gpsd.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
<!--
This file is Copyright (c) 2010 by the GPSD project
BSD terms apply: see the file COPYING in the distribution root for details.
-->
<!DOCTYPE refentry PUBLIC
   "-//OASIS//DTD DocBook XML V4.1.2//EN"
   "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
<!ENTITY gpsdsock	 "/var/run/gpsd.sock">
]>
<refentry id='gpsd.8'>
<refentryinfo><date>9 Aug 2004</date></refentryinfo>
<refmeta>
<refentrytitle>gpsd</refentrytitle>
<manvolnum>8</manvolnum>
<refmiscinfo class="source">The GPSD Project</refmiscinfo>
<refmiscinfo class="manual">GPSD Documentation</refmiscinfo>
</refmeta>
<refnamediv id='name'>
<refname>gpsd</refname>
<refpurpose>interface daemon for GPS receivers</refpurpose>
</refnamediv>

<refsynopsisdiv id='synopsis'>

<cmdsynopsis>
  <command>gpsd</command>
      <arg choice='opt'>-F <replaceable>control-socket</replaceable></arg>
      <arg choice='opt'>-S <replaceable>listener-port</replaceable></arg>
      <arg choice='opt'>-b </arg>
      <arg choice='opt'>-l </arg>
      <arg choice='opt'>-G </arg>
      <arg choice='opt'>-n </arg>
      <arg choice='opt'>-N </arg>
      <arg choice='opt'>-h </arg>
      <arg choice='opt'>-P <replaceable>pidfile</replaceable></arg>
      <arg choice='opt'>-D <replaceable>debuglevel</replaceable></arg>
      <arg choice='opt'>-V </arg>
      <arg rep='repeat'>
	   <group><replaceable>source-name</replaceable></group>
      </arg>
</cmdsynopsis>
</refsynopsisdiv>

<refsect1 id='quickstart'><title>QUICK START</title>

<para>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:</para>

<programlisting>
gpsd /dev/ttyUSB0
</programlisting>

<para>For the lowest-numbered serial port:</para>

<programlisting>
gpsd /dev/ttyS0
</programlisting>

<para>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 <option>-b</option> (which see).</para>

<para>On Linux systems supporting udev, <application>gpsd</application>
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.</para>

<para>For your initial tests set your GPS hardware to speak NMEA, as
<application>gpsd</application> is guaranteed to be able to process
that. If your GPS has a native or binary mode with better performance
that <application>gpsd</application> knows how to speak,
<application>gpsd</application> will autoconfigure that mode.</para>

<para>You can verify correct operation by first starting
<application>gpsd</application> and then
<application>xgps</application>, the X windows test client.</para>

<para>If you have problems, the GPSD project maintains a FAQ to
assist troubleshooting.</para>

</refsect1>
<refsect1 id='description'><title>DESCRIPTION</title>

<para><application>gpsd</application> 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
<application>gpsd</application> 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,
<application>gpsd</application> discovers the correct port speed and
protocol for it.</para>

<para><application>gpsd</application> 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.</para>

<para>The GPS reporting formats supported by your instance of
<application>gpsd</application> 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 <command>gpsd -l</command></para>

<para><application>gpsd</application> 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
<application>gpsd</application> as an intermediary, applications
avoid contention for serial devices.</para>

<para><application>gpsd</application> 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 <application>gpsd</application> 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
<xref linkend='accuracy'/> and <xref linkend='files'/> for discussion.</para>

