GPS satellites

In past centuries and up to the 1990s, optical instruments such as the sextant were used to measure the angular positions of stars from which an observer’s geodetic position could be calculated. With the success of achieving accurate locational fixes by the Global Positioning System (GPS), optical instruments, being very dependent on the skills of the observer and with subsequent data reduction procedures, are now superceded. Small hand-held GPS detector systems are readily available which are capable of determining an operator’s position to within a few metres.
The GPS system involves some 32 artificial satellites, each carrying a code and an accurate clock. Their transmissions provide the code with the embedded time accurate to a microsecond. The onboard clocks are regularly updated and corrected from a ground control station, to an accuracy that includes effects involving the principles of general relativity.
The receiver picks up the transmitted signals at the ground and decodes them. After comparing the apparent times provided by each clock, the time differences allow the various satellite–observer distances to be determined. With the detection of four satellites, the information is sufficient to
determine the four unknowns of latitude, longitude, height and the time. The observer’s coordinates are referred to maps based on the World Geodetic Survey (WGS84), the new terrestrial frame adopted in 1987 as reference for the broadcast orbits. The WGS84 frame takes the Earth’s equatorial radius as 6378·137 km with a value of  of 1/298·257 223 563.
Geodetic positions based on older standard maps are related to the regional geodetic datum represented by the spheroid that approximates to the geoid of the locality. Data for the reduction of GPS geodetic coordinates to the local conventional systems are continually undergoing revision. Care, therefore, must be taken in comparing the latitudes and longitudes provide by GPS relative to printed maps as a result of the differences between the reference systems. In the UK, the systematic difference between the Ordinance Survey positions and a GPS value can have an apparent location shift
of about 140 m.