Differential GPS is an enhancement to the standard Global Positioning System that provides refined location accuracy. By adding a stationary local reference station, it augments satellite information and continuously supplies positional corrections to the rover receiver in the field. Through these ground-based corrections, DGPS overcomes the inherent errors of standard GPS and meets the demands of precise navigation and positioning applications.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is Differential GPS?
Differential GPS (DGPS) enhances positional data available from global navigation satellite systems (GNSSs). It can increase accuracy of positional data by about a thousandfold because most GPS errors affect each receiver nearly equally and can thus be cancelled out in the calculations.
Differential Global Positioning System is a Ground-Based Augmentation System in which a stationary unit at a precisely known location compares calculated GPS position with the true geodetic mark, computes the error, and broadcasts a correction signal to the rover. Applying this correction cancels out a large percentage of the different pseudorange errors from each satellite signal, so the rover receives adjusted data and its GPS receiver can increase the accuracy of its position.
What is the purpose of differential GPS?
The purpose of DGPS is that it gives a GNSS receiver the accuracy demanded for surveying, maritime guidance and engineering work. Users want to place a point within centimeters, not tens of meters, so a reference system fixed at a known position measures the errors still present in each satellite signal. It broadcasts a correction signal to the GPS receiver for each satellite, allowing the receiver to apply that correction to every measurement it makes. This cancels out a large percentage of the different pseudorange errors, which brings positional data to the centimeter level - an increase in accuracy of roughly a thousandfold. Land-based networks like Southern Positioning Augmentation Network, an Australian Satellite-Based Augmentation System, stream the same kind of corrections automatically, giving continuous, higher-accuracy positioning for GNSS users who once relied solely on standalone receivers prone to multipath and orbit errors.
How does differential GPS work?
Differential GPS works by having a reference station at precisely surveyed coordinates calculate the instantaneous range error for every satellite in view and continuously broadcast the resultant corrections locally using ground-based transmitters. Non-fixed mobile receivers apply each correction to the matching pseudorange measurement it makes, canceling a large percentage of the satellite-specific errors that otherwise produce a degraded position. The process is real-time: the base station measures what the signal is versus what it actually is, packages that difference into a data message, and the listening rover uses it to raise its own solution toward the sub-meter level.
How accurate is differential GPS? With the majority of common-mode errors thus removed, differential GPS typically places the receiver within one metre of its true coordinates when the base station is within a hundred kilometres or so. Corrections refine their accuracy by reducing the largest error sources - ionospheric delay, satellite ephemeris inaccuracies, and clock drift - leaving residual multipath and atmospheric delay as the dominant limits.
We used a support base placed on a known, fixed location on the coast. This installation computed the discrepancy between its exactly known position and the position given by the basic GPS communications it obtained. This distinction, known as the compensation parameter, was then transmitted to our rover on the survey ship. The rover used the corrections sent from the control base to its personal GPS computations. I noted how this real-time information flow permitted our gear to correct for miscalculations established by satellite time inexactitudes and atmospheric lags. Our reported location stayed systematically within a couple of centimeters of our true course, producing positional accuracy I had not thought feasible with basic GPS only.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What are the specifications of differential GPS?
The specifications of differential GPS are given in the table below.
