Lidar (LIght Detection and Ranging) is an active remote sensing technology first developed in the mid 1960s. It is similar to radar with the main difference being the portion for the electromagnetic spectrum employed. Radar uses wavelengths from a few millimeters to meters whereas lidar uses wavelengths in the visible to near-infrared part of the spectrum. The wavelength that an instrument uses will determine the size of the object that it is able to detect.
An example of lidar data. The vertical axis irepresents altitude (0-60 km) and the horizontal axis represents time. Brighter the color, the more light was returned to the instrument. In this case, clouds are seen passing by over the instrument.
In 1993 Goddard Spaceflight center constructed a prototype MPL that operated at 523 nm, the MPL000. This was soon replaced by new and improved models also operating at 523 nm or similar. Years later, the University of Arizona received the MPL000 in order to modify the system to operate at 1047 nm.
A 532 nm system can be calibrated by looking at a portion of data that is known to have minimal aerosols, i.e. high elevation, and comparing these data to what is predicted. This molecular prediction based off of Rayleigh scattering and is fairly well known. A system operating at 1047 on the other hand does not have this advantage. This wavelength is more sensitive to aerosols and much less sensitive to the molecular component of the atmosphere. This means that a system at 1047 nm will be much more difficult to calibrate since its output cannot be compared with a well-known prediction, but will have a much better signal for detecting aerosols.