Orbits and passes
What is the direction of an ascending pass for an amateur satellite?
From south to north.
Ascending means moving northward. Descending is the opposite. It matters because Doppler and antenna pointing both depend on which way the satellite is going, and because most tracking software labels passes this way.
What are Keplerian elements?
Parameters that define the orbit of a satellite.
The compact orbital description from the Technician track, which tracking software consumes.
What type of satellite appears to stay in one position in the sky?
Geostationary.
An equatorial orbit at about 35,800 km, where the orbital period matches the Earth’s rotation. Fixed antennas work, and there is no Doppler — the great convenience, at the cost of a much longer path.
Mode designators
What is meant by the “mode” of an amateur radio satellite?
The satellite’s uplink and downlink frequency bands.
What do the letters in a satellite’s mode designator specify?
The uplink and downlink frequency ranges.
Uplink first, downlink second — U/V is 70 cm up, 2 m down, from the Technician track.
What do the terms “L band” and “S band” specify?
The 23- and 13-centimeter bands.
Adding to the letter set: V = 2 m, U = 70 cm, L = 23 cm, S = 13 cm. A mode L/S satellite uplinks at 1.2 GHz and downlinks at 2.4 GHz.
Linear transponders
Which of the following types of signals can be relayed through a linear transponder?
All these choices are correct — FM and CW, SSB and AM, and any modulation conforming to the transponder’s bandwidth.
A linear transponder does not demodulate anything. It takes a slice of spectrum, shifts it, amplifies it, and retransmits — so anything in that slice comes out the other side. Several contacts share one transponder simultaneously, which is why linear satellites are so much more useful than FM ones.
How is an upload signal processed by an inverting linear transponder?
The signal is mixed with a local oscillator signal and the difference product is transmitted
Which of the following is characteristic of an inverting linear transponder?
All these choices are correct — the passband is inverted, LSB uplink becomes USB downlink, and Doppler shift is reduced.
Taking the difference frequency inverts the passband: tune your uplink upward and your downlink moves downward. Three consequences:
- Sideband flips. LSB up becomes USB down.
- Doppler partly cancels. Uplink and downlink Doppler shifts act in opposite directions on an inverting transponder, so the net shift is smaller than either — a genuine operational benefit.
- Tuning feels backwards until you are used to it.
Uplink power discipline
Why should effective radiated power (ERP) be limited to a satellite that uses a linear transponder?
To avoid reducing the downlink power to all other users
The Technician answer, restated with the mechanism. A linear transponder’s total output is fixed and shared among every signal it hears; one loud station takes most of it.
The self-check remains the same: your downlink should be about as strong as the beacon.
Spin modulation
What type of antenna can be used to minimize the effects of spin modulation and Faraday rotation?
A circularly polarized antenna.
Two problems, one solution. Spin modulation is the satellite rotating; Faraday rotation is the ionosphere rotating the polarisation of a signal passing through it, by an amount that varies unpredictably. Both make a linearly polarised antenna fade deeply, and a circularly polarised one is insensitive to the orientation entirely.
That is what the crossed-Yagi arrangement from the antennas module is for.
Store-and-forward
What is the purpose of digital store-and-forward functions on an amateur radio satellite?
To hold digital messages in the satellite for later download.
A satellite in low Earth orbit sees any two stations only briefly, and often not at the same time. Store-and-forward lets it collect messages on one pass and deliver them on another — a mailbox in orbit, and how satellite messaging worked before real-time transponders were common.
Check yourself
- A satellite is mode V/U with an inverting transponder. You transmit LSB on 2 metres — what do you receive on 70 cm, and in which direction does your downlink move as you tune up?
- Why does an inverting transponder reduce net Doppler shift?
- Your downlink is much louder than the beacon. What are you doing to everyone else?
Answers
- USB, and your downlink moves down as you tune up — the passband is inverted.
- The uplink and downlink Doppler shifts act in opposite directions through the inversion, so they partly cancel.
- Taking a disproportionate share of the transponder’s fixed output power, and reducing the downlink power available to every other user. Reduce until you match the beacon.