It would appear that the measured mile is measured using GPS technology and then lasers are positioned at the points along the course - the run must then be completed in the opposite direction (over come bias of favourable winds etc). An average of the 2 runs is then taken.
GPS technology is used to measure the land speed record. Lasers are positioned along the course of the measured mile and an average of two runs is taken.
A pretty pony
A frog with a tophat runs along side of the car and tells the people how fast it was going, they use this method instead of the 'GPS' I've known it can also be a pretty pony aswell! It is interesting because where they drive the cars/rockets is on a salt plain, >fish and chips with a pinch of SALT...nom nom nom< Each car used needs to specially modified so it can be able to start properly no this is important othewise they wouldn't finish the distance....etc. :D .