Lecture notes on Satellite Communication systems

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Ch01_Roddy\ks 5/29/01 12:09 PM Page 1 Chapter 1 Overview of Satellite Systems 1.1 Introduction The use of satellites in communications systems is very much a fact of everyday life, as is evidenced by the many homes which are equipped with antennas, or “dishes,” used for reception of satellite television. What may not be so well known is that satellites form an essential part of telecommunications systems worldwide, carrying large amounts of data and telephone traffic in addition to television signals. Satellites offer a number of features not readily available with other means of communications. Because very large areas of the earth are visible from a satellite, the satellite can form the star point of a com- munications net linking together many users simultaneously, users who may be widely separated geographically. The same feature enables satellites to provide communications links to remote communities in sparsely populated areas which are difficult to access by other means. Of course, satellite signals ignore political boundaries as well as geo- graphic ones, which may or may not be a desirable feature. To give some idea of cost, the construction and launch costs of the Canadian Anik-E1 satellite (in 1994 Canadian dollars) were 281.2 million, and the Anik-E2, 290.5 million. The combined launch insur- ance for both satellites was 95.5 million. A feature of any satellite sys- tem is that the cost is distance insensitive, meaning that it costs about the same to provide a satellite communications link over a short dis- tance as it does over a large distance. Thus a satellite communications system is economical only where the system is in continuous use and the costs can be reasonably spread over a large number of users. Satellites are also used for remote sensing, examples being the detection of water pollution and the monitoring and reporting of weather conditions. Some of these remote sensing satellites also form 1 Copyright 2001 The McGraw-Hill Companies Click Here for Terms of Use TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 2 2 Chapter One a vital link in search and rescue operations for downed aircraft and the like. A good overview of the role of satellites is given by Pritchard (1984) and Brown (1981). To provide a general overview of satellite systems here, three different types of applications are briefly described in this chapter: (1) the largest international system, Intelsat, (2) the domestic satellite system in the United States, Domsat, and (3) U.S. National Oceanographic and Atmospheric Administration (NOAA) series of polar orbiting satellites used for environmental monitoring and search and rescue. 1.2 Frequency Allocations for Satellite Services Allocating frequencies to satellite services is a complicated process which requires international coordination and planning. This is carried out under the auspices of the International Telecommunication Union. To facilitate frequency planning, the world is divided into three regions: Region 1: Europe, Africa, what was formerly the Soviet Union, and Mongolia Region 2: North and South America and Greenland Region 3: Asia (excluding region 1 areas), Australia, and the south- west Pacific Within these regions, frequency bands are allocated to various satel- lite services, although a given service may be allocated different fre- quency bands in different regions. Some of the services provided by satellites are Fixed satellite service (FSS) Broadcasting satellite service (BSS) Mobile satellite services Navigational satellite services Meteorological satellite services There are many subdivisions within these broad classifications; for example, the fixed satellite service provides links for existing tele- phone networks as well as for transmitting television signals to cable companies for distribution over cable systems. Broadcasting satellite services are intended mainly for direct broadcast to the home, some- times referred to as direct broadcast satellite (DBS) service in Europe it may be known as direct-to-home (DTH) service. Mobile satellite ser- TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 3 Overview of Satellite Systems 3 vices would include land mobile, maritime mobile, and aeronautical mobile. Navigational satellite