Satellite Comms Link Budget

Friis + receiver G/T + Boltzmann noise, with an ITU-R P.618 simplified rain term (P.838 k/α) and P.676 gaseous absorption. A first-order budget for antenna, power, and frequency trades - run the full ITU-R P.618 method for a licensing-grade availability figure.

This satellite link budget calculator runs the whole accounting sheet in the browser: transmit effective isotropic radiated power (EIRP), free-space path loss (FSPL), rain attenuation from the ITU-R P.618 simplified method with P.838 k/α coefficients, P.676 gaseous absorption, receiver gain-to-noise-temperature (G/T), and Boltzmann noise - down to the carrier-to-noise-density ratio (C/N₀), the achieved energy-per-bit (Eb/N₀), and the link margin. Antennas can be entered as dish diameters or as gain / G/T figures directly, and range as altitude-plus-elevation geometry or a straight slant distance. A first-order budget for antenna, power, and frequency trades - verify survivors with a full ITU-R P.618 availability run and a modem-vendor threshold.

A link budget is the accounting sheet of a radio link: it adds up every gain and subtracts every loss between the transmitter's power amplifier and the receiver's demodulator, and asks whether what arrives is strong enough to carry the data. The bottom line is the link margin - the decibels of headroom the achieved signal has above the minimum the modem needs. A positive margin means the link closes; a negative one means it does not.

The link starts with EIRP - the transmit power (in dBW) plus the transmit-antenna gain, less feed loss. A parabolic dish gain is G = η·(πD/λ)², so gain climbs with the square of both diameter and frequency (a 3 m Ka-band dish is over 50 dBi). From EIRP the signal loses free-space path loss, FSPL = 20·log₁₀(4πR/λ) - the dominant term, ~180 dB for a LEO pass and ~205 dB from GEO - plus atmospheric, rain, pointing, and polarization losses. The receiver's sensitivity is captured by G/T, its antenna gain divided by system noise temperature. Putting it together gives the carrier-to-noise-density ratio C/N₀ = EIRP − losses + G/T − k, where k = −228.6 dBW/K/Hz is Boltzmann's constant. Dividing out the bit rate, Eb/N₀ = C/N₀ − 10·log₁₀(R_b), gives the achieved energy-per-bit, and the margin is that minus the required Eb/N₀ and the modem implementation loss.

The term that makes or breaks a high-band link is rain attenuation. This tool uses the ITU-R P.618 simplified method: the specific attenuation γ_R = k·R^α in dB/km, with the frequency-dependent regression coefficients k and α from ITU-R P.838, is multiplied by the effective slant path through the rain cell (the geometric slant path scaled by a path-reduction factor that accounts for a storm cell being smaller than the beam's footprint). Below about 10 GHz - S, C, and X band - rain fade is a fraction of a decibel and links are effectively rain-immune. Above it - Ku, Ka, Q, V - γ_R climbs steeply, and at Ka (20-30 GHz) a temperate 0.01% rain rate of ~40-50 mm/hr produces a fade of tens of decibels. That is why Ka systems live or die on their rain-availability allocation and lean on adaptive coding and modulation and ground-station site diversity. A smaller gaseous absorption term (oxygen + water vapour, ITU-R P.676, peaking near the 22 GHz water line) is added on top.

What this tool does not do: it is a first-order budget, not a licensing-grade link analysis. The rain and gaseous terms are the ITU-R simplified parametrisations, not a site-specific P.618 availability run against measured P.837 rain statistics; there is no scintillation, cloud, or melting-layer term, no interference/adjacent-satellite budget, no polarization-diversity or XPD treatment, and no specific-MODCOD threshold table - you supply the required Eb/N₀ for your coding. Use the numbers to size antennas, power, and frequency and to see how much rain a link can take, then verify the survivors with a full ITU-R P.618 run and a modem-vendor threshold.