Satellite Communications

Satellite Communications

Definitions:
Satellite
Is a celestial body that orbits around a planet (e.g., the moon is the earth satellite )
Satellite Communications
Is a microwave repeater in the sky that consist of a diverse combination of one or more of the following: receiver, transmitter, amplifier,
regenerator, filter, onboard computer, multiplexer, demultiplexer, antenna and other electronic communication circuits.
Transponder
It’s a satellite repeater, sometimes this term also used to specify the circuit capacity of the satellite.
1 Transponder is equal to 36 MHz Bandwidth
Satellite System
Is a system consisting of one or more satellite space vehicles, ground-based station to control the operation of the system, and a user network of
earth stations that provides the interface facilities for transmission and reception of terrestrial communication traffic through the satellite system.

Satellite Orbit

Inclined Orbit
Are virtually all orbits except those travel directly above the equator or directly over the North and South Poles.

Equatorial Orbit
When the satellite rotates in an orbit directly above the equator, usually in a circular path. It has an angle of inclination of 0º.

Polar Orbit
When the satellite rotates in a path that takes it over the North and South Poles in an orbit perpendicular to the equatorial plane. It has an angle of
inclination of 90º.

Types of Satellite

LEO ( Low Earth Orbit )

It’s a low orbiting satellite approximately 480 miles above the earth’s surface.
MEO ( Medium Earth Orbit )
Satellite with an altitude between 5000 km to 15,000 km
GEO ( Geosynchronous Earth Orbit )
Are high-latitude earth –orbit satellite with an altitude of 35,786 km
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Satellite Orbital Patterns

Apogee
The point in an orbit that is located farthest from Earth.
Perigee
The point in an orbit that is located closest to Earth.
Major Axis
The line joining the perigee and apogee through the center of Earth: sometimes called line of apsides
Minor Axis
The line perpendicular to the major axis and halfway between the perigee and apogee ( Half the distance of the minor axis is called the semi-minor
axis.)

Kepler’s Law

A satellite remains in orbit because the centrifugal force caused by its rotation around earth is counterbalanced by earth’s gravitational pull.

First Law
The orbit of a planet around the sun is an ellipse.

Second Law
A straight line from the planet to the center of the sun sweeps out equal areas in equal time intervals as if goes around the orbit; the planet moves
faster when closer to the sun and slower when distant.

Third Law
The square of the period ( in years ) for one revolution about the sun equals the cube of the mean distance from the sun center, measured in an
astronomical units.

Kepler’s Formula

Period = C x distance 1.5

Where :

C is a constant approximately equal to 1/100

Distance is the earth’s radius plus the distance of the satellite above the earth in km
Period is in second
Earth radius is equal to 6378 km

Example:

What is the period of the moon, according to Kepler’s Law?

Solution:
The moon is located approximately 384,000 km above the earth.
Applying Kepler’s Law

100 
Period  1 384, 000  6378 
1.5

Period  2, 439, 090sec

Period  1month
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Uplink and Downlink

Uplink Model
The primary component within the uplink section of a satellite system is the earth station transmitter. A typical earth station transmitter consist of
an IF modulator, and an IF-RF microwave up- converter, a high-power amplifier(HPA) and a filter.
Note: A High frequency signal is transmitted in an Uplink Band.

B B H
MODULATOR MIXER
P P P
F F A

OSCILLATOR

Transponder
It refers to the satellite itself consisting of an input bandlimiting devices, an input low-noise amplifier, a frequency translator, a low level power
amplifier, and an output bandpass filter.

L B B L
N P Mixer P N
A F F A

Oscillator

Downlink Model
An Earth station receiver includes an input BPF, an LNA and an RF-to-IF down converter and an IF demodulator.
Note: A Low frequency signal is transmitted in a Downlink Band.

B L B
P N MIXER P DEMODULATOR
F A F

OSCILLATOR

Satellite Frequency Band

The frequency reserve for satellite microwave communication are in GHz range. Each satellite send and receives over two different frequency band.
Transmission from earth to the satellite is called the uplink. Transmission from the satellite to the earth is called the downlink.
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Band Downlink, GHz Uplink, GHz Bandwidth, MHz

L 1.5 1.6 15
S 1.9 2.2 70
C 4.o 6.0 500
Ku 11.0 14.0 500
Ka 20.0 30.0 3500

Access Method

Frequency Division Multiple Access (FDMA)

The user on this access is differentiated by means of carrier. Each channel on its frequency band has its own carrier.

