SATCOM Glossary: A–F
This glossary provides concise engineering definitions for satellite communications terminology used throughout the satcomindex knowledge base. Each term includes a definition, a note on why it matters in practice, and links to related reference pages.
The definitions are written for engineers, technicians, and technical decision-makers working with satellite systems. They complement the detailed explanations in the basics section and the deployment-specific context in the solutions pages.
A
ACM (Adaptive Coding and Modulation)
A technique used in DVB-S2 and DVB-S2X systems that dynamically adjusts the modulation order and forward error correction (FEC) code rate on a per-terminal basis, responding to real-time link conditions such as rain fade or interference.
ACM maximizes throughput during clear-sky conditions by using higher-order modulation, then automatically shifts to more robust settings when the link degrades. This allows the system to maintain connectivity without manual intervention, directly impacting service availability.
Antenna Gain
A measure of how effectively an antenna focuses RF energy in a particular direction compared to an isotropic radiator, expressed in dBi. Gain increases with antenna aperture size and operating frequency.
Antenna gain is a primary variable in the link budget. A larger antenna with higher gain allows smaller BUC output power to achieve the same EIRP, or provides greater receive sensitivity (G/T) for improved downlink performance.
Azimuth
The horizontal compass bearing from a terminal antenna to the target satellite, measured in degrees clockwise from true north. Together with elevation and polarization skew, azimuth defines the antenna pointing direction.
Correct azimuth is essential for antenna alignment. An error of even a few degrees can cause the antenna to miss the satellite beam entirely, resulting in no link or severe cross-polarization interference with adjacent satellites.
B
Bandwidth
The range of frequencies occupied by a signal or allocated to a communication channel, measured in Hertz (Hz). In satellite systems, bandwidth refers to both the RF spectrum allocated on the transponder and the data throughput capacity assigned to a service.
Satellite bandwidth is a finite and expensive resource. The amount of bandwidth allocated to a terminal, combined with the modulation and coding scheme, determines the maximum achievable data rate. Bandwidth management is a core function of satellite network operations.
Beam (Spot Beam)
A focused area of satellite coverage created by the satellite antenna. Wide beams cover large regions (e.g., continental footprints), while spot beams cover smaller areas with higher power density. High-throughput satellites (HTS) use many spot beams to increase total capacity through frequency reuse.
Spot beams enable higher EIRP and G/T within each coverage zone, supporting higher data rates per terminal. They also allow the satellite to reuse the same frequency across non-overlapping beams, multiplying the aggregate system capacity.
BUC (Block Upconverter)
An outdoor RF component that converts the intermediate-frequency (IF) or L-band signal from the satellite modem to the transmit frequency band (e.g., Ku-band or Ka-band) and amplifies it for uplink to the satellite.
BUC output power directly determines the terminal EIRP when combined with antenna gain. Undersizing the BUC results in insufficient uplink power, degrading the return-link throughput and potentially causing the carrier to fall below the gateway demodulation threshold.
C
C-band
A satellite frequency band operating at approximately 4–8 GHz (downlink 3.7–4.2 GHz, uplink 5.925–6.425 GHz). C-band is used for wide-area coverage and is resistant to rain fade, making it suitable for tropical and equatorial regions.
C-band requires larger antennas (typically 2.4 m to 3.8 m for remote terminals) due to lower frequency, but its rain-fade resilience makes it the preferred band for high-availability links in regions with heavy precipitation.
Carrier-to-Noise Ratio (C/N)
The ratio of the received carrier signal power to the noise power within the channel bandwidth, expressed in dB. C/N0 is the carrier-to-noise-density ratio, which normalizes noise to a 1 Hz bandwidth for comparison across different channel widths.
C/N determines whether a modem can successfully demodulate the received signal. The required C/N depends on the modulation and coding scheme in use — higher-order modulation requires higher C/N. Monitoring C/N is a primary method for assessing link health.
Cross-Polarization (Cross-Pol)
Unwanted coupling of signal energy from one polarization plane to the orthogonal polarization plane. In dual-polarization satellite systems, the same frequency is reused across two orthogonal polarizations (e.g., horizontal and vertical, or left-hand and right-hand circular).
Poor cross-polarization isolation — caused by incorrect feed alignment (skew), antenna deformation, or rain depolarization — allows signals on the opposite polarization to interfere with the wanted signal, degrading C/N and potentially causing errors.
Contention Ratio
The ratio of the total number of terminals sharing a bandwidth pool to the capacity that would be needed if every terminal transmitted at maximum rate simultaneously. A contention ratio of 20:1 means 20 terminals share bandwidth dimensioned for one terminal at full rate.
