Download Satellite Communication

Satellite Communication
17.2
Satellite Networks
Orbits
Three Categories of Satellites
GEO Satellites
MEO Satellites
LEO Satellites
Figure 17.13
Satellite orbits
Example 1
What is the period of the moon according to Kepler’s
law?
Solution
The moon is located approximately 384,000 km above
the earth. The radius of the earth is 6378 km. Applying
the formula, we get
Period = (1/100) (384,000 + 6378)1.5 = 2,439,090 s
= 1 month
Example 2
According to Kepler’s law, what is the period of a
satellite that is located at an orbit approximately 35,786
km above the earth?
Solution
Applying the formula, we get
Period = (1/100) (35,786 + 6378)1.5 = 86,579 s = 24 h
A satellite like this is said to be stationary to the earth.
The orbit, as we will see, is called a geosynchronous
orbit.
Figure 17.14
Satellite categories
Figure 17.15
Satellite orbit altitudes
Table 17.1 Satellite frequency band
Band
Downlink,
GHz
Uplink, GHz
Bandwidth,
MHz
L
1.5
1.6
15
S
1.9
2.2
70
C
4
6
500
Ku
11
14
500
Ka
20
30
3500
Figure 17.16
Satellites in geosynchronous orbit
Figure 17.17 Triangulation
Figure 17.18
GPS
Figure 17.19
LEO satellite system
Figure 17.20
Iridium constellation
Note:
The Iridium system has 66 satellites in
six LEO orbits, each at an
altitude of 750 km.
Note:
Iridium is designed to provide direct
worldwide voice and data
communication using handheld
terminals, a service similar to cellular
telephony but on a global scale.
Figure 17.21 Teledesic
Note:
Teledesic has 288 satellites in 12 LEO
orbits, each at an altitude of 1350 km.
Satellite Components
• Satellite Subsystems
– Telemetry, Tracking, and Control
– Electrical Power and Thermal Control
– Attitude Control
– Communications Subsystem
Satellite Orbits
• Equatorial
• Inclined
• Polar
Orbital Mechanics
Without
Force
Gravity
Effect of
Gravity
Here’s the Math…
• Gravity depends on the mass of the earth,
the mass of the satellite, and the distance
between the center of the earth and the
satellite
• For a satellite traveling in a circle, the
speed of the satellite and the radius of the
circle determine the force (of gravity)
needed to maintain the orbit
But…
• The radius of the orbit is also the distance
from the center of the earth.
• For each orbit the amount of gravity
available is therefore fixed
• That in turn means that the speed at which
the satellite travels is determined by the
orbit
Let’s look in a Physics Book…
• From what we have deduced so far, there
has to be an equation that relates the orbit
and the speed of the satellite:
r3
T  2
4 1014
T is the time for one full revolution around the orbit, in seconds
r is the radius of the orbit, in meters, including the radius of the
earth (6.38x106m).
The Most Common Example
• “Height” of the orbit = 22,300 mile
• That is 36,000km = 3.6x107m
• The radius of the orbit is
3.6x107m + 6.38x106m = 4.2x107m
• Put that into the formula and …
The Geosynchronous Orbit
• The answer is T = 86,000 sec (rounded)
• 86,000 sec = 1,433 min = 24hours
(rounded)
• The satellite needs 1 day to complete an
orbit
• Since the earth turns once per day, the
satellite moves with the surface of the
earth.
