G/T (pronounced “G over T”) is one of the most important performance metrics for a satellite ground station receiver. It combines two things that determine how well a station can hear weak signals from space: antenna gain (G) and system noise temperature (T). A higher G/T generally means better receive sensitivity, higher link margin, and more reliable demodulation—especially for faint downlinks, higher data rates, and challenging weather conditions.
Table of contents
- What Is G/T?
- Why G/T Matters in Satellite Links
- How G/T Affects C/N0 and Eb/N0
- What Determines G/T?
- How to Improve G/T
- G/T vs EIRP: What’s the Difference?
- G/T in Real Ground Stations: What Changes It
- Measuring and Verifying G/T
- Common Mistakes When Optimizing G/T
- G/T FAQ
- Glossary
What Is G/T?
G/T is the ratio of an antenna’s receive gain (G) to the system’s effective noise temperature (T). It is typically expressed in dB/K. Conceptually:
• Higher gain helps collect more signal energy from the satellite.
• Lower noise temperature reduces the noise added by the antenna environment and receiver electronics.
Because it combines both effects, G/T is a compact way to describe how “sensitive” a receiving station is. Two stations with different antenna sizes and different RF chains can be compared directly by G/T.
Why G/T Matters in Satellite Links
Satellite downlinks arrive extremely weak. A better G/T improves the station’s ability to maintain lock, decode with fewer errors, and sustain higher-order modulation or higher coding rates. In operations, this can translate into:
• More usable contact time during LEO passes (especially at lower elevation angles).
• Higher throughput for payload downlinks when modulation can stay aggressive longer.
• Better resilience to rain fade, scintillation, and interference by increasing receive margin.
When a link is “downlink-limited,” improving G/T is often the most direct lever you have on ground performance.
How G/T Affects C/N0 and Eb/N0
Engineers use C/N0 (carrier-to-noise density) and Eb/N0 (energy per bit to noise density) to predict whether a modem can demodulate and decode a signal at a given data rate.
G/T is a key input to C/N0. All else equal:
Higher G/T → higher C/N0 → higher Eb/N0 → more robust decoding or higher data rates.
That’s why ground network operators treat G/T as a headline spec for receive performance, especially for high-rate downlink services.
What Determines G/T?
G/T is driven by both the antenna and the noise environment:
Antenna gain (G): Depends on dish diameter (or aperture size), frequency, antenna efficiency, surface accuracy, feed illumination, and pointing quality. At a fixed frequency, larger effective aperture generally yields higher gain.
System noise temperature (T): Is the combined noise seen by the receiver, including:
• Antenna noise temperature: sky noise, atmospheric noise, spillover to warm ground, nearby objects, and sun/moon contributions.
• Front-end noise figure: LNA quality and how much loss occurs before amplification (feeds, waveguide, coax, connectors, filters).
• Downstream receiver noise: mixers, converters, and baseband stages (usually less critical if the LNA dominates as it should).
A common rule: loss before the LNA is expensive because it directly degrades system noise temperature.
How to Improve G/T
Improving G/T means increasing gain, lowering noise temperature, or both. Practical levers include:
Increase antenna gain (G)
Use a larger antenna: The most direct way to raise gain, especially for a fixed band.
Improve antenna efficiency: better feed design, correct illumination, reduced blockage, accurate surface, and high-quality radome (if used).
Improve pointing and tracking: mispointing reduces effective gain, particularly at higher frequencies with narrow beams.
Reduce system noise temperature (T)
Use a better LNA: lower noise figure front ends can yield meaningful improvements, especially in higher bands.
Minimize pre-LNA loss: shorten waveguide/coax runs, use low-loss components, keep connectors clean, and place the LNA as close to the feed as possible.
Control spillover and local noise: avoid warm ground pickup, keep reflective/hot objects out of sidelobes, and choose a clean site with good horizon clearance.
Reduce environmental degradation: water ingress, corrosion, and temperature extremes can increase loss and noise over time.
Operate smarter
Use elevation masks: avoid very low elevation angles when they are dominated by atmospheric loss, clutter, and multipath.
Schedule around known noise sources: sun outage windows, local RF emitters, or nearby operations that elevate noise.
Monitor continuously: trending C/N0, noise power, and pointing error helps catch slow drift before it becomes an outage.
G/T vs EIRP: What’s the Difference?
G/T describes how well a station receives. EIRP (Effective Isotropic Radiated Power) describes how strong a station transmits in the direction of the satellite.
In many systems:
• Downlink performance is often improved by higher ground G/T or higher satellite EIRP.
• Uplink performance is often improved by higher ground EIRP or higher satellite G/T.
Keeping these two concepts separate helps diagnose whether you’re limited on receive sensitivity or transmit power.
G/T in Real Ground Stations: What Changes It
Even if the hardware doesn’t change, effective G/T can vary:
Weather: rain and wet snow add attenuation and can raise noise temperature at higher frequencies.
Elevation angle: low angles increase atmospheric noise and spillover effects.
Thermal state: LNAs and RF components can drift with temperature and power conditions.
Contamination: ice, water films, bird droppings, and surface damage reduce gain and efficiency.
Mispointing: small tracking errors can cost real dB at Ku/Ka.
Measuring and Verifying G/T
Operators verify G/T through a mix of methods:
Carrier-based checks: using known beacon or downlink carriers and measuring C/N0 under controlled conditions.
Y-factor methods: comparing noise power under “hot” and “cold” reference conditions to estimate system noise temperature.
Trend monitoring: tracking noise floor, C/N0, and pointing error over time to detect degradation.
The goal is not only to measure once, but to maintain G/T across seasons and over equipment lifetime.
Common Mistakes When Optimizing G/T
• Upgrading the dish but ignoring losses: a bigger antenna can be undermined by long waveguide runs and poor connectors.
• Putting filters before the LNA without accounting for noise penalty: pre-LNA insertion loss directly hurts T.
• Neglecting pointing calibration: especially at higher bands where a small error can cost significant gain.
• Treating G/T as static: seasonal weather, contamination, and drift can turn a “passing” spec into an operational problem over time.
G/T FAQ
What is a “good” G/T?
It depends on frequency band, antenna size, and mission needs. The right benchmark is the G/T required to meet your target C/N0 and throughput at your worst-case elevation angles and weather conditions.
Is increasing antenna size always the best way to improve G/T?
It’s the most direct way to increase gain, but not always the most cost-effective. Reducing pre-LNA loss, improving pointing, or upgrading the LNA can deliver meaningful improvements at lower cost and complexity.
Why does pre-LNA loss matter so much?
Loss before the first low-noise amplification stage reduces the signal and effectively increases system noise temperature. That hurts G/T and is hard to recover later in the chain.
Does G/T affect uplink performance?
Not directly. Uplink performance is more tied to ground EIRP and the satellite’s receive sensitivity. G/T is primarily a receive-side metric.
Glossary
G/T: Gain-to-noise-temperature ratio (dB/K), describing receive sensitivity of an earth station.
Antenna gain (G): The antenna’s ability to concentrate received energy from a direction, typically expressed in dBi.
System noise temperature (T): The effective noise seen at the receiver input, expressed in Kelvin (K).
C/N0: Carrier-to-noise density ratio, a common measure of received signal quality.
Eb/N0: Energy per bit to noise density ratio, used to assess demodulation/decoding performance at a given data rate.
LNA: Low-noise amplifier, the first receive-stage amplifier designed to add minimal noise.
Insertion loss: Signal loss introduced by a component (cable, filter, connector), typically in dB.
EIRP: Effective Isotropic Radiated Power, a measure of transmit strength in the direction of peak antenna gain.
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