services include global positioning sys- tems, and satellites intended for the meterorological services often provide a search and rescue service. Table 1.1 lists the frequency band designations in common use for satellite services. The Ku band signifies the band under the K band, and the Ka band is the band above the K band. The Ku band is the one used at present for direct broadcast satellites, and it is also used for certain fixed satellite services. The C band is used for fixed satellite services, and no direct broadcast services are allowed in this band. The VHF band is used for certain mobile and navigational services and for data transfer from weather satellites. The L band is used for mobile satellite services and navigation systems. For the fixed satellite ser- vice in the C band, the most widely used subrange is approximately 4 to 6 GHz. The higher frequency is nearly always used for the uplink to the satellite, for reasons which will be explained later, and common practice is to denote the C band by 6/4 GHz, giving the uplink fre- quency first. For the direct broadcast service in the Ku band, the most widely used range is approximately 12 to 14 GHz, which is denoted by 14/12 GHz. Although frequency assignments are made much more pre- cisely, and they may lie somewhat outside the values quoted here (an example of assigned frequencies in the Ku band is 14,030 and 11, 730 MHz), the approximate values stated above are quite satisfactory for use in calculations involving frequency, as will be shown later in the text. Care must be exercised when using published references to fre- quency bands because the designations have developed somewhat dif- ferently for radar and communications applications; in addition, not all countries use the same designations. The official ITU frequency TABLE 1.1 Frequency Band Designations Frequency range, GHz Band designation 0.1–0.3 VHF 0.3–1.0 UHF 1.0–2.0 L 2.0–4.0 S 4.0–8.0 C 8.0–12.0 X 12.0–18.0 Ku 18.0–27.0 K 27.0–40.0 Ka 40.0–75 V 75–110 W 110–300 mm 300–3000 m TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 4 4 Chapter One band designations are shown in Table 1.2 for completeness. However, in this text the designations given in Table 1.1 will be used, along with 6/4 GHz for the C band and 14/12 GHz for the Ku band. 1.3 INTELSAT INTELSAT stands for International Telecommunications Satellite. The organization was created in 1964 and currently has over 140 member countries and more than 40 investing entities (see http://www.intelsat.com/ for more details). Starting with the Early Bird satellite in 1965, a succession of satellites has been launched at intervals of a few years. Figure 1.1 illustrates the evolution of some of the INTELSAT satellites. As the figure shows, the capacity, in terms of number of voice channels, increased dramatically with each suc- ceeding launch, as well as the design lifetime. These satellites are in geostationary orbit, meaning that they appear to be stationary in rela- tion to the earth. The geostationary orbit is the topic of Chap. 3. At this point it may be noted that geostationary satellites orbit in the earth’s equatorial plane and that their position is specified by their longitude. For international traffic, INTELSAT covers three main regions, the Atlantic Ocean Region (AOR), the Indian Ocean Region (IOR), and the Pacific Ocean Region (POR). For each region, the satellites are positioned in geostationary orbit above the particular ocean, where they provide a transoceanic telecommunications route. The coverage areas for INTELSAT VI are shown in Fig. 1.2. Traffic in the AOR is about three times that in the IOR and about twice that in the IOR and POR combined. Thus the system design is tailored mainly around AOR requirements (Thompson and Johnston, 1983). As of May 1999, there were three INTELSAT VI satellites in service in the AOR and two in service in the IOR. TABLE 1.2 ITU Frequency Band Designations Frequency range Metric Band (lower limit exclusive, Corresponding abbreviations number Symbols upper limit inclusive) metric subdivision for the bands 4 VLF 3–30 kHz Myriametric waves B.Mam 5LF 30–300 kHz Kilometric waves B.km 6 MF 300–3000 kHz Hectometric waves B.hm 7HF 3–30 MHz Decametric waves B.dam 8 VHF 30–300 MHz Metric waves B.m 9 UHF 300–3000 MHz Decimetric waves B.dm 10 SHF 3–30 GHz Centimetric waves B.cm 11 EHF 