Differential GPS (DGPS) is a technique that improves standard GPS accuracy from roughly 5 meters down to 1 to 3 meters by using a fixed reference station to calculate and broadcast error corrections to nearby GPS receivers. The core idea is simple: if you know the exact location of one GPS receiver, you can figure out how much error GPS is introducing at any given moment, then pass that correction along to other receivers in the area.
How Standard GPS Introduces Error
A standard GPS receiver determines your position by measuring the time it takes signals to travel from multiple satellites. But several factors distort those signals before they reach you. Charged particles in the upper atmosphere (the ionosphere) slow the signal down, adding delay. Weather conditions in the lower atmosphere do the same. The satellite’s own clock drifts slightly from the predicted time. The satellite’s actual orbital position differs from where the system predicts it should be. And signals can bounce off buildings, terrain, or other surfaces before reaching the receiver, a problem called multipath error.
Each of these errors is small on its own, but together they typically put a standard GPS reading within about 5 meters of your true position. For casual navigation, that’s fine. For surveying land, guiding a tractor, or navigating a ship through a harbor channel, it’s not.
How the Correction Process Works
DGPS starts with a reference station: a GPS receiver installed at a precisely surveyed location. Because the station already knows its exact coordinates, it can compare those coordinates to the position GPS calculates at any moment. The difference is the error.
There are two main approaches to packaging that error. The simpler method calculates the difference in latitude and longitude between the station’s known position and its GPS-derived position, then sends that offset to nearby receivers to apply directly. The more accurate method works at the individual satellite level, calculating the error in the measured distance (called a pseudorange) to each satellite in view. The nearby receiver then applies the appropriate correction for whichever satellites it’s using. This second method is more flexible because the mobile receiver doesn’t need to use the same satellites as the reference station.
The key insight behind DGPS is that most GPS errors are “common mode,” meaning they affect nearby receivers in nearly the same way. If the ionosphere is adding 3 meters of delay at the reference station, it’s adding roughly 3 meters of delay at your receiver 50 kilometers away. By subtracting the known error, you eliminate the largest sources of inaccuracy in one step.
What You Need to Run DGPS
A DGPS setup has two sides. The base station includes a GPS receiver, a GPS antenna (often mounted on a tripod with a choke ring to reduce signal reflections), a radio modem to broadcast corrections, a whip antenna for the radio link, batteries, and sometimes a laptop running base station software. The rover unit, which is the receiver in the field, carries its own GPS receiver and antenna, a radio modem to receive the corrections, a data collector or handheld computer, and its own battery pack. In professional setups, the rover gear fits into a backpack with an antenna pole.
The radio link between base and rover is the critical piece. Corrections need to arrive quickly because GPS errors change over time. If the correction is stale by even 30 seconds, its value starts to degrade. The industry standard format for these corrections is maintained by the Radio Technical Commission for Maritime Services (RTCM), with the current version being RTCM 10403.4. This protocol defines exactly how correction data is structured so that equipment from different manufacturers can work together.
Local Area vs. Wide Area DGPS
Traditional DGPS uses a single reference station covering an area of several dozen to several hundred square kilometers. This is called Local Area Differential GPS (LADGPS). Accuracy degrades as you move farther from the station because atmospheric conditions become less similar to those at the reference point.
Wide Area Differential GPS (WADGPS) solves this by using a network of reference stations spread across a large region. Instead of sending a single correction, the network calculates separate corrections for satellite orbits, satellite clocks, and ionospheric conditions, then combines them into a correction model valid across a much broader area. Systems like WAAS (Wide Area Augmentation System) in North America and EGNOS (European Geostationary Navigation Overlay Service) in Europe take this a step further by broadcasting corrections from geostationary satellites, so any receiver with a clear view of the sky can apply them without a dedicated radio link. These satellite-based systems improve accuracy to around 1 to 2 meters.
DGPS vs. RTK: When Meters Aren’t Enough
DGPS typically delivers 1 to 3 meter accuracy. For applications that need centimeter-level precision, there’s a related but fundamentally different technique called Real-Time Kinematic (RTK) positioning. The distinction comes down to what part of the GPS signal each system measures.
DGPS works with the “code” embedded in the GPS signal, a relatively coarse measurement. RTK works with the carrier wave itself, which has a much higher frequency and therefore a much finer resolution. By counting the exact number of wave cycles between a satellite and the receiver, and resolving the fractional part of the last cycle (a step called integer ambiguity resolution), RTK can pin down distances with millimeter-level precision. The tradeoff is range: RTK corrections are only valid within about 30 kilometers of the base station, compared to the broader coverage of code-based DGPS.
In practice, DGPS and RTK sit on a spectrum. DGPS is simpler, works over longer distances, and is good enough for navigation and mapping. RTK is more complex and range-limited but essential for land surveying, construction grading, and machine control where centimeters matter.
Where DGPS Gets Used
Precision agriculture was one of the earliest and most widespread applications. Farmers use corrected GPS signals for field mapping, soil sampling, tractor guidance, variable rate application of fertilizer and seed, and yield mapping. With sub-meter corrections, an auto-steer system can drive parallel passes across a field with minimal overlap, saving fuel, chemicals, and time. More advanced operations use RTK for tasks like strip-till planting where rows need to line up year after year.
Maritime navigation was the original driver behind government DGPS services. Harbors, shipping channels, and coastal waters demand better accuracy than standalone GPS provides. The U.S. Coast Guard operated a network of maritime DGPS broadcast stations along coastlines and major waterways for decades, transmitting corrections on marine radio beacon frequencies.
Surveying, GIS data collection, construction, and fleet tracking all rely on some form of differential correction. Even consumer-grade GPS receivers have benefited indirectly, as satellite-based augmentation systems like WAAS deliver free differential corrections to any compatible device.
The Decline of Traditional DGPS Networks
The ground-based DGPS infrastructure that governments built in the 1990s and 2000s has been steadily winding down. The U.S. Department of Transportation shut down 37 Nationwide DGPS sites in August 2016, including both Coast Guard and inland DOT stations. In March 2018, the Coast Guard announced discontinuation of its remaining 38 maritime DGPS sites, with closures phased between September 2018 and September 2020.
The reason is straightforward: GPS itself has gotten better. The removal of Selective Availability in 2000 (an intentional signal degradation the Department of Defense once applied for security reasons) immediately improved civilian GPS accuracy. Newer GPS satellites broadcast on additional frequencies, and modernized receiver designs handle multipath and noise more effectively. Meanwhile, satellite-based augmentation systems deliver corrections without the cost of maintaining hundreds of ground-based radio transmitters. For users who need centimeter accuracy, RTK networks using cellular data links have largely replaced traditional DGPS infrastructure.