SATCOM Glossary: M–R
This section of the glossary covers terms from M through R, including modulation techniques, network operations and management, orbital classifications, propagation effects, quality-of-service concepts, and reliability engineering terminology used in satellite communication systems.
Each term includes a concise definition, a note on practical relevance, and cross-links to related pages in the basics and solutions sections of the satcomindex knowledge base.
M
MEO (Medium Earth Orbit)
An orbital altitude range between approximately 2,000 km and 35,786 km above Earth. MEO constellations such as O3b (now SES mPOWER) operate at roughly 8,000 km, providing a latency and capacity balance between LEO and GEO systems.
MEO offers round-trip latency of approximately 120–150 ms, significantly lower than GEO while requiring fewer satellites than LEO for regional coverage. MEO is used for high-throughput trunking and enterprise-grade services where GEO latency is unacceptable but full LEO global coverage is not required.
Modem (Satellite Modem)
The indoor electronic unit that performs modulation, demodulation, coding, and decoding of satellite signals. The modem converts IP traffic into an RF-compatible baseband signal for transmission and reverses the process on reception. Common platforms include iDirect, Comtech, Newtec, and Hughes.
The modem is the protocol boundary between the IP network and the satellite link. Its supported modulation schemes, throughput limits, and acceleration features determine the achievable data rates and application performance across the satellite hop.
Modulation
The process of encoding digital data onto an RF carrier by varying its amplitude, frequency, or phase. In satellite communications, phase-shift keying (PSK) variants are predominant: QPSK, 8PSK, 16APSK, and 32APSK as defined by DVB-S2 and DVB-S2X standards.
The modulation order determines spectral efficiency — how many bits are carried per hertz of bandwidth. Higher-order modulation increases throughput but requires a higher carrier-to-noise ratio (C/N). ACM systems select the optimal modulation in real time based on link conditions.
Multiplexing
A technique for combining multiple data streams into a single transmission channel. In satellite systems, time-division multiplexing (TDM) is used on the forward/outbound link to combine traffic for multiple terminals into one continuous carrier, while frequency-division multiplexing (FDM) separates carriers across the transponder bandwidth.
Multiplexing efficiency directly affects how many terminals and services can share the available satellite bandwidth. The choice of multiplexing scheme interacts with the access method (MF-TDMA, SCPC) and determines the overhead, latency, and scalability characteristics of the network.
MTTR / MTTD (Mean Time to Repair / Detect)
Reliability metrics used in satellite network operations. MTTR is the average time required to restore a failed component or link to operational status. MTTD is the average time between a fault occurring and its detection by the monitoring system.
MTTR and MTTD are key inputs to availability calculations. Reducing MTTD through automated fault detection and reducing MTTR through sparing strategies and pre-positioned equipment directly improves the overall service availability percentage.
N
NOC (Network Operations Center)
A centralized facility staffed by operations personnel who monitor and manage the satellite network in real time. The NOC uses NMS platforms, spectrum analyzers, and alarm systems to detect faults, manage capacity, and coordinate maintenance across the ground and space segments.
The NOC is the operational nerve center of a satellite service. Its monitoring coverage, escalation procedures, and staffing model determine how quickly faults are detected and resolved, directly affecting service-level agreement (SLA) compliance.
NMS (Network Management System)
Software that provides centralized monitoring, configuration, fault detection, and performance reporting for the satellite network. The NMS collects telemetry from hubs, modems, and RF equipment, presenting it through dashboards and alarm interfaces used by NOC operators.
The NMS enables proactive network management by aggregating data from thousands of terminals and network elements. Its alarm correlation, trending, and automated remediation capabilities determine the operational efficiency and responsiveness of the satellite service.
Network Management
The discipline encompassing all processes, tools, and policies used to plan, operate, monitor, and optimize a satellite communication network. It covers bandwidth allocation, QoS enforcement, fault management, configuration control, and performance reporting — often organized under the FCAPS framework.