Time Division Multiple Access (TDMA)

The user on this access is differentiated by means of time slot. Multiple user can share on a single frequency or carrier but on different time slots.

Code Division Multiple Access (CDMA)

In CDMA, multiple user is differentiated by means of code. Transmission can spread throughout the entire bandwidth. Each earth or space vehicle
transmission is encoded with a unique code word called Chip Code.

Fixed Assignment Multiple Access (DAMA)

The assignment of capacity is distributed in a fixed manner among multiple stations.

Demand Assignment Multiple Access (DAMA)

Capacity assignments is changed as needed to respond optimally to demand changes among the multiple stations.

Satellite Orientation
Antenna Look Angles
Azimuth Angle
An Azimuth angle is the horizontal angular distance from the reference direction, either the southern or northern most point of the horizon. It is
defined as the horizontal pointing angle of an earth station antenna.
Elevation Angle
Angle of Elevation is the vertical angle formed between the direction of travel of an electromagnetic wave radiated from the earth station antenna
pointing directly toward satellite and horizontal plane.

Satellite Parameters
Back-Off loss
The amount of loss due to traveling wave tube or transmission lines.
Transmit Power and Bit Energy
Eb  PT
t b
Where:
Eb is energy of a single bit ( joules per bit )
Pt is total saturated output power ( watt or joules per second )
Tb is the time of a single bit ( second )

Effective Isotropic Radiated Power

EIRP  Pin At
Where:
Pin is the antenna input power
At is the transmit antenna gain
EIRP is the Effective Isotropic Radiated Power in watts

Equivalent Noise Temperature

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N  KTB
Where:
N= total noise power
K= boltzman’s constant 1.38 x 10-23 joules/ per kelvin
B= bandwidth
T= temperature in Kelvin

Noise Density
N KTE B
NO    KTE
B B
Where:
No is the noise density ( watts/ per hertz) generally expressed as simply watts; the per hertz is implied in the definition of No.
N is the total noise power
B is the bandwidth
K= boltzman’s constant 1.38 x 10-23 joules/ per Kelvin
TE= temperature in Kelvin
Carrier-to-Noise Density Ratio
C C C
 or ( dB)  C (dB)  NO (dB )
N O KTE NO

Where:
C is the carrier power
No is the noise density ( watts/ per hertz) generally expressed as simply watts; the per hertz is implied in the definition of No.
N is the total noise power
B is the bandwidth
K= boltzman’s constant 1.38 x 10-23 joules/ per Kelvin
TE= temperature in Kelvin

Energy of Bit-to-Noise Ratio

Eb  C   B 
    Eb C B
NO  N   fb  or ( dB)  ( dB0  (dB)
NO N fb

Where:
C is the carrier power
N is the total noise power
B is the bandwidth
Eb is energy of a single bit ( joules per bit )
fb is the input bit rate

Gain-to-Equivalent Noise Temperature Ratio

G
 G  10 log(TS )
TE

Where:
Ts  Ta  Tr
Ta is the antenna temperature in kelvin
Tr is the receiver effective input noise temperature in kelvin
G is the receiver antenna gain in dB
Ts is the operating or system temperature in Kelvin
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QUESTION AND ANSWERS:

Topic: Satellite Communications

1.A circular orbit around the equator with a 24-h period is called a(n)
Ans: geostationary orbit

2. The earth area covered by a satellite radio beam.

Ans: Footprint

3. What is Keppler's first law?

Ans: planetary orbits are elliptical with sun at the focus

4. Geostationary satellites are located ______ with respect to the equator.

Ans: 0 degrees longitude

5. The complete bandwidth of a typical multi-purpose satellite is

Ans: 5 KHz

6. What circuit is responsible in activating and deactivating adjacent antenna elements in a mobile satellite array?

Ans: Radial divider

7. Refers to the satellite orientation with respect to the earth.

Ans: Satellite altitude

8. With FDMA, transponder bandwidth is subdivided into bandwidths which are capable of carrying one voice channel.

Ans: Smaller

9. For satellite communication, the lower limit of frequency is

Ans: 10 MHz

10. In a geosynchronous satellite orbital disturbances are caused by which of the following?

 Sun  
 Earth
 Moon
 All of these  (Correct Answer)

