Provided by: gpsd_3.25-4ubuntu1_amd64 
NAME
gpsd - interface daemon for GPS receivers
SYNOPSIS
gpsd [OPTIONS]
gpsd -h
gpsd -V
QUICK START
If you have a GPS attached on the lowest-numbered USB port of a Linux system, and want to
read reports from it on TCP/IP port 2947, it will normally suffice to do this, as root:
# gpsd /dev/ttyUSB0
For the lowest-numbered serial port:
# gpsd /dev/ttyS0
gpsd may be started as a normal user, or by using sudo, but some functionality will be
lost.
Change the device number as appropriate if you need to use a different port. Command-line
flags enable verbose logging, a control port, and other optional extras but should not be
needed for basic operation; the one exception, on very badly designed hardware, might be
-b (which see).
On Linux systems supporting udev, gpsd is normally started automatically when a USB plugin
event fires (if it is not already running) and is handed the name of the newly active
device. In that case no invocation is required at all.
For your initial tests set your GPS hardware to speak NMEA, as gpsd is guaranteed to be
able to process that. If your GPS has a native or binary mode with better performance that
gpsd knows how to speak, gpsd may autoconfigure that mode.
You can verify correct operation by first starting gpsd. Then xgps, the X windows test
client or cgps the curses based terminal client.
If you have problems, the GPSD project maintains helpful information online. Some links
are at the end of this man page.
DESCRIPTION
gpsd is a monitor daemon that collects information from GPSes, differential-GPS radios, or
AIS receivers attached to the host machine. Each GPS, DGPS radio, or AIS receiver is
expected to be directly connected to the host via a USB or RS232C serial device. The
serial device may be specified to gpsd at startup, or it may be set via a command shipped
down a local control socket (e.g. by a USB hotplug script). Given a GPS device by either
means, gpsd discovers the correct port speed and protocol for it.
gpsd should be able to query any GPS that speaks either the standard textual NMEA 0183
protocol, or the (differing) extended NMEA dialects used by MKT-3301, iTrax, Motorola
OnCore, Sony CXD2951, Ashtech/Thales and some other devices. It can also interpret the
binary protocols used by EverMore, Garmin, Javad, Navcom, Rockwell/Zodiac, SiRF, Trimble,
and u-blox devices. Under Linux it can read NMEA2000 packets through the kernel CAN
socket. It can read heading and attitude information from the Oceanserver 5000 or TNT
Revolution digital compasses.
The GPS reporting formats supported by your binary of gpsd may differ depending on how it
was compiled; general-purpose versions support many, but it can be built with protocol
subsets down to a singleton for use in constrained environments. For a list of the GPS
protocols supported by your binary, see the output of gpsd -l
gpsd effectively hides the differences among the GPS types it supports. It also knows
about and uses commands that tune these GPSes for lower latency. By using gpsd as an
intermediary, applications avoid contention for serial devices.
gpsd can use differential-GPS corrections from a DGPS radio or over the net, from a ground
station running a DGPSIP server or a Ntrip broadcaster that reports RTCM-104 data; this
may shrink position errors. When gpsd opens a serial device emitting RTCM-104, it
automatically recognizes this and uses the device as a correction source for all connected
GPSes that accept RTCM corrections (this is dependent on the type of the GPS; not all
GPSes have the firmware capability to accept RTCM correction packets).
Client applications typically communicate with gpsd via a TCP/IP port, port 2947 by
default. Both IPv4 and IPv6 connections are supported and a client may connect via either.
OPTIONS
The program accepts the following options:
-?, -h, ---help
Display help message and terminate.
-b, --readonly
Broken-device-safety mode, otherwise known as read-only mode. A few bluetooth and USB
receivers lock up or become totally inaccessible when probed or reconfigured; see the
hardware compatibility list on the GPSD project website for details. This switch
prevents gpsd from writing to a receiver. This means that gpsd cannot configure the
receiver for optimal performance, but it also means that gpsd cannot break the
receiver. A better solution would be for Bluetooth to not be so fragile. A platform
independent method to identify serial-over-Bluetooth devices would also be nice.
-D LVL, --debug LVL
Set debug level. Default is 0. At debug levels 2 and above, gpsd reports incoming
sentence and actions to standard error if gpsd is in the foreground (-N) or to syslog
if in the background. See LOGGING below.
