nmea —
NMEA 0183 timedelta sensor
pseudo-device nmea [count]
This line discipline interfaces NMEA devices, such as GPS receivers attached to
a serial or USB port.
The line discipline is enabled by the following sequence:
#include <sys/ioctl.h>
int ldisc = NMEADISC, fildes; ...
ioctl(fildes, TIOCSETD, &ldisc);
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 sysctl(8)
interface.
The nmea line discipline decodes the
following NMEA 0183 sentences:
- 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.
- GGA
- Current fix data. The altitude in meters is extracted.
Messages source are recognised by the first two characters of the
NMEA sentence according to the following prefixes:
- GP
- GPS
- GA
- Galileo
- BD
- BeiDou
- GL
- Glonass
- GN
- Generic GNSS
The quality of the timedelta is reported as the sensor status:
- OK
- The time information and position are valid. The timedelta is safe to use
for applications like ntpd(8).
- 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.
- 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.
The status of a second sensor is used to report the status of the
device itself:
- OK
- The clock is synchronized, e.g. a GPS receiver has a fix.
- WARN
- The device issued a warning condition, e.g. a GPS receiver has no
fix.
The nmea interface first appeared in
OpenBSD 4.0.