Global Satellite Services

GEO, MEO and LEO Satellites, IntelSat, and Global Positioning

Systems

What is a telecommunications satellite?

Telecommunications satellites

• Space-based cluster of radio repeaters (called transponders)

• Link – terrestrial radio transmitters to satellite

receiver (uplink)– Satellite transmitters to terrestrial receivers

(downlink)

Orbits

• Mostly geostationary (GEO)– Circular orbit– 22,235 miles above earth– Fixed point above surface – Almost always a point on Equator

• Must be separated by at least 4 degrees

Satellite services• Wide Area Broadcasting

– Single transmitter to multiple receivers

• Wide Area Report-Back– Multiple transmitters to a single receiver– Example VSATs (very small aperture terminals)

• Also have microwave transmitters and receivers– Allows for spot-beam transmission (point- to-point data

communications)

• Can switch between beams upon request (Demand Assigned Multiple Access –DAMA)

• Multi-beam satellites link widely dispersed mobile and fixed point users

Earth-based equipment

• Original microwave transmitters and receivers were large installations– Dishes

measuring 100 feet in diameter

• Modern antennas about 3 feet in diameter

A Modern GEO satellite (IntelSat 900 series)

• May have more than 72 separate microwave transponders

• Each transponder handles multiple simultaneous users (protocol called Time Division Multiple Access)

• Transponder consists of– Receiver tuned to frequency of uplink– Frequency shifter (to lower frequency to that of

transmitter)– Power amplifier

IntelSat 902 (launched August 30, 2001)

Frequency ranges

• Most transponders operate in 36MHz bandwidth

• Use this bandwidth for– voice telephony (400 2-way

channels/transponder)– Data communication (120Mbs)– TV and FM Radio

C-band, Ku-band, Ka-band

• Most GEO satellites operate in the C-Band frequencies– Uplink at 6 GHz– Downlink at 4 GHz

• Ku-band also used– Uplink at 14 GHz– Downlink at 11 GHz

• Above bands best suited for minimal atmospheric attenuation

• Few slots left… forcing companies to look at Ka band (uplink:30 GHZ , downlink: 20 GHz)

Intelsat

• Began as an inter-governmental consortium in 1964• Launched world’s first commercial communication

satellite in 1965• By 1969, had a fleet• Broadcast live Neil Armstrong’s first steps on the moon• In 1974, establish the “hot-line” between the Kremlin and

the White House• In mid-1980s, developed more powerful satellites –

allowed smaller ground equipment for live broadcasts• In 1997, introduced pay-as-you-go, shared access

satellite coverage for low demand, rural areas.• 2001 – became a private company providing turnkey

connectivity solutions

Intelsat’s fleet

• 20 Geosynchronous satellites• Cover 200 countries• 4 nines reliability (99.997%)• 18000 earth stations, and millions of VSATs• Offer

– Bandwidth-on-demand– Point-to-point– Point-to-multipoint– C- and Ku-band capacity

Latest Satellites launched

• Last of the Intelsat IX series launched Feb 15, 2003 (went live in March) – Offers more powerful

C-band coverage for • Europe

• Africa

• The Americas

– Ku spot beam coverage for Europe and Africa

Global Positioning Satellites• 24 MEO satellites owned by

US DoD (last launched in 1994)

• 21 active, 3 spare

• 11,000 miles above earth

• Five control stations around the world make sure satellites operating correctly

• Can tell your position within 300 feet

• Possible to get as accurate as 3 feet using special calculations.

How GPS works

• 24 satellites, each takes 12 hours to orbit the earth

• You can receive signals from 6 from any point on earth

• Satellites have up to 4 cesium and rubidium clocks (accurate to 3 billionths of a second)

The signals

• GPS satellites transmit 2 low power radio signals – L1 and L2 (civilians can only use L1)– 25-50 watts (compare to 100,000 watts for an FM radio station)

• L1 sends 3 pieces of data– Pseudorandom code (ID of transmitting satellite)– Ephemeris data (tells current data and time)– Almanac data (tells GPS receiver where every satellite should

be at any time of day)• The data indicates when the signal was transmitted from

the satellite• The receiver indicates when it was received.• It is now possible to calculate distance from satellite• Three more such signals and you can be pinpointed.

Galileo

• The EU wants to compete

• Galileo will be under civilian control

• 30 satellites (27+3 spare)

• Inter-operable with GPS

• Dual frequency design allows positioning within 1 meter.

Then there was LEO

• Iridium project– Initially began as a plan for 77 LEO satellites

(atomic number 77 = Iridium)– Scaled down to 66 (should be called

Dysporium)– Cost $5billion (Mostly Motorola)– Sold 2 years ago for $25 million