The history of LiDAR

The oldest known variation of modern LiDAR systems evolved in nature millions of years ago. The bat used a guidance system now known as SONAR (SOund Navigation And Ranging). They emit short 'chirps' from their noses and receive an echo through their ears in the form of two antennae. This provides the bat with a three-dimensional view of the surrounding area, allowing them to avoid obstacles and to find their prey.

Humans started to develop similar systems in the beginning of the 20th century. Christian Huelsmeyer's "Telemobiloscope", developed in 1904, was the first form of RADAR (RAdio Detection And Ranging) sensor. This used radio waves outside the audible range. It consisted of an antenna, a receiver and a transmitter. Its original use was to detect metallic objects, in particular ships at sea, as a form of collision avoidance. This early form of RADAR had a distance of 3000m, much less than today’s modern alternatives. On the detection of an object it would sound a bell until the object left its path. The method of determining distance was later solved by aiming the beam at any level of elevation. Taking into account the height of the transmitting antenna and the angle of vertical elevation of the detected object allowed simple calculation to determine the distance of the object from the transmitter.

RADARs transmit a narrow, rectangular shaped pulse modulating in a sine wave carrier. Distance is measured by the time it takes the pulse to travel to and from the target. It is also possible to use a continuous waveform showing the Doppler frequency shift to measure the targets velocity. The Doppler effect, named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the waves.

The principle of RADAR is also used in optics. There are only two major distinctions between the signals. These are:

• The wavelengths are different. Light has a wavelength of a minute fraction of a millimetre, whereas radars use wavelengths between 1mm and 100m.
• Humans don’t have the necessary sensors for the wavelength bands. Our senses cover the optical band (perceived as light) and the far infrared which is perceived as heat.

LiDAR 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.

LiDAR has been used extensively for atmospheric research and meteorology due to its excellent resolution. It was only with the deployment of Global Positioning Systems (GPS) in the 1980s, allowing the precise positioning of aircraft, that LiDAR made airborne surveying possible. Since then many downward looking LiDAR instruments have been developed for aircraft and satellite use.