| Attribute) | Details |
|---|---|
| Accuracy | 1-3 metres |
| Frequency Range | 283.5 kHz - 2.95 MHz |
| Bandwidth | 150 Hz, 250 Hz |
| L2 Frequency | 1227.6 MHz |
| Transmission Rate | 300 Bd |
| Signal Availability | 99.0% of the time |
| Correction Rate | Exceed 20 seconds for 1-sigma error under 1.6 metres (5.25 feet) |
| Average 1-sigma Horizontal Radial Position Error | 7 metres (22.97 feet) after 30s |
| Antenna Location | 0.89 m (2.92 ft) left, 10.01 m (32.84 ft) aft and 3.38 m (11.09 ft) up from IRU; 0.71 m (2.33 ft) right, 9.83 m (32.25 ft) aft and 3.10 m (10.17 ft) up from static pressure sensor |
| Integrity Monitoring Warning | Within 10 seconds |
| Corrections Quality | Falls with distance |
| Receiver Channels | 72 channels or more |
| Supported Constellations | GPS (NAVSTAR), GLONASS |
| RTK Capability | Yes |
| Post Processing Capability | Yes |
| Reference System | Located at known location |
DGPS can achieve accuracies of 1-3 metres (3.3-9.8 feet) depending on factors, and when real-time kinematic methods are added the same service can reach accuracies of 1-3 centimetres (0.4-1.2 inches), while the GDGPS system delivers sub-decimeter (<10 cm) (<3.9 inches) positioning accuracy anywhere in the world. Static operations routinely provide 5 mm + 0.5 ppm horizontal (0.197 in + 0.5 ppm) and 10 mm + 1.0 ppm vertical (0.394 in + 1.0 ppm) accuracy or higher. Kinematic surveys deliver 10 mm + 1.0 ppm horizontal (0.394 in + 1.0 ppm) and 20 mm + 1.0 ppm vertical (0.787 in + 1.0 ppm) accuracy or higher.
The correction signal is transmitted on 283.5 kHz - 2.95 MHz, and the U.S. Coast Guard previously ran DGPS on longwave radio frequencies between 285 kHz and 325 kHz. The data stream occupies about 150 Hz bandwidth, while modulation emission designators of 150 Hz and 250 Hz are also used. Stations normally run with 300 Bd baud, corrections are normally sent every few seconds, and the transmission rate must exceed 20 seconds for 1-sigma horizontal radial position error to stay under 1.6 metres (5.25 feet). Integrity monitoring warning is issued within ten seconds when performance degrades.
Field equipment typically pairs a NovAtel OEM4 dual frequency (L1/L2) receiver with a Sensor Systems model # S67-1575-76 L1/L2 antenna. Dual-frequency receivers can receive the encrypted L2 frequency at 1227.6 MHz intended for military use, while single-frequency units rely on L1. Modern GPS receivers have 72 channels or more, support GPS (NAVSTAR) and GLONASS constellations, are DGPS, WAAS and EGNOS enabled, and can switch automatically between stations because overlap of effective coverage areas is planned.
What are the components of differential GPS?
DGPS uses two GNSS receivers in different locations. A base station receiver is set up on a precisely known location and calculates its position based on satellite signals. It compares this location to the known location and computes the error. For each satellite, a correction signal is generated and transmitted to the GPS receiver.
The rover receiver uses this data to correct its position. Mobile GPS receivers receive GPS signals from satellites and decode them. Ground-based reference stations, known as fixed receivers or known points, broadcast correction data locally using ground-based transmitters of shorter range.
The monitoring station calculates the difference between the signals and transmits correction data. Dual-frequency receivers receive signals on two frequencies simultaneously, allowing the receiver to determine very precise positions. The control segment monitors and controls the satellite system continuously, predicts satellite ephemerides, and updates the navigation message periodically.
What are the types of differential GPS?
The types of differential GPS are listed below.
- Local Area DGPS (LADGPS): LADGPS, known as Conventional DGPS (CDGPS), operates within a small geographical area. It is most effective when user receivers are in close proximity to the RS, ensuring high correlation of error components.
- Wide Area DGPS (WADGPS): WADGPS extends the coverage area by using a network of reference stations. These stations continuously monitor GNSS signals, evaluate errors, and transmit corrections to users over a large region.
DGPS transmits data in frames categorized by Message Types. Signals are broadcast in two formats: MSK Minimum-Shift Keying and 200-baud MSK Minimum-Shift Keying. Beyond ground-based services, Satellite-Based Augmentation Systems like WAAS for northern American countries, EGNOS for Europe, GAGAN and MSAS constitute space-borne complements to DGPS. Surveying applications further split into two techniques - kinematic and static - while PPK offers differential corrections when processing GNSS after the observation, in which receivers are motionless on the Earth.
