It is well known that stratospheric ozone is important because it blocks out the harmful UV rays of the sun. Canada, and other countries, have been involved in global ozone observations for nearly 40 years. Nowadays, part of that monitoring involves the use of Canadian technology on ER-2 high altitude planes flown by NASA on scientific missions. Dr Tom McElroy of Environment Canada works with the spectrometer, the only non-American instrument on the plane. The spectrometer is mounted inside a pod on the wing of the plane, where a missile detector used to be when versions of these aircraft, then known as U-2, were flown on spy missions in the 1950s and 1960s. The instruments measure how much energy is hitting the air, the light coming from the horizon and from the sun itself.