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[2.0] International Navigation Satellite Systems

v3.0.3 / chapter 2 of 2 / 01 feb 08 / greg goebel / public domain

* The Soviet Union followed the American lead in navigation satellites, fielding a system known as "Parus / Tsikada" comparable to Transit in the 1970s and then developing a GPS-like system named "GLONASS" in the 1980s. Other nations are now getting into the navigation satellite business, with Japan and China now operating their own "navsats" and Europe now working on the "Galileo" system to compete with GPS.


[2.1] PARUS-TSIKADA
[2.2] GLONASS
[2.3] JAPANESE, EUROPEAN, & CHINESE NAVIGATION SATELLITE EFFORTS
[2.4] FOOTNOTE (1): NAVIGATING ON OTHER WORLDS
[2.5] FOOTNOTE (2): THE COSPAS-SARSAT NETWORK
[2.6] COMMENTS, SOURCES, & REVISION HISTORY

[2.1] PARUS-TSIKADA

* The Soviets set up a network of navigation satellites similar to the US Transit system, using Doppler location technology and with comparable accuracies. As with the US Transit system, the primary rationale was to provide navigational data to ballistic-missile submarines. Investigations began in the late 1950s, leading to start of a formal development program in 1962. Launch of an initial series of "Tsyklon" experimental satellites began in 1967, with a total of 29 launched into 1978, not counting at least two launch failures.

The development program was protracted because it proved difficult to obtain the required location accuracy. Meeting specification required launch of geodetic studies satellites to obtain a more precise gravity map of the Earth. The last 13 Tsyklons were operational prototypes, close to production spec, and were also called "Zaliv".

   _______________________________________________________________________

   15 may 67   Cosmos 158 -- dummy payload
   27 sep 67   Tsyklon dummy payload launch attempt -- failure
   23 nov 67   Cosmos 192 
   07 may 68   Cosmos 220 
   14 aug 69   Cosmos 292 
   21 oct 69   Cosmos 304 
   11 apr 70   Cosmos 332 
   20 aug 70   Cosmos 358 -- went into unusual, possibly wrong, orbit
   12 oct 70   Cosmos 371
   12 dec 70   Cosmos 385 
   22 may 71   Cosmos 422 
   15 dec 71   Cosmos 465 
   25 feb 72   Cosmos 475
   06 may 72   Cosmos 489
   16 aug 72   Cosmos 514 
   26 jan 73   Cosmos 546 

   25 may 73   first Zaliv launch attempt -- failure
   20 jun 73   Cosmos 574 
   14 sep 73   Cosmos 586 
   29 dec 73   Cosmos 627 
   17 jan 74   Cosmos 628 
   27 jun 74   Cosmos 663
   19 oct 74   Cosmos 689 
   23 apr 75   Cosmos 729 
   03 feb 76   Cosmos 800 
   03 jun 76   Cosmos 823 
   29 jul 76   Cosmos 846 
   20 jan 77   Cosmos 890
   28 oct 77   Cosmos 962
   15 mar 78   Cosmos 994 
   27 jul 78   Cosmos 1027 
   _______________________________________________________________________

The Tsyklon series was followed by the fully operational "Tsyklon-B" or "Parus" system. Unlike the wildly varying Transit satellites, the Tsyklon and Parus satellites had similar configurations. Both were in the form of a drum covered with solar cells, with a weighted mast on top for gravity-gradient stabilization, and a what appears to have been an antenna hung off one side near the bottom. They were all launched from the Plesetsk Northern Cosmodrome by the standard Soviet Kosmos 3M medium-lift booster, one satellite per shot, though a few did include small secondary payloads. The developed Parus satellites had a launch weight of about 810 kilograms (1,785 pounds), and were placed into a near-circular orbit of about 1,000 kilometers (620 miles) at a near-polar inclination of 83 degrees.

95 Parus satellites have been launched since 1974, not counting two failures. The system was formally accepted into service in 1976, but it appears that the full constellation required 22 satellites and that wasn't achieved until 1980. The Parus satellites also provided a "store-dump" communications relay service for Red Navy surface vessels and submarines. Parus satellites continue to be launched, and some suspect that military communications is now the primary rationale for the constellation.

