BeiDou Performances

COMPASS
Title	BeiDou Performances
Edited by	GMV
Level	Basic
Year of Publication	2011

The Compass Navigation Satellite System (CNSS), also named BeiDou-2, is China’s second-generation satellite navigation system that will be capable of providing positioning, navigation, and timing services to users on a continuous worldwide basis. The system is being deployed using an incremental approach and full operational capability is expected to be reached by 2020. The COMPASS System has been designed to reach accuracy levels similar to those of GPS and Galileo Open Service: positioning accuracy within 10 meters, timing accuracy within 20 ns and velocity accuracy within 0.2 meters per second.

COMPASS Performances

The BeiDou-1 system was established in 2003 for the area of the greater China and provided navigation, communication and timing services with performance characteristics as follows::

• Positioning: 100m (1σ); 20m (1σ, with reference stations).
• Regional Service short message service (SMS): up to 120 Chinese characters.
• Timing: 20 ns.

On November 2, 2006, China announced the second generation Chinese navigation system, COMPASS, that in the completion of its first phase by 2012 would offer an open service with an accuracy of 10 meters, timing of 0.2 nanoseconds, speed of 0.2 meter/second. As of December 2011, ten satellites for COMPASS have been launched and the system went into operation on a trial basis with a constellation of 10 satellites (5 GEO satellites and 5 IGSO satellites). With this constellation the system can offer positioning to within 25 meters but with the launch of 6 more satellites during 2012 the accuracy will be enhanced to within 10 meters and then the open service performances will reach the levels described before. The global COMPASS system will be built by 2020.

The performances for COMPASS Services would be:

• Open service: a free service for civilian users with positioning accuracy of within 10 meters, velocity accuracy of within 0.2 m/s and timing accuracy of within 20 nanoseconds;

• Authorized service: a licensed service with higher accuracy even in complex situations for authorized and military users only.

• Wide area differential positioning service: with positioning accuracy of 1 m.

• Short message service (SMS): up to 120 Chinese characters.

Table 1: COMPASS Signals Characteristics

Preliminary performance figures were presented in the COMPASS Workshop, integrated in the ION GNSS 2011 Conference. The COMPASS constellation available at the time (referred to as "3+3", standing for 3 GEO Satellites and 3 IGSO satellites), operational in July 2011, includes the signals depicted in Table 1.

These results show that a combined COMPASS("3+3")+GPS constellation provides:

• higher availability: up to 14 satellites were visible in the test conditions (compared to up to 9 with GPS only);
• better geometry: the PDOP for the combined constellation was lower than the PDOP for each individual constellation.

The results show that the accuracy of the combined solution (GPS and current COMPASS) is in general worse than the single GPS Solution - even though in the same order of magnitude. This was explained by the differences between systems and the preliminary status of the COMPASS constellation which is still expected to improve their satellite clocks and orbits accuracy as it reaches full operational capability.