-F FILE, --sockfile FILE
Create a control socket for device addition and removal commands. Default is None. You
must specify a valid pathname on your local filesystem; this will be created as a
Unix-domain socket to which you can write commands that edit the daemon’s internal
device list.
-f FRAME, --framing FRAME
Fix the framing to the GNSS device. The framing parameter is of the form:
[78][ENO][012]. Most GNSS are 8N1. Some Trimble default to 8O1. The default is to
search for the correct framing.
-G, --listenany
This flag causes gpsd to listen on all addresses (INADDR_ANY) rather than just the
loop back (INADDR_LOOPBACK) address. For the sake of privacy and security, gpsd
information is private by default to the local machine until the user makes an effort
to expose this to the world.
-l, --drivers
List all drivers compiled into this gpsd instance. The letters to the left of each
driver name are the gpsd control commands supported by that driver. Then exit.
-n, --nowait
Don’t wait for a client to connect before polling whatever GPS is associated with it.
Some RS232 GPSes wait in a standby mode (drawing less power) when the host machine is
not asserting DTR, and some cellphone and handheld embedded GPSes have similar
behaviors. Accordingly, waiting for a watch request to open the device may save
battery power. (This capability is rare in consumer-grade devices). You should use
this option if you plan to use gpsd to provide reference clock information to ntpd or
chronyd. This option will also enable clients to see data from the receiver sooner on
connection.
-N, --foreground
Don’t daemonize; run in foreground. This switch is mainly useful for debugging.
-p, --passive
Passive mode. Do not autoconfigure the receiver, but allow manual configuration
changes.
-P FILE, --pidfile FILE
Specify the name and path to record the daemon’s process ID.
-r, --badtime
Use GPS time even with no current fix. Some GPSs have battery powered Real Time Clocks
(RTC’s) built in, making them a valid time source even before a fix is acquired. This
can be useful on a Raspberry Pi, or other device that has no battery powered RTC, and
thus has no valid time at startup. Use with caution.
--port PORT, -S PORT
Set TCP/IP port on which to listen for GPSD clients (default is 2947).
-s SPEED, --speed SPEED
Fix the serial port speed to the GNSS device. Allowed speeds are: 4800, 9600, 19200,
38400, 57600, 115200, 230400, 460800 and 921600. The default is to autobaud. Note that
some devices with integrated USB ignore port speed.
-V, --version
Dump version and exit.
Arguments are interpreted as the names of data sources. Normally, a data source is the
device pathname of a local device from which the daemon may expect GPS data. But there are
three other special source types recognized, for a total of four:
Local serial or USB device
A normal Unix device name of a serial or USB device to which a sensor is attached.
Examples: /dev/ttyS0, or /dev/ttyUSB0.
Local PPS device
A normal Unix device name of a PPS device to which a PPS source is attached. The
device name must start with "/dev/pps" and a local serial or USB GPS device must also
be available. Example: /dev/pps0.
TCP feed
A URI with the prefix "tcp://", followed by a hostname, a colon, and a port number.
The daemon will open a socket to the indicated address and port and read data packets
from it, which will be interpreted as though they had been issued by a serial device.
Example: tcp://data.aishub.net:4006.
UDP feed
A URI with the prefix "udp://", followed by a hostname, a colon, and a port number.
The daemon will open a socket listening for UDP datagrams arriving in the indicated
address and port, which will be interpreted as though they had been issued by a serial
device. Example: udp://127.0.0.1:5000.
Ntrip caster
A URI with the prefix "ntrip://" followed by the name of an Ntrip caster (Ntrip is a
protocol for broadcasting differential-GPS fixes over the net). For Ntrip services
that require authentication, a prefix of the form "username:password@" can be added
before the name of the Ntrip broadcaster. For Ntrip service, you must specify which
stream to use; the stream is given in the form "/streamname". An example DGPSIP URI
could be "dgpsip://dgpsip.example.com" and a Ntrip URI could be
"ntrip://foo:bar@ntrip.example.com:80/example-stream". Corrections from the caster
will be sent to each attached GPS with the capability to accept them.