30–300 GHz Millimetric waves B.mm 12 300–3000 GHz Decimillimetric waves SOURCE: ITU Geneva. TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 5 5 Figure 1.1 Evolution of INTELSAT satellites. (From Colino 1985; courtesy of ITU Telecommunications Journal.) TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 6 6 Chapter One Figure 1.2 INTELSAT VI coverage areas. (From P. T. Thompson and E. C. Johnston, INTELSAT VI: A New Satellite Generation for 1986–2000, International Journal of Satellite Communications, vol. 1, 3–14. © John Wiley & Sons, Ltd.) The INTELSAT VII-VII/A series was launched over a period from October 1993 to June 1996. The construction is similar to that for the V and VA/VB series shown in Fig. 1.1 in that the VII series has solar sails rather than a cylindrical body. This type of construction is described more fully in Chap. 7. The VII series was planned for service in the POR and also for some of the less demanding services in the AOR. The antenna beam coverage is appropriate for that of the POR. Figure 1.3 shows the antenna beam footprints for the C-band hemi- spheric coverage and zone coverage, as well as the spot beam coverage possible with the Ku-band antennas (Lilly, 1990; Sachdev et al., 1990). When used in the AOR, the VII series satellite is inverted north for south (Lilly, 1990), minor adjustments then being needed only to opti- mize the antenna patterns for this region. The lifetime of these satel- TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 7 Overview of Satellite Systems 7 Figure 1.3 INTELSAT VII coverage (Pacific Ocean Region; global, hemispheric, and spot beams). (From Lilly, 1990, with permission.) lites ranges from 10 to 15 years depending on the launch vehicle. Recent figures from the INTELSAT Web site give the capacity for the INTELSAT VII as 18,000 two-way telephone circuits and 3 TV chan- nels; up to 90,000 two-way telephone circuits can be achieved with the use of “digital circuit multiplication.” The INTELSAT VII/A has a capacity of 22,500 two-way telephone circuits and 3 TV channels; up to 112,500 two-way telephone circuits can be achieved with the use of digital circuit multiplication. As of May 1999, four satellites were in service over the AOR, one in the IOR, and two in the POR. The INTELSAT VIII-VII/A series of satellites was launched over a period February 1997 to June 1998. Satellites in this series have sim- ilar capacity as the VII/A series, and the lifetime is 14 to 17 years. It is standard practice to have a spare satellite in orbit on high-relia- bility routes (which can carry preemptible traffic) and to have a ground TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 8 8 Chapter One spare in case of launch failure. Thus the cost for large international schemes can be high; for example, series IX, described below, represents a total investment of approximately 1 billion. The INTELSAT IX satellites are the latest in the series (Table 1.3). They will provide a much wider range of services than previously and promise such services as Internet, direct-to-home (DTH) TV, tele- medicine, tele-education, and interactive video and multimedia. In addition to providing transoceanic routes, the INTELSAT satel- lites are also used for domestic services within any given country and regional services between countries. Two such services are Vista for telephone and Intelnet for data exchange. Figure 1.4 shows typical Vista applications. 1.4 U.S. Domsats Domsat is an abbreviation for domestic satellite. Domestic satellites are used to provide various telecommunications services, such as voice, data, and video transmissions, within a country. In the United States, all domsats are situated in geostationary orbit. As is well known, they make available a wide selection of TV channels for the home entertainment market, in addition to carrying a large amount of commercial telecommunications traffic. U.S. Domsats which provide a direct-to-home television service can be classified broadly as high power, medium power, and low power (Reinhart, 1990). The defining characteristics of these categories are shown in Table 1.4. The main distinguishing feature of these categories is the equivalent isotropic radiated power (EIRP). This is explained in more detail in Chap. 12, but for present purposes it should be noted that the upper limit of EIRP is 60 dBW for