Differential Global Positioning System

DGPS reference antenna ( choke ring antenna )

Differential GPS ( DGPS ; German "Differential Global Positioning System" ) refers to methods by the broadcasting of correction data (path and time system) the accuracy of GNSS - navigation can increase. The name is derived from the American GPS , although signals from other satellite navigation systems are now also included in the process.

Working principle

Positioning using GNSS is based on differences in transit time between the signals from several satellites to a receiver, see hyperbola navigation .

It is assumed that the distance differences are proportional to the time differences.

In practice, inaccuracies arise because the signal speed in the troposphere and ionosphere varies slightly in time and space, see GNSS . In addition, the orbits and clock errors of the satellites are not exactly known to the receiver. The influences of these effects are the same for neighboring recipients and can be eliminated using suitable evaluation methods.

As the distance between the receivers increases, the correction becomes less accurate. Larger distances can be bridged by interpolating between several reference stations . In addition, the procedures differ according to whether positions or pseudoranges are corrected for individual satellites.

Reference station

Mobile reference station Baseline HD from Claas for use in satellite-based steering systems in agriculture

In the DGPS, stationary GNSS antennas, so-called reference stations , are used, the exact position of which was determined by classical surveying methods. The actual transit times of the signals for each satellite can be determined very precisely from the deviation between the actual and the received position. The differences between the theoretical and the actual signal propagation times are transmitted to the DGPS receivers in the vicinity.

DGPS receiver

The DGPS receivers correct their position with these correction signals, which means that the position of the receiver can be calculated much more precisely. The receiving antenna required for the correction signals is often integrated into the GNSS antenna. If the (radio) connection to the DGPS transmitter system fails, the receiver switches to the uncorrected GNSS mode with normal accuracy.

How much the accuracy can be increased depends mainly on the distance of the DGPS receiver from the reference station. The achievable accuracy , depending on the quality of the receiver and the correction data, is between 0.3 m and 2.5 m for the position ( x , y ) and between 0.2 m and 5 m for the height. High-quality systems also evaluate the phase shift of the carrier wave (as is common with geodetic receivers, for example ) and thus achieve accuracies of a few millimeters (± 1 mm to ± 10 mm per km of distance to the reference system).

Offline method (post processing)

The measurement data (the received satellite signals) can either be recorded for a subsequent evaluation of all measurement points ( offline ) or the position corrections of the base station can be transmitted online to all rovers. The former method was mainly used in the early days of GPS, but is still used in some cases for precise surveying networks today .

The calculation is carried out by means of spatial network balancing , which is based either on the signal transit times or on their phase measurement . If accuracies in the decimeter to meter range are sufficient, it is also sufficient to adjust the positions calculated directly by the receivers.

For extensive survey networks, it may be necessary to divide them into overlapping sections, the so-called sessions . With the existing receivers, some of the points and one to three reference points are measured at the same time; by means of the latter, the entire network can be uniformly balanced a posteriori. It is also possible to subsequently “ connect ” individual power supplies.

In the first decade of GPS, when the receivers were still very expensive, methods to increase accuracy with only one receiver were developed ("single receiver methods"), including the qGPS ( quasi-difference GPS) of the Vienna University of Technology, which carries out the individual measurement points repeated visits to a centrally located reference point stiffened against each other.

The Garmin GPS II receiver (1995) proved to be particularly suitable for this purpose , with which, despite data obfuscation ( selective availability ), the accuracy could be improved from about ± 50 m to a few meters.

The repeated measurements on the reference point or node (called "reoccupation") enabled not only a more precise network through suitable adjustment , but also the adjustment of any temporal trends in the determined GPS coordinates of the measuring points.

Online methods (correction signals)

In general, however, the correction data from the reference station (s) are broadcast directly to all recipients or - in the case of regional permanent stations - also distributed via the Internet.

Each rover can immediately increase its positioning accuracy by transmitting the correction data from a base station by telephone or radio. A finer correction can also be made afterwards, when the rover and base station record all the data for determining the position (post-processing).