DGPSIP server
A URI with the prefix "dgpsip://" followed by a hostname, a colon, and an optional
colon-separated port number (defaulting to 2101). The daemon will handshake with the
DGPSIP server and read RTCM2 correction data from it. Corrections from the server will
be set to each attached GPS with the capability to accept them. Example:
dgpsip://dgps.wsrcc.com:2101.
Remote gpsd feed
A URI with the prefix "gpsd://", followed by a hostname and optionally a colon and a
port number (if the port is absent the default gpsd port will be used). Then followed
optionally by a second colon and the remote device name The daemon will open a socket
to the indicated address and port and emulate a gpsd client, collecting JSON reports
from the remote gpsd instance that will be passed to local clients. Example:
gpsd://gpsd.io:2947:/dev/ttyAMA0.
NMEA2000 CAN data
A URI with the prefix "nmea2000://", followed by a CAN devicename. Only Linux socket
CAN interfaces are supported. The interface must be configured to receive CAN messages
before gpsd can be started. If there is more than one unit on the CAN bus that
provides GPS data, gpsd chooses the unit from which a GPS message is first seen.
Example: nmea2000://can0.
(The "ais:://" source type supported in some older versions of the daemon has been retired
in favor of the more general "tcp://".)
Additionally, two serial device name have a side effect, if your binary was compiled with
the MAGIC_HAT option:
/dev/ttyAMA0
The UART device on a Raspberry Pi. Has the side effect of opening /dev/pps0 for
RFC2783 1PPS data.
/dev/gpsd0
Generic GPS device 0. Has the side effect of opening /dev/pps0 for RFC2783 1PPS data.
Note, however, that if /dev/pps0 is the fake "ktimer" PPS, then /dev/pps1 will be used
instead.
Internally, the daemon maintains a device pool holding the pathnames of devices and remote
servers known to the daemon. Initially, this list is the list of device-name arguments
specified on the command line. That list may be empty, in which case the daemon will have
no devices on its search list until they are added by a control-socket command. Daemon
startup will abort with an error if no devices and no control socket is specified.
When a device is activated (i.e. a client requests data from it), gpsd attempts to execute
a hook from /etc/gpsd/device-hook with first command line argument set to the pathname of
the device and the second to ACTIVATE. On deactivation, it does the same passing
DEACTIVATE for the second argument.
gpsd can export data to client applications in three ways: via a sockets interface, via a
shared-memory segment, and via D-Bus. The next three major sections describe these
interfaces.
LOGGING
When gpsd is running as a daemon (not in the foreground) is sends all of its logging to
syslog(3). Logging is sent with ident "gpsd" using facility "LOG_USER".
If you are using syslog-ng, then you can send all gpsd logs to a file /var/log/gpsd. Put
this at the end of your /etc/syslog-ng/syslog-ng.conf file:
destination gpsdf { file("/var/log/gpsd"); };
filter f_gpsd { program("gpsd"); };
log { source(src); filter(f_gpsd); destination(gpsdf); };
THE SOCKET INTERFACE
Clients may communicate with the daemon via textual request and responses over a socket.
It is a bad idea for applications to speak the protocol directly: rather, they should use
the libgps client library and take appropriate care to conditionalize their code on the
major and minor protocol version symbols.
The request-response protocol for the socket interface is fully documented in
gpsd_json(5).
SHARED-MEMORY AND DBUS INTERFACES
gpsd has two other (read-only) interfaces.
Whenever the daemon recognizes a packet from any attached device, it writes the
accumulated state from that device to a shared memory segment. The C and C++ client
libraries shipped with GPSD can read this segment. Client methods, and various
restrictions associated with the read-only nature of this interface, are documented at
libgps(3). The shared-memory interface is intended primarily for embedded deployments in
which gpsd monitors a single device, and its principal advantage is that a daemon instance
configured with shared memory but without the sockets interface loses a significant amount
of runtime weight.
