Lecture 3, Earth Segment

Presentation / Lecture 3, Earth Segment

Date Submitted: 06 June 2001

Written by RPC Telecommunications. Website: http://www.rpctelecom.com

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This is the third in the series of general satcom tutorial lectures submitted by RPC Telecommunications.

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 Printable Version
-Section 1
Types and components of an Earth Station
-Section 2
General Construction
-Section 3
Antenna Theory
-Section 4
Radiation Patterns
-Section 5
Polarisation
-Section 6
Noise
-Section 7
Low-noise Amplifiers
-Section 8
Power Amplifiers
-Section 9
Tracking
Noise
There are two main sources of noise in a satellite system:
  • Noise arising in the satellite and earth-station equipment, and
  • Noise collected by the satellite and earth-station antennas

The antenna receives noise from the sky and from the earth.

Sky noise comprises cosmic (galactic) noise and noise resulting from absorption and re-radiation of energy by water and oxygen molecules in the atmosphere

Noise power radiated by the Earth is collected by the sidelobes of the earth-station antenna and the main beam of the satellite antenna.

Noise power N is related to an equivalent noise temperature by the expression:

N = k . T . B watts

Where:

K is Boltzmann's constant (1.38 x 10-23 J/K)
B is the bandwidth in which the noise is measured (in Hz)

 

G/T
The ratio between the receive gain of an earth-station and its noise temperature is a measure of the "quality" with which it is able to receive signals

(Also known as the "Figure of Merit")

G/T ranges from around 37 dB/K for high-gain low-noise FSS antennas to about -23 dB/K for low-gain high-noise mobile terminals.

Noise Temperature
The noise power collected by an antenna comprises:
  • Atmospheric attenuation noise caused by absorption and re-radiation of signal energy by water and oxygen molecules in the atmosphere; this noise increases rapidly with decreasing antenna elevation angle because the signal has to travel further through the atmosphere; it also increases when it is raining.
  • Noise radiated by the Earth - collected through the antenna sidelobes.
  • Cosmic (galactic) noise - only contributes a few K to the noise temperature.
  • Ohmic losses - i.e. losses due to the resistance of the feed system and the antenna reflectors.
  • Attenuation between the antenna and the LNA - contributes about 7K for every 0.1 dB of loss, so it is essential to put the LNA very close to the antenna.

Next: Section 7 - Low-noise Amplifiers

 
 

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