Satellite

communication

system done by Mahmoud Zaher

Farouk el Kanawati

Amr Safwat

Aly Mahmoud

Islam Ayman

Ziad maged

Active satellites

Passive satellites

Kepler’s laws of planetary motion

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.

Effect of Nonlinearity

Effect of Nonlinearity

Intermodulation

Intermodulation

Phase Noise

Phase Noise

Phase Noise

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

Thank you

any questions ?

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