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<ul>
<li><a href='#verify'>How can I verify operation of a new GPS?</a></li>
<li><a href='#bug-reporting'>How do I report bugs in GPSD?</a></li>
<li><a href='#startup'>Why does getting a fix take so long after powerup?</a></li>
<li><a href='#timelag'>Why does GPS time lag wall time by 11-15 seconds?</a></li>
<li><a href='#timelag2'>Why is my GPS time off by exactly one second?</a></li>
<li><a href='#speed'>Why does my receiver report wildly fluctuating speed?</a></li>
<li><a href='#gpsdrive'>Why do I get implausibly low speeds when using gpsdrive?</a></li>
<li><a href='#kismet'>Why do I get no results when I try to use <code>gpsd</code> with Kismet?</a></li>
<li><a href='#bluetooth'>Why do I have to restart <code>gpsd</code> whenever I power-cycle my Bluetooth device?</a></li>
<li><a href='#lockup'>My <code>gpsd</code> sometimes stops responding overnight</a></li>
<li><a href='#why_not_parse_nmea'>Why use the <code>gpsd</code> protocol rather than parsing raw NMEA?</a></li>
<li><a href='#interfacing'>How should I interface my application with <code>gpsd</code>?</a></li>
<li><a href='#agps'>Can GPSD use Assisted GPS data from cellphone networks?</a></li>
<li><a href='#accuracy'>How can I improve fix accuracy from my GPS?</a></li>
<li><a href='#tiocmwait'>PPS is failing to work under Linux</a></li>
<li><a href='#time'>How can I improve time reference accuracy from my GPS?</a></li>
<li><a href='#sleep'>Why does my GPS get lost when I sleep/wake my laptop?</a></li>
<li><a href='#web'>How do I get gpsd data into a web page?</a></li>
<li><a href='#baud'>Why is there no option to fix baud rate?</a></li>
<li><a href='#willitwork'>Will this GPS work? It's not on the hardware list.</a></li>
<li><a href='#conflict'>Why does GPSD open non-GPS USB devices?</a></li>
<li><a href='#efficiency'>What is gpsd's CPU and power overhead?</a></li>
<li><a href='#usblockup'>My USB port becomes unavailable after gpsd uses it.</a></li>
<li><a href='#nmea2000'>How do I configure GPSD to use NMEA2000?</a></li>
<li><a href='#android'>Android reports that gpsd is draining my battery. Help!</a></li>
</ul>
<h1 id='verify'>How can I verify operation of a new GPS?</h1>
<p>1. First, check that the GPS has power. If it is a USB device, it
needs to be cabled to a USB port to have power. All Bluetooth GPSes
and some serial GPSes are powered by an on-board battery; check that
the battery is present and charged. The GPS may have an on-off switch
which needs to be in the 'on' position.</p>
<p>2. Most GPSes have a power-on LED; it should be continuously on
or blinking once a second. If it is continuously off, your GPS is dead
or disconnected. Check that it has power.</p>
<p>3. For anything other than a serial (RS232) device, there should be
a discovery utility that allows you to check that the device is connected.</p>
<p>3a. For a USB device, run <code>"lsusb"</code> before and after
connecting your GPS; after, you should see an additional line
indicating a new device. Expect the new line to describe a
serial-to-USB adapter chip, often (but not always) the Prolific
Technology PL2303. Then run "dmesg", looking for a message indicating
a new USB device of that kind and giving you the device path -
<code>/dev/ttyUSBn</code> for some number n.</p>
<p>3b. For a Bluetooth device, see our <a href="bt.html">Bluetooth
instructions</a>.</p>
<p>4. If you have installed a GPSD binary package on a Linux system and
are using a USB GPS, you should not need to start gpsd manually,
because the hotplug system will have done it for you. You should be
able to start a test client (such as <code>cgps</code> or <code>xgps</code>)
and watch it report fixes.</p>
<p>5. If your test client fails to run, a good test to try is to run
<code>gpsmon</code> on the device (give it the device path as an
argument). If yours reports no data at all, you probably have some
low-level system problem with serial or USB that you'll need to fix
before <code>gpsd</code> will operate.</p>
<p>6. USB GPSes actually emulate RS232 serial using converter chips.
Under Linux, the usbserial kernel module must be loaded for correct
operation of this class of device. Normally this module load should
happen automatically when the device activates, but if you don't
receive data check for it with <b>lsmod(1)</b>.</p>
<p>On Linux systems with module autoloading disabled or misconfigured,
it is possible you may need to load the module manually with a command
such as <code>sudo modprobe usbserial vendor=0×1a86
product=0×7523</code>. Do not copy those hex numbers slavishly, they
are examples. To get the right numbers, you will need to dig up the
vendor and product ID of your USB-serial converter device.</p>
<p>For more detailed troubleshooting instructions, <a
href="troubleshooting.html">see the Troubleshooting Guide</a>.</p>
<h1 id='bug-reporting'>How do I report bugs in GPSD?</h1>
<blockquote>
<p>When you have a problem with gpsd, here are some steps you can take to
help get your bug resolved as quickly as possible.</p>
<h3>1. Read this whole FAQ first</h3>
<p>First, read this whole FAQ before reporting apparent misbehavior as a
bug. You may find a solution here.</p>
<h3>2. Make sure it's not a problem with your toolchain or OS</h3>
<p>See our page on <a href='upstream-bugs.html'>upstream bugs</a>.</p>
<h3>3. Make sure it's not a problem in your client software</h3>
<p>Make sure it is a real gpsd bug and not a problem with
your client software. A good way to do this is to run your client and
the gpsd test client (xgps) side by side. If xgps seems to report
good numbers but your client does not, you have a client problem.