Contention ratios determine per-user throughput during peak usage. Lower contention ratios (e.g., 5:1) provide more consistent performance but at higher cost. Understanding the contention ratio is essential when evaluating satellite service plans and SLA commitments.
D
Downlink
The signal path from the satellite to the ground — either to a gateway earth station or directly to a user terminal. The downlink operates at a different frequency than the uplink to avoid self-interference at the satellite transponder.
Downlink performance determines what the terminal can receive. The received signal level depends on the satellite EIRP toward the terminal location, the path loss at the operating frequency, atmospheric attenuation, and the terminal antenna G/T.
DVB-S2 / DVB-S2X
Digital Video Broadcasting – Satellite Second Generation (and its extension, S2X) is the standard that defines the physical layer for the forward link in most commercial VSAT systems. It specifies modulation formats (QPSK through 256APSK), FEC code rates, and framing structures.
DVB-S2/S2X provides the framework for ACM operation, enabling the system to adapt throughput to link conditions in real time. S2X extends the standard with finer granularity in modulation/coding combinations and support for very low SNR operation.
Diversity (Gateway Diversity)
The use of multiple geographically separated gateway earth stations to protect against site-specific outages. If one gateway experiences rain fade, equipment failure, or power loss, traffic is rerouted through an alternate gateway.
Gateway diversity is a primary technique for achieving high service availability, particularly for Ka-band systems where rain fade at the gateway can interrupt service to all terminals served by that gateway. Diversity design must account for the statistical independence of rain events at each site.
E
Eb/No
Energy per bit to noise spectral density ratio, expressed in dB. Eb/No normalizes the signal quality measurement to be independent of data rate and bandwidth, making it the standard metric for comparing modulation and coding performance.
Eb/No is the fundamental link quality metric that determines the bit error rate (BER) for a given modulation and coding scheme. Satellite modems report Eb/No as the primary indicator of link health, and ACM thresholds are defined in terms of Eb/No.
Earth Station
A ground-based facility equipped with antenna and RF systems that communicates with one or more satellites. The term encompasses both large gateway earth stations (teleports) and smaller user terminals (VSAT), though in common usage it often refers to the gateway side.
Earth stations are the ground-side endpoints of every satellite link. Gateway earth stations in particular are single points of failure for all terminals they serve, making their design, redundancy, and operational practices critical to network availability.
EIRP (Effective Isotropic Radiated Power)
The product of transmitter output power and antenna gain, expressed in dBW. EIRP represents the total radiated power in the direction of the satellite as if it were radiated equally in all directions by an isotropic antenna.
EIRP is the single most important transmit-side parameter in the link budget. Satellite operators specify maximum EIRP density limits to protect adjacent satellites from interference. Terminal EIRP must be high enough to close the return link while remaining within these limits.
F
Fade Margin
The additional signal level (in dB) designed into a satellite link budget above the minimum required for demodulation under clear-sky conditions. Fade margin accounts for atmospheric attenuation, rain fade, scintillation, and other propagation impairments.
Insufficient fade margin causes link outages during rain events or atmospheric degradation. The required fade margin depends on the frequency band (Ka-band requires more than Ku-band), the geographic climate zone, and the target availability percentage.
Feed Horn
The component at the focal point of a parabolic antenna that collects or radiates RF energy. The feed horn illuminates the reflector on transmit and captures the reflected energy on receive. It connects to the BUC (transmit) and LNB/LNA (receive) via waveguide or directly.
Feed horn design affects the antenna illumination pattern, sidelobe levels, and polarization purity. An improperly aligned or damaged feed horn degrades antenna gain and cross-polarization performance, directly impacting link quality.
Frequency Band
A designated range of radio frequencies allocated for satellite communication. The primary bands used in commercial SATCOM are L-band (~1–2 GHz), C-band (~4–8 GHz), Ku-band (~12–18 GHz), and Ka-band (~26.5–40 GHz). Each band has distinct propagation characteristics, regulatory allocations, and hardware requirements.
Band selection is one of the earliest and most consequential decisions in satellite system design. It determines antenna size, rain-fade margin requirements, available satellite capacity, regulatory licensing, and overall system cost.
FSS (Fixed-Satellite Service)
An ITU-defined radiocommunication service between earth stations at specified fixed locations using one or more satellites. FSS allocations in C-band, Ku-band, and Ka-band form the regulatory basis for most commercial VSAT and gateway operations.
FSS frequency allocations determine what spectrum is legally available for satellite operations in each region. Operators must coordinate their transmissions within FSS allocations and comply with ITU coordination procedures to avoid harmful interference with other services.