The satellite that went quiet for 41 days

At 3:12 a.m. on May 21, the green streak disappeared from the blue waterfall on my laptop. That was Yeoul-1's last signal. Or so we believed for 41 days.

A satellite the size of a milk carton

Yeoul-1 is a 1U cubesat: 10 cm on each side, 1.18 kg. Eleven of us built it over three years, and on May 12, 2026 it went up into a 520 km sun-synchronous orbit. It had one job: circle the Earth and send our school's name down at 9,600 bps.

Day 9: the signal stopped

The first eight days went well. On each of the 4-6 daily passes over Seoul, the rooftop antenna picked up the beacon. But the battery voltage was slipping a little every day.

On the early pass on day 9 the signal cut out after three minutes. Nothing on the next pass, or the next day. The nights on the roof staring at a laptop began.

What we heard for 41 days

We didn't give up. Over 41 days we recorded every one of the 203 passes over Seoul, and emailed 51 amateur radio stations in 23 countries asking them to tell us if they heard anything.

One email from Chile

On the morning of day 42, an email came from Marta Vidal, a radio amateur in Punta Arenas, Chile. The attached waterfall showed a thin, slanted green streak. The subject line: "Is this yours? It is beeping."

"The beeps were too regular to be a person. So I looked up the orbit tables." - Marta Vidal

That night we re-ran every recording from the past 41 days as Morse. The decoder was forty lines of Python.

# Yeoul-1 safe-mode beacon: decoding 0.1 W Morseimport numpy as npdef cw_envelope(iq, rate, tone_hz, win=0.02):    t = np.arange(len(iq)) / rate    mixed = iq * np.exp(-2j * np.pi * tone_hz * t)    # pull the drifting tone down to 0 Hz    n = int(win * rate)    return np.convolve(np.abs(mixed), np.ones(n) / n, "same")def to_morse(env, rate, wpm=12):    dot = 1.2 / wpm                                   # at 12 WPM one dot is 0.1 s    on = env > np.percentile(env, 80) * 0.5    edges = np.flatnonzero(np.diff(on.astype(int))) + 1    runs = np.diff(np.r_[0, edges, len(on)]) / rate    return "".join("." if r < 2 * dot else "-" for r, o in zip(runs, on[np.r_[0, edges]]) if o)

It was rebooting 45 times a day

The first decoded line read "YEOUL1 SAFE BAT 3.41 RST 1847": battery at 3.41 V and 1,847 restarts. Over 41 days that is about 45 a day, roughly once an orbit.

Getting one command up

The heater was the cause, and the fix was simple: one command to switch it off in shadow. The catch was that in safe mode the satellite only listens for commands for 40 seconds per pass.

If we built it again

What we heard for 41 days wasn't noise. The satellite was talking the whole time; we were just listening in the wrong language. Next time we'll keep one more ear open from the start.