The daemon may be configured to emit a D-Bus signal each time an attached device delivers
a fix. The signal path is "path /org/gpsd", the signal interface is "org.gpsd", and the
signal name is "fix". The signal payload layout is as follows:
Table 1. Satellite object
┌─────────────────┬──────────────────────────────────┐
│ │ │
│Type │ Description │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Time (seconds since Unix epoch) │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_INT32 │ mode │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Time uncertainty (seconds). │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Latitude in degrees. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Longitude in degrees. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Horizontal uncertainty in meter. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Altitude MSL in meters. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Altitude uncertainty in meters. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Course in degrees from true │
│ │ north. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Course uncertainty in meters │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Speed, meters per second. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Speed uncertainty in meters per │
│ │ second. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Climb, meters per second. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_DOUBLE │ Climb uncertainty in meters per │
│ │ second. │
├─────────────────┼──────────────────────────────────┤
│ │ │
│DBUS_TYPE_STRING │ Device name │
└─────────────────┴──────────────────────────────────┘
Uncertainty values are provided by the GNSS receiver. Check your receiver documentation to
see if is specifies what its "uncertainty" means.
GPS DEVICE MANAGEMENT
gpsd maintains an internal list of GPS devices (the "device pool"). If you specify devices
on the command line, the list is initialized with those pathnames, otherwise the list
starts empty. Commands to add and remove GPS device paths from the daemon’s device list
must be written to a local Unix-domain socket which will be accessible only to programs
running as root. This control socket will be located wherever the -F option specifies it.
A device may will also be dropped from the pool if GPSD gets a zero length read from it.
This end-of-file condition indicates that the device has been disconnected.
When gpsd is installed along with working hotplug notifier scripts feeding it device-add
commands over the control socket, gpsd should require no configuration or user action to
find hotplug devices.
Sending SIGHUP to a running gpsd forces it to close all GPSes and all client connections.
It will then attempt to reconnect to any GPSes on its device list and resume listening for
client connections. This may be useful if your GPS enters a wedged or confused state but
can be soft-reset by pulling down DTR.
When gpsd is called with no initial devices (thus, expecting devices to be passed to it by
notifications to the control socket), and reaches a state where there are no devices
connected and no subscribers after some devices have been seen, it shuts down gracefully.
It is expected that future device hotplug events will reactivate it.
To point gpsd at a device that may be a GPS, write to the control socket a plus sign ('+')
followed by the device name followed by LF or CR-LF. Thus, to point the daemon at
/dev/foo. send "+/dev/foo\n". To tell the daemon that a device has been disconnected and
is no longer available, send a minus sign ('-') followed by the device name followed by LF
or CR-LF. Thus, to remove /dev/foo from the search list, send "-/dev/foo\n".
To send a control string to a specified device, write to the control socket a '!',
followed by the device name, followed by '=', followed by the control string.
To send a binary control string to a specified device, write to the control socket a '&',
followed by the device name, followed by '=', followed by the control string in paired hex
digits.
Your client may await a response, which will be a line beginning with either "OK" or
"ERROR". An ERROR response to an 'add' command means the device did not emit data
recognizable as GPS packets, an ERROR response to a remove command means the specified
device was not in gpsd's device pool. An ERROR response to a '!' command means the daemon
did not recognize the devicename specified.
The control socket is intended for use by hotplug scripts and other device-discovery
services. This control channel is separate from the public gpsd service port, and only
locally accessible, in order to prevent remote denial-of-service and spoofing attacks.
ACCURACY
The base User Estimated Range Error (UERE) of GPSes is 8 meters or less at 66% confidence,
15 meters or less at 95% confidence. Actual horizontal error will be UERE times a dilution
factor dependent on current satellite position. Altitude determination is more sensitive
to variability in ionospheric signal lag than latitude/longitude is, and is also subject
to errors in the estimation of local mean sea level, base error is 12 meters at 66%
confidence, 23 meters at 95% confidence. Again, this will be multiplied by a vertical
dilution of precision (VDOP) dependent on satellite geometry, and VDOP is typically larger
than HDOP. Users should not rely on GPS altitude for life-critical tasks such as landing
an airplane.
These errors are intrinsic to the design and physics of the GPS system. gpsd does its
internal computations at sufficient accuracy that it will add no measurable position error
of its own.
DGPS correction may reduce UERE, provided you are within about 100 miles (160 km) of a
DGPS ground station from which you are receiving corrections.
