Topography is essential to safe lunar landings. On both a local and global scale, knowledge of the lunar surface topography is vital to landing site selection. Topography, surface slopes and surface roughness also preserve a scientific record of the evolution of the lunar surface.
The Lunar Orbiter Laser Altimeter (LOLA) has two primary objectives. First, LOLA will produce a high-resolution global topographic model and geodetic framework that will assist with precise targeting, safe landing, and surface mobility for future scientific and exploration activities. LOLA will also characterize the polar illumination environment and image the Moon’s PSRs to identify possible locations of surface ice crystals in shadowed polar craters. To achieve these primary objectives, LOLA will make three measurements:
1) the distance between the surface and the spacecraft 2) the spreading of the returned laser pulse 3) the transmitted and returned laser energies
LOLA is a pulse detection time-of-flight altimeter that incorporates a five-spot pattern that measures the precise distance to the lunar surface at 5 spots simultaneously, thus providing 5 profiles across the lunar surface. Each spot within the five-spot pattern has a diameter of five meters; the spots are 25 meters apart, and form a cross pattern (Fig. 1). The 5-spot pattern enables the surface slope to be derived in the along-track and across track directions; the pattern is rotated approximately 26 degree to provide five adjacent profiles, 10 to 12 meters apart over a 50 to 60 meter swath, with combined measurements in the along track direction every 10 to 12 meters.LOLA will provide a 10 cm local topography in a center-of-mass coordinate system for regions displaying low slopes. The body fixed center-of-mass measurements will have a nominal accuracy of a fiducial position of ∼50 m on the surface and 1 m elevation. In addition, each 5-meter spot provides a measure of the surface roughness to ∼30 cm within the (flat) spot, derived from the spreading of the laser pulse. LOLA will also measure the relative surface reflectance within each 5-m spot at the wavelength of the laser to a 5% precision, enabling the detection of highly reflective material on the lunar surface (such as water ice crystals).
Fig. 1 Pulse detection time-of-flight altimeter 5-spot pattern. Red represents the laser spots on the ground while the grey circles represent the receiver FOV
LOLA’s instrument design (Fig. 2) has five laser beams and five receiver channels. LOLA’s laser transmitter consists of a single stage diode-pumped and Q-switched Nd:YAG laser with a 1064 nm wavelength, a 2.7 mJ pulse energy, a 6 ns pulse, a 28 Hz pulse rate, and a 100 μrad beam divergence angle. A diffractive optics element made of fused silica with an etched-in diffraction pattern is used to split the single incident laser beam into five off-pointed beams, creating the 50 meter diameter 5-spot cross-pattern on the lunar surface. The reflected signal is collected by a 14-cm diameter telescope with a 5-optical-fiber array at the focal plane. Each of the five optical fibers collects the reflected signal from one of the five laser spots on the lunar surface, and delivers it to one of the five avalanche photodiodes. The transmitted laser pulse and the five received laser pulses are time tamped with respect to the spacecraft mission elapsed time using a set of time-to-digital converters at <0.5 ns precision. In addition, LOLA measures the transmitted and received pulse by integrating the pulse waveforms. The on-board science algorithm, running on an embedded microprocessor, autonomously adjusts the receiver detection threshold levels and detector gain to keep the range window tracking the lunar surface returns[8].
Fig. 2 LOLA instrument design
The key instrument parameters are listed in Table 1.
Table 1 LOLA instrument overview