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With navigation satellite constellations rapidly becoming a must-have for any major world power, China has joined Europe and Russia as an aspirant to match US sat nav technology. The People's Republic has announced that it will upgrade its localised Beidou satellite system to global coverage by 2015.

The Chinese state Xinhua news service reports that Zhang Xiaojin of the state-owned China Aerospace Science and Technology Corporation revealed the plans on Chinese television yesterday.

The only worldwide sat nav constellation currently in service is the American Global Positioning Service (GPS), funded by the US military. The unencrypted, free-to-use GPS civil signal is used by the vast majority of satnav kit worldwide. The former "selective availability" option, in which the accuracy of the civil GPS signal could be degraded*, has now been discontinued. However, America retains the option of denying GPS service at places and times of its own choosing.

Many people worldwide don't care to be reliant on the civil GPS signal, and can't get access to the encrypted military signal (though this latter option is open to American allies). One application of sat nav often seen as strategically critical is the guidance of intercontinental ballistic missiles (ICBMs). A modern ICBM requires no assistance to strike a city-sized target with a nuclear warhead, but if a more difficult task such as knocking out a hardened enemy launch silo is contemplated then sat nav assistance is necessary.

Quite apart from ICBMs, modern military operations are highly dependent on satnav for hosts of other purposes, with applications now descending to the level of infantry squads or even individual footsoldiers.

Thus the French military in particular are very keen to see the European Galileo system deployed, offering some independence from the use of American GPS. Unlike the UK, France makes its own ICBMs but it has no independent sat nav. Similarly, Russia is currently upgrading its GLONASS constellation, which fell into disrepair after the collapse of the Soviet Union.

Quite apart from military applications, sat nav is now becoming more and more widespread in civil life, and it is argued by some that a loss of service - if the current pace of adoption continues - could soon be economically devastating to developed countries. GPS is particularly significant to the aviation and shipping industries at present, and will be more so as the US moves to GPS-based air traffic control, but ordinary groundbased applications are also growing in popularity.

In Europe, the development of the Galileo system is also seen as a chance to develop high-tech space industry and related user electronics.

All this has plainly not been lost on the overlords of modern China. It now appears that the existing five Beidou satellites, with orbits chosen to provide best coverage mainly over the People's Republic, will be joined by a world-girdling fleet of 30.

The sat nav users of the future will be spoilt for choice, it appears. With more than 100 spacecraft from the GPS, Galileo, GLONASS and Beidou systems in orbit, coverage and accuracy - at least for devices able to use more than one system - should be excellent. ®

*Though there were effective workarounds such as "Differential GPS", in which a fixed ground station - knowing its own location accurately - could work out the GPS error and transmit corrections to a suitably equipped mobile unit in near-real time.

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