Link budgets

A link budget is an accounting exercise. You start with the power a transmitter radiates, subtract everything the signal loses on the way, add what the receiver recovers, and compare the result against what the demodulator needs. If there is margin left over, the link closes.

The chain

Every term is in decibels, so the accounting is addition rather than multiplication. If that convention is unfamiliar, read Decibels and the link budget first.

\frac{C}{N_0} = \mathrm{EIRP} - L_{\mathrm{fs}} - L_{\mathrm{atm}} + \frac{G}{T} - k

Term	Name	Units	Meaning
$\mathrm{EIRP}$	Effective isotropic radiated power	dBW	Transmit power plus antenna gain, minus feed loss
$L_{\mathrm{fs}}$	Free-space path loss	dB	Spreading of the wavefront over the slant range
$L_{\mathrm{atm}}$	Atmospheric loss	dB	Gases, cloud, rain, and scintillation
$G/T$	Receive figure of merit	dB/K	Receive antenna gain over system noise temperature
$k$	Boltzmann’s constant	dBW/K/Hz	$-228.6$ dBW/K/Hz
$C/N_0$	Carrier to noise density	dBHz	What the demodulator has to work with

Free-space path loss is geometry, not absorption. The energy is not consumed; it is spread over an expanding sphere.

L_{\mathrm{fs}} = 20\log_{10}\left(\frac{4\pi d}{\lambda}\right)

where $d$ is slant range in meters and $\lambda$ is wavelength in meters.

Slant range is not altitude

The single most common error in a first link budget is using orbital altitude as the distance to the ground station. A satellite directly overhead is at its altitude. A satellite near the horizon is much further away.

For a circular orbit at altitude $h$ above Earth radius $R_E$ , seen at elevation angle $\varepsilon$ :

d = \sqrt{(R_E + h)^2 - \left(R_E\cos\varepsilon\right)^2} - R_E\sin\varepsilon

At 550 km altitude, the range at 10 degrees elevation is roughly 2.5 times the range at zenith, which costs about 8 dB. A budget that closes overhead and is never checked at the minimum elevation mask is a budget that fails in service.

Varaha evaluates the link across the pass and quotes the worst case within the elevation mask, not the best.

Availability, not weather

Rain attenuation is not a single number. It is a statistical distribution, so the honest question is not “how much rain loss” but “how much loss is exceeded for no more than $p$ percent of an average year”.

That is why link results are quoted against an availability target. A 99.9 percent target permits roughly 8.8 hours of outage per year; 99.99 percent permits roughly 53 minutes and can cost many more decibels at Ka band.

What the tool computes for you

You supply	Varaha derives
Station coordinates and minimum elevation	Access windows, slant range, and elevation across each pass
Frequency and antenna description	Free-space path loss, antenna gain patterns, polarization terms
Availability target and climate zone	Gaseous, cloud, rain, and scintillation attenuation at that exceedance
Transmit power and system noise temperature	EIRP, G/T, C/N0, and margin against the required threshold

The output quotes margin at the availability target, not at clear sky. A clear-sky margin is a number that describes a day you will not always have.

Common failure modes

Quoting margin at zenith. Always evaluate at the minimum elevation in the mask. This is the difference between a link that works and one that drops on every pass edge.

Ignoring rain-driven noise rise. Rain both attenuates the carrier and warms the receive path, raising system noise temperature. Counting only the attenuation understates the loss, and the error grows with frequency.

Using a datasheet antenna gain at every angle. Peak boresight gain applies on boresight. Off-pointing costs real decibels, and a phased array’s gain rolls off with scan angle.

Next steps

Decibels and the link budget for the arithmetic from first principles.
Accuracy and limitations for what these predictions are and are not good for.

Question? Give us feedbackDocuments Varaha Constellation Designer main (pre-release)