How satellites took over

In the second instalment of our two-part deep dive into the emergence of electronic navigation, satellite technology begins to emerge in the 1950s. It would soon have a major impact on marine navigation, initially for naval use and later for merchant shipping.

The satellite navigation concept

In October 1957, two American physicists observed the Doppler shift of radio signals from the USSR’s Sputnik, the world’s first satellite. They realised they could use those signals to determine its orbit.

In March of the following year, the chairman of the Johns Hopkins Applied Physics Laboratory (APL) Research Centre suggested that if a satellite's position was known and predictable, its Doppler shift could also be used to locate a receiver on Earth. He proposed a satellite system to implement this principle. The Doppler effect caused an apparent compression of the carrier's wavelength as the satellite approached the receiver, and a stretching as the satellite receded.

Transit

The Transit system, also known as NAVSAT or NNSS (Navy Navigation Satellite System), was the first to use the principle. It was developed for the US Navy to provide accurate location information for its Polaris ballistic missile submarines, so that their rockets would start from a precisely known position. As the submarines used an inertial navigation system – involving gyros and accelerometers measuring continuous movement – they suffered errors over time caused by bias in the devices. By using Transit, the submarine could regularly update its inertial generated position.

The Transit 1A satellite was launched in September 1959 but failed to reach orbit. A second, Transit 1B, was successfully launched in April 1960, with the first successful tests of the system made that year. The system entered naval service in 1964 and would later become available for civilian use.

The satellites were placed in low polar orbits, at an altitude of about 600 miles. With a speed of approximately 17,000 mph, the orbital period – the time it took to orbit Earth – was about one hour and 46 minutes.

Five satellites were required to provide reasonable global coverage. While the system was operational, at least 10 satellites – one spare for each satellite in the basic constellation – were usually kept in orbit.

A Transit satnav receiver (Credit: Ebay)

Position fixing

A network of ground stations, whose locations were accurately known, continually tracked the Transit satellites and measured their Doppler shift to fix their location. The satellite’s Doppler shift could then be used to fix the location of any other object on land or sea.

Since only one satellite was usually visible at any given time, fixes could be made only when it was above the horizon. At the equator, this delay between fixes was several hours; at mid-latitudes, the delay decreased to an hour or two. This was viewed as acceptable as an updating system for ballistic missile launch, as the submarines needed the periodic fixes to reset their inertial system. With later improvements, the system provided single-pass accuracy of roughly 200m.

One of the new capability-improved GPS Block III satellites (Credit: EGNOS)

GPS

The Global Positioning System (GPS) project was started by the US in 1973 to overcome the deficiencies of the previous system, by providing a continuous position fix. Developed by the US Department of Defence for use by the United States military, the first experimental GPS satellite was launched in 1978. Civilian use was allowed from the 1980s, and the system became fully operational in 1995.

Initially, the highest-quality signal the system generated was reserved for military use only. The signal available for civilian use was intentionally degraded, in a policy known as Selective Availability. This was changed on 1 May 2000, when President Clinton signed a policy to turn off Selective Availability to provide the same accuracy to civilians as the military had. The change was made because of the widespread use of differential GPS services by private industry to improve civilian accuracy. The US military was also developing technologies to deny GPS service to potential adversaries on a regional basis.

The satellites operate at an altitude of approximately 12,550 miles, and each circles the Earth twice a day. In total, 31 satellites make up the operational group, with a further nine in reserve, though only 24 are required to provide the minimum acceptable coverage. The system has become a universal navigation system on land, sea and air, although other systems are in operational use including the European Union’s Galileo and Russia’s GLONASS.

John Barnes is a journalist and author and former editor of Marine Engineers Review.