Scenario 4 - Set and Forget

Session: Sunday, 16:34 UTC

Station: BKYD-YAGI — crossed yagi, feed harness now correct

Operator: You

Riley: Here, and about to hand over the checkbox

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Riley’s Note

Eighty-three degrees tonight. Nearly straight overhead — the strongest pass this rig has ever seen, and also the nastiest Doppler, because the geometry changes fastest when the bird is closest.

Here’s the deal I made with you two sessions ago. Fly the first half by hand, like you did on Friday. Then, once I’ve seen you do it, the tape comes off the AFC checkbox and you can spend the back half of the pass watching the loop do your job better than you do.

Then I want you to tell me when it would fail you. Because it will.

— Riley

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Tonight’s Pass

Satellite	AOS	Max Elevation	LOS
CUBEHOP-1	16:38	83.1° at 16:45	16:51

Note

Closest approach is the natural moment to switch modes: the range rate passes through zero there, so the carrier briefly stops drifting. Hand over to the loop while the frequency is standing still and the transition is clean.

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Link Reference

Signal	Frequency	Bandwidth	Mode
CUBEHOP-1 FM downlink	435.250 MHz	15 kHz	QPSK 1/2

Doppler excursion is the same ±10 kHz you fought on Friday — but it arrives faster on a high pass, because the satellite sweeps through its closest approach in less time.

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What AFC Actually Does

Automatic Frequency Control is a loop, and it is not clever. Every update it reads exactly one number — the carrier’s offset from the center of your channel, the same OFFSET readout you were using by hand — and slews the VFO to re-center whatever it is currently locked to.

Property	Value
Input	Measured carrier offset (Hz)
Slew rate limit	200 Hz per update
Deadband	±10 Hz (inside this it holds still)
Requirement	There must be a carrier to measure

Caution

That last row is the whole limitation. AFC re-centers a signal it already has. It cannot find one, cannot predict one, and cannot recover from a deep fade — if the carrier leaves the passband, the loop has nothing to measure and simply stops. A human operator knows where the signal will be in ten seconds. The loop only knows where it was.

It will also happily follow the wrong signal. If a stronger carrier wanders into your channel, AFC re-centers on that instead, with no opinion about whether it’s the thing you wanted.

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Tonight’s Sequence

• Program-track on CUBEHOP-1 before AOS
• AFC off. Lock the downlink and hold it 90 seconds by hand
• At the top of the pass, enable AFC
• Hands off the tuning: hold 120 seconds at 10 dB C/N or better
• Be ready to say when you’d turn it back off

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End of Briefing

Automation on a ground station is not a reward for good behavior — it’s a tool with a specific failure mode, and the only operators who can be trusted with it are the ones who can work without it.

You’ve done that twice now. Take the checkbox.

Just don’t forget it’s a loop, not a pilot.