Encoding large (Type 5) messages causes NMEA max message length to exceed 82 characters.

[bgercken]

Was running into an issue in encode.py ais_to_nmea_0183 where it was causing messages to exceed the NMEA max size.

On line 54 (in encode.py) I changed max_len to 60, which seems to have resolved it.

Thanks.

[M0r13n]

@bgercken Thank you for bringing this up. I will look at this but as soon as time allows. But it might take a couple of days.

[bgercken]

No hurry on my part. Thank you very much.

[M0r13n]

@bgercken

Could you provide me some example that led to this issue? I am not able to reproduce the described behavior.

[bgercken]

Below is output for encoding the following data.

data: {'type': 5, 'repeat': 0, 'mmsi': '538008122', 'ais_version': 1, 'imo': 9500091, 'callsign': 'V7GS3', 'shipname': 'CLIPPER BAROLO', 'shiptype': 70, 'to_bow': 166, 'to_stern': 30, 'to_port': 22, 'to_starboard': 10, 'epfd': 1, 'month': 5, 'day': 16, 'hour': 0, 'minute': 30, 'draught': 6.6, 'destination': 'BALTIMORE', 'dte': 0}

sentence: !AIVDO,2,1,0,A,5815J>T2@uKeKLM?<00<hU10E:0858tht0000000DhNF:5H0N@PPC52CClQ@0,0*28 (81)

sentence: !AIVDO,2,2,0,A,0000000000,2*16 (30)

UPDATE: actually 81 is too long. When you append the NMEA terminator it exceeds the max sentence length.

[bgercken]

I had closed this but it still appears to be an issue.

Updated test fails:

from pyais import encode_dict

data = {
    'type': 5, 'repeat': 0, 'mmsi': '259725000', 'ais_version': 1, 'imo': 9103128, 'callsign': 'LAXP4',
    'shipname': 'STAR HANSA', 'shiptype': 70, 'to_bow': 177, 'to_stern': 22, 'to_port': 17, 'to_starboard': 14,
    'epfd': 1, 'month': 5, 'day': 20, 'hour': 6, 'minute': 0, 'draught': 7.4, 'destination': 'US BAL', 'dte': 0
}


# Maximum sentence length, including the $ and <CR><LF> is 82 bytes.
NMEA_MAX_LEN = 82
NMEA_TERMINATOR = "\r\n"


def test_encode_type5():
    encoded = encode_dict(data)
    for i in range(len(encoded)):
        encoded[i] += NMEA_TERMINATOR
        assert len(encoded[i]) <= NMEA_MAX_LEN


if __name__ == "__main__":
    test_encode_type5()

[M0r13n]

Thank you. I am pretty sure that I forgot to include the final <LF> terminator when calculating the total length of a message. I will fix this tomorrow.

[M0r13n]

@bgercken

You were right. The max payload length should be set to 60 and not 61. The docstring of the relevant method actually mentions this:

This method takes care of splitting large payloads (larger than 60 characters)
into multiple sentences. With a total of 80 maximum chars excluding end of line
per sentence, and 20 chars head + tail in the nmea 0183 carrier protocol, 60
chars remain for the actual payload.

But then the implementation sets max_len to 61 (see).

[M0r13n]

Should be fixed with version 2.2.1