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The development of Compass is slowly gathering pace. It is a project by the People's Republic of China to develop a global satellite navigation system. The previous system, called Beidou, consisted of four geostationary satellites over China, and give coverage from about 70°E to 140°E, and from 5°N to 55°N. Positioning accuracy is reportedly 20 metres or so.

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The Beidou satellites are modified communications satellites and have a fully duplex communications protocol, i.e. both mobile and satellite are required to transmit and receive. The mobile receiver on the ground sends a message to the satellite requesting a position fix. There is an interaction between sender and receiver, and the satellite then sends the position to the receiver.

The system does not lend itself to the construction of lightweight receivers, but it is valuable in giving engineers experience in satellite navigation.

Compass will work in more or less the same fashion as GPS, Glonass and Galileo. It will have 35 satellites. Five will remain in geostationary orbit, and 30 will be placed in medium (20,000 km) orbits.

The first satellite was launched into geostationary orbit on 14 April 2007. It is also known as Compass M1, Beidou 4, or Beidou 2A. The second satellite was launched 15 May 2009. It is known as Compass M2, Beidou 5 or Beidou 2G, depending on which source one quotes, although it appears Chinese authorities have now settled on the Compass M designations.

Signals from Compass M1 were received by Centre National d'Etudes Spatiales – the French aerospace agency – Standford University and the Belgian company, Septentrio. Septentrio has built a receiver – evidently the first by any company in the West.

It broadcasts on four frequencies (called E1, E2, E5 and E5b). The E1 signal has not yet been detected. E2 is at 1561.099 MHz, E6 is at 1278.75 MHz and E5b is at 1207.14 MHz.

They overlap existing GNSS signals. E1 and E2 will sit on either side of the GPS L1 signal, and right across the Galileo L1. Compass E6 is essentially identical to Galileo E6. Compass E5b cuts across Galileo E5b.

Currently, a great deal of work is being undertaken to ensure compatibility between these systems. Having common frequencies makes the design of receivers easier, but it also means that radio engineers have to be careful to ensure that their signals do not degrade other signals on the same frequency.

Such issues dominated the third meeting of the system providers hosted by the International Committee on GNSS (ICG-3) in the US in December.

China’s plan to transmit Compass signals on the same frequencies as Galileo’s Public Regulated Service (PRS) and interoperability issues with Glonass at L1 were high on the agenda.

Working parties are currently studying these issues.

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