On a 4800bps connection, the time latency of fixes provided by gpsd will be one second or
less 95% of the time. Most of this lag is due to the fact that GPSes normally emit fixes
once per second, thus expected latency is 0.5sec. On the personal-computer hardware
available in 2005 and later, computation lag induced by gpsd will be negligible, on the
order of a millisecond. Nevertheless, latency can introduce significant errors for
vehicles in motion, at 50 km/h (31 mi/h) of speed over ground, 1 second of lag corresponds
to 13.8 meters change in position between updates.
The time reporting of the GPS system itself has an intrinsic accuracy limit of 14
nanoseconds, but this can only be approximated by specialized receivers using that send
the high-accuracy PPS (Pulse-Per-Second) over RS232 to cue a clock crystal. Most GPS
receivers only report time to a precision of 0.01s or 0.001s, and with no accuracy
guarantees below 1sec.
If your GPS uses a SiRF chipset at firmware level 231, reported UTC time may be off by the
difference between whatever default leap-second offset has been compiled in and whatever
leap-second correction is currently applicable, from startup until complete subframe
information is received. Firmware levels 232 and up don’t have this problem. You may run
gpsd at debug level 4 to see the chipset type and firmware revision level.
There are exactly two circumstances under which gpsd relies on the host-system clock:
In the GPS broadcast signal, GPS time is represented using a week number that rolls over
after 2^10 or 2^13 weeks (about 19.6 years, or 157 years), depending on the spacecraft.
Receivers are required to disambiguate this to the correct date, but may have difficulty
due to not knowing time to within half this interval, or may have bugs. Users have
reported incorrect dates which appear to be due to this issue. gpsd uses the startup time
of the daemon detect and compensate for rollovers while it is running, but otherwise
reports the date as it is reported by the receiver without attempting to correct it.
If you are using an NMEA-only GPS (that is, not using SiRF or Garmin or Zodiac binary
mode), gpsd relies on the system clock to tell it the current century. If the system clock
returns an invalid value near zero, and the GPS does not emit GPZDA at the start of its
update cycle (which most consumer-grade NMEA GPSes do not) then the century part of the
dates gpsd delivers may be wrong. Additionally, near the century turnover, a range of
dates as wide in seconds as the accuracy of your system clock may be referred to the wrong
century.
USE WITH NTP
gpsd can provide reference clock information to ntpd or chronyd, to keep the system clock
synchronized to the time provided by the GPS receiver.
On Linux, gpsd includes support for interpreting the PPS pulses emitted at the start of
every clock second on the carrier-detect lines of some serial GPSes, this pulse can be
used to update NTP at much higher accuracy than message time provides. You can determine
whether your GPS emits this pulse by running at -D 5 and watching for carrier-detect state
change messages in the logfile. In addition, if your kernel provides the RFC 2783 kernel
PPS API then gpsd will use that for extra accuracy.
gpsd communicates with ntpd using shared memory segments (SHMs). The SHMs are numbered
SHM(0) to SHM(7). and have a name (key) of NTP0 to NTP7. The first local time source on
the command line may use NTP0 and NTP1. The second may use NTP2 and NTP3, etc.
You can see the output of gpsd to ntpd in real time with the ntpshmmon command.
Other daemons, such as ptp4l may also be using the same SHMs to talk to ntpd.
Detailed instructions for using GPSD to set up a high-quality time service can be found
among the documentation on the GPSD website.
USE WITH D-BUS
On operating systems that support D-BUS, gpsd can be built to broadcast GPS fixes to
D-BUS-aware applications. As D-BUS is still at a pre-1.0 stage, we will not attempt to
document this interface here. Read the gpsd source code to learn more.
SECURITY AND PERMISSIONS ISSUES
gpsd, if given the -G flag, will listen for connections from any reachable host, and then
disclose the current position. Before using the -G flag, consider whether you consider
your computer’s location to be sensitive data to be kept private or something that you
wish to publish.
gpsd must start up as root in order to open the NTPD shared-memory segment, open its
logfile, and create its local control socket. Note that starting gpsd with sudo is not the
same as starting as root. Before doing any processing of GPS data, it tries to drop root
privileges by setting its UID to "nobody" (or another configured userid) and its group ID
to the group of the initial GPS passed on the command line — or, if that device doesn’t
exist, to the group of /dev/ttyS0.
