Algebra
Smart software using simple maths calculations derived from the position info of the gps works it out. if you imagine a chess board and the gps places you at white kings rook or A1 the next position fix is at white king 4 or D4, then your track, assuming the board is north south, would be appx 050, you can work it out exactly if you know the dimensions of the squares etc. If you stand still with a portable it will fluctuate in heading and speed. Start moving in a straight line and you will soon have heading and speed indications. So the unit needs to be moving to give you this info. Hope that helps PG.