BeiDou Receivers

COMPASS
Title	BeiDou Receivers
Author(s)	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.

Although the upgrade of its regional navigation system towards a global solution started in 1997, the formal approval by the Government of the development and deployment of BeiDou-2/CNSS was done in 2004. The system is currently under development evolving from a regional system called BeiDou-1, and in the first phase will provide high-accuracy positioning services for users in China and its neighboring regions by 2012. In a second stage, the system will evolve to provide global navigation services by 2020, similarly to the GPS, GLONASS or Galileo systems.

As of August 2011, nine satellites for Compass have been launched, the first eight of which completed the deployment foreseen for the first phase of BeiDou-2.

COMPASS Receiver

The COMPASS User Segment consists of COMPASS/Beidou user terminals. The first generation of BeiDou-1 terminals emerged in 2003 with full RDSS function and communication terminal based on General Purpose ICs. The second generation appeared in 2009 based in ASIC and comprising already GPS integration.

Particularities

Each GNSS system uses a specific Reference Frame; although a multi-constellation receiver is able to convert all information to the same common frame, a COMPASS-only receiver uses the China Geodetic Coordinate System 2000 (CGCS2000), consistent with the International Terrestrial Reference System (ITRS).

In an analogous way, each system has its own time reference defined by the respective control segments; the time reference for COMPASS is called “BeiDou Time” (BDT) and it is aligned with UTC.

Each GNSS System transmits its own navigation message, defined in the respective Signal In Space Interface Control Documents, SIS ICD. As an example, the satellites transmit information that allows the receiver to compute their positions. For the case of COMPASS there is no information at this stage on the contents of the navigation message, but its SIS ICD is expected to be released in 2011.

GNSS signals modulation, structure, navigation message contents and formats are often different among signals from the same system and from different systems. Most of these characteristics are easily implemented at the receiver (e.g. requiring only “software modifications”, such as the use of different PRN codes or the ability to cope with different message structures). The main difference among GNSS receivers falls into the specific characteristics that have impact at RF level, such as the Multiple Access Techniques employed. COMPASS (as GPS and Galileo) uses CDMA techniques allowing a simpler RF module (than for example GLONASS), since all signals in the same frequency band have a common carrier. Nevertheless, COMPASS supports a regional short message service, which allows the user to send information to the stations. This additional communication link adds complexity to the receiver, and therefore potentially higher costs.

It should be noted that the current trend consists on facilitating the access of each system to the receivers, i.e. fomenting multi-constellation receivers. Hence, most discussions and agreements among the systems’ responsibles are conducted in the sense of taking this effort out of the user segment, focusing on compatibility and interoperability aspects in the system design.