gpsd.xml

gpsd, a popular GPS daemon.

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE refentry PUBLIC 
   "-//OASIS//DTD DocBook XML V4.1.2//EN"
   "docbook/docbookx.dtd" [
<!ENTITY gpsdsock         "/var/run/gpsd.sock">
]>
<refentry id='gpsd.8'>
<refmeta>
<refentrytitle>gpsd</refentrytitle>
<manvolnum>8</manvolnum>
<refmiscinfo class='date'>9 Aug 2004</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>GPS-devicename</replaceable></arg>
      <arg choice='opt'>-F <replaceable>control-socket</replaceable></arg>
      <!-- arg choice='opt'>-R
      <replaceable>rtcm-listener-port</replaceable></arg -->
      <arg choice='opt'>-S <replaceable>listener-port</replaceable></arg>
      <arg choice='opt'>-b </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='description'><title>DESCRIPTION</title>

<para><application>gpsd</application> is a monitor daemon that watches
a TCP/IP port (2947 by default), waiting for applications to request
information from GPSes or differential-GPS radios attached to the host machine.
Each GPS or radio is expected to be direct-connected to the host via a USB or
RS232C serial port.  The port 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
binary Rockwell protocol used by EarthMate and some other GPSes, the
TSIP binary protocol used by Trimble GPSes, or the binary SiRF
protocol used by SiRFstar chipsets, or the Garmin binary protocol used
by the USB version of the Garmin 18 and other Garmin USB GPSes, or the
binary protocol used by Evermore chipsets, or the extended NMEA used
by iTrax. <application>gpsd</application> effectively hides the
differences among these. It also knows about and uses commands that
tune the GPS for lower latency, decreased bandwidth usage, or
increased accuracy on the San Jose Navigation FV18, the Sony CXD2951,
the uBlox, and the Motorola OnCore GT+.  It can read heading and
attitude information from the True North Technologies Revolution 2X
Digital compass.</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-S104
data; this will improve user error 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.  <!-- If no server is
specified, <application>gpsd</application> will hunt for one.--> See
<xref linkend='accuracy'/> and <xref linkend='files'/> for
discussion.</para>

<!-- para><application>gpsd</application> may itself serve DGPSIP data
from an attached RTCM-104 source to other instances of
<application>gpsd</application> connecting on port 2101.</para -->

<para>The program accepts the following options:</para>
<variablelist remap='TP'>
<varlistentry>
<term>-f</term>
<listitem>
<para>Set a GPS device name.  This option is deprecated and may be
removed in a future version.  Each command-line argument will be
treated as a device to be probed for the presence of a GPS; that,
rather than the -f option, is the preferred way of specifying GPS
devices at startup.</para>
</listitem>
</varlistentry>
<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>-R</term>
<listitem><para>Set TCP/IP port on which to listen for DGPSIP clients
(default is 2101). The option -R 0 will disable serving DGPSIP
clients.</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>-d</term>
<listitem><para>This is a deprecated option which takes a differential-GPS 
source as argument.  See the description of argument interpretation
below.</para></listitem>
</varlistentry>
<varlistentry>
<term>-b</term>
<listitem><para>Broken-device-safety, otherwise known as read-only mode. Some
popular bluetooth and USB receivers lock up or become totally inaccessible when
probed or reconfigured. 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.</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.  The wait is a feature; many
serial GPSes go to a standby mode (not drawing power) before the host
machine asserts DTR, so waiting for the first actual request can save
valuable battery power on portable equipment.  This option combines
well with -D2 to enable monitoring of the GPS data stream.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>-N</term>
<listitem><para>Don't daemonize; run in foreground.  Also suppresses 
privilege-dropping.  This switch is mainly useful for debugging.
Its meaning may change in future versions.</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 name of a local serial device from
which the daemon may expect GPS data.</para>

<para>A data source name may also be a URL pointing to a specific
differential-GPS service (DGPSIP server or Ntrip broadcaster).If the
URL starts with "ntrip://" Ntrip will be used; if the URL starts with
"dgpsip://", DGPSIP will be used.  <application>Gpsd</application>
also defaults to DGPSIP if no protocol is defined.  For Ntrip services
that require authentication, a prefix of the form "username:password@"
can be added before the name of the Ntrip broadcaster.  If a suffix of
the service name begins with ":" it is interpreted as a port number,
overriding the default IANA-assigned port of 2101.  For Ntrip service
you also need to specify which stream to use; the stream is given in
the form "/streamname". So, an example DGPSIP URL could be
"dgpsip://dgpsip.example.com" and a Ntrip URL could be
"ntrip://foo:[e-mail]:80/example-stream". <!-- If this
option is not given, <application>gpsd</application> will hunt for a
DGPSIP server. --></para>



<para>Internally, the daemon maintains a device list holding the
pathnames of GPSes 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>At any given time, each client will be listening to only one of
the GPSes on the device list.  By default, a client's device is the
one that most recently shipped information to the daemon at the time
the client first requests GPS information (that is, issues any command
other than F, K, W=0 or R=0).</para>

