NMEA (GPS) and AIS Parser / Decoder for Ruby (nmea_plus)

NMEA Plus is a Ruby gem for parsing and decoding "GPS" messages: NMEA, AIS, and any other similar formats of short messaging typically used by marine equipment. It provides convenient access (by name) to the fields of each message type, and a stream reader designed for use with Ruby Blocks.

Install

gem install nmea_plus

Example

In its most basic use, you can decode messages as follows:

require 'nmea_plus'

decoder = NMEAPlus::Decoder.new
message = decoder.parse("$GPGLL,4916.45,N,12311.12,W,225444,A*00")
# message data -- specific to this message type
puts message.latitude     # prints 49.27416666666666666666
puts message.longitude    # prints -123.18533333333333333
puts message.fix_time     # prints <today's date> 22:54:44 <your local time zone offset>
puts message.valid?       # prints true
puts message.faa_mode     # prints nil

# metadata
puts message.checksum_ok? # prints false -- because this checksum is made up
puts message.original     # prints "$GPGLL,4916.45,N,12311.12,W,225444,A*00"
puts message.data_type    # prints "GPGLL" -- what was specified in the message
puts message.interpreted_data_type  # prints "GLL" -- the actual container used

# metadata that applies to multipart messages (also works for single messages)
puts message.all_messages_received? # prints true
puts message.all_checksums_ok?      # prints false -- checksum is still made up

# safer way to do what we did above
if "GPGLL" == message.data_type     # Alternately, if "GLL" == message.interpreted_data_type
  puts message.latitude     # prints 49.27416666666666666666
  puts message.longitude    # prints -123.18533333333333333
  puts message.fix_time     # prints <today's date> 22:54:44 <your local time zone offset>
  puts message.valid?       # prints true
  puts message.faa_mode     # prints nil
end

Of course, decoding in practice is more complex than that. Some messages can have multiple parts, and AIS messages have their own complicated payload. NMEAPlus provides a SourceDecoder class that operates on IO objects (anything with each_line support) -- a File, SerialPort, etc. You can iterate over each message (literally each_message), or receive only fully assembled multipart messages by iterating over each_complete_message.

require 'nmea_plus'

input1 = "!AIVDM,2,1,0,A,58wt8Ui`g??r21`7S=:22058<v05Htp000000015>8OA;0sk,0*7B"
input2 = "!AIVDM,2,2,0,A,eQ8823mDm3kP00000000000,2*5D"
io_source = StringIO.new("#{input1}\n#{input2}")  # source decoder works on any IO object

source_decoder = NMEAPlus::SourceDecoder.new(io_source)
source_decoder.each_complete_message do |message|
  puts message.all_checksums_ok?                  # prints true -- the full message set has good checksums
  puts message.all_messages_received?             # prints true -- taken care of by each_complete_message
  if "AIVDM" == message.data_type
    puts message.ais.message_type                   # prints 5
    puts message.ais.repeat_indicator               # prints 0
    puts message.ais.source_mmsi                    # prints 603916439
    puts message.ais.ais_version                    # prints 0
    puts message.ais.imo_number                     # prints 439303422
    puts message.ais.callsign.strip                 # prints "ZA83R"
    puts message.ais.name.strip                     # prints "ARCO AVON"
    puts message.ais.ship_cargo_type                # prints 69
    puts message.ais.ship_cargo_type_description    # prints "Passenger, No additional information"
    puts message.ais.ship_dimension_to_bow          # prints 113
    puts message.ais.ship_dimension_to_stern        # prints 31
    puts message.ais.ship_dimension_to_port         # prints 17
    puts message.ais.ship_dimension_to_starboard    # prints 11
    puts message.ais.epfd_type                      # prints 0
    puts message.ais.eta                            # prints <this year>-03-23 19:45:00 <your local time zone offset>
    puts message.ais.static_draught                 # prints 13.2
    puts message.ais.destination.strip              # prints "HOUSTON"
    puts message.ais.dte?                           # prints false
  end
end

NMEA (GPS) Parsing

This gem was coded to accept the standard NMEA messages defined in the official and unoffical specs found here:

• http://www.catb.org/gpsd/NMEA.txt
• http://www.nmea.org/Assets/100108_nmea_0183_sentences_not_recommended_for_new_designs.pdf

Because the message types are standard, if no override is found for a particular talker ID then the message will parse according to the command (the last 3 characters) of the data type. In other words, $GPGLL will use the general GLL message type. Currently, the following standard message types are supported:

AAM, ALM, APA, APB, BEC, BER, BOD, BPI, BWC, BWR, BWW, DBK, DBS, DBT, DCN, DPT, DRU, DTM, FSI, GBS, GDA, GDF, GDP, GGA, GLA, GLC, GLF, GLL, GLP, GOA, GOF, GOP, GNS, GRS, GSA, GST, GSV, GTD, GXA, GXF, GXP, HCC, HCD, HDG, HDM, HDT, HFB, HSC, HTC, HVD, HVM, IMA, ITS, LCD, MDA, MHU, MMB, MSK, MSS, MTA, MTW, MWH, MWS, MWV, OLN, OLW, OMP, ONZ, OSD, Rnn (R00, R01, ...), RMA, RMB, RMC, RNN, ROT, RPM, RSA, RSD, RTE, SBK, SCD, SCY, SDB, SFI, SGD, SGR, SIU, SLC, SNC, SNU, SPS, SSF, STC, STN, STR, SYS TDS, TEC, TEP, TFI, TGA, TIF, TPC, TPR, TPT, TRF, TRP, TRS, TTM, VBW, VCD, VDR, VHW, VLW, VPE, VPW, VTA, VTG, VTI, VWE, VWR, VWT WCV, WDC, WDR, WFM, WNC, WNR, WPL, XDR, XTE, XTR, YWP, YWS, ZCD, ZDA, ZEV, ZFI, ZFO, ZLZ, ZPI, ZTA, ZTE, ZTG, ZTI, ZWP, ZZU

Support for proprietary NMEA messages is also possible. PASHR is included as proof-of-concept.

AIS Decoding

AIS message type definitions were implemented from the unofficial spec found here: http://catb.org/gpsd/AIVDM.html

And some binary subtypes from the ITU spec found here: https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.1371-4-201004-S!!PDF-E.pdf

The AIS payload can be found in the payload field of a VDM message (aka !AIVDM, !ABVDM, !SAVDM) or VDO message (aka !AIVDO). Currently, the following AIS message types are supported:

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 19, 20, 21, 24, 27

Type 6 subtypes for DAC/FID: 1/0, 1/2, 1/3, 1/4, 1/5, 235/10, 1022/61

Type 8 subtypes for DAC/FID: 1/0, 1/22, 1/31, 200/10, 366/56, 366/57

Disclaimer

This module was written primarily from information scraped together on the web, with minimal testing on an actual device (an SR161 AIS receiver). A lack of test cases -- especially for more obscure message types -- is a barrier to completeness. Please don't entrust your life or the safety of a ship to this code without doing your own rigorous testing.

Author

This gem was written by Ian Katz (ianfixes@gmail.com) in 2015. It's released under the Apache 2.0 license.

See Also

• Contributing