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       gpsdecode - decode RTCM or AIVDM streams into a readable format


       gpsdecode [-c] [-d] [-e] [-j] [-u] [-D debuglevel] [-V]


       This tool is a decoder/encoder for various binary packet formats
       associated with GPS and differential-correction services. It produces a
       text dump on standard output from binary on standard input, or binary
       packets on standard output from text on standard input, and aims to be
       100% information-preserving in both directions. As well as data, the
       decoder also prints decoder status messages to standard error as

       Two of the supported formats are RTCM 2 and 3, a pair of obscure and
       complicated serial protocol used for broadcasting pseudorange
       corrections from differential-GPS reference stations. You can use this
       mode of the tool with nc(1) to examine RTCM feeds from DGPSIP servers
       or Ntrip broadcasters. The decoder dump formats for RTCM2 are described
       in rtcm(5); these lines go to standard output.

       Another supported format is AIVDM. This is the sentence format used by
       the marine Automatic Identification System. This can be decoded, but
       not yet encoded.


       The -d option tells the program to decode packets presented on standard
       input to a text dump on standard output. This is the default behavior.

       RTCM2 will be dumped in one of the formats of rtcm-104(5) on standard

       The -e option option tells the program to encode a text dump in one of
       the formats of rtcm-104(5) to standard output. This option is a
       placeholder: support for RTCM2 encoding from the Sager format has been

       The -u suppresses scaling of AIS data to float quantities and text
       expansion of numeric codes. A dump with this option is lossless.

       The -j sets the dump format to JSON, with each each field preceded by a
       quoted label and colon and the entire dump line wrapped in curly

       The -c sets the AIS dump format to CSV. Fields are dumped in the order
       they occur in the AIS packet. Numerics are not scaled. Strings are
       unpacked from six-bit to full ASCII

       The -V option directs the program to emit its version number, then

       The -D option sets a debug verbosity level. It is mainly of interest to


       Without the -j option, dump lines are values of AIS payload fields,
       comma-separated, in the order that they occur in the payload. Spans of
       fields expressing a date are emitted as an ISO8601 timestamp (look for
       colons and the trailing Z indicating Zulu/UTC time), and the 19-bit
       group of TDMA status fields found at the end of message types 1-4 are
       are dumped as a single unsigned integer (in hex preceded by "0x").
       Unused regional-authority fields are also dumped (in hex preceded by
       "0x"). Variable-length binary fields are dumped as an integer bit
       length, followed by a colon, followed by a hex dump.

       By default, certain scaling and conversion operations are performed for
       the output. Latitudes and longitudes are scaled to decimal degrees
       rather than the native AIS unit of 1/10000th of a minute of arc. Ship
       (but not air) speeds are scaled to knots rather than tenth-of-knot
       unit. Navigation status and positioning-system type are dumped as text
       strings rather than IAS numeric codes. Rate of turn may appear as "nan"
       if is unavailable, or as one of the strings "fastright" or "fastleft"
       if it is out of the IAS encoding range; otherwise it is quadratically
       mapped back to the turn sensor number in degrees per minute. Vessel
       draughts are converted to decimal meters rather than native AIS

       With the -j option, the AIS dump format changes to JSON. Data fields
       are handled as described above in scaled and unscaled modes, but are
       values attached to JSON attributes as described in AIVDM/AIVDO protocol


       The applicable standard for V2 is RTCM Recommended Standards for
       Differential NAVSTAR GPS Service RTCM Paper 194-93/SC 104-STD.

       Note that gpsdecode presently recognizes only the 2.1 level of RTCM;
       the protocol was revised up to a version 2.3 including additional
       messages relating to GLONASS and real-time kinematics before being
       deprecated in favor of V3. It is now semi-obsolete.

       The applicable standard for V3 is RTCM Standard 10403.1 for
       Differential GNSS Services - Version 3 RTCM Paper 177-2006-SC104-STD.

       Ordering instructions for the RTCM standards are accessible from the
       website of the Radio Technical Commission for Maritime Services[2]
       under "Publications".

       The applicable standard for AIVDM is ITU-R M.1371: ITU Recommendation
       on the Technical Characteristics for a Universal Shipborne Automatic
       Identification System (AIS) using Time Division Multiple Access in the
       Maritime Mobile Band, A more accessible description can be found at
       AIVDM/AIVDO protocol decoding[1] on the references page of the GPSD
       project website.


       AIDVM decoding presently only parses fields for the sentence types 1-21
       and 24. Decoding of types 6-8, 12-17, and 20-21 is unverified. AIVDM
       encoding is not yet supported.

       RTCM3 decoding is buggy and incomplete.

       RTCM2 represents floating-point quantities as an integer multiple of a
       fixed scale factor. Editing an RTCM2 dump can produce numbers that are
       not an integer multiple of the scale factor for their field. If you do
       this, the value actually packed into binary RTCM2 will be rounded down
       to the nearest scale unit, and dumping will show slightly different
       numbers than those you entered. This bug could be fixed by supporting
       the -u option to suppress scaling.

       The RTCM2 decoder logic is sufficiently convoluted to confuse some
       compiler optimizers, notably in GCC 3.x at -O2, into generating bad


       gpsd(8), gps(1), libgps(3), libgpsd(3), gpsprof(1), gpsfake(1),


       Eric S. Raymond This is a somewhat hacked version of
       an RTCM decoder originally written by Wolfgang Rupprecht. There is a
       project page for gpsd here[3].


        1. AIVDM/AIVDO protocol decoding

        2. Radio Technical Commission for Maritime Services

        3. here