Pass scheduler

A satellite is not always overhead. The pass scheduler predicts when one will be, and moves the bridge between four states so that power and airtime are spent when a message can actually leave.

The four modes

Mode	When	What it means
Idle	no pass within the pre-wake window	nothing to do, stay cheap
Pre-wake	a pass starts soon	warm up the modem so it is ready at acquisition
Active	between acquisition and loss of signal	send now
Post-pass	a grace period after loss of signal	finish what was in flight

Pre-wake exists because a cold modem takes time to acquire. Waking at the moment the satellite arrives means missing the start of a window that may only be a few minutes long.

Post-pass exists because loss of signal is predicted, not observed. The grace period covers a pass that runs slightly longer than the prediction rather than cutting a send off mid-message.

Where the predictions come from

From TLE orbital elements, through a predictor interface the scheduler depends on rather than implements. That indirection is deliberate: it breaks a package cycle, and it means the scheduler can be tested against fixed predictions instead of against the sky.

What this is not

This is scheduling, not queueing. The scheduler decides when the window is; the dead letter queue decides what goes into it and in what order.