Scenario 2 - The Slippery Bird

Session: Friday evening, 16:14 UTC — fourteen minutes after WXSAT set

Station: BKYD-YAGI — crossed yagi on a TV rotator

Operator: You

Riley: Outside, bolting the rotator down, shouting

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Riley’s Note

You earned the fence post. Here’s the rotator.

Different antenna, different band, and — this is the part nobody warns you about — a completely different problem. Last hour you tuned once and sat on your hands for fifteen minutes. Try that tonight and you’ll lose the bird ninety seconds in.

CUBEHOP-1 transmits on 70cm. Same orbital speed, three times the frequency, three times the Doppler shift. It does not fit the channel.

There IS a checkbox on this console that solves it automatically. You are not allowed to touch it tonight. You will learn what it does for you by doing it yourself first — that’s the deal.

— Riley

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Tonight’s Pass

Satellite	AOS	Max Elevation	LOS
CUBEHOP-1	16:18 (T+4 min)	48.2° at 16:24	16:30

A 48° pass — comfortable geometry, strong signal. Nothing about tonight is hard except the frequency, which will not hold still.

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Link Reference

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

The Numbers That Matter

Quantity	Value
Doppler excursion across the pass	±10 kHz
Receiver channel width	30 kHz
Tuning steps available	±1 kHz (fine), ±10 kHz (coarse)

Twenty kilohertz of total travel. The carrier starts about 10 kHz high while the satellite is approaching, slides down through the nominal frequency at closest approach, and finishes about 10 kHz low on the way out.

Caution

The drift is not linear. It is slowest at the very top of the pass — at closest approach the range rate passes through zero, so for a few seconds the frequency barely moves at all — and fastest near the horizon at both ends. Expect to work hardest right after AOS and right before LOS.

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The Rig

Component	Value	Notes
Antenna	Crossed yagi, 12 dBi, ~40° beam	Needs pointing — this one is not a fence post
Polarization	RHCP	Matches CUBEHOP-1
Rotator	TV rotator, ~6°/s	Program-track flies it for you
Parked at	Az 180°, El 5°	Where the last session left it

Note

Set the tracking mode to program-track first, then pick the satellite. Changing the mode clears the selected target — do it the other way round and the rotator sits there doing nothing while your pass goes by.

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The Console’s Offset Readout

The DECODER section shows OFFSET in hertz: how far the carrier sits from the center of your channel. Positive means the carrier is above your VFO — tune up. Negative means tune down.

That readout is exactly what the checkbox you’re not allowed to touch reads. Tonight you are the loop.

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Working the Pass

• Rotator in program-track, target CUBEHOP-1, before AOS
• Signal on the waterfall at 435.250 — expect it high of nominal at first
• Lock the downlink
• Hold that lock for two solid minutes, retuning as the carrier walks
• AFC stays off the entire time

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End of Briefing

Every satellite operator alive has, at some point, sat with one hand on a tuning knob following a signal down a waterfall because there was nothing clever available to do it for them.

It is the least efficient way to work a pass and the single best way to understand what Doppler actually is. Two minutes. Then you get the checkbox — in a couple of sessions.

Bird’s up in four minutes. Go.