The cart has a PNI V2XE compass module  http://www.pnicorp.com/products/all/v2xe with a comm board to manage serial communication. The compass is mounted on the underside of the roof of the cart.

Device:

/dev/compass9600 8N1

Inputs:

nonenone

Outputs:

headingdecimal degrees (0-360) corrected for declination, increasing positive values indicate clockwise rotation. 0 degrees is north

Source:

source:embedded_systems/sensor/compass/trunk/compass.py

Reference:

source:embedded_systems/sensor/compass/trunk/doc/commboard.pdf

source:embedded_systems/sensor/compass/trunk/doc/V2Xe.pdf