Privilege-dropping is a hedge against the possibility that carefully crafted data, either
presented from a client socket or from a subverted serial device posing as a GPS, could be
used to induce misbehavior in the internals of gpsd. It ensures that any such compromises
cannot be used for privilege elevation to root.
The assumption behind gpsd's particular behavior is that all the tty devices to which a
GPS might be connected are owned by the same non-root group and allow group read/write,
though the group may vary because of distribution-specific or local administrative
practice. If this assumption is false, gpsd may not be able to open GPS devices in order
to read them (such failures will be logged).
In order to fend off inadvertent denial-of-service attacks by port scanners (not to
mention deliberate ones), gpsd will time out inactive client connections. Before the
client has issued a command that requests a channel assignment, a short timeout (60
seconds) applies. There is no timeout for clients in watcher or raw modes; rather, gpsd
drops these clients if they fail to read data long enough for the outbound socket write
buffer to fill. Clients with an assigned device in polling mode are subject to a longer
timeout (15 minutes).
LIMITATIONS
If multiple NMEA talkers are feeding RMC, GLL, and GGA sentences to the same serial device
(possible with an RS422 adapter hooked up to some marine-navigation systems), a 'TPV'
response may mix an altitude from one device’s GGA with latitude/longitude from another’s
RMC/GLL after the second sentence has arrived.
gpsd may change control settings on your GPS (such as the emission frequency of various
sentences or packets) and not restore the original settings on exit. This is a result of
inadequacies in NMEA and the vendor binary GPS protocols, which often do not give clients
any way to query the values of control settings in order to be able to restore them later.
Some receivers do not report VDOP/TDOP/GDOP figures and associated error estimates. In
that case they may be computed by gpsd instead. This computation does not exactly match
what chips do internally, which includes some satellite weighting using parameters gpsd
cannot see.
Autobauding on the Trimble GPSes can take as long as 20 seconds, or more, if the device
speed is not matched to the GPS speed. Use the -s option to avoid autobaud delays.
Generation of position error estimates (eph, epv, epd, eps, epc) from the incomplete data
handed back by GPS reporting protocols involves both a lot of mathematical black art and
fragile device-dependent assumptions. This code has been bug-prone in the past and
problems may still lurk there.
AIDVM decoding of types 16-17, 22-23, and 25-26 is unverified.
GPSD presently fully recognizes only the 2.1 level of RTCM2 (message types 1, 3, 4, 5, 6,
7, 9, 16). The 2.3 message types 13, 14, and 31 are recognized and reported. Message types
8, 10-12, 15-27, 28-30 (undefined), 31-37, 38-58 (undefined), and 60-63 are not yet
supported.
The ISGPS used for RTCM2 and subframes decoder logic is sufficiently convoluted to confuse
some compiler optimizers, notably in GCC 3.x at -O2, into generating bad code.
Devices meant to use PPS for high-precision timekeeping may fail if they are specified
after startup by a control-socket command, as opposed to on the daemon’s original command
line. Root privileges are dropped early, and some Unix variants require them in order to
set the PPS line discipline. Under Linux the POSIX capability to set the line discipline
is retained, but other platforms cannot use this code.
USB GPS devices often do not identify themselves through the USB subsystem; they typically
present as the class 00h (undefined) or class FFh (vendor-specific) of USB-to-serial
adapters. Because of this, the Linux hotplug scripts must tell gpsd to sniff data from
every USB-to-serial adapter that goes active and is known to be of a type used in GPSes.
No such device is sent configuration strings until after it has been identified as a GPS,
and gpsd never opens a device that is opened by another process. But there is a tiny
window for non-GPS devices not opened; if the application that wants them loses a race
with GPSD its device open will fail and have to be retried after GPSD sniffs the device
(normally less than a second later).
FILES
/dev/ttyS0
Prototype TTY device. After startup, gpsd sets its group ID to the owning group of
this device if no GPS device was specified on the command line does not exist.
/etc/gpsd/device-hook
Optional file containing the device activation/deactivation script. Note that while
/etc/gpsd is the default system configuration directory, it is possible to build the
GPSD source code with different assumptions. See above for further details on the
device-hook mechanism.
