to the sixth degree of freedom 34 the distance measuring device 80 supplies a voltage proportional to the distance covered by the machine from the point of origin in this example it has a wheel towed behind the machine and an associated incremental angular encoder as the wheel turns the encoder generates an electrical signal in the form of a series of pulses the encoder may be any commercially available device subject to calibration beforehand counting these pulses provides a way of computing the distance covered by the machine from the point of origin fig2 is a diagram showing the main parts of the laser receivers used in this embodiment of the present invention each receiver 30 i embodies four lenses 31 through 34 focusing the laser beam on four receivers photodetectors 311 321 331 and 341 each at the focus of a respective lens 31 through 34 the photodetectors employed are os d 60 0 silicon photodiodes manufactured and marketed by the company centronic limited of surrey great britain the lenses employed are 01 lpx 111 plano convex lenses 25 u00d7 25 mm 2 with a focal length of 50 mm marketed by the company melles griot of irvine calif their usable light receiving diameter is approximately 50 mm the point at which the lenses 31 through 34 intersect defines the axis o i of the.