fundamentals_satellite_communication_part_1

Fundamentals of Satellite
Communications, Part 1

Howard Hausman
President
MITEQ, Inc.
Hauppauge, NY 11788
May 29, 2008

Fundamentals of Satellite
Communications Part 1
Satellite Communications Introduction
Types of Satellite Services
Satellite Configurations
Geostationary Satellites
Non-Geostationary Satellites
Satellite Configurations
Frequency Reuse / Polarization
Earth Station Antennas
Major Earth Stations Components
Satellite Communications Summary
Part 2 – Communication System Link Analysis
Part 3 – Specifying Subsystems for Data
Communications -
05/29/08 Howard Hausman, MITEQ, Inc. 2

Satellite Communications
Introduction
Are Satellites a Cost Effective Means of
Communications? Answer is NO.
Limited Frequency Spectrum
Limited Spatial Capacity (Orbital Slots)
High Equipment Cost
Land lines and Fiber are Much More
Cost Effective
Fiber Reuses Spectrum, Multi-Fiber Cables
A few multi-fiber cables can have as much
information as all the satellites in orbit
Land Line Infrastructure is In Place -


Satellites Provides Capabilities Not Available
with Terrestrial Communication Systems

Adaptable to the needs of different customers
Variable Information Rates
Mobility
Cost advantage over building land lines for a limited
population
Versatility in use Paging, Voice, Data, Video
No geographical obstructions that prohibit landlines
Quick implementation – e.g. News Gathering
Alternate routing or redundancy as required
Cost is independent of distance
Cost effective for short term requirements e.g.
Sporting Events -

Types of Satellite Services
Fixed Service Satellites (FSS)
Communication to non-moving satellites.
Generally Earth Station is not moving when in use.
Low Cost Tracking antennas are making
communication on the move a reality
Types of service
Video:
Broadcast: Television network distribution
Satellite News Gathering (SNG).
Voice:
Telecommunications traffic / connecting
cells.
Connecting cellular telephone to small
islands
Data: Internet, Business to Business -


Satellite Industry Issues and Concerns
Frequency spectrum
Orbital Slots
Regulatory inconsistencies
Signals reach multiple countries
Consolidation of manufacturers
Multiple standards
Quality control versus production lead time
Rapid change in telecommunications
requirements
Digital Television
Rapid deployment of Fiber optics -


Satellite Configurations & Stabilization

-

•Propulsion System
Satellite Components •Telemetry
•Attitude Control
•Fuel
LNA
•Batteries
Down Converter
•Power & Thermal Control
High Power Amplifier
•Solar Arrays
Filter

Up Link Down Link
Path Path
Loss Loss

Up Link
Frequency Always
Higher than Down
Link Frequency -


Spin Stabilized Satellites
Satellite Body is Spin
Spin Stabilized (60-100 RPM)
Stabilized
Gyroscopic stability

Spins to minimize thermal
effects

1/2 the solar cells face the
sun at one time

More efficient for smaller
satellites

Antenna must de-spun -

Body Stabilized Satellites

All solar cells face the
sun
Thermal control more
difficult
Requires more
stabilization control
More solar cells than
Spin Stabilized Satellites
Body Stabilized Better design for larger
satellites -


Geo-Stationary Satellites
In a British magazine, "Wireless World", May, 1945, Arthur
Clark, a renown science fiction author, wrote a paper
predicting that three geo-stationary satellites would
provide complete global telecommunications coverage.

Sir Arthur Clark
wrote ‘2001, A
Space Odyssey’
Died this year
2008 -


Geo-Stationary Satellites (Continued)
Clark Orbit, which is Equatorial Circular
35,800km (22,300 Miles ) above the Earth traveling
at a speed of 11,000km/hour
One Orbit takes 24 hours
Synchronous with the spinning of the Earth
Satellites headed for geosynchronous orbit first go
to a Geostationary Transfer Orbit (GTO)
Elliptical orbit with apogee about 23,000 miles.
Firing the rocket engines at apogee then makes the
orbit circular.
A Single Satellite is visible from ≈ 1/3 of the earth's
surface, excluding extreme Polar Regions.
Cannot see latitudes greater than ± 77° °
Orbital locations are regulated by the International
telecommunications Union (ITU) -

Geo-Stationary Satellites Beams
Generally satellites must be spaced at least
1.5° to 2° apart (2° ≈ 911 miles or 1,466 Km).
° ° °
Earth Station antenna will illuminate multiple
satellites if they are spaced closer
Orbital slots are measured in degrees going
East from Greenwich meridian = 0° °
• Satellite
antenna beam
width is 17.3° °
for full earth
coverage -


