Satellite communications dennis roddy

Satellite communications dennis roddy

Satellite Communications

Dennis Roddy

Fourth Edition

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2.9.8 The topocentric-horizon coordinate system 62

2.9.9 The subsatellite point 64

Chapter 6 Antennas 137

7.9 Morelos and Satmex 5 227

8.4 Community Antenna TV System 244

11.7 Link Parameters Affected by Coding 331

12.9.1 Uplink rain-fade margin 377

13.2.5 Passband interference 407

TDMA

15.4 Asynchronous Transfer Mode (ATM) 494

15.10 Enhancing TCP Over Satellite Channels Using Standard Mechanisms (RFC-2488)

16.13.1 HDTV displays 554

17.7 Iridium 576

As with previous editions, the fourth edition provides broad coverage of satellite communications, whilemaintaining sufficient depth to lay the foundations for more advanced studies Mathematics is used as adescriptive tool and to obtain numerical results, but lengthy mathematical derivations are avoided In setting

worked examples in the text are presented in normal algebraic notation, so that other programs, includingprogrammable calculators can be used

The main changes compared to the previous edition are as follows In Chap 1 the sections on INTELSATand polar orbiting satellites, including environmental and search and rescue, have been updated.Sun-synchronous orbits have been treated in more detail in Chap 2 A new section on planar antennas andarrays, including reflectarrays, and array switching has been added in Chap 6 Chapter 8 includes additionaldetails on C-band reception of television signals In Chap 11, a more detailed description is given of linear

block codes, and new sections on the Shannon capacity and turbo and low density parity check (LDPC) codes have been included Chapter 12 has a new section on intersatellite links (ISLs), including optical links Chapter 15 has been extensively rewritten to include more basic details on networks and asynchronous

transfer mode (ATM) operation Chapter 16 covers high definition television (HDTV) in more detail, and the

Iridium mobile satellite system, which is now under new ownership, is described in Chap 17

In this age of heightened security concerns, it has proved difficult to get detailed technical information onsatellite systems and equipment Special thanks are, therefore, due to the following people and organizationsthat provided copies of technical papers, diagrams, and figures for the topics listed:

Planar antennas and arrays, reflectarrays, and array switching: Jacquelyn Adams, Battelle/GLITeC; Dr.

Luigi Boccia, Universita della

Calabria; Thomas J Braviak, Director, Marketing Administration, Aeroflex/KDI-Integrated Products; Dr.Michael Parnes, Ascor, Saint-Petersburg, Russia; Dharmesh Patel, Radar Division Naval ResearchLaboratory; Professor David Pozar, Electrical and Computer Engineering, University of Massachusetts atAmherst; Dr Bob Romanofsky, Antenna, Microwave, and Optical Systems Branch, NASA Glenn ResearchCenter; Dr Peter Schrock, Webmaster/Publications Representative, JPL.

ATM: William D Ivancic, Glenn Research Center, and Lewis Research Center, NASA; Dr.-Ing Petia

Todorova, Fraunhofer Institut FOKUS, Berlin

Turbo and LDPC codes: Dr Alister G Burr, Professor of Communications, Dept of Electronics, University

of York; Tony Summers, Senior Applications Engineer, Comtech AHA

HDTV: Cathy Firebrace, Information Officer, the IEE, London U.K.

INTELSAT: Travis S Taylor, Corporate Communications, Intelsat, Washington, DC.

Iridium system: Liz DeCastro, Corporate Communications Director, Iridium Satellite.

