Compass navigation system
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This article contains information about a scheduled or expected future product. It may contain preliminary information that does not reflect the final version of the product. |
The Compass system (also known as Beidou-2) is a project by China to develop an independent global satellite navigation system. Compass is not an extention to the previously deployed Beidou-1, but a new GNSS system similar in principles to GPS and Galileo.
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[edit] General
The new system will be a constellation of 35 satellites, which include 5 geostationary orbit (GEO) satellites and 30 medium Earth orbit (MEO) satellites, that will offer complete coverage of the globe. The ranging signals are based on the CDMA principle and have complex structure typical to Galileo or modernized GPS. Similarly to the other GNSS, there will be two levels of positioning service: open and restricted (military). The public service shall be available globally to general users. When all the currently planned GNSS systems are deployed, the users will benefit from the use of a total constellation of 75+ satellites, which will significantly improve all the aspects of positioning, especially availability of the signals in so-called “urban canyons” [1].
Frequencies for Compass are allocated in four bands: E1, E2, E5B, and E6 and overlap with Galileo. The fact of overlapping could be convenient from the point of view of the receiver design, but on the other hand raises the issues of inter-system interference, especially within E1 and E2 bands, which are allocated for Galileo’s publicly-regulated service [2].
Although almost nothing has yet been officially announced by Chinese authorities about the signals of the new system, the launch of the first Compass satellite permitted independent researchers not only to study general characteristics of the signals but even to build a Compass receiver.
[edit] Compass-M1
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This article or section documents a current or recent spaceflight. Details may change as the mission progresses. |
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Compass-M1 is an experimental satellite launched for signal testing and validation and for the frequency filing on April 14, 2007. The role of Compass-M1 for Compass is similar to the role of GIOVE satellites for Galileo. The signals of Compass-M1 are to a great extent unraveled by independent research. The orbit of Compass-M1 is nearly circular, has an altitude of 21,150 km and an inclination of 55.5 degrees.
Compass-M1 is transmitting in 3 bands: E2, E5B, and E6. In each frequency band two coherent sub-signals have been detected with a phase shift of 90 degrees (in quadrature). These signal components are further referred to as “I” and “Q”. The “I” components have shorter codes and are likely to be intended for the open service. The “Q” components have much longer codes, are more interference resistive, and are probably intended for the restricted service.
The investigation of the transmitted signals started immediately after the launch of COMPASS-M1 on April 14 2007. Already in June engineers at CNES reported the spectrum and structure of the signals [3]. Next month researchers from the Stanford University reported complete decoding of the “I” signal scomponents[4][5]. The knowledge of the codes allowed a group of engineers at Septentrio to build the COMPASS receiver[6] and report tracking and multipath characteristics of the “I” signals on E2 and E5B [7].
| Parameters | E2-I | E2-Q | E5B-I | E5B-Q | E6-I | E6-Q | GPS L1-CA | |
|---|---|---|---|---|---|---|---|---|
| Native notation | B1 | B1 | B2 | B2 | B3 | B3 | --- | |
| Code modulation | BPSK(2) | BPSK(2) | BPSK(2) | BPSK(10) | BPSK(10) | BPSK (10) | BPSK (1) | |
| Carrier frequency, MHz | 1561.098 | 1561.098 | 1207.14 | 1207.14 | 1268.52 | 1268.52 | 1575.42 | |
| Chip rate, Mchips/sec | 2.046 | 2.046 | 2.046 | 10.230 | 10.230 | 10.230 | 1.023 | |
| Code period, chips | 2046 | ?? | 2046 | ?? | 10230 | ?? | 1023 | |
| Code period, msec | 1.0 | >400 | 1.0 | >160 | 1.0 | >160 | 1.0 | |
| Symbols/sec | 50 | ?? | 50 | ?? | 50 | ?? | 50 | |
| Navigation frames, sec | 6 | ?? | 6 | ?? | ?? | ?? | 6 | |
| Navigation sub-frames, sec | 30 | ?? | 30 | ?? | ?? | ?? | 30 | |
| Navigation period, min | 12.0 | ?? | 12.0 | ?? | ?? | ?? | 12.5 |
Characteristics of the “I” signals on E2 and E5B are generally similar to the civilian codes of GPS (L1-CA and L2C), but Compass signals have somewhat greater power. The notation of Compass signals used in this page follows the naming of the frequency bands and agrees with the notation used in the Western literature on the subject, but the notation used by the Chinese seems to be different and is quoted in the first raw of the table.
[edit] References
- ^ G. Gibbons. China GNSS 101. Compass in the rearview mirror. InsideGNSS, January/February 2008, p. 62-63[1]
- ^ Galileo, Compass on collision course, GPS World, April 2008, p. 27
- ^ T. Grelier, J. Dantepal, A. Delatour, A. Ghion, L. Ries, Initial observation and analysis of Compass MEO satellite signals, Inside GNSS, May/June 2007[2]
- ^ G. Xingxin Gao, A. Chen, S. Lo, D. De Lorenzo, P. Enge, GNSS over China. The Compass MEO satellite codes. Inside GNSS, July/August 2007, pp. 36-43[3]
- ^ G. Xingxin Gao, A. Chen, Sh. Lo, D. De Lorenzo and Per Enge, Compass-M1 broadcast codes and their application to acquisition and tracking, Proceeedings of the ION National Technical Meeting 2008, San Diego, California, January 2008.[4]
- ^ W. De Wilde, F. Boon, J.-M. Sleewaegen, F. Wilms, More Compass points. Tracking China’s MEO satellite on a hardware receiver. Inside GNSS, July/August 2007, pp. 44-48. [5]
- ^ A. Simsky, D. Mertens, Wim De Wilde, Field Experience with Compass-M1 E2 and E5B Signals. Proceedings of ENC GNSS 2008, Toulouse, 22-25 April 2008.
[edit] External Sources
- Compass on the Chinese Defence Today website
- Compass-M1 orbit details can be found in the Space-Track catalogue with a name "Beidou M1"
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