I believe that IFR-certified GPS systems have the added capability of factoring Differential GPS. This is an additional GPS beacon (aside from the satellites) originating from the airport you're landing at, which have a location that's very accurately known.
High-price civilian GPS receivers are capable of this -- except you rarely have access to such a beacon. I mean, the restaurant I'm going to eat at isn't going to provide my little TomTom with a differential signal. With aviation, it's important enough that high-traffic airports provide the DGPS beacons.
This isn't true. IFR certified GPS units generally don't have Differential receivers. The main reason for this as far as I can tell is integrity. To be IFR certified, the receiver has to be able to tell when a satellite is giving bad data. In the older units, this was done with RAIM, which basically compares what is coming in from all of the satellites, and determines if all of the satellites are providing positioning that is relatively consistent. DGPS is a relatively simple system, and there's no way to check the DGPS signal for integrity. Without the ability to check for integrity, the additional accuracy is useless, as the design of instrument approaches is based on worst-case scenarios.
Now newer IFR certified units do have WAAS, which is similar to DGPS, except the signal is transmitted from a satellite, and is based on measurements from a couple dozen ground stations, the correction interpolated in the unit.
There has been a proposal to implement a system called LAAS for a number of years, which would be like DGPS, but would have the integrity checks that are required for aviation. However, it never really got off the ground.
As it stands, with WAAS, you can be guided to 200 feet above the runway, but this is not as good as the old analog radio ILS, which, if setup in a sophisticated way, can take a plane all the way to the surface.
On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,[59] eventually making the policy permanent.[60]
