Satellite & Terrestrial Link Budget
Compute probabilistic link margins using ITU-R propagation models (P.618 rain, P.676 gaseous, P.840 cloud). Enter EIRP, G/T, frequency, and site location. Get a full line-by-line budget, availability curve, and Monte Carlo confidence intervals over rain, pointing, and EIRP/G·T uncertainties.
How It Works
A link budget calculates whether received signal power is sufficient for reliable communications. The fundamental equation is:
C/N₀ = EIRP + G/T − FSPL − A_rain − A_gas − A_cloud − 10 log₁₀(k)
where FSPL = 20 log₁₀(4πdf/c) and k = 1.38 × 10⁻²³ J/K (Boltzmann constant).
Rain attenuation (ITU-R P.618) dominates at Ku-band (12 GHz) and above. The model computes specific attenuation γ_R = k × R^α from the rain rate at 0.01% availability for the site's climate zone, then integrates over the slant path geometry through the rain layer.
Gaseous absorption (ITU-R P.676) from oxygen and water vapour is significant above 20 GHz and at low elevation angles.
The Monte Carlo sweeps EIRP (±0.3 dB), G/T (±0.3 dB), pointing loss (exponential mean 0.2 dB), rain rate (log-normal σ = 0.5), and scintillation (normal σ = 0.4 dB) to produce margin confidence intervals at the target availability.
Migrating from STK Cloud?
Ansys/AGI are sunsetting STK Cloud in March 2026. This tool covers the link-budget piece of STK Cloud (ITU-R propagation + Monte Carlo confidence bands). For orbit propagation (SGP4/TLE, pass schedules, Doppler) pair this with Skyfield or our satellite pass predictor. For programmatic access to the same ITU-R math, see ITU-Rpy.
Methodology & References
References
- ITU-R P.618-13 — Propagation data and prediction methods required for the design of Earth-space telecommunication systems link
- ITU-R P.676-13 — Attenuation by atmospheric gases link
- ITU-R P.840-8 — Attenuation due to clouds and fog link
- ITU-R P.838-3 — Specific attenuation model for rain for use in prediction methods link
- ITU-R P.530-18 — Propagation data and prediction methods required for the design of terrestrial line-of-sight systems link
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Formulas implemented directly from the ITU-R recommendations above. Test vectors from ITU-R P.618 Annex 2 examples are used in the backend test suite.
Related Calculators
Read the Guide
Satellite Link Budget: Rain, Absorption & Availability
How to use the Satellite Link Budget Analyzer to compute a complete Ka-band downlink budget using ITU-R P.618 rain attenuation, P.676 gaseous absorption, and.
Migrating from STK Cloud: Free Alternatives for Link Budget and Orbit Analysis
Ansys is sunsetting STK Cloud in March 2026. Here are the free open-source replacements for the two things it did best — ITU-R link budgets and orbital pass prediction.
Sizing a 9600-baud UHF Downlink for a 3U CubeSat: Full Walkthrough
End-to-end link budget for an amateur-band 3U cubesat: EIRP, ground-station G/T, ITU-R propagation losses, and Monte Carlo availability. Uses the Amateur CubeSat preset.
FAQ
What is link availability and why does 99.5% matter?+
99.5% availability means the link is operational for 99.5% of the year — it experiences outages for 0.5% of time, which is 43.8 hours/year. This is typically quoted as an annual statistic and is dominated by rain fade events. The remaining 0.05% (4.4 hours/year) represents exceptional storm events. Broadcast satellite services typically target 99.5–99.7%. Critical infrastructure links target 99.99% or better.
Why does lowering the elevation angle increase rain attenuation?+
The satellite signal passes through more of the atmosphere at shallow elevation angles. The slant path length through the rain layer scales as 1/sin(elevation). At 5° elevation, the path is 11× longer than at 90°. This dramatically increases the rain attenuation. For low-elevation or terrestrial links, rain attenuation is the binding constraint for availability.
What do the Monte Carlo confidence bands mean?+
The p5/p50/p95 bands represent the distribution of link margin outcomes when all uncertain parameters are drawn from their statistical distributions. The p5 margin is exceeded by 95% of operating scenarios — it is the conservative design point for worst-case margin allocation. If p5 margin is negative, 5% of operating scenarios will be outages in addition to the rain fade outage budget.