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Resume Wikipedia de Global Positioning System

Le Global Positioning System (GPS) (en français Système mondial de positionnement [littéralement] ou Géo-positionnement par satellite), aussi connu sous le nom de Navstar est un système de géolocalisation fonctionnant au niveau mondial et reposant sur l’exploitation de signaux radio émis par des satellites dédiés. En 2011, il est avec GLONASS, un système de positionnement par satellites entièrement opérationnel et accessible au grand public.
Ce système est mis en place par le département de la Défense des États-Unis à des fins militaires. Il est très rapidement apparu que des signaux transmis par les satellites pouvaient être librement reçus et exploités, et qu’ainsi un récepteur pouvait connaître sa position sur la surface de la Terre, avec une précision sans précédent, dès l’instant qu’il était équipé des circuits électroniques et du logiciel nécessaires au traitement des informations reçues. Une personne munie de ce récepteur peut ainsi se localiser et s’orienter sur terre, sur mer, dans l’air ou dans l’espace au voisinage proche de la Terre.
Le GPS a connu un grand succès dans le domaine civil et engendré un énorme développement commercial dans de nombreux domaines : navigation maritime, sur route, localisation de camions, randonnée, etc. De même, le milieu scientifique a su développer et exploiter des propriétés des signaux transmis pour de nombreuses applications : géodésie, transfert de temps entre horloges atomiques, étude de l’atmosphère, etc.
Le GPS utilise le système géodésique WGS 84, auquel se réfèrent les coordonnées calculées grâce au système. Le premier satellite expérimental fut lancé en 1978, mais la constellation de 24 satellites ne fut opérationnelle qu’en 1995.

GPSD(8) GPSD Documentation GPSD(8)

NAME

gpsd – interface daemon for GPS receivers

SYNOPSIS

gpsd [-F control-socket] [-S listener-port] [-b] [-l] [-G] [-n] [-N] [-h] [-P pidfile] [-D debuglevel] [-V] [[source-name]…]

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:

gpsd /dev/ttyUSB0

For the lowest-numbered serial port:

gpsd /dev/ttyS0

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
will autoconfigure that mode.

You can verify correct operation by first starting gpsd and then xgps,
the X windows test client.

If you have problems, the GPSD project maintains a FAQ to assist
troubleshooting.

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
direct-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, and
Ashtech/Thales devices. It can also interpret the binary protocols used
by EverMore, Garmin, Navcom, Rockwell/Zodiac, SiRF, Trimble, and u-blox
ANTARIS 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 instance 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 instance, 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 will shrink position
errors by roughly a factor of four. 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). See
the section called “ACCURACY” and the section called “FILES” for
discussion.

Client applications will communicate with gpsd via a TCP/IP port, 2947
by default). Both IPv4 and IPv6 connections are supported and a client
may connect via either.

The program accepts the following options:

-F
Create a control socket for device addition and removal commands.
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.

-S
Set TCP/IP port on which to listen for GPSD clients (default is
2947).

-b
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.

-G
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, TPV information is now private to the
local machine until the user makes an effort to expose this to the
world.

-l
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.

-n
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).

-N
Don’t daemonize; run in foreground. This switch is mainly useful
for debugging.

-h
Display help message and terminate.

-P
Specify the name and path to record the daemon’s process ID.

-D
Set debug level. 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.

-V
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. Example: /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 on 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 send 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 colony and a port number (if the port is absent the
default gpsd port will be used). 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.

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 then 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://”.)

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 (see the
section called “GPS DEVICE MANAGEMENT” for details on this). Daemon
startup will abort with an error if neither any devices nor a control
socket are 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.

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. 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 │
│ │ meters, 95% │
│ │ confidence. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_DOUBLE │ │
│ │ Altitude in meters. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_DOUBLE │ │
│ │ Altitude │
│ │ uncertainty in │
│ │ meters, 95% │
│ │ confidence. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_DOUBLE │ │
│ │ Course in degrees │
│ │ from true north. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_DOUBLE │ │
│ │ Course uncertainty │
│ │ in meters, 95% │
│ │ confidence. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_DOUBLE │ │
│ │ Speed, meters per │
│ │ second. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_DOUBLE │ │
│ │ Speed uncertainty │
│ │ in meters per │
│ │ second, 95% │
│ │ confidence. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_DOUBLE │ │
│ │ Climb, meters per │
│ │ second. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_DOUBLE │ │
│ │ Climb uncertainty │
│ │ in meters per │
│ │ second, 95% │
│ │ confidence. │
├─────────────────┼────────────────────────────┤
│DBUS_TYPE_STRING │ │
│ │ Device name │
└─────────────────┴────────────────────────────┘

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 properly installed along with hotplug notifier scripts
feeding it device-add commands over the control socket, gpsd should
require no configuration or user action to find 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
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 will reduce UERE by a factor of 4, provided you are
within about 100mi (160km) 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 50km/h (31mi/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, 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.

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. 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.

When using SiRF chips, the VDOP/TDOP/GDOP figures and associated error
estimates are computed by gpsd rather than reported by the chip. The
computation does not exactly match what SiRF chips do internally, which
includes some satellite weighting using parameters gpsd cannot see.

Autobauding on the Trimble GPSes can take as long as 5 seconds if the
device speed is not matched to the GPS speed.

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 tbe 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.

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.

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-200E, GLOBAL POSITIONING SYSTEM WING
(GPSW) SYSTEMS ENGINEERING & INTEGRATION, INTERFACE SPECIFICATION
IS-GPS-200 Revision E. The format understood by this program is defined
in Section 20 (Appendix II) of the IS-GPS-200E, GPS NAVIGATION DATA
STRUCTURE FOR DATA, D(t)

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

SEE ALSO

gpsdctl(8), gps, libgps, gpsd_json(5), libgpsd, gpsprof,
gpsfake(1), gpsctl(1), gpscat,

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
.

The GPSD Project 9 Aug 2004 GPSD(8)