Cospas-Sarsat: Cheryl Bertoia, Principal Operations Officer, Deputy Head, Cospas-Sarsat Secretariat,

London, U.K., and Hannah Bermudez also of the Cospas-Sarsat Secretariat

And from the third edition, thanks to Dr Henry Driver of Computer Sciences Corporation for detailsrelating to the calculation of geodetic position in Chap 2 Thanks also to the users and reviewers who madesuggestions for corrections, additions, and improvements Errors should not occur, but they do, and the authorwould be grateful if these are drawn to his attention He can be reached at droddy@tbayel.com

The editorial team at McGraw-Hill has contributed a great deal in getting the fourth edition into print:thanks are due to Steve Chapman, the sponsoring editor; Diana Mattingly, editorial assistant; and Gita Raman,project manager, for their help, and their gentle but persistent reminders to keep the book on schedule

Dennis Roddy

Thunder Bay, Ontario

be so well known is that satellites form an essential part of munications systems worldwide, carrying large amounts of data andtelephone traffic in addition to television signals.

telecom-Satellites offer a number of features not readily available with othermeans of communications Because very large areas of the earth are vis-ible from a satellite, the satellite can form the star point of a commu-nications net, simultaneously linking many users who may be widelyseparated geographically The same feature enables satellites to providecommunications links to remote communities in sparsely populatedareas that are difficult to access by other means Of course, satellite sig-nals ignore political boundaries as well as geographic ones, which may

or may not be a desirable feature

To give some idea of cost, the construction and launch cost of theCanadian Anik-E1 satellite (in 1994 Canadian dollars) was $281.2million, and that of the Anik-E2, $290.5 million The combined launchinsurance for both satellites was $95.5 million A feature of any satel-

lite system is that the cost is distance insensitive, meaning that it

costs about the same to provide a satellite communications link over

a short distance as it does over a large distance Thus a satellite munications system is economical only where the system is in contin-uous use and the costs can be reasonably spread over a large number

com-of users

Satellites are also used for remote sensing, examples being thedetection of water pollution and the monitoring and reporting of

weather conditions Some of these remote sensing satellites also form

a vital link in search and rescue operations for downed aircraft andthe like

A good overview of the role of satellites is given by Pritchard (1984)and Brown (1981) To provide a general overview of satellite systemshere, three different types of applications are briefly described in thischapter: (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 andrescue

1.2 Frequency Allocations

for Satellite Services

Allocating frequencies to satellite services is a complicated processwhich requires international coordination and planning This is car-

ried out under the auspices of the International Telecommunication

Region 2: North and South America and Greenland

Region 3: Asia (excluding region 1 areas), Australia, and the west Pacific

south-Within these regions, frequency bands are allocated to various lite services, although a given service may be allocated different fre-quency bands in different regions Some of the services provided bysatellites are:

satel-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 FSS provides links for existing telephone networks

as well as for transmitting television signals to cable companies fordistribution over cable systems Broadcasting satellite services areintended mainly for direct broadcast to the home, sometimes referred

to as direct broadcast satellite (DBS) service [in Europe it may be known

as direct-to-home (DTH) service] Mobile satellite services would include

land mobile, maritime mobile, and aeronautical mobile Navigational

satellite services include global positioning systems (GPS), and

satel-lites intended for the meteorological services often provide a searchand rescue service

Table 1.1 lists the frequency band designations in common use forsatellite services The Ku band signifies the band under the K band, andthe Ka band is the band above the K band The Ku band is the one used

at present for DBS, and it is also used for certain FSS The C band isused for FSS, and no DBS is allowed in this band The very high fre-quency (VHF) band is used for certain mobile and navigational servicesand for data transfer from weather satellites The L band is used formobile satellite services and navigation systems For the FSS in the Cband, the most widely used subrange is approximately 4 to 6 GHz Thehigher frequency is nearly always used for the uplink to the satellite,for reasons that will be explained later, and common practice is to denotethe C band by 6/4 GHz, giving the uplink frequency first For the directbroadcast service in the Ku band, the most widely used range is approxi-mately 12 to 14 GHz, which is denoted by 14/12 GHz Although frequencyassignments are made much more precisely, and they may lie somewhatoutside the values quoted here (an example of assigned frequencies inthe Ku band is 14,030 and 11,730 MHz), the approximate values statedare 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 frequencybands, because the designations have been developed somewhat differ-ently for radar and communications applications; in addition, not allcountries use the same designations