Assignment
• How long does a Low Earth Orbit Satellite
need for one orbit at a height of 200miles
= 322km = 3.22x105m
• Do this:
– Add the radius of the earth, 6.38x106m
– Compute T from the formula
– Change T to minutes or hours
r3
T  2
4 1014
GEO Coverage
• Altitude is about 6 times the earth’s radius
• Three satellite can cover the surface of the
earth
Orbit Examples
• Geostationary
– Equatorial and Geosynchronous
• Inclined Geosynchonous
– Satellite moves north/south relative to the
earth station
• Polar LEO
– Satellite group covers the entire earth
LEOS Coverage
• Altitude is 1/6 of the earth’s radius
Communication Frequencies
• Uplink (Earth to Satellite)
– C Band: around 6 GHz
– Ku Band: around 14 GHz
– Ka Band: around 30 GHz
• Downlink (Satellite to Earth)
– C Band: around 4 GHz
– Ku Band: around 12 GHz
– Ka Band: around 20 GHz
Sputnik I
Sputnik I -- 60 cm (about 2 ft.) diam. sphere with straight-wire antennas
Explorer I
Explorer I -- 1 m. long and 20 cm in diam., spin
stabilized (like a gyroscope), with flexible antennas
A generic military/meteorological/
communications satellite
1-3 m. on each side, stabilized with internal
gyroscopes or external thrusters
Dual-spin stabilized satellite
1-3 m. in diameter, up to several meters tall; lower section spins
to provide gyroscopic stability, upper section does not spin
LIONSAT
Local IONospheric Measurements
SATellite
•will measure ion distrib. in ram and
wake of satellite in low orbit
•student-run project
(funded by Air Force, NASA and AIAA)
•www.psu.edu/dept/aerospace/lionsat
Hubble Space Telescope
http://www.stsci.edu/hst/proposing/documents/cp_cy12/primer_cyc12.pdf
Propulsion
• Provides force needed to change satellite’s orbit.
• Includes thrusters and propellant.
Spacecraft Propulsion
Subsystem
• Uses of onboard propulsion systems
– Orbit Transfer
• (Low Earth Orbit) LEO to (Geosynchronous
Earth Orbit) GEO
• LEO to Solar Orbit
– Drag Makeup
– Attitude Control
– Orbit Maintenance
Types of Propulsion
– Chemical Propulsion
• Performance is energy limited
• Propellant Selection
– Electric Propulsion
• Electrostatic—Ion Engine
• Electrothermal—ArcJet
• Electomagnetic—Rail gun
Types of Propulsion
– Solar Sails
• Would use large (1 sq. km.) reflective sail (made of
thin plastic)
• Light pushes on the sail to provide necessary force
to change orbit.
• Still on the drawing board, but technologically
possible!
– Nuclear Thermal
Power
• Provides, stores, distributes, and controls
electrical power.
• Need power for (basically everything)
communications, computers, scientific
instruments, environ. control and life
support, thermal control, and even for
propulsion (to start the rocket engine)
Power
• Solar array: sunlight  electrical power
– max. efficiency = 17% (231 W/m2 of array)
– degrade due to radiation damage 0.5%/year
– best for missions less than Mars’ dist. from Sun
• Radioisotope Thermoelectric Generator (RTG):
nuclear decay  heat  electrical power
– max. efficiency = 8% (lots of waste heat!)
– best for missions to outer planets
– political problems (protests about launching 238PuO2)
• Batteries – good for a few hours, then recharge
Power
• Dynamic Power Sources
– Like power plants on Earth.
• Fuel Cells
– Think of these as refillable batteries.
– The Space Shuttle uses hydrogen-oxygen
fuel cells.
Power
• The design is highly dependent on:
– Space Environment (thermal, radiation)
– Shadowing
– Mission Life
Thermal
• Thermal Control System
– Purpose—to maintain all the items of a
spacecraft within their allowed temperature
limits during all mission phases using
minimum spacecraft resources.
Thermal
• Passive
– Coatings (control amt of heat absorbed & emitted)
• can include louvers
– Multi-layer insulation (MLI) blankets
– Heat pipes (phase transition)
Thermal
• Active (use power)
– Refrigerant loops
– Heater coils
Communications
• Transmits data to ground or to relay
satellite (e.g. TDRS)
• Receives commands from ground or relay
satellite
Communications
• Radios (several for redundancy)
– voice communications if humans onboard
– data sent back to Earth from scientific
instruments
– instructions sent to s/c from Earth
• Video (pictures of Earth, stars, other
planets, etc.)