Effective network management maximizes the utilization of expensive satellite capacity while meeting SLA commitments. It spans the full lifecycle from network planning and commissioning through day-to-day operations and capacity optimization.
Noise Figure
A measure of how much noise an electronic component (such as an LNB or amplifier) adds to the signal, expressed in dB. Noise figure is the ratio of the input signal-to-noise ratio to the output signal-to-noise ratio. A lower noise figure indicates a quieter, higher-quality component.
The noise figure of the first active element in the receive chain (typically the LNB) dominates the overall system noise temperature and therefore the G/T. Selecting low-noise-figure components is essential for maximizing receive sensitivity, especially on Ka-band links with tight link budgets.
O
ODU / IDU (Outdoor Unit / Indoor Unit)
The standard division of a VSAT terminal into two subsystems. The ODU comprises the antenna, feed, BUC, and LNB mounted outdoors. The IDU is the satellite modem and associated indoor equipment that interfaces with the local network.
The ODU/IDU split defines the physical installation architecture and the IF cabling requirements between them. Understanding this separation is essential for site surveys, installation planning, and troubleshooting — determining whether a fault lies in the RF path (ODU) or the baseband/network path (IDU).
Orbit
The curved path a satellite follows around Earth under the influence of gravity. Satellite communication systems use three primary orbital regimes: GEO (35,786 km, geostationary), MEO (2,000–35,786 km), and LEO (300–2,000 km). Each regime presents different trade-offs in latency, coverage area, and constellation size.
The orbital regime is the single most consequential architectural decision in satellite system design. It determines latency, the number of satellites required for coverage, handover complexity, link budget parameters, and the ground segment architecture needed to support the service.
Outage
A period during which a satellite communication link or service is unavailable, whether caused by equipment failure, atmospheric attenuation beyond the link margin, planned maintenance, or interference. Outage duration is the primary metric for calculating service availability percentages.
Service-level agreements define maximum permissible outage per month or year. Understanding the distinction between rain-induced outages (predictable, mitigated by link margin) and equipment outages (mitigated by redundancy and sparing) is critical for availability engineering.
P
Polarization
The orientation of the electric field vector of an electromagnetic wave. Satellite signals use linear polarization (horizontal/vertical) or circular polarization (left-hand/right-hand). Dual-polarization allows two independent signals on the same frequency, effectively doubling the available spectrum.
Correct polarization alignment at the terminal is essential for maximizing the desired signal and minimizing cross-polarization interference from the orthogonal polarization. Polarization skew adjustment is a standard step in antenna commissioning, especially for linearly polarized signals.
Pointing Error
The angular deviation between the actual antenna boresight direction and the ideal direction toward the target satellite, measured in degrees or fractions of a degree. Pointing error can result from inaccurate installation, wind loading, structural settling, or platform motion on mobile terminals.
Pointing error directly reduces antenna gain, degrading both EIRP (uplink) and G/T (downlink). For a typical 1.2 m Ku-band antenna, a pointing error of 0.5° can reduce gain by 3 dB or more. Accurate initial pointing and periodic re-peaking are essential for maintaining link performance.
Propagation Delay
The time required for an electromagnetic signal to travel from the transmitter to the receiver, determined by the distance and the speed of light. For a GEO satellite, the one-way propagation delay is approximately 270 ms; for LEO, it ranges from 1 to 13 ms depending on altitude and elevation angle.
Propagation delay is the irreducible component of satellite link latency. Unlike queuing or processing delays, it cannot be reduced by engineering — only the choice of orbital altitude changes it. Two-way propagation delay plus processing overhead defines the minimum achievable round-trip time.
Q
QoS (Quality of Service)
A set of mechanisms that classify, prioritize, and manage network traffic to meet defined performance targets for different service types. In satellite networks, QoS policies are configured at the hub and enforce bandwidth guarantees, latency bounds, and priority levels across traffic classes.
