Follow the rocket in the sights to apogee, release the trigger to lock the reading. It is that easy. Use two Altitraks for greater accuracy.From the ManufacturerUse the NEW Altitrak to instantly tell how high a rocket flies. Fully assembled and ready to use right out of the package, the Altitrak measures altitudes as high as 1,300 ' . Simply aim Altitrak at the rocket before lift-off, squeeze and hold the trigger , then follow the rockets flight, releasing the trigger at apogee. Read the height instantly in the pointer?s window or use the degree scale to make your own calculations. Scale reads in meters. Convert to ' using the simple formula or the angle table printed on the package to figure how high the rocket flew. Easy to operate for all ages. Great for contests, clubs or schools.