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schwehr / libais

C++ decoder for Automatic Identification System for tracking ships and decoding maritime information
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How should I deal with UTC time decoded AIS data ? #196

Open DanAlz opened 5 years ago

DanAlz commented 5 years ago

I've used this library to decode AIS data but I don't know how I should work with time. some records have UTC time and some data have just timestamp. these time aren't understandable to me. for example, I don't know ('utc_hour': 17, 'utc_min': 10, 'utc_spare': 0 ) what exactly means. I'll annex a file that you could see. 1.txt

schwehr commented 5 years ago 
{'id': 1, 'repeat_indicator': 0, 'mmsi': 538004431, 'nav_status': 0, 'rot_over_range': False, 
 'rot': 0.0, 'sog': 11.699999809265137, 'position_accuracy': 1,
 'x': 57.63802833333333, 'y': 25.3185, 'cog': 120.5, 'true_heading': 120, 'timestamp': 17,
 'special_manoeuvre': 0, 'spare': 0, 'raim': False, 'sync_state': 0,
 'slot_timeout': 4, 'slot_number': 670}

AIS is a terrible system when it comes for time. Life is better when the metadata wrapping the logged NMEA message contains a complete timestamp. Then you only have to contend with the time delta between when an transmitter created a message, when it was received, and the clock offset of the receiver (e.g. the USCG NAIS was sometimes off by up to 48 hours in its receivers)

For AIS messages 1, 2, and 3: If you only have this decoded in-message content, then you get different time info depending on what slot state is. Sometimes the message will have the hour and minute, sometimes not. It always has second of the minute.

Message 4 always has the year, month.

So if you don't have good metadata, you'll need to write a fancy filter to keep track of time and assign a datetime to each message based on those around it.