satellite communication - guc · satellite transponder a communications satellite's...
TRANSCRIPT
Satellite
communication
system done by Mahmoud Zaher
Farouk el Kanawati
Amr Safwat
Aly Mahmoud
Islam Ayman
Ziad maged
Low Earth Orbits
Although the Earth's pull due to gravity on the surface of the Earth is approximately considered equal to the gravity effect on the LEO, objects in orbit experience weightlessness because they are in free fall and all bodies attract each other in space.
A low Earth orbit satellite is simple and cheap to place. It provides high bandwidth and low communication time lag (latency), but LEO satellites are not geostationary and are not visible from any certain point on the Earth at all times.
First Law: The Law of Ellipses Two– body elliptic system
Any ellipse has two Focal points, and the center of mass of Earth
(the first body) is always centered on one of the foci so the satellite
which is the second body follows an elliptic path around Earth.
The mass of the satellite is extremely smaller than the mass of
earth, the center of mass will always conform in attraction with the
center of Earth.
An ellipse
R Is the distance from the planet center of mass to the satellite.
The linear eccentricity (ε) is the distance between the center and the focus (or one of
the two foci).
The latus rectum l=(semi-major axis length)*(1- ε^2) is the chord parallel to the
directrix and passing through the focus (or one of the two foci).
The focal parameter (p) is the distance from the focus (or one of the two foci) to the
directrix.
Second Law: The Law of Equal
Areas Any line segment joining a satellite and the Earth produces out equal areas during
equal intervals of time.
An example on the earth and sun:
The orbital radius and angular velocity of the planet in the elliptical orbit will vary. The
planet travels faster when closer to the sun (relative to the arc x), then slower when
farther from the sun (relative to distance y). Kepler's second law states that the blue
sector has constant area. Area X= Area Y.
Proof is done using the angular momentum equations and Newton’s law F=ma.
However, we don’t need the derivations.
Third Law: The Law of
Harmonies The square of the orbital period of a satellite is proportional to the
cube of the semi-major axis of its orbit which is the mean distance
between the primary point and the satellite.
After applying Newton’s law of universal gravitation, the more
general form of Kepler’s second law is:
Uplink and Downlink The Uplink is the signal from the
base station to the satellite, while
the Downlink is the signal from the
satellite back to base station.
Satellite Frequency Bands
Satellites operate at certain frequency
bands such as C, X, Ku, Ka, EHG and V-
band. To help in preventing
interference caused by multiple sites
transmitting on the same frequency,
and this is controlled by the
government.
Why is the Uplink frequency
higher?!
High frequency is needed for the signal to penetrate the atmosphere.
Satellites are limited by its weight compared to base stations.
Bigger antennas in base stations is used to receive the relatively low power transmitted by the satellite.
Small antenna in the satellite is sufficient to receive the higher power transmitted from base stations.
Uplink and downlink model
The term uplink chain is
used to refer to the series
of equipment that are
used to produce a radio
frequency signal for
sending out data. The
downlink chain is built
using nearly the same
equipment in reverse
order.
Uplink chain
1. Digital data is sent to the modulator which takes the data and converts it into a modulated signal in the Intermediate Frequency range (70-140 MHz).
2. The Intermediate Frequency is piped to an "up converter" which mixes the intermediate frequency with a higher frequency to produce a final frequency which carries the modulated data.
3. Noise is removed from the signal via either a band pass filter or other means and then it is amplified in a Klystron, Traveling Wave Tube or Solid State amplifier.
4. The final cleaned signal is transmitted down the wave guide to the dish.
5. The feed horn at the focal point of the dish emits the high frequency radio transmission, which the dish focuses into a directional transmission at the satellite.
Downlink chain
1. The satellite transmits a signal containing data.
2. The signal is received at the satellite dish.
3. The signal is amplified and fed to the Down Converter.
4. The Down Converter down mixes the signal to create an intermediate frequency.
5. The intermediate frequency is fed to the demodulator and converted into a data.
6. The data stream is forwarded into the network via a router.
Satellite transponder
A communications satellite's transponder is the series of
interconnected units that form a communications channel
between the receiving and the transmitting antennas. It is
mainly used in satellite communication to transfer the
received signals.
A transponder is typically composed of:
An input band limiting device (a band pass filter).
An input low-noise amplifier (LNA), designed to amplify
the (normally very weak, because of the large distances
involved) signals received from the earth station.
A frequency translator (normally composed of an
oscillator and a frequency mixer) used to convert the
frequency of the received signal to the frequency
required for the transmitted signal.
An output band pass filter.
A power amplifier (this can be a traveling-wave tube or a
solid state amplifier).
Device Limitations
The low-noise amplifier (LNA) introduces nonlinearity to
the system.
The non-ideality of the mixer introduces phase noise.
The combined effect of these drawbacks together with
the non-ideality of the band pass filters used (especially
at high frequency before the mixer) decreases the
overall SIR.
Iridium satellite phone communication
6 orbital planes, 11 acive satellites in each orbit.
86.4 degrees inclination.
Orbital velocity of about 27000 km/h
Height of approximately 781 km above Earth.
Ka band is used for inter-satellite links.
4 inter-satellite links for each satellite.
Military applications
Reconnaissance (spy) satellites:
Missile early warning
Photo surveillance
Radar imaging
Electronic reconnaissance
References
Dr. Mohamed Abd el Ghany Lecture slides.
www.applicationstrategy.com/communications_simulation.htm
www.inetdaemon.com/tutorials/telecom/satellite/uplink_chain.shtml
www.webopedia.com/TERM/T/transponder.html