Differential GPS and Wide Area Augmentation System (WAAS)

Improving precision of the GPS navigation

Differential GPS (DGPS) provides more accurate position data. Wide Area Augmentation System (WAAS) is a similar technology.
Augmentation messages are sent by ground-based radio relay stations (in WAAS, the satellites are used to distribute the signal).
There are several different augmentation systems and providers available.
Most modern receiver units can work with DGPS signal, if there is one available in the area.

How differential GPS works

There are several fixed GPS receivers with a precisely known location - reference stations.
Each reference station computes the position difference between satellite-derived fix and the reference fix.
This difference is accurate throughout a wide area, because the satellite signal errors and ionosphere-induced signal distortions change slowly over space and time.
The computed difference is sent to other receivers via radio, which adjust their fix data accordingly.
This allows to achieve less than 10m accuracy 95% of the time. Typical precision is less than 1m if the receiver is closer than 100-150 km to the base station.

What's the difference between DGPS and WAAS?

Both WAAS and DGPS are based on GPS and cannot function without it.
Both WAAS and DGPS use the same principle of fixed reference stations on the ground.
WAAS covers larger areas with slightly less precision. DGPS covers smaller areas with greater precision.
WAAS distributes augmentation signal from satellites and requires satellite in view. DGPS uses medium frequency radio waves which follow terrain to a certain extent.
WAAS is mostly designed for aviation use. Sea- or ground-based users are supposed to use DGPS.

What was the original purpose of DGPS?

DGPS was invented to circumvent the intentional civilian GPS signal distortion (which was in effect before year 2000). That was possible because the artificial error in the signal changed slowly over time.
During the development they found that other types of errors are also eliminated pretty well and really good precision (less than one meter) can be achieved.