It was early 1981, and they were testing a new technology: a vertically mounted single-beam laser that would hit the trees in a straight line and occasionally would reach the ground, giving enough data points to extrapolate the contour lines of the rough surface. The bulky equipment was an Airborne Profile Recorder (APR) called Geodolite 3A and manufactured by Spectra Physics and equipped with an analog printer attached to it.
Juan Plaza shares the untold story of the Airborne Profile Recorder (APR), the precursor to lidar in Adventures in the Amazon (Part 1) | Geo Week News.