A satellite link budget is a structured way to predict whether a radio link will work—how strong the signal will be when it arrives, how much noise it will compete with, and how much margin you have in real conditions. It’s called a “budget” because you’re accounting for every gain and loss from transmitter to receiver.
This guide walks through a simple, practical link budget from end to end, explains what each term means, and shows how operators use link budgets to choose antennas, power levels, modulation/coding, and availability targets.
Table of contents
- What a Link Budget Is
- What You Need Before You Start
- Step 1: Define the Link and the Units
- Step 2: Calculate EIRP
- Step 3: Add Free-Space Path Loss
- Step 4: Add Atmospheric and Pointing Losses
- Step 5: Add Receiver Gain and G/T
- Step 6: Calculate Carrier-to-Noise (C/N)
- Step 7: Convert to Eb/No or C/N0
- Step 8: Compare to Required Performance
- Step 9: Add Fade Margin and Availability
- Common Mistakes and Practical Tips
- Link Budget FAQ
- Glossary
What a Link Budget Is
A link budget answers a basic question: when the signal gets there, will it be good enough? “Good enough” depends on your modem and service goals: the receiver needs enough signal quality to demodulate and decode with acceptable error rates.
A link budget is used to size antennas, select amplifiers, choose modulation/coding, plan operating margins, and estimate availability in rain or other fading. It’s also how you compare design options without guessing.
What You Need Before You Start
Before doing math, you need a clear definition of the link:
Direction: uplink (ground → space) or downlink (space → ground).
Frequency band: because losses and antenna gains depend on frequency.
Orbit geometry: LEO/MEO/GEO and the slant range at the elevations you’ll operate.
Data rate and waveform: modulation/coding requirements determine the required Eb/No or C/N.
Hardware assumptions: antenna sizes, amplifier power, receiver noise figure or system temperature.
Step 1: Define the Link and the Units
Link budgets are typically done in decibels (dB) because gains and losses add cleanly. Power is often expressed in dBW (decibels relative to 1 watt) or dBm (relative to 1 milliwatt).
The practical trick: in dB space, you add gains and subtract losses. That’s why engineers love it.
Step 2: Calculate EIRP
EIRP (Effective Isotropic Radiated Power) is how strong your transmitter appears in the direction of peak antenna gain.
In plain terms, EIRP combines:
Transmitter output power (after losses) + transmit antenna gain − feed/cable losses.
If you increase amplifier power, improve antenna gain, or reduce line losses, EIRP goes up—making the signal easier to receive.
Step 3: Add Free-Space Path Loss
Free-space path loss (FSPL) is the loss you get simply from the signal spreading out over distance. It increases with:
Distance: farther means more spreading loss.
Frequency: higher frequency means higher FSPL for the same distance (in the standard FSPL form).
For LEO, distance changes a lot across a pass (especially at low elevations). For GEO, distance is relatively stable.
Step 4: Add Atmospheric and Pointing Losses
Real links have extra losses beyond FSPL:
Atmospheric gases: absorption by oxygen and water vapor.
Rain fade: especially important at Ku/Ka, sometimes relevant elsewhere in severe conditions.
Cloud/fog loss: can matter at higher frequencies.
Polarization mismatch: if the transmit and receive polarization aren’t aligned.
Pointing loss: if the antenna is not perfectly aimed (especially at higher bands or narrow beams).
Radome or de-ice loss: if the signal passes through protective materials or ice accumulation.
These losses vary with weather and elevation angle, so they’re often modeled for both “clear sky” and “worst-case” availability targets.
Step 5: Add Receiver Gain and G/T
On the receive side, the key concepts are antenna gain and system noise.
Receive antenna gain is how well the antenna collects energy from the signal direction. Bigger antennas usually mean more gain.
G/T (gain-to-noise-temperature) is a common figure of merit for receive systems. It combines:
G = antenna gain, and T = system noise temperature (how “noisy” the receiver system is).
Higher G/T means better receive performance: either more gain, less noise, or both.
Step 6: Calculate Carrier-to-Noise (C/N)
After you’ve accounted for transmit EIRP and all the losses, you can estimate the received carrier power relative to the noise.
