Much of the light passes sidewards out of the crystal, but light moving along the axial zone of the cylinder encounters other Cr ions as it reflects repeatedly. Other laser materials include gallium arsenide and excited gases such as neon or helium. Both visible and infra-red wavelength light can be generated in this way. Laser altimetry has been conducted from aircraft platforms for several decades. Aircraft can be scheduled to fly in good weather, offsetting the main disadvantage of using lasers coming from cloud interference with the light beam.
Lasers are also reliable and very accurate tools for determining distances to satellites in orbit.
Even at their high orbital speeds, these spacecraft can be targeted to intercept a laser beam pulse directed at them from Earth-based stations which is then bounced back by the reflectors. Along with GPS, this is one way in which the orbital position of a satellite can be precisely fixed. The astronauts in later Apollo missions left retroreflectors on the Moon's surface as a means of learning more about its orbital motions, including its recession from Earth. Like lasers, the signals are dispatched as short pulses, but at the longer radar wavelengths these signals can penetrate cloud cover. Again, using roundtrip times the instrument acts as a ranger to determine distance to target.