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

<para>The program accepts the following options:</para>
<variablelist remap='TP'>
<varlistentry>
<term>-F</term>
<listitem>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>-S</term>
<listitem><para>Set TCP/IP port on which to listen for GPSD clients
(default is 2947).</para></listitem>
</varlistentry>
<varlistentry>
<term>-b</term>
<listitem><para>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
<application>gpsd</application> cannot configure the receiver for
optimal performance, but it also means that
<application>gpsd</application> 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.</para></listitem>
</varlistentry>
<varlistentry>
<term>-G</term>
<listitem><para>This flag causes <application>gpsd</application> 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.</para></listitem>
</varlistentry>
<varlistentry>
<term>-l</term>
<listitem><para>List all drivers compiled into this
<application>gpsd</application> instance. The letters to the left of
each driver name are the <application>gpsd</application>
control commands supported by that driver.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>-n</term>
<listitem>
<para>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).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>-N</term>
<listitem><para>Don't daemonize; run in foreground. 
This switch is mainly useful for debugging.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>-h</term>
<listitem><para>Display help message and terminate.</para></listitem>
</varlistentry>
<varlistentry>
<term>-P</term>
<listitem>
<para>Specify the name and path to record the daemon's process ID.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>-D</term>
<listitem>
<para>Set debug level. At debug levels 2 and above,
<application>gpsd</application> reports incoming sentence and actions
to standard error if <application>gpsd</application> is in the foreground
(-N) or to syslog if in the background.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>-V</term>
<listitem>
<para>Dump version and exit.</para>
</listitem>
</varlistentry>
</variablelist>

<para>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:</para>

<variablelist>
<varlistentry>
<term>Local serial or USB device</term>
<listitem>
<para>A normal Unix device name of a serial or USB device to which a
sensor is attached. Example:
<filename>/dev/ttyUSB0</filename>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>TCP feed</term>
<listitem>
<para>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:
<filename>tcp://data.aishub.net:4006</filename>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>UDP feed</term>
<listitem>
<para>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:
<filename>udp://127.0.0.1:5000</filename>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Ntrip caster</term>
<listitem>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DGPSIP server</term>
<listitem>
<para>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:
<filename>dgpsip://dgps.wsrcc.com:2101</filename>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Remote gpsd feed</term>
<listitem>
<para>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 <application>gpsd</application> port will be used). The daemon
will open a socket to the indicated address and port and emulate a
<application>gpsd</application> client, collecting JSON reports from
the remote <application>gpsd</application> instance that will be
passed to local clients.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>NMEA2000 CAN data</term>
<listitem>
<para>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 <application>gpsd</application> can be started. If
there is more then one unit on the CAN bus that provides GPS data,
<application>gpsd</application> chooses the unit from which a GPS message
is first seen. Example: <filename>nmea2000://can0</filename>.
</para>
</listitem>
</varlistentry>
</variablelist>

<para>(The "ais:://" source type supported in some older versions of
the daemon has been retired in favor of the more general
"tcp://".)</para>

<para>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 <xref linkend='devices'/> for
details on this).  Daemon startup will abort with an error if neither
any devices nor a control socket are specified.</para>

<para>When a device is activated (i.e. a client requests data from it),
gpsd attempts to execute a hook from
<filename>/etc/gpsd/device-hook</filename> with first command line argument
set to the pathname of the device and the second to
<option>ACTIVATE</option>. On deactivation it does the same passing
<option>DEACTIVATE</option> for the second argument.</para>

<para><application>gpsd</application> 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.</para>

</refsect1>
<refsect1 id='sockets'><title>THE SOCKET INTERFACE</title>

<para>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
<application>libgps</application> client library and take appropriate
care to conditionalize their code on the major and minor protocol
version symbols.</para>

<para>The request-response protocol for the socket interface is fully 
documented in
<citerefentry><refentrytitle>gpsd_json</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
  
</refsect1>

<refsect1 id='shm'><title>SHARED-MEMORY AND DBUS INTERFACES</title>

<para><application>gpsd</application> has two other (read-only)
interfaces.</para>

<para>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
<citerefentry><refentrytitle>libgps</refentrytitle><manvolnum>3</manvolnum></citerefentry>. The
shared-memory interface is intended primarily for embedded deployments
in which <application>gpsd</application> 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.</para>

<para>The daemon may be configured to emit a D-Bus signal each time an
attached device delivers a fix.  The signal path is <filename>path
/org/gpsd</filename>, the signal interface is "org.gpsd", and the
signal name is "fix".  The signal payload layout is as follows:</para>

