Satellite Communication Terminology Guide

This page explains key terminology related to satellite communication technology. Let’s dive into some essential terms:

Antenna Characteristics

Antenna Gain

Antenna gain is the ratio, expressed in decibels (dB), of the power radiated by an antenna in a specific direction to the power that would be radiated in the same direction by a hypothetical isotropic antenna. An isotropic antenna radiates equally in all directions and is defined to have a gain of 0 dBi (dB relative to an isotropic antenna).

• Units: dBi
• Important Note: The gain value is specific to a particular direction and frequency.

Antenna Noise Temperature

This is a measure of the total external noise picked up by a receiving antenna, expressed in Kelvin (K). It’s influenced by factors like:

• Antenna diameter
• Elevation angle
• Antenna polarization

Larger antennas tend to have lower noise temperatures. Major noise sources include:

• Cosmic Noise: Caused by the sun, moon, and stars.
• Ground Noise: Resulting from noise energy radiated by the ground.

Power and Performance Metrics

EIRP (Effective Isotropic Radiated Power)

EIRP indicates the total power transmitted from an antenna, taking into account the antenna’s gain.

$EIRP = Power + Antenna\ Gain$

Both EIRP and Power are typically expressed in dBW (decibel-watts), while the antenna gain is in dBi.

LNA (Low Noise Amplifier) Noise Temperature

This measures the amount of noise generated by the LNA itself, measured in Kelvin (K). Lower values indicate better performance (less noise). Typical values range from 30K to 70K.

LNA noise performance can also be specified using Noise Figure (NF), measured in dB:

$NF = 10 * log_{10}(1 + \frac{T}{290})$

Where:

• NF is the Noise Figure in dB
• T is the noise temperature in Kelvins.

G/T (Gain-to-Noise Temperature Ratio)

G/T is a figure of merit for an earth station, expressed in dB/K (dB per Kelvin). It represents the ratio of the antenna gain to the system noise temperature.

$G/T = Antenna\ Gain - 10 * log_{10}(System\ Noise\ Temperature)$

Higher G/T values are desirable. You can improve G/T by:

• Using a higher gain antenna.
• Employing a lower noise temperature LNA.

The earth station’s G/T directly impacts the received carrier-to-noise ratio (C/N). Increasing the station’s G/T improves the C/N of the received signal.

Amplifier Operation

Output Back-Off (OPBO)

OPBO describes the signal level at the output of an amplifier relative to its maximum possible output level.

Example: If the maximum output level is +40 dBm and the measured output is +34 dBm, then the OPBO is 6 dB.

Input Back-Off (IPBO)

IPBO describes the signal level at the input of an amplifier relative to the input level that would cause maximum output.

Example: If an input level of -20 dBm results in maximum output, and the actual input level is -25 dBm, the IPBO is 5 dB.

Both IPBO and OPBO are commonly used to determine the optimal operating levels within a satellite transponder’s Traveling Wave Tube Amplifier (TWTA).

Signal Characteristics

Saturation Flux Density (SFD)

Flux density measures the signal strength at a specific point in space, expressed in Watts/meter² or dBW/meter². It’s typically used when referring to signals received at a satellite.

Polarization

Polarization

Polarization refers to the orientation of the electric field vector (E) of an RF wave. In satellite systems, polarization is used to separate signals at the same frequency, enabling frequency reuse.

Types of polarization include:

• Circular Polarization
• Linear Polarization (with horizontal and vertical orientations)