TABLE 1.1 Frequency Band Designations

The official ITU frequency band designations are shown in Table 1.2 forcompleteness 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 membercountries and more than 40 investing entities (see http://www.intelsat.com/for more details) In July 2001 INTELSAT became a private companyand in May 2002 the company began providing end-to-end solutions

through a network of teleports, leased fiber, and points of presence (PoPs)

around the globe Starting with the Early Bird satellite in 1965, a sion of satellites has been launched at intervals of a few years Figure 1.1illustrates the evolution of some of the INTELSAT satellites As thefigure shows, the capacity, in terms of number of voice channels,increased dramatically with each succeeding launch, as well as the

succes-design lifetime These satellites are in geostationary orbit, meaning that

they appear to be stationary in relation to the earth The geostationaryorbit is the topic of Chap 3 At this point it may be noted that geosta-tionary satellites orbit in the earth’s equatorial plane and 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) and what is

termed Intelsat America’s Region For the ocean regions the satellites

are positioned in geostationary orbit above the particular ocean, wherethey provide a transoceanic telecommunications route For example,INTELSAT satellite 905 is positioned at 335.5° east longitude The foot-

prints for the C-band antennas are shown in Fig 1.2a, and for the band spot beam antennas in Figs 1.2b and c.

Ku-TABLE 1.2 ITU Frequency Band Designations

0 °

Figure 1.2 INTELSAT satellite 905 is positioned at 335.5 ° E

longitude (a) The footprints for the C-band antennas; (b) the Ku-band spot 1 beam antennas; and (c) the Ku-band spot 2

beam antennas.

The INTELSAT VII-VII/A series was launched over a period fromOctober 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 solarsails rather than a cylindrical body This type of construction is described

in more detail in Chap 7 The VII series was planned for service in thePOR and also for some of the less demanding services in the AOR Theantenna beam coverage is appropriate for that of the POR Figure 1.3shows the antenna beam footprints for the C-band hemispheric cover-age and zone coverage, as well as the spot beam coverage possible withthe Ku-band antennas (Lilly, 1990; Sachdev et al., 1990) When used inthe AOR, the VII series satellite is inverted north for south (Lilly, 1990),minor adjustments then being needed only to optimize the antenna pat-terns for this region The lifetime of these satellites ranges from 10 to

15 years depending on the launch vehicle Recent figures from theINTELSAT Web site give the capacity for the INTELSAT VII as 18,000two-way telephone circuits and three TV channels; up to 90,000 two-waytelephone circuits can be achieved with the use of “digital circuit mul-tiplication.” The INTELSAT VII/A has a capacity of 22,500 two-waytelephone circuits and three TV channels; up to 112,500 two-way tele-phone circuits can be achieved with the use of digital circuit multipli-cation As of May 1999, four satellites were in service over the AOR, one

in the IOR, and two in the POR

Figure 1.2 (Continued).

The INTELSAT VIII-VII/A series of satellites was launched over theperiod February 1997 to June 1998 Satellites in this series have similarcapacity 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 reliability routes (which can carry preemptible traffic) and to have aground spare in case of launch failure Thus the cost for large internationalschemes can be high; for example, series IX, described later, represents

high-a tothigh-al investment of high-approximhigh-ately $1 billion

Table 1.3 summarizes the details of some of the more recent of theINTELSAT satellites These satellites provide a much wider range ofservices than those available previously, including such services asInternet, DTH TV, tele-medicine, tele-education, and interactive videoand multimedia Transponders and the types of signals they carry are

Figure 1.3 INTELSAT VII coverage (Pacific Ocean Region; global, hemispheric, and

spot beams) (From Lilly, 1990, with permission.)

described in detail in later chapters, but for comparison purposes itmay be noted that one 36 MHz transponder is capable of carrying about

9000 voice channels, or two analog TV channels, or about eight digital

TV channels

In addition to providing transoceanic routes, the INTELSAT satellitesare also used for domestic services within any given country and regionalservices between countries Two such services are Vista for telephone andIntelnet 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, theymake available a wide selection of TV channels for the home enter-tainment market, in addition to carrying a large amount of commercialtelecommunications traffic

