path: root/HACKING

This is the Hacker's Guide to gpsd.  If you're viewing it with Emacs, try
doing Ctl-C Ctl-t and browsing through the outline headers. Ctl-C Ctl-a
will unfold them again.

If you're looking for things to hack on, first see the TODO file.

** Goals and philosophy of the project

If the GPSD project ever needs a slogan, it will be "Hide the ugliness!"
GPS technology works, but is baroque, ugly and poorly documented.  Our
job is to know the grotty details so nobody else has to.

*** Audience and supported hardware

Our paradigm user, the one we have in mind when making design choices,
is running navigational or wardriving software on a Linux laptop or
PDA. Some of our developers are actively interested in supporting
GPS-with-SBC (Single-Board-Computer) hardware such as is used in
balloon telemetry, marine navigation, and aviation.  

These two use cases have similar issues in areas like client
interfaces and power/duty-cycle management.  The one place where
they differ substantially is that in the SBC case we generally
know in advance what devices will be connected and when.  Thus, by
designing for the less predictable laptop/PDA environment, we
cover both. It is not by accident that the source code can be built 
with support for only any single GPS type compiled in.

While we will support survey-grade GPSes when and if we have that
hardware for testing, our focus will probably remain on inexpensive
and readily-available consumer-grade GPS hardware.

*** The time and location service

The primary aim of the GPSD project is to support a simple
time-and-location service for users and their geographically-aware
applications.

A GPS is a device for delivering fourteen numbers: x, y, z, t, vx, vy, vz,
and error estimates for each of these seven coordinates.  The gpsd daemon's
job is to deliver these numbers to user applications with minimum fuss.

'Minimum fuss' means that the only action the user should have to take
to enable location service is to plug in a GPS.  The gpsd daemon, and its
associated hotplug scripts or local equivalent, is responsible for
automatically configuring itself. That includes autobauding,
handshaking with the device, determining the correct mode or protocol
to use, and issuing any device-specific initializations required.

Features (such as GPS type or serial-parameter switches) that would
require the user to perform administrative actions to enable location
service will be rejected.  GPSes that cannot be autoconfigured will
not be supported.  99% of the GPS hardware on the market in 2005 is
autoconfigurable, and the design direction of GPS chipsets is such
that this percentage will rise rather than fall; we deliberately
choose simplicity of interface and zero administration over 100%
coverage.

Here is a concrete example of how this principle applies.  At least
one very low-end GPS chipset does not deliver correct checksums on the
packets it ships to a host unless it has a fix.  Typically, GPSes do
not have a fix when they are plugged in, at the time gpsd must
recognize and autoconfigure the device.  Thus, supporting this chipset
would require that we either (a) disable packet integrity checking in
the autoconfiguration code, making detection of other more
well-behaved devices unreliable, or (b) add an invocation switch to
disable packet integrity checking for that chipset alone.  We refuse
to do either, and do not support this chipset.

*** The testing and tuning tools

Another principal goal of the GPSD software is that it be able to
demonstrate its own correctness, give technical users good tools for
measuring GPS accuracy and diagnosis of GPS idiosyncrasies, and
provide a test framework for gpsd-using applications.

Accordingly, we support the gpsfake tool that simulates a GPS using
recorded or synthetic log data.  We support gpsprof, which collects
accuracy and latency statistics on GPSes and the GPS+gpsd combination.
And we include a comprehensive regression-test suite with the package.
These tools are not accidents, they are essential to ensure that the
basic GPS-monitoring code is not merely correct but *demonstrably*
correct.

We support a tool, sirfmon, which is a low-level packet monitor and
diagnostic tool for the chipset with 80% market share.  sirfmon is
capable of tuning some device-specific control settings such as the
SiRF static-navigation mode.  A future direction of the project is to
support diagnostic monitoring and tuning for other chipsets.

*** The upgrading tools

A secondary aim of the GPSD project is to support GPS firmware
upgrades under open-source operating systems, freeing GPS users from
reliance on closed-source vendor tools and closed-source operating
systems.

We have made a first step in that direction with the initial alpha
version of gpsflash, currently supporting SiRF chips only.  A future
direction of the project is to have gpsflash support firmare upgrades 
for other chipsets.

