The Global Positioning System (GPS) was first tested in the USA in the 1960s, and was then developed as part of the Cold War defence strategy. By the year 2000, it ceased to have solely military importance, and was made available for civil use.
There are now a total of 32 satellites in use, all in carefully controlled orbits at an altitude of about 20,000 km, and an observer anywhere on earth can see nine of them. A satellite weighs about a tonne, is solar-powered, and orbits the earth in about 12 hours. Each satellite repeatedly sends a radio signal giving the satellite's position (“ephemeris”) and the time at which it sent the message; both position and time are known with great accuracy.
The GPS receiver built into every Smartphone receives the radio signals transmitted by the satellites, and calculates the distance between the receiver and the satellites, using the delay between the time the message was sent and the time of its reception. Radio waves travel at the speed of light (1 km in about 3 microseconds) and it is a simple calculation, but it must be carried out on each satellite.
If the distance from three satellites is known, then the position of the receiver can be found by a process of “3D triangulation”. In practice, more than three satellites must be used, as additional data is needed to get the best possible accuracy.
For the meridian experiment, the GPS device used was the one fitted in a Samsung Galaxy S2 phone. A free App called “GPS Test” was downloaded (doubtless there are other Apps).
The App lets you use the GPS in the most basic way - it shows your current position as numbers on the screen (see illustration), together with other numbers - height above sea-level (ASL), the speed at which you are walking, number of satellites in view, current time, etc. To get a good answer, you should be out-of-doors with no tree canopy and a large number of satellites in view. The receiver is often slow to get a “first fix”, after which it responds quickly.
The Meridian updated
In 1884, an International Meridian Conference was held in Washington to establish a prime meridian for navigation. It selected the Greenwich Meridian devised by George Airy, the Astronomer Royal, as it was already widely used.
In 1984, a new meridian was established which was better suited to the GPS satellite system. It resulted in WGS84 (World Geodetic System 1984) as the common standard for GPS measurements. It does not differ greatly from the old meridian (it is only a few metres away), and is the default option on all GPS devices.
The first satellite was launched about 40 years ago, and the system has been constantly updated; the latest system ("IIIa") will be phased in from 2017. The original GPS receivers weighed over 23kg, but the latest types reside in a single silicon chip inside a mobile phone.
There are a number of competing systems, such as the European "Galileo" system, working but incomplete; in 2016 it had 18 out of the planned 30 satellites in orbit, and the system is expected to be complete by 2020. The first satellite was launched in 2005. Early funding problems have been overcome, and many nations now have a stake in the project.