Lecture 3, Earth Segment

 Presentation / Lecture 3, Earth Segment Date Submitted: 06 June 2001 Written by RPC Telecommunications. Website: http://www.rpctelecom.com ExcellentGoodAveragePoorVery Poor   Rate This Article This is the third in the series of general satcom tutorial lectures submitted by RPC Telecommunications. Comment On This Article
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

 Copyright © 2018 Satcom Online (About This Site)