Decode raw payload data

[xerpi]

I have a GMSK demodulator that provides me with raw AIS payload data. For example, from the following message:

!AIVDM,1,1,,A,86vcLMv0<f=fN3oJalUUu<bOJvL7,0*0D

What I get from the demodulator is:

20 6f ab 71 df 80 32 e3 6e 78 3d da a7 49 65 f4 ca 9f 6b e7 07

This payload has the following fields: (Message ID = 8, Source ID (MMSI) = 468376695)

I would like to use the pyais.messages.Payload class (or equivalent) to pass the raw data bytes and be able to decode and get the fields of the message payload.

[xerpi]

I think we would need a constructor for AISSentence that takes a bitarray. In a way, it would be like starting from here: https://github.com/M0r13n/pyais/blob/master/pyais/messages.py#L458

For now, the following seems to work:

msg_id = pyais.util.get_int(decoded_bitarray, 0, 6)
message = pyais.messages.MSG_CLASS[msg_id].from_bitarray(decoded_bitarray)

[M0r13n]

Hey @xerpi,

do I understand correctly: The demodulator provides the binary payload of the AIS message as a sequence of bytes? So no NMEA protocol, no multi part messages, no fragments, no checksum, etc? Also I suppose that there isn't any ASCII6 armour?

In this case your approach seems absolutely fine. The full code would then be:

import bitarray

from pyais.messages import MSG_CLASS
from pyais.util import get_int

payload = b'\x20\x6f\xab\x71\xdf\x80\x32\xe3\x6e\x78\x3d\xda\xa7\x49\x65\xf4\xca\x9f\x6b\xe7\x07'

bit_array = bitarray.bitarray()
bit_array.frombytes(payload)

ais_id: int = get_int(bit_array, 0, 6)
msg = MSG_CLASS[ais_id].from_bitarray(bit_array)

print(msg)

[xerpi]

Hi @M0r13n, thank you very much for your response.

My signal generator, given !AIVDM,1,1,,A,86vcLMv0<f=fN3oJalUUu<bOJvL7,0*0D as an input, produces a GMSK signal. After demodulation, I notice that only 86vcLMv0<f=fN3oJalUUu<bOJvL7, and not the other fields, forms part of the 168-bit long AIS payload ("Data bits - Payload" in the image below, the bits between the preamble + start flag and the CRC + end flag) (more info here).

Is this the standard behavior, or should the other fields (!AIVDM,1,1,,A, and ,0*0D) also be sent?

[M0r13n]

Hi @xerpi,

That behavior is completely normal. Your demodulator yields the raw AIS payload. The additional fields you're referring to are part of an optional NMEA protocol layer that can encapsulate AIS payloads, functioning as a protocol within a protocol. However, this outer NMEA protocol layer is not required for the AIS message to be valid; the AIS payload itself is valid without it. The NMEA protocol is often used to encapsulate AIS data for standardized communication between marine electronic devices, but the core AIS messages themselves are valid and interpretable without this additional layer

[xerpi]

Hi @xerpi,

That behavior is completely normal. Your demodulator yields the raw AIS payload. The additional fields you're referring to are part of an optional NMEA protocol layer that can encapsulate AIS payloads, functioning as a protocol within a protocol. However, this outer NMEA protocol layer is not required for the AIS message to be valid; the AIS payload itself is valid without it. The NMEA protocol is often used to encapsulate AIS data for standardized communication between marine electronic devices, but the core AIS messages themselves are valid and interpretable without this additional layer

Thank you for the detailed explanation! Then, I'll use MSG_CLASS[ais_id].from_bitarray(bit_array) manually! Thanks for this amazing library!