Andy Stamp Department of Communication & Electronic Engineering 2002

SATELLITE COMMUNICATIONS A BRIEF HISTORY In May 1945 Arthur C. Clarke described a novel, feasible communications system and proposed the use of several satellite microwave repeater stations to provide a reliable, global communications system (see ref.1). In his article (in Wireless World, a popular periodical at the time), Clarke proposed a system using satellites orbiting at 35,786 km (roughly 22000 miles) above the surface of equatorial Earth. This height, which was calculated using Newtonian mechanics and Keppler's laws, was chosen because the satellites would orbit synchronously with Earth - thus avoiding the need for complex tracking systems. Sometimes described as following a Clarke orbit, the satellites orbit Geosynchronously (and appear to be Geostationary to the casual observer). This vision of the future prompted many early research programmes into the possible use for satellite systems. The notion was based on a marriage of disparate technologies emerging out of work done to put an early end to World War 2. Developments in: communications technology, microwave engineering, rocketry and space-flight were all new and would all be needed by Clarke's novel scheme. Consequently, many experimental systems appeared around the beginning of the 1950's and 60's initially using the reflective properties of our natural satellite (the moon) to re-direct a microwave beam and connecting widely separated regions of Earth. The first voice transmissions by Earth-Moon-Earth were successfully received in 1954. Shortly later, in 1957, the CIS (then USSR) launched Sputnik 1 that took 90 minutes to orbit. This satellite, which was the first active satellite to provide a communications link only operated for 21 days. In the USA experiments even included the use of 30 metre diameter meteorological balloons, which acted as reflectors positioned at 1600 km above the planet (Echo 1). Other notable systems to emerge around this time were the American Explorer and Score satellites (in low earth elliptic orbit). Political advantage was taken in 1958 when Score was used to broadcast the American President Eisenhower's Christmas speech. Perhaps the best known active satellite series around in the sixties was the Telstar series. Much was learnt from Telstar1 which had a short operational life due to radiation damage. Telstar 2 (launched 7.51963) was radiation hardened and lasted months rather than weeks. Telstar became well known to the public because it was used for transatlantic Television links. The Clarke orbit was initially avoided because of the high complexity and cost of satellites, the round trip delay time was also an issue. Consequently, many early communications satellites followed elliptical orbits. However, with just one satellite in elliptical orbit the need for complex tracking systems, loss of visibility and considerable Doppler correction were serious drawbacks. The first Geosynchronous satellite was in use was Syncom, with the third flight vehicle in service by August 1963. Because of the obvious merits of Syncom, just two years later (1965) many international organisations had evolved to exploit the potential offered by satellite communications. SATELLITE COMMUNICATIONS

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These were (and still are) categorised into regional (e.g. European) or International (e.g. an alliance of eastern or western nations). INTERNATIONAL INTELSAT INTERSPUTNIK REGIONAL EUTELSAT ARABSAT

INMARSAT NIPPONSAT

MEASAT

Intelsat is the International Telecommunications Satellite organisation that was formed in 1964 by eleven member nations. There are currently about 200 member signatories (representing interests from nations, territories and dependencies in every continent). Intelsat is however, essentially under the control of the Americans. EARLY-BIRD was the first flight vehicle in the Intelsat series, it later became known as INTELSAT-1. 'BIRD' was an in-vogue term for satellite. Early Bird provided a transoceanic link for telephony and television (by way of 240 [4 kHz voice] telephone channels and one television channel). Later came Intelsat II, III, IV, V, VI, K and VII, VIII, & IX. This type of service uses fixed Earth stations (as opposed to mobile ones etc.) and is classified as a Fixed Service System (FSS). To see the current range of Intelsat vehicle look under ref.2. Study of the evolution of the Intelsat satellites gives a real insight into the way the technology advanced and how market forces have dictated the choice of supplier. A study of Intelsat I to Intelsat IX is included in the lecture series.

Intelsat IX Courtesy of Intelsat

Built by Space Systems/Loral and designed to replace the INTELSAT VI satellites, the INTELSAT IX series reflects a total investment of approximately $1 billion and is comprised of four satellites with increased flexibility, advanced design, and higher capacity than any other INTELSAT satellite series. In addition, the INTELSAT IX satellites will provide high quality digital carriers at better than international standards and comparable to fibre cables. Because of their high power, INTELSAT IX satellites will also reduce earth segment costs and facilitate services such as SNG, DAMA, Internet, DTH and VSAT Networks.

"Regional" systems such as EUTELSAT (the European Telecommunications Satellite organisation) covers smaller areas and have fewer sovereign signatories compared with "international" (EUTELSAT is an organisation with around 40 signatories), but these still use the system for telephony, television and business communications services. Television is currently tending to occupy much more of the available link bandwidth compared with the lower revenue service. Broadcast and experimental services are discussed separately later in the course. At this stage the focus will be on Fixed Satellite services or FSS systems. Mobile Service Systems (MSS) are also in use, one example being INMARSAT (the INternational MARitime SATellite organisation). This developed from experimental use of TAC(com)SAT and FLEETSAT (FLTSAT), which provide ships with a non-fading communication link that was truly almost global in coverage. Before this, the best available system for such coverage was HF, which is well known for fading links. INMARSAT uses MARISATS and MARECS as its flight vehicles.