   _______________________________________________________________________

   26 dec 74   Cosmos 700
   11 apr 75   Cosmos 726
   14 aug 75   Cosmos 755
   04 nov 75   Cosmos 778
   20 jan 76   Cosmos 789
   29 oct 76   Cosmos 864
   28 dec 76   Cosmos 887
   21 feb 77   Cosmos 894
   13 jul 77   Cosmos 928
   13 sep 77   Cosmos 951
   23 dec 77   Cosmos 971
   17 jan 78   Cosmos 985
   28 feb 78   Cosmos 991
   28 mar 78   Cosmos 996
   23 may 78   Cosmos 1011
   20 dec 78   Cosmos 1064
   16 jan 79   Cosmos 1072
   21 mar 79   Cosmos 1089
   07 apr 79   Cosmos 1091
   31 may 79   Cosmos 1104
   16 oct 79   Cosmos 1141
   14 jan 80   Cosmos 1150
   25 jan 80   Cosmos 1153
   20 may 80   Cosmos 1181
   05 dec 80   Cosmos 1225
   12 feb 81   Cosmos 1244
   04 jun 81   Cosmos 1275
   12 aug 81   Cosmos 1295
   18 sep 81   Cosmos 1308
   14 jan 82   Cosmos 1333
   24 mar 82   Cosmos 1344
   08 apr 82   Cosmos 1349
   18 jun 82   Cosmos 1380
   07 jul 82   Cosmos 1386
   19 oct 82   Cosmos 1417
   12 jan 83   Cosmos 1428
   30 mar 83   Cosmos 1448
   06 may 83   Cosmos 1459
   24 may 83   Cosmos 1464
   08 dec 83   Cosmos 1513
   11 jan 84   Cosmos 1531
   02 feb 84   Cosmos 1535
   11 may 84   Cosmos 1550
   27 jun 84   Cosmos 1577
   13 sep 84   Cosmos 1598
   11 oct 84   Cosmos 1605
   15 nov 84   Cosmos 1610
   01 feb 85   Cosmos 1627
   14 mar 85   Cosmos 1634
   23 oct 85   Launch failure.
   28 nov 85   Cosmos 1704
   19 dec 85   Cosmos 1709
   16 jan 86   Cosmos 1725
   23 may 86   Cosmos 1745
   18 jun 86   Cosmos 1759
   24 nov 86   Cosmos 1802
   17 dec 86   Cosmos 1808
   18 feb 87   Cosmos 1821
   06 jul 87   Cosmos 1864
   14 oct 87   Cosmos 1891
   23 dec 87   Cosmos 1904
   22 mar 88   Cosmos 1934
   18 jul 88   Cosmos 1959
   22 feb 89   Cosmos 2004
   04 apr 89   Cosmos 2016
   07 jun 89   Cosmos 2026
   25 jul 89   Cosmos 2034
   20 mar 90   Cosmos 2061
   20 apr 90   Cosmos 2074
   14 sep 90   Cosmos 2100
   26 feb 91   Cosmos 2135
   16 apr 91   Cosmos 2142
   22 aug 91   Cosmos 2154
   27 nov 91   Cosmos 2173
   17 feb 92   Cosmos 2180
   15 apr 92   Cosmos 2184
   01 jul 92   Cosmos 2195
   29 oct 92   Cosmos 2218
   09 feb 93   Cosmos 2233
   01 apr 93   Cosmos 2239
   02 nov 93   Cosmos 2266
   26 apr 94   Cosmos 2279
   22 mar 95   Cosmos 2310
   06 oct 95   Cosmos 2321, did not reach operational orbit
   16 jan 96   Cosmos 2327
   05 sep 96   Cosmos 2334* 
   20 dec 96   Cosmos 2336
   17 apr 97   Cosmos 2341
   23 sep 97   Cosmos 2346* 
   24 dec 98   Cosmos 2361
   26 aug 99   Cosmos 2366
   08 jun 01   Cosmos 2378
   28 may 02   Cosmos 2389
   04 jun 03   Cosmos 2398
   22 jul 04   Cosmos 2407
   20 jan 05   Cosmos 2414*
   11 sep 07   Cosmos 2429 
   _______________________________________________________________________

   (*) Indicates other payloads in launch.

