.
JonRB said: GLONASS is the Russian equivalent, if I remember correctly?
I see what you're saying, but half the reason for GNSS-2 (Gallileo) is to get a quantum leap in accuracy, not just to break the USA's monopoly of global positioning (I exclude GLONASS from the term 'global positioning' because, as far as I am aware, it doesn't have global coverage. Do correct me if I'm wrong though).
Having said that, the USA's repeated statements that it's thier ball and they'll take it away whenever they want is probably a fairly big motivation for Gallileo too.
That's if the Europeans can get it into space in one piece given Ariane's recent problems - why can't we just be the US's best mate and use GPS instead of forking out billions for a few more bits of space junk?
Well, if they're going to switch it on again they haven't done it yet. I've never understood the point anyway - differential GPS is widely available (though illegal- at least in the States don't know about here) My GPS is typically accurate to 5x5m which I would have thought would be accurate enough for a home made fire and forget system-presumably the point in introducing offsets ?
JonRB said: GLONASS is the Russian equivalent, if I remember correctly?
(I exclude GLONASS from the term 'global positioning' because, as far as I am aware, it doesn't have global coverage. Do correct me if I'm wrong though).
Yes you do remember correctly GLONASS is the Russian equivalent.
The GPS and GLONASS sats transit around the world about twice a day (GPS every 12 hours minus 4 mins {sidereal time - includes the effect of the earth moving around the sun}, GLONASS 17 times every 8 days [to reduce the gravitational effect on the orbit of passing over the same land track twice a day]) and as such both provide global navigation, however the GLONASS system is short of sats at present so that 3D nav is not possible all over the world at all times of the day, 3D nav is possible anywhere in the world at times during the day when enough sats are in view
jondokic said: Well, if they're going to switch it on again they haven't done it yet. I've never understood the point anyway - differential GPS is widely available (though illegal- at least in the States don't know about here) My GPS is typically accurate to 5x5m which I would have thought would be accurate enough for a home made fire and forget system-presumably the point in introducing offsets ?
Differential GPS is not illegal in the States.
SA was switched off partly because of the situation in USA where the DoD were spending money and effort to apply SA (and to stop any research that came close to predicting the clock and range errors), whilst the Department of Transport was spending a fortune equipping the entire coast with differential GPS stations for public use. The DoT was also funding the work on providing the wide area differential system via IMARSAT (WASS).
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