Lecture 9 2006

Communication Systems Lecture 9 Professor A.K.Brown

Satellite Communications Lecture 9 2006

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• Basic parameters of any communications link: – Bandwidth – Power(signal strength) – Noise – Delay (Round Trip Time)

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Wireless channels:some key points • Electromagnetic waves: – Spectrum – Wavelength and frequency relationship

• Link Budget (power) – Antenna Gain – Range loss

• Noise Temperature Lecture 9 2006

Mobile User Link (MUL)

Gateway Link (GWL)

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MUL GWL

small cells (spotbeams)

base station or gateway

Cellular Network

Public Telephone System

ISDN

Cellular Network

User data

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Satellite Communications

courtesy Ecliptic Enterprises Inc Lecture 9 2006

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Orbits

Keplers Law: In general satellites have elliptic orbits ( circular a special case of ellipse) Newton: Gravitation attraction must equal centripetal force

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Circular Orbit

GME ms = ms v2 r2 r

v=2π Τ Τ is the orbital period Lecture 9 2006

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Orbits • So for circular orbit, the orbital period, T is: T2= 4π2 r3 GME With G the Universal Gravitational Constant ME the mass of the earth Lecture 9 2006

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Some consequences • Low orbits mean satellite period decreases • The rotational period, T does not depend on the mass of the satellite • If we choose the right height and place the satellite above the earths equator we can make the orbital period equal one rotation of the earth – Geostationary orbit Note in practice higher order effects become important (eg gravitational effect of the moon ) Lecture 9 2006

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Sidereal Day • Consider a point on the earths equator directly below a object fixed and in the equatorial plane .The sidereal day is the time the earth takes to rotate once so that the object returns to being directly overhead. • The mean Sidereal day is 23hrs 56min 4.1sec NOT 24 hrs!

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Geostationary orbit • Occurs when: Orbital period, T = Sidereal Day

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Satellite period satellite period [h]

24 20 16 12 8 4 synchronous altitude 10

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40 x106 m

radius Lecture 9 2006

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Some Circular Orbits • LEO (Low Earth Orbit): ~ 500 - 1500 km • MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): ~ 6000 20000 km • GEO: geostationary orbit, ~ 35500 km above earth surface in equatorial plane Lecture 9 2006

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GEO HEO

MEO (ICO)

LEO

inner and outer Van Allen belts

earth 1000 10000

35768 km

Van-Allen-Belts:ionized particles 2000 - 6000 km and 15000 - 30000 km above earth surface Lecture 9 2006

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• Satellites are used for a wide range of applications including: – Navigation – Weather Forecasting – Environmental Monitoring – Communications

• Different applications use different orbits

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Courtesy Boeing/Hughes Space Systems Lecture 9 2006

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A common misconception • ‘Satellites act as mirrors’. THEY DO NOT! • Satellites act as transponders. The simplest is a ‘bent pipe’ satellite- the signal is received, frequency converted and retransmitted

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‘Bent Pipe’ Configuration

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Regenerative Transponder

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Some basic ParametersGeostationary Satellite Link Uplink Path length or Range, R

Altitude

Downlink Path length or Range, R

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Delay Time Delay time: given a range(distance to satellite) of R Delay time = R per path c With c the velocity of light Each path comprises of a path to satellite then back to ground So immediately below satellite we have Delay time ~ 35,500,000 x2 ~ 237 milliseconds 3 x 108 So an RTT of 2x237 = 474 millisecs Lecture 9 2006

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• Consider now the link budget (in dBs) Pr=Pt-Lt+Gt- 20x Log10 4πR + Gr - Lr λ Where Pt is transmitted power ,Gt is the antenna gain at the transmitter, Lt loss in transmitter, R is the satellite RANGE (NOT orbit radius) Gr receive Gain, Lr receiver losses

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Range Loss • Communications satellites typically operate either 4 to 6 GHz or 11 to 14 GHz • Range loss in free space can be in excess of 195dB • To this must be added loss due to atmosphere (typically 0.5 to 1db) and losses due to bad weather (can be 10dB)

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• Now Power transmitted (Pt) is limited as the satellite is powered by batteries running off solar cells. Also a satellite has to handle many simultaneous users. • So, to get a certain bit rate we need to consider high antenna gains.

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• Antenna Gain is related to the concentration of power in angular termsthe higher the gain the narrower the beam. • High Ground Station gains mean big antennas –expensive for many users, but are used extensively commercially • Is there a way of helping to get lower cost ground stations?

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Satellite footprint • As the satellite is not moving with respect to the earth one way to help is to use large, sophisticated antennas on the satellite . These can then be tailored to give antenna beams shaped to cover particular countries or continents. • The narrower the coverage area the higher the gain but requires bigger antennas on the satellite. Lecture 9 2006

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Advantages of Geostationary Satellites

• Remains essentially fixed in space with respect to the user- major advantage • Only three satellites needed to cover the globe(approximately 42% of earth surface per satellite, but cant cover poles) • Well proven, simple architecture Lecture 9 2006

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Disadvantages • Geostationary orbit is a long way away! • Need high power from the satellite • Need high gain antennas on satellite and ground • Delay time significant.

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What about lower orbits? • As the satellite now moves over the earths surface, to maintain communications requires a constellation. • There is a highly complex handover requires between satellites • Complex satellites with low life but can be relatively small

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Iridium – in service?

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Inter Satellite Link (ISL)

Mobile User Link (MUL)

MUL

Gateway Link (GWL)

GWL

small cells (spotbeams)

base station or gateway

footprint

Public Telephone System

ISDN

Cellular Network

User data

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Summary • Satellite Communications allows wide areas of the globe to be provided with low infrastructure cost • Three main types of orbit: geostationary, low earth and intermediate In principle elliptic orbits may also be used

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Summary (cont) • Geostationary has major advantages in relatively low complexity, ‘stationary satellites’ • BUT the orbit is high above the earthdelay time issues, needs gain in both ground and satellite (note some low rate rate services can be provided with low gain ground stations)

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Summary(cont) • Both LEO and MEO (ICO) orbits reduce delay problem substantially and range loss • BUT need complex systems and satellites • So far two LEO constellations have been proven technically but financially not successful

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