Parus was a secret military system, but it was followed into service by a simplified system for civilian use, known as "Tsikada", also launched by the Kosmos 3M booster. Parus is sometimes referred to as "Tsikada Military" or "Tsikada-M". The Tsikada system was accepted into service in 1979 and reached its full complement of satellites in 1986. Tsikada was heavily used by the Soviet merchant marine. A few launches involved secondary payloads; in particular, the Tsikada satellite Cosmos 2123 was also fitted with two Russian amateur radio communications transponders.
   _______________________________________________________________________

   15 dec 76   Cosmos 883 
   08 jul 77   Cosmos 926 
   31 mar 78   Cosmos 1000 
   12 apr 79   Cosmos 1092 
   18 mar 80   Cosmos 1168 
   10 dec 80   Cosmos 1226 
   04 sep 81   Cosmos 1304 
   18 feb 82   Cosmos 1339 
   26 oct 83   Cosmos 1506 
   17 may 84   Cosmos 1553 
   30 may 85   Cosmos 1655 
   23 jan 86   Cosmos 1727 
   13 nov 86   Cosmos 1791 
   29 jan 87   Cosmos 1816 
   23 jun 87   Cosmos 1861 
   05 feb 91   Cosmos 2123*
   10 mar 92   Cosmos 2181 
   12 jan 93   Cosmos 2230 
   24 jan 95   Tsikada*
   05 jul 95   Cosmos 2315 
   _______________________________________________________________________

   (*) Indicates other payloads in launch.
Although the last Tsikada satellite as such was launched in 1995, the constellation continues to be replenished with a slightly different type of spacecraft. A series of Tsikada-type satellites were fitted with an auxiliary COSPAS-SARSAT rescue beacon locator payload, described in more detail at the end of this chapter, and were given the name "Nadezhda (Hope)". An evaluation prototype was launched in 1982, followed by the first launch of an operational satellite in 1983. From the mid-1990s the Nadezhdas were fitted with an improved "Kurs" rescue locater system, and these improved satellites were designated "Nadezhda-M".
   _______________________________________________________________________

   30 jun 82   Cosmos 1383 (evaluation prototype)

   24 mar 83   Cosmos 1447
   21 jun 84   Cosmos 1574
   04 jul 89   Nadezhda 1 
   27 feb 90   Nadezhda 2
   12 mar 91   Nadezhda 3 
   14 jul 94   Nadezhda 4 
   10 dec 98   Nadezhda 5 (Nadezhda M)*
   28 jun 00   Nadezhda 6 (Nadezhda M)*
   26 sep 02   Nadezhda 7 (Nadezhda M)
   _______________________________________________________________________

   (*) Indicates other payloads in launch.
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[2.2] GLONASS

* Although both the Parus and Tsikada / Nadezhda systems are still in operation, the Soviets went on to develop a GPS-like system, with the English name of "Global Navigation Satellite System (GLONASS)". Like GPS, the full GLONASS network includes 24 satellites, consisting of 21 operational satellites and three spares.

All the satellites transmit identical codes but at different frequencies, exactly the reverse of the scheme used for GPS. Actually, some satellites do transmit on the same frequencies, but they are placed in "antipodal" orbits so they can't be picked up by a receiver at the same time. The GLONASS satellites provide a "High Precision (HP)" signal for military location purposes and a "Standard Precision (SP)" signal for civil location purposes. The orbits are at an altitude of 19,100 kilometers (11,865 miles), slightly lower than that of the GPS satellites, with the satellites placed in three orbital planes, each containing eight satellites and with the planes separated by 120 degrees. Each satellite completes an orbit in 11 hours 15 minutes. The planes have orbital inclinations of 64.8 degrees. GLONASS is supposed to have location accuracy capabilities roughly similar to those of GPS.

GLONASS satellites, also known by the name "Uragan (Hurricane)", are launched in triplets by Proton heavy-lift boosters. They have a configuration roughly along the lines of that of US GPS Navstar satellites, with a central module carrying antenna arrays and twin solar panels. GLONASS launches began in 1982, apparently with prototype launches into 1985. An Etalon geodetic satellite was launched in place of one of the GLONASS triplets in two launches in 1989 to validate the GLONASS orbit.

The improved GLONASS-M / Uragan-M spacecraft has now been introduced, featuring better signal characteristics and a design lifetime of seven years, instead of the three year design lifetime of the original series. Older GLONASS satellites were launched along with GLONASS-M spacecraft for a time, apparently to burn up existing inventory.

