Small and lightweight laser retro-reflector arrays for lunar landers

Calibration method of the right-angle error of a hollow corner-cube retroreflector based on an independent autocollimator

Hollow corner-cube retroreflectors (HCCRs) are an essential reflection component of next-generation lunar laser-ranging technology. The verticality among the three reflectors, known as the right-angle error, is a critical parameter that affects the emission performance, and thus, should be correctly measured and calibrated. However, conventional methods measure the three right-angle errors separately, which can induce error superposition during the measurement process. A one-time measurement method was developed for the three right-angle errors of the HCCR using a single autocollimator (AC). The method establishes a mathematical relationship between the right-angle error of the HCCR and the angle offset of the reflected beam, and it considers the observation coordinates of the AC simultaneously to perform the coordinate transformation of the relationship parameters. The corresponding measurement equation was derived to extract the three-plane right-angle error of the HCCR using the measured readings of a single AC. In addition, a HCCR was designed to freely adjust the angle of the two reflective surfaces and used to simulate the different states of the three right-angle errors in practice. The measurement results show that the root-mean-square error of the proposed method in all right-angle error states is smaller than 16 ' ' .

Low-Latency Beaming Display: Implementation of Wearable, 133 μs Motion-to-Photon Latency Near-Eye Display

This paper presents a low-latency Beaming Display system with a 133 μs motion-to-photon (M2P) latency, the delay from head motion to the corresponding image motion. The Beaming Display represents a recent near-eye display paradigm that involves a steerable remote projector and a passive wearable headset. This system aims to overcome typical trade-offs of Optical See-Through Head-Mounted Displays (OST-HMDs), such as weight and computational resources. However, since the Beaming Display projects a small image onto a moving, distant viewpoint, M2P latency significantly affects displacement. To reduce M2P latency, we propose a low-latency Beaming Display system that can be modularized without relying on expensive high-speed devices. In our system, a 2D position sensor, which is placed coaxially on the projector, detects the light from the IR-LED on the headset and generates a differential signal for tracking. An analog closed-loop control of the steering mirror based on this signal continuously projects images onto the headset. We have implemented a proof-of-concept prototype, evaluated the latency and the augmented reality experience through a user-perspective camera, and discussed the limitations and potential improvements of the prototype.

Early ICESat-2 on-orbit Geolocation Validation Using Ground-Based Corner Cube Retro-Reflectors

The Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), an Earth-observing laser altimetry mission, is currently providing global elevation measurements. Geolocation validation confirms the altimeter’s ability to accurately position the measurement on the surface of the Earth and provides insight into the fidelity of the geolocation determination process. Surfaces well characterized by independent methods are well suited to provide a measure of the ICESat-2 geolocation accuracy through statistical comparison. This study compares airborne lidar data with the ICESat-2 along-track geolocated photon data product to determine the horizontal geolocation accuracy by minimizing the vertical residuals between datasets. At the same location arrays of corner cube retro-reflectors (CCRs) provide unique signal signatures back to the satellite from their known positions to give a deterministic solution of the laser footprint diameter and the geolocation accuracy for those cases where two or more CCRs were illuminated within one ICESat-2 transect. This passive method for diameter recovery and geolocation accuracy assessment is implemented at two locations: White Sands Missile Range (WSMR) in New Mexico and along the 88°S latitude line in Antarctica. This early on-orbit study provides results as a proof of concept for this passive validation technique. For the cases studied the diameter value ranged from 10.6 to 12 m. The variability is attributed to the statistical nature of photon-counting lidar technology and potentially, variations in the atmospheric conditions that impact signal transmission. The geolocation accuracy results from the CCR technique and airborne lidar comparisons are within the mission requirement of 6.5 m.

Optical characterization of laser retroreflector arrays for lunar landers

The Laser Retroreflector Array for Lunar Landers (LRALL) is a small optical instrument designed to provide a target for precision laser ranging from a spacecraft in lunar orbit, enabling geolocation of the lander and its instrument suite and establishing a fiducial maker on the lunar surface. Here we describe the optical performance of LRALL at visible and near-infrared wavelengths. Individual corner cube reflectors (CCRs) within LRALL were tested for surface flatness and dihedral angle values. We also imaged the far-field diffraction patterns of individual CCRs as well as the entire retroreflector array over the range of possible incident angles to extract the optical cross section as a function of viewing angle. We also measured the optical properties of one of the CCRs over the lunar temperature range (100-380 K) and found no significant temperature-dependent variance. The test results show LRALL meets the design criteria and can be ranged to elevation angles above 30° with respect to the instrument base from an orbital laser altimeter such as the Lunar Orbiter Laser Altimeter on the Lunar Reconnaissance Orbiter. This work summarizes the test data and serves as a guide for future laser ranging to these retroreflector arrays.