Advantages/Disadvantages
of Geo-Synchronous Orbits
Advantages:
No ground station tracking required
No inter-satellite handoff, permanently in view
Three satellites give full earth coverage
Almost no Doppler shift, yields reduced complexity
receivers
Disadvantages:
35786 km orbits imply long transmission latencies
Weak received signal
Poor coverage at high latitudes (>77 degrees) -


Satellite Link Delays
Satellite to the surface of the Earth is 22,300 miles
Two way transmission is 44,600 miles
C = speed of light = 186,282 miles per second

239 milliseconds Minimum
delay
Transmit and receive
stations on the equator at
the same longitude as >120 ms
the satellite. >120 ms
Typical signals may required
two hops through the 120 ms
satellite, i.e. around
500mSec
500mS echo is barely
distinguishable -


Elevation & Slant-Range
Not All Satellites are located above the Earth Station
At Higher Elevation Angles
Signals Traverse Less Atmosphere
Overall Slant-Range is Reduced.
Signal Strength is Inversely Proportional to the Square of
the Distance
Atmospheric Effects are Significant at Low elevation

Top of the Atmosphere
Note the difference
in traversed
atmosphere -


Elevation Angles & Atmospheric Effects
Atmospheric Effects are
critical to signal path Loss
22,300 miles Earth to
Satellite
1st 5 miles is most critical
High > 5 miles at low angles of
Elevation elevation
Angle Low
Atmospheric Problems
Elevation
Angle Potential interference
from terrestrial sources.
Increased atmospheric
absorption
Minimum Elevation Angles Partially depolarizes
C-Band Elevations ≥ 5°
° signal -
Ku-Band Elevations ≥ 10°
°


Geo-Stationary Satellite Movement

North-South East-West

North-South perturbations
Due to gravitational pulls of the Sun and Moon (Similar
to Tidal Effects)
North-South perturbations are the largest
Most demanding on satellite fuel reserves
East-West perturbations
Due to lumpiness of the Earth
Incorrect satellite velocity & altitude -

North-South Satellite Movement

An Object not exactly over the equator, transcribes a figure
eight pattern to an observer on Earth
Satellite lateral errors can be as much as ± 3 ° at end of life
Looks like an up and down motion two times every 24
hours
Large Earth Station Antennas must track the motion -


Satellite Orbit Stabilization & Life
Positioning is regularly corrected to within ± 0.1° °
Without correction the movement in the North-South
direction will be about 0.85 ° per year
± 15° over the satellites typical 12 year lifetime
°
Life of satellite is determined by how much fuel is stored to
correct its position
Last remaining fuel is used to move the satellite out of
orbit
Satellite fuel capacity is typically designed for 13 years,
because the satellite technology becomes obsolete.
Satellite life can be extended by making less frequent
position corrections
Allowing ± 3° latitude shift can extend the satellite life as
°
much as 3 years. -


Solar Outages Due to a Solar Eclipse
Satellites experience a solar eclipse two times a year
Vernal & Autumnal equinoxes for about 6 weeks each year.
Satellites are in the earth's shadow for a few minutes to as
much as 65 minutes on the day of the equinox.

During Eclipse
No Solar
power - Need
Equinox
battery power
No Solar
warming -
65
Requires
Minutes heaters. -


Satellite Eclipse of the sun
For about five days during the eclipse season (65
days) the sun passes behind the satellite with respect
to the earth station.
The background noise builds up as this event unfolds
Plateaus for about 10 minutes.
The satellite may be unusable (due to lower C/N) for
this period
Traffic may have to be switched to another satellite.
Outage occurs around noon
Larger diameter receive antennas (small beam width) exhibit
this a shorter time and fewer days
Tracking: mode is
usually turned off to
prevent the Earth Station
from the tracking the Sun,
which at this time is a
higher source of energy. -

Non-Geostationary Satellites
Medium Earth Orbits (MEO)
Low Earth Orbits (LEO)

Types of Orbits

Polar
Equatorial
Inclined -


Orbit Shapes

Eliptical
Orbit
Circular

Elliptical -
Earth Circular
Orbit

Satellite
Orbital Shapes


Polar Circular Orbital Characteristics
Full global coverage with a single satellite
Continuous communications requires many satellites
Iridium uses 66 satellites
Transfer of information between satellites
Information is handed off from satellite to satellite
like a cellular system
Satellite moves and customer stays relatively still
Constellation of satellites
separated in time and angle
Every customer is always in
the foot print
Higher orbits require fewer
satellites
More terrestrial up-link
(User) RF power -