*** The GPS/GNSS monitoring tools

Another secondary goal of the project is to provide open-source tools
for diagnostic monitoring and accuracy profiling not just of
individual GPSes but of the GPS/GNSS network itself. The protocols
(such as IS-GPS-200 for satellite downlink and RCTM104 for
differential-GPS corrections) are notoriously poorly documented, and
open-source tools for interpreting them have in the past been hard
to find and only sporadically maintained.

We aim to remedy this.  Our design goal is to provide lossless
translators between these protocols and readable, documented
text-stream formats.  

We currently provide a tool for decoding RTCM104 reports on satellite
health, almanacs, and pseudorange information from differential-GPS
radios and reference stations.  A future direction of the project is
to support an RTCM encoder; also to support DGPSIP service via gpsd.

** Contribution guidelines

*** Send patches in diff -u or -c format

We prefer diff -u format, but diff -c is acceptable.  Do not send
patches in the default (-e or ed) mode, as they are too brittle.

Before shipping a patch, you should go through the following checklist:

(1) If you are introducing a new feature or driver, include a
    documentation patch.

(2) Use the regression-test suite -- "make testregress" -- to check
    that your patch doesn't break the handling of any already-supported GPS.

(3) If you have valgrind(1) on your development system, run
    valgrind-audit and look out for reports of memory leaks and
    other dynamic-allocation problems (see <http://valgrind.org> for
    a description of this tool if you don't already know about it).  
    If you can't run valgrind. tell us that you couldn't do it.

(4) If you have splint(1) on your development system, make sure the
    patched code displays no warnings when you run 'make
    splint' (see <http://www.splint.org> for further description of 
    this tool if you don't already know about it). If you can't run
    splint. tell us that you couldn't do it.

If you are contributing a driver for a new GPS, please also do the
following things:

(5) Send us a representative sample of the GPS output for future 
    regression-testing.

(6) Write a hardware entry describing the GPS for the hardware page at
    <http://gpsd.berlios.de/hardware.html>.

There's a whole section on adding new drivers later in this document.

*** The license on contributions

The GPSD libraries are under the BSD license.  Please do not send 
contributions with GPL attached!

The reason for this policy is to avoid making people nervous about
linking the GPSD libraries to applications that may be under other
licenses (such as MIT, BSD, AFL, etc.).

*** Don't add invocation options!

If you send a patch that adds a command-line option to the daemon, it
will almost certainly be refused.  Ditto for any patch that requires
gpsd to parse a dotfile.  

One of the major objectives of this project is for gpsd *not to
require administration* -- under Linux, at least.  It autobauds,
it does protocol discovery, and it's activated by the hotplug
system.  Arranging these things involved quite a lot of work, 
and we're not willing to lose the zero-configuration property
that work gained us.

Instead of adding a command-line option to support whatever feature
you had in mind, try to figure out a way that the feature can
autoconfigure itself by doing runtime checks.  If you're not clever
enough to manage that, consider whether your feature control might be
implemented with an extension to the gpsd protocol or the
control-socket command set.

*** Don't use malloc!

The best way to avoid having dynamic-memory allocation problems is
not to use malloc/free at all.  The gpsd daemon doesn't (though the
client-side code does).  Thus, even the longest-running instance 
can't have memory leaks.  The only cost for this turned out to be
embedding a PATH_MAX-sized buffer in the gpsd.h structure.

Don't undo this by using malloc/free in a driver or anywhere else.

** Understanding the code

*** Debugging

For debugging purposes, it may be helpful to configure with --disable-shared.
This turns off all the shared-library crud, making it somewhat easier to
use gdb.

There is a script called logextract in the distribution that you can use
to strip clean NMEA out of the log files produced by gpsd.  This can be
useful if someone ships you a log that they allege caused gpsd to 
misbehave.

gpsfake enables you to repeatedly feed a packet sequence to a gpsd
instance running as non-root.  Watching such a session with gdb should
smoke out any repeatable bug pretty quickly.

The parsing of GPGSV sentences in the NMEA driver has been a
persistent and nasty trouble spot, causing more buffer overruns and
weird secondary damage than all the rest of the code put together.
Any time you get a bug report that seems to afflict NMEA devices
only, suspicion should focus here.

*** Profiling

There is a barely-documented Z command in the daemon will cause it to emit
a $ clause on every request.  The $ clause contains four
space-separated fields:

(1) An identifing sentence tag.

(2) The character length of the sentence containing the timestamp data.

