High Spectral Resolution Lidar 2

The NASA Langley airborne High Spectral Resolution Lidar 2 (HSRL) is used to characterize clouds and small particles in the atmosphere, called aerosols. From an airborne platform, the HSRL2 scientist team studies aerosol size, composition, distribution and movement.
 

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Cloud Lidar System

The CLS is flown on the ER-2 to conduct cloud radiation and severe storm field experiments. Designed to operate at high altitudes in order to obtain measurements above the highest clouds, the instrument provides the true height of cloud boundaries and the density structure of less dense clouds. The height structure of cirrus, cloud top density and multiple cloud layers may also be profiled. The system specifications are as follows:

Laser Type: Nd:YAG I,II
Wavelength: 1064, 532 nanometer
Pulse Energy: 90, 30 mJ
Pulse repetition frequency: 10 Hz
Beam width: 1 mrad

Diameter: 0.15 m
Beam width: 1.4 mrad
Polarization: vert. and horiz.

Sample rate: Measurements at 20 m intervals at 200 m/s aircraft speed
Range Resolution: 7.5 m
Number of Channels: 4
Samples per Channel: 3310
Record Capacity: 8 hours

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Multicenter Airborne Coherent Atmospheric Wind Sensor

MACAWS is an airborne side-scanning Doppler laser radar (lidar) which measures two dimensional wind fields, vertical wind profiles, and aerosol backscatter from clear air and clouds. Range varies from 10-30 km depending on aerosol abundance and cloud attenuation. Upon exiting the aircraft, the lidar beam is completely eye-safe. MACAWS is developed and operated cooperatively by the atmospheric lidar remote sensing groups of NASA Marshall Space Flight Center, NOAA Environmental Technology Laboratory, and Jet Propulsion Laboratory.

MACAWS consists of: a frequency-stable pulsed transverse-excited atmospheric pressure carbon dioxide laser emitting 0.5-1.0 J per pulse at 10.6 micron wavelength at a nominal pulse repetition frequency (PRF) of ~20 Hz; a coherent receiver employing a cryogenically-cooled HgCdTe detector; a 0.3 m off-axis paraboloidal telescope shared by the transmitter and receiver in a monostatic configuration; a ruggedized optical table and three-point support structure; a scanner using two counter-rotating germanium wedges to refract the transmitted beam in the desired direction; an inertial navigation system (INS) for frequent measurements of aircraft attitude and speed; data processing, display, and storage devices; and an Operations Control System (OCS) to coordinate all system functions.

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Differential Absorption Lidar

The NASA Langley Airborne Differential Absorption Lidar (DIAL) system uses four lasers to make DIAL O3 profile measurements in the ultraviolet (UV) simultaneously with aerosol profile measurements in the visible and IR. Recent changes incorporate an additional laser and modifications to the receiver system that will provide aerosol backscatter, extinction, and depolarization profile measurements at three wavelengths (UV, visible, and NIR). For SEAC4RS, the DIAL instrument will include for the first time aerosol and cloud measurements implementing the High Spectral Resolution Lidar (HSRL) technique [Hair, 2008]. The modifications include integrating an additional 3-wavelength (355 nm, 532 nm, 1064 nm) narrowband laser and the receiver to make the following measurements; depolarization at all three wavelengths, aerosol/cloud backscatter and extinction at 532 nm via the HSRL technique, and aerosol/cloud backscatter at the 355 and 1064 nm via the standard backscatter lidar technique. Integration of the aerosol extinction profile at 532nm above and below the aircraft also provides aerosol optical depth (AOD) along the aircraft flight track.

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High Spectral Resolution Lidar

The NASA Langley airborne High Spectral Resolution Lidar (HSRL) is used to characterize clouds and small particles in the atmosphere, called aerosols. From an airborne platform, the HSRL scientist team studies aerosol size, composition, distribution and movement.

The HSRL instrument is an innovative technology that is similar to radar; however, with lidar, radio waves are replaced with laser light. Lidar allows researchers to see the vertical dimension of the atmosphere, and the advanced HSRL makes measurements that can even distinguish among different aerosol types and their sources. The HSRL technique takes advantage of the spectral distribution of the lidar return signal to discriminate aerosol and molecular signals and thereby measure aerosol extinction and backscatter independently.

The HSRL provides measurements of aerosol extinction at 532 nm and aerosol backscatter and depolarization at 532 and 1064 nm. The HSRL measurements of aerosol extinction, backscattering, and depolarization profiles are being used to:

1) characterize the spatial and vertical distributions of aerosols
2) quantify aerosol extinction and optical thickness contributed by various aerosol types
3) investigate aerosol variability near clouds
4) evaluate model simulations of aerosol transport
5) assess aerosol optical properties derived from a combination of surface, airborne, and satellite measurements.

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Doppler Aerosol WiNd Lidar

DAWN (Doppler Aerosol WiNd lidar) is a pulsed laser, 2-micron, and solid-state. It pulses at 10 Hertz with 250 mJ pulses that are 200 ns long full width at half maximum (FWHM). Using the wedge scanner, five different azimuth angles are measured: 1) to end up with five equations for the three unknown components of wind vs. altitude, 2) to mitigate cloud obscurations, and 3) to measure the atmospheric variability.