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Remember however, that satellites can be used for the following applications:FIXED SATELLITE SERVICE (FSS) - point to point communications BROADCAST SATELLITE SERVICE (BSS) - point to multi-point communications MOBILE SATELLITE SERVICE (MSS) - Voice, data, computer and network services METREOROLOGICAL SERVICES RADIO ASTRONOMY RADIO NAVIGATION SYSTEMS REMOTE SENSING (EARTH RESOURCE EXPLORATION) SATELLITE RELAY SYSTEMS STANDARD FREQUENCY AND TIME SIGNALLING INTELLIGENCE GATHERING MILITARY COMMUNICATIONS

CURRENT TECHNOLOGY In addition to the conventional Geosynchronous systems, a new way of thinking about satellite communications systems was evident as early as 1987, when Iridium was proposed. Apparently as a result of dissatisfaction of the wife of a highly placed Motorola executive, who wanted to use her mobile telephone anywhere on the planet. After all, the Global Positioning System (GPS) was available and could be used anywhere on the planet to find the users current location (more on this later), so why not a mobile 'phone. The constellation for Iridium (originally 77 satellites in low Earth orbit) was filed with the American authorities. Around the same time, in the UK a 'Direct to Home' Television broadcast service 'Astra' was launched. Until this time satellites had been used for T.V. , but always under the control of the government posts and telecommunications organisation, but consumers could now subscribe to 'Sky Television' (which offered many more channels than the terrestrial broadcasters). British Satellite Broadcasting also emerged as a rival to Astra, but with much higher levels of sophistication. After much wrangling, the two companies merged to become B -Sky-B, which is now successfully introducing a digital service. Astra (SES) has recently launched a second and third series (i.e. they now have Astra 1 A-H, Astra 2A-B and Astra 3A), and also provide 'radio' to supplement their T.V. services

FOOTPRINT FOR ASTRA 1H Courtesy of Astra BskyB

The Iridium constellation was reduced to 66 satellites with little reduction in service. The concept was to offer a truly global mobile 'phone service. In essence the system was cellular, the 'static' user was connected from one mobile (satellite) cell to the next. Losses and delays would be relatively small because the satellites were in such a low orbit. The same concept was employed by Globalstar (Vodaphone) formed in 1991, but with a slightly less complete coverage. By 1996, many of Iridium's satellites had been launched with the full complement in full operation by 1999. However, the system wasn't commercially successful at that time. SATELLITE COMMUNICATIONS

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Although a very different product to terrestrial 'cellular' 'phones, Iridium 'phones were bound to be compared with them by the uninformed. As such they were judged to be very expensive, bulky and they 'didn't even work indoors'. The merits of true global operations (including uninhabited places, oceans, mountain ranges, wilderness etc) were not deemed worth the expenditure and Iridium went bankrupt in the summer of 2000. The story does not die here as Globalstar has taken off and offers a basic satellite service (voice, data messaging). Since the July 2000, Globalstar has had a viable World-wide subscriber base. On the horizon is another system (founded in 1990, with FCC approval in 1997, merged with ICO may 2000) is now called Teledesic -ICO. The system originally planned a constellation of 840 satellites which was later rationalised to 288 satellites. Teledesic is planned to offer a fibre like service and driven by Craig McCaw and Bill Gates to start services in 2005. The system will use fast packet switching at a minimum data rate of 16 kbps. It will also offer T1 (at 1.544Mbps) and a normal maximum data rate of 2.048 Mbps (but with the option for a special data rate of 1.24416 Gbps). Make no mistake, converging technology will lead to an integrated communications infra-structure that makes great use of satellite technology. INTRODUCTION TO ORBIT REQUIREMENTS With such a variety of satellite applications, one would expect a wide range of satellite orbit types. Modern satellite systems demand sophisticated orbits. All orbits may be classified as elliptical (circular orbits are a special case of elliptical orbits) they are often additionally classified by height and angle of inclination with respect to the equatorial plane. Examples of systems in current use are in the following table.

