Differential GNSS

Did You Know?

• The US Coast Guard's NDGPS was jointly administered by the Coast Guard and the Army Corps of Engineers.
• During the Gulf War of 1990–1991, Selective Availability was temporarily turned off because Allied troops were using commercial GPS receivers.
• The Canadian Coast Guard discontinued its DGPS system on December 15, 2022.

History

When GPS first entered service, the US military worried that adversaries might use the globally available signals for weapon guidance. The coarse acquisition (C/A) signal was originally expected to yield only about 100 m accuracy, but improved receiver designs achieved 20 to 30 m. In March 1990, to counter this unexpected precision, the military deliberately degraded the C/A signal by randomly offsetting its clock, equivalent to about 100 m of distance—a technique called Selective Availability (SA). Dual-frequency receivers that also accessed the encrypted L2 frequency could obtain more accurate guidance, but only authorized users held the decryption keys. Civilian users relied on ground-based radio navigation systems like LORAN, VOR, and NDB, which cost millions annually to maintain. A GNSS could offer far better performance at lower cost, but SA made that impractical. The FAA, USCG, and DOT repeatedly asked the military to disable SA for civilian use, but security concerns kept it in place. During the early to mid 1980s, several agencies worked on a solution. Because SA changed slowly, its offset was relatively fixed over a wide area—for instance, a 100-meter offset to the east would hold true regionally. Broadcasting that offset to local receivers could cancel SA, yielding around 15 m accuracy. Additionally, correcting for ionospheric transmission delays could improve accuracy to about 5 m, sufficient for most civilian needs.

Receivers

A user obtains a DGPS correction data stream, often via a separate radio receiver, from a nearby station and merges it with GPS observations. Users may be up to 200 nautical miles (370 km) from the station. In the simplest real-time setup, correction data is applied to measurements as they are taken, commonly used for navigating moving vehicles like aircraft and ships. If the corrections are of the pseudorange type, the receiver adjusts apparent pseudoranges before calculating position, requiring special receiver support. If the corrections are of the position type, the receiver applies them after calculating position normally. Real-time kinematic is a more advanced method that uses carrier wave phase measurements from both station and receiver for higher-resolution pseudorange estimates, typically employed in surveying.

Networks

Differential GNSS (DGNSS) correction data reaches receivers through ground-based radio stations (known as Ground-Based Augmentation Systems, or GBAS), satellites, or the internet. Airports often use short-range VHF radio services called GBAS or LAAS to guide approaching aircraft. The US Coast Guard reported that 47 countries once ran networks similar to the US Nationwide DGPS (NDGPS). In the UK and Ireland, the system was introduced as a maritime navigation aid after the Decca Navigator System shut down in 2000. The General Lighthouse Authorities—Trinity House, the Northern Lighthouse Board, and the Commissioners of Irish Lights—established a network of 12 transmitters around the coastline and three control stations in 1998. Operating on the 300-kHz band, it underwent testing, had two more transmitters added, and was declared operational in 2002; it was eventually decommissioned in March 2022. In the United States, the Department of Transportation, along with the Federal Highway Administration, the Federal Railroad Administration, and the National Geodetic Survey, designated the US Coast Guard as the agency responsible for maintaining the NDGPS. This system expanded on the earlier Maritime DGPS (MDGPS), which the Coast Guard began developing in the late 1980s and completed in March 1999. MDGPS only covered coastal waters, the Great Lakes, and the Mississippi River inland waterways, whereas NDGPS provides full coverage of the continental US. The USCG Navigation Center in Alexandria, Virginia, serves as the centralized command and control hub, and the network currently consists of 85 sites managed by the US Department of Homeland Security Navigation Center.

Related methods

Differential GPS post-processing improves the accuracy of unknown locations by linking them to known survey markers. Receivers store measurement data, which is later transferred to a computer running specialized software. This software calculates baselines—three-dimensional lines between antenna positions—using simultaneous data from at least two receivers. Because most GPS errors affect all receivers nearly the same way, these errors cancel out during calculations, yielding more precise positions. However, multiple receivers are not always required. In the 1990s, when handheld units were costly, quasi-differential methods emerged. These used a single receiver moved quickly through a series of positions or loops of three to ten survey points to achieve similar improvements.