Amateur Satellites

Why this is a Technician topic

Satellite work sits entirely in the VHF and UHF bands, where a Technician holds full privileges. A handheld with a small directional antenna genuinely works satellites — this is one of the few places where the entry-level licence gets the whole experience with no restriction at all.

Orbits

What does the term LEO mean in reference to communication satellites?

Low Earth Orbit, which has a period of around 100 minutes.

Roughly a hundred minutes per orbit means a pass over your location lasts about ten to fifteen minutes, and the satellite is moving fast enough that everything below follows from it: you must track it, and its frequency shifts while you do.

Mode designations

Satellites name their bands with a two-letter code: uplink first, downlink second.

What does it mean if a satellite is operating in U/V mode?

The satellite uplink is in the 70-centimeter band and the downlink is in the 2-meter band.

The letters: U is 70 cm (UHF), V is 2 m (VHF), L is 23 cm, S is 13 cm. So U/V means transmit on 70 cm, listen on 2 m; V/U is the reverse.

Uplink and downlink are on different bands so that the satellite can receive while transmitting — the same reason a repeater uses an offset, taken further.

Amateur satellites use all the common transmission modes — the pool answers “all these choices are correct” for FM, SSB, and CW.

Doppler shift

What is Doppler shift in reference to satellite communications?

An observed change in signal frequency caused by relative motion between the satellite and Earth station

A satellite approaching you compresses the wave — the received frequency reads high. Departing, it reads low. The shift passes through zero at closest approach.

The magnitude scales with frequency: a few kilohertz on 70 cm, a fraction of that on 2 m. In practice you continuously retune during a pass, or let software do it. This is why an SSB satellite contact requires an active hand on the dial, and why FM satellites are more forgiving — an FM receiver tolerates a few kHz of offset.

Spin fading

What causes spin fading of satellite signals?

Rotation of the satellite and its antennas.

Many satellites spin for stabilisation, which sweeps their antenna pattern and polarisation past you. The result is a periodic rise and fall of signal, at the spin rate. It is a characteristic of the satellite, not a fault in your station.

Beacons and telemetry

What is a satellite beacon?

A transmission from a satellite that contains status information.

What telemetry information is typically transmitted by satellite beacons?

Health and status of the satellite — battery voltage, temperatures, mode.

And a rule worth knowing because it is unusual:

Who is permitted to receive telemetry from an amateur radio satellite?

Anyone.

No licence needed to receive it, and no restriction on decoding or sharing it. Amateur satellite telemetry is deliberately open, and networks of unlicensed listeners routinely contribute to satellite health monitoring.

Setting uplink power

This is the etiquette question that matters most, and the pool asks it twice.

What is the impact of using excessive effective radiated power on a satellite uplink?

Blocking access by other users.

A linear transponder shares its limited output power among every signal it hears. One station running far too much power takes most of that output and everyone else drops out. It is not merely rude; it denies the satellite to everybody in the footprint.

The self-check is elegant:

Which of the following is a way to determine whether your satellite uplink power into a linear transponder satellite is neither too low nor too high?

Your signal strength on the downlink should be about the same as the beacon

Listen to your own downlink and compare it with the satellite’s beacon. Louder than the beacon means back off. Much quieter means you may add a little. It costs nothing and it is entirely self-administered.

Tracking

Which of the following are inputs to a satellite tracking program?

The Keplerian elements.

A compact set of orbital parameters, published and updated regularly, from which a program computes where the satellite is at any moment.

Which of the following are provided by satellite tracking programs?

All these choices are correct — the time, azimuth, and elevation of the satellite’s rise and set; the maximum elevation of a pass; and the Doppler shift during a pass.

Azimuth and elevation tell you where to point. Rise and set times tell you when to be there. Maximum elevation tells you whether a pass is worth the effort — a pass topping out at ten degrees is scraping the horizon.

Check yourself

1. A satellite is in V/U mode. Which band do you transmit on?
2. Your downlink signal is noticeably stronger than the satellite’s beacon. What should you do?
3. Your received signal rises and falls smoothly a few times during a pass. Fault or feature?