Orbit Type

Example Spacecraft

Geosynchronous, Geostationary (GEO) Low Earth Orbit (LEO), usually circular Big LEO Intermediate Circular Orbit (ICO) Sun Synchronous Polar (small LEO's use this) Highly Elliptic Orbits (HEO)

Astra Iridium Globalstar Odyssey Radarsat PoSat Molniya Sirius

Orbital Height

Inclination

35,850 km 0O 780 km Various 1410 km Various 10,355 km 798 km 98.6o 822km -800 km 90o 39,354km -1000 km V. inclined. As above 60o

Geostationary Orbits Use a series of satellites (3 or more) orbiting earth synchronously at a point above the equator (e.g. latitude = 0o and sub-satellite longitude spacing >100o). From Earth, each satellite (which has an identical velocity to that of Earth, i.e. one orbit = 1 sidereal day (23 hrs 56 minutes and 4.191 seconds) appears stationary in space above the equator. This is Geostationary orbit. The height satellites need to be above earth for stable circular orbits can be found by calculating the attractive force acting between (the mass of) Earth and (the mass of) the satellite, then equating this with the force due to centripetal acceleration on the satellite. Only when these two forces are equal, places the satellite at equilibrium (and orbits earth in one sidereal day) SATELLITE COMMUNICATIONS

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Low Earth Orbits Traditionally, low orbits (such as those used by RCA satellites and TELSTAR) were considered to have major drawbacks. For example - communication with a single satellite for about only 15 minutes out of every 90 minutes. Telstar 1; Telstar 2; RCA;

Earth orbit Earth orbit Earth orbit

2 Hrs 38 mins 3 Hrs 35 mins 90 mins

at at at

600 - 3800 miles 600 - 6200 100 - 300 miles

But these orbits have made a major resurgence because of clever use of dynamic satellite constellations which offer world-wide cover with much less loss than Geosynchronous. The path loss to a Geosynchronous satellite (at 10GHz) is typically over 200dB each way, and the two way time delay is over 250mS. This would imply high power and bulky systems and delay sensitive systems may not work effectively. Clearly the path loss for Low earth orbit systems is much reduced, as is the propagation delay time and a viable systems operates using many satellites (with a hand-over protocol) at much less power. The following equations are traditionally used for Geostationary systems, but may be used to calculate the orbital height based on the desired orbital period (or vice-versa) for any of the circular orbit systems Calculations for Circular Orbits

FA =

GM E M S

………………………………………...(1)

d2

FC = MASS X ACCELERATION

FC =

MASS.(VELOCITY )2 DISTANCE

æ 2πd ö MS ç ÷ T ø è FC = d

2

æ 2πd ö MS ç ÷ GM E M S T ø è = d d2

…………………………………………..(2)

2 …………………….... Equating (1) & (2)

So d3 =

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GM ET 2

(2π )2

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The distance is given by

d =3

GM ET 2

(2π )

metres

2

Where G Me Ms d Fc FA req T

(Universal Gravitational constant) = 6.672.10-11 Nm2/kg2 (mass of Earth) = 5.977.1024 kg (mass of satellite) (distance between centre of masses) : centripetal force : attractive force : equatorial radius of earth = 6378 km : time for one sidereal day = 86164.2 seconds Note G.ME = 3.986 . 1014

Elliptic Orbits Polar orbits and sun synchronous orbits are elliptic. The polar orbit has an inclination of 90o, whilst other elliptic The USSR make use of a highly inclined elliptic orbit; known as a Molniya (sometimes Molnya) orbit. A satellite (and there about 30 at present; 1989) on a Molniya orbit provides coverage for extreme latitudes (> 81o , North-East regions of the USSR, which covers 5 time zones) and central regions simultaneously. High orbit inclination means that these satellites spend the greatest proportion of their time over Northerly extremes with reduced propagation than Geosynchronous (i.e. < 270 mS) and reduced path loss. With all . R ), but this is affects elliptic systems most orbiting satellites a Doppler shift occurs ( f d = λ

because of the varying amount of shift.

10,355km

Peak radiation bands in the Van Allen belts High Energy Protons

20,200km

Diagram of simplified orbit types (After Wood, L) 35,786km

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Realates websites

1. Clarke's Paper 2. Intelsat 3. Spaceflight 4. Teledesic 5. Astra 6. JPL 7. Spacesim 8. Nasa 9. Goddard 10. ESA 11. History explore 12. Rockets

http://www.lsi.usp.br/~rbianchi/clarke/ACC.ETRelays.html http://intelsat.int/ http://www.spaceflightno.com/index.html http://teledesic.com/ http://astra.lusatellites/systems.index.html http://leonardo.jpl.nasa.gov.msl/QuickLooks/ http://satellites.spacesim.org/english/engineer/copy/orbit/ http://www.nasa.gov/ http://www.gsfc.nasa.gov/education/education_home.html http://www.esrin.esa.it/htdocs/esa/progs/mstp.html http://www.c3.Ianl.gov/~cjhamil/SolarSystem/history.html http://www.c3.Ianl.gov/~cjhamil/SolarSystem/rocket.html

Useful references 1. Roddy 2. Maral & Bosquet 3. Evans (IEE) 4. Williams 5. ITU

Satellite Communications Satellite Communications Satellite Communications The Communications Satellite The Satellite Handbook (FSS)

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The photographs follow courtesy of ; JPL, Hughes, TRW, Lockheed, Iridium and SaVi.

Intelsat 1

Intelsat 2

Intelsat 3

Intelsat 4

Intelsat 4a

Intelsat 5

Intelsat 6

Intelsat 7, 7a

Intelsat 5a

Intelsat 8 Intelsat k

Iridium SATELLITE COMMUNICATIONS

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