In order to map the direction of the transmitter, the transmitter heading needs to be relative to a compass heading.  When in a moving vehivicle that can be obtained from GPS.  A real compass is necessary for stationary readings.  I started playing with the HMC5883L and MLX90393 magnetometers.   The sensors do function as compasses but they require software and calibration.  There are Adafruit and Grove libraries for them and some others as well.  The Adafruit library for the HMC5883 yielded the best results but the libraries didn't play well with the other interrupts.   In order to spare a major detour I plan on using a spendy HMC6643 compass module that includes it own microcontroller and comes calibrated.    

To calibrate, the transmitter will be placed as far away as possible from the front of the car and will need a high quality reading.  The raw measured value is replaced by the compass is reading.