China: Researchers at the National Astronomical Observatories, part of the Chinese Academy of Sciences, have formulated a cheaper way for developing a navigation system. They propose to transmit the highly accurate time signals from the ground and simply route them through geostationary satellites that are already in orbit. Receivers can then triangulate their position in the same way. Researchers Hai-Fu Ji claims that this is cheaper since it uses existing communications infrastructure
China tested this idea in 2005 using communications satellites to provide positioning signals for several Chinese cities.
There is a problem, however. Geostationary satellites sit exactly in the equatorial plane so their signals can only give an east-west position. To get a north-south position requires satellites above and below the equatorial plane.
Hai-Fu and his team of researchers have solved this problem too. Throughout their lives, geostationary satellites are constantly buffeted by a variety of forces that push them off station. These are the result of the Sun and Moon”s gravity, the aspherical shape of the earth, solar radiation and so on. So these satellites expend most of their fuel keeping station.
Towards the end of the satellites” lives, however, operators conserve fuel by keeping station only in the east-west direction. This allows the satellites to move into an inclined plane so that they appear to map out a figure-of-eight shape from the ground.
It is this north-south movement that Hai Fu wants to exploit. The researchers want to use the communications capabilities of these end of life satellites to broadcast time signals that allow receivers in the ground to triangulate their position in three dimensions.
They say there are various advantages over and above the simplicity and cost. Communications satellites are ideally suited to broadcasting other information too so the positioning signals could be combined with other data such as weather info. The satellites can also broadcast over a range of frequencies which can get around issues of interference. And since the ground-based equipment can always be upgraded to offer state of art time signals, the system would offer a positioning resolution in the centimetre range.
Perhaps most interesting of all, the communications link can be two-way, which would allow users to transmit data too–perhaps their position in times of distress.
Source: Technology Review