If you are going to use a PIC, wouldnt it just be easier to have the PIC take the NMEA data from the GPSr on its own, skipping the D/A conversion? Beyond that, you would need to convert the decimal serial output of the GPSr into a binary value which you could then apply to a D/A conversion for input to the PIC. However, this has the added problem that you need to know the range and possibly how to scale the value before you apply it to the D/A, that is probably the hardest part -- in fact, it would be best to use a micro to do this.