(3) The timestamp associated with the sentence, in seconds since
    the Unix epoch (this time *is* leap-second corrected, like UTC).
    This timestamp may be zero.  If nonzero, it is the base time for
    the packet.

(4) An offset from the timestamp telling when gpsd believes the
    transmission of the current packet started (this is actually 
    recorded just before the first read of the new packet).  If
    the sentence timestamp was zero, this offset is a full timestamp 
    and the base time of the packet.

(5) An offset from the base time telling when gpsd received the last
    bytes of the packet.

(6) An offset from the base time telling when gpsd decoded the data.

(7) An offset from the base time taken just before encoding the
    response -- effectively, when gpsd was polled to transmit the data.

(8) An offset from the base time telling when gpsd transmitted 
    the data.

The Z figures measure components of the latency between the GPS's time
measurement and when the sentence data became available to the
client. For it to be meaningful, the GPS has to ship timestamps with
sub-second precision. SiRF-II and Evermore chipsets ship times with
millisecond resolution.  Your machine's time reference must also be
accurate to subsecond precision; I recommend using ntpd, which will
normally give you about 15 microseconds precision (two orders of
magnitude better than GPSes report).

Note, some inaccuracy is introduced into the start- and end-of-packet
timestamps by the fact that the last read of a packet may grab a few
bytes of the next one.

The distribution lincludes a Python script, gpsprof, that uses the 
Z command to collect profiling information from a running GPS instance.
You can use this to measure the latency at each stage -- GPS to daemon,
daemon to client library -- and to estimate the portion of the latency 
induced by serial transmit time.  The gpsprof script creates latency
plots using gnuplot(1).  It can also report the raw data.

*** Porting to weird machines: endianness, width, and signedness issues.

The gpsd code is well-tested on 32- and 64-bit IA chips, also on PPCs.
Thus, it's known to work on mainstream chips of either 32 or 64 bits
and either big-endian or little-endian representation with IEE754
floating point.

Handling of NMEA devices should not be sensitive to the machine's
internal numeric representations, However, because the binary-protocol
drivers have to mine bytes out of the incoming packets and mung them
into fixed-width integer quantities, there could potentially be issues
on weird machines.  The regression test should spot these.

If you are porting to a true 16-bit machine, or something else with
an unusual set of data type widths, take a look at bits.h.  We've
tried to collect all the architecture dependencies here.  If splint
gives you warnings, it is possible you may need to adjust the -D
directives in .splintrc that are used to define away fixed-width typedefs.

(No, we don't know why splint doesn't handle these natively.)

*** Architecture and how to hack it

gpsd is not a complicated piece of code.  Essentially, it spins in a loop 
polling for input from one of three sources:

1) A client making requests over a TCP/IP port.

2) A set of GPSes, connected via serial or USB devices.

3) A DGPS server issuing periodic differential-GPS updates.

The daemon only connects to a GPS when clients are connected to it.
Otherwise all GPS devices are closed and the daemon is quiescent, but
retains fix and timestamp data from the last active period. 

All writes to client sockets go through throttled_write().
This code addresses two cases.  First, client has dropped the connection.
Second, client is connected but not picking up data and our buffers are
backing up.  If we let this continue, the write buffers will fill and 
the effect will be denial-of-service to clients that are better behaved.

Our strategy is brutally simple and takes advantage of the fact that
GPS data has a short shelf life.  If the client doesn't pick it up 
within a few minutes, it's probably not useful to that client.  So if
data is backing up to a client, drop that client.  That's why we set
the client socket to nonblocking.

GPS input updates an internal data structure which has slots in it for
all the data you can get from a GPS.  Client commands mine that
structure and ship reports up the socket to the client.  DGPS data is
passed through, raw, to the GPS.

The trickiest part of the code is the handling of input sources in gpsd.c 
itself.  It had to tolerate clients connecting and disconnecting at random
times, and the GPS being unplugged and replugged, without leaking file 
descriptors; also arrange for the GPS to be open when and only when clients 
are active.

*** Autoconfiguration

One of the design goals for gpsd is to be as near zero-configuration
as possible.  Under most circumstances, it doesn't require either
the GPS type or the serial-line parameters to connect to it to be
specified.  Presently, here's how the autoconfig works.

