John F. Cavanaugh

Initial observations from the Lunar Orbiter Laser Altimeter (LOLA)

As of June 19, 2010, the Lunar Orbiter Laser Altimeter, an instrument on the Lunar Reconnaissance Orbiter, has collected over 2.0 × 10^9 measurements of elevation that collectively represent the highest resolution global model of lunar topography yet produced. These altimetric observations have been used to improve the lunar geodetic grid to ~10 m radial and ~100 m spatial accuracy with respect to the Moons center of mass. LOLA has also provided the highest resolution global maps yet produced of slopes, roughness and the 1064-nm reflectance of the lunar surface. Regional topography of the lunar polar regions allows precise characterization of present and past illumination conditions. LOLAs initial global data sets as well as the first high-resolution digital elevation models (DEMs) of polar topography are described herein.

Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles

Wet Mercury Radar observations of Mercurys poles in the 1990s revealed regions of high backscatter that were interpreted as indicative of thick deposits of water ice; however, other explanations have also been proposed (see the Perspective by Lucey). MESSENGER neutron data reported by Lawrence et al. (p. 292, published online 29 November) in conjunction with thermal modeling by Paige et al. (p. 300, published online 29 November) now confirm that the primary component of radar-reflective material at Mercurys north pole is water ice. Neumann et al. (p. 296, published online 29 November) analyzed surface reflectance measurements from the Mercury Laser Altimeter onboard MESSENGER and found that while some areas of high radar backscatter coincide with optically bright regions, consistent with water ice exposed at the surface, some radar-reflective areas correlate with optically dark regions, indicative of organic sublimation lag deposits overlying the ice. Dark areas that fall outside regions of high radio backscatter suggest that water ice was once more widespread. Spacecraft data and a thermal model show that water ice and organic volatiles are present at Mercury’s north pole. [Also see Perspective by Lucey] Measurements of surface reflectance of permanently shadowed areas near Mercury’s north pole reveal regions of anomalously dark and bright deposits at 1064-nanometer wavelength. These reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near-surface water ice. Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice, whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation. Impacts of comets or volatile-rich asteroids could have provided both dark and bright deposits.

Airborne lidar for profiling of surface topography

A lidar system is described that measures laser pulse time-of-flight and the distortion of the pulse waveform for reflection from earth surface terrain features. This instrument system is mounted on a high-altitude aircraft platform and operated in a repetitively pulsed mode for measurements of surface elevation profiles. The laser transmitter makes use of recently developed short-pulse diode-pumped solid-state laser technology. Aircraft position in three dimensions is measured to submeter accuracy by use of differential Global Positioning System receivers. Instrument construction and performance are detailed.

Aerosol and cloud backscatter at 1.06, 1.54, and 0.53 µm by airborne hard-target-calibrated Nd:YAG/methane Raman lidar

A lidar instrument was developed to make simultaneous measurements at three distinct wavelengths in the visible and near infrared at 0.532, 1.064, and 1.54 mum with high cross-sectional calibration accuracy. Aerosol and cloud backscatter cross sections were acquired during November and December 1989 and May and June 1990 by the NASA DC-8 aircraft as part of the Global Backscatter Experiment. The instrument, methodology, and measurement results are described. A Nd:YAG laser produced 1.064- and 0.532-mum energy. The 1.54-mum transmitted pulse was generated by Raman-shifted downconversion of the 1.064-mum pulse through a Raman cell pressured with methane gas. The lidar could be pointed in the nadir or zenith direction from the aircraft. A hard-target-based calibration procedure was used to obtain the ratio of the system calibration between the three wavelengths, and the absolute calibration was referenced to the 0.532-mum lidar molecular backscatter cross section for the clearest scattering regions. From the relative wavelength calibration, the aerosol backscatter cross sections at the longer wavelengths are resolved for values as small as 1% of the molecular cross section. Backscatter measurement accuracies are better than 10(-9) (m sr)(-1) at 1.064 and 1.54 mum. Results from the Pacific Ocean region of the multiwavelength backscatter dependence are presented. Results show extensive structure and variation for the aerosol cross sections. The range of observed aerosol cross section is over 4 orders of magnitude, from less than 10(-9) (m sr)(-1) to greater than 10(-5) (m sr)(-1).