• various antennas: dish, dipole, helix
Attitude Sensing and Control
• Senses and controls the orientation of the
spacecraft.
Attitude Sensing
• star sensor –
– The light from stars and compares it to a star
catalog.
Attitude Sensing
• sun sensor measures angle between "sun line"
Attitude Sensing
• gyroscopes -- spinning disk maintains its
orientation with respect to the fixed stars -onboard computer determines how the s/c
is oriented with respect to the spinning
disk.
Attitude Control
• Thrusters -- fire thrusters (small rockets) in pairs
to start rotation, then fire opposite pair to stop the
rotation.
Attitude
• gyroscopes -- use electric motor in s/c
satellite
wheel
motor
Attitude Determination and Control
y
• Sensors
–
–
–
–
–
Earth sensor
Earth sensor (0.1o to 1o)
Sun sensor (0.005o to 3o)
star sensors (0.0003o to 0.01o)
magnetometers (0.5o to 3o)
Inertial measurement unit (gyros)
• Active control (<
0.001o)
– thrusters (pairs)
– gyroscopic devices
• reaction & momentum wheels
– magnetic torquers (interact with
Earth’s magnetic field)
• Passive control (1o to 5o)
– Spin stabilization (spin entire sat.)
– Gravity gradient effect
x
field of view
photocells
rotation
satellite
wheel
motor
• Motor applies torque to wheel (red)
• Reaction torque on motor (green)
causes satellite to rotate
Command and Data Handling
• Principal Function
– Processes and distributes commands;
processes, stores, and formats data
• Other Names
– Spacecraft Computer System
– Spacecraft Processor
Command and Data Handling
• Commands
– Validates
– Routes uplinked commands to subsystems
• Data
– Stores temporarily (as needed)
– Formats for transmission to ground
– Routes to other subsystems (as needed)
• Example: thermal data routed to thermal controller, copy
downlinked to ground for monitoring
Command and Data Handling
• provide automatic capability for s/c, reducing
dependence on expensive ground control
• must include backups or redundant computers if
humans onboard
• need to be protected from high-energy radiation
• cosmic rays can alter computer program (bit flip)
without human ground controllers realizing it.
Structure
• Not just a coat-rack!
• Unifies subsystems
• Supports them during launch
– (accel. and vibrational loads)
• Protects them from space debris, dust, etc.
Launch Vehicle
• Boosts satellite from Earth’s surface to space
• May have upper stage to transfer satellite to
higher orbit
• Provides power and active thermal control
before launch and until satellite deployment
Creates high levels of accel. and vibrational
loading
Launch System
• System selection process
– Analyze capable systems
– Maximum accelerations
– Vibration frequencies and amplitudes
– Acoustic frequencies and amplitudes
– Temperature extremes
– LV/satellite interface
– Kick motor needed?
Delta II Rocket
Image:http://www.boeing.com/companyoffices/gallery/images/space/del
ta_ii/delta2_contour_08.htm
Titan IV Rocket
Image: www.spaceline.org/galleries/cpx-40-41/blowup41.jpg.html
Ground Control
• MOCC (Mission Operations Control Center)
– Oversees all stages of the mission (changes in orbits,
deployment of subsatellites, etc.)
• SOCC (Spacecraft Operations Control Center)
– Monitors housekeeping (engineering) data from sat.
– Uplinks commands for vehicle operations
• POCC (Payload Operations Control Center)
– Processes (and stores) data from payload (telescope
instruments, Earth resource sensors, etc.)
– Routes data to users
– Prepares commands for uplink to payload
• Ground station – receives downlink and transmits uplink
Payload Operations Control
Center
NASA Marshall Space Flight Center, Huntsville Alabama
Mission Control Center
NASA Johnson Spaceflight Center, Houston Texas