The errors in determining the distances to GPS satellites are approximately the same for a small, limited area. Therefore, with the help of a reference GPS receiver located at a point with previously known coordinates, it is possible to determine the so-called differential corrections to the absolute range. And with their help, to clarify the position determined by any GPS-receiver operating in the area.
Additional means of increasing the accuracy of the GPS receiver, satellite differential subsystems WAAS, EGNOS, MSAS.
This technology is called differential GPS, or DGPS. Of course, the closer the receiver is to the reference location, the higher the accuracy, so the range of such a system is limited. Corrections are transmitted in a special format RTCM SC-104 (Radio Technical Commission for Maritime Services). In the simplest case, a radio modem is used to transmit the correction.
Depending on the transmission methods, various networks of DGPS stations are created, both free and commercial, or corporate. The most widespread is the free DGPS maritime network, transmitting corrections by radio at frequencies of 285–325 kHz. Originally created by the US and Canada Coast Guard on the basis of former radio beacons, it is deployed along the coasts and inland waterways of many countries..
GPS differential system structure.
When talking about ordinary DGPS receivers, they mean exactly this system. With its help, accuracy increases to 5 meters. In addition, the differential correction will tell you about emergency deviations of the GPS receiver signal, providing control of navigation continuity and increasing its reliability. The frequency of the nearest differential station is entered manually or automatically searched. Differential correction receivers compatible with most consumer GPS navigators can be found on the websites of many manufacturers..
Commercial differential systems exist, in particular, in some European countries. In many cases, the cost of services depends on the level of accuracy ordered. Most often, the correction is transmitted in the VHF range, the signals are encoded, the equipment for their reception and subscription with decryption keys are supplied by the owners of the systems. Some systems use satellite transmission. Corporate differential systems are created to provide any work, such as surveying, as well as sports competitions, accurate speed measurements, and so on..
The cost of an OEM differential correction board along with a GPS board, for example, from Motorola does not exceed several hundred dollars. For data transmission, you can use any modem connection, radio modem, wired, radio, cellular modem, and so on. In any case, the limitations of DGPS remain a limited range and the need for a separate GPS receiver for corrections.
Wide Range Satellite Differential Subsystems WAAS, EGNOS, MSAS.
More modern, working since 2003, the American WAAS (Wide Area Augmentation System) and its analogues use a slightly different method of calculating corrections. GPS signal deviation data is not collected by a single differential station for a small area, but by a whole network of ground stations in the United States – 25 stations, and transmitted to two main stations where they are processed and corrections are calculated for the entire coverage area.
The received data is transmitted to consumers through one or two geostationary satellites. The signal frequency is the same as GPS, so a separate receiver is not needed to receive the WAAS correction. Work with WAAS supports most modern models of GPS-receivers, even portable ones. WAAS is a free system, designed to improve the accuracy and reliability of civilian aircraft approaches (precise approaches), but is available to all civilian users. High location accuracy – up to 3 meters in horizontal and vertical directions, using WAAS is provided for 95% of the time.
There are two limitations to using WAAS. Unlike GPS satellites, signals from geostationary satellites, especially at low elevation angles, can often be obscured by local objects and not received throughout the coverage area. Therefore, first of all, this is a system for open areas, for aviation and the sea. On the other hand, WAAS signals can also be received outside the coverage area for which corrections are actually calculated. And one cannot count on improving accuracy in such an area. The creators of the system plan to expand the coverage area in Alaska and Mexico and create on its basis the Global Navigation Satellite Landing System (GLS).
Coverage areas of satellite differential subsystems WAAS, EGNOS, MSAS.
At present, the European analogue of WAAS is being deployed, the EGNOS (European Geostationary Navigation Overlay Service) system is a joint project of the European Space Agency, the European Commission and Eurocontrol, an organization for air traffic safety. The European system uses the same principles and signals as WAAS, so that owners of a GPS receiver capable of receiving and decoding WAAS signals will be able to obtain an accuracy of about 5 meters in its area of operation and significantly improve navigation reliability.
In addition to GPS, the system will transmit corrections to GLONASS signals. The system consists of three geostationary satellites, a network of 34 ground stations (RIMS-Ranging and Integrity Monitoring Station), 6 transmitting stations and 4 control centers (MCC-Master Control Center). The cost of the system is about 300 million euros.
Other ways to improve GPS accuracy.
In conclusion, we briefly mention other technologies used to improve the accuracy and control of the continuity of civilian GPS navigation, mainly in professional applications. In addition to the mentioned dual-frequency civil receivers and GPS-GLONASS combined receivers, the following can be mentioned.
Post-processing or Post-processing.
Sometimes jokingly called “Poor Man DGPS” (“DGPS for the Poor”), it’s actually an effective, high-precision technology widely used in surveying. A pair of special GPS receivers is used, which have the ability to store measured pseudorange in memory. After making measurements on the ground, the data of both receivers for each moment of time are processed by special software and based on the data of one, differential corrections for the GPS receiver are calculated. This allows you to achieve high relative accuracy, up to centimeters.
Frequency-phase measurements (Carrier-phase tracking).
In addition to determining the range to satellites using a code, the carrier signal itself can be used to refine measurements. Measurements of the phase shift of the carrier signal are used for high-precision (order of centimeters) positioning, and the measurement of Doppler frequency shift – to clarify the speed and direction. The technology for measuring the phase difference for two or three antennas is used to determine the direction in publicly accessible marine “satellite compasses”.
RAIM-Receiver Autonomous Integrity Monitoring.
Receiver with autonomous navigation continuity control. This technology does not increase the accuracy of navigation, but only allows the GPS receiver to detect an invalid GPS satellite signal using its own means. For this, the receiver uses an excess of satellites in excess of the required four and calculates the average position according to the data of different constellations of satellites.
Satellites whose data differ markedly from average values are ignored. Almost all modern stationary aviation GPS receivers have this capability, including for small aircraft. Excess satellites can also be used to further correct atmospheric refraction in single-frequency GPS receivers..
Based on materials from the book All About GPS Navigators.
Naiman V.S., Samoilov A.E., Ilyin N.R., Sheinis A.I..