Overview

python-gpsdshm provides a read-only(!) Python interface to gpsd’s shared memory. It provides a single class Shm that exposes the fields of the shared memory structure as attributes. The python-gpsdshm API is (loosely) modelled on gpsd version 3.16 (API 6.1). gpsd releases earlier than 3.0 are not supported.

python-gpsdshm is implemented using Swig and requires the gpsd header files for compilation.

Many Linux distributions ship the gpsd package without shared memory support. See Compiling gpsd with shared memory support for details how to build gpsd with shared memory support.

Example

The example below shows all attributes and typical values (of a stationary GPS, placed about one metre inside a window).

>>> import gpsdshm
>>> gpsd_shm = gpsdshm.Shm()
>>> gpsd_shm.set
TODO: <Swig Object of type 'gps_mask_t *' at 0x133bf50>
>>> gpsd_shm.online               # True if GPS is online
gpsd_shm.online
TODO: Out[6]: 1454057376.6934643
>>> gpsd_shm.status               # Do we have a fix (True/False)?
gpsd_shm.status
TODO Out[7]: 1
>>> gpsd_shm.skyview_time         # Skyview timestamp
>>> gpsd_shm.satellites_visible   # Number of satellites in view
TODO: nan
>>> gpsd_shm.fix.time             # Time of update
1454057448.0
>>> gpsd_shm.fix.mode             # Mode of fix (0=not seen, 1=no fix, 2=2D, 3=3D)
3
>>> gpsd_shm.fix.ept              # Expected time uncertainty
0.005
>>> gpsd_shm.fix.latitude         # Latitude in degrees (valid if mode >= 2)
-37.75859333333334
>>> gpsd_shm.fix.epy              # Latitude position uncertainty, meters
14.853882022256698
>>> gpsd_shm.fix.longitude        # Longitude in degrees (valid if mode >= 2)
144.99911
>>> gpsd_shm.fix.epx              # Longitude position uncertainty, meters
10.040781879264545
>>> gpsd_shm.fix.alitude          # Altitude in meters (valid if mode == 3)
68.2
>>> gpsd_shm.fix.epv              # Vertical position uncertainty, meters
19.55
>>> gpsd_shm.fix.track            # Course made good (relative to true north)
44.45
>>> gpsd_shm.fix.epd              # Track uncertainty, degrees
nan
>>> gpsd_shm.fix.speed            # Speed over ground, meters/sec
0.3446777748
>>> gpsd_shm.fix.eps              # Speed uncertainty, meters/sec
29.707764044513397
>>> gpsd_shm.fix.climb            # Vertical speed, meters/sec
0.0
>>> gpsd_shm.fix.epc              # Vertical speed uncertainty
nan
>>> gpsd_shm.dop.xdop
0.6693854586176363
>>> gpsd_shm.dop.ydop
0.9902588014837799
>>> gpsd_shm.dop.pdop
1.17
>>> gpsd_shm.dop.hdop
0.99
>>> gpsd_shm.dop.vdop
0.82
>>> gpsd_shm.dop.tdop
1.2548625723457216
>>> gpsd_shm.dop.gdop
2.4342743978108503

Information about satellites is contained in the satellites list.

>>> gpsd_shm.satellites[0].ss         # Signal-to-noise ratio (dB)
16.0
>>> gpsd_shm.satellites[0].used       # Used in solution?
False
>>> gpsd_shm.satellites[0].prn        # PRNs of satellite
6
>>> gpsd_shm.satellites[0].elevation  # Elevation of satellite, degrees
56
>>> gpsd_shm.satellites[0].azimuth    # Azimuth, degrees
59

Compiling gpsd with shared memory support

$ git clone git://git.sv.gnu.org/gpsd.git   # git clone http://git.savannah.gnu.org/r/gpsd.git
$ cd gpsd
$ git tag
$ git checkout release-3.16
$ scons prefix=/usr/local shm_export=yes
$ sudo scons install

Then run start gpsd and check whether the shared segment has been created.

$ sudo /usr/local/sbin/gpsd -n /dev/ttyAMA0
$ ipcs -m | grep 0x47505344
------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x47505344 163844     root       666        31616      1

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