If xgps reports the same sort of bad numbers as your client, you
have a real <code>gpsd</code> bug.</p>
<h3>4. Check the latest version of <code>gpsd</code> for the bug.</h3>
<p>If you are using an old version of <code>gpsd</code>, it is
possible your bug has already been fixed. Download the latest public
version from the <a href='@MAINPAGE@'>project page</a> and test it.
To be really helpful, check out the <a href='@BROWSEREPO@'>git head</a>
and test that. We don't mind getting bug reports that say "I saw
version foo had the following bug, but you've fixed it."</p>
<h3>5. Capture a log that triggers the problem</h3>
<p>If we can reproduce your gpsd problem, we can usually fix it very
rapidly. If we can't reproduce it, you might get lucky or you might
not — and we try hard, but all too often the result is 'not'.</p>
<p>Therefore the most important step you can take is to capture a log
of some receiver output that reproduces the bug; <code>gpscat</code> will
help you.</p>
<h3>6. Trim the capture log that reproduces the problem</h3>
<p>Your next step should be to feed the log you just captured to a
<code>gpsd</code> instance through <code>gpsfake</code> to verify that
the log does in fact reproduce the bug.</p>
<p>Once you have the log, trim it to the smallest span of data that
reproduces the bug. A systematic way to do this is to cut the log in
half at the middle and test each half. If one half doesn't reproduce
the bug but the other does, throw away the half that doesn't. Repeat
this procedure on each half that tickles the bug until you can't make
it any smaller. Then send us that.</p>
<p>If possible, use the -l option of gpsfake to pin down the sentence
or packet that produces the bug, and tell us that.</p>
<h3>7. Look at <code>gpsd</code> log output to see if it gives you a clue</h3>
<p>You may get a better handle on your problem by running gpsd in
foreground (-N option) with the -D option set to a high level (-D 5 is
often good). If the transcript has anything that looks like a clue
in it, send it along with your bug report. When in doubt about
whether it holds a clue, send it.</p>
<p>One of the things this should tell you, if the chip reports it at
all, is the firmware version. You will want that for your report.</p>
<h3 id="logformat">8. Annotate the capture log and send us a copy</h3>
<p>We'll describe the annotation steps here for completeness, but the
easiest way to do this is with <a href="@WEBFORM@">our web form</a>.</p>
<p>A logfile should consist of an identifying header followed by a
straight unencoded dump of receiver data, whether NMEA or binary. The
header should consist of text lines beginning with # and ending with LF.
Here is the beginning of one log file I already have:</p>
<pre>
# Name: Magellan eXplorist 210
# Chipset: unknown
# Submitter: "Paul B van den Berg" <paulberg@wanadoo.nl>
# Date: 2006-05-26
# Location: Groningen, NL, 53.2N 6.6E
#
# mode V2.1 GSA
# Lines up to but not including the first GPGLL are
# `cat /dev/ttyACM0` at startup
# Following lines are
# `cat /dev/ttyACM0` stationary
$GPGSV,3,1,00,,,,,,,,,,,,,,,,*7B
$GPGSV,3,2,00,,,,,,,,,,,,,,,,*78
$GPGSV,3,3,00,,,,,,,,,,,,,,,,*79
$PMGNST,01.75,3,F,816,11.1,+00000,20*5E
$GPGSV,3,1,00,,,,,,,,,,,,,,,,*7B
$GPGSV,3,2,00,,,,,,,,,,,,,,,,*78
$GPGSV,3,3,00,,,,,,,,,,,,,,,,*79
$PMGNST,01.75,3,F,816,11.1,+00000,20*5E
$GPGSV,3,1,00,,,,,,,,,,,,,,,,*7B
$GPGSV,3,2,00,,,,,,,,,,,,,,,,*78
$GPGSV,3,3,00,,,,,,,,,,,,,,,,*79
$PMGNST,01.75,3,F,822,11.2,+00000,20*5A
$GPGSV,3,1,00,,,,,,,,,,,,,,,,*7B
$GPGSV,3,2,00,,,,,,,,,,,,,,,,*78
$GPGSV,3,3,00,,,,,,,,,,,,,,,,*79
$PMGNST,01.75,3,F,822,11.2,+00000,20*5A
$GPGSV,3,1,12,09,76,287,,17,38,073,36,26,34,163,,05,33,230,*72
$GPGSV,3,2,12,29,27,161,,18,24,256,,22,24,299,,28,11,055,*73
$GPGSV,3,3,12,14,08,319,,11,03,017,,30,02,232,,24,00,084,*71
$PMGNST,01.75,3,F,822,11.2,-00673,20*5E
$GPGLL,5313.2228,N,00634.4228,E,200619.295,A*35
$GPGGA,200619.30,5313.2228,N,00634.4228,E,1,05,2.6,00000,M,,,,*2C
$GPRMC,200619.30,A,5313.2228,N,00634.4228,E,00.0,000.0,200506,00,W*59
$GPGSA,A,3,26,05,22,09,18,,,,,,,,05.1,02.6,04.4*03
$GPGSV,3,1,10,09,78,288,39,17,38,071,,05,34,230,45,26,33,163,39*77
$GPGSV,3,2,10,29,26,162,,18,24,255,42,22,24,298,44,28,10,056,*75
$GPGSV,3,3,10,14,09,319,,11,03,016,,136,27,157,,124,28,162,*71
</pre>
<p>The way to fill in the Name, Submitter, and Date
headers should be pretty obvious.</p>
<p>Chipset should include the name and (if possible) model and/or
firmware revision of the chipset in the GPS.</p>
<p>Please also include a Location header giving your city,
state/province, country code, and a rough latitude/longitude.</p>
<p>A log file is most useful when it contains (a) some sentences
generated when the GPS has no fix, (b) some sentences representing
a fix with the GPS stationary, and (c) some sentences representing
a fix with the GPS moving.</p>
<p>If you have notes or comments on the logfile or the GPS, or any
additional information you think might be helpful, add them as
additional # comments (not containing a colon) after these headers.