1. At each baud rate gpsd grabs packets until it sees either a
   well-formed and checksum-verified NMEA packet, a well-formed and
   checksum-verified packet of one of the binary protocols, or it sees
   one of the two special trigger strings EARTHA or ASTRAL, or it
   fills a long buffer with garbage (in which case it steps to the
   next baud rate).

2. If it finds a SiRF packet, it queries the chip for firmware
   version.  If the version is < 231.000 it drops back to SiRF NMEA.
   We're done.

3. If it finds a Zodiac binary packet (led with 0xff 0x81), it
   switches to the Zodiac driver.  We're done.

4. If it finds an Evermore binary packet (led with DEL=0x10 followed
   by STX=0x02) it switches to Evermore binary protocol.  We're done.

5. If it finds a TSIP binary packet (led with 0x10=DLE), it
   switches to the TSIP driver.  We're done.

6. If it finds n iTrax binary packet (led with <* ), it
   switches to the iTrax driver.  We're done.

7. If it finds EARTHA, it selects the Earthmade driver, which then
   flips the connection to Zodiac binary mode.  We're done.

8. If it finds ASTRAL, it feeds the TripMate on the other end what
   it wants and goes to Tripmate NMEA mode.  We're done.

9. If it finds a NMEA packet, it selects the NMEA driver.  This
   initializes by shipping all vendor-specific initialization strings
   to the device.  The objectives are to enable GSA, disable GLL, and
   disable VTG.  Probe strings go here too, like the one that turns 
   on SiRF debugging output in order to detect SiRF chips.

10. Now gpsd reads NMEA packets.  If it sees a driver trigger string it
   invokes the matching driver.  Presently there is really only one of
   these: "$Ack Input 105.\r\n", the response to the SiRF probe. On
   seeing this, gpsd switches from NMEA to SiRF binary mode, probes
   for firmware version, and either stays in binary or drops back 
   to SiRF NMEA.

The outcome is that we know exactly what we're looking at, without any
driver-type or baud rate options.

*** Error modeling

To estimate errors (which we must do if the GPS isn't nice and reports
them in meters with a documented confidence interval), we need to
multiply an estimate of User Equivalent Range Error (UERE) by the
appropriate dilution factor,

The UERE estimate is usually computed as the square root of the sum of
the squares of individual error estimates from a physical model.  The
following is a representative physical error model for satellite range
measurements:

From R.B Langley's 1997 "The GPS error budget". 
GPS World , Vol. 8, No. 3, pp. 51-56

Atmospheric error -- ionosphere                 7.0m
Atmospheric error -- troposphere                0.7m
Clock and ephemeris error                       3.6m
Receiver noise                                  1.5m
Multipath effect                                1.2m

From Hoffmann-Wellenhof et al. (1997), "GPS: Theory and Practice", 4th
Ed., Springer.

Code range noise (C/A)                          0.3m
Code range noise (P-code)                       0.03m
Phase range                                     0.005m

We're assuming these are 2-sigma error ranges. This needs to
be checked in the sources.  If they're 1-sigma the resulting UEREs
need to be doubled.

See http://www.seismo.berkeley.edu/~battag/GAMITwrkshp/lecturenotes/unit1/
for discussion.

Carl Carter of SiRF says: "Ionospheric error is typically corrected for 
at least in large part, by receivers applying the Klobuchar model using 
data supplied in the navigation message (subframe 4, page 18, Ionospheric 
and UTC data).  As a result, its effect is closer to that of the 
troposphere, amounting to the residual between real error and corrections.

"Multipath effect is dramatically variable, ranging from near 0 in
good conditions (for example, our roof-mounted antenna with few if any
multipath sources within any reasonable range) to hundreds of meters in
tough conditions like urban canyons.  Picking a number to use for that
is, at any instant, a guess."

"Using Hoffman-Wellenhoff is fine, but you can't use all 3 values.
You need to use one at a time, depending on what you are using for
range measurements.  For example, our receiver only uses the C/A
code, never the P code, so the 0.03 value does not apply.  But once
we lock onto the carrier phase, we gradually apply that as a
smoothing on our C/A code, so we gradually shift from pure C/A code
to nearly pure carrier phase.  Rather than applying both C/A and
carrier phase, you need to determine how long we have been using
the carrier smoothing and use a blend of the two."

