*-- Author :    Stephen J. Maxfield   22/04/94
      FUNCTION FRDT2D(TS, V0, V1, S1)
*                                                                                                             
*     The time-to-distance function.                                                                          
*     TS is time in ticks scaled by average drift velocity in cm/tick                                         
*     V0, V1 (corrected) velocities at wire and 1st knot point                                                
*     S1 distance in cm (corrected) to 1st knot point                                                         
*                                                                                                             
*     Return drift distance in cm.                                                                            
*                                                                                                             
*      Time taken to arrive at first knot point...                                                            
*