Cloud detection/screening is a fundamental step in satellite data analysis. For the Ice, Cloud, and land Elevation Satellite (ICESat) and its successor ICESat-2, clouds can significantly affect the accuracy of the surface elevation retrievals. This paper proposes a new method for cloud screening in support of the ICESat-2 mission with focus on the polar ice sheet regions. The method utilizes the apparent surface reflectance (ASR) at the backscattering direction as the cloud screening test. The basis of this method is that clouds produce a strong signal by significantly decreasing the ASR. We show that depending on the height and microphysics of the cloud, the ASR decreases 8%–17% for clouds with an optical depth of 0.1 and 57%–85% for clouds with an optical depth 1.0. Data from ICESat's 1064-nm channel is used to demonstrate the feasibility of the method. It is shown that cloud detectability is a function of surface reflectance variability. Generally, the smaller the surface reflectance variability, the more accurate is cloud detection. Unlike ICESat, which used a 1064-nm laser, ICESat-2 adopts a 532-nm photon counting system for its laser altimeter. With both modeling studies and results from the Moderate Resolution Imaging Spectroradiometer (MODIS), we demonstrate that the ASR variability is much smaller for the 532-nm channel than that for the 1064-nm channel. Hence, the 532-nm channel is better suited for cloud screening than the 1064-nm channel.

This paper establishes a framework that simulates the behavior of a spaceborne 532-nm micropulse photon-counting lidar in cloudy and clear atmospheres in support of the ICESat-2 mission. Adopted by the current mission design, the photon-counting system will be used to obtain surface altimetry for ICESat-2. To investigate how clouds affect surface elevation retrievals, a 3-D Monte Carlo radiative transfer model is used to simulate the photon path distribution and the Poisson distribution is adopted for the number of photon returns. Since the photon-counting system only registers the time of the first arriving photon within the detector “dead time,” the retrieved average surface elevation tends to bias toward higher values. This is known as the first photon bias. With the scenarios simulated here, the first photon bias for clear sky is about 6.5 cm. Clouds affect surface altimetry in two ways: 1) Cloud attenuation lowers the average number of arriving photons and hence reduces the first photon bias, and 2) cloud forward scattering increases the photon path length and makes the surface appear further away from the satellite. Compared with that for clear skies, the average surface elevation detected by the photon-counting system for cloudy skies with optical depth of 1.0 is 4.0-6.0 cm lower for the simulations conducted. The effect of surface roughness on the accuracy of elevation retrievals is also discussed.

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.

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.

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

GLAS is a space lidar on NASA's ICESat Mission. GLAS was qualified and delivered and ICEsat is scheduled for launch in December 2002. This talk summarizes the as-built characteristics of GLAS and its predicted measurement performance.

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.

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.