   _______________________________________________________________________

   12 oct 82   Cosmos 1413:1415   dummy GLONASS x 3
   10 aug 83   Cosmos 1490:1492   GLONASS prototype x 3
   29 dec 83   Cosmos 1519:1521   GLONASS prototype x 3
   19 may 84   Cosmos 1554:1556   GLONASS prototype x 3
   04 sep 84   Cosmos 1593:1595   GLONASS prototype x 3
   18 may 85   Cosmos 1650:1652   GLONASS prototype x 3
   25 dec 85   Cosmos 1710:1712   GLONASS prototype x 3

   16 sep 86   Cosmos 1778:1780   GLONASS x 3
   24 apr 87   Cosmos 1838:1840   GLONASS x 3, launch failure
   16 sep 87   Cosmos 1883:1885   GLONASS x 3
   17 feb 88   Cosmos 1917:1919   GLONASS x 3, launch failure
   21 may 88   Cosmos 1946:1948   GLONASS x 3
   16 sep 88   Cosmos 1970:1972   GLONASS x 3
   10 jan 89   Cosmos 1987:1989   GLONASS x 2, Etalon
   31 may 89   Cosmos 2022:2024   GLONASS x 2, Etalon
   19 may 90   Cosmos 2079:2081   GLONASS x 3
   08 dec 90   Cosmos 2109:2111   GLONASS x 3
   04 apr 91   Cosmos 2139:2141   GLONASS x 3
   30 jan 92   Cosmos 2177:2179   GLONASS x 3
   30 jul 92   Cosmos 2177:2179   GLONASS x 3
   17 feb 93   Cosmos 2234:2236   GLONASS x 3
   11 apr 94   Cosmos 2275:2277   GLONASS x 3
   11 aug 94   Cosmos 2287:2289   GLONASS x 3
   20 nov 94   Cosmos 2294:2296   GLONASS x 3
   07 mar 95   Cosmos 2306:2309   GLONASS x 3
   24 jul 95   Cosmos 2316:2319   GLONASS x 3
   14 dec 95   Cosmos 2323:2225   GLONASS x 3
   30 dec 98   Cosmos 2362:2364   GLONASS x 3
   13 oct 00   Cosmos 2374:2376   GLONASS x 3

   01 dec 01   Cosmos 2380:2382   GLONASS x 2, GLONASS-M x 1 (COSMOS 2382)
   25 dec 02   Cosmos 2394:2396   GLONASS x 3
   01 dec 03   Cosmos 2402:2404   GLONASS x 2, GLONASS-M x 1 (COSMOS 2404)
   26 dec 04   Cosmos 2411:2413   GLONASS x 2, GLONASS-M x 1 (COSMOS 2413)
   26 dec 04   Cosmos 2411:2413   GLONASS x 2, GLONASS-M x 1 (COSMOS 2413)
   25 dec 05   Cosmos 2417:2419   GLONASS x 1 (COSMOS 2417), GLONASS-M x 2
   24 dec 06   Cosmos 2424:2426   GLONASS-M x 3
   26 oct 07   Cosmos 2431:2433   GLONASS-M x 3
   25 dec 07   Cosmos 2435:2437   GLONASS-M x 3

   _______________________________________________________________________

Launches of GLONASS spacecraft have been spotty, not surprising considering the unsettled nature of the Russian state in the wake of the collapse of the USSR, and the constellation still remains incomplete. However, the Russians have been bringing in money with oil sales and finally have had the funds to get moving on GLONASS. They managed to get 18 operational spacecraft into orbit by the end of 2007, bringing the constellation up to a minimal operating capability, and want to have a full constellation of 24 operational spacecraft in orbit by 2010.

The Russians are now working on a third-generation GLONASS spacecraft, the "GLONASS-K", which will be smaller and will have a design lifetime of ten years. First launches are expected in 2008. India has signed a partnership deal with the Russians for a share in the GLONASS-K development program, with the Indians performing some GLONASS-M and GLONASS-K launches.

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[2.3] JAPANESE, EUROPEAN, & CHINESE NAVIGATION SATELLITE EFFORTS

* Both Japan and the European Space Agency (ESA) are now working on GPS augmentation systems, with the main function of both networks being air traffic control.

The Japanese system is known as "MTSAT (Multifunction Transport SATellite) Space-based Augmentation System" or "MSAS", and is being implemented by the Japanese Meteorological Agency and the Japanese Ministry of Transport, hence the name of the satellite. The MTSAT spacecraft are a combination meteorological and communications satellite, and are being placed in geostationary orbit over the eastern Pacific.

