Historically, laser ranging using a laser/lidar and a distant retroreflecting mirror (corner cube) has been used to measure the distance from the Earth to the Moon for over 60 years, and has been used to measure continental drift and urban mapping for decades. Most of these past measurements used classical Nd:YAG 1.0642 µm lasers with a Q-switched laser pulse length of about 10 ns which translates to a range resolution of about 1.5 m. Recently, however, there has been considerable interest in using picosecond (ps) lasers in a lidar system to greatly enhance the range accuracy to on the order of mm. Toward this end, an in-depth optical analysis and multi-wavelength laser/lidar performance measurements of compact retroreflector arrays suitable for moon landings have been made by a lunar landing group at NASA Goddard, SSAI, KBR Tech, and MIT. The group had previously designed and tested a compact retroreflecting sphere consisting of eight 0.5 inch retroreflecting arrays contained on the surface of a 5 cm diameter hemisphere. This initial system was designed for use in the vacuum and solar radiation flux found on the lunar surface and was mostly concerned with the mechanical reliability due to high-g launch vibrations, degradation of the optical coatings from radiation, and alignment of the individual retros. Based upon these early successes, the group has now expanded the research to include the spectral response, optical characterization, and longer range testing of the arrays for a wider range of laser wavelengths (0.542 and 1.064 µm), polarizations, and far-field diffraction patterns over the lunar temperature range (100 to 380 K) . Of importance is that they have performed the optical characterization of the arrays over the spectral range of the visible to 3.5 µm, determined the equivalent of the array optical cross sections (approaching 0.5 million m2
), and developed and compared Al coated vs Total Internal Reflection retroreflecting corner cubes which showed that Al coated surfaces increased the return signal by about a factor of 3. They also used a high resolution camera to map the optical retroreflected image to see the pixel-to-pixel variation, and related the imaging data to the actual optical center of the array (where the retro axis crossed) to a precision of 0.1 mm in order to better determine the lidar return signal timing accuracy. For lidar measurements, they used a 1064 nm fiber coupled pulsed laser with a pulse length of 48 ps. This pulse length showed a centroid position accuracy of 39 ps, which corresponds to a lidar range accuracy of about 6 mm (i.e., Δt·c/2). Their research shows that the use of a lidar and several retroreflecting arrays placed strategically on the lunar surface should provide accurate and precision lunar landings at ranges out to about 300 km.
Lastly, it is astounding to think that this means that such a lunar landing system using a ps laser/lidar will be able to map and triangulate in 3D its position with an accuracy on the order of a few millimeters and thus almost duplicate what a satellite based GPS system does for us on Earth (and with greater accuracy).
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