On Carl's advice we would apply tropospheric error twice, and use
the largest Wellenhof figure:

UERE = sqrt(0.7^2 + 0.7^2 + 3.6^2 + 1.5^2 + 1.2^2 + 0.3^2) = 4.1

DGPS corrects for atmospheric distortion, ephemeris error, and satellite/
receiver clock error.  Thus:

UERE =  sqrt(1.5^2 + 1.2^2 + 0.3^2) = 1.8

which we round up to 2 (95% confidence).

Due to multipath uncertainty, Carl says 4.1 is too low and recommends
a non-DGPS UERE estimate of 8 (95% confidence).  That's what we use.

** Known trouble spots

*** The Y2.1K problem and other calendar issues

Because of limitations in various GPS protocols (e.g., they were
designed by fools who weren't looking past the ends of their noses) 
this code unavoidably includes some assumptions that will turn around
and bite on various future dates. 

The two specific problms are:

1) NMEA delivers only two-digit years.

2) SiRF chips at firmware level 231 deliver only GPS time in binary mode,
not leap-second-corrected UTC.

See the timebase.h file for various constants that will need to
be tweaked accasionally to cope with these problems.

Note that gpsd does not rely on the system clock in any way.  This
is so you can use it to set the system clock.

*** Hotplug interface problems

The hotplug interface works pretty nicely for telling gpsd which
device to look at, at least on my FC3 Linux machines.  The fly in the
ointment is that I'm using a deprecated version of the interface, the
old-style /etc/hotplug version with usermap files.

It is unlikely this interface will be dropped by distro makers any
time soon, because it's supporting a bunch of popular USB cameras.
Still, it would be nice not to be using a deprecated interface.

I tried moving to the new-style /etc/hotplug.d interface, but I ran
into a nasty race condition.  My hotplug agent got woken up on a USB
add event as it should, but in the new interface the creation of
/dev/ttyUSB* can be delayed arbitrarily long after the wakeup event.
Thus, it may not be there when gpsd goes to probe it unless I
busy-wait in the script.

Ultimately this should all be done through udev.  The problem is that at
the current state of udev, we'd need to do it through a script that would
fire every time a tty activates.  Because of virtual consoles firing up at
boot time, this would introduce significant boot lag.

This would be antisocial and I'm not willing to do it, so udev needs
to grow better filtering before I'll use it.

When and if udev supports HOTPLUG and ACTION keys, this will work:

# The Prolific Technology 2303 (commonly in tandem with SiRF chips)
BUS="usb" SYSFS{vendor}="067b" SYSFS{product}="2303" \
		NAME="gps%e" \
		HOTPLUG="/usr/bin/gps-probe"
# FTDI 8U232AM
BUS="usb" SYSFS{vendor}="0403" SYSFS{product}="6001" \
		NAME="gps%e" \
		HOTPLUG="/usr/bin/gps-probe"
# Cypress M8/CY7C64013 (DeLorme uses these)
BUS="usb" SYSFS{vendor}="1163" SYSFS{product}="0100" \
		NAME="gps%e" \
		HOTPLUG="/usr/bin/gps-probe"

More generally, the hotplug code we have is Linux-specific.  OpenBSD
(at least) features a hotplug daemon with similar capabilities.

*** Security Issues

Between versions 2.16 and 2.20, hotplugging was handled in the most
obvious way, by allowing the F command to declare new GPS devices for
gpsd to look at.  Because gpsd runs as root, this had problems:

1) A malicious client with non-root access on the host could use F to
point gpsd at a spoof GPS that was actually a pty feeding bogus
location data.

2) A malicious client could use repeated probes of a target tty or
other device to cause data loss to other users.  This is a potential
remote exploit! Not too bad if the bytes he steals are your mouse, it
would just get jumpy and jerky -- but suppose they're from an actual
tty and sections drop out of a serial data stream you were relying on?

The conclusion was inescapable.  Switching among and probing devices
that gpsd already knows about can be an unprivileged operation, but 
editing gpsd's device list must be privileged.  Hotplug scripts 
should be able to do it, but ordinary clients should not.

Adding an authentication mechanism was considered and rejected (can you
say "can of big wriggly worms"?).  Instead, there is a separate
control channel for the daemon, only locally accessible, only
recognizing "add device" and "remove device" commands.

The channel is a Unix-domain socket owned by root, so it has
file-system protection bits.  An intruder would need root permissions
to get at it, in which case you'd have much bigger problems than a
spoofed GPS.