The test machinery that interprets the headers will ignore these and
any empty comment lines.</p>
<h3>9. If it's a dual-mode GPS, see if the problem reproduces in NMEA mode</h3>
<p>If you're using a SiRF, Evermore, iTalk or u-blox GPS in binary mode
(which you can tell from the -D 4 output), switch back to NMEA mode
using the N command (or a vendor-provided tool) and see if the bug is
still reproducible.</p>
<h3>10. If your bug core-dumps gpsd, send us a stack trace.</h3>
<p>Though it happens seldom (we've had only 2 such reports since about
mid-2005), badly-formed input from a device with poor standards
compliance has been known to core-dump gpsd. If your gpsd has
core-dumped, try to use gdb or whatever your local symbolic debugger
is to generate a stack trace ("bt full") of the crash, and send us
that.</p>
<p>Very occasionally we have also received reports of core dumps in
gpsfake and gpson. A stack trace is also useful in those cases.</p>
<p>A good technique in all such cases is to try to reproduce the bug
by feeding a log with the bad packet in it to gpsdecode. This utility
exercises the same packet decoders as gpsd/gpsfake/gpsmon but without
involving nearly as much other code. If you can send a test log that
crashes gpsdecode, you can expect the bug to be fixed very quickly.</p>
<h3>11. Try to determine what release introduced the bug</h3>
<p>If you have upgraded from a previous version of <code>gpsd</code>,
and the upgrade broke something that was working previously, the
most useful thing you can do is pin down the release in which the
bug was introduced.</p>
<p>How efficiently you can do this depends on whether or not you have
a client for the git version control system. If you don't, all
you can do is download and test named releases. If you do, you can
pin down the exact change that introduced the bug. The latter is
far more helpful to us and will get your bug fixed faster, so we'll
describe that procedure here.</p>
<ol>
<li><p>Follow <a href='@CLONEREPO@'>these instructions</a> to clone
a copy of the source repository.</p></li>
<li><p>Use <a
href="http://www.kernel.org/pub/software/scm/git/docs/git-bisect.html">git
bisect</a> to locate the exact revision that introduced your bug.
This will happen very quickly, as the number of tests required will be
the log to the base 2 of the number of revisions in your original
span. Even if there are (say) 500 revisions in the span you should
only require 9 tests to nail down the exact change in
question.</p></li>
</ol>
<h3>12. Include the vendor, mode, and firmware version in your report.</h3>
<p>Always include with your bug report the receiver vendor and model. Try
to include the firmware version as well. This should be in your xgps
display if your device makes it available; in a form field before
2.35, or as the window title in 2.35 and later. If the ID string is
too long to fit, let the daemon run for a few minutes and issue an "I"
command to it. Alternatively, running the daemon at -D 4 may reveal
the version.</p>
<h3>13. Report it on the GPSD bugtracker</h3>
<p>There is a <a
href="@BUGTRACKER@">GPSD bug tracker</a>.
Use that. If your bug narrative does not fit the tracker template well,
it's acceptable to send an email report to the gpsd-dev list.</p>
</blockquote>
<h1 id="startup">Why does getting a fix take so long after powerup?</h1>
<p>On a Linux machine, the <code>gpsd</code> daemon normally takes
between 0.1 and 0.6 seconds to handshake with your hardware. After
that you will receive GPS reports within a second of when the sensor
issues them. GPSD itself adds <a href="performance.html">almost no
measurable latency</a>, but RS-232 transmission time to
<code>gpsd</code> can be more significant; you can cut this time by
increasing the baud rate.</p>
<p>Longer handshake delays have been reported from other platforms.
