Satellite & Terrestrial Link Budget
Compute probabilistic link margins using ITU-R propagation models (P.618 rain, P.676 gaseous, P.840 cloud) with climate data read from the ITU-R maps at your site. Enter EIRP, G/T, frequency, and site latitude and longitude. 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).
Climate at the site. The rain rate exceeded 0.01% of the time (P.837-7), the rain height (P.839-4), the cloud liquid water (P.840-8), the water vapour (P.836-6), the surface temperature (P.1510-1) and the station height (P.1511-1) are read from the ITU-R digital maps at the latitude and longitude you enter. Every value used is listed with the result.
Rain attenuation (ITU-R P.618-13) dominates at Ku-band (12 GHz) and above. The specific attenuation γ_R = k × R^α uses the P.838-3 coefficients for your polarisation, over the slant path through the rain layer with the P.618 horizontal and vertical reduction factors, then scaled from 0.01% to your availability target.
Gaseous absorption (ITU-R P.676-12 Annex 2) from oxygen and water vapour is significant above 20 GHz and at low elevation angles. Cloud attenuation follows P.840-8. The losses are combined as P.618-13 section 2.5 specifies: below 1% exceedance, gas and cloud are taken at 1%. Scintillation is not included — P.618 needs the antenna diameter and efficiency for it.
A terrestrial link uses the P.530-17 path-reduction method over the path length, with ground-level gas absorption times the length. Multipath fading is not modelled, and the result says so.
The Monte Carlo is an uncertainty model around the ITU-R nominal budget, not an ITU-R method: it varies EIRP (σ 0.3 dB), G/T (σ 0.3 dB), pointing loss (exponential, mean 0.2 dB), the rain rate (log-normal σ = 0.5) and a fading term (|normal|, σ 0.4 dB) to give 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.676-12 — Attenuation by atmospheric gases (Annex 2, slant paths) 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.530-17 — Propagation data and prediction methods required for the design of terrestrial line-of-sight systems link
- ITU-R P.837-7, P.839-4, P.836-6, P.1510-1, P.1511-1 — Rain rate, rain height, water vapour, surface temperature and topography digital maps link
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Ported from ITU-Rpy 0.4.0 and checked against it at 12 sites worldwide: slant-path rain, cloud and gas attenuation, P.530 terrestrial rain, the climate map values and the P.838 coefficients all agree within 0.001%.
Related Calculators
Read the Guide
Satellite Link Budget: Rain, Absorption & Availability
A Ka-band direct-to-home downlink in London through the Satellite Link Budget Analyzer: ITU-R climate at the site, P.618 rain, gas and cloud, and availability.
Migrating from STK Cloud: Free Alternatives for Link Budget and Orbit Analysis
Ansys is sunsetting STK Cloud in March 2026. Here are free open-source replacements for what 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, slant range, ground-station G/T, propagation losses and Monte Carlo margin.
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.
Why does the tool need my longitude?+
Rain rate, rain height, cloud water and water vapour are read from the ITU-R climate maps, and those vary with longitude as much as with latitude — New York and Madrid are both near 40.5° N with very different rain climates. A request without a longitude is refused rather than answered for a guessed location.