The correction data can be generated by the user himself if a second GPS receiver is available. In order to be able to do without second devices, however, many countries have set up permanent reference stations that are operated by user groups or the official national surveying (e.g. the SAPOS network of the German federal states). This means that highly precise position determinations are possible with just one receiver, and with the appropriate hardware even practically in real time .

For Germany was SAPOS HEPS (High precision real time positioning service) was developed. It offers a positional accuracy of approx. 1–2 cm and a height accuracy of approx. 2–3 cm.

For measurements in the SAPOS system you need rover equipment with a geodetic, RTK- capable GNSS receiver, as well as a modem / cell phone to receive the SAPOS data. You can move from point to point (while maintaining satellite contact and mobile phone connection) without having to re-initialize the receiver each time. This enables flexible work and you immediately get the coordinate of a point in the ETRS coordinate system. 5–20 seconds are sufficient as observation time per point.
Advantage: cost-effectiveness due to low expenditure of time and personnel. Coordinates directly available without post-processing in-house. No dependence on time of day or weather.
Disadvantage: Coordinate determination in a precise position only through coordinate transformation.

In other countries, similar data services have been set up, which are either officially operated by surveying services or by utility companies. In Austria there are v. a. Power plant operators and the dGPS of the engineering offices, in Switzerland the swipos of the country's topography , in Germany alongside Sapos providers such as ALF, AMDS or ascos.
With the pseudorange correction method, the base station calculates the errors in the routes to the satellites and transmits them to the rover. Correction is also possible if different satellites are being received by the base station and the rover. Accuracies <1 m are possible.
In the case of very precise measurements, the phase position of the satellite signals is also evaluated. This enables accuracies of 1–10 mm per km to the base station to be achieved.
Lower accuracy is sufficient at sea, but radio broadcasting is useful. For the Federal Republic of Germany, differential stations are operated by the Federal Waterways and Shipping Administration (WSV). They work according to the international IALA standard and send out correction data on medium wave for coastal and inland areas. The central technical authority is the WSV's specialist office for traffic technology in Koblenz.

Dissemination of the correction signals

Wireless

The correction signals are usually transmitted to the DGPS receiver via radio or, for special applications, via other data transmission paths. Since the runtime differences of the individual GNSS signals change only slowly, this transmission is not time-critical. For simple DGPS correction, a correction every three seconds is sufficient; for high-precision DGPS correction, much higher rates in the range of 0.1 seconds are necessary.

In addition to the regional DGPS correction data, which are each derived and transmitted by a single reference station, there is also correction data for large areas that are distributed via geostationary satellites ( SBAS ). These data are derived from the received data in a network of reference stations and transmitted via geostationary satellites for continent-wide distribution. In Europe this satellite-based DGPS system is called EGNOS , in America WAAS , in Japan MSAS and in India GAGAN .

Ground-based systems ( GBAS ) are operated in the Federal Republic of Germany as part of the satellite positioning service of the German national survey (SAPOS). Another operator of reference stations is the Federal Waterways and Shipping Administration (WSV). Its stations work in accordance with the international standard of the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) and transmit correction data on medium wave for coastal and inland areas. The central technical authority is the WSV's specialist office for traffic technology in Koblenz.

The following reference stations can or could be used in Germany:

simple correction transmitters (every three seconds)
- Out of service since January 8, 2012: Lake Constance transmitter ( medium wave : transmission frequency 666 kHz, transmission was carried out according to the AMDS process parallel to the radio program).

Internet

Like any other real-time data format, DGPS data can be made available over the Internet using a wide variety of methods. The NTRIP protocol offers a method standardized by RTCM for the transmission of DGPS and other navigation data. Since navigation mostly takes place outside of wired Internet connections, what is meant here is distribution via WLAN and, in particular, mobile communications .

