Doppler was an Austrian mathematician and physicist. He was born in Salzburg, Austria, the son of a stone mason. After completing high school Doppler studied astronomy and mathematics in Vienna and Salzburg and started work at the Prague Polytechnic. At the age of 39 Doppler published his most famous work, "Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels" (On the colored light of the binary stars and some other stars of the heavens). In this works Doppler suggested his principle that the observed frequency of a wave depends on the relative speed of the source and the observer. He tried to use this theory for explaining the colors of binary stars. LiDAR (Light Detection And Ranging) sensors work on the same principle as RADAR, firing a wavelength at an object and timing the delay in its return to the source to measure the distance between the two points. Because laser light has a much shorter wavelength it is possible to accurately measure much smaller objects, such as aerosols and cloud particles, which makes it especially suitable for airborne terrain mapping. The LiDAR instrument fires rapid pulses of laser light at a surface, some at up to 150,000 pulses per second. A sensor on the instrument measures the amount of time it takes for each pulse to bounce back. Light moves at a constant and known speed so the LiDAR instrument can calculate the distance between itself and the target with high accuracy. By repeating this in quick succession the instrument builds up a complex 'map' of the surface it is measuring. With airborne LiDAR other data must be collected to ensure accuracy. As the sensor is moving height, location and orientation of the instrument must be included to determine the position of the laser pulse at the time of sending and the time of return.