How are GPS apps developed

History and technology of the Global Positioning System (GPS)

History and technology of the Global Positioning System (GPS)
Image: The development and construction of the GPS satellite system, from the starting shot in 1973 to full operational readiness in 1995, took over 20 years and was by no means a picture-perfect story. The budget of the program was cut several times and increased again, the number of the originally planned 24 satellites was cut down to 18 and then increased again after it became clear that the function of the system could otherwise not be guaranteed. Developed by the US Department of Defense, GPS was the first functional GNSS (Global Navigation Satellite System). Although there are now widespread civil applications of the tracking system, it is still used by the U.S. Air Force operated.

Over 20 years of GPS: From the start into the future

After initial system tests in 1974, the first-generation GPS satellites - the so-called Block I satellites - were sent into orbit around 20,000 kilometers above the ground from 1978 onwards. The first Block II satellite was installed in 1989 and the Block II satellites have now completely replaced the previous generation.

At the beginning of 2016, a total of 30 functioning satellites were in orbit. With a term of 7.5 years for older models and 12 years for the newer Block II satellites, the models are continuously being further developed and replaced piece by piece with more recent satellites. The next generation of GPS satellites, the Block III satellites, went into space in May 2017 after long delays.

Approval for civil use from 2000 onwards brings navigation boom

GPS Block III satellite
Graphics: GPS / Lockheed Martin A turning point in the history of GPS is considered to be the shooting down of the Korean Airlines Flight 007 liner, which was shot down by Soviet interceptors in 1983 after it went off course and flown into Soviet airspace. In response to this tragic incident, the then US President Reagan announced that the GPS system should also be released for civil use after its completion.

The so-called first operational readiness of GPS (IOC - Initial Operational Capability) did not take place until 1993, however. In that year the release for civil use was definitely decided. After the satellite constellation was completed with another Block II satellite in 1994, the US Department of Transportation (DoT) announced full operational capability (FOC) in 1995.

History and technology of the Global Positioning System (GPS)
Image: For the civil use of GPS, initially only an artificially deteriorated service (SA - Selective Availability) was provided, which allowed a position to be determined with an accuracy of only about 100 meters. Selective availability was only switched off in 2000. Since then, position determinations with an accuracy of around 10 meters and more reliable height determinations have also been possible for civil users. Switching off selective availability is generally seen as a prerequisite for the success of civil navigation systems.

Additional ground stations improve accuracy

The accuracy of GPS position determination can be further improved with extension systems. The systems called SBAS (Satellite Based Augmentation System) use ground stations with exactly known positions. This enables them to calculate correction signals for GPS, with which freely available services can also determine locations with an accuracy of about one to three meters. The globally available commercial service OmniSTAR even offers accuracies in the range of ten centimeters.

EGNOS satellite service
Graphic: ESA The correction signals relate to errors such as fluctuations in the orbits of the satellites or disturbances in the transit time of the signals in the ionosphere and are transmitted via geostationary satellites. There are various SBAS systems that are compatible with one another, such as WAAS (Wide Area Augmentation System) for America, EGNOS (European Geostationary Navigation Overlay Service) for Europe, MSAS (Multi-Functional Satellite Augmentation System) for Japan or GAGAN (GPS Aided Geo Augmented Navigation) for India. EGNOS works with around 30 ground stations and three geostationary satellites (Artemis and Inmarsat). In addition to aviation, the SBAS systems also support a more precise application for navigation in road traffic.

A-GPS: Faster GPS location through cellular networks

A-GPS (Assisted GPS) combines position determination by satellite with assistance information that is sent out via cellular networks. The individual GPS receiver is relieved with A-GPS: It only has to measure the arrival times of the satellite signals. A reference receiver connected to the A-GPS system is responsible for receiving further information such as error correction data. The advantage of A-GPS is, in addition to a much faster position determination (TTFF - Time To First Fix), the better localization option even in environments with poor GPS reception such as in cities, which has a positive effect on daily navigation with the smartphone.

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