More generally, certainly gpsd needs to treat command input as
untrusted and for safety's sake should treat GPS data as untrusted
too (in particular this means never assuming that either source won't
try to overflow a buffer).

Daemon versions after 2.21 drop privileges after startup, setting UID
to "nobody" and GID to whichever group owns the GPS device specified
at startup time -- or, if it doesn't exist, the system's
lowest-numbered TTY device named in PROTO_TTY.  It may be necessary to
change PROTO_TTY in gpsd.c for non-Linux systems.

** Adding new GPS types

This section explains the conventions drivers for new devices should follow.

*** Driver architecture

Internally, gpsd supports multiple GPS types.  All are represented by
driver method tables; the main loop knows nothing about the driver
methods except when to call them.  At any given time one driver is
active; by default it's the NMEA one.  

To add a new device, populate another driver structure and add it to
the null-terminated array in drivers.c.

Unless your driver is a nearly trivial variant on an existing one,
it should live in its own C source file named after the driver type.
Add it to the libgps_c_sources name list in Makefile.am

The easiest way write a driver is probably to copy the driver_proto.c
file in the source distribution, change names appropriately, and write
the guts of the analyzer and writer functions.  Look in gpsutils.c
before you do; driver helper functions live there.  Also read some
existing drivers for clues.

*** When not to add a driver

It is not necessary to add a driver just because your NMEA GPS wants
some funky initialization string.  Simply ship the string in the
initializer for the default NMEA driver.  Because vendor control
strings live in vendor-specific namespaces (PSRF for SiRF, PGRM for
Garmin, etc.)  your initializing control string will almost certainly
be ignored by anything not specifically watching for it.

*** Initializing time and date

Some mode-changing commands have time field that initializes the GPS
clock.  If the designers were smart, they included a control bit that
allows the GPS to retain its clock value (and previous fix, if any)
and for you to leave those fields empty (sometimes this is called "hot
start").

If the GPS-Week/TOW fields are required, as on the Evermore chip,
don't just zero them.  GPSes do eventually converge on the correct
time when they've exchanged handshakes with enough satellites, but the
time required for convergence is proportional to how far off the
initial value is.  So make a point of getting the GPS week right.

*** How drivers are invoked

Drivers are invoked in one of three ways: (1) when the NMEA driver
notices a trigger string associated with another driver. (2) when the
packet state machine in packet.c recognizes a special packet type, or
(3) when a probe function returns true during device open.

Each driver may have a trigger string that the NMEA interpreter
watches for.  When that string is recognized at the start of a 
line, the interpreter switches to its driver.  

When a driver switch takes place, the old driver's wrapup method is
called.  Then the new driver's initializer method is called.

A good thing to send from the NMEA initializer is probe strings.  These are
strings which should elicit an identifying response from the GPS that
you can use as a trigger string for a native-mode driver.

Don't worry about probe strings messing up GPSes they aren't meant for.
In general, all GPSes have rather rigidly defined packet formats with
checksums.  Thus, for this probe to look legal in a different binary
command set, not only would the prefix and any suffix characters have
to match, but the checksum algorithm would have to be identical.

Incoming characters from the GPS device are gathered into packets
by an elaborate state machine in packet.c.  The purpose of this 
state machine is so gpsd can autobaud and recignize GPS types
automatically. The other way for a driver to be invoked is for 
the state machine to recognize a special packet type associated
with the driver.

If you have to add a new packet type to packet.c, add tests for the
type to the TESTMAIN code. Also, remember to tell gpsfake how to
gather the new packet type so it can handle logs for regression
testing.  The relevant function in gpsfake is packet_get().  It
doesn't have to deal with garbage or verify checksums, as we assume
the logfiles will be clean packet sequences,

Probe functions are interpreted for drivers that don't use the packet
getter because they read from a device with special kernel support.
See the Garmin binary driver for an example.

*** Where to put the data you get from the GPS

Your driver should put new data from each incoming packet or sentence
in the 'newdata' member of the GPS, and return a validity flag mask
telling what members were updated.  There is driver-independent code
that will be responsible for merging that new data into the existing
fix.  To assist this, the CYCLE_START_SET flag is special.  Set this
when the driver returns the first timestamped message containing fix
data in in an update cycle.  (This excludes satellite-picture messages
and messages about GPS status that don't contain fix data.)