the high-power category and 37 dBW for the low-power category, a difference of 23 dB. This represents an increase in 2.3 received power of 10 or about 200:1 in the high-power category, which allows much smaller antennas to be used with the receiver. As noted in TABLE 1.3 INTELSAT Series IX Geostationary Satellites Satellite Projected location Capacity Launch window 901 62°E Up to 96 units of 36 MHz First quarter 2001 902 60°E Up to 96 units of 36 MHz First quarter 2001 903 335.5°E Up to 96 units of 36 MHz Second quarter 2001 904 342°E Up to 96 units of 36 MHz Third quarter 2001 905 332.5°E Up to 96 units of 36 MHz Fourth quarter 2001 to first quarter 2002 906 332.5°E Up to 92 units of 36 MHz To be determined 907 328.5°E Up to 96 units of 36 MHz To be determined TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 9 9 Figure 1.4 (a) Typical Vista application; (b) domestic/regional Vista network with standard A or B gateway. (From Colino, 1985; courtesy of ITU Telecommunication Journal.) TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 10 10 Chapter One TABLE 1.4 Defining Characteristics of Three Categories of United States DBS Systems High power Medium power Low power Band Ku Ku C Downlink frequency 12.2–12.7 11.7–12.2 3.7–4.2 allocation, GHz Uplink frequency allocation, GHz 17.3–17.8 14–14.5 5.925–6.425 Space service BSS FSS FSS Primary intended use DBS Point to point Point to point Allowed additional use Point to point DBS DBS Terrestrial interference possible No No Yes Satellite spacing, degrees 9 2 2–3 Satellite spacing determined by ITU FCC FCC Adjacent satellite No Yes Yes interference possible? Satellite EIRP range, dBW 51–60 40–48 33–37 ITU: International Telecommunication Union; FCC: Federal Communications Commission. SOURCE: Reinhart, 1990. the table, the primary purpose of satellites in the high-power category is to provide a DBS service. In the medium-power category, the primary purpose is point-to-point services, but space may be leased on these satellites for the provision of DBS services. In the low-power category, no official DBS services are provided. However, it was quickly discov- ered by home experimenters that a wide range of radio and TV pro- gramming could be received on this band, and it is now considered to provide a de facto DBS service, witness to which is the large number of TV receive-only (TVRO) dishes which have appeared in the yards and on the rooftops of homes in North America. TVRO reception of C-band signals in the home is prohibited in many other parts of the world, part- ly for aesthetic reasons because of the comparatively large dishes used, and partly for commercial reasons. Many North American C-band TV broadcasts are now encrypted, or scrambled, to prevent unauthorized access, although this also seems to be spawning a new underground industry in descramblers. As shown in Table 1.4, true DBS service takes place in the Ku band. Figure 1.5 shows the components of a direct broadcasting satellite sys- tem (Government of Canada, 1983). The television signal may be relayed over a terrestrial link to the uplink station. This transmits a very narrowbeam signal to the satellite in the 14-GHz band. The satel- lite retransmits the television signal in a wide beam in the 12-GHz frequency band. Individual receivers within the beam coverage area will receive the satellite signal. Table 1.5 shows the orbital assignments for domestic fixed satellites for the United States (FCC, 1996). These satellites are in geostation- ary orbit, which is discussed further in Chap. 3. Table 1.6 shows the TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 11 Overview of Satellite Systems 11 Figure 1.5 Components of a direct broadcasting satellite system. (From Government of Canada, 1983, with permission.) U.S. Ka-band assignments. Broadband services such as Internet (see Chap. 15) can operate at Ka-band frequencies. In 1983, the U.S. Federal Communications Commission (FCC) adopted a policy objec- tive setting 2° as the minimum obital spacing for satellites operating in the 6/4-GHz band and 1.5° for those operating in the 14/12-GHz band (FCC, 1983). It is clear that interference between satellite cir- cuits is likely to increase as satellites are positioned closer together. These spacings represent the minimum presently achievable in each band at acceptable interference levels. In fact, it seems likely that in some cases home satellite receivers in the 6/4-GHz band may be sub- ject to excessive interference where 2° spacing is employed. 