Re: War and GPS
Posted by Joel Briggs on March 20, 2003 at 13:06:08:
In Reply to: War and GPS posted by jason on March 19, 2003 at 23:27:31:
I was going to stay out of this one, but...
There are THREE primary levels (L*) of service for GPS satellites. L1 is a single frequency, with L2 performing dual frequency capability. L3 – classified and being deployed on the newer satellites. The L2 frequency is encrypted so our civilian GPS receiver can't use their signals – sometimes (in some cases they will turn this off, and your unit will be accurate up to 2’). All this is to prevent spoofing (the deliberate transmission of fake GPS signals with the disruptive intent to fool a receiver.) of critical applications, to control GPS usage per a grid (selective availability), and to allow the military to have precise positioning service (more accurate than a centimeter). In some cases, these spoofing transmission can be used against the sender, thus, providing fake coordinates.
With your regular off the shelf GPS, you get Standard Position Service (SPS). This is a L1 frequency; also know as the Civilian Code or C/A (Clear/Acquisition Code). Now, I hate to bust your bubble, this L1 is still not 100% accurate and in fact, can be off up to 15 meters – even though your GPS says accuracy 18 feet or something of that nature. Don’t ask me why, but it just does. In most cases, these units are accurate up to 5 meters.
Now, SA or Selective Availability is pretty cool. Last AT (annual training) I was exposed to how this works to include anti-spoofing and pro-spoofing – long story and you can read about them on the internet using Google. What this does, it degrades the GPS to an accuracy of about 100 meters, but not those units with +L2 capability. Currently, this is being used in Iraq and some other parts of the world today. Basically, when applied to a grid (section of the world), your GPS is off 100 meters. The last time this was used in the US was after 9-11 and was quickly restored when they started to plot out the damage done. How fast can this be applied to a “selective area”, in about 10 seconds.
An example of this: when you hook up your GPS to your laptop and you start down the road, if you zoom in, in some cases you will see your plot trail not lining up or being on the road. This is because you don’t have pulse per second capability. I might add, pulse per second is much fast than a second. In some cases, accuracy can be up too 1/100th of a second. You are still plotting, but not accurately. For the average user, this is fine.
This is how it was explained to me last year at AT (annual training with the US Army Reserve).
Now, I have both PPS and pulse per second with my GPS unit in my FJ40 Landcruiser. Now before you call the military and other law enforcement agencies, I have the necessary paper work and a unique identifier for my unit. It can be turned off via satellite at will by the DoD. The pulse per second output is connected to a laptop that has mapping software. If need be, I can pull down, via satellite, additional information if need be. Nice feature when off roading. My GPS unit both pulls and pushes. That is to say, I can receive and transmit my coordinates and text messages from anywhere in the world and have them stored or displayed on some far away computer systemn (right Scottertrash and Tom D.). Very similar to your car navigation systems, but with more bells and whistles.
When comparing my unit against the L1 units, the best accuracy I have seen was 14’.
Again, this is how it was explain to me and some of what I wrote may be off and I am open to learning. But I think I covered the basics.