In this paper, we describe the development of a prototype differential absorption fluorescence lidar for nighttime tropospheric formaldehyde (H2CO) concentration profiling. H2CO has a strong absorption band in the 352-357nm region and fluoresces strongly in the 390-500nm region. Here, we obtain high sensitivity (∼0.1ppb) measurements of H2CO profiles from differential fluorescence signals obtained by injection seeding a Nd:YVO laser and tuning its wavelength on and off the peak of a strong absorption line. The fluorescence signal strength is further improved by using a multi-line bandpass filter whose pass-bands are aligned to multiple fluorescence peaks of H2CO. A H2CO filled photo-acoustic absorption cell is utilized for tuning the seed laser wavelength to the center of absorption line.
In this paper, we describe the development of a three-beam elastic lidar that utilizes aerosol backscatter correlation to measure three-component wind profiles for detecting and tracking aircraft wake vortices; turbulence intensity and wind shear profiles. High-resolution time-resolved wind information can currently be obtained with ultrasonic or hot-wire anemometers suitable for local point measurements, or with Doppler wind lidars that only measure line-of-sight wind speeds and have to be scanned over large measurement cone angles for obtaining three-component winds. By tracking the motion of aerosol structures along and between three near-parallel laser beams, our lidar obtains three-component wind speed profiles along the field of view (FOV) of the lidar beams. Our prototype lidar wind profiler (LWP) has three 8-inch transceiver modules placed in a near-parallel configuration on a two-axis pan-tilt scanner to measure winds up to 2km away. Passively q-switched near-infrared (1030nm) Yb:YAG lasers generate 12 - 18ns wide pulses at high repetition rate (about 10KHz) that are expanded and attenuated to eye-safe levels. Sensitive low noise detection is achieved even in daytime using a narrow FOV receiver, together with narrowband interference filters and single photoncounting Geiger-mode Si detectors. A multi-channel scaler retrieves the lidar return with 7.8ns bins (∼1.2m spatial resolution) and stores accumulated counts once every 50ms (20 profiles/sec). We adapted optical flow algorithms to obtain the movement of aerosol structures between the beams. The performance of our prototype LWP was validated using sonic anemometer measurements.
Global measurement of tropospheric winds is a key
measurement for understanding atmospheric
dynamics and improving numerical weather
prediction. Global wind profiles remain a high
priority for the operational weather community and
also for a variety of research applications including
studies of the global hydrologic cycle and transport
studies of aerosols and trace species. In addition to
space based winds, high altitude airborne Doppler
lidar systems flown on research aircraft, UAV's or
other advanced sub-orbital platforms would be of
great scientific benefit for studying mesoscale
dynamics and storm systems such as hurricanes. The
Tropospheric Wind Lidar Technology Experiment
(TWiLiTE) is a three year program to advance the
technology readiness level of the key technologies and
subsystems of a molecular direct detection wind lidar
system by validating them, at the system level, in an
integrated airborne lidar system. The TWiLiTE
Doppler lidar system is designed for autonomous
operation on the WB57, a high altitude aircraft
operated by NASA Johnson. The WB57 is capable of
flying well above the mid-latitude tropopause so the
downward looking lidar will measure complete
profiles of the horizontal wind field through the
lower stratosphere and the entire troposphere. The
completed system will have the capability to profile
winds in clear air from the aircraft altitude of 18 km
to the surface with 250 m vertical resolution and < 3
m/s velocity accuracy. Progress in technology
development and status of the instrument design will
be presented.
We have developed simplified conical scanning telescopes using Holographic Optical Elements (HOEs) to reduce the size, mass, angular momentum, and cost of scanning lidar systems. This technology enables wide-angle scanning and three-dimensional measurements of atmospheric backscatter when used in airborne instruments, and high temporal and spatial resolution observations of atmospheric dynamic structure, including wind profiles from ground-based facilities. We deployed the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE) on the ground at the Department Of Energy's (DOE) central site in northern Oklahoma during their most recent Atmospheric Radiation Measurement (ARM) program Water Vapor Intensive Operating Period (WVIOP) in September-October 2000, in order to take advantage of the many coincident atmospheric measurements taking place at that time while collecting data with which to develop data reduction algorithms. We are evaluating the HARLIE technology and scanning techniques with an eye toward their application into other types of lidar systems, including Raman and Doppler lidar systems.
We report the results of three campaigns in which the horizontal wind vector at cloud altitudes was measured using the holographic, conical-scan lidar HARLIE in its zenith-viewing mode. Measurements were made during the HOLO-1 and HOLO-2 tests in Utah and New Hampshire in March and June 1999, respectively, and at the DoE-ARM site in Oklahoma in September/October 2000. A novel algorithm facilitates the wind vector analysis of the HARLIE data. Observed wind velocity and direction were compared with radiosonde records and with other data obtained from video cloud imagery and independent lidar ranging. The results demonstrate good agreement between HARLIE data and the results of other methods. The conically scanning holographic lidar opens up new possibilities for obtaining the vertical profile of horizontal winds.