Under OpenBSD, time to handshake with some binary GPSes (including
SiRFs) can be up to two minutes. This seems to reflect some bad
interaction between the autobauding code in <code>gpsd</code> and the
operating system's tty layer; when <code>gpsd</code> is compiled to
use a fixed port speed, handshake times drop to a fraction of a
second.</p>
<p>If you are starting a GPS for the first time, or after it has been
powered off for more than two weeks, this is a 'cold start'; it needs
to get a new satellite ephemeris to do its job. The satellites
broadcast this information very slowly (at 50bps) on a fixed schedule,
and it can take up to 20 minutes.</p>
<p>Warm start on a modern GPS with a good skyview (4 or more sats
visible) normally takes about 30 seconds. (Vendor spec sheets fib by
quoting this time only, leaving out the cold-start lag to fetch
ephemeris.) If it's taking longer, the first thing to suspect is that
your skyview is poor. Especially if you're indoors.</p>
<p>The best advice is: go outside and be patient for a few minutes.</p>
<h1 id="timelag">Why does GPS time lag wall time by 11-15 seconds?</h1>
<p>Your GPS may have dropped its leap-second offset. You can tell you
have this problem if your sentence timestamps look wrong at startup.
Wait 20 minutes or so; the lag should go away.</p>
<p>GPS satellites broadcast time using a clock without a leap-second
correction. They broadcast a leap-second correction once each complete
reporting cycle along with the satellite ephemerides; it's up to the
GPS firmware to add that correction to the time it puts in reports.
If your GPS has forgotten the current correction, you'll have to wait
until the next ephemerides message for it.</p>
<p>GPSes are supposed to retain the leap-second correction along with
the last fix in NVRAM when they power down, but we've observed that
many seem prone to occasionally drop this information. All you can do
is wait for the problem to resolve itself.</p>
<p>SiRF-based GPSes have a more specific timelag problem. 4800 is just
a bit too slow for SiRF binary at full flood; the device can actually
fail to ship all its reports before the next once-per-second fix,
producing an accumulating stall. The symptom of this is sentence times that
look right at startup but gradually fall behind clock time. To fix
this, bump your speed to 9600 or higher.</p>
<h1 id="timelag2">Why is my GPS time off by exactly one second?</h1>
<p>This can happen if (a) there has been a recent <a
href="ftp://maia.usno.navy.mil/ser7/tai-utc.dat">leap-second
adjustment</a>, (b) you have a version of GPSD that was built before
the adjustment, and (c) your GPS doesn't ship the current leap-second
offset in a form GPSD can see. If this happens, GPSD will fall back
to a compiled-in default that is off by one</p>
<p>Some SiRFs have a particularly insidious version of this. They are
supposed to ship a MID52 sentence (which GPSD knows how to interpret)
containing the current leap-second offset. But there is at least one
firmware revision that ships a damaged version of MID52 with a garbled
start sequence or a zero length field. This GPSD cannot handle.</p>
<p>The bad revision is 2.3.2-GSW2-2.05.024-C1Prod1.1; there may be
others. Suspect this if you have persistent off-by-one errors. If you
are able to identify other bad firmeare versions, please let us
know about it.</p>
<h1 id='speed'>Why does my receiver report wildly fluctuating speed?</h1>
<p>If your problem is wildly fluctuating speed reports on a SiRF,
switching on static navigation mode using <code>gpsmon</code>. Static
navigation mode will freeze your position if your speed is below 1.2
m/s for three seconds, and will begin updating your position again
when speed exceeds 1.4 m/s. This prevents multipath, weak signals, or
poor constellation geometry from dragging your solutions around too
much. Other receivers may suffer the same problem and may have a
similar solution.</p>
<h1 id='gpsdrive'>Why do I get implausibly low speeds when using gpsdrive?</h1>
<p>This is a gpsdrive bug, as you can verify by running
<code>xgps</code> alongside it.</p>
<h1 id='kismet'>Why do I get no results when I try to use <code>gpsd</code> with Kismet?</h1>
<p>Your Kismet configuration has to include the setting
"gps=true" This is a surprisingly easy detail to forget.</p>
<h1 id='bluetooth'>Why do I have to restart <code>gpsd</code> whenever I power-cycle my Bluetooth device?</h1>
<p>The Bluetooth stack returns 0 for a read from a missing device,
rather than -1, and doesn't set errno. This is wrong and needs to be
fixed at OS level.</p>
<h1 id='lockup'>My <code>gpsd</code> sometimes stops responding overnight</h1>
<p>At one point in the development of <code>gpsd</code> we got a
report of the daemon ceasing to respond to queries when run for
more than a day or so; the user, quite reasonably, suspected some sort
of resource leak in the daemon. On the other hand, other users reported
good operation over much longer periods with the same version of
the software. That suggests a bug at the level of the user's operating
system or local site configuration.</p>
<p>Nevertheless, the possibility of a resource-leak bug alarmed us
enough that after 2.26 one of us (ESR) built an entire test framework
for auditing the code's dynamic behavior and used it to apply <a
href="http://valgrind.org">Valgrind</a>. You can look at the
resulting script, valgrind-audit, in the source distribution. This
turned up a couple of minor leaks, but nothing sufficient to explain
the report.</p>
<p>One of our senior developers, Rob Janssen, has seen
<code>gpsd</code> interact badly with overnight backups, pushing the
system load average through the roof. He says: "when you copy many
