William H. Hunt

Overview of the CALIPSO Mission and CALIOP Data Processing Algorithms

Abstract The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) is a two-wavelength polarization lidar that performs global profiling of aerosols and clouds in the troposphere and lower stratosphere. CALIOP is the primary instrument on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite, which has flown in formation with the NASA A-train constellation of satellites since May 2006. The global, multiyear dataset obtained from CALIOP provides a new view of the earth’s atmosphere and will lead to an improved understanding of the role of aerosols and clouds in the climate system. A suite of algorithms has been developed to identify aerosol and cloud layers and to retrieve a variety of optical and microphysical properties. CALIOP represents a significant advance over previous space lidars, and the algorithms that have been developed have many innovative aspects to take advantage of its capabilities. This paper provides a brief overview of the CALIPSO mission, the CA...

Fully Automated Detection of Cloud and Aerosol Layers in the CALIPSO Lidar Measurements

Abstract Accurate knowledge of the vertical and horizontal extent of clouds and aerosols in the earth’s atmosphere is critical in assessing the planet’s radiation budget and for advancing human understanding of climate change issues. To retrieve this fundamental information from the elastic backscatter lidar data acquired during the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, a selective, iterated boundary location (SIBYL) algorithm has been developed and deployed. SIBYL accomplishes its goals by integrating an adaptive context-sensitive profile scanner into an iterated multiresolution spatial averaging scheme. This paper provides an in-depth overview of the architecture and performance of the SIBYL algorithm. It begins with a brief review of the theory of target detection in noise-contaminated signals, and an enumeration of the practical constraints levied on the retrieval scheme by the design of the lidar hardware, the geometry of a space-based remote sensing pl...

CALIPSO Lidar Description and Performance Assessment

Abstract This paper provides background material for a collection of Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) algorithm papers that are to be published in the Journal of Atmospheric and Oceanic Technology. It provides a brief description of the design and performance of CALIOP, a three-channel elastic backscatter lidar on the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite. After more than 2 yr of on-orbit operation, CALIOP performance continues to be excellent in the key areas of laser energy, signal-to-noise ratio, polarization sensitivity, and overall long-term stability, and the instrument continues to produce high-quality data products. There are, however, some areas where performance has been less than ideal. These include short-term changes in the calibration coefficients at both wavelengths as the satellite passes between dark and sunlight, some radiation-induced effects on both the detectors and the laser when passing through the South Atlant...

CALIPSO Lidar Calibration Algorithms. Part I: Nighttime 532-nm Parallel Channel and 532-nm Perpendicular Channel

Abstract The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission was launched in April 2006 and has continuously acquired collocated multisensor observations of the spatial and optical properties of clouds and aerosols in the earth’s atmosphere. The primary payload aboard CALIPSO is the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), which makes range-resolved measurements of elastic backscatter at 532 and 1064 nm and linear depolarization ratios at 532 nm. CALIOP measurements are important in reducing uncertainties that currently limit understanding of the global climate system, and it is essential that these measurements be accurately calibrated. This work describes the procedures used to calibrate the 532-nm measurements acquired during the nighttime portions of the CALIPSO orbits. Accurate nighttime calibration of the 532-nm parallel-channel data is fundamental to the success of the CALIOP measurement scheme, because the nighttime calibration is used to infer...

Status and performance of the CALIOP lidar

The Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) is the primary instrument on the CALIPSO satellite, which is scheduled to launch in 2005. CALIOP will provide profiles of total backscatter at two wavelengths, from which aerosol and cloud profiles will be derived. The instrument also measures the linear depolarization of the backscattered return, allowing discrimination of cloud phase and the identification of the presence of non-spherical aerosols. CALIOP is complete and has been tested in a ground-based configuration. This paper provides information on basic characteristics and performance of CALIOP.