<table frame="all" pgwide="0"><title>Satellite object</title>
<tgroup cols="2" align="left" colsep="1" rowsep="1">
<thead>
<row>
	<entry>Type</entry>
	<entry><para>Description</para></entry>
</row>
</thead>
<tbody>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Time (seconds since Unix epoch)</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_INT32</entry>
        <entry><para>mode</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Time uncertainty (seconds).</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Latitude in degrees.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Longitude in degrees.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Horizontal uncertainty in meters, 95% confidence.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Altitude in meters.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Altitude uncertainty in meters, 95% confidence.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Course in degrees from true north.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Course uncertainty in meters, 95% confidence.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Speed, meters per second.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Speed uncertainty in meters per second, 
	95% confidence.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Climb, meters per second.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_DOUBLE</entry>
        <entry><para>Climb uncertainty in meters per second, 
	95% confidence.</para></entry>
</row>
<row>
	<entry>DBUS_TYPE_STRING</entry>
        <entry><para>Device name</para></entry>
</row>
</tbody>
</tgroup>
</table>

</refsect1>

<refsect1 id='devices'><title>GPS DEVICE MANAGEMENT</title>

<para><application>gpsd</application> 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.</para>

<para>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.</para>

<para>When <application>gpsd</application> is properly installed along
with hotplug notifier scripts feeding it device-add commands over the
control socket, <application>gpsd</application> should require no
configuration or user action to find devices.</para>

<para>Sending SIGHUP to a running <application>gpsd</application>
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.</para>

<para>To point <application>gpsd</application> 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
<filename>/dev/foo</filename>. 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 <filename>/dev/foo</filename> from the search
list. send "-/dev/foo\n".</para>

<para>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.</para>

<para>To send a binary control string to a specified device, write to the
control socket a '&amp;', followed by the device name, followed by '=',
followed by the control string in paired hex digits.</para>

<para>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
<application>gpsd</application>'s device pool. An ERROR response to a
! command means the daemon did not recognize the devicename
specified.</para>

<para>The control socket is intended for use by hotplug scripts and
other device-discovery services.  This control channel is separate
from the public <application>gpsd</application> service port, and only
locally accessible, in order to prevent remote denial-of-service and
spoofing attacks.</para>

</refsect1>
<refsect1 id='accuracy'><title>ACCURACY</title>

<para>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 <emphasis>not</emphasis> rely on GPS altitude for
life-critical tasks such as landing an airplane.</para>

<para>These errors are intrinsic to the design and physics of the GPS
system.  <application>gpsd</application> does its internal
computations at sufficient accuracy that it will add no measurable
position error of its own.</para>

<para>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.</para>

<para>On a 4800bps connection, the time latency of fixes provided by
<application>gpsd</application> 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 <application>gpsd</application> 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.</para>

<para>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.</para>

<para>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 <application>gpsd</application> at
debug level 4 to see the chipset type and firmware revision
level.</para>

<para>There are exactly two circumstances under which
<application>gpsd</application> relies on the host-system
clock:</para>

<para>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.  <application>gpsd</application> 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.</para>

<para>If you are using an NMEA-only GPS (that is, not using SiRF or
Garmin or Zodiac binary mode), <application>gpsd</application> 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
<application>gpsd</application> 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.</para>
</refsect1>

<refsect1 id='ntp'><title>USE WITH NTP</title>

<para>gpsd can provide reference clock information to
<application>ntpd</application>, to keep the system clock synchronized
to the time provided by the GPS receiver.</para>

<para>On Linux, <application>gpsd</application> 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
<application>gpsd</application> will use that for extra
accuracy.</para>

<para>Detailed instructions for using GPSD to set up a high-quality
time service can be found among the documentation on the GPSD
website.</para>
</refsect1>

<refsect1 id='dbus'><title>USE WITH D-BUS</title>

<para>On operating systems that support D-BUS,
<application>gpsd</application> 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
<application>gpsd</application> source code to learn more.</para>

</refsect1>
<refsect1 id='security'><title>SECURITY AND PERMISSIONS ISSUES</title>

<para><application>gpsd</application>, 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.</para>

<para><application>gpsd</application> 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 &mdash; or, if that device doesn't exist,
to the group of <filename>/dev/ttyS0</filename>.</para>

<para>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 <application>gpsd</application>.
It ensures that any such compromises cannot be used for privilege
elevation to root.</para>