1.5 Polar Orbiting Satellites Polar orbiting satellites orbit the earth in such a way as to cover the north and south polar regions. (Note that the term polar orbiting does not mean that the satellite orbits around one or the other of the poles). Figure 1.6 shows a polar orbit in relation to the geostationary orbit. Whereas there is only one geostationary orbit, there are, in theory, an TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 12 12 Chapter One TABLE 1.5 FCC Orbital Assignment Plan (May 7, 1996) Location Satellite Band/polarization 139°W.L. Aurora II/Satcom C-5 4/6 GHz (vertical) 139°W.L. ACS-3K (AMSC) 12/14 GHz 137°W.L. Satcom C-1 4/6 GHz (horizontal) 137°W.L. Unassigned 12/14 GHz 135°W.L. Satcom C-4 4/6 GHz (vertical) 135°W.L. Orion O-F4 12/14 GHz 133°W.L. Galaxy 1-R(S) 4/6 GHz (horizontal) 133°W.L. Unassigned 12/14 GHz 131°W.L. Satcom C-3 4/6 GHz (vertical) 131°W.L. Unassigned 12/14 GHz 129°W.L. Loral 1 4/6 GHz (horizontal)/12/14 GHz 127°W.L. Galaxy IX 4/6 GHz (vertical) 127°W.L. Unassigned 12/14 GHz 125°W.L. Galaxy 5-W 4/6 GHz (horizontal) 125°W.L. GSTAR II/unassigned 12/14 GHz 123°W.L. Galaxy X 4/6 GHz (vertical)/12/14 GHz 121°W.L. EchoStar FSS-2 12/14 GHz 105°W.L. GSTAR IV 12/14 GHz 103°W.L. GE-1 4/6 GHz (horizontal) 103°W.L. GSTAR 1/GE-1 12/14 GHz 101°W.L. Satcom SN-4 (formerly 4/6 GHz (vertical)/12/14 GHz Spacenet IV-n) 99°W.L. Galaxy IV(H) 4/6 GHz (horizontal)/12/14 GHz 97°W.L. Telstar 401 4/6 GHz (vertical)/12/14 GHz 95°W.L. Galaxy III(H) 4/6 GHz (horizontal)/12/14 GHz 93°W.L. Telstar 5 4/6 GHz (vertical) 93°W.L. GSTAR III/Telstar 5 12/14 GHz 91°W.L. Galaxy VII(H) 4/6 GHz (horizontal)/12/14 GHz 89°W.L. Telestar 402R 4/6 GHz (vertical)/12/14 GHz 87°W.L. Satcom SN-3 (formerly 4/6 GHz (horizontal)/12/14 GHz Spacenet III-R)/GE-4 85°W.L. Telstar 302/GE-2 4/6 GHz (vertical) 85°W.L. Satcom Ku-1/GE-2 12/14 GHz 83°W.L. Unassigned 4/6 GHz (horizontal) 83°W.L. EchoStar FSS-1 12/14 GHz 81°W.L. Unassigned 4/6 GHz (vertical) 81°W.L. Satcom Ku-2/ 12/14 GHz unassigned 79°W.L. GE-5 4/6 GHz (horizontal)/12/14 GHz 77°W.L. Loral 2 4/6 GHz (vertical)/12/14 GHz 76°W.L. Comstar D-4 4/6 GHz (vertical) 74°W.L. Galaxy VI 4/6 GHz (horizontal) 74°W.L. SBS-6 12/14 GHz 72°W.L. Unassigned 4/6 GHz (vertical) 71°W.L. SBS-2 12/14 GHz 69°W.L. Satcom SN-2/Telstar 6 4/6 GHz (horizontal)/12/14 GHz 67°W.L. GE-3 4/6 GHz (vertical)/12/14 GHz 64°W.L. Unassigned 4/6 GHz (horizontal) 64°W.L. Unassigned 12/14 GHz 62°W.L. Unassigned 4/6 GHz (vertical) 62°W.L. ACS-2K (AMSC) 12/14 GHz 60°W.L. Unassigned 4/6 GHz 60°W.L. Unassigned 12/14 GHz NOTES: FCC: Federal Communications Commission; W.L.: west longitude; E.L.: east longitude. TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 13 Overview of Satellite Systems 13 TABLE 1.6 Ka-Band Orbital Assignment Plan (FCC, May 9, 1997) Location Company Band 147°W.L. Morning Star Satellite Company, L.L.C. 20/30 GHz 127°W.L. Under consideration 20/30 GHz 125°W.L. PanAmSat Licensee Corporation 20/30 GHz 121°W.L. Echostar Satellite Corporation 20/30 GHz 115°W.L. Loral Space & Communications, LTD. 20/30 GHz 113°W.L. VisionStar, Inc. 20/30 GHz 109.2°W.L. KaStar Satellite Communications Corp. 20/30 GHz 105°W.L. GE American Communications, Inc. 20/30 GHz 101°W.L. Hughes Communications Galaxy, Inc. 20/30 GHz 99°W.L. Hughes Communications Galaxy, Inc. 20/30 GHz 97°W.L. Lockheed Martin Corporation 20/30 GHz 95°W.L. NetSat 28 Company, L.L.C. 20/30 GHz 91°W.L. Comm, Inc. 20/30 GHz 89°W.L. Orion Network Systems 20/30 GHz 87°W.L. Comm, Inc. 20/30 GHz 85°W.L. GE American Communications, Inc. 20/30 GHz 83°W.L. Echostar Satellite Corporation 20/30 GHz 81°W.L. Orion Network Systems 20/30 GHz 77°W.L. Comm, Inc. 20/30 GHz 75°W.L. Comm, Inc. 20/30 GHz 73°W.L. KaStar Satellite Corporation 20/30 GHz 67°W.L. Hughes Communications Galaxy, Inc. 20/30 GHz 62°W.L. Morning Star Satellite Company, L.L.C. 20/30 GHz 58°W.L. PanAmSat Corporation 20/30 GHz 49°W.L. Hughes Communications Galaxy, Inc. 20/30 GHz 47°W.L. Orion Atlantic, L.P. 20/30 GHz 21.5°W.L. Lockheed Martin Corporation 20/30 GHz 17°W.L. GE American Communications, Inc. 20/30 GHz 25°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 28°E.L. Loral Space & Communications, LTD. 20/30 GHz 