Differential GPS (DGPS) represents the first major improvement over standard GPS accuracy. This technique uses a network of fixed ground-based reference stations at precisely known locations. These stations compare their known position with the GPS-calculated position, determining the error in real-time. This correction data is then broadcast to nearby DGPS receivers, which apply the corrections to improve their positioning accuracy.
What is a Differential Global Positioning System (DGPS)?
Differential Global Positioning System (DGPS) enhances standard GPS by eliminating pseudorange errors. Ground-based reference stations, whose positions are known precisely, continuously compare the theoretical satellite distance with the real distance measurement; the difference they find is formatted into real-time corrections and broadcast to mobile units. Because every correction is computed against a fixed, verified location, DGPS survey can reduce positioning uncertainty to decimetre or even sub-decimetre levels, thereby refining accuracy compared to standalone GPS.
What is differential correction GPS?
Differential correction is a class of techniques which improves the accuracy of GPS positioning by comparing measurements taken by two or more receivers. Differential GPS measurements can be computed in real time by GPS receivers which receive a correction signal using a separate radio receiver, or post-processing is used in Differential GPS to obtain precise positions of unknown points.
Differential correction is a process that removes shared errors by comparing two receivers, one fixed base station placed at a pre-surveyed point and one mobile rover, so that the corrected position is calculated by the mobile receiver at the same instant. Real-time differential GPS corrects GPS data as you collect it through radio links that send differential corrections every few seconds. These corrections are later applied to the position calculated by the mobile receiver and cancel out a large percentage of pseudorange errors. Most GPS receivers support differential correction through satellite-based augmentation systems. Trimble's RTX system is a satellite-based real-time differential correction service broadcast over the internet or directly through satellites and can produce as good as 4 cm (1.57 in) real-time accuracy with compatible receivers.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
How can differential GPS improve accuracy?
DGPS improves GPS system accuracy by using ground-based reference stations. It can calculate corrections. The underlying premise of differential GPS (DGPS) is that any two receivers that are relatively close together will experience similar atmospheric errors. DGPS requires that a GPS receiver be set up on a precisely known location. This GPS receiver is the base or reference station. The base station receiver calculates its position based on satellite signals and compares this location to the known location. The difference is applied to the GPS data recorded by the second GPS receiver, which is known as the roving receiver. The corrected information can be applied to data from the roving receiver in real time in the field using radio signals or through postprocessing after data capture using special processing software.
What is a differential GPS antenna?
A differential GPS antenna is a part used in Differential Global Positioning Systems (DGPSs), which supplement and enhance the positional data available from global navigation satellite systems (GNSSs), increase accuracy of positional data from approximately 15 metres, and increase accuracy of positional data by about a thousandfold.
A differential GPS antenna is the antenna that listens to the DGPS correction signal. That signal is a supplementary correction stream used by GPS receivers. A correction signal is sent to the GPS receiver every few seconds and mobile receivers use it to correct their position. When the antenna picks up this update, the non-fixed mobile receivers use the data to correct their position by the same amount.
The antenna itself is not immune to error. Multipath error is caused by reflections of GPS signals from metal objects near the antenna, and a DGPS user does not have the option to minimize multipath error. Once the reflected energy enters the antenna along with the straight-line signal, the reference system that measures the remaining error can only note it, not remove it. Thus the differential GPS sensor is the entire receiver-and-antenna assembly that applies the correction from the base, while the antenna is merely the first stage where both the wanted correction and the unwanted echo arrive.
What is a differential GPS fix in DGNSS?
A differential GPS fix, flagged by the letter D on receivers like u-blox, is the outcome of applying real-time corrections to ordinary GNSS data. DGNSS is an enhancement to the primary GNSS constellations in which a network of ground-based reference stations, each situated at a precisely known point, continuously compares its calculated position with the position given by the satellites. The difference yields an error vector that is broadcast locally through ground-based transmitters of shorter range. Mobile receivers use this data to correct their position by the same amount. Traditional code-based DGNSS makes the simple assumption that satellite errors and local atmospheric errors are the same over small areas, so a single correction value is valid for every rover within the transmitter footprint. In this way the infrastructure provides a Differential GNSS fix to receivers, refining stand-alone accuracy without altering the original satellite signals.
What is the range of DGPS?
DGPS users are located as far as 200 nautical miles (370 km) from the reference station. Accuracy decreases as the distance between the reference station and the user station increases. Within a few tens of kilometres, DGPS can achieve accuracies of 1m (3.28 ft). Trial measurements produced error rates of 0.5-1.5 m (1.64-4.92 ft) within a radius of 250 km (155.34 miles) and 1-3 m (3.28-9.84 ft) within 600 km (372.82 miles). Local-area DGPS is effective typically within a few hundred kilometres of the base station, while Wide-Area DGPS covers regional or continental areas. The digital correction signal is broadcast over ground-based transmitters of shorter range, and correction data must not be older than approximately 10 to 60 seconds.
What are differential GPS stations?
Differential GPS stations form a network of ground-based reference stations that DGPS uses to broadcast differential information to the non-fixed (mobile) receivers named rovers. One GNSS receiver is located in a precise, known location and is used as the base, or reference station. This receiver receives signals from the satellites, compares them to its known position, and calculates differential corrections for its own location and time. These reference stations contain very accurate clocks, and the stations broadcast this data locally using ground-based transmitters of shorter range.
The corrected information can be applied in real-time in the field using radio signals, and non-fixed mobile receivers use it to correct their position by the same amount. Network coverage is extensive: for example, USCG's national DGPS consists of 85 broadcast sites that provide dual coverage to almost the entire US coastline and inland navigable waterways including Alaska, Hawaii, and Puerto Rico. Australia runs three DGPSes, of which one is mainly for marine navigation, broadcasting its signal on the long-wave band. Taken together, DGPS leverages this network of stations so that mobile users receive continuously updated corrections, refining positioning accuracy wherever the ground-based reference network reaches.
What is the difference between differential GPS and WAAS?
The difference between differential GPS and WAAS is that in DGPS, a single coast or harbour beacon broadcasts short-range corrections intended for local mariners. The receiver must be within the beacon's 100-300 km (62-186 mi) footprint and possess an additional radio modem. WAAS, the FAA's Wide Area Augmentation System, applies the same ground-reference idea but links dozens of fixed stations across North America into a master network. Measurements from these stations are processed centrally, then uplinked so that three commercial geostationary satellites re-broadcast the correction stream. Because the signal comes from space, WAAS is available over a much wider area compared to DGPS and the same satellite transmitter broadcasts cover the entire United States and most of Canada without extra hardware.
A non-WAAS GPS relies solely on the transmissions from the GPS satellites for its position, whereas any WAAS-enabled GPS receiver can read the signal without other costly modifications. The space-born correction also acts as additional pseudo-GPS signals, adding ranging sources and refining geometry. DGPS normally reaches 1-3 m (3.3-9.8 ft), while WAAS improves accuracy to the sub-10-metre level (sub-32.8 ft) and typically keeps the error within 30 m (98.4 ft) or less. Thus, DGPS remains the choice for local, high-precision harbour approaches, while WAAS gives seamless, continent-wide corrections suited primarily for aviation but freely usable by every compatible receiver.