gigabytes of data from disk to disk, the [Linux] kernel's buffer
management goes completely haywire. [...] I think this is caused both
by allocation of many buffers for reading files, and by accumulation
of many dirty buffers that still have to be written. At some point,
programs like gpsd (but also all interactive programs and the X
display manager) come to a complete standstill while the system is
swapping like mad."</p>
<p>If Rob's analysis is correct, <code>gpsd</code> is a canary in a
coal mine. If your <code>gpsd</code> locks up after a long period of
operation, you should look at your logs and see if you can connect the
point at which it stopped responding to some kind of resource crisis
brought on by lots of I/O activity.</p>
<p>Another thing to try is running <code>gpsd</code> under Valgrind overnight
and seeing if it reports any leaks.</p>
<h1 id='why_not_parse_nmea'>Why use the <code>gpsd</code> protocol rather than parsing raw NMEA?</h1>
<p>Some applications that use <code>gpsd</code> start raw mode
and parse the NMEA directly. This is not a good idea.</p>
<p>One problem with raw mode is that NMEA is a poorly specified
standard. There are, for example, two different and incompatible
variants of GPVTG. Another issue is that implementations vary as to
whether they leave fields they don't supply empty or fill them in with
a special value such as 0.0. Interpretation of the different NMEA
status fields is a black art.</p>
<p>It is all too easy to write an NMEA parser that works well on one
variant but breaks on another, delivering subtly incorrect results or
even crashing your application. Because <code>gpsd</code> specializes
in the job, we collect knowledge on all variants and do parsing that
is much less likely to get tripped up.</p>
<p>Another issue is that some of the reports your application would
like to have are not generated by all GPSes. Estimated position error
in meters is the most obvious example; climb/sink is another. When a GPS
doesn't supply these, <code>gpsd</code> can fill them in using the
same sorts of computation that more capable GPSes use.</p>
<h1 id='interfacing'>How should I interface my application with <code>gpsd</code>?</h1>
<p>The <code>gpsd</code> package provides two ways for C code to get
data from a GPS or AIS receiver. Both go through the libgps.a library,
which supports two sets of entry points. The <a
href="libgpsd.html">low-level interface</a> talks directly to the GPS.
The <a href="libgps.html">high-level interface</a> communicates with
an instance of <code>gpsd</code>, which uses its own copy of libgps.a
to talk to the device.</p>
<p>A third way would be to open a socket to <code>gpsd</code> and
interpret <code>gpsd</code> protocol or raw NMEA in your application.
Before 2.0, all <code>gpsd</code>-aware applications had to do this
because libgps.a didn't exist. Now that it does, the exercise is
rather pointless. Using libgps.a will probably simplify your code a
lot.</p>
<p>You will almost always want to use the high-level interface and go
through the daemon; among other things, this means more than one
application will be able to query the GPS without causing confusion.
The only exception will be in very space-constrained single-user
scenarios, perhaps on embedded systems or PDAs. On those it may be
appropriate to use the low-level interface directly, probably with a
build from source that conditions out all but one of the drivers.</p>
<p>For Python programmers, there is a gps.py module the high-level
interface. It exports a class that encapsulates a GPS session.</p>
<h1 id='agps'>Can GPSD use Assisted GPS data from cellphone networks?</h1>
<p>Sadly, no. There are both hardware and software barriers to this.</p>
<p>Using AGPS would requires picking up and interpreting information
broadcast by cellphone towers, so it would at minimum need another
receiver (separate from the GPS mouse) operating in those frequency
bands. Of course, a cellphone has one of those built in. Standalone
GPS sensors don't.</p>
<p>There's another level of the problem, too. Supposing we had the
right sort of receiver, AGPS systems are carrier-dependent. As far as
we know there aren't any published standards for the format of the
corrections. So even if we had the signals, GPSD couldn't know what to
do with them.</p>
<h1 id='accuracy'>How can I improve fix accuracy from my GPS?</h1>
<p>Use an antenna, and place the sensor (and/or antenna) properly.</p>
<p>A real antenna can help, especially if you're using PPS as a time
reference. It should be particularly helpful for reducing timing
jitter.</p>
<p>One common error is to place the GPS or antenna as high as
possible. This will increase <a
href="http://en.wikipedia.org/wiki/Global_Positioning_System#Multipath_effects">multipath
effects</a> due to signal bounce from the ground or water, which can
cause the GPS to mistake its position and the time signal. The
correct location for a boat GPS antenna is on the gunwale rail or
pushpit rail, close to the water and as far from the mast as possible
(to reduce signal bounce from the mast). If you're outside or in a
fixed location, put the GPS antenna as far from buildings as possible,
and on the ground. If you're in a car, don't put the GPS antenna on
the roof, put it on the towbar or some similar location.</p>
<p>If you're driving in a heavily built up area, you're going to get
signal bounce off buildings and reduced accuracy. That's just how
the physics works. Note, however, that as your velocity goes up it
becomes easier for the convergence filters in your GPS to spot
and discard delayed signal, so multipath effects are proportionally
less important in fast-moving vehicles.</p>
