What is the accuracy of dgps?

A Differential Global Positioning System (DGPS) is a system designed to improve the accuracy of Global Navigation Satellite Systems (GNSS) by measuring infinitesimal changes in variables to provide satellite positioning corrections.

In general, a Reference Station calculates differential corrections which are valid for that exact location (zero baseline) at that particular epoch (age of corrections zero). However, DGPS users may be located as far as 200 nm away from the Reference Station and some of the errors compensated for by the Reference Station vary with space, namely satellite ephemeris, tropospheric and ionospheric errors. Therefore, the corrections calculated at the Reference Station suffer certain accuracy degradation as the separation distance increases, because of a decreasing relevance of the Reference Station data to the user.

The error growth with increasing distance to the beacon is accentuated by the inability of Reference Station and user to see the same satellites, commonly termed the lack of intervisibility. The error growth with distance is the most important factor determining DGPS accuracy, but surprisingly very little has been done to assess it. US official documents and IALA state that the achievable accuracy degrades at an approximate rate of 1 m for each 150 km (80 nm) distance from the broadcast site, but this value is based on a theoretical prediction, made back in 1993.

To estimate the error growth with real data, 6 DGPS receivers were placed along the Portuguese coastline at approximately 50 nm intervals from Sagres Broadcast Station, in a South – North direction. This paper describes the results of the trial.

detail:
Two or more receivers observe the same set of satellites, taking similar measurements that produce similar errors when positioned closely together. A reference receiver, placed at a known location, calculates its theoretical position and compares it to the measurements provided by the navigation satellite signals. The difference between the two values reveals the measurement error. The reference receiver then transmits a corrected signal to any number of receivers at unknown positions within the area covered by the DGPS. Accuracy of global satellite positioning is thereby increased from 15 meters to within a few meters. This technique compensates for errors in the satellite navigation system itself but may not always correct errors caused by the local environment when satellite navigation signals are reflected off of tall buildings or nearby mountains, creating multi-path signals. The accuracy of DGPS decreases with asynchronous measurement caused by spatial and temporal error decorrelation when the system receivers are set at greater distances apart.

More sophisticated DGPS techniques can increase positioning accuracy to within a few millimeters. Raw measurements recorded by the reference receiver and one or more roving receivers can be processed using specially designed software that calculates the errors. The corrections may then be transmitted in real time or after the fact (post-processing). By applying the corrections and recalculating the position, accuracy from within several meters to within a few millimeters is achieved, depending on the specific methodology used and the quality of the real-time data link.

DGPS Methods
Satellite navigation receivers calculate position by measuring pseudo distances from the positioning satellites. These measurements are taken in several different ways. The most common method, used by all receivers, is to calculate the difference between the time a signal is transmitted from a satellite and the time it is recorded by the receiver, using the code embedded in the satellite's signal. This measurement is called code phase, and produces non-ambiguous meter-level results.

There are three types of DGPS using code phase measurement methods. DGPS and LADGPS (Local Area DGPS) typically cover an area up to several tens of kilometers. The coverage area is greatly increased up to several thousand kilometers by a more sophisticated method known as WADGPS (Wide Area DGPS). WADGPS classifies errors into position-dependent and position-independent components creating a secondary set of measurements that are transmitted to the rover receivers. The rover receivers are then able to reconstruct the pseudo range correction most applicable to their actual position and compute an accurate differential position.

A second method serves to compliment code phase measurement by measuring the carrier phase of the satellite carrier wave. This method provides millimeter-level resolution with measurements that are ambiguous to about 19 centimeters.

DGPS using the carrier phase achieves maximum accuracy only when measurement ambiguities are resolved in some way. The static method of ambiguity resolution is related to stationary receivers, with rover receiver point occupation from 30 minutes to several hours or even several days. The rapid static method reduces occupation periods to several minutes, while the kinematic method allows rover receivers to move without constraint.

The principle of DGPS is separated into the following classifications:

Measurement Type	Real-time or Post-processing	System Type	Accuracy	Coverage Area
Code phase	Post-processing	Post-processed DGPS, post-processed LADGPS or post-processed WADGPS	from < 1 m to ~10 m	From several x 10 km to several x 1000 km
Code phase	Real time	DGPS, LADGPS or WADGPS	from < 1 m to ~10 m	From several x 10 km to several x 1000 km
Carrier phase	Post-processing	Kinematic, rapid static or static	from < 1 cm to several cm	From several km to several x 1000 km
Carrier phase	Real time	Real-time kinematic	from < 1 cm to several cm	From several km to several x 10 km