The satellites provide voice and data links between aircraft and ground controllers; hand off relay GPS augmentation and integrity data to aircraft; and provide ground controllers with precision aircraft location. They replace the long-standing Japanese "Himawari (Sunflower)" or "Geostationary Meteorological Satellite (GMS)" series in the weather surveillance role, using a observation payload to track clouds and storms, and a relay system to pass data from surface stations on to central weather analysis centers.

The first MTSAT was launched by a Japanese H-2 booster on 15 November 1999, but the spacecraft failed to reach orbit. A replacement spacecraft, designated "MTSAT-1R", was successfully launched by an H-2A booster on 26 February 2005, with a second satellite designated "MTSAT-2" following on 18 February 2006. MTSAT-1 and MTSAT-1R were built by Space Systems / Loral, were based on standard Loral satellite buses, and had a launch weight of 2,900 kilograms (6,400 pounds). MTSAT-2 was built by Mitsubishi Electric with assistance from Boeing Satellite Systems and Alcatel Space, and had a larger launch mass of 4,535 kilograms (10,000 pounds).

* The ESA network is known as the "European Geostationary Navigation Overlay System (EGNOS)". Like MSAS, EGNOS transmits augmentation and integrity data to aircraft through geostationary communications satellites. It went into service in July 2005, using the INMARSAT AOR-E and IOR commercial communications satellites, along with the European Space Agency Artemis experimental communications satellite. The satellites provide coverage of subpolar areas ranging from the east coast of the United States to Japan, using 43 ground stations in 22 countries. It is being used as an approach and landing system comparable to WAAS.

* There has been some effort towards building receivers that can obtain signals from both GPS and GLONASS, providing substantially greater accuracy than would be possible from either by itself. Use of two satellite systems also gives users a "backup" operational capability if one of the systems is disabled. The European Community is now implementing the "Global Navigation Satellite System 1 (GNSS-1)", which will integrate services from GPS, GLONASS, and various augmentation networks.

One of the problems in combining use of GPS and GLONASS is that they use different global coordinate systems. GPS uses a coordinate system named "WGS-84", in which the precise location of the North Pole (which drifts a bit) is fixed at its location in 1984. GLONASS uses a coordinate system named "PZ-90", in which the precise location of the North Pole is given as an average of its position from 1900 to 1905. Trying to link the two coordinate systems has proven difficult, particularly because there are far fewer GLONASS receivers than GPS receivers.

GNSS-1 is supposedly a stepping stone to a completely independent European "GNSS-2". GNSS-2, or "Galileo" as it has been named, is to be based on an entirely new satellite constellation, consisting of 30 satellites, including three on-orbit spares, placed in three orbital planes at an altitude of 26,616 kilometers (16,530 miles). The orbital system will be integrated from the start with ground augmentation networks. The Galileo satellites will also carry COSPAS-SARSAT search and rescue payloads.

Unlike GPS, Galileo will be under complete civilian control. It is being implemented through a cooperative relationship between the ESA and the European Union (EU) organization. European military forces have expressed interest in making use of Galileo, but so far have not offered to help with funding. India bought into a share of the program in late 2003.

The Galileo group plans to offer four types of service packages: an "open" service available to all at no cost; a "safety of life" service that provides alerts when the system's accuracy or integrity is compromised; a commercial service using encrypted signals; and a public regulated service for government users. The Galileo system uses a different scheme from the US GPS system, but work was done to make sure the two systems dovetailed well enough to prevent mutual interference and allow users to pick up both systems with a single receiver.

The green light for the demonstration-validation ("dem-val") phase was given in the summer of 2003. Current plans envision flight of two "Galileo Test-Bed Satellites (GTBS)", with the spacecraft known more specifically by the name of "Galileo In-Orbit Validation Element (GIOVE), followed by launch of four dem-val spacecraft.

