There are a couple of other questions associated with that topic.
1. How do they do it ?
2. Under which circumstances are all systems affected ?
3. Does it matter where you are ?
Without going into too much detail, the GPS NAVSTAR satellites are sending out two codes, the C/A code (coarse acquisition) for civilian use and the P/Y code (precision encrypted) to be used by the military. There are two frequencies, L1 with 1575 and L2 with 1375 MHz. The civilian code is being transferred only on L1, while the P/Y military code is being transferred on both frequencies, which allows better acquisition under certain ionospheric disturbances.
Now, to make civilian use more precise, geostationary satellites have been added to allow for WAAS (and EGNOS for that matter).
My question here is:
What do they do exactly ? Do they “only” make civilian positioning less precise, i.e. by adding the so-called PRN code (pseudo random noise) to the civilian code – or by switching off the L1 frequency altogether ? Or do they make the satellites temporarily unavailable for acquisition ? Are they really jamming or faking the frequencies (which the Russians have already threatened to do) ?
Are we in Europe affected at all ? Any modern GPS receiver (at least over the land mass of Europe) receives signals from 3 systems: GPS Navstar, Galileo and the russion Glonass, plus the geostationary wide area augmentation service birds as well (they are also sending valid positioning signals).
So, in what scenario will there be a total outage ? I assume that a military exercise in the US won’t affect us but in which case exactly are we talking about a complete “darkness” ?
I assume they just can’t switch it off completely. There would be too much civilian damage, no ?
@EuroFlyer, it’s a bit more complicated. Firstly, NGA (the authority over GPS) is able to “warp” the civilian map at select locations (typically the areas of NATO military operations), but L1 is never switched off. (By the way, there is also a new frequency for civilan signals called L5). Secondly, a third party can jam the GPS signal locally (which is relatively common and easy to achieve even for all GNSS systems at once, you can even buy a low-power jammer on Chinese e-commerce sites). Thirdly, a third party can also spoof the signal, as Russia is known to do around the Kremlin and possibly in the North Atlantic, but this is a considerably more sophisticated attack than mere jamming, so it is usually limited to broadcasting one fixed GNSS position over a certain area, so that wherever you go within that area, you are still under impression of standing still. Spoofing the signals to warp the map in a manner indistinguishable to the user (like NGA does) is an extremely complicated task.
When you say “warp the map”, it means it warps the WGS84 ellipsoid ? So that the horizontal and vertial position the receiver thinks it has in relation to the ellipsoid doesn’t correspond with the real position in relation to earth ? Doesn’t that mean they need to change the almanach data in the satellites ? Or are they changing the time code ?
Fiddling with almanacs is a favourite.
EuroFlyer wrote:
the P/Y military code is being transferred on both frequencies, which allows better acquisition under certain ionospheric disturbances.The major source of GPS inaccuracy is the variable signal delay in the ionosphere. The delay is different for different frequencies, so by using two frequencies it is possible to measure the delay and compensate for it. I guess this could improve the precision of the pseudorange, thus allowing better acquisition, but that is not the main reason for having two frequencies.
Do they “only” make civilian positioning less precise, i.e. by adding the so-called PRN code (pseudo random noise) to the civilian code – or by switching off the L1 frequency altogether ?
PRN means “Pseudo-random number” and are specific numbers unique to each satellite. The satellites’ transmissions are encoded using their PRNs. PRNs are used by the receiver to distinguish the signal from different satellites as they all transmit on the same frequency.
You may be thinking of SA (Selective availability) which was a deliberate pseudorandom offset to the satellite clocks used to degrade the accuracy of civilian receivers. The SA was turned off during the Gulf war when the US military needed more GPS receivers quickly and had to resort to civilian units. The US has officially stated that SA will never be used again.
It is 100% certain that there is a secret agreement between the USA and Europe that should the USA come under attack (a proper war of some sort) the civilian Galileo signal will be turned off, and vice versa.
My recollection of the reason for turning off SA is different: the USA became hugely dependent on GPS, and other ways to make the civilian signal useless for missile guidance were developed. With SA it is almost impossible to do satnav, because the c. 100m error makes it impossible to place the vehicle on the right road if there is another one nearby. The whole parcel delivery business as we know it today would grind to a halt, for a start.
Peter wrote:
It is 100% certain that there is a secret agreement between the USA and Europe
It’s not even secret. The Europeans agreed to make changes to Galileo specifically to allow the US to jam Galileo while leaving their own military GPS signal unaffected, and this is all on the public record.
alioth wrote:
The Europeans agreed to make changes to Galileo specifically to allow the US to jam Galileo while leaving their own military GPS signal unaffected, and this is all on the public record.Do you have a reference?
That would make the Galileo project even more ridiculous… Isn’t the whole point of it to be independent from the US GPS?
