The GPS preview is a GTK based project to create a usable GUI tool with the simple output (no maps). It is built and tested around the gllin driver but should work with any other driver via pipe at /tmp/nmeaNP. It detects GPRMC and GPGGA messages and constantly shows the following data on the screen:
- Speed in kilometers per hour (converts from knots).
- Curved distance from the initially marked point (uses advanced IERC 2003 geodetic reference to convert degrees into kilometers).
- Number of the satellites.
- Time (UTM, not a local time).
The source code of this project is available in the project page (http://projects.openmoko.org/projects/gpv/), under LGPL. The project uses GTK framework and C programming language.
The GPS Preview program (gpv) is released either in .ipk file or in the form of the source code. The released installer still needs the gllin to run, but, if needed, can start it itself.
Inside the project
The most interesting part of this project was to keep the external driver thread and GUI thread sufficiently happy together. We cannot just read from the pipe in a loop as this blocks the GUI repainting! Instead, the code schedules the GTK timer, and then it needs to check if any input from the pipe is available - again, without blocking the current thread for too long. This is done using select function which can check the given stream for the availability of input, passing the certain time-out duration. This time-out seems also tricky: if the value or 0.1 s or less is passed on the Neo, function never reports any input present. However too large values block GUI in the input check step. The code now has a kind of adaptation, gradually increasing the duration of this check if there is no input available for a while.
The distance counter tries to track the curved distance, not the "bird flight" distance between two points. It adds the shifts in space, computing the length of the lat/long degrees into meters in accordance to the IERS 2003 goedetic reference - very new and not yet widely used in geography.
The distance counter may have its own problems, adding minor random changes of the reported location just due measurement error. The algorithm accepts these errors as an actual "walking step back and forward", so the distance counter may slowly go up while standing in place. This, however, should only be noticeable during slow walking - cycling is already fast enough to make this problem minor.
Testing the device
The device has been tested outdoors where it shows the correct location and altitude. The speed indicator has been tested on the train, correctly showing the speed of about 100 km/h.
Nobody yet tested, how accurate the distance counter is (please update this information if you ever tried).
The project uses CVS to host the most recent source code and all build environment and sometimes does releases of .ikp binaries and plain gzipped source code. Project also uses Wiki page to describe various technical aspects.