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.\"	$OpenBSD: nmea.4,v 1.28 2019/01/27 07:15:57 jmc Exp $
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.\" Copyright (c) 2006, 2007 Marc Balmer <mbalmer@openbsd.org>
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.Dd $Mdocdate: January 27 2019 $
.Dt NMEA 4
.Os
.Sh NAME
.Nm nmea
.Nd NMEA 0183 timedelta sensor
.Sh SYNOPSIS
.Cd "pseudo-device nmea" Op Ar count
.Sh DESCRIPTION
This line discipline interfaces NMEA devices,
such as GPS receivers attached to a serial or USB port.
.Pp
The line discipline is enabled by the following sequence:
.Bd -literal -offset indent
#include <sys/ioctl.h>
int ldisc = NMEADISC, fildes; ...
ioctl(fildes, TIOCSETD, &ldisc);
.Ed
.Pp
The byte stream is unaltered by the line discipline which
maintains timedelta and position sensors using the NMEA data.
The sensors will appear as nmea* in the list.
The timedelta (nanoseconds difference between the received time
information and the local time), position (calculated latitude
and longitude in degrees), altitude and speed can be accessed
through the
.Xr sysctl 8
interface.
.Pp
The
.Nm
line discipline decodes the following NMEA 0183 sentences:
.Bl -tag -width "RMCXX"
.It RMC
Recommended Minimum Specific GPS/TRANSIT Data.
The time and date information, position and speed are extracted.
The warning indication is used to provide the sensor status (see below).
If the attached device sends the RMC message in the 13-field format,
the operation mode of the GPS device is reported in the sensor description.
The sensor timestamp is copied from the tty timestamp if a device with PPS
is being used and tty timestamping has been turned on.
Otherwise the sensor timestamp is taken when the initial `$' character of
a message block is received from the NMEA device.
.It GGA
Current fix data.
The altitude in meters is extracted.
.El
.Pp
Messages source are recognised by the first two characters of the NMEA
sentence according to the following prefixes:
.Pp
.Bl -tag -width "XXXXX" -offset indent -compact
.It GP
GPS
.It GA
Galileo
.It BD
BeiDou
.It GL
Glonass
.It GN
Generic GNSS
.El
.Sh SENSOR STATES
The quality of the timedelta is reported as the sensor status:
.Bl -tag -width "CRITICALXX" -offset indent
.It OK
The time information and position are valid.
The timedelta is safe to use for applications like
.Xr ntpd 8 .
.It WARN
The attached GPS receiver has been indicating a warning condition
for at least the last ten minutes.
The timedelta should be used with care.
.It CRITICAL
tty timestamping has been turned on but there is no PPS signal present or the
GPS receiver indicated a warning condition for at least the last twenty
minutes.
Check your hardware.
Some GPS units need PPS to be manually turned on.
.El
.Pp
The status of a second sensor is used to report the status of the
device itself:
.Bl -tag -width "CRITICALXX" -offset indent
.It OK
The clock is synchronized, e.g. a GPS receiver has a fix.
.It WARN
The device issued a warning condition, e.g. a GPS receiver has no fix.
.El
.Sh SEE ALSO
.Xr tty 4 ,
.Xr ldattach 8 ,
.Xr ntpd 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
interface first appeared in
.Ox 4.0 .
.Sh AUTHORS
The
.Nm
line discipline was written by
.An Marc Balmer Aq Mt mbalmer@openbsd.org .