More information on reliability

Systems based on radio are inherently unsafe and can fail due to radio interference. In addition to natural radio problems, GNSS also offers the possibility of unannounced encryption or deliberate errors in the GNSS signals by the respective operator, as was the case with the American GPS system for a long time. In order to enable safety-relevant applications such as the navigation of aircraft, in addition to the correction signals, signals about the current GNSS quality can also be transmitted from the reference station to the DGPS receiver. If the DGPS receiver can receive the GNSS signals, the correction signal from the reference station and also the information contained therein that the GNSS satellites are sending unadulterated signals, it can assume a reliable position measurement. If one of these conditions is not met, the position data must not be used for safety-relevant applications; in this case, for example, a pilot has to switch off GNSS-based navigation systems and automatic take-off and landing systems and replace them with other methods.

application

The method is used, for example, in the military, in shipping, in precision agriculture and on large areas for surveying in geodesy . Many GNSS receivers for the end user have already implemented DGPS in the lowest price range.

aviation

Since 1998, route navigation in air traffic using GPS has been permitted in Germany. To do this, the approved devices must carry out a reliability test of the data ( RAIM ), which requires reception from at least five satellites. Modern receivers outside of aviation today have 12 or more reception channels.

Non-precision approaches can be completed using GNSS as the only navigation method. However, only the horizontal information may be used; for a precision landing, the height measurement is not accurate enough for guidance on the glide path . Ground-based navigation aids are not required for this.

With the help of the SBAS called EGNOS, precision approaches up to Category I should be possible, with a signal refresh every 500 ms. In the case of higher requirements or difficult reception conditions, ground-based systems such as SAPOS should be used. The high-precision real-time positioning service ( HEPS ) of this system , which is subject to a charge, enables a horizontal position accuracy of 1 to 2 cm and a height accuracy of 2 to 3 cm.

Angle measurement

GNSS-based methods for angle measurement are based on the same signals as location determination, but use a completely different measuring principle: Two antennas are mounted on an antenna carrier at a defined distance, and their received signals are compared with one another. No reference signal is required from a stationary reference system, but the angle of the antenna connection axis to the satellite and, with the ephemeris of the satellite, the angle to the north can be determined directly from the antenna distance and phase shift . Measurement accuracies of 0.01 ° to 0.1 ° can be achieved.

Devices of this type are sometimes referred to as an electronic compass or GPS compass. They are relatively expensive and not very widespread due to their higher equipment costs and small numbers. They are used, for example, on ships or construction machinery.

Web links

Commons : Differential Global Positioning System - collection of images, videos and audio files

Department of the Waterways and Shipping Administration for traffic engineering Technical information on GPS / DGPS in shipping
u-blox .de: Essentials of Satellite Navigation (PDF; 6.0 MB)
DGPS @ home. Archived from the original on March 7, 2017 ; accessed on March 7, 2017 (project idea that wants to collect and distribute DGPS data via crowd-sourcing, using consumer technology).

Individual evidence

↑ Martin Asbeck, Stefan Drüppel, Klaus Skindelies, Markus Stein: Surveying and Geoinformation . Specialist book for surveyors and geomatists. Ed .: Michael Gärtner. Gärtner, Solingen 2012, ISBN 978-3-00-038273-4 , p. 117 .
↑ Wolfgang Augath, Wolfgang Lechner, Stefan Baumann: LORAN-C / EUROFIX / EGNOS integration. (Pdf, 249 kB) tu-dresden.de, accessed on August 9, 2015 .

[1] Martin Asbeck, Stefan Drüppel, Klaus Skindelies, Markus Stein: Surveying and Geoinformation . Specialist book for surveyors and geomatists. Ed .: Michael Gärtner. Gärtner, Solingen 2012, ISBN 978-3-00-038273-4 , p. 117 .

[2] Wolfgang Augath, Wolfgang Lechner, Stefan Baumann: LORAN-C / EUROFIX / EGNOS integration. (Pdf, 249 kB) tu-dresden.de, accessed on August 9, 2015 .