Firmware TX Scheduler

The transmitter does not fire at random. A small scheduler decides when each burst starts, how many times it repeats, and when the radio goes back to sleep.

The beacon cycle

In BEACON mode with TX enabled, the firmware runs a loop:

1. Wake from deep sleep.
2. Build the payload (message + callsign + coordinates).
3. Transmit the burst (repeat count times).
4. Sleep for the configured interval.
5. Repeat.

Timing parameters

Parameter	Default	Effect
Sleep interval	10 s	Gap between bursts
Repeat count	1	Bursts per wake
Tone speed	12 WPM	Length of each burst

Duty cycle math

At 12 WPM, an SOS message with a callsign and coordinates is roughly 25 to 35 seconds of key-down time. With a 10-second sleep, the duty cycle would exceed legal limits for continuous operation. The scheduler therefore enforces a minimum gap between transmissions and warns when the configured values would exceed the band’s duty cycle limit (see Duty Cycle Rule).

Timing integrity

The scheduler uses millis() deltas, never delay(), for the gaps between bursts. This matters because the GPS and OLED work while the radio idles. The deep sleep wake-up uses the ESP32’s real-time clock, which keeps time during sleep.

Emergency behavior

In EMERGENCY mode the schedule changes: the sleep interval is shortened to the minimum the radio allows and the repeat count rises, because the legal limits on normal operation are explicitly suspended for a genuine emergency.

Related pages

• Firmware TX Cycle for what happens inside one burst
• Duty Cycle Rule for the legal constraint
• Power Budget and Runtimes for what the schedule costs in battery