---
name: "gpsdecode"
section: "1"
platform: "linux"
package: "gpsd"
version: "3.27.5-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/gpsdecode"
---

# gpsdecode(1)

decode GPS, RTCM or AIS streams into a readable format

## NAME

gpsdecode - decode GPS, RTCM or AIS streams into a readable format

## SYNOPSIS

**gpsdecode** \[OPTIONS\]

**gpsdecode** -h

**gpsdecode** -V

## DESCRIPTION

**gpsdecode** tool is a batch-mode decoder for NMEA and various binary packet formats associated with GPS, AIS, and differential-correction services.  It produces a JSON dump on standard output from binary on standard input. The JSON is the same format documented by **gpsd**; this tool uses the same decoding logic as **gpsd**, but with a simpler interface intended for batch processing of data files.

All sensor-input formats known to the GPSD project can be decoded by this tool. These include: NMEA, AIVDM (the NMEA-derived sentence format used by AIS, the marine Automatic Identification System), RTCM2, and all supported GPS binary formats (notably including SiRF). See [**gpsd**(8)](/gpsd/8.md) for applicable standards and known limitations of the decoding logic.

You can use this tool with [**nc**(1)](/nc/1.md) to examine AIS feeds from AIS pooling services, RTCM feeds from RTCM receivers or NTRIP broadcasters.

## OPTIONS

- **-?**, **-h**, **--help**

  Output a usage mssage, then exit.

- **-c**, **--json**

  Sets the AIS dump format to separate fields with an ASCII pipe symbol. Fields are dumped in the order they occur in the AIS packet. Numerics are not scaled (**-u** is forced). Strings are unpacked from six-bit to full ASCII

- **-d**, **--decode**

  Decode packets presented on standard input to standard output. This is the default behavior.

- **-D LVL**, **--debug LVL**

  Set the debug verbosity level to LVL. It is mainly of interest to for troubleshooting.

- **-e**, **--encode**

  Encode JSON on standard input to JSON on standard output. This option is only useful for regression-testing of the JSON dumping and parsing code.

- **-j**, **--json**

  Sets the output dump format to JSON (the default behavior).

- **-m**, **--minlength**

  Dump minimum lengths for each packet type in the input (ignoring comment packets). This is probably of interest only to GSD developers.

- **-n**, **--nmea**

  Dump the generated pseudo-NME0183.

- **-s**, **--split24**

  Report AIS Type 24 sentence halves separately rather than attempting to aggregate them.

- **-t TYPES**, **--types TYPES**

  A comma-separated list of numeric types. Packets with a numeric AIS, RTCM2, or RTCM3 type are passed through and output only if they match a type in the list. Packets of other kinds (in particular GPS packets) are passed through unconditionally.

- **-u**, **--unsscaled**

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

- **-v**, **--verbose**

  Dump textual packets to output as they are received on input, immediately preceding corresponding output.

- **-V**, **--version**

  Print version number, then exit.

## AIS DSV FORMAT

With the **-c** or **--ais** option, dump lines are values of AIS payload fields, pipe-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.

## RETURN VALUES

**0**

on success.

**1**

if the action was unknown or the write to the control socket failed,

## SEE ALSO

[**gpsd**(8)](/gpsd/8.md), [**gps**(1)](/gps/1.md)

## RESOURCES

**Project web site:**  \<https://gpsd.io/\>

## COPYING

This file is Copyright 2013 by the GPSD project\
 SPDX-License-Identifier: BSD-2-clause

## AUTHOR

Eric S. Raymond