E2A

Amateur satellites and orbits

Explain ascending passes, orbit types, inverting transponders, mode letters, circular polarization and store-and-forward.

13 pool questions1 on the exam

Syllabus: Amateur radio in space: amateur satellites; orbital mechanics; frequencies and modes; satellite hardware; satellite operations

The orbit sets the lap time and the pass direction

Keplerian elements are the parameters that define an orbit (size, shape, tilt, timing). Feed them to tracking software and it predicts every pass.

Ascending = heading north. Descending = heading south. East and west do not matter.

A pass heading from south to north is ascending. Bigger orbit, longer lap. At one special height a lap takes exactly one Earth turn, so a geostationary satellite (over the equator) hangs over one spot. A LEO satellite sweeps past in minutes.

Animated 60 min per second, zoomed to fit. The dot on Earth is you.

Altitude550 km

RememberAscending = south to north. Geostationary = fixed in the sky. LEO, HEO and the rest all move.

A linear transponder is a flipping mirror for a whole band

A linear transponder amplifies a whole passband without demodulating it, so any mode (SSB, CW, SSTV, PSK, packet, FM) passes through. An inverting one mixes the uplink with a local oscillator and sends the difference: the band is mirrored, so USB becomes LSB. Doppler on the uplink and downlink then push opposite ways, which reduces the net shift.

Downlink = oscillator minus uplink. The whole band is flipped.

Your uplink frequencylow in band

The downlink power is shared. A strong uplink grabs more of it and reduces downlink power for everyone else, so keep your ERP low.

RememberInverting: band flips, sidebands swap, difference product is sent. Keep power low.

A mode name is two band letters: up, then down

The mode of a satellite is its uplink and downlink frequency bands. Each letter is a range. L band is 23 cm and S band is 13 cm.

A mode name is two band letters: uplink first, downlink second.

RememberMode = uplink band / downlink band. L = 23 cm, S = 13 cm.

Circular polarization shrugs off twisting waves

A spinning satellite (spin modulation) and the ionosphere (Faraday rotation) twist the wave's polarization. A linear antenna fades as the twist passes through 90°. A circularly polarized antenna does not care.

Spin or Faraday rotation twists the wave. Circular polarization ignores the twist.

RememberTwisting signal? Use circular polarization.

Store-and-forward is a mailbox in orbit

Digital satellites hold messages on board for later download. You upload on one pass; the satellite carries it and delivers on a later pass. The technique is called store-and-forward.

The satellite is a mailbox in the sky: hold now, deliver later.

RememberStore-and-forward = hold a message, deliver it later.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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