/@RUNDIR@/chrony.XXX.sock
Optional Unix domain socket provided by chronyd configured with a SOCK reference
clock, where XXX is the name of the serial device (e.g. ttyS0). When gpsd is started,
it will try to connect to this socket. If connected, it will be sending PPS timing
information to chronyd in order to synchronize the system clock.
/@RUNDIR@/chrony.clk.XXX.sock
Similar to chrony.XXX.sock, but will be used to pass timing information to chronyd
that is derived from the serial (likely NMEA) timing instead of PPS.
ENVIRONMENT VARIABLES
By setting the environment variable GPSD_SHM_KEY, you can control the key value used to
create the shared-memory segment used for communication with the client library. This will
be useful mainly when isolating test instances of gpsd from production ones.
RETURN VALUES
0
on success.
1
on failure
APPLICABLE STANDARDS
The official NMEA protocol standards for NMEA0183 and NMEA2000 are available from the
National Marine Electronics Association, but are proprietary and expensive; the
maintainers of gpsd have made a point of not looking at them. The GPSD project website
links to several documents that collect publicly disclosed information about the protocol.
gpsd parses the following NMEA sentences: RMC, GGA, GLL, GSA, GSV, VTG, ZDA, GBS, HDT,
DBT, GST. It recognizes these with either the normal GP talker-ID prefix, or with the GN
prefix used by GLONASS, or with the II prefix emitted by Seahawk Autohelm marine
navigation systems, or with the IN prefix emitted by some Garmin units, or with the EC
prefix emitted by ECDIS units, or with the SD prefix emitted by depth sounders, or with
the HC and TI prefix emitted by some Airmar equipment. It recognizes some vendor
extensions: the PGRME emitted by some Garmin GPS models, the OHPR emitted by Oceanserver
digital compasses, the PTNTHTM emitted by True North digital compasses, the PMTK omitted
by some San Jose Navigation GPSes, and the PASHR sentences emitted by some Ashtech GPSes.
Note that gpsd JSON returns pure decimal degrees, not the hybrid degree/minute format
described in the NMEA standard.
Differential-GPS corrections are conveyed by the RTCM protocols. The applicable standard
for RTCM-104 V2 is RTCM Recommended Standards for Differential GNSS (Global Navigation
Satellite) Service RTCM Paper 136-2001/SC 104-STD. The applicable standard for RTCM-104 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 under "Publications".
AIS is defined by ITU Recommendation M.1371, Technical Characteristics for a Universal
Shipborne Automatic Identification System Using Time Division Multiple Access. The
AIVDM/AIVDO format understood by this program is defined by IEC-PAS 61162-100, Maritime
navigation and radiocommunication equipment and systems. A more accessible description of
both can be found at AIVDM/AIVDO Protocol Decoding, on the references page of the GPSD
project website.
Subframe data is defined by IS-GPS-200, GLOBAL POSITIONING SYSTEM WING (GPSW) SYSTEMS
ENGINEERING & INTEGRATION, INTERFACE SPECIFICATION IS-GPS-200. The format understood by
this program is defined in Section 20 (Appendix II),
JSON is specified by RFC 7159, The JavaScript Object Notation (JSON) Data Interchange
Format.
The API for PPS time service is specified by RFC 2783, Pulse-Per-Second API for UNIX-like
Operating Systems, Version 1.0
AUTHORS
Authors: Eric S. Raymond, Chris Kuethe, Gary Miller. Former authors whose bits have been
plowed under by code turnover: Remco Treffcorn, Derrick Brashear, Russ Nelson.
This manual page by Eric S. Raymond esr@thyrsus.com.
SEE ALSO
gpsctl(1), gps(1), gpsprof(1), gpsfake(1). gpscat(1), *ntpshmmon(1),
libgps(3), libgpsmm(3)
gpsd_json(5)
chronyd(8), gpsd(8), gpsdctl(8), ntpd(8)
RESOURCES
Project web site: https://gpsd.io/ GPSD FAQ: https://gpsd.io/faq.html GPSD Time Service
HOWTO: https://gpsd.io/gpsd-time-service-howto.html Introduction to Time Service:
https://gpsd.io/time-service-intro.html
COPYING
This file is Copyright 2013 by the GPSD project
SPDX-License-Identifier: BSD-2-clause