U.S Domsats, which provide a DTH television service, can be fied broadly as high power, medium power, and low power (Reinhart,1990) The defining characteristics of these categories are shown inTable 1.4

classi-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

TABLE 1.3 INTELSAT Geostationary Satellites

of EIRP is 60 dBW for the high-power category and 37 dBW for the power category, a difference of 23 dB This represents an increase inreceived power of 102.3or about 200:1 in the high-power category, whichallows much smaller antennas to be used with the receiver As noted inthe table, the primary purpose of satellites in the high-power category

low-is to provide a DBS service In the medium-power category, the primarypurpose is point-to-point services, but space may be leased on thesesatellites for the provision of DBS services In the low-power category,

no official DBS services are provided However, it was quickly discovered

by home experimenters that a wide range of radio and TV programmingcould 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 that have appeared in the yards and on the rooftops

of homes in North America TVRO reception of C-band signals in thehome is prohibited in many other parts of the world, partly for aestheticreasons, because of the comparatively large dishes used, and partly forcommercial reasons Many North American C-band TV broadcasts arenow encrypted, or scrambled, to prevent unauthorized access, althoughthis also seems to be spawning a new underground industry in descram-blers As shown in Table 1.4, true DBS service takes place in the Ku band.Figure 1.5 shows the components of a DBS system (Government ofCanada, 1983) The television signal may be relayed over a terrestriallink to the uplink station This transmits a very narrow beam signal tothe satellite in the 14-GHz band The satellite retransmits the televisionsignal in a wide beam in the 12-GHz frequency band Individual receiverswithin the beam coverage area will receive the satellite signal

TABLE 1.4 Defining Characteristics of Three Categories of United States DBS Systems

possible?

Commission.

SOURCE : Reinhart, 1990.

Table 1.5 shows the orbital assignments for domestic fixed satellitesfor the United States (FCC, 1996) These satellites are in geostationaryorbit, which is discussed further in Chap 3 Table 1.6 shows the U.S.Ka-band assignments Broadband services, such as Internet (see Chap.15), can operate at Ka-band frequencies In 1983, the U.S FCC adopted

a policy objective, setting 2° as the minimum orbital spacing for lites operating in the 6/4-GHz band and 1.5° for those operating in the14/12-GHz band (FCC, 1983) It is clear that interference between satel-lite circuits is likely to increase as satellites are positioned closertogether These spacings represent the minimum presently achievable

satel-in each band at acceptable satel-interference levels In fact, it seems likely that

in some cases home satellite receivers in the 6/4-GHz band may be ject to excessive interference where 2° spacing is employed

sub-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 1.5 Components of a direct broadcasting satellite system (From Government of

Canada, 1983, with permission.)

TABLE 1.5 FCC Orbital Assignment Plan (May 7, 1996)

Location, degrees

TABLE 1.6 Ka-Band Orbital Assignment Plan (FCC December 19, 1997)

Figure 1.6 shows a polar orbit in relation to the geostationary orbit.Whereas there is only one geostationary orbit, there are, in theory, aninfinite number of polar orbits The U.S experience with weather satel-lites has led to the use of relatively low orbits, ranging in altitudebetween 800 and 900 km, compared with 36,000 km for the geostation-

ary orbit Low earth orbiting (LEO) satellites are known generally by

the acronym LEOSATS

In the United States, the National Polar-orbiting Operational

Environmental Satellite System (NPOESS) was established in 1994 to

consolidate the polar satellite operations of the Air Force, NASA

(National Aeronautics and Space Administration) and NOAA (National

Oceanic and Atmospheric Administration) NPOESS manages the

TABLE 1.6 Ka-Band Orbital Assignment Plan (FCC December 19, 1997) (Continued )

Geostationary orbit and one possible polar orbit.

