gpsd.xml

gpsd, a popular GPS daemon.

<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>

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

<para><application>gpsd</application> maintains an internal list of
GPS devices.  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>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>Your client may await a response, which will be a line beginning
with either "OK" or "ERROR".  An ERROR reponse 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 list. 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 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 to atmospheric signal lag than latitude/longitude, 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 from roughly 8 meters to
roughly 2 meters, provided you are within about 100mi (160km) of a
DGPS ground station.</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, 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>

</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.  This facility is
only available when the daemon is started from root.  If you're going
to use <application>gpsd</application> you probably want to run it
<option>-n</option> mode so the clock will be updated even when no
clients are active.</para>

<para>Note that deriving time from messages received from the GPS is
not as accurate as you might expect.  Messages are often delayed in
the receiver and on the link by several hundred milliseconds, and this
delay is not constant.  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. On OpenBSD <application>gpsd</application>
makes use of the nmea(4) line discipline and the tty(4) timestamping
facilities to export PPS time via the sensors framework. OpenBSD's ntpd
uses these sensors to adjust the hardware clock and frequency. To make
use of this feature, <application>gpsd</application> must be started
as root so it can activate the timestamping and line discipline; after
attempting to set up PPS, it will relinquish root privileges.</para>

<para>When <application>gpsd</application> receives a sentence with a
timestamp, it packages the received timestamp with current local time
and sends it to a shared-memory segment with an ID known to
<application>ntpd</application>, the network time synchronization
daemon.  If <application>ntpd</application> has been properly
configured to receive this message, it will be used to correct the
system clock.</para>

<para>Here is a sample <filename>ntp.conf</filename> configuration
stanza telling <application>ntpd</application> how to read the GPS
notfications:</para>

<programlisting>
server 127.127.28.0 minpoll 4 maxpoll 4
fudge 127.127.28.0 time1 0.420 refid GPS

server 127.127.28.1 minpoll 4 maxpoll 4 prefer
fudge 127.127.28.1 refid GPS1
</programlisting>

<para>The magic pseudo-IP address 127.127.28.0 identifies unit 0 of
the <application>ntpd</application> shared-memory driver; 127.127.28.1
identifies unit 1.  Unit 0 is used for message-decoded time and unit 1
for the (more accurate, when available) time derived from the PPS
synchronization pulse.  Splitting these notifications allows
<application>ntpd</application> to use its normal heuristics to weight
them.</para>

<para>With this configuration, <application>ntpd</application> will
read the timestamp posted by <application>gpsd</application> every 16
seconds and send it to unit 0.  The number after the parameter time1
is an offset in seconds.  You can use it to adjust out some of the
fixed delays in the system.  0.035 is a good starting value for the
Garmin GPS-18/USB, 0.420 for the Garmin GPS-18/LVC.</para>

<para>After restarting ntpd, a line similar to the one below should
appear in the output of the command "ntpq -p" (after allowing a couple
of minutes):</para>

<screen> 
remote           refid      st t when poll reach  delay    offset  jitter
=========================================================================
+SHM(0)          .GPS.      0 l   13   16  377    0.000    0.885   0.882
</screen>

<para>If you are running PPS then it will look like this:</para>

<screen> 
remote           refid      st t when poll reach  delay    offset  jitter
=========================================================================
-SHM(0)          .GPS.      0 l   13   16  377    0.000    0.885   0.882
*SHM(1)          .GPS1.     0 l   11   16  377    0.000   -0.059   0.006
</screen>

<para>When the value under "reach" remains zero, check that gpsd is
running; and some application is connected to it or the '-n' option was
used.  Make sure the receiver is locked on to at least one satellite,
and the receiver is in SiRF binary, Garmin binary or NMEA/PPS mode.  Plain
NMEA will also drive ntpd, but the accuracy as bad as one second.  When
the SHM(0) line does not appear at all, check the system logs for error
messages from ntpd.</para>

<para>When no other reference clocks appear in the NTP configuration,
the system clock will lock onto the GPS clock.  When you have previously
used <application>ntpd</application>, and other reference clocks appear
in your configuration, there may be a fixed offset between the GPS clock
and other clocks.  The <application>gpsd</application> developers would
like to receive information about the offsets observed by users for each
type of receiver.  Please send us the output of the "ntpq -p" command
and the make and type of receiver.</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> 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
userid as set by configure) 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), an 'O' 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>If your GPS uses a SiRF chipset at firmware level 231, and it is
after 31 May 2007, reported UTC time may be off by the difference
between 13 seconds and whatever leap-second correction is currently
applicable, from startup until complete subframe information is
received (normally about six seconds).  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>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>If you are using an NMEA-only GPS (that is, not using SiRF or
Garmin or Zodiac binary mode) and the GPS does not emit GPZDA at the
start of its update cycle (which most consumer-grade NMEA GPSes do
not) and it is after 2099, then the century part of the dates
<application>gpsd</application> delivers will be wrong.</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 owner of this
device if no GPS device was specified on the command line does not
exist.</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 standard is available on paper from
the <ulink url='http://www.nmea.org/pub/0183/'>National Marine
Electronics Association</ulink>, but is proprietary and expensive; the
maintainers of <application>gpsd</application> have made a point of
not looking at it.  The <ulink url="http://gpsd.berlios.de/">GPSD
website</ulink> 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.  It recognizes these
with either the normal GP talker-ID prefix, or with the II prefix
emitted by Seahawk Autohelm marine navigation systems, or with the IN
prefix emitted by some Garmin units.  It recognizes one vendor
extension, the PGRME emitted by some Garmin GPS models.</para>

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

</refsect1>
<refsect1 id='see_also'><title>SEE ALSO</title>
<para>
<citerefentry><refentrytitle>gps</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>libgps</refentrytitle><manvolnum>3</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>,
<citerefentry><refentrytitle>rtcm-104</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para>
</refsect1>

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

<para>Remco Treffcorn, Derrick Brashear, Russ Nelson, Eric S. Raymond,
Chris Kuethe.  This manual page by Eric S. Raymond
<email>[e-mail]</email>.  There is a project site at <ulink
url="http://gpsd.berlios.de/">here</ulink>.</para>
</refsect1>

</refentry>