China’s Regional Navigation Satellite System – CAPS

Binghao Li and Andrew G. Dempster University of New South Wales, Australia NASA photo NASA

When it comes to China’s satellite navigation efforts, most attention is focused on the Compass (Beidou-2) GNSS that is under development. However, since 2002, that nation has also been working on a regional system called CAPS — for Chinese Area Positioning System. Although CAPS operates on C-band frequencies, rather than L-band in which most GNSS systems transmit, it has a very similar signal structure as GPS. This article discusses the history, system architecture, and advantages and disadvantages of CAPS.

any navigation satellite sys- A less well-known RNSS is the Chi- Communication Satellites tems have emerged since nese Area Positioning System, or CAPS. 2000. CAPS is a “passive” one-way system M GPS is the first global nav- similar to most navigation satellite sys- igation satellite system (GNSS), followed tems: satellites broadcast the navigation by Russia’s GLONASS, reinvigorated in messages; receivers are only “listeners.” recent years. Europe is developing Gali- (Beidou-1 is a two-way system, the user leo, and China is expanding the regional also sends messages to the control center system Beidou to a global system called through satellites). Compass (also referred to as Beidou-2). But CAPS is also different from all Not all countries can afford to devel- the other navigation satellite systems in Receiver Ground op a GNSS, mostly because of the high that the navigation messages are gener- Station cost, but also because they may not need ated on the ground and uploaded to the global coverage. Several regional naviga- communication satellites, with the sat- FIGURE 1 The principle of CAPS tion satellite systems (RNSS) are under ellites acting only as transponders (see development, such as Japan’s QZSS, Figure 1). cal Observatories of China (NAOC), India’s IRNSS, and the first phase of Bei- Chinese Academy of Science (CAS). dou (with completion of an east Asian History and Basic Concepts The CAPS constellation consists of coverage area currently projected for CAPS was initiated in 2002 based on a commercial geostationary (GEO) com- 2012). proposal from the National Astronomi- munication satellites and inclined geo- www.insidegnss.com june 2010 InsideGNSS 59 caps

synchronous orbit (IGSO) communication satellites. These CAPS Signal Structure spacecraft are not traditional navigation satellites: all the navi- In GEO communication satellite systems, C-band and Ku-band gation-related facilities are all located on the ground. are popular for communications, while L band is rare. Because This system has at least three major advantages: of the significant rain attenuation of Ku-band signals, C-band • Operation does not require the launch of specific naviga- is the only suitable choice for a navigation satellite system. tion satellites, but instead bandwidth can be rented on com- Although most such systems use L-band, C-band has mercial communications satellites. Hence, the cost is much advantages and disadvantages for navigation: a C-band signal lower, and the deployment of the system can occur much is less affected by ionospheric delay, less carrier phase noise, and more rapidly. better resistance of multipath. However, as with Ku-band, C- • Because all the navigation related facili- ties, especially the atomic clocks, are Unlike GNSS systems, the CAPS satellites located in a ground station, the size of the themselves do not carry atomic clocks. However, clocks and their operational environment the pseudoranges used for positioning are the are no longer big issues. The system can use a larger but more accurate and reli- measurement between the satellite and the user; able atomic clock (for the whole system hence, the signal transmission time for that part of rather than one clock for each satellite), the signal path must be known. and maintenance is obviously much eas- ier. band has larger rain attenuation and larger tropospheric delay • A receiver of this system by definition must also be able use than L-band. the communications facilities of these satellites, meaning CAPS uses C-band for navigation. The two carrier fre- that versatile new applications can be considered for this quencies (downlink) are C1=4143.15 MHz and C2=3826.02 system that are not inherent in other navigation satellite MHz. CAPS also needs two uplink carriers, which are