What is the difference between differential GPS and GPS?
The difference between differential GPS and GPS is that GPS uses one receiver and absolute GPS provides location information based on signals from satellites. DGPS is more accurate than GPS because a stationary unit computes the error and broadcasts a correction. GPS is less expensive than DGPS. Handheld GPS units are compact portable devices commonly used in field reconnaissance and navigation. Mapping grade GPS systems are typically more accurate than handheld GPS systems.
How does assisted GPS differ from differential GPS?
Assisted GPS differs from differential GPS in purpose and method. A-GPS uses the cellular network to obtain GPS satellite information, so it is faster in finding the location and helps with availability, yet it does not enhance accuracy. The server-assisted scheme involves a location server with satellite data. The cellular network sends the satellite almanac and ephemeris to the phone, the pseudorange data is sent back to the server, and the final position is processed at the server. Because A-GPS relies on external infrastructure, it is most beneficial in cities and is present in cellphones, cameras, and vehicles, but it is not a separate GPS system and does not use fixed base stations.
Differential GPS enhances accuracy. DGNSS uses base stations whose fixed, known positions let them compute real-time corrections for each satellite range measurement. These corrections are broadcast to nearby mobile receivers, so RTK GPS uses carrier-phase tracking to achieve centimetre-level precision. Thus, while A-GPS speeds the first fix through the cellular network, differential systems use ground-based infrastructure to refine the fix itself.
How to use differential gps?
Differential GPS is used by letting the base station find the difference between its known position and the GPS-derived position, then broadcasting RTCM corrections every few seconds. The rover receives these corrections from the base station and applies them to each measurement, so pseudorange errors are cancelled out. DGPS can be applied using either pseudorange corrections or coordinate corrections, and it works even with single-frequency receivers. After the rover applies the corrections, accuracy improves from 5-10 m (16.4-32.8 ft) to 1-3 m (3.3-9.8 ft). With precise implementations, DGPS achieves accuracy better than 1 cm (0.4 in). To enable the service, the GPS receiver must output NMEA-0813 GGA so the base can generate the correction signal that the rover uses.
Begin by installing a fixed base at a precisely surveyed point and the base antenna must be set to receive satellite signals continuously. Next, power the base receiver and allow it to lock onto the same spacecraft constellation the rover will use; it will compute the error between its known coordinates and the satellite-derived position. While the base steadies, place the rover unit on the survey platform and verify that its antenna can detect both satellites and the wireless correction stream. Once both receivers show a steady engagement, the base incessantly broadcasts rectification parameters over the chosen wireless channel and no post-processing is required. Pilot the rover along planned transects and record each seafloor position. The infrastructure guarantees that every data point is geo-referenced with centimeter-level exactness.
What is the difference between local area differential GPS and wide area differential GPS?
The difference between local area differential GPS and wide area differential GPS is that Local Area Differential GPS (LADGPS) is used when the baselines from a single base station to the roving receivers using the service are less than a couple of hundred kilometers, whereas Wide Area Differential GPS (WADGPS) uses a network of base stations and distributes corrections over a larger area that may even be continental in scope.
Local Area DGPS (LADGPS) is a form of differential GPS that provides GPS signal corrections within a relatively limited coverage area, usually within the vicinity of a single reference station. The method establishes ground reference stations and uses a single, fixed reference station that knows its exact position. These stations broadcast corrections locally to mobile receivers, which then apply the corrections to achieve positioning accuracy within one to three meters, sometimes even better. The coverage area for LADGPS typically ranges from 10 to several hundred kilometers, depending on the transmission power of the reference station and the geography of the area. LADGPS applications include surveying, navigation, and construction, where precise positioning is pivotal.