<p>If you're using <code>gpsd</code> with software that plots your
position on a map, and you seem to be getting latitude/longitude that
is at a fixed offset from reality, it is possible the base datum of
the map is something other than the WGS84 GPS uses. A
frequently-occurring case of this is older maps in the United States
based on NAD27 (e.g., USGS topo maps); you may see a displacement of
as much as 100-150m with respect to WGS84. While modern datums (e.g.,
NAD83) are almost all very close to WGS84, typically each area of
world has an older datum that only agrees at the 100m level.</p>
<h1 id="tiocmwait">PPS is failing to work under Linux</h1>
<p>You may have tripped over an <a
href="upstream-bugs.html#tocmwait">obscure kernel bug</a>. There is a
workaround; use stty to preset your port baud rate to the correct
value for the device, so the autobaud loop never runs and
ioctl(TIOCMWAIT) is never called.</p>
<h1 id='time'>How can I improve time reference accuracy from my GPS?</h1>
<p>All the measures you'd take to improve <a href="#accuracy">fix
accuracy</a> will help. Time referencing at accuracies below
0.01sec has its own set of issues related to latency in your
sensor and computer.</p>
<p>In particular, USB transport is not suited for anything
time-related that requires sub-millisecond accuracy. If a
USB-connected GPS unit wants to tell the computer it's 12:23:45, it
will have to wait for the computer to poll it. This polling happens
1000 times per second, introducing variable latency averaging a
half-millisecond. Furthermore, the USB chip might run at anything
close to 1000 Hz. If it runs at 1000.1 Hz, every 10th second the next
time-report will come 1 whole millisecond earlier. This will have NTP
observing something like a 1ms-high sawtooth in the time-reports.</p>
<p>That being said, we've observed excellent 1 millisecond accuracy
timing performance over USB with a uBlox6+PL2303 combination featuring
1PPS wired to DCD_IN. NaviSys sells this as the GR601W, aka the
Macx-1.</p>
<p>For accurate time reference, use a PPS line over RS232 triggering an
interrupt. Serial bus interrupt latencies on modern hardware on the
order of 10 microseconds, roughly a hundred-fold improvement over
USB.</p>
<p>Don't confuse PPS with conventional, non-PPS data transport over
RS232; the latency on that is much higher. At one character per ten
bits (counting framing at stopbits) a 9600-bps serial link introduces
about a millisecond of latency <em>per character</em>; furthermore,
the Linux kernel will normally delay delivery of characters to your
application until the next timer tick, about every 4 milliseconds in
modern kernels. Both USB and RS232 will incur that approximately
5ms-per-char latency overhead. You'll have to deal with this
latency even on chips like the Venus 6 that claim the beginning of
their reporting burst is synced to PPS.</p>
<p>GPS chips that do not sync their start of reporting to PPS can have
long-period jitters in the latency between 1PPS and the start of
reporting. SiRFs, in particular, wobble by 100-170 milliseconds.</p>
<h1 id='sleep'>Why does my GPS get lost when I sleep/wake my laptop?</h1>
<p>This is not a GPSD problem, but a result of the way Linux handles
USB serial devices. In a default Linux configuration, USB serial
device name do not depend on which physical port you plug the
USB/serial adaptor, but on what order you plug devices in: 1st device
gets /dev/ttyUSB0, 2nd gets /dev/ttyUSB1, etc....</p>
<p>This collides with what happens during a suspend/resume. If you
suspends while <code>gpsd</code> has a device active, it will hold the
device open while your laptop is asleep - but, meanwhile, the suspend
logic is shutting down hotpluggable devices to be recreated at
resume time. On resume, Linux will see that the old device is open
<em>and recreate one with a different name</em>, leaving <code>gpsd</code>
looking at a bad file descriptor.</p>
<p>There is a solution to this problem: create a stable gps-usb device
that is actually a symlink which gets modified by hotplug events, and
give <code>gpsd</code> that device when you invoke it. You'll need <a
href="70-persistent-usb-gps.rules">these replacement udev rules</a>,
and the experience required to patch them so the vendor ID in the last
one matches your GPS hardware (look in your lsusb output).</p>
<h1 id='web'>How do I get gpsd data into a web page?</h1>
<p>The <code>gpsd</code> source-code distribution now includes a PHP
script that makes this easy. The script shows current fix data, a
satellite view, and can even incorporate a Google Maps display
if you have a Google API key. Look at the end of the file INSTALL for
instructions.</p>
<p>Another way is to use a perl CGI script that leverages
Net::GPSD like this...</p>
<pre><code>
#!/usr/bin/perl -w
use strict;
use Net::GPSD;
my $g=new Net::GPSD;
my $p=$g->get;
print qq{
<?xml version='1.0'?>
<Report xmlns:gml="http://www.opengis.net/gml">
<items>
<Item>
<ID>35643563245</ID>
<position>
<gml:Point srsDimension="2"
srsName="urn:ogc:def:crs:EPSG:6.6:4326">
<gml:pos>}. $p->lat. " ". $p->lon. q{</gml:pos>
</gml:Point>
</position>
</Item>
</items>
</Report>
};
</code>
</code></pre>
<p>This will return something like:</p>
<pre><code>
<?xml version='1.0'?>
<Report xmlns:gml="http://www.opengis.net/gml">
<items>
<Item>
<ID>35643563245</ID>
<position>
<gml:Point srsDimension="2" srsName="urn:ogc:def:crs:EPSG:6.6:4326">
<gml:pos>38.865960 -77.108624</gml:pos>
</gml:Point>
</position>
</Item>
</items>
</Report>
</code></pre>
<h1 id='baud'>Why is there no option to fix baud rate?</h1>
<p>There is no option to fix baud rate because <code>gpsd</code> is
designed to (a) be autoconfiguring, and (b) handle multiple devices.