11. The physical location of a satellite is determined by its

Ans: Latitude and longitude

12. The farthest distance from earth, a satellite orbit reaches is called

Ans: Apogee

13. Which of the following is the most effective anti-jamming technique?

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Ans: Spread-spectrum modulation

14. Long distance communication system via satellite uses frequency in the range of

Ans: 3 to 6 GHz

15. Before transmitting to the satellite, the wide-band satellite signal is amplified by

Ans: Traveling wave tube

16. It is the typical bandwidth of a satellite band

Ans: 500 MHz

17. Essentially a satellite ______ is a radio repeater in the sky.

Ans: Transponder

18. The energy to power electronic equipment on board in satellite is invariably supplied by

Ans: Solar cells

19. Which of the following statements is correct regarding a communication satellite?

Ans: It is a repeater between many transmitting stations and many receiving stations

20. The lifetime of a geosynchronous satellite is about

Ans: 10 years

21. The most common carrier frequencies used for satellite communications are

Ans: 6/4 and 14/12 GHz bands

22. The frequency of Ku band for satellite communications.

Ans: 14/11 GHz

23. Repeaters inside communications satellites are known as

Ans: Transponders

24. For direct reception of TV signal from a communication satellite, the type of antenna required is

Ans: Chicken-mesh antenna

25. A satellite repeater serves which of the following purposes?

Ans: It translates the signal to new frequency band

26. The signal path from satellite to earth-based receiver

Ans: Downlink signal

27. The ______ angle measured the satellite position clockwise from the direction of true north.

Ans: Azimuth

28. For a satellite with time period of 2 hours, the height above the surface must be

Ans: 2000 km

29. The key electronic component in a communications satellite is the

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Ans: Transponder

30. The satellite communications channels in a transponder are defined by the

Ans: Bandpass filter

31. As the height of a satellite orbit gets lower, the speed of the satellite

Ans: Increases

32. What is the basic technique used to stabilized a satellite?

Ans: Spin

33. The round-trip propagation delay between two earth stations through a geosynchronous satellite is

Ans: 500 to 600 ms

34. For satellite communication, the frequency should be ______ the critical frequency of ionosphere.

Ans: More than

35. In this multiple access technique, each earth stations transmissions are assigned specific uplink and downlink frequency bands alloted satellite
channel bandwidth.

Ans: frequency division multiple accessing

36. Sputnik 1 was orbiting the earth in

Ans: 90 minutes

37. The distance between the two antennas of a geostationary satellite system is 22,300 miles and is operating at a downlink frequency of 12.2
GHz, what is the free space loss in dB?

Ans: 205 dB

38. A type of satellite that simply reflects a signal back to earth.

Ans: Passive satellite

39. The main function of a communications satellite is as a/an

Ans: Repeater

40. Shannon's law relates which of the following?

Ans: Information-carrying capacity to S/N ratio

41. What is the approximate path loss from satellite-to-earth station?

Ans: 200 dB

42. _____ is the first satellite transponder.

Ans: Moon

43. What is the local oscillator (mixer) frequency of the satellite with an uplink frequency in GHz band?

Ans: 2225 MHz

44. The ______ requires a larger receiving antenna and receivers are often only able to receive C-band signals

Ans: C-band
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45. GPS means

Ans: Global Positioning Satellite

46. The first Intelsat satellite that was launched in 1965 was named

Ans: Early Bird I

47. A satellite link can be made to carry ______ information when operating digitally than in analog fashion.

Ans: More

48. Transponder bandwidth commonly used in satellite is

Ans: 36 MHz

49. A satellite battery that has more power but lighter.

Ans: Lithium

50. How can multiple earth stations share a satellite on the same frequencies?

Ans: Frequency reuse