[dhutererprats]

Closes #370

Summary

Adds an opt-in way for a downlink action to end when the satellite's data buffer becomes empty. This lets a variable-interval environment request another action without waiting for the configured downlink duration to expire.

Enable it with:

act.Downlink(variable_duration_downlink=True)

Existing act.Downlink() configurations keep their current fixed-duration behavior.

Behavior

• If storage is nonempty when downlink begins, the event checks at the FSW cadence and requests retasking when the buffer reaches the configured empty threshold.
• If storage is already empty when downlink begins, early termination is held until at least minimum_duration seconds have elapsed. The default is 10 seconds. If the simulator step is longer, the action waits at least one simulator step.
• empty_storage_threshold_bits defaults to 1 bit to account for small residual values.
• The configured action duration remains a hard maximum. If it is shorter than the empty-buffer minimum, that maximum takes precedence.
• Selecting another action removes the downlink event. An episode reset creates a fresh simulator and clears the action’s saved event state.

Example:

act.Downlink(
    duration=180.0,
    variable_duration_downlink=True,
    empty_storage_threshold_bits=0.5,
    minimum_duration=10.0,
)

Changes

• Extend Downlink with opt-in variable-duration settings and an empty-buffer terminal event.
• Use Basilisk’s callable event API and a shared action cleanup hook.
• Add focused unit and Basilisk integration coverage.
• Document the feature in the development release notes.

Validation

• Full test suite: 1,147 passed, 2 skipped.

[LorenzzoQM]

Looks good!