Your packet parser must return field-validity mask bits (using the
_SET macros in gps.h), suitable to be put in session->gpsdata.valid.
The watcher-mode logic relies on these as its way of knowing what to
publish.  Also, you must ensure that gpsdata.fix.mode is set properly to
indicate fix validity after each message; the framework code relies on
this.  Finally, you must set gpsdata.status to indicate wheen DGPS 
fixes are available, whether through RTCM or WAAS/Egnos.

Your packet parser is also responsible for setting the tag field 
in the gps_data_t structure.  This is the string that will be emitted
as the first field of each $ record for profiling.  The packet getter
will set the sentence-length for you; it will be raw byte length, 
including both payload and header/trailer bytes.

Note, also, that all the timestamps your driver puts in the session
structure should be UTC (with leap-second corrections) not just Unix
seconds since the epoch.  The report-generator function for D
does *not* apply a timezone offset.

*** Report errors with a 95% confidence interval

gpsd drivers are expected to report position error estimates with
a 95% confidence interval.  A few devices (Garmins and Zodiacs)
actually report error estimates.  For the rest we have to compute them 
using an error model.

Here's a table that explains how to convert from various
confidence interval units you might see in vendor documentation.

sqr(alpha)  Probability  Notation
-----------------------------------------------------------------------
 1.00            39.4%        1-sigma or standard ellipse
 1.18            50.0%        Circular Error Probable (CEP)
 1.414           63.2%        Distance RMS (DRMS)
 2.00            86.5%        2 sigma ellipse
 2.45            95.0%        95% confidence level
 2.818           98.2%        2DRMS
 3.00            98.9%        3 sigma ellipse
------------------------------------------------------------------------ 

There are constants in gpsd.h for these factors.

*** Log files for regression testing

Any time you add support for a new GPS type, you should also send us a
representative log for your GPS.  This will help ensure that support
for your device is never broken in any gpsd release, because we will
run the full regression before we ship.

A logfile should consist of an identifying header followed by a
straight unencoded dump of GPS 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:

# Name: Holux GM-210
# Cycle-time: 1-second
# Start-of-cycle: ?
# Pause-noted: ?
# Well-behaved: N
# Submitted-by: "Patrick L. McGillan" <pmcgillan@pateri.com>
# Date: 4 Apr 2005
$GPGGA,012519.563,4131.7353,N,09336.8150,W,0,00,50.0,280.2,M,-31.6,M,0.0,0000*7D
$GPGSA,A,1,,,,,,,,,,,,,50.0,50.0,50.0*05
$GPRMC,012519.563,V,4131.7353,N,09336.8150,W,0.00,,050405,,*14
$GPGGA,012520.563,4131.7353,N,09336.8150,W,0,00,50.0,280.2,M,-31.6,M,0.0,0000*77
$GPGSA,A,1,,,,,,,,,,,,,50.0,50.0,50.0*05
$GPGSV,3,1,09,14,65,034,00,01,55,291,43,25,53,210,37,22,45,125,00*7E
$GPGSV,3,2,09,30,29,096,00,11,25,294,32,05,20,056,00,18,14,127,00*73
$GPGSV,3,3,09,15,08,176,00*4C
$GPRMC,012520.563,V,4131.7353,N,09336.8150,W,0.00,,050405,,*1E
$GPGGA,012521.563,4131.7353,N,09336.8150,W,0,00,50.0,280.2,M,-31.6,M,0.0,0000*76
The way to fill in the Name, Cycle-Time, Submitted-by, and Date
headers should be pretty obvious.  

Start-of-cycle should be the name of the NMEA sentence (or, in a
packet protocol, the numeric type ID of the packet) that is emitted
first in each cycle.

Pause-Noted should be Y or N as there is or is not a visible pause
between cycles.

Well-behaved should by Y if all sentences in the same cycle have the
same timestamp, N otherwise.

New log files should include after Date an additional Location header
giving the submitter's city, state/province, country code, and a rough
latitude/longitude.  A good one for the above file might look like
this:

Location: Osceola, Iowa, US, 41N93W

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.

See the header comment of the gpsfake.py module for more about the
logfile format.

An ideal log file would include an initial portion during which the
GPS has no fix, a portion during which it has a fix but is stationary,
and a portion during which it is moving.

If your GPS is SiRF-based, it's easy to capture packets using the
'l' command of sirfmon.