<para>The request protocol for <application>gpsd</application> clients
is very simple.  Each request normally consists of a single ASCII
character followed by a newline.  Case of the request character is
ignored.  Each request returns a line of response text ended by a
CR/LF.  Requests and responses are as follows, with %f standing for a
decimal float numeral and %d for decimal integer numeral:</para>

<variablelist>
<varlistentry>
<term>a</term>
<listitem><para>The current altitude as "A=%f", meters above mean sea level.</para></listitem>
</varlistentry>

<varlistentry>
<term>b</term>
<listitem><para>The B command with no argument returns four fields
giving the parameters of the serial link to the GPS as "B=%d %d %c
%d"; baud rate, byte size, parity, (N, O or E for no parity, odd, or
even) and stop bits (1 or 2).  The command "B=%d" sets the baud rate,
not changing parity or stop bits; watch the response, because it is
possible for this to fail if the GPS does not support a
speed-switching command.  In case of failure, the daemon and GPS will
continue to communicate at the old speed.  The B= form is rejected
if more than one client is attached to the channel.</para></listitem>
</varlistentry>

<varlistentry>
<term>c</term>
<listitem>
<para>C with no following = asks the daemon to return the cycle time
of the attached GPS, if any.  If there is no attached device it will
return "C=?".</para>

<para>If the driver has the capability to change sampling rate the
command "C=%f" does so, setting a new cycle time in seconds. The "C="
form is rejected if more than one client is attached to the
channel.</para>

<para>If the driver has the capability to change sampling rate, this
command always returns "C=%f %f" giving the current cycle time in
seconds and the minimum possible cycle time at the current baud rate.
If the driver does not have the capability to change sampling rate,
this returns, as "C=%f", the cycle time in seconds only.</para>

<para>Either number may be fractional, indicating a GPS cycle shorter
than a second; however, if >1 the cycle time must be a whole number. Also
note that relatively few GPSes have the ability to set sub-second
cycle times; consult your hardware protocol description to make sure
this works.</para>
 
<para>This command will return "C=?" at start of session, before the
first full packet has been received from the GPS, because the GPS type is
not yet known. To set up conditions for a real answer, issue it after
some command that reads position/velocity/time information from the
device.</para></listitem>

</varlistentry>

<varlistentry>
<term>d</term>
<listitem><para>Returns the UTC time in the ISO 8601 format,
"D=yyyy-mm-ddThh:nmm:ss.ssZ". Digits of precision in the fractional-seconds
part will vary and may be absent.</para></listitem>
</varlistentry>

<varlistentry>
<term>e</term>
<listitem>
<para>Returns "E=%f %f %f": three estimated position errors in meters
&mdash; total, horizontal, and vertical (95% confidence
level).  Note: many GPSes do not supply these numbers.  When the
GPS does not supply them, <application>gpsd</application> computes
them from satellite DOP using fixed figures for expected non-DGPS 
and DGPS range errors in meters.  A value of '?' for any of these
numbers should be taken to mean that component of DOP is not available.
See also the 'q' command.</para>
</listitem>
</varlistentry>

<varlistentry>
<term>f</term>
<listitem><para>Gets or sets the active GPS device name. The bare
command 'f' requests a response containing 'F=' followed by the name
of the active GPS device.  The other form of the command is 'f=', in
which case all following printable characters up to but not including
the next CR/LF are interpreted as the name of a trial GPS device. If
the trial device is in <application>gpsd</application>'s device list,
it is opened and read to see if a GPS can be found there.  If it can,
the trial device becomes the active device for this client.</para>

<para>The 'f=' command may fail if the specified device name is not on
the daemon's device list.  This device list is initialized with the
paths given on the command line, if any were specified.  For security
reasons, ordinary clients cannot change this device list; instead,
this must be done via the daemon's local control socket declared with
the -F option.</para>

<para>Once an 'f=' command succeeds, the client is tied to the
specified device until the client disconnects.</para>

<para>Whether the command is 'f' or 'f=' or not, and whether it succeeds
or not, the response always lists the name of the client's device.</para>

<para>(At protocol level 1, the F command failed if more than one
client was attached, and multiple devices were not supported.)</para>
</listitem>
</varlistentry>

<varlistentry>
<term>g</term>
<listitem>
<para>With =, accepts a single argument which may have either of the
values 'gps' or 'rtcm104', with case ignored.  This specifies the 
type of information the client wants and forces a device assignment.  
Without =, forces a device assignment but doesn't force the type.
This command is optional; if it is not given, the client will be
bound to whatever available device the daemon finds first.</para>

<para>This command returns either '?' if no device of the specified
type(s) could be assigned, otherwise a string ('GPS' or 'RTCM104')
identifying the kind of information the attached device returns.</para>
</listitem>
</varlistentry>

<!--
<varlistentry>
<term>h</term>
<listitem>
<para>Heading, "H=%f" in degrees from true north.  Only available from
True North Technologies or comparable digital compass devices (c.f. 't')</para>