The lunar orbiter laser altimeter (LOLA) on NASA's lunar reconnaissance orbiter (LRO) mission

The Lunar Orbiter Laser Altimeter (LOLA) instrument on NASAs Lunar Reconnaissance Orbiter (LRO) mission, scheduled to launch in October 2008, will provide a precise global lunar topographic map using laser altimetry. LOLA uses short 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 polar regions of the lunar surface to identify possible locations of surface ice crystals in shadowed polar craters.

Optical system design and integration of the Mercury Laser Altimeter

The Mercury Laser Altimeter (MLA), developed for the 2004 MESSENGER mission to Mercury, is designed to measure the planets topography by laser ranging. A description of the MLA optical system and its measured optical performance during instrument-level and spacecraft-level integration and testing are presented.

Optical system design and integration of the Lunar Orbiter Laser Altimeter.

The Lunar Orbiter Laser Altimeter (LOLA), developed for the 2009 Lunar Reconnaissance Orbiter (LRO) mission, is designed to measure the Moons topography via laser ranging. A description of the LOLA optical system and its measured optical performance during instrument-level and spacecraft-level integration and testing are presented.

Spaceborne laser instruments for high-resolution mapping

We discuss past, present and future spaceborne laser instruments for high-resolution mapping of Earth and planetary surfaces. Previous spaceborne-laser-altimeters projected and imaged a single laser spot for surface-height measurements. In contrast, the recent Lunar Orbiter Laser Altimeter (LOLA) instrument on the Lunar Reconnaissance Orbiter (LRO) uses a non-scanning multi-beam system for surface topography mapping. The multi-beam instrument facilitates surface slope measurement and reduces the time-to-completion for global high-resolution topographic mapping. We discuss our first-year progress on a three-year swath-mapping laser-altimetry Instrument Incubator Program (IIP) funded by the NASA Earth Science Technology Office (ESTO). Our IIP is a technology development program supporting the LIdar Surface Topography (LIST) space-flight mission that is a third-tier mission as recommended by the National Research Council (NRC) for NASAs Earth Science programs.

Preliminary spaceflight results from the uncooled infrared spectral imaging radiometer (ISIR) on shuttle mission STS-85

In August 1997 an infrared spectral imaging radiometer (ISIR) based on uncooled microbolometer array technology was flown on space shuttle mission STS-85. In this paper the design of the instrument and experimental goals are presented, and initial results from the flight mission are described. The ISIR instrument provided 1/4 km resolution imagery at four wavelengths that were selected for cloud remote sensing. A major goal of the work is development of compact and less costly cloud imagers for small satellite missions. A large data set of earth imagery and test operations was obtained from the mission. In most regards the ISIR functioned within its design parameters.

Mapping the Topography of Mercury with MESSENGER Laser Altimetry

The Mercury Laser Altimeter onboard MESSENGER involves unique design elements that deal with the challenges of being in orbit around Mercury. The Mercury Laser Altimeter (MLA) is one of seven instruments on NASAs MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. MESSENGER was launched on 3 August 2004, and entered into orbit about Mercury on 18 March 2011 after a journey through the inner solar system. This involved six planetary flybys, including three of Mercury. MLA is designed to map the topography and landforms of Mercurys surface. It also measures the planets forced libration (motion about the spin axis), which helps constrain the state of the core. The first science measurements from orbit taken with MLA were made on 29 March 2011 and continue to date. MLA had accumulated about 8.3 million laser ranging measurements to Mercurys surface, as of 31 July 2012, i.e., over six Mercury years (528 Earth days). Although MLA is the third planetary lidar built at the NASA Goddard Space Flight Center (GSFC), MLA must endure a much harsher thermal environment near Mercury than the previous instruments on Mars and Earth satellites. The design of MLA was derived in part from that of the Mars Orbiter Laser Altimeter on Mars Global Surveyor. However, MLA must range over greater distances and often in off-nadir directions from a highly eccentric orbit. In MLA we use a single-mode diode-pumped Nd:YAG (neodymium-doped yttrium aluminum garnet) laser that is highly collimated to maintain a small footprint on the planet. The receiver has both a narrow field of view and a narrow spectral bandwidth to minimize the amount of background light detected from the sunlit hemisphere of Mercury. We achieve the highest possible receiver sensitivity by employing the minimum receiver detection threshold.