**HOW GOOD IS GPS?**

**GPS Positioning Modes**

The main positioning modes can now be identified:

**ABSOLUTE or POINT positioning: **coordinates in
relation to a well-defined global reference system.

**DIFFERENTIAL or RELATIVE positioning:** coordinates
in relation to some other fixed point. In GPS surveying this is referred
to as baseline determination.

**STATIC positioning: **coordination of stationary
points, either in the absolute or relative mode. Relative static positioning
is generally synonymous with the SURVEYING mode of operation, and is based
on the analysis of carrier phase observations.

**KINEMATIC positioning:** coordination of moving
points, either in the absolute or relative mode. This is the NAVIGATION
mode of positioning, and is based on the use of pseudo-range observations.

The basic GPS positioning modes.

**Comment:**

- Real-time positioning in the kinematic mode is the type of positioning
for which GPS was explicitly developed. Only low to moderate navigation
accuracies are possible.
- Pseudo-range data can be used for both absolute and differential positioning.
Carrier phase data on its own is used in precise differential positioning.
"Smoothing" the relatively noisy pseudo-range data with carrier
phase is a useful technique for enhancing absolute or differential pseudo-range
results.
- With respect to differential positioning, the dividing line between
the two extremes of static positioning with carrier phase data (conventional
"GPS surveying"), and kinematic positioning using pseudo-range
data (conventional "precise navigation") is becoming increasingly
blurred. It is possible to now speak of "rapid" static positioning,
or high precision phase-based kinematic positioning (section
5.5.1).

*© Chris Rizos, SNAP-UNSW, 1999*