The contract for the first testbed satellite, GIOVE A, was issued to Surrey Satellite Technology LTD in the UK in July 2003. GIOVE A was launched from Baikonur by a Soyuz-Fregat booster on 28 November 2005; the satellite had a launch mass of 600 kilograms (1,327 pounds). Galileo Industries, a European consortium led by Astrium, Alcatel Space, and Alenia, is developing the larger GIOVE B satellite, with this spacecraft to be launched by a Soyuz booster from Baikonur in the spring of 2006. A contract for the four dem-val or "In-Orbit Validation (IOV)" spacecraft was signed in 2006 with Galileo Industries. Initial launch of an IOV satellite will be in 2008, with the full 30-satellite constellation in orbit by the end of 2010 and introduction to operational service in 2011. Galileo was designed to provide locations with a meter of error.

However, Galileo has suffered from delays and cost overruns, partly due to persistent bickering over the program among the member states. Critics have blasted the program, calling it a classic "Euro-boondoggle", based on a completely bogus business model and amounting to little more than an overpriced attempt to acquire a "me-too" GPS system. There have been loud calls for its cancellation, though for the time being the program remains alive.

* China is now operating their own first-generation satellite navigation system, named "Compass". The first "Beidou" satellite, where "Beidou" is the "North Dipper", the Chinese name for the constellation of the Big Dipper, was launched in the fall of 2000, with five satellites launched to date:

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   31 oct 00   Beidou 1                  Long March 3
   21 dec 00   Beidou 1B                 Long March 3
   25 may 03   Beidou 3                  Long March 3
   02 feb 07   Beidou 4                  Long March 3
   14 apr 07   Beidou 5 AKA Compass 1M   Long March 3
   ___________________________________________________

The first four satellites were labelled as experimental, with the fifth being the first operational spacecraft. The satellites were based on the Chinese DFH-3 geostationary communications satellite and had a launch weight of 1,000 kilograms (2,200 pounds) each. The first two geostationary satellites provided navigational coverage over the entire country. There was some impression initially, partly because the spacecraft looked so much like communications satellites, that they provided error corrections of GPS signals much as did MTSAT, but as it turned out, they provided locations without use of other navsat systems.

The scheme is referred to as the "TwinStar" system; it was demonstrated using two DFH-2A communications satellites in 1989, leading to authorization of full development of the Compass system in 1993. The developed system involves a ground-based control center sending an interrogation signal to a user's ground-based navigation receiver through the Beidou satellites, with the receiver then sending back a response through two satellites. The "time delay of arrival (TDOA)" of the response signal back to each satellite allows the position of the receiver to be determined by triangulation, with the position estimate refined at the control center by cross-referencing to a China terrain altitude database. The position data is relayed back to the receiver using an encrypted channel -- of course, Compass was designed with military applications in mind, just as were US and Soviet navsat systems -- and users can also send text messages with up to 120 Chinese characters through the spacecraft.

The system operates in a band around 2.491 GHz. There was an ambiguity in the use of two satellites, in that the position might be north or south of the Equator, but since all of China is well north of the Equator and Compass was intended at least at the outset as a national system, that's not a problem. Accuracy is only about 100 meters (330 feet), though it can be improved to 20 meters (66 feet) or better using ground-based augmentation systems. Since two-way communications are required, the Compass receivers are generally bigger than GPS / GLONASS receivers, and the maximum number of users runs to about 540,000 over the course of an hour.

Military users began to utilize the Compass system in late 2001, with civilian users getting on board in April 2004. Compass was clearly designed to provide a useful navsat system under Chinese control at much lower cost than fielding a full GPS / GLONASS-type global navsat constellation. Only five satellites will be needed to provide a low-resolution global satellite network, with a high-resolution system under consideration that would use 30 medium-orbit satellites. China is also cooperating on development of the Galileo system, possibly as an alternate plan.

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[2.4] FOOTNOTE (1): NAVIGATING ON OTHER WORLDS

* Robot and human explorers visiting other worlds in the Solar System such as the Moon or Mars would find a navigation satellite system useful to find their away around. While Earth-based tracking systems can do an amazingly good job of tracking a spacecraft in flight to or in orbit around another world, navigating around on the surface of the world requires a less cumbersome approach.

GPS is used for navigation on Earth orbiting spacecraft, but these vehicles are only in low orbit and so they might as well still be on the ground as far as the GPS constellation far above is concerned. Since GPS satellites point their antennas toward the Earth, once a spacecraft flies beyond the orbit of the constellation the signals start to become difficult to pick up. The best reception would be from satellites just coming over the Earth's horizon, but that would limit the number of satellites that could be observed at any one time for a position fix.

