term% cat index.txt GPSFS(8) System Manager's Manual GPSFS(8)
NAME
gpsfs, gpsevermore - GPS time and position service
SYNOPSIS
aux/gpsfs [ -d device ] [ -b baud ] [ -s srvname ] [ -m mntpt ]
aux/gpsevermore [ -d device ] [ -b baud ] [ -n baud ] [ -l location ]
DESCRIPTION
Aux/gpsfs reads an NMEA-compatible serial GPS (Global Positioning Sys‐
tem) device and provides time and position through a file system, by
default mounted on /mnt and implementing /mnt/gps.
It implements four files in the gps directory: position, time, satel‐
lites, and raw.
The read-only position file contains one line of information in 9 tab-
separated fields:
fix quality
0 means position data invalid, 1 means a 2D position is avail‐
able, 2 means a 3D position is available. The value is 8, 9, or
10, respectively, when the fix data comes from a file rather
than an actual GPS.
zulu time
universal coordinated time encoded as hhmmss followed by the
character 'Z'.
system time
time and date converted to the format of time(2).
longitude
in degrees, east of Greenwich is positive, west negative.
latitude
in degrees, positive is north, negative south of the equator.
altitude
above sea level, in meters.
course degrees, clockwise from true north.
ground speed
in km/h
magnetic deviation
(not provided by all GPSs), in degrees, positive is westerly,
negative easterly.
The read-only time file contains one line of information in 4 tab-sepa‐
rated fields:
gps time
in time(2) format.
gps time
in nsec (see time(2)) format (ms accuracy).
system time
in nsec format. This is the system time at the time of the gps
time sample. The difference between this and the previous field
is used in clock synchronization. See timesync(8).
validity
the character A meaning sample valid and usable for clock syn‐
chronization. The other values are not usable for clock sync: B
means valid sample from file playback, V means invalid sample,
and W means invalid playback sample.
The read-only satellites file contains information about the current
satellite constellation. It consists of one line of general informa‐
tion, followed by zero or more lines, one for each satellite in use.
The first line contains two fields:
fix quality
same as in the position file.
satellites in view
number of satellites above the horizon
Subsequent lines have four fields:
prn satellite ID
elevation
above the horizon, degrees.
azimuth
direction, degrees from true north
snr Signal to noise ratio, 0 - 99 dB
The contents of these files are refreshed once per second when reading
from an actual GPS, and once per 100 ms (giving a speed up of a factor
10) when playing back from file.
The read-only raw file can be read to obtain a copy of the raw NMEA GPS
output. Gpsfs keeps an internal buffer of 8KB, so the reader must keep
up with the output (typically 500 or so bytes per second).
The -d flag establishes the device the GPS samples are read from. If
the device file is not a serial interface, gpsfs assumes playback from
file and modifies quality parameters as such.
The -b flag specifies the baud rate of the serial line. The standard
baud rate for NMEA GPS is 4800 baud, but many device allow changing to
higher speeds.
The -s flag specifies the name under which the gpsfs service is posted
in /srv.
The -m flag specifies a mount other than /mnt.
Evermore
Aux/gpsevermore is used to configure GPSs using an Evermore chipset.
The -d flag specifies the serial device to the GPS.
The -b flag specifies the baud rate of the serial line. The standard
baud rate for NMEA GPS is 4800 baud, but many device allow changing to
higher speeds.
The -n flag specifies the speed to set the GPS to. When the command
finishes, the GPS should be read (and configured) at the new speed.
The -l flag is sued to specify the location to initialize the GPS to.
The format is dd:mm:ssX or dd:mm.mmmX or dd.dddX, where dd stands for
degrees (one or more digits), mm for minutes and ss for seconds of arc.
X is one of W, E, N or S. Longitudes come with W or E, latitudes with
N or S. The -l flag is followed by two such fields, one for longitude,
one for latitude. They may be given in a single argument (separated by
white space), or in two arguments, in either order. Initialization
time is taken from time(2).
SEE ALSO
timesync(8), time(2)
FILES
/mnt/gps/position
position, time, speed and heading
/mnt/gps/satellites
satellites in view
/mnt/gps/time
GPS time (millisecond accuracy)
/dev/eia0
default GPS device
SOURCE
/sys/src/cmd/aux/gps
GPSFS(8)