systems. C1u=6368.15MHz, C2u=6051.02MHz. It took three years to develop a validation system for CAPS. Similar to GPS, the two frequencies can be used together to This system used four commercial GEO communication sat- correct the ionospheric delay. Despite the difference in carrier ellites. (At least three satellite-receiver ranges are needed for frequencies, CAPS has a very similar signal structure to GPS. a position fix; a fourth range can increase the coverage and It also has similar types of codes: coarse acquisition (C/A) and provide redundant measurements.) precise (P) codes. A third code — a modulated binary offset This type of constellation cannot provide 3D positioning carrier (BOC) signal, has also been designed and is believed because the satellites are all located in orbit over the equator. to be in use. Consequently, CAPS equipment designers incorporated a The C/A-code uses a Gold code at a rate of 1.023 Mcps, and barometer into receivers to provide a height estimate. P-code employs a special secure code at a rate of 10.23 Mcps. The ground station collects pressure and temperature data The broadcast messages consist of ephemeris, time, virtual from weather stations around China and broadcasts the infor- clock parameters, the pressure and temperature information, mation as part of the navigation messages, which helps cali- wide/local area differential corrections, and integrity informa- brate this barometer. The positioning principle is the same as tion relating to GNSSes. GPS, except for barometer estimating height to provide a 3D positioning result: Virtual Atomic Clock The standard time reference for CAPS is provided by the mas- ter clock — a hydrogen maser, which is located at the ground station. The master clock (UTC) of the National Time Service Center (NTSC) of China calibrates the CAPS clock. The time offset between the master clocks may be broadcast in the navi- gation messages. Unlike GNSS systems, the CAPS satellites themselves do not carry atomic clocks. However, the pseudoranges used for where ρi is the pseudorange, (xi, yi, zi) is the satellite’s position, positioning are the measurement between the satellite and the

(xu, yu, zu) is the user’s position, ∆tu is the receiver’s clock error. user; hence, the signal transmission time for that part of the a and b are the semi major axis and semi minor axis of the signal path must be known. The actual range that is measured earth respectively. by the receiver is that from the ground station, which transmits If the IGSO satellites are available, at least one more pseudo- the broadcast messages to the satellite, plus that from the satel- range measurement can be used rather than the altitude from lite to the receiver. the barometer. The relationship of the various pseudoranges can be expressed as

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6 but many factors — such as 90 gravitational effects of celes- tial bodies on the GEO —can 75 5 change the orbits once con- 60 trol is relaxed. Consequently, 45 4 GEO satellites always carry 30 propellant fuel with which to 15 maintain their orbital loca-

0 3 Vs tion — a technique referred #S -15 to as station-keeping. Latitude (Deg) -30 Most of the time, the 2 -45 amount of fuel transported aboard the satellite deter- -60 mines the lifetime of the -75 1 GEO. In many cases, even -90 -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 if the satellite is otherwise operating successfully, the 0 satellite has to be terminated Longitude (Deg) once it has consumed all its

FIGURE 2 Average number of visible satellites (above a five-degree elevation cutoff angle) of the six-satellite CAPS fuel. constellation, which consists of two GEO satellites and four SIGSO satellites with inclination of seven degrees If the satellite is no longer controlled, its position will However, when the virtual clock error drift. The drift in south-north direction and ephemeris error are considered is more significant than that in east-west where ρ is the pseudorange from the together in positioning, the combina- direction. Indeed, 90 percent of the fuel satellite to the user (required for posi- tion of these two is smaller than the on a GEO is used to maneuver a satellite

tioning), ρ0 is the pseudorange from the individual virtual clock error or the against disturbance in the south-north ground station to the user (measured), ephemeris error, leading to more accu- direction.

and ρ1 is the pseudorange from the rate positioning. When a GEO satellite approaches the ground station to the satellite, a term end of its life but its position in the east- which needs to be removed. SIGSO west direction can still be maintained, the

The equivalent expression of ρ1 is If there are only GEO satellites in CAPS, GEO in effect becomes a SIGSO satellite. . is the correction of a vir- the 3D position dilution of precision Because the satellite is no longer using tual clock that could be considered to be (PDOP) is poor, so that 3D positioning fuel to maintain its north-south position operating on the satellite. This correction is impossible. This has led to the use of but only the 10 percent portion needed for is obtained by measuring the broadcast the receiver barometer in the early con- east-west station-keeping, the remaining message from the satellite at the ground figuration of the system. lifetime of a SIGSO can be extended to station (where it originated) to obtain the Two methods have been proposed to 10 times before controllers need to place round trip delay and then subtracting the improve the CAPS PDOP. The first is to it into a graveyard orbit. Such a SIGSO downlink delay which includes the iono- launch several IGSO satellites (at least satellite can be used in CAPS. spheric delay, tropospheric delay, and the three), which have “figure-8” ground The beauty of a SIGSO is that it has a delay caused by the receive channel on tracks, as used by other systems such as “figure-8” ground track, meaning it can the ground station. IRNS and Japan’s Quasi-Zenith Satel- improve PDOP because it introduces a This virtual clock correction includes lite System (QZSS). (This approach also north-south component into the con- the uplink atmospheric delay, satellite includes Compass, where three IGSO stellation geometry. The larger the incli- receiving and broadcasting delay, and satellites are proposed). Use of IGSOs nation of the SIGSO satellite, the better so forth. The virtual clock correction can decrease the PDOP significantly. the PDOP that can be achieved. Another model is then created and the coeffi- However, IGSO launches are not advantage is that a retired GEO satellite cients of the model are included in the cheap; so, a second method was pro- can be acquired cheaply. navigation message. posed — using retired GEO communi- A simulation of the CAPS constel- The major error in the virtual clock cation satellites by maneuvering them to lation, assuming two GEO satellites correction is caused by the imperfect so-called slightly inclined geostationary- (located at 59°E and 163°E) and four ephemeris prediction of satellite loca- satellite orbits (SIGSO). SIGSO satellites (located at 87.5°E, tion because the location is used to GEO satellites are normally con- 110.5°E, 125°E and 142°E respectively) calculate the signal downlink delay. trolled to be “static” at their orbital slot, with an inclination of seven degrees.