Furthermore, GPS signals are already weak, and at the distance of the Moon they would be so weak as to be very hard to pick up; in addition their triangulation angles would be narrow, limiting positioning accuracy. NASA researchers have had has some ideas for setting up a network of lunar ground stations to provide augmentation, though of course that would be a useless exercise on the Moon's far side, where there is no line of sight to the Earth or anything orbiting close to it.

Earth-based GPS signals would be impossible to pick up on Mars. NASA has rejected the idea of trying to duplicate a GPS constellation around Mars because of the cost and the difficulty of maintaining the timing system. Since Mars rovers don't move very fast, there would be no need to obtain continuous accurate positioning coordinates anyway. NASA sees that orbiters providing communications relay services to Mars ground stations and rovers could do the job, using a system similar to the classic DME aircraft navigation aid: the orbiter would send a short signal to the station or rover, which would immediately shoot it back, with the round-trip time giving the distance between the two. Since the position of the orbiter can be precisely determined by Earth stations, several such ranging transactions could give the precise ground location of the station or rover. Nobody envisions the number of ground stations or rovers as being very large, not remotely in the same league as the number of GPS receivers on Earth, and so the scheme would be manageable.

NASA and DARPA have been worked together on a particularly exotic notion for a space navigation system: mapping the positions of X-ray pulsars (XRPs) in the sky. An XRP is a superdense, rapidly spinning neutron star that produces bursts of X rays on a well-defined period that changes little on a human timescale, with different XRPs in different positions in the sky having different periods. Although the Earth's ozone layer prevents X rays from reaching the ground, the atmosphere of Mars presents little obstacle to X rays and the airless Moon presents no obstacle at all, allowing a map of known XRPs to be used to get a position fix. The trick to design an X-ray detector system that is compact and cheap enough to be practical, since traditionally an X-ray telescope is a hefty, complicated, and expensive piece of gear.

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[2.5] FOOTNOTE (2): THE COSPAS-SARSAT NETWORK

* As mentioned, the Soviet-Russian Nadezhda satellites carry an auxiliary "COSPAS-SARSAT" payload for search and rescue operations, which is an interesting technology in its own right. The "SARSAT" in the name stands for "Search And Rescue Satellite-Aided Tracking", which describes exactly what it is. (The "COSPAS" part is a Russian acronym for "Space System for the Search of Vessels in Distress".)

The US got interested in the use of civil radio rescue beacons in 1970, when a light aircraft carrying two US congressman disappeared over the wilds of Alaska. Given the vast uninhabited regions of the state, the search was like hunting for a needle in a haystack, and the plane and its passengers were never found. In response, the US Congress passed a law requiring that all aircraft operating in the United States carry a "Emergency Locator Transmitter (ELT)". The ELT would be activated in a crash and broadcast a distress signal at 121.5 MHz, the international aircraft distress frequency band.

The original ELT scheme was workable but far from satisfactory. Not only did the ELT have limited range, but its radio band was cluttered and noisy, and none of the signals operating on the band provided an identification feature to allow search and rescue (SAR) teams to sort between them. As a result, the US began to consider a space-based system that overcame these limitations. Other nations were interested in the same idea, and the US, Canada, and France banded together to work on the SARSAT concept. The USSR was working along a separate track on the COSPAS concept, but in 1979 the Western and Soviet groups decided to band together to form the COSPAS-SARSAT collaboration.

The new COSPAS-SARSAT rescue beacons operated on a band around 406 MHz that was reserved strictly for their use, and provided a coded digital ID signal so that a specific beacon could be identified. The digital ID signal was normally 112 bits in length and provided a country code, an ID code, and a field for supplementary data. An optional 32-bit data field could be added, extending the ID signal to 144 bits.

Satellites were fitted with secondary payloads that would monitor the rescue band and determine its location with enough accuracy to allow SAR teams to fly into the area and home in on the beacon signal. The satellites also monitored the old 121.5 MHz band, since the old ELTs were still in widespread use. The first satellite with a COSPAS-SARSAT payload was launched in 1982 and the network was fully operational by 1985. Dozens of countries eventually signed on to the effort.