Kepler’s laws of Planetary Motion
In the early 17th century, Kepler
discovered the three laws of
Time Kepler's Law planetary motion:
T1 of Planetary
The orbits of the planets have
Motion
the same physics as earth
satellites.
Area A1
1.The law of orbits: Planets
Same Time - move in elliptical orbits with the
Area A1 Sun at one of the foci.
Earth 2.The law of areas: the line
from the Sun to a planet
sweeps out equal areas in
equal times.
Time Satellite 3.The law of periods: The
T1 Eliptical Orbit square of the period is
proportional to the cube of the
ellipse's major axis.

Elliptical Satellite Orbits
Inclined Elliptical Orbits allow asymmetrical time
coverage over different sectors of the Earth
Geostationary satellites do not
provide coverage for the Polar
regions
Elliptical orbits cover the same
area per unit time in all parts of
the ellipse, Satellite travel
slower further away
To serve Polar Regions
establish an Inclined Orbit with
the apogee over the Polar
regions
Most of the satellites orbital
time is over the Polar region -


Low Earth Orbit
Advantages/Disadvantages
Advantages:
Reduced launch costs to place in low Earth orbit
e.g., airplane/booster launched
Reduced pass loss
Lower Power, Lower cost satellite ($0.5-2M)
Much shorter transmission delays
Disadvantages:
Short visibility from any point on earth, as little as 15 minutes
Potentially large constellations
Radiation effects reduce solar cells and electronics lifetimes
Van Allen radiation belts limit orbit placement
Belt 1: 1500-5000 km
Belt 2: 13000-20000 km -


LEO Example: IRIDIUM
Voice (4.8 kbps), Data (2.4 kbps), Fax, Location
Services
66 satellites in 6 polar orbits (780 km)
Iridium has an atomic number 77
Original design called for 77 satellites
48 spot beams per satellite forming “cells”
230 simultaneous duplex conversations
Satellite-to-satellite links as well as to ground
Ka band @20 GHz to gateways & crosslinks,
L band at 1.5GHz to handheld units
FDMA uplink, TDMA downlink
Supports satellite handoff during calls -


Satellite Configurations: Bent Pipe
Keep the satellite simple
RF to RF – Frequency Translator
C-Band Earth Station Transmits typically at 5.925
GHz to 6.425GHz
Earth Station Receives signals at 3.7GHz to 4.2 GHz
Satellite has a fixed Local Oscillator at 2.225 GHz
Satellite transmits at a lower frequency (Less loss) -
Low Noise Local Oscillator
High Power
Amplifier Amplifier
EIRP
Mixer
G/T Band Pass Band Pass
Filter Filter

Satellite Transponder

On-Board Processing vs. Bent Pipe
More control over signal routing
Higher Cost, more complicated, Higher failures
Typical Configurations
RF to IF Microwave switch matrix IF to RF
Allows Changing Signal Path Transponders
RF to IF Demodulator Baseband
Modulator IF to RF
Reprocessing eliminated accumulative noise
Intersatellite Links, Handing off Signals
some LEO's
Military satellites
NASA TDRSS system -


Typical C-Band Link
This Example Ignores Line, Atmospheric, and Other Losses.
Does Not Include C/N Requirement Data.

Satellite HPA is smaller -

Adjust Power
for weather


Satellite Transponders
Satellite Bandwidth (Typically 500 MHz to 750MHz) is
broken up into segments called transponders

Frequency

Transponders are characterized by:
Center Frequency
Bandwidth
Down Link power (Satellite EIRP)
Different bandwidths have different power
All signals to a satellite must have the same
power spectral density
Polarization -

Frequency Reuse by Polarization
Electromagnetic Wave Behavior
Direction of
Propagation Non-Polarized
Electromagnetic
Direction of
Direction
Vibration Wave Viewed
of Head-On
Vibration
Direction of
Vibration

Direction
of
Vibration

Polarization Refers to the orientation of the
electric field vector as a function of time. -

Linear Polarization

Vertically Horizontally Electromagnetic
Polarized Polarized Wave Transmits
Wave Wave in a single plane
E l e c t r o m a g n e t i c W a v e

Linear Polarization requires precision alignment
of the transmitter and receiver, i.e. Satellite & Earth
Station -


Circular Polarization
Electromagnetic Plane
rotates clockwise with
time
Right Hand
Polarization
Electromagnetic Plane
rotates counterclockwise
with time Right Hand
Left Hand Polarization Polarization

