Satellite Microwave

Agenda

Propagation in Soft ChannelsSatellite MicrowaveOrbitsClassesBandsUsesLimitations

Propagation in Soft ChannelsLine-of-Sight (LOS)

Signals occupy spectrum above 30MHz

EM waves in this frequency band or spectrum are not reflected back to the Earth and easily pass through the atmosphere.

102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015

Power &Telephone

Radio Micro-wave

Infrared VisibleLight

Frequency (Hz)Kilo Mega Giga Tera Peta

LOS to satellite.LOS is blocked.

Satellite Microwave

After WWII, a British engineer named Arthur C. Clarke thought of putting microwave repeaters in space.

Satellites are microwave repeaters that receive transmissions on one frequency band (uplink), amplify or repeat the signal, and transmit them on another frequency (downlink).

A single satellite operates on several frequency bands, called transponder channels or transponders.

Satellite Microwave

Downlink is broadcast.

Uplink is “directed” using antenna.

Footprint: the area the satellite

can “cover”.This can be very

large.

Satellite Microwave

If a satellite orbited in the same plane as the equator and if its velocity took it around the Earth every 24 hours then…

The satellite would appear “stationary” from the point of view of an observer on the Earth.

Satellite Orbits

A Geostationary Orbit seen from above one of the poles.

Earth Satellite makes one circuit per day.

Observer on Earth makes one circuit per day.

Relative E/W positions of observer and satellite never changes.

Satellite Orbits

Orbits don’t have to be circular!

Earth

An elliptical orbit.

Satellite is constantly changing distance from the observer and speed.

Moving slowestat apogee.

Moving fastestat perigee.

Satellite Orbits

A Geostationary Orbit seen from above the equator

N

S

Angle up/down in the sky of observer and satellite never changes.

From the POV of an Earth-bound observer, what happens to position in the sky of the satellite as the observer moves North?

The orbit is “equatorial”.

Satellite Orbits

Orbits can also be “inclined”.

N

S

Plane of the equator

An inclined orbitA “polar” orbithighly inclined

Some satellites have orbits that are both elliptical and inclined.

Satellite Orbits Three satellites, orbiting in the same plane,

35,579 Km from the Earth, 120o apart can fully “cover” the planet’s surface.

Earth

Each satellite’s footprint covers 1/3 of the Earth’s surface.

Satellite Orbits

Geosynchronous: A satellite with an orbital period equal to the earth's rotation period and approximately 35,579 Km (22, 237 miles) above the earth's surface.

Geostationary: Geosychnronous with an orbital plane in the earth's equatorial plane (i.e. zero inclination). Satellites in Geostationary orbit are always moving with the rotation of the earth.

Number of satellite “slots” in an orbital plane is limited by 4o angular displacement requirement.

Satellite Classes

Geosynchronous or Geostationary (GEOs) Low Earth Orbit (LEOs)

Motorola's 66-satellite Iridium system

Globalstar 48-satellite system from Loral/Qualcomm

Medium Earth Orbit (MEOs) Highly Elliptical Orbit (HEOs)

a.k.a Molnya Orbit

GEOs

Orbit is about 35,579 Km above the equator and orbital period equals the Earth's rotation period. Therefore satellite is stationary with respect to observers

on the Earth. Finding and tracking is relatively easy. No problem with frequency changes as satellites

move closer/farther from the observer. Long propagation delays. Very large footprints - wasteful of spectrum.

LEOs

Orbit at 500-2,200 Km. Each orbits the Earth every 1-2 hours. Not

geosynchronous or geostationary! Therefore receivers must be “passed-off” from satellite to satellite.

A large constellation of satellites is required to guarantee continuous coverage.

Much stronger signals are received for a given power of transmission than from a GEOs. Can use hand-held receivers instead of dish antennas.

Smaller footprint than GEOs.

MEOs

Orbit at 5,000-15,000 Km. Longer orbital period than LEOs. Not geosynchronous or

geostationary. However, receivers need not be passed-off as often.

Fewer satellites are required for complete coverage than for a constellation of LEOs.

Signals received will be weaker than for LEOs but stronger than for GEOs. Can use hand-held receivers instead of dish antennas.

HEOs

Near the poles, GEOs appear near the horizon in the sky. Signals have farther in the atmosphere to travel and

can encounter obstacles.

Some Russian satellites are in highly inclined (65o above the equator), highly elliptical orbits.

Each satellite orbits in about 12 hours, spending most of its time above the northern hemisphere.

Greater distance/attenuation than GEOs.

Satellite Microwave

As the frequencies used get higher… Attenuation increases so more amplification is

necessary. This means more powerful transmitters are required.

Wavelength gets shorter. The dish antennas used can become smaller for the

same amount of amplification

Satellite BandsC Band - 4/6 GHz

Uplink frequencies: 5.925-6.425 GHz Downlink frequencies: 4.2-4.7 GHz First choice for satellites because it was also used for terrestrial

microwave. Equipment and expertise were available. Interference with terrestrial

microwave Used for video delivery, VSAT,

news gathering, telephony Band is saturated

Uplink frequencies: 14-14.5 GHz Downlink frequencies: 11.7-12.2 GHz Up/downlink freqencies differ for US and Europe. Smaller/cheaper earth stations used than for C Band. Rain produces attenuation problems so more powerful

transmitters are needed. Used for video delivery, VSAT, news gathering, telephony, direct-

to-home video/audio, internet access, voice, video, data Band will become saturated.

Satellite BandsKu Band - 12/14 GHz

Some Other Bands

L Band 1.530-2.700 GHz Voice/low speed data to mobile terminals.

S Band 2.7-3.5 GHz Cellular telephony, data, paging

Ka Band 18-31 GHz Internet access, voice, video, data, videoconferencing Very powerful transmitters to deal with attenuation

Satellite Uses

Used for television (both network and DSB), long-distance telephone, and private business networks.

Particularly suitable for creating networks where cabling is not practical. Low subscriber densities Mobile stations

Satellite Limitations

Limited bandwidth, even with dish antennas directionality is limited. Satellites cannot be placed close to each other

unless they are processing the same signals.

Long propagation delays due to distance. Distracting for voice communications. Data communications requires special protocols.