• 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)

(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.

• 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.