A Right Hand Polarized
Satellite signal is Left
Hand Polarized at the
Earth Station Left Hand
Mirror Image - Polarization


Creating Circular Polarization
Power
Diveder Circular Sum Circular polarization
RF
In
Polarization RF is achieved
90 Degree
Circular Polarization
Delay Out Splitting the linearly
polarized signal into
Siganl A
two orthogonal
vectors
Signal B
Delaying one with
Time
#3 respect to the other
#2

#1
Delayed by a quarter wave
90 Degrees

#1
length (90°)
°
#2
Summing the
vectors -


Polarization and Frequency Reuse
Frequency Reuse is receiving and transmitting signals at the
same frequency, but with orthogonal polarization.
Linear polarization needs absolute alignment
Circular requires no alignment but more effected by rain
Transponder Frequencies are offset to minimize interference

Frequency Reuse Depolarization
The same Frequency Band is used twice causes
Separated by orthognol polarizations
transponder
Vertical or
interference -
Right Hand
Circular Frequency

Horizontal
or Left Hand
Circular


Spatial Reuse Ka Band uses multiple
– Spot Beams narrow beams
Focused beams cover a
much smaller area
Hundreds of miles
across, rather than
thousands of miles with
Lower Frequency FSS
Form coverage cells
Adjacent cells use
different frequency
ranges
•Each color is a different Frequency range
frequency range reused many times over
•Similar colors don’t touch - a wide geographical
area

Advantage of Spot Beams
Large increase in overall capacity
Spot beams provide 30 to 60 times the system
capacity of the FSS
Capacity of 30 Gbits/Sec makes satellite broadband
services a long-term, economically viable business
Flexible Spatial Redundancy with Phase Array Spot
Beams

Service is
restored by
moving beams to
effected areas -


Earth Station Antennas
Antenna Mounts
Fixed:
views one
satellite
Inexpensive
Elevation-Azimuth:
Vertical and
horizontal
movement
Narrow Beam
Width
High Gain -


Azimuth & Elevation Angles
Azimuth is the axis of angular rotation
Elevation is the Angle with respect to the
horizon -

Azimuth Elevation

Top View


Parabolic Reflector Geometry
Signals are fed
from a point
source
Feed Horn is the
antenna
Reflector
Feed Horn Dish is a
Reflector
Geometry is such
that all signals
are reflected in
parallel -


Parabolic Antenna Types

Prime Focus
Feed
Simplest
Antenna Design
Cassegrain Feed
Allows for
Shorter
Feedlines
Offset Feed
Minimizes Feed
Blockage -

Antenna Beam-Width
Antenna Beam Width
Beam width is the angle where
the antenna power is within 3
dB of the peak
Beam Mid-Point: Boresight
Beam width is a solid angle
Beam width ≈ 21 / (F*D) in
degrees (Parabolic dish)
F = Frequency in GHz
D = diameter of the dish in
Example: 3
Meters
Meter Antenna @ 4
GHz has a 1.75° ° For a parabolic dish D is the
Beam width (-3 dB) - same in all directions


Antenna Radiation Pattern

Main Beam 3dB
HPBW
Beam-width
First side lobe Angle
F/B
≈ -25dB Gain
Back Lobe

Isotropic Level
Bore sight = 0 degrees
-180 0 +180
All angles are referenced to Bore-sight
is the ½ Power (3dB) Beam Width
Side Lobes: The antenna patterns are repeated at
lower gains on either side of the main beam -

Side Lobe Radiation Problem

Side Lobe Energy
Limits: Limit
interference to nearby Angle Off
Boresight
Bore
satellites Sight

IESS Spec: Side Lobe < 0.5 dB

Max: ≤ 29 - 25* Log 10
(A) in dB
A = the angle off
boresight. -


Antenna Side Lobes Limits
Antenna shapes are
often skewed to lower the
side lobes
Increases the beam
width
Lowers the antenna gain
-


Major Earth Stations Components
Typical
Earth Station
HPA’s
Configuration
Up Conv.
UPC
Antenna
MITEQ 70MHz/140MHz Modulators
L-Band For each carrier
Non-MITEQ
Down Conv. De-Modulators
LNA’s

Tracking System
Control
/ Monitoring
-
Beacon Receivers

Satellite Communications Summary

Broadcasting
One Transmitter to millions of receivers
Voice, Data, Internet, etc. access everywhere
On the move
In the Air
Isolated locations
Communications with minimal infrastructure
Satellite Communications is versatile enough
to let your imagination runaway with ideas


fundamentals_satellite_communication_part_1

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