Integrated Program Office (IPO) and the Web page can be found at http://www.ipo.noaa.gov/ As of 2005, a four-orbit system is in place,

consisting of two U.S Military orbits, one U.S Civilian orbit and one

EUMETSAT/METOP orbit Here, METSAT stands for meteorological

satellite and EUMETSAT stands for the European organization for the exploration of the METSAT program METOP stands for meteorologi- cal operations These orbits are sun synchronous, meaning that they

cross the equator at the same local time each day For example, thesatellites in the NPOESS (civilian) orbit will cross the equator, goingfrom south to north, at times 1:30 P.M., 5:30 P.M., and 9:30 P.M Sun-synchronous orbits are described in more detail in Chap 2, but briefly,the orbit is arranged to rotate eastward at a rate of 0.9856°/day, to

make it sun synchronous In a sun-synchronous orbit the satellite

crosses the same spot on the earth at the same local time each day, sothat the same area of the earth can be viewed under approximately thesame lighting conditions each day A sun-synchronous orbit is inclinedslightly to the west of the north pole 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 polar orbits are almost circular, and as previously mentionedthey are at a height of between 800 and 900 km above earth The polarorbiters are able to track weather conditions over the entire earth, andprovide a wide range of data, including visible and infrared radiometerdata for imaging purposes, radiation measurements, and temperatureprofiles They carry ultraviolet sensors that measure ozone levels, andthey can monitor the ozone hole over Antarctica The polar orbiterscarry a NOAA letter designation before launch, which is changed to anumeric designation once the satellite achieves orbit For example,NOAA M, launched on June 24, 2002, became NOAA 17 when success-

fully placed in orbit The series referred to as the KLM satellites carry

much improved instrumentation Some details are shown in Table 1.7.Most of the polar orbiting satellites used in weather and environ-mental studies, and as used for monitoring and in search and rescue,have a “footprint” about 6000 km in diameter This is the size of theantenna spot beam on the surface of the earth As the satellite orbits theearth, the spot beam sweeps out a swath on the earth’s surface about

6000 km wide passing over north and south poles The orbital period ofthese satellites is about 102 min Since a day has 1440 min, the number

of orbits per day is 1440/102 or approximately 14 In the 102 min theearth rotates eastward 360° × 102/1440 or about 25° Neglecting for themoment the small eastward rotation of the orbit required for sun syn-chronicity, the earth will rotate under the subsatellite path by thisamount, as illustrated in Fig 1.7

TABLE 1.7 NOAA KLM Satellites

NOAA-L: September 21, 2000 NOAA-M: June 24, 2000 NOAA-N: March 19, 2005 (tentative) NOAA-N’: July 2007

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)

Figure 1.7 Polar orbiting satellite: (a) first pass; (b) second pass, earth having

1.6 Argos System

The Argos data collection system (DCS) collects environmental data radioed up from platform transmitter terminals (PTT) (Argos, 2005).

The characteristics of the PTT are shown in Table 1.8

The transmitters can be installed on many kinds of platforms, ing fixed and drifting buoys, balloons, and animals The physical size ofthe transmitters depends on the application These can weigh as little

includ-as 17 g for transmitters fitted to birds, to track their migratory patterns.The PTTs transmit automatically at preset intervals, and those withinthe 6000 km swath are received by the satellite As mentioned, the satel-lite completes about 14 orbits daily, and all orbits cross over the poles APTT located at the polar regions would therefore be able to deliverapproximately 14 messages daily At least two satellites are operational

at any time, which doubles this number to 28 At the equator the tion is different The equatorial radius of the earth is approximately

situa-6378 km, which gives a circumference of about 40,074 km Relative tothe orbital footprint, a given longitude at the equator will thereforerotate with the earth a distance of 40074 × 102/1440 or about 2839 km.This assumes a stationary orbital path, but as mentioned previously theorbit is sun synchronous, which means that it rotates eastward almost

1° per day (see Sec 2.8.1), that is in the same direction as the earth’s tion The overall result is that an equatorial PTT starting at the west-ern edge of the footprint swath will “see” between three and four passesper day for one satellite Hence the equatorial passes number betweensix and seven per day for two satellites During any one pass the PTT is

rota-in contact with the satellite for 10 mrota-in on average The messages received

at the satellite are retransmitted in “real time” to one of a number ofregional ground receiving stations whenever the satellite is within range.The messages are also stored aboard the satellites on tape recorders, andare “dumped” to one of three main ground receiving stations These arelocated at Wallops Island, VA, USA, Fairbanks, Alaska, USA, andLannion, France The Doppler shift in the frequency received at thesatellite is used to determine the location of the PTT This is discussedfurther in connection with the Cospas-Sarsat search and rescue satellites