30°E.L. Morning Star Satellite Company, L.L.C. 20/30 GHz 36°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 38°E.L. Lockheed Martin Corporation 20/30 GHz 40°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 48°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 54°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 56°E.L. GE American Communications, Inc. 20/30 GHz 78°E.L. Orion Network Systems, Inc. 20/30 GHz 101°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 105.5°E.L. Loral Space & Communications, LTD. 20/30 GHz 107.5°E.L. Morning Star Satellite Company, L.L.C. 20/30 GHz 111°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 114.5°E.L. GE American Communications, Inc. 20/30 GHz 124.5°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 126.5°E.L. Orion Asia Pacific Corporation 20/30 GHz 130°E.L. Lockheed Martin Corporation 20/30 GHz 149°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 164°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 173°E.L. Hughes Communications Galaxy, Inc. 20/30 GHz 175.25°E.L. Lockheed Martin Corporation 20/30 GHz TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 14 14 Chapter One Figure 1.6 Geostationary orbit and one possible polar orbit. infinite number of polar orbits. The U.S. experience with weather satellites has led to the use of relatively low orbits, ranging in altitude between 800 and 900 km, compared with 36,000 km for the geosta- tionary orbit. In the United States, the National Oceanic and Atmospheric Administration (NOAA) operates a weather satellite system. Their Web page can be found at http://www.noaa.gov/. The system uses both geo- stationary satellites, referred to as geostationary operational environ- mental satellites (GOES), and polar operational environmental satellites (POES). There are two of these polar satellites in orbit at any one time. The orbits are circular, passing close to the poles, and they are sun syn- chronous, meaning that they cross the equator at the same local time each day. The morning orbit, at an altitude of 830 km, crosses the equa- tor going from south to north at 7:30 A.M. each day, and the afternoon orbit, at an altitude of 870 km, at 1:40 P.M. The polar orbiters are able to track weather conditions over the entire earth and provide a wide range of data, including visible and infrared radiometer data for imaging pur- poses, radiation measurements, and temperature profiles. They carry ultraviolet sensors that measure ozone levels, and they can monitor the ozone hole over Antarctica. The polar orbiters carry a NOAA letter designation before launch, which is changed to a numeric designation once the satellite achieves orbit. NOAA-J, launched in December 1994, became NOAA-14 in oper- ation. The new series, referred to as the KLM satellites, carries much improved instrumentation. Some details are shown in Table 1.7. The TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 15 Overview of Satellite Systems 15 TABLE 1.7 NOAA KLM Satellites Launch date (callup basis) NOAA-K (NOAA-15): May 13, 1998 NOAA-L: September 14, 2000 NOAA-M: May 2001 NOAA-N: December 2003 NOAA-N: July 2007 Mission life 2 years minimum Orbit Sun-synchronous, 833 ± 19 km or 870 ± 19 km Sensors Advanced Very High Resolution Radiometer (AVHRR/3) Advanced Microwave Sounding Unit-A (AMSU-A) Advanced Microwave Sounding Unit-B (AMSU-B) High Resolution Infrared Radiation Sounder (HIRS/3) Space Environment Monitor (SEM/2) Search and Rescue (SAR) Repeater and Processor Data Collection System (DCS/2) Argos data collection system (DCS) collects environmental data radioed up from automatic data collection platforms on land, on ocean buoys, and aboard free-floating balloons. The satellites process these data and retransmit them to ground stations. The NOAA satellites also participate in satellite search and rescue (SAR) operations, known generally as Cospas-Sarsat, where Cospas refers to the payload carried by participating Russian satellites and Sarsat to the payloads carried by the NOAA satellites. Sarsat-6 is car- ried by NOAA-14, and Sarsat-7 is carried by NOAA-15. The projected payloads Sarsat-8 to Sarsat-10 will be carried by NOAA-L to NOAA-N. The Cospas-Sarsat Web page is at http://www.cospas-sarsat.org/. As of January 2000, there were 32 countries formally associated with Cospas- Sarsat. Originally, the system was designed