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This configuration provided CAPS PDOP (6 degree mask) more than 80 percent of the territory of China with a 90 11 PDOP of less than 7 (assum- 75 ing a five-degree elevation 60 cutoff angle). With CAPS, 10 45 the majority of South-East 30 Asian countries and parts 9 of China and Australia 15 0 Vs can enjoy PDOPs less than 8 #S 6. Korea, part of Japan, -15 and half of the territory of Latitude (Deg) -30 7 Australia obtained PDOPs -45 below 7. -60 Figure 2 6 shows the aver- -75 age number of visible satel- -90 lites (above a five-degree -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 5 elevation cutoff angle) of the six-satellite CAPS constella- Longitude (Deg) tion. Figure 3 gives the aver- age PDOP value a 24 hour FIGURE 3 Average PDOP value of the six-satellite CAPS constellation over a 24-hour period; white area indicates that period, and Figure 4 shows the PDOP value is higher than 12. the variation of PDOP value at several cities in the Asia-Oceania area CAPS PDOP over 24 hours (5 degree mask) over a 24-hour period. More IGSO sat- 15 ellites are still needed to provide even 14 better PDOP. 13 CAPS: Advantages and Disadvantages 12 As with any complex system in which numerous trade-offs must be analyzed 11 to achieve an optimal solution, the Chi- 10 nese Area Positioning System has both. PDOP The advantages of the CAPS system 9 design include the following: 1. CAPS is much cheaper than other 8 navigation satellite systems, for two 7 reasons: the system uses GEO com- munication satellites, especially 6 retired ones and generates the nav- igation messages on the ground. 5 5 10 15 20 Building a CAPS-like system that Elapsed Time (hours) covered one-third of the Earth’s surface would only cost an esti- FIGURE 4 Variation of PDOP value of the 6 satellite CAPS constellation at Singapore, Sydney, New mated US$300 million — about one Delhi, Perth, and Beijing over a 24 hour period percent of the cost of a full-fledged GNSS. munications satellites, which do not munication satellites, they can be used 2. The system can be deployed very need a special design. To build the not only for navigation but also for quickly. All the important equip- validation system only took three navigation-related communications ment in a traditional navigation sat- years. or redeployed primarily for general ellite can be deployed on the ground. 3. Use by CAPS extends the lifetime of communications at any time. Only several more IGSO satellites are some communication satellites — in 5. CAPS increases the number of navi- potentially needed in the constella- effect, “recycling in space.” gation satellites visible in the Asia- tion and those are basically com- 4. Because the CAPS spacecraft are com- Oceania area.

