Computing Position Problem

I'm building a program outside of flight simulator that emulates the ND features of the 737NG using FSUIPC as its interface point. I have almost all the data presented I need except for the computed position.The ND display is oriented in the Heading up or Track up modes. From the tip of the aircraft pointer extends a 3-dash line where the end of each line is at 30 seconds from the previous point, the tip being 90 seconds from the aircraft's present position. During a high-rate turn, it would form a semi-circle and in a high-enough turn, would show a circle.My question is how to compute the value of position. I know it would involve groundspeed and turn rates, but how they compute position is a little out of reach of my mind. Maybe i'm burned out from programming so much that my mind doesn't want to comprehend this problem.Anyone got any bright ideas?