<para>The assumption behind <application>gpsd</application>'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,
<application>gpsd</application> may not be able to open GPS devices in
order to read them (such failures will be logged).</para>

<para>In order to fend off inadvertent denial-of-service attacks by
port scanners (not to mention deliberate ones),
<application>gpsd</application> 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,
<application>gpsd</application> 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).</para>

</refsect1>
<refsect1 id='limitations'><title>LIMITATIONS</title>

<para>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.</para>

<para><application>gpsd</application> 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.</para>

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

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

<para>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.</para>

<para>AIDVM decoding of types 16-17, 22-23, and 25-26 is unverified.</para>

<para>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.</para>

<para>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.</para>

<para>Devices meant to 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.</para>

<para>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
<application>gpsd</application> 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 <application>gpsd</application> 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).</para>

</refsect1>
<refsect1 id='files'><title>FILES</title>

<variablelist>
<varlistentry>
<term><filename>/dev/ttyS0</filename></term>
<listitem>
<para>Prototype TTY device. After startup,
<application>gpsd</application> sets its group ID to the owning group of this
device if no GPS device was specified on the command line does not
exist.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>/etc/gpsd/device-hook</filename></term>
<listitem>
<para>Optional file containing the device activation/deactivation script.
Note that while <filename>/etc/gpsd</filename> is the default system
configuration directory, it is possible to build the GPSD source code
with different assumptions.</para>
</listitem>
</varlistentry>
<!--
<varlistentry>
<term><filename>/usr/share/gpsd/dgpsip-servers</filename></term>
<listitem>
<para>A text file listing DGPSIP servers worldwide.  If no DGPSIP
server is specified at startup (via the -d option)
<application>gpsd</application> will look here to find the
nearest one.  Each line has three space-separated fields:
latitude (decimal degrees), longitude (decimal degrees) and
a server name (optionally followed by a colon and a port number).
Text following # on a line is ignored.  Blank lines are ignored.</para>
</listitem>
</varlistentry>
-->
</variablelist>

</refsect1>
<refsect1 id='standards'><title>APPLICABLE STANDARDS</title>

<para>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
<application>gpsd</application> 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.</para>

<para><application>gpsd</application> parses the following NMEA
sentences: 
RMC, GGA, GLL, GSA, GSV, VTG, ZDA, GBS, HDT, DBT.  
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.  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, and the PASHR sentences emitted by
some Ashtech GPSes.</para>

<para>Note that <application>gpsd</application> JSON returns pure decimal
degrees, not the hybrid degree/minute format described in the NMEA
standard.</para>

<para>Differential-GPS corrections are conveyed by the RTCM
protocols. The applicable standard for RTCM-104 V2 is <citetitle>RTCM
Recommended Standards for Differential GNSS (Global Navigation
Satellite) Service</citetitle> RTCM Paper 136-2001/SC 104-STD.  The
applicable standard for RTCM-104 V3 is <citetitle>RTCM Standard
10403.1 for Differential GNSS Services - Version 3</citetitle> 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".</para>

<para>AIS is defined by ITU Recommendation M.1371,
<citetitle>Technical Characteristics for a Universal Shipborne
Automatic Identification System Using Time Division Multiple
Access</citetitle>. The AIVDM/AIVDO format understood by this program
is defined by IEC-PAS 61162-100, <citetitle>Maritime navigation and
radiocommunication equipment and systems</citetitle>. A more accessible
description of both can be found at <citetitle>AIVDM/AIVDO Protocol
Decoding</citetitle>, on the references page of the GPSD project
website.</para>

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

<para>The API for PPS time service is specified by RFC 2783,
<citetitle>Pulse-Per-Second API for UNIX-like Operating Systems,
Version 1.0</citetitle></para>

</refsect1>
<refsect1 id='see_also'><title>SEE ALSO</title>
<para>
<citerefentry><refentrytitle>gpsdctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gps</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>libgps</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpsd_json</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>libgpsd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpsprof</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpsfake</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpsctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpscat</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
</para>
</refsect1>

<refsect1 id='maintainer'><title>AUTHORS</title>

<para>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
<email>esr@thyrsus.com</email>.</para>
</refsect1>

</refentry>