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
AIS DUMP FORMATS
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
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 on the references page of the GPSD
BUGS AND LIMITATIONS
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 firstname.lastname@example.org. This is a somewhat hacked version of
an RTCM decoder originally written by Wolfgang Rupprecht. There is a
project page for gpsd here.
1. AIVDM/AIVDO protocol decoding
2. Radio Technical Commission for Maritime Services