*** Throughput computation for baud rate changes

At low baud rates it is possible to try to push more characters of
NMEA through per cycle than the time to transmit will allow.  Here
are the maxima to use for computation:

GLL       51
GGA       82
VTG       40
RMC       75
GSA       67
GSV       60 (per line, thus 180 for a set of 3)
ZDA       34

The transmit time for a cycle (which must be less than 1 second)
is the total character count multiplied by 10 and divided by the 
baud rate.  

A typical budget is GGA, RMC, GSA, 3*GSV = 82+75+67+(3*60) = 404.

When you write a driver that includes the capability to change
sampling rates, you must fill in the cycle_chars member with 
a maximum character length so the daemon framework code will
be able to compute when a sample-rate change will work.  If
you have to estimate this number, err on the high side.

** Blind alleys

Things we've considered doing and rejected.

*** Reporting fix data only once per cycle

Considered in the abstract, the cleanest thing for a
position/velocity/time oracle to return is a 14-tuple including
position components in all four dimensions, velocity in three, and
associated error estimates for all seven degrees of freedom.  This is
what the O message in GPSD protocol attempts to deliver.  Ideally, we
want exactly one report like this per cycle. That's what we get from
SiRF protocol (all PVT data is in packet type 02) and with the Zodiac
protocol (all PVT data is in the type 1000 packet).  These, together,
account for a share of the GPS market that is 80% and rising in 2005.

Unfortunately NMEA devices are a different story. NMEA in general,
rather badly designed for approximating this.  For historical reasons,
NMEA splits 2-D position info and altitude in two different messages,
each issued once during the normal 1-second send cycle.  Some vendor
binary protocols have similar problems.

This means that gpsd may execute one of two policies.  Either it
must issue a separate report on each sentence or packet containing PVT
information, or it must accumulate data across an entire send cycle
and issue a unified report at end of cycle.  Report-per-packet is
simpler, and adds minimum reporting latency, but means that O
responses may issue more than once per second with complementary
sets of data that add up to a complete report.  The accumulating
policy means there will be exactly one report per cycle and it will
contain full PVT information, but it may have additional latency
of up to one second.

Presently gpsd implements mainly report-per-packet, accumulating data
during the cycle and clearing it at the beginning of each cycle.
Thus, later packet reports include buffered data from earlier ones.

The reason we have not attempted a fully accumulating policy is that
it would require gpsd to know which sentence ends the GPS send
cycle, so as to know the one point at which to ship accumulated data
to the client.  Which sentence this is does, in fact, vary across GPS
types. One of gpsd's design goals, zero configuration, means that we
can't count on the user or administrator passing gpsd this
information; instead, gpsd must somehow be able to autodetect it.

We can't even count on all send cycles of the same device having the
same end sentence, so the naive plan of waiting one cycle to see 
what it is won't work. Devices like the Garmin 48 have two different
cycle sequences with different start and end sentences.

With great effort and a lot of complex code, this problem might be
overcome.  Presently, it doesn't seem like a good idea for a marginal
improvement in reporting from 20% of GPSes.

*** Allowing clients to ship arbitrary control strings to a GPS

Tempting -- it would allow us to do sirfmon-like things with the
daemon running -- but a bad idea.  It would make denial-of-service 
attacks on applications using the GPS far too easy.  For example,
suppose the control string were a baud-rate change?

*** Using libusb to do USB device discovery

There has been some consideration of going to the cross-platform libusb
library to do USB device discovery. This would create an external
dependency that gpsd doesn't now have, and bring more complexity on
board than is probably desirable.

We've chosen instead to rely on the local hotplug system.  That way
gosd can concentrate solely on knowing about GPSes.

*** Setting FIFO threshold to 1 to reduce jitter in serial-message times

When using gpsd as a time reference, one of the things we'd like to do
is make the amount of lag in the message path from GPS to GPS small
and with as little jitter as possible, so we can correct for it with
a constant offset.

A possibility we considered is to set the FIFO threshold on the serial
device UART to 1 using TIOCGSERIAL/TIOCSSERIAL.  This would, in
effect, disable transmission buffering, increasing lag but decreasing
jitter.

But it's almost certainly not worth the work.  Rob Janssen, our timekeeping
expert, reckons that at 4800bps the UART buffering can cause at most
about 15msec of jitter.  This is, observably, swamped by other less
controllable sources of variation.

Local variables:
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