* There are three different classes of homing beacons:

The signals emitted by the beacons are picked up by transponders on satellites that relay them to a ground station. Transponders for beacons are carried on three classes of satellites:

The TIROS and Nadezhda satellites are referred to as "Low Earth Orbit SAR (LEOSAR)" satellites. They can determine the location of a beacon using Doppler processing, something like the old Transit navigation scheme turned on its head. The satellites travel in a known orbit, and the Doppler shifts of a rescue beacon due to the motion of the satellite can be used to provide the general location of the beacon, allowing SAR teams to get close enough to pick up the beacon.

As the satellite moves towards the beacon, the Doppler shift increases the received frequency of the beacon signal; when the satellite passes over the beacon, there is no Doppler shift; when the satellite moves away from the beacon, the Doppler shift decreases the frequency of the beacon signal. If the beacon is offset to one side of the satellite's orbital path the same pattern of frequency shifts occur -- but over a decreasing range of frequencies as the sideways distance between the beacon and the orbital path increases.

There is an ambiguity here in determining which side of the orbital path the beacon is on with a single pass of a satellite, but two satellite observations along nearby tracks can cross-bracket the signal to a specific location. 406 MHz beacons result in greater location accuracy than the old 121.5 MHz beacons because the 406 MHz beacons are designed to tighter frequency specifications. Interestingly, even with observations from only one satellite, the system can give a "best guess" for which side of the flight track the beacon was on that often proves accurate. If the satellite were simply flying in a straight line over a flat immobile surface that would be impossible, but the real world is not flat and it is rotating under the satellite track, "skewing" the position vectors so that the change in angle of the vectors as the satellite passes is compressed slightly on the "correct" side of the track and expanded slightly on the "wrong" side of the track.

The GOES satellites, which are referred to as "Geostationary SAR (GEOSAR)" satellites, do not move relative to the Earth's surface and so cannot provide location information on their own, but they can relay alerts to the ground SAR network. In 1997, a new generation of 406 MHz beacons was introduced that could transmit Global Positioning Satellite coordinates as part of their coded signal, allowing precision location using any class of COSPAS-SARSAT satellite.

As mentioned, the European Galileo navigation satellites are supposed to carry COSPAS-SARSAT payloads. These spacecraft will be in medium Earth orbit, so it is a bit unclear what class of services they will provide for the COSPAS-SARSAT system.

* The ground stations of the COSPAS-SARSAT network are known as "Local User Terminals (LUTs)". They relay their information to a regional "Mission Control Center (MCC)", which deciphers the ID code on the beacon signal and correlates the event with other useful information, which is then passed on to the proper regional "SAR Point of Contact (SPOC)" or "Rescue Coordination Center (RCC)", which dispatches SAR teams to deal with the emergency.

Incidentally, apparently some numbers of 243 MHz beacons, like the 121.5 MHz first-generation beacons but operating at twice frequency, were built and some satellites in the COSPAS-SARSAT network can pick up 243 MHz signals. 243 MHz is the military emergency band and so these beacons are used by military forces.

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[2.6] COMMENTS, SOURCES, & REVISION HISTORY

* Sources include:

Launch histories and some details were obtained from Mark Wade's online ENCYCLOPEDIA ASTRONAUTICA, and Gunter Kreb's GUNTER'S SPACE PAGE in Germany. The Russian military maintains a GLONASS page, but at least the English materials are sketchy. ESA's pages on Galileo provided a fair update on the current status of a program that has been a moving target. The China Defense website gave the only explanation I could find of the Compass system.

* Revision history:

   v1.0   / 05 dec 96 / gvg / Introduced as "The Global Positioning System".
   v1.1   / 07 dec 98 / gvg / Minor cosmetic update.
   v2.0   / 01 jun 99 / gvg / GNSS, Y2K problems, general rewrite.
   v2.1   / 01 jun 01 / gvg / More details on WAAS & so on, plus GPS jamming.
   v2.2.0 / 01 nov 01 / gvg / Added updates, data on other systems, new title.
   v2.2.1 / 01 mar 02 / gvg / Minor fixes.
   v2.2.2 / 01 jul 02 / gvg / GPS III comments.
   v2.2.3 / 01 dec 02 / gvg / Comments on GPS constellation degradation.
   v3.0.0 / 01 dec 04 / gvg / Split into two chapters, overall update.
   v3.0.1 / 01 jan 05 / gvg / Minor cleanup, Galileo fixes.
   v3.0.2 / 01 apr 06 / gvg / Various updates.
   v3.0.3 / 01 feb 08 / gvg / Minor updates, some rearrangement.
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