During the development of the Geoscience Laser Altimeter System (GLAS), launched in 2003, it was understood that the values of surface pusle reflectance was an important design parameter for satellite laser altimeters and that the magnitude of observed pulse reflectance would have valuable applications. Thus a significant effort was made to obtain calibrated surface pulse reflectance from GLAS measurements. In this paper we describe the calibration and atmospheric correction procedures and present global measurement results for earth surface laser pulse reflectance.

Satellite and aircraft observations have revealed that remarkable changes in the Earth's polar ice cover have occurred in the last decade. The impacts of these changes, which include dramatic ice loss from ice sheets and rapid declines in Arctic sea ice, could be quite large in terms of sea level rise and global climate. NASA's Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), currently planned for launch in 2015, is specifically intended to quantify the amount of change in ice sheets and sea ice and provide key insights into their behavior. It will achieve these objectives through the use of precise laser measurements of surface elevation, building on the groundbreaking capabilities of its predecessor, the Ice Cloud and Land Elevation Satellite (ICESat). In particular, ICESat-2 will measure the temporal and spatial character of ice sheet elevation change to enable assessment of ice sheet mass balance and examination of the underlying mechanisms that control it. The precision of ICESat-2's elevation measurement will also allow for accurate measurements of sea ice freeboard height, from which sea ice thickness and its temporal changes can be estimated. ICESat-2 will provide important information on other components of the Earth System as well, most notably large-scale vegetation biomass estimates through the measurement of vegetation canopy height. When combined with the original ICESat observations, ICESat-2 will provide ice change measurements across more than a 15-year time span. Its significantly improved laser system will also provide observations with much greater spatial resolution, temporal resolution, and accuracy than has ever been possible before.

Multispectral thermal imagery acquired from low Earth orbit was used to develop a method of cloud-height determination that applies image brightness temperature histograms and region-of-interest (ROI) image segmentation as a processing step prior to stereo-height retrieval. The National Aeronautics and Space Administration's Infrared Spectral Imaging Radiometer (ISIR) acquired all imagery during the August 1997 STS-85 mission of the Space Shuttle Discovery. ISIR, the first Earth-observing spectroradiometer to employ an uncooled large-format microbolometer-array focal plane, provided continuous coverage in four spectral bands along track in an 80-km-wide swath. ROI segmentation created binary cloud masks ranging over brightness temperatures in the imagery for which the parallax was determined by a two-dimensional correlation method, allowing stereo heights to be determined using the standard parallax equations. A subpixel parallax algorithm allowed stereo heights to be determined with a precision of roughly plusmn0.39 km for clouds in the observed altitude range of 0.5-10 km

MPLNET is a network of lidar systems that provide long term observations of aerosol and cloud properties at multiple sites around the globe. Each site in the network uses an elastic-scattering lidar co-located with a sunphotometer to provide data products of aerosol optical and physical properties. Expansion of the network is based on partnering with research groups interested in joining MPLNET. Results have contributed to a variety of studies including aerosol transport processes and satellite calibration and validation efforts

In this presentation, we will show results from intercomparison case studies of Cloud Physics Lidar under-flights of GLAS orbit tracks during the GLAS Validation Experiment in October 2003. Compared products include Rayleigh backscatter profiles, calibrated attenuated backscatter, layer identification, optical depth, extinction, and backscatter cross section. We will show examples of GLAS operational optical data products.

Multiangle remote sensing provides a wealth of information for Earth and climate monitoring, such as the ability to measure the height of cloud tops through stereoscopic imaging. As technology advances so do the options for developing spacecraft instrumentation with sufficient versatility to meet the demands associated with multiangle measurements. One such instrument is the Infrared Spectral Imaging Radiometer, which flew as part of Mission STS-85 of the Space Shuttle Columbia in 1997 and was the first Earth-observing radiometer to incorporate an uncooled microbolometer array detector as its image sensor. Specifically, a method for computing cloud-top height with a precision of 620 m from the multispectral stereo measurements acquired during this flight has been developed, and the results are compared with coincident direct laser ranging measurements from the Shuttle Laser Altimeter. Mission STS-85 was the first space flight to combine laser ranging and thermal IR camera systems for cloud remote sensing.

The first polar orbiting satellite lidar instrument, the Geoscience Laser Altimeter System (GLAS), was launched in 2003 and is approaching six months of data operations. As part of the NASA Earth Observing System (EOS) project, the GLAS instrument is intended as a laser sensor fulfilling complementary requirements for several Earth science disciplines including atmospheric and surface applications on the Ice, Cloud and Land Elevation Satellite. In this paper we present examples of atmospheric measurement results and explain data products now accessible for the science community

A key aspect to the recently-launched GLAS instrument is understanding its radiometric performance using on-orbit verification approaches. One of these approaches compares the reported backscattered signal from a surface to a predicted signal from that same surface. This returned signal depends on both the atmospheric transmittance and the surface reflectance in the backscatter direction. This work describes three methods that have been used to derive the backscatter surface reflectance for the prediction of the hard-target signal at the GLAS sensor. The first relies on a portable field spectrometer carried over a representative area of the surface and the reflected solar irradiance is measured with reference to a known field standard of reflectance. The difficulty with this approach is that it does not measure the backscatter reflectance. Thus, a second method using a laser-based system operating on the ground has been used to determine the surface reflectance. Both of these cases require that ground personnel be present near in time to the GLAS overpass. A third method, relying on reflectances derived from the MODIS sensor increases the available number of data sets for comparison to GLAS. Early results from all three approaches indicate agreement at better than 10% with the GLAS sensor.