There are some more philosophical reasons as well; read the
<a href="hacking.html">Hacker's Guide</a> for discussion.</p>
<p>Unfortunately, this causes problems with some devices (notably
Bluetooth GPSes) that are designed to operate at fixed baud rates.
Some of these go catatonic if you try to set the baud rate, which is
why we have a -b option that prevents <code>gpsd</code> from trying to
configure the GPSes it talks to.</p>
<p>On Linux systems, there's a trick you can play to simulate fixing the
baud rate. It utilizes the fact that under Linux, setting baud rate 0
is interpreted as "use the hardware port's existing baud rate without
change". The autoconfiguring hunt loop in <code>gpsd</code> always starts
with this behavior.</p>
<p>Accordingly, you can short-circuit the autobaud if you use stty(1) to set
the bit rate just before starting gpsd. For example, suppose you know that
your GPS is on serial port 0 and operates at a fixed bps of 54600. You
can set that up like this:</p>
<pre><code>
stty speed 54600 </dev/ttyS0
gpsd -nN /dev/ttyS0
</code></pre>
<p>On *BSD systems, baud rate 0 doesn't have this special meaning -
the ports open at a fixed speed, usually 9600. So to suppress the
hunt loop you have to actually compile at a fixed baud rate and then
use the stty trick.</p>
<h1 id='willitwork'>Will this GPS work? It's not on the hardware list.</h1>
<p>Probably.</p>
<p>GPSD's support for the NMEA protocol is mature and stable. If the
specification for your receiver says "NMEA 0183" (maybe with a version 2.x
or 3.x qualifier) it should just work.</p>
<p>Beware of receivers that do not say "NMEA" somewhere in the specification;
while it may indicate that the receiver only uses a binary protocol, it often
means that the receiver cannot be used as data source for a computer, as is
usually the case with car navigation devices.</p>
<p>We also support many proprietary protocols, in case your receiver
doesn't emit NMEA. Datasheets often indicate which chip the receiver
is based on, for example a <i>NavCorp NX666</i>. Check to see if other
<i>NavCorp</i> receivers are listed, either as a vendor or a
chipset. Compare this with the output of <code>gpsd -l</code> which
will list the protocols compiled into gpsd. If your receiver doesn't
support NMEA and we don't have a special driver for the chipset, talk
to us. But it'll probably just work.</p>
<p>Assuming the receiver has a USB interface, do a web search to see
if someone has tried it with linux already, eg. "<code>NavCorp NX666
linux</code>". Search for the product and "driver install" to find
instructions on installing Windows drivers for the product - these
often hint at which bridge chip is used, if the specifications don't
say so. A receiver claiming Mac compatibility is usually based on one
of the common bridge chips from FTDI, Prolific or Silicon
Laboratories. These just work.</p>
<h1 id='conflict'>Why does GPSD open non-GPS USB devices?</h1>
<p>Most USB devices have a defined <a
href="http://www.usb.org/developers/defined_class">device class</a> -
mass storage, video, hub, and human interface device are three of the
more common ones. Using GPSD will never interfere with such devices,
nor will they interfere with GPSD.</p>
<p>Unfortunately, there is no device class for USB GPSes. Nor is there
a device class for USB-to-serial adapter chips. Both are assigned the
catch-all class 0xFF, "Vendor Specific". Every USB GPS we've ever seen
consists of a GPS module with TTL-level RS232 outputs connected to an
RS232-to-USB adapter chip, and presents the class ID 0xFF to your USB
subsystem.</p>
<p>When you install GPSD, it puts in place rules that watch for
hotplug activation events from devices that might be GPSes. When such
a hotplug event happens, a <code>gpsd</code> instance is started (if
one is not already running) and puts a copy of the device path in an
internal stash list. Later, if a client application requests GPS data,
<code>gpsd</code> will try to read from the device, and discard it
from the stash list if it is not emitting data that <code>gpsd</code>
recognizes.</p>
<p>GPSD's notion of "might be a GPS" depends on the fact that all USB
GPSes are made with one of a small number of USB-to-serial adapter
chips, the most common of which is the Prolific Logic 2303. GPSD's
hotplug rules expect that anything exhibiting the USB vendor:product
ID pair of one of these chips will be a GPS.</p>
<p>A problem can arise if you have other devices connected to your
system through one of these specific adaptor chips. When these
activate, the GPSD hotplug rules will believe they are GPSes, and
<code>gpsd</code> will try to read from them when a GPS-ware client
application requests data. We've had reports of this happening with
USB modems and various microcontroller kits. It's not a problem we can
solve with clever programming, the devices simply don't yield
enough information about themselves to avoid conflicts.</p>
<p>Under Linux, <code>gpsd</code> at 3.1 and later versions checks
each serial and USB device after opening it to see if any other
