Send a GMDSS distress alert

Reinforce IMO GMDSS from radio-operator training. The DSC alert is not sent until the nature and position are set, and voice follows on the distress frequency.

1. Confirm distress: the vessel or a person is in grave and imminent danger.
2. Select the correct DSC distress nature.
3. Confirm the GPS position in the alert.
4. Send the DSC distress alert.
5. Transmit the MAYDAY voice call on the associated distress frequency.
6. Listen for acknowledgment and follow the rescue-coordination schedule.
7. Cancel the alert if it was sent in error, using the taught cancellation.