TABLE 1.8 Platform Transmitter Terminals (PTT) Characteristics

1.7 Cospas-Sarsat*

COSPAS is an acronym from the Russian Cosmicheskaya Sistyema Poiska

Avariynich Sudov, meaning space system for the search of vessels in

dis-tress and SARSAT stands for Search and Rescue Satellite-Aided Tracking (see http://www.equipped.com/cospas-sarsat_overview.htm) The initial

Memorandum of Understanding that led to the development of thesystem was signed in 1979 by agencies from Canada, France, the USA,and the former USSR There are (as of November 2004) 37 countries andorganizations associated with the program Canada, France, Russiaand the USA provide and operate the satellites and ground-segmentequipment, and other countries provide ground-segment support A fulllist of participating countries will be found in Cospas-Sarsat (2004).The system has now been developed to the stage where both low earth

orbiting (LEO) satellites and geostationary earth orbiting (GEO)

satel-lites are used, as shown in Fig 1.8

The basic system requires users to carry distress radio beacons, whichtransmit a carrier signal when activated A number of different beacons

are available: emergency locator transmitter (ELT) for aviation use;

emer-gency position indicating radio beacon (EPIRB) for maritime use; and sonal locator beacon (PLB) for personal use The beacons can be activated

per-manually or automatically (e.g., by a crash sensor) The transmittedsignal is picked up by a LEO satellite, and because this satellite is moving

relative to the radio beacon, a Doppler shift in frequency is observed In

effect, if the line of sight distance between transmitter and satellite isshortened as a result of the relative motion, the wavelength of the emit-ted signal is also shortened This in turn means the received frequency

is increased If the line of sight distance is lengthened as a result of the

LEOSAR satellites

GEOSAR satellites

Figure 1.8 Geostationary orbit search and rescue (GEOSAR) and low earth orbit search

and rescue (LEOSAR) satellites (Courtesy of Cospas-Sarsat Secretariat.)

relative motion the wavelength is lengthened and therefore the receivedfrequency decreased It should be kept in mind that the radio-beaconemits a constant frequency, and the electromagnetic wave travels at con-stant velocity, that of light Denoting the constant emitted frequency by

f0, the relative velocity between satellite and beacon, measured along the

line of sight as v, and the velocity of light as c, then to a close mation the received frequency is given by (assuming v c):

approxi-(1.1)

The relative velocity v is positive when the line of sight distance is

decreasing, (satellite and beacon moving closer together) and negativewhen it is increasing (satellite and beacon moving apart) The relative

velocity v is a function of the satellite motion and of the earth’s rotation.

The frequency difference resulting from the relative motion is

(1.2)The fractional change is

The time at which Δf is zero is known as the time of closest approach.

Figure 1.9 shows how the beacon frequency, as received at the lite, varies for different passes In all cases, the received frequency goesfrom being higher to being lower than the transmitted value as thesatellite approaches and then recedes from the beacon The longestrecord and the greatest change in frequency are obtained if the satellitepasses over the site, as shown for pass no 2 This is so because the satel-lite is visible for the longest period during this pass Knowing the orbitalparameters for the satellite, the beacon frequency, and the Doppler shiftfor any one pass, the distance of the beacon relative to the projection ofthe orbit on the earth can be determined However, whether the beacon

satel-is east or west of the orbit cannot be determined easily from a singlepass For two successive passes, the effect of the earth’s rotation on theDoppler shift can be estimated more accurately, and from this it can

be determined whether the orbital path is moving closer to, or movingaway from the beacon In this way, the ambiguity in east-west posi-tioning is resolved The satellite must of course get the informationback to an earth station so that the search and rescue operation can becompleted, successfully one hopes The SARSAT communicates on a

f

f0  v c

f  f  f0 v c f0

f Q1 v c R f0