to operate only with satel- lites in low earth orbits (LEOs), this part of the search and rescue system being known as LEOSAR. Later, the system was complemented with geostationary satellites, this component being known as GEOSAR. Figure 1.7 shows the combined LEOSAR-GEOSAR system. The nominal space segment of LEOSAR consists of four satellites, although as of January 2000 there were seven in total, three Cospas and four Sarsat. In operation, the satellite receives a signal from an emer- gency beacon set off automatically at the distress site. The beacon trans- mits in the VHF/UHF range, at a precisely controlled frequency. The satellite moves at some velocity relative to the beacon, and this results in a Doppler shift in frequency received at the satellite. As the satellite approaches the beacon, the received frequency appears to be higher than the transmitted value. As the satellite recedes from the beacon, the received frequency appears to be lower than the transmitted value. Figure 1.8 shows how the beacon frequency, as received at the satellite, varies for different passes. In all cases, the received frequency goes from TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 16 16 Chapter One LEOSAR Satellites GEOSAR Satellites Figure 1.7 Geostationary Orbit Search and Rescue (GEOSAR) and Low Earth Orbit Search and Rescue (LEOSAR) satellites. (Courtesy Cospas-Sarsat Secretariat.) Figure 1.8 Polar orbiting satellite: (a) first pass; (b) second pass, earth having rotated 25˚. Satellite period is 102 min. TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 17 Overview of Satellite Systems 17 Figure 1.9 Showing the Doppler shift in received frequency on successive passes of the satellite. ELT  emergency locator transmitter. being higher to being lower than the transmitted value as the satellite approaches and then recedes from the beacon. The longest record and the greatest change in frequency are obtained if the satellite passes over the site, as shown for pass no. 2. This is so because the satellite is visible for the longest period during this pass. Knowing the orbital para- meters for the satellite, the beacon frequency, and the Doppler shift for any one pass, the distance of the beacon relative to the projection of the orbit on the earth can be determined. However, whether the beacon is east or west of the orbit cannot be determined easily from a single pass. For two successive passes, the effect of the earth’s rotation on the Doppler shift can be estimated more accurately, and from this it can be determined whether the beacon is approaching or receding from the orbital path. In this way, the ambiguity in east-west positioning is resolved. Figure 1.9 illustrates the Doppler shifts for successive passes. The satellite must of course get the information back to an earth station so that the search and rescue operation can be completed, successfully one hopes. The Sarsat communicates on a downlink fre- quency of 1544.5 MHz to one of several local user terminals (LUTs) established at various locations throughout the world. TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 18 18 Chapter One In the original Cospas-Sarsat system, the signal from the emer- gency locator transmitters (ELTs) was at a frequency of 121.5 MHz. It was found that over 98 percent of the alerts at this frequency were false, often being caused by interfering signals from other services and by inappropriate handling of the equipment. The 121.5-MHz sys- tem relies entirely on the Doppler shift, and the carrier does not carry any identification information. The power is low, typically a few tenths of a watt, which limits locational accuracy to about 10 to 20 km. There are no signal storage facilities aboard the satellites for the 121.5-MHz signals, which therefore requires that the distress site (the ELT) and the local user terminal (LUT) must be visible simulta- neously from the satellite. Because of these limitations, the 121.5- MHz beacons are being phased out. Cospas-13, planned for launch in 2006, and Sarsat-14, planned for launch from 2009, will not carry 121.5-MHz beacons. However, all Cospas-Sarsat satellites launched prior to these will carry the 121.5-MHz processors. (Recall that Sarsat-7 is NOAA-15, Sarsat-8 is NOAA-L, Sarsat-9 is NOAA-M, and Sarsat-10 is NOAA-N). The status of the 121.5-MHz LEOSAR system as