process has that device open; if so, it's dropped. This should at
least keep GPSD from usurping data sources belonging to other service
daemons.</p>
<p>Note that GPSD never tries to configure USB devices until it has
identified them as sensors of a known type. Also, it tries to open with
TIOCEXCL and thus will not open devices that another process already
has open. So the worst case is a race between GPSD and another process
to open a device not in use, in which the other process's open fails.</p>
<p>If this sort of conflict becomes a problem, you can work around
it by disabling the GPSD hotplug rules. Unfortunately, this means
you will have to start <code>gpsd</code> manually with a device-path
argument when you want to use a GPS.</p>
<h1 id='efficiency'>What is gpsd's CPU and power overhead?</h1>
<p>The daemon, <code>gpsd</code>, is designed to run light so it can be
used in low-power embedded-systems deployments. Data throughput from GPSes
and other navigational sensors is low and tends to be concentrated in bursts
at about one-second intervals; accordingly, <code>gpsd</code> spends most
of its time simply waiting in select(2) at effectively zero CPU cost. Even
with a client session and a GPS active, we have measured gpsd's CPU load
at steadily less than 1% on a low-power, low-speed ARM SBC.</p>
<h1 id='usblockup'>My USB port becomes unavailable after gpsd uses it.</h1>
<p>Occasionally we get a bug report from a user who says a USB port
locks up or becomes unavailable after <code>gpsd</code> has closed it.
Such problems may persist until the USB port is unplugged and
replugged, or until all devices in that root hub are unplugged and
replugged, or even until a reboot.</p>
<p>This is not a GPSD bug. Bugs in the serial-device layer of your
operating system, tickled by gpsd's unusual autobauding loop and
serial-parameter changes, can cause problems like this. They may be
driver-specific bugs, or they may be due to bad interactions between
ioctl() and select() in the kernel's generic tty code. Bugs in the USB
chipset on your motherboard or in a hub can do it, too.</p>
<p>Here are some possible fixes:</p>
<dl>
<dt><b>1. Suppress the autobauding loop</b></dt>
<dd>The autobauding hunt loop in <code>gpsd</code> stresses chipsets and
drivers in unusual ways, which is why these sorts of bugs show up more
often under <code>gpsd</code> than most other USB-using software. The first
thing to try is to <a href="#baud">suppress the autobauding loop</a>.</dd>
<dt><b>2. Upgrade your kernel</b></dt>
<dd>Upgrading your kernel may help. Obscure tty-layer kernel bugs pop
up relatively often and are usually fixed pretty quickly.</dd>
<dt><b>3. Try a different USB-to-serial chip</b></dt>
<dd>Another thing to try is a GPS with a different USB-to-serial chip.
You probably do not have a a chip-specific problem if you're using
a PL23203, as those drivers have been tested a lot. But we've seen
reports that were definitely chip-specific on less common chipsets
such as FTDI. The CP210x chips are also known to be problematic,
mainly because the vendor refused to release enough programming
information to allow a decent open-source driver to be written.</dd>
<dt><b>4. Replace your USB hub</b></dt>
<dd>Some USB hubs are flaky. You may need to replace yours.</dd>
</dl>
<p>When you have had such a problem with <code>gpsd</code>, and are
able to work around it or fix it, please inform us so we can
improve this FAQ.</p>
<h1 id='nmea2000'>How do I configure GPSD to use NMEA2000?</h1>
<p>This feature requires kernel-level support for CAN (Controller Area
Network) hardware. Presently this is available only under Linux using
the Socket CAN subsystem. The nmea2000 driver of gpsd has been tested
with 2.6.38 and newer kernels.</p>
<p>GPSD expects your CAN interface to be configured to receive CAN
data at the nmea2000 baudrate of 250kBit/s. The distribution includea
a <tt>cansetup</tt> script that will set this up for some of the more
common interface types.</p>
<h1 id='android'>Android reports that gpsd is draining my battery. Help!</h1>
<p>Yes, Android 4.0 and later versions use gpsd to interpret the data
stream from the onboard GPS. No, the battery drain is not actually
due to a GPSD bug or design error.</p>
<p>You probably have a badly-written or buggy app installed that is
running in background, keeping gpsd awake by keeping a client session
to it constantly open; this can happen if the app is using "Detailed
Localization" rather than computing its location the cheaper way using
cell-tower signal strengths. What you need to do is identify that app
and remove it.</p>
<p>See <a href="http://www.bitdirect.nl/?p=47">this blog post</a> for
a procedure for hunting down the bad app. In this particular case it
was something called <code>buienradar</a>.
<p>On the Samsung Galaxy SIII, vendor firmware provides a "Remote
Location" service with a similar bug. You can disable this through
Settings. See <a
href="http://blog.gadgethelpline.com/gpsd-stop-samsung-galaxy-siiis-battery-life/">this
blog post</a> for a fix.</p>
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