Satellite bandwidth is expensive and shared across many users. Without QoS, bulk data transfers can consume all available capacity, starving latency-sensitive traffic such as VoIP or SCADA. Proper QoS configuration is essential for meeting SLA commitments across diverse traffic profiles.
QPSK (Quadrature Phase-Shift Keying)
A digital modulation scheme that encodes two bits per symbol by using four distinct phase states of the carrier signal. QPSK is the baseline modulation for most satellite communication standards (DVB-S2, DVB-S2X) and is used when link conditions are poor or maximum robustness is needed.
QPSK provides 2 bits/symbol at relatively low C/N requirements (approximately 1–3 dB for coded QPSK with modern FEC). It is the fallback modulation in ACM systems, ensuring connectivity is maintained even during deep rain fades or low-elevation passes.
Queueing
The process of buffering packets in a queue before transmission, managed by the modem or hub according to QoS policies. Queueing algorithms such as weighted fair queueing (WFQ) or priority queueing determine the order in which packets from different traffic classes are transmitted on the satellite link.
Queueing behavior directly affects per-class latency and jitter on the satellite link. Misconfigured queueing can cause priority inversion — low-priority bulk traffic blocking high-priority real-time packets — or excessive buffer delay that degrades interactive application performance.
R
Rain Fade
The attenuation of satellite signals caused by absorption and scattering as they pass through rain, snow, or other hydrometeors in the atmosphere. Rain fade increases with frequency — Ka-band signals experience significantly more attenuation per kilometer of rain path than Ku-band or C-band signals.
Rain fade is the dominant availability-limiting factor for Ku-band and Ka-band satellite services in tropical regions. Link budgets must include sufficient fade margin to maintain the link during rain events that statistically occur for the target availability percentage (e.g., 99.5% or 99.9%).
Redundancy (1+1, N+1)
A reliability strategy that deploys additional backup components alongside the primary active components. In a 1+1 configuration, one standby unit protects one active unit with automatic switchover. In N+1 configurations, one spare protects a group of N active units, offering a cost-efficient alternative.
Redundancy is the primary mechanism for achieving high availability in ground segment equipment. Gateway HPA/BUC assemblies, upconverters, modems, and routers are commonly deployed in redundant configurations to ensure that a single equipment failure does not cause a service outage.
Remote Terminal
The complete VSAT station deployed at the end-user site, comprising the antenna, ODU (BUC and LNB), IDU (satellite modem), and associated cabling and mounting hardware. Remote terminals range from small auto-pointing flat-panel antennas to large fixed 2.4 m dishes depending on the service requirements.
The remote terminal is the user-facing element of the satellite network. Its antenna size, BUC power, and modem capabilities define the achievable throughput and link availability at each site. Terminal selection and installation quality are the most common variables affecting end-user service quality.
RFI (Radio Frequency Interference)
Unwanted RF emissions from external sources that degrade satellite link performance. RFI can originate from terrestrial wireless systems (e.g., 5G in C-band), adjacent satellite operators, industrial equipment, or faulty VSAT installations that transmit on incorrect frequencies or with excessive power.
RFI is a growing operational concern as terrestrial wireless networks expand into frequency bands adjacent to satellite allocations. Diagnosing and resolving RFI requires spectrum monitoring, geolocation techniques, and coordination with regulatory authorities and adjacent operators.
Routing (Traffic Routing)
The process of directing IP traffic between the satellite network and end-user devices or connected terrestrial networks. In VSAT networks, routing at the hub and terminal involves static routes, dynamic routing protocols (OSPF, BGP), and hub-managed traffic policies that determine which traffic traverses the satellite link.
Correct routing ensures that only intended traffic crosses the satellite link, preventing waste of expensive bandwidth on unnecessary traffic such as broadcast storms or misdirected updates. Routing configuration at the terminal also controls local LAN segmentation and VPN tunnel establishment.