Satellite link parameters calculator

How much to turn the VFO

Through a satellite in low orbit the signal arrives frequency-shifted: at the start of a pass the spacecraft is coming towards you, at the end going away. On two metres the shift is still tolerable; on seventy centimetres it is such that working a linear transponder without automatic correction is practically impossible.

At AOS and LOS the radial velocity is at its greatest; at the zenith it is zero

Orbit	Enter the altitude by handTypical LEO, 500 kmISS ≈ 420 km / cubesats ≈ 600 kmSun-synchronous, 800 kmMedium orbit, 1400 kmGeostationary, 35,786 km
Orbit altitude (km)
Elevation (°)
Frequency (MHz)	Enter by hand2 m — 145.9 MHz70 cm — 435.3 MHz23 cm — 1268 MHz13 cm — 2400 MHz (QO-100 uplink)3 cm — 10,489 MHz (QO-100 downlink)
Frequency, manual (MHz)
Radial velocity (km/s)
Orbit
Orbital velocity
Orbital period
Slant range at the given angle
Range to the horizon
Radius of the footprint on the ground
One-way signal delay

📐 The formulas

Orbit:

v = √( μ / (R + h) )                μ = 398,600.4418 km³/s²
T = 2π · √( (R + h)³ / μ )

Slant range at an elevation E:

d = R · [ √( ((R+h)/R)² − cos²E ) − sin E ]

Footprint:

β = arccos( R / (R + h) )
footprint radius = R · β

Doppler:

Δf = f · v_radial / c

A check point (500 km altitude):

v = 7.617 km/s, T = 94.5 min, range at the horizon 2573 km, footprint radius 2446 km. At a radial velocity of 7.2 km/s the Doppler shift is ±3.50 kHz on 145.9 MHz and ±10.45 kHz on 435.3 MHz.

Where this calculator stops:

• The orbit is taken as circular: for elliptical ones (Molniya, some amateur satellites) velocity and range vary round the orbit
• Radial velocity is entered by hand — an exact figure comes only from a TLE calculation for the moment in question
• For a geostationary satellite the Doppler is essentially zero: set the velocity field to zero
• The rotation of the Earth is not accounted for, and it adds a few percent to the radial velocity

🎯 In practice

• Preparing for a pass: how much frequency correction to be ready for.
• Estimating the length of the window from the footprint radius.
• A link budget to a satellite: the slant range is what goes into the FSPL.

Practical advice:

1. On 2 metres Doppler can be handled by hand; on 70 cm it cannot.
2. With an inverting transponder the uplink and downlink shift in opposite directions.
3. Correct the uplink, not the downlink: that way your signal stays put for everyone else.
4. At the zenith the radial velocity is zero — that is where the frequency passes through its nominal value.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

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UR3PKI. «Satellite link parameters calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/satellite (licence CC BY-NC-SA 4.0).

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