Petrel User Manual 0.1
7
AIS Transmit Mode
In AIS Transmit Mode, the Petrel will maintain its GPS lock and activate its AIS functionality to
transmit and receive AIS messages. While in this mode, the unit’s Bluetooth functionality is
inactive.
The Petrel will continuously receive AIS messages and interpret them in order to build an internal
AIS slot map. This slot map is used to avoid slot collisions with other AIS devices when the Petrel
chooses to transmit.
The Petrel will wait at least 1 minute before transmitting to ensure it has fully mapped which AIS
slots are in use. Once this process is complete, it will use its slot map and slot selection algorithm
to find two consecutive free slots. Two slots are required because the Petrel transmits AIS
message type 21, which fills two AIS slots. If at any time the unit is not able to find two
consecutive slots, it will wait for one minute and try again.
A Petrel’s AIS transmissions contain
an MMSI and an AIS name. The AIS name is made of up to 20
characters, which can be a combination of uppercase letters (A-Z) or numbers (0-9). The AIS name
can be set by the user. The MMSI is set during the assembly phase and cannot be changed by the
user.
The Petrel AIS slot selection algorithm is designed to prioritize a specific subset of slots based on
the unit’s serial number. This is to avoid potential slot collisions if multiple Petrels are in use in a
localized environment. Each Petrel prioritizes a subset based on its serial number modulus 25. As
an example, Petrels with serial numbers 60 to 84 will all have unique, non-overlapping prioritized
subsets. However, a unit with serial number 85 will prioritize the same subset as serial number
60. If no pair of slots is available in a unit’s prioritized subset, it will expand
its search to the full
set of AIS slots.
A Petrel’s first transmission will always be on AIS channel A. It will then proceed to alternate
between channels B and A for successive transmissions.
The Petrel’s AIS system performs various integrity checks at d
ifferent times to ensure it is
transmitting AIS messages appropriately. If any of these integrity checks fail, the Petrel will
perform a complete reset of its internal AIS system. This may cause the unit’s transmission period
to deviate from the usual 3 minutes, and cause it to perform consecutive channel A
transmissions.