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6. It has a similar signal structure to located at 87.5°E and 110.5°E longi- CAPS. Science in China Series G: Physics, Mechan- GPS (in a different band), making the tude, two SIGEOs located at 134°E ics & Astronomy, 2009, 52 (3), pp. 45-457 development of an all-in-one receiver and 142°E, plus a receiver barom- [3] Hu, Y., and Y. Hua, L. Hou, J. Wei, and J. Wu, (for GPS, Galileo, Compass, QZSS, eter), the static PVT accuracies are Design and implementation of the CAPS receiver, and so forth) easier, while making the as follows: Science in China Series G: Physics, Mechanics & equipment more robust against inter- a. for standard service: position ~15– Astronomy, 2009, 52 (3) pp 445-457 ference at any given radio frequency. 25 meters (1σ) horizontal, ~1–3m [4] Li, X., and H. Wu, Y. Bian, and D. Wang, “Sat- 7. CAPS was developed for China, but a vertical; velocity, ~0.13~0.3 meters/ ellite Virtual Atomic Clock with Pseudorange much larger Asian-Oceania area can second; time, 160 nanoseconds Difference Function,” Science in China Series G: benefit from it, especially South East b. for precision service: position Physics Mechanics & Astronomy, 2009, 52(3), pp. 353-359 Asia and part of Australia. Japan’s ~5–10 meters (1σ) horizontal, ~1–3 QZSS has already attracted much meters vertical; velocity, ~0.15–0.17 [5] 卢晓春, 吴海涛, 边玉敬, 等. 中 attention from some Asia-Ocea- meters/second;, time, 13 nanosec- 国区域定位系统信号体制. 中国科学 Ｇ辑: 物理学 力学 天文学 (Science in nia countries, including Australia. onds. China Series G: Physics, Mechanics & Astronomy), Because the CAPS constellation has After the launch of IGSO satellites, 2008, 38(12), pp. 1634—1647 more satellites and is a civilian sys- the accuracy is expected to improve [6] 韩延本, 马利华, 乔琪源, 尹志强, tem, it also has potential for interna- several times. 施浒立, 艾国祥, 退役GEO 通信卫星对 tional collaboration. 7. China has a working regional mili- 改善CAPS 系统 PDOP 的作用, 中国科学 On the other hand, CAPS has a num- tary satellite navigation system Bei- Ｇ辑: 物理学 力学 天文学 (Science in ber of disadvantages: dou and it is on the way to expand China Series G: Physics, Mechanics & Astronomy), 1. C-band is used for the data link, this into a dual-use global system, 2008, 38(12), pp. 1738-1749 making interoperability more com- Compass, which consists of five GEO plicated than for systems operating satellites, three IGSO satellites, and at L-band. 27 MEO satellites. Even those people Authors 2. The system faces the possibility of who are working on CAPS agree that Binghao Li is a research same-frequency interference from Compass will have priority, cast- associate in the School other communication satellites. To ing a shadow over the future of the of Surveying and Spatial solve this problem will require the CAPS. Information Systems, collaboration of many organizations. The University of New 3. Because the satellite is only a tran- Conclusion South Wales, Sydney, Australia. Li obtained the sponder, the navigation signal relies Compass is a GPS-like system. The cost B.Sc. in electrical & mechanical engineering from on the ground station. If anything of this system will be tens of billions of Northern Jiaotong University, P.R. China and the happens to the ground station or the dollars, and a debate is still underway in M.Sc. in civil engineering from Tsinghua Univer- uplink, the satellite loses its naviga- China about whether it is worth develop- sity, P.R. China. He received his Ph.D. from the tion function. Compared with the ing such a GNSS. The technical barriers, University of New South Wales, Sydney, Australia. other navigation satellite systems, such as the on-board atomic clocks, are His research areas include pedestrian navigation, therefore, CAPS is more vulnerable. a great challenge for the development of new positioning technologies, and network-RTK. 4. A barometer may be needed to pro- Compass. Andrew Dempster is direc- vide altitude. Local pressure and As a far cheaper alternative, CAPS tor of research in the temperature data around China has attracted many supporters. No mat- School of Surveying and must be collected and broadcast by ter what we argue about the good or bad Spatial Information Sys- the satellites. If other countries want elements of this system, it is very impres- tems at the University of to use this system, this requirement sive to build a RNSS within three years. New South Wales is likely to become a problem. Other Where CAPS goes from here may not (UNSW), Sydney. He has a BE and MEngSc from UNSW and a Ph.D. from countries far from China would have live up to this early success. University of Cambridge.He was project manager to build their own ground station to and system engineer on the first GPS receiver generate the broadcast messages. Additional Resources developed in Australia in the late 1980s and is 5. Generally, the PDOP of CAPS is not [1] Ai, G., and H. Shi, H. Wu, Y. Yan, Y. Bian, Y. named on three patents arising from that work. He as good as that of GNSS systems, Hu, Z. Li, J. Guo, and X. Cai, “A Positioning Sys- has taught GPS-related topics for more than a even if three IGSO satellites were tem Based Satellite Communication and Chinese decade at the University of Westminster, London, launched. Area Positioning System (CAPS),” Chinese Journal and at UNSW. His current research interests are 6. Positioning, velocity, and time (PVT) of Astronomy and Astrophysics, 2008, 8(6), pp. GNSS signal processing, software-defined receiv- accuracy is not as good as for GNSS 611-635 er design, and new location technologies. systems. Based on the CAPS vali- [2] Han, Y., and L. Ma, Q. Qiao, Z. Yin, and G. Ai, dation system (two GEO satellites “Selection of Satellite Constellation Framework of

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