Results of two lidar measurement campaigns are presented, HOLO-1 (Utah, March 1999) and HOLO-2 (New Hampshire, June 1999). These tests demonstrate the ability of lidars utilizing holographic optical elements (HOEs) to determine tropospheric wind velocity and direction at cloud altitude. Several instruments were employed. HOLO-1 used the 1.064 mm transmission-HOE lidar (HARLIE, Goddard Space Flight Center), a zenith-staring 532 nm lidar (AROL-2, Utah State University), and a wide-field video camera (SkyCam) for imagery of clouds overhead. HOLO-2 included these instruments plus the 532 nm reflection-HOE lidar (PHASERS, St. Anselm College). HARLIE and PHASERS scan the sky at constant cone angles of 45° and 42° from normal, respectively. The progress of clouds and entire cloud fields across the sky is tracked by the repetitive conical scans of the HOE lidars. AROL-2 provides the altitude information enabling the SkyCam cloud images to be analyzed for independent data on cloud motion. Data from the HOE lidars are reduced by means of correlations, visualization by animation techniques, and kinematic diagrams of cloud feature motion. Excellent agreement is observed between the HOE lidar results and those obtained with video imagery and lidar ranging.
A new technique combining active and passive remote sensing instruments for the estimation of surface latent heat flux over the ocean is presented. This synergistic method utilizes aerosol lidar backscatter data, multi-channel infrared radiometer data and microwave scatterometer data acquired onboard the NASA P-3B research aircraft during an extended field campaign over the Atlantic ocean in support of the Lidar In-space Technology Experiment in September of 1994. The 10 meter wind speed derived from the scatterometers and the lidar-radiometer inferred near- surface moisture are used to obtain an estimate of the surface flux of moisture via bulk aerodynamic formulae. The results are compared with the Special Sensor Microwave Imager (SSM/I) daily average latent heat flux and show reasonable agreement. However, the SSM/I values are biased high by about 30 W/m2. In addition, the MABL height, entrainment zone thickness and integrated lidar backscatter intensity are computed from the lidar data and compared with the magnitude of the surface fluxes. The results show that the surface latent heat flux is most strongly correlated with entrainment zone top, bottom and the integrated MABL lidar backscatter, with corresponding correlation coefficients of 0.62, 0.67 and 0.61, respectively.
Two scanning lidar systems have been built using holographic optical elements (HOE) that function as a scanning telescope primary optic. One is a ground based lidar using a reflection HOE, and uses a frequency doubled Nd:YAG laser transmitter. The other system is an airborne/ground based system that uses a transmission HOE and operates at the 1064 nm fundamental of the Nd:YAG laser. Each HOE has a focal spot on the center- line, normal to the flat disk holding the hologram, and a field of view (FOV) that points approximately 45 degrees from the normal. Rotating the disk effects a conical scan of the FOV. In both systems, the same HOE is also used to collimate and steer the transmitted laser beam. The utility of using the HOEs to save weight and size in scanning lidars is evidenced by the atmospheric backscatter data collected with these systems. They also will lower the cost of commercial systems due to the low cost of replicating HOEs and the simplified mechanical scanning systems. Development of airborne scanning lidar altimeters and other lidars and passive instruments using holographic optics are underway, including the development of a one meter diameter, space qualified holographic scanning telescope for use in the ultraviolet.
The first lidar system to employ a holographic optical element as the receiver telescope and scanning mirror has been developed to measure cloud and aerosols from a ground based platform with the primary objective of demonstrating technology that offers the advantage of decreased system complexity, size and weight so that satellite based lidar systems can be made smaller, lighter, and cheaper.
Until a few years ago, virtually all NASA''s remote sensing was done passively. NASA is now working to develop active remote sensing systems, making use of the very rapid advances occurring in laser and radar technology. To be deployed in the difficult space environment, laser instruments must be rugged enough to withstand vibrations and cold while being able to operate automatically without retuning or realigning the instrument by hand. This paper describes several major NASA research efforts in lidar remote sensing, including hardware and key sensor issues, along with results and expectations.
Narrowband radiation is produced from a pulsed alexandrite laser when injection seeded with the output of a low-power, tunable, continuous-wave, single-mode diode laser. Injection seeded power oscillators are easier to frequency stabilize than etalong narrowed lasers, are more efficient, and less prone to optical damage. AlGaAs diode lasers are available with wavelengths from 760 to 770 nm in the oxygen A band that can be used for differential absorption lidar remote sensing of atmospheric pressure and temperature. Diodes with room temperature output at 740 nm may be cooled sufficiently to emit in the water vapor absorption band at 720-730 nm for humidity remote sensing. The diodes are driven with approximately 70 mA of current with a 0.5 mA peak to peak sinusoidal dither which imparts a 3 GHz frequency modulation to the output. A photoacoustic cell is utilized to generate a negative feedback signal to keep the diode frequency centered on the atmospheric absorption feature. The diode laser linewidth of 200 MHz is sufficient to seed 2 or 3 longitudinal modes of the multitransverse mode alexandrite laser, giving the pulsed laser a bandwidth of 0.007 to 0.014 cm-
The design and operation of a differential absorption lidar (Light Detection and Ranging) system capable of remotely measuring
the vertical stmcture oftroposphenc pressure and temperature is described. The measurements are based on the absorption
by atmospheric oxygen of the spectrally narrowband output of two pulsed alexandrite lasers. Detailed laser output spectral
characteristics, which are critical to successful lidar measurements, are presented.
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