Engineers typically compute either:
C/N0 (carrier-to-noise density, in dB-Hz): useful because it separates the RF link from the data rate.
C/N (carrier-to-noise in a specific bandwidth): useful for a particular channel configuration.
C/N0 is often the most convenient stepping stone, especially when you want to compare performance across different bit rates.
Step 7: Convert to Eb/No or C/N0
Digital modems usually care about Eb/No (energy per bit to noise density). It connects link physics to bit-level performance.
In plain terms:
Eb/No tells you whether you have enough “signal energy per bit” for your modulation and coding to work at an acceptable error rate.
You can relate C/N0 to Eb/No by accounting for your bit rate and any implementation/overhead factors. Higher bit rates generally reduce Eb/No for a fixed C/N0, which is why pushing throughput often requires higher EIRP, better G/T, more bandwidth, or more efficient coding/modulation—usually a combination.
Step 8: Compare to Required Performance
Every waveform has a required Eb/No (or required C/N) to meet a target error rate (BER/FER) after coding. Your modem vendor or waveform spec gives you those thresholds.
The difference between what your link provides and what the waveform requires is your margin.
Positive margin means you should close the link under the assumed conditions.
Negative margin means you need changes: more power, more gain, lower rate, different coding/modulation, or reduced losses.
Step 9: Add Fade Margin and Availability
Clear-sky margin is not the same as operational reliability. If your service needs high availability, you add fade margin to survive rain, pointing errors, temperature drift, and other real-world degradations.
Availability targets force decisions:
For TT&C: conservative margins are common because losing command/telemetry can be mission-threatening.
For payload downlink: you may accept lower availability if you can retry on the next pass.
For broadband: you often need engineered mitigation (ACM, power control, diversity) to maintain user-visible uptime.
Common Mistakes and Practical Tips
Common operator mistakes include:
Mixing units: dBW vs dBm, Hz vs kHz/MHz, or forgetting to convert bandwidth terms.
Ignoring elevation angle effects: LEO passes at low elevation have more atmospheric loss and longer path length.
Skipping implementation losses: real modems have non-ideal losses (phase noise, quantization, nonlinearity).
Assuming clear-sky equals availability: rain fade and operational variability matter—especially at Ku/Ka.
Not modeling pointing loss: narrow beams and fast tracking can cost you dB you didn’t plan for.
A practical approach is to run at least three cases: best, typical, and worst (or a target availability case), then confirm that your operations plan can actually enforce the constraints the budget assumes.
Link Budget FAQ
Is a link budget only for engineers?
You don’t need to be an RF specialist to understand the logic. A link budget is just a structured checklist of what helps the signal and what hurts it. Operators, program managers, and regulators often rely on the results to validate feasibility and risk.
What’s the difference between uplink and downlink budgeting?
The math structure is similar, but the constraints differ. Uplinks are often limited by ground transmit power and regulatory EIRP limits, while downlinks can be limited by spacecraft power, antenna size, and the receiving station’s G/T.
Why does frequency matter so much?
Frequency affects antenna gain, path loss, atmospheric attenuation, and susceptibility to rain. It also changes hardware and operational complexity, which is why band selection and link budgeting go hand in hand.
What’s a “good” link margin?
There isn’t one universal number. It depends on required availability, band, climate, pointing accuracy, and mission criticality. The right margin is the one that meets your service target with acceptable cost and operational complexity.
Glossary
Link budget: An accounting of gains and losses used to predict received signal quality and margin.
EIRP: Effective Isotropic Radiated Power—apparent transmit strength in the direction of maximum antenna gain.
FSPL: Free-space path loss—spreading loss over distance in free space.
G/T: Gain-to-noise-temperature—receive system figure of merit combining antenna gain and system noise temperature.
C/N: Carrier-to-noise ratio in a defined bandwidth.
C/N0: Carrier-to-noise density (dB-Hz), independent of the channel bandwidth.
Eb/No: Energy per bit to noise density—maps RF link quality to digital performance.
Margin: The difference between achieved and required performance (in dB).
Fade margin: Extra headroom reserved to survive weather and other degradations.
ACM: Adaptive Coding and Modulation—changing waveform parameters to maintain service as conditions change.
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