No abstract

More precise lunar and Martian ranging will enable unprecedented tests of Einstein's theory of general relativity as well as lunar and planetary science. NASA is currently planning several missions to return to the Moon, and it is natural to consider if precision laser ranging instruments should be included. New advanced retroreflector arrays at carefully chosen landing sites would have an immediate positive impact on lunar and gravitational studies. Laser transponders are currently being developed that may offer an advantage over passive ranging, and could be adapted for use on Mars and other distant objects. Precision ranging capability can also be combined with optical communications for an extremely versatile instrument. In this paper we discuss the science that can be gained by improved lunar and Martian ranging along with several technologies that can be used for this purpose.

The Slope Imaging Multi-polarization Photoncounting Lidar is an airborne instrument developed to demonstrate laser altimetry measurement methods that will enable more efficient observations of topography and surface properties from space. The instrument was developed through the NASA Earth Science Technology Office Instrument Incubator Program with a focus on cryosphere remote sensing. The SIMPL transmitter is an 11 KHz, 1064 nm, plane-polarized micropulse laser transmitter that is frequency doubled to 532 nm and split into four push-broom beams. The receiver employs single-photon, polarimetric ranging at 532 and 1064 nm using Single Photon Counting Modules in order to achieve simultaneous sampling of surface elevation, slope, roughness and depolarizing scattering properties, the latter used to differentiate surface types. Data acquired over ice-covered Lake Erie in February, 2009 are documenting SIMPL's measurement performance and capabilities, demonstrating differentiation of open water and several ice cover types. ICESat-2 will employ several of the technologies advanced by SIMPL, including micropulse, single photon ranging in a multi-beam, push-broom configuration operating at 532 nm.

Abstract. Hyperspectral, multiangle reflected radiances were obtained using the airborne advanced solid-state array spectroradiometer (ASAS) over Boreal Ecosystem-Atmosphere Study (BOREAS) sites in Canada during four field campaigns in 1994. Atmospherically corrected bidirectional reflectance factors and estimates of spectral hemispherical reflectance for three canopies in the BOREAS southern study area (old aspen, old black spruce, and old jack pine) are presented. The multiangle spectral reflectance factors derived from data acquired July 21, 1994 (for a limited solar zenith angle range of 340-37 ø) for the forested sites showed distributions of high backscatter and consistently low forward scatter due to shadowing. Position and shape of the retrosolar maximums for the three sites varied. Spectral distinction among the sites was evident in the red, where reflectance factors for the jack pine exceeded those for spruce and aspen, and in the near infrared (NIR), where the aspen reflectance factors were twice those for the conifers. Data presented here suggest that spectral reflectances acquired at 26 ø backscatter in the principal plane would provide better discrimination among the major cover types than those obtained from a nadir view. Red reflectance was strongly dependent on view geometry for the spruce and jack pine sites due to the varying amounts of deep shadow and red-reflecting materials observed as a function of view azimuth and zenith. At the aspen site the red reflectance displayed much less variation with changing view zenith and azimuth. Accordingly, angular effects on the normalized difference vegetation index were large for the spruce and jack pine canopies but small for the aspen site.

Laser altimeter measurements of forest stands with distinct structures and compositions have been acquired at 532 nm (green) and 1064 nm (near-infrared) wavelengths and parallel and perpendicular polarization states using the Slope Imaging Multi-polarization Photon Counting Lidar (SIMPL). The micropulse, single photon ranging measurement approach employed by SIMPL provides canopy structure measurements with high vertical and spatial resolution. Using a height distribution analysis method adapted from conventional, 1064 nm, full-waveform lidar remote sensing, the sensitivity of two parameters commonly used for above-ground biomass estimation are compared as a function of wavelength. The results for the height of median energy (HOME) and canopy cover are for the most part very similar, indicating biomass estimations using lidars operating at green and near-infrared wavelengths wiIl yield comparable estimates. The expected detection of increasing depolarization with depth into the canopies due to volume multiplescattering was not observed, possibly due to the small laser footprint and the small detector field of view used in the SIMPL instrument. The results of this work provide pathfinder information for NASA's ICESat-2 mission that will employ a 532 nm, micropulse, photon counting laser altimeter.