The Micro-Pulse Lidar NETwork (MPLNET) uses elastic-scattering lidars stationed at sites around the globe to produce aerosol and cloud vertical profiles on a continuous year-round basis. Processing of MPLNET data requires a correction for the lidar overlap function in the 0-6 km range, to take into account the loss in near-field receiver efficiency. This correction is normally determined from recording horizontal profiles that require a /spl sim/10 km clear line-of-sight and homogeneous atmospheric conditions, limiting the practicality in which successful corrections can be obtained. An alternative overlap correction method using a secondary receiver is considered that eliminates the need for horizontal measurements. A review of both methods is presented, including a discussion of signal uncertainties.

The Geoscience Laser Altimeter System launched in early 2003 is the first satellite instrument in space to globally observe the distribution of clouds and aerosol through laser remote sensing. The instrument is a basic backscatter lidar that operates at two wavelengths, 532 and 1064 nm. The mission data products for atmospheric observations include the calibrated, observed, attenuated backscatter cross section for cloud and aerosol; height detection for multiple cloud layers; planetary boundary layer height; cirrus and aerosol optical depth and the height distribution of aerosol and cloud scattering cross section profiles. The data is expected to significantly enhance knowledge in several areas of atmospheric science, in particular the distribution, transport and influence of atmospheric aerosol. Measurements of the coverage and height of polar and cirrus cloud should be significantly more accurate than previous global measurement. Initial result from the first several months of operation will be presented.

In this presentation, we will show new optical data processing results from the Cloud Physics Lidar during SAFARI-2000. Retrieved products include aerosol and cloud layer location and identification, layer optical depths, vertical extinction profiles, and extinction-to-backscatter (S) ratios for 532 and 1064 nm. The retrievals will focus on the persistent and smoky planetary boundary layer and occasional elevated aerosol layers found in southern Africa during August and September 2000.

A new era in atmospheric remote sensing is about to begin. Global monitoring of clouds and aerosols from space using a backscatter lidar system will greatly add to our knowledge in areas such as polar cloud climatology, aerosol loading and transport, and the planetary boundary layer. The Geoscience Laser Altimeter System (GLAS) will produce nearly 5 GB of data per day. A challenge is to create autonomous algorithms that will analyze the data in near-real time. This paper briefly discusses the algorithms and presents results of testing with simulated GLAS data sets.

A ground-based, autonomous, low-power atmospheric lidar instrument developed at NASA Goddard Space Flight Center has been returned from an extended period on the Antarctic polar plateau. We report on the pre- and post-deployment performance of the AGO Lidar, compare the instrument performance with lidar models, and compare results with the anticipated performance of the new Automatic Weather Station (AWS) Lidar.

In In 1993 the first of the eye safe visible wavelength lidar systems known now as Micro Pulse Lidar (MPL) became operational. Since that time there have been several dozen of these systems produced and applied for full time profiling of atmospheric cloud and aerosol structure. There is currently an observational network of MPL sites to support global climate research. In the course of application of these instruments there have been significant improvements in understanding, design and performance of the systems. There are addition potential and applications beyond current practice for the high repetition rate, eye safe designs. The MPL network and the current capability, design and future potential of MPL systems are described.

Estimates of the effect of pulse stretching on satellite laser altimetry, such as planned for the Geoscience Laser Altimeter System (GLAS), by cloud multiple scattering were made from an analytical method and with Monte Carlo simulations. Altimetry is dependent on the time required for a laser pulse to complete the roundtrip to the surface and return to the transmitter. Since a transmitted Gaussian pulse will be stretched by the effects of multiple scattering, the use of the pulse centroid as the receive time will produce a biased measurement or an apparent delay in the receive time. The magnitude of this delay was found to be dependent on several factors including cloud height, cloud optical depth, cloud particle size, particle shape, and receiver field of view. The delay was found to be largest for low-level clouds with particle radii of 3-20 /spl mu/m, potentially amounting to altimetry biases of tens of cm. Alternate methods for measuring the receive time, such as a simple Gaussian fit of the return pulse peak reduce the path delay estimates for all cloud conditions. Since GLAS is a dual mode instrument that includes an atmospheric lidar channel, altimeter measurements that are likely to be significantly contaminated by multiple scattering can be identified.

An eye safe, compact, solid-state lidar for profiling atmospheric cloud and aerosol scattering is described. The transmitter of the micro pulse lidar is a diode pumped microjoule pulse energy, high-repetition-rate Nd:YLF laser. Eye safety is obtained through beam expansion. The receiver uses a photon counting solid-state Geiger mode avalanche photodiode detector. Data acquisition is by a single card multichannel scaler. Daytime background induced quantum noise is controlled by a narrow receiver field-of-view (FOV) and a narrow bandwidth temperature controlled interference filter. Dynamic range of the signal is limited by optical geometric signal compression. Signal simulations and initial atmospheric measurements indicate that systems built on the micro pulse lidar concept are capable of detecting and profiling all significant cloud and aerosol scattering through the troposphere and into the stratosphere. The intended applications are scientific studies and environmental monitoring. which require full-time unattended measurements of the cloud and aerosol height structure.>

Advances in the development and application of lidar as a tool for the remote sensing of atmospheric aerosols and clouds are reviewed. The lidar sensing technique is described, and various approaches for solving the lidar equation to retrieve aerosol properties are summarized. Examples are presented of lidar applications of aerosol and cloud sensing in both the troposphere and stratosphere. These include environmental monitoring, atmospheric boundary layer studies, retrieval of aerosol optical and physical properties, sensing of clouds, and investigation of volcanic effects in the stratosphere. Comments are offered regarding the future outlook for aerosol and cloud sensing by both ground-based and spaceborne lidar.>