Satellite Downlink Budget Margin Calculator

Calculate the deterministic link margin for a satellite downlink by combining EIRP, aggregate path losses, receive G/T, throughput, and any implementation penalties.

Satellite communications planning aid—validate against full link budget software before committing to service-level agreements.

Examples

  • EIRP 52 dBW, losses 198 dB, G/T 2 dB/K, data rate 50 Mbps, required Eb/N0 7 dB, implementation loss 1 dB ⇒ Available C/N0: 84.60 dB-Hz. Available Eb/N0: 7.61 dB. Required Eb/N0 plus implementation penalties: 8.00 dB. Resulting downlink margin: -0.39 dB.
  • EIRP 48 dBW, losses 194 dB, G/T 0 dB/K, data rate 20 Mbps, required Eb/N0 5 dB, implementation loss 0.5 dB ⇒ Available C/N0: 82.60 dB-Hz. Available Eb/N0: 9.59 dB. Required Eb/N0 plus implementation penalties: 5.50 dB. Resulting downlink margin: 4.09 dB.

FAQ

What if my modem specification quotes required C/N rather than Eb/N0?

Convert the requirement to Eb/N0 by adding 10·log10(data rate / symbol rate) or directly subtract 10·log10(noise bandwidth) from the quoted C/N. Enter the equivalent Eb/N0 so the calculator can compare like terms.

How should I treat adaptive coding and modulation schemes?

Run the calculation per modulation and coding pair, using the required Eb/N0 for each profile. The link margin reveals at which rain loss or pointing offset each profile drops below zero, informing fade mitigation plans.

Can I include uplink interference or intermodulation products?

Yes. Add those degradations to the implementation loss input. If they vary with traffic load, model a worst-case penalty or perform a sensitivity sweep to bracket the expected margin.

Why is Boltzmann’s constant represented as -228.6 dB?

In logarithmic form Boltzmann’s constant equals -228.6 dBW/K/Hz. Subtracting this negative quantity adds 228.6 dB to the carrier-to-noise-density expression, simplifying link budget arithmetic.

Additional Information

  • Result unit: decibels of link margin after subtracting required Eb/N0 and implementation penalties.
  • The calculation assumes noise bandwidth equals the data rate; adjust the input when using excess bandwidth factors.
  • G/T must be in dB/K and losses in positive dB to maintain consistent sign conventions.

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