Estimating random errors due to shot noise in backscatter lidar observations

We discuss the estimation of random errors due to shot noise in backscatter lidar observations that use either photomultiplier tube (PMT) or avalanche photodiode (APD) detectors. The statistical characteristics of photodetection are reviewed, and photon count distributions of solar background signals and laser backscatter signals are examined using airborne lidar observations at 532 nm using a photon-counting mode APD. Both distributions appear to be Poisson, indicating that the arrival at the photodetector of photons for these signals is a Poisson stochastic process. For Poisson- distributed signals, a proportional, one-to-one relationship is known to exist between the mean of a distribution and its variance. Although the multiplied photocurrent no longer follows a strict Poisson distribution in analog-mode APD and PMT detectors, the proportionality still exists between the mean and the variance of the multiplied photocurrent. We make use of this relationship by introducing the noise scale factor (NSF), which quantifies the constant of proportionality that exists between the root mean square of the random noise in a measurement and the square root of the mean signal. Using the NSF to estimate random errors in lidar measurements due to shot noise provides a significant advantage over the conventional error estimation techniques, in that with the NSF, uncertainties can be reliably calculated from or for a single data sample. Methods for evaluating the NSF are presented. Algorithms to compute the NSF are developed for the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations lidar and tested using data from the Lidar In-space Technology Experiment.

Lidar In-Space Technology Experiment (LITE) - NASA's first in-space lidar system for atmospheric research

The Lidar In-Space Technology Experiment (LITE) is being developed by NASA/Langley Research Center for flight on the Space Shuttle. The system will detect stratospheric and tropospheric aerosols, probe the planetary boundary layer, measure cloud top heights, and measure atmospheric temperature and density in the range of 10 to 40 km. The system consists of a nominal 1 m diameter telescope receiver, a three-color neodymium: YAG laser transmitter, and the system electronics. The instrument makes extensive use of Space Shuttle resources for electrical power, thermal control, and command and data handling. The instrument will fly on the Space Shuttle in mid-1993. This paper presents the engineering aspects of the design, fabrication, integration, and operation of the instrument. A companion paper by members of the LITE Science Steering Group that details the science aspects of LITE is in preparation and will be published at a later time.

Elevation information in tail (EIT) technique for lidar altimetry.

A technique we refer to as Elevation Information in Tail (EIT) has been developed to provide improved lidar altimetry from CALIPSO lidar data. The EIT technique is demonstrated using CALIPSO data and is applicable to other similar lidar systems with low-pass filters. The technique relies on an observed relation between the shape of the surface return signals (peak shape) and the detector photo-multiplier tube transient response (transient response tail). Application of the EIT to CALIPSO data resulted in an order of magnitude or better improvement in the CALIPSO land surface 30-meter elevation measurements. The results of EIT compared very well with the National Elevation Database (NED) high resolution elevation maps, and with the elevation measurements from the Shuttle Radar Topography Mission (SRTM).

Measurement of cloud solar reflected radiance and extinction from space lidar

A method of obtaining the reflected solar radiance from clouds with space lidar is described. The lidar telescope and detector are used effectively together as a visible radiometer at the lidar wavelength, the background signal on a lidar backscatter profile being proportional to the observed radiance in the lidar field of view. A DC-coupled output from the telescope detector is required for this method. The RMS background noise signal is also proportional to the observed radiance, where a DC-coupled detector is still effective. However, an AC-coupled detector could also be used. Both the methods are used here to measure the relative radiance along a part of one orbit of the Lidar In-Space Technology Experiment (LITE) on Space Shuttle Discovery. This orbit crossed over Typhoon Melissa where the cloud was optically thick and the reflectance at 532 nm was estimated by normalization at maximum values to previously observed GMS satellite values over similar clouds. Retrieval of the radiance from the internal lidar parameters is also being investigated. Some profiles of extinction coefficient below cloud top near the center of Melissa were also retrieved, showing an increase in extinction below cloud top to at least a depth of 1 km.

The On-Orbit Performance of the CALIOP Lidar on CALIPSO

CALIPSO is a joint NASA – CNES satellite currently in its third year of operation in low earth orbit. The satellite is making optical measurements of the Earth’s atmosphere to help quantify the impact of aerosols and clouds on the Earth’s radiation budget. To do this, it carries three instruments: CALIOP, a two-wavelength polarization-sensitive elastic backscatter lidar; the IIR a three band thermal imaging radiometer; and the WFC a visible single-band imager. CALIOP utilizes a Nd:YAG laser which incorporates a harmonic crystal to provide laser light at both 1064 nm and 532 nm. This beam is expanded and transmitted into the atmosphere at near nadir. The laser light scattered from clouds and aerosols back to the satellite, along with any solar background light, is collected by a one meter diameter beryllium telescope. The captured light is separated into its two wavelengths and optically filtered. The 1064 nm component is detected with an avalanche photodiode, while the 532 nm component is further resolved into two linear polarization components which are then detected by matching photomultiplier tubes. This presentation will describe the lidar and give examples of its on-orbit performance.