DAWN can provide vertical profiles of u, v, and w components of 3-D wind in the region below the aircraft. Various vertical and horizontal resolutions are possible. DAWN can also provide vertical profiles of line of sight (LOS) wind for the five (5) azimuth angles; vertical profiles of relative aerosol backscatter in the region below the aircraft, for the five (5) azimuth angles; vertical profiles of wind turbulence in the region below the aircraft, for the five (5) azimuth angles; and correlations of the data products vs. height.

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Lidar Atmosphere Sensing Experiment

The Differential Absorption Lidar uses the backscatter of two simultaneous laser wavelengths through zenith and nadir windows to measure the vertical profiles of H2O and aerosols/clouds.

NASA's Lidar Atmospheric Sensing Experiment (LASE) system is an airborne DIAL (Differential Absorption Lidar) system used to measure water vapor, aerosols, and clouds throughout the troposphere. LASE probes the atmosphere using lasers to transmit light in the 815-nm absorption band of water vapor. Pulses of laser light are fired vertically below the aircraft. A small fraction of the transmitted laser light is reflected from the atmosphere back to the aircraft and collected with a telescope receiver. The received light indicates the amount of water vapor along the path of the laser beam.

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Airborne Laser Terrain Mapper Experiment

Detailed topographic maps of very high accuracy are produced by airborne laser altimeter terrain mapping. The unique capabilities of this new technique yield more comprehensive and precise topographic information than traditional methods. Airborne laser altimeter data can be used to accurately measure the topography of the ground, even where overlying vegetation is quite dense. The data can also be used to determine the height and density of the overlying vegetation, and to characterize the location, shape, and height of buildings and other manmade structures.

The method relies on measuring the distance from an airplane, or helicopter, to the Earth’s surface by precisely timing the round-trip travel time of a brief pulse of laser light. The travel-time is measured from the time the laser pulse is fired to the time laser light is reflected back from the surface. The reflected laser light is received using a small telescope that focuses any collected laser light onto a detector. The travel-time is converted to distance from the plane to the surface based on the speed of light. Typically a laser transmitter is used that produces a near-infrared laser pulse that is invisible to humans. The laser light reaching the ground surface is completely safe. It can not cause any eye damage to a person who might be looking up at the plane as it flies overhead.

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Aerosol Lidar

The Aerosol Lidar system measures profiles of aerosol and/or cloud backscatter at 532 and 1064 nm and aerosol/cloud depolarization at 532 nm. Backscatter profiles at these two wavelengths provide information on the relative concentration and spatial distribution of aerosol/cloud particles. Comparison of aerosol/cloud backscatter at the two wavelengths provides some indication of particle size. Measurement of the depolarizing effect of the particles (that is, the degree to which the polarization of the backscattered light from the particles differs from the linear polarization of the transmitted laser light) provides an indication of particle phase.

The Aerosol Lidar is a piggy-back instrument on AROTEL lidar fielded by John Burris and Tom McGee of NASA Goddard Space Flight Center. The light source for the aerosol measurements is a Continuum 9050 Nd:YAG laser operating at 50 shots per second. The laser transmits approximately 600 mJ at 1064 nm, 250 mJ at 532 nm, and 350 mJ at 355 nm. AROTEL also employs an excimer laser transmitting at 308 nm and uses the molecular and Raman backscatter from the 355 and 308 beams to measure ozone and temperature. Backscattered light at all wavelengths is collected by a 16-inch diameter Newtonian telescope with a selectable field stop. In the aft optics assembly following the telescope and field stop, the UV signals are separated from the 532- and 1064-nm signals by a dichroic beam splitter. The UV signals are directed to the AROTEL receiver assembly and the 532- and 1064-nm signals are directed to the Aerosol Lidar receiver assembly. In the Aerosol Lidar receiver, a rotating shutter blocks the very strong near-range 532- and 1064-nm signals in order to reduce distortion in the relatively weaker signals from higher altitudes. The 532- and 1064-nm signals are separated by a dichroic beam splitter and the 532-nm signal is further separated into orthogonal polarization components using a polarizing beam cube. A computer-controlled half-wave plate in front of the polarizing beam cube is rotated so that the polarization of the 532 signals are parallel and perpendicular to the polarization of the transmitted laser pulses. The signals at both wavelengths and both 532-nm polarizations are transmitted to detectors at the Aerosol Lidar data acquisition rack via fiber optic cables. Each optical signal, the 1064-nm total backscatter and the 532-nm parallel and perpendicularly polarized backscatter, is directed to two separate detectors, with 10% going to one detector and 90% to the other, in order to more accurately measure the signals over their full dynamic range. The 532-nm returns are measured with photo-multiplier tubes and the 1064-nm returns are measured with avalanche photo-diodes. Because of the high optical signal levels, all data are acquired in analog mode, using 12-bit analog-to-digital converters. The instrument operates under both daytime and nighttime lighting conditions, with a slight degradation in data quality during the daytime.

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Thickness from Offbeam Returns

THOR stands for THickness from Offbeam Returns. This Lidar system is designed to estimate the thickness of clouds by measuring the size of the reflected halo resulting from a laser entering a cloud. A refractive telescope with approximately 7.5-inch (19.05-centimeter) aperture is used to gather the returned light and collect it into a custom designed fiber optic bundle. The fiber optic bundle routes specific sections of the light focused by the telescope into ten Hamamatsu detectors.

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