of January 2000 consisted of repeaters on seven polar orbiters, 35 ground receiving sta- tions (referred to as LEOSAR local user terminals, or LEOLUTs), and 20 mission control centers (MCCs). The MCC alerts the rescue coordi- nation center (RCC) nearest the location where the distress signal orig- inated, and the RCC takes the appropriate action to effect a rescue. There are about 600,000 distress beacons, carried mostly on aircraft and small vessels. Newer beacons operating at a frequency of 406 MHz are being intro- duced. The power has been increased to 5 Watts, which should permit locational accuracy to 3 to 5 km (Scales and Swanson, 1984). These are known as emergency position indicating radio beacons (EPIRBs). Units for personnel use are also available, known as personal locator beacons (PLBs). The 406-MHz carrier is modulated with information such as an identifying code, the last known position, and the nature of the emergency. The satellite has the equipment for storing and for- warding the information from a continuous memory dump, providing complete worldwide coverage with 100 percent availability. The polar orbiters, however, do not provide continuous coverage. The mean time between a distress alert being sent and the appropriate search and rescue coordination center being notified is estimated at 27 min satel- lite storage time plus 44 min waiting time for a total delay of 71 min (Cospas-Sarsat, 1994b). The nominal frequency is 406 MHz, and originally, a frequency of 406.025 MHz was used. Because of potential conflict with the TLFeBOOKCh01_Roddy\ks 5/29/01 12:09 PM Page 19 Overview of Satellite Systems 19 GEOSTAR system, the frequency is being moved to 406.028 MHz. Beacons submitted for type approval after January 1, 2000 may oper- ate at the new frequency, and after January 1, 2001, all beacons sub- mitted for type approval must operate at a frequency of 406.028 MHz. However, beacon types approved before the January 2001 date and still in production may continue to operate at 406.025 MHz. The power of the 406 MHz beacons is 5 watts. As shown in Figure 1.7, the overall system incorporates GEOSAR satellites. Because these are stationary, there is no Doppler shift. However, the 406-MHz beacons for the GEOSTAR component carry posi- tional information obtained from the Global Positioning Satellite (GPS) system. The GPS system is described in Chap. 17. It should be noted that the GEOSAR system does not provide coverage of the polar regions. As mentioned previously, the NOAA satellites are placed in a low earth orbit typified by the NOAA-J satellite. The NOAA-J satellite will orbit the earth in approximately 102.12 min. The orbit is arranged to rotate eastward at a rate of 0.9856°/day, to make it sun- synchronous. Sun-synchronous orbits are discussed more fully in Chap. 2, but very briefly, in a sun-synchronous orbit the satellite crosses the same spot on the earth at the same local time each day. One advantage of a sun-synchronous orbit is that the same area of the earth can be viewed under approximately the same lighting condi- tions each day. By definition, an orbital pass from south to north is referred to as an ascending pass, and from north to south, as a descending pass. The NOAA-J orbit crosses the equator at about 1:40 P.M. local solar time on its ascending pass and at about 1:40 A.M. local solar time on its descending pass. Because of the eastward rotation of the satellite orbit, the earth rotates approximately 359° relative to it in 24 h of mean solar time (ordinary clock time), and therefore, in 102.12 min the earth will have rotated about 25.59° relative to the orbit. The satellite “footprint” is dis- placed each time by this amount, as shown in Fig. 1.7. At the equator, 25.59° corresponds to a distance of about 2848 km. The width of ground seen by the satellite sensors is about 5000 km, which means that some overlap occurs between passes. The overlap is greatest at the poles. 1.6 Problems 1.1. Describe briefly the main advantages offered by satellite communica- tions. Explain what is meant by a distance-insensitive communications system. 1.2. Comparisons are sometimes made between satellite and optical fiber communications systems. State briefly the areas of application for which you feel each system is best suited. TLFeBOOK

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