The Lunar Orbiter Laser Altimeter (LOLA) instrument [1-3] on NASA’s Lunar Reconnaissance Orbiter (LRO) mission, launched on June 18th, 2009, from Kennedy Space Center, Florida, will provide a precise global lunar topographic map using laser altimetry. LOLA will assist in the selection of landing sites on the Moon for future robotic and human exploration missions and will attempt to detect the presence of water ice on or near the surface, which is one of the objectives of NASA’s Exploration Program.
Our present knowledge of the topography of the Moon is inadequate for determining safe landing areas for NASA’s future lunar exploration missions. Only those locations, surveyed by the Apollo missions, are known with enough detail. Knowledge of the position and characteristics of the topographic features on the scale of a lunar lander are crucial for selecting safe landing sites. Our present knowledge of the rest of the lunar surface is at approximately 1 km kilometer level and in many areas, such as the lunar far side, is on the order of many kilometers. LOLA aims to rectify that and provide a precise map of the lunar surface on both the far and near side of the moon.
LOLA uses short (6 ns) pulses from a single laser through a Diffractive Optical Element (DOE) to produce a five-beam pattern that illuminates the lunar surface. For each beam, LOLA measures the time of flight (range), pulse spreading (surface roughness), and transmit/return energy (surface reflectance). LOLA will produce a high-resolution global topographic model and global geodetic framework that enables precise targeting, safe landing, and surface mobility to carry out exploratory activities. In addition, it will characterize the polar illumination environment, and image permanently shadowed regions of the lunar surface to identify possible locations of surface ice crystals in shadowed polar craters.
The first NASA Ice, Cloud and land Elevation Satellite (ICESat) was launched in January 2003 and placed into a nearpolar
orbit whose primary mission was the global monitoring of the Earth's ice sheet mass balance. ICESat has
accumulated over 1.8 B shots in space and provided a valuable dataset in the study of ice sheet dynamics over the past
few years. NASA is planning a follow-on mission ICESat-2 to be launched tentatively in 2015. In this paper we will
discuss the development effort of the laser transmitters for the ICESat-2 mission.
The Fizeau Interferometer Testbed (FIT) is a ground-based laboratory experiment at Goddard Space Flight Center (GSFC) designed to develop and test technologies that will be needed for future interferometric spacecraft missions. Specifically, the research from this experiment is a proof-of-concept for optical accuracy and stability, closed-loop control algorithms, optimal sampling methodology of the Fourier UV-plane, computational models for system performance, and image synthesis techniques for a sparse array of 7 to 30 mirrors. It will assess and refine the technical requirements on hardware, control, and imaging algorithms for the Stellar Imager (SI), its pathfinder mission, and other sparse aperture and interferometric imaging mission concepts. This ground-based optical system is a collaborative effort between NASA's GSFC, Sigma Space Corporation, the Naval Research Laboratory, and the University of Maryland. We present an overview of the FIT design goals and explain their associated validation methods. We further document the design requirements and provide a status on their completion. Next, we show the overall FIT design, including the optics and data acquisition process. We discuss the technologies needed to insure success of the testbed as well as for an entire class of future mission concepts. Finally, we compare the expected performance to the actual performance of the testbed using the initial array of seven spherical mirrors. Currently, we have aligned and phased all seven mirrors, demonstrated excellent system stability for extended periods of time, and begun open-loop operations using "pinhole" light sources. Extended scenes and calibration masks are being fabricated and will shortly be installed in the source module. Installation of all the different phase retrieval/diversity algorithms and control software is well on the way to completion, in preparation for future tests of closed-loop operations.
The Fizeau Interferometer Testbed (FIT) is a ground-based system that will be used for the development and testing of technologies relevant to Stellar Imager (SI) and other sparse aperture/Fizeau imaging interferometer mission concepts. The testbed will utilize image-based wavefront sensing and control to co-phase and maintain closed-loop control over a Sparse Aperture Array (SAA) consisting of spherical mirror elements. The SAA is a re-configurable assembly baselined to incorporate between seven (initially) and thirty 12.5mm diameter (R = 4000mm) mirror elements. In this paper we describe the fabrication, alignment, and initial calibration of the phase I (7 primary elements) FIT hardware and discuss various factors impacting the performance and stability of the testbed.
Stellar Imager (SI) is a potential NASA space-based UV imaging interferometer to resolve the stellar disks of nearby stars. SI would consist of 20 - 30 separate spacecraft flying in formation at the Earth-Sun L2 libration point. Onboard wavefront control would be required to initially align the formation and maintain alignment during science observations and after array reconfiguration. The Fizeau Interferometry Testbed (FIT) is a testbed currently under development at the NASA/Goddard Space Flight Center to develop and study the wavefront control methodologies for Stellar Imager and other large, sparse aperture telescope systems. FIT consists of 7 articulated spherical mirrors in a Golay pattern, expandable up to 30 elements, and reconfigurable into multiple array patterns. FIT’s purpose is to demonstrate image quality versus array configuration and to develop and advance the wavefront control for SI. FIT uses extended scene wavelength, focus and field diversity to estimate the wavefront across the set of apertures. The recovered wavefront is decomposed into the eigenmodes of the control matrix and actuators are moved to minimize the wavefront piston, tip and tilt. Each mirror’s actuators are 3 degrees of freedom, however, they do not move each of the mirrors about a point on each mirrors surface, thus the mapping from wavefront piston, tip/tilt to mirror piston, tip/tilt is not diagonal. We initially estimate this mapping but update it as part of wavefront sensing and control process using system identification techniques. We discuss the FIT testbed, wavefront control methodology, and show initial results from FIT.