22-01-2013, 12:19 PM
FUNDAMENTALS OF SATELLITES COMMUNICATION
[attachment=47989]
Geostationary Satellites
•Communications satellites are like very tall relay towers
–Most communications satellites use Geosynchronous Equatorial Orbits
–Key concept: GEO sats appear to stand motionless in the sky
•Satellites are easy to find, track
•No handoffs required
•Fixed full-period coverage areas
–Very valuable orbital slots
•Orbit must be circular
•Orbit Must be equatorial
•Only one altitude (for 24 hours)
–GEO is high; 1/10thof way to moon
•Speed of light c = 3 ×108m/sec
•Round trip signal time: ~0.25 sec
Transmitters and HPAs
•Satellites and terminals employ multiple stages of amplifiers
–9 to 12 orders of magnitude of amplification common in GEO satellites
–Last stage of amplifier chain is called High Power Amplifier (HPA)
•Several types of high-freq amplifiers, with names like Klystron, Magnetron
–Very popular one: Traveling Wave Tube Amplifier (TWTA)
–TWTA’s widely used in satellites; good efficiency, high power, reliable
•Up to 100’s watts in satellites, 1000’s of watts in large terminals
–Solid-state amplifiers (SSA’s) also; rugged, but less power, efficiency
–HPA’sgenerally: The higher the frequency, the harder to do
Wideband Satellites
•High gain antennas transmit, collect lots of power
–Supports high capacity
–But high bandwidth also needed
–So higher bands with more total frequency used
•X-band: 500 MHz
•Ka-band: 1000 MHz
•Terminals can reuse frequencies because the beams are so narrow
–Have to stop and point: no COTM
–Exception: Navy, some aircraft
•Satellites: (DSCS, GBS, WGS): Tradeoff between coverage and capacity1
[attachment=47989]
Geostationary Satellites
•Communications satellites are like very tall relay towers
–Most communications satellites use Geosynchronous Equatorial Orbits
–Key concept: GEO sats appear to stand motionless in the sky
•Satellites are easy to find, track
•No handoffs required
•Fixed full-period coverage areas
–Very valuable orbital slots
•Orbit must be circular
•Orbit Must be equatorial
•Only one altitude (for 24 hours)
–GEO is high; 1/10thof way to moon
•Speed of light c = 3 ×108m/sec
•Round trip signal time: ~0.25 sec
Transmitters and HPAs
•Satellites and terminals employ multiple stages of amplifiers
–9 to 12 orders of magnitude of amplification common in GEO satellites
–Last stage of amplifier chain is called High Power Amplifier (HPA)
•Several types of high-freq amplifiers, with names like Klystron, Magnetron
–Very popular one: Traveling Wave Tube Amplifier (TWTA)
–TWTA’s widely used in satellites; good efficiency, high power, reliable
•Up to 100’s watts in satellites, 1000’s of watts in large terminals
–Solid-state amplifiers (SSA’s) also; rugged, but less power, efficiency
–HPA’sgenerally: The higher the frequency, the harder to do
Wideband Satellites
•High gain antennas transmit, collect lots of power
–Supports high capacity
–But high bandwidth also needed
–So higher bands with more total frequency used
•X-band: 500 MHz
•Ka-band: 1000 MHz
•Terminals can reuse frequencies because the beams are so narrow
–Have to stop and point: no COTM
–Exception: Navy, some aircraft
•Satellites: (DSCS, GBS, WGS): Tradeoff between coverage and capacity1