The recommended design approach for the 3D Tropospheric Winds mission is a hybrid Doppler lidar which combines the best elements of both a coherent aerosol Doppler lidar operating at 2 μm and a direct detection molecular Doppler lidar operating at 0.355 μm. In support of the mission, we built a novel, compact, light-weighted multi-field of view transceiver where multiple telescopes are used to cover the required four fields of view. A small mechanism sequentially selects both the “transmit” and “receive” fields of view. The four fields are combined to stimulate both the 0.355 μm receiver and the 2 μm receiver. This version is scaled (0.2 m diameter aperture) from the space-based version but still demonstrates the feasibility of the hybrid approach. The primary mirrors were conventionally light-weighted and coated with dielectric, high reflectivity coatings with high laser damage thresholds at both 2 μm and 0.355 μm. The mechanical structure and mounts were fabricated from composites to achieve dimensional stability while significantly reducing the mass. In the laboratory, we demonstrated the system level functionality at 0.355 μm and at 2 μm, raising the Technology Readiness Level (TRL) from 2 to 4.
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 designed and built two versions of a space-qualifiable, single-frequency Nd:YAG laser. Our approach to frequency stabilization of the seeded oscillator is a variation of the “ramp and fire” technique. In this design, the length of the pulsed laser cavity is periodically varied until a resonance with the seed laser is optically detected. At that point the pulsed laser is fired, ensuring that it is in resonance with the seed laser. For one of the lasers the resulting single frequency pulses are amplified and frequency tripled. This system operates at 50 Hz and provides over 50 mJ/pulse of single-frequency 355 nm output. It has been integrated into the GLOW (Goddard Lidar Observatory for Winds) mobile Doppler lidar system for field testing. The second laser is a 20o Hz oscillator only system that is frequency doubled for use in the High Spectral Resolution Lidar (HSRL) system being built at NASA Langley Research Center. It provides 4 mJ of single-frequency 532 nm output that has a spectral purity of >10,000. In this paper we describe the design details, environmental testing, and integration of these lasers into their respective lidar systems.
The Goddard Lidar Observatory for Wind (GLOW) has participated three field campaigns since Sept. 2000. Near 300 hours of wind measurement under a wide variety of conditions including day and night operation, high and low altitude, line of sight and horizontal winds, have been obtained from the GroundWinds intercomparison, Harlie-GLOW intercomparison ( HARGLO) and the International H2O Project (IHOP) field campaigns. The GLOW system has demonstrated the capability to atmospheric wind from ground up to 30 km with an accuracy of 0.4-6m/s.
In this paper we present a detailed analysis of the performance of the Goddard Lidar Observatory for Winds (GLOW) Doppler lidar. GLOW is a mobile direct detection Doppler lidar system which uses the double edge technique to measure the Doppler shift of the molecular backscattered laser signal at a wavelength of 355 nm. The lidar has been used in several recent field experiments to measure wind profiles from the surface into the lower stratosphere. Simulations of detected signal levels predicted using a realistic instrument model and representative atmospheric model will be presented and compared with range resolved signals detected with the photon counting data acquisition system. A detailed analysis of wind errors observed with the system will also be reported. The analysis begins by propagating ideal shot noise limited errors through the analysis algorithms used to calculate winds. The resulting shot noise limited errors are compared with statistical standard deviations obtained by averaging multiple independent wind profiles taken at various temporal and spatial sampling scales. An assessment of other instrumental and atmospheric effects contributing to the wind error will also be given.
A field campaign featuring three collocated Doppler wind lidars was conducted over ten days during September 2000 at the GroundWinds Observatory in New Hampshire. The lidars were dissimilar in wavelength and Doppler detection method. The GroundWinds lidar operated at 532 nm and used fringe-imaging direct detection, while the Goddard Lidar Observatory for Winds (GLOW) ran at 355 nm and employed double-edge filter direct detection, and the NOAA mini-MOPA operated at 10 microns and used heterodyne detection. The objectives of the campaign were (1) to demonstrate the capability of the GroundWinds lidar to measure winds while employing several novel components, and (2) to compare directly the radial wind velocities measured by the three lidars for as wide a variety of conditions as possible. Baseline wind profiles and ancillary meteorological data (temperature and humidity profiles) were obtained by launching GPS radiosondes from the observatory as frequently as every 90 minutes. During the final week of the campaign the lidars collected data along common lines-of-sight for several extended periods. The wind speed varied from light to jet stream values, and sky conditions ranged from clear to thick clouds. Intercomparisons of overlapping lidar and radiosonde observations show that all three lidars were able to measure wind given sufficient backscatter. At ranged volumes containing thicker clouds, and those beyond, the wind sensing capability of the direct detection lidars was adversely affected.
The Goddard Lidar Observatory for Winds (GLOW) is a mobile Doppler lidar system which uses direct detection Doppler lidar techniques to measure wind profiles from the surface into the lower stratosphere. GLOW is intended to be used as a field deployable system for studying atmospheric dynamics and transport and can also serve as a testbed to evaluate candidate technologies developed for use in future spaceborne systems. In September of 2000 GLOW particpated in a three week intercomparison experiment at the GroundWinds facility in North Glen, NH. More than 50 hours of line-of-sight wind profile data were obtained in a wide variety of conditions including both day and night operation. Typical clear air lidar wind profiles extended to altitudes of 20 km with a 1 km vertical resolution and 1 minute averaging. A description of the mobile system is presented along with the examples of lidar wind profiles obtained with the Goddard system during the New Hampshire experiment.
A variation of the direct detection Doppler Lidar method known as the edge technique is discussed. This new method uses a frequency-agile laser transmitter to alternate the outgoing laser pulse between the two edges of a high resolution Fabry-Perot etalon. The difference in sign of the two slopes of the edge allows the unwanted molecular return signal to be eliminated as a background error source. This technique is similar to that of the 'double-edge' technique, with the main advantage being reduced complexity and cost for the system. By eliminating the error in the wind velocity measurement due to the molecular return, transmitter powers, within the eye-safety range, may be utilized to measure winds within the planetary boundary layer (PBL) with reasonable accuracy for many applications.
GLOW (Goddard Lidar Observatory for Winds) is a mobile Doppler lidar system which uses direct detection Doppler lidar techniques to measure wind profiles from the surface intO the lower stratosphere. The system is contained in a modified van to allow deployment in field operations. The lidar system uses a Nd:YAG laser transmitter to measure winds using either aerosol backscatter at 1064 nm or molecular backscatter at 355 nm. The receiver telescope is a 45 cm Dall-Kirkham which is fiber coupled to separate Doppler receivers, one optimized for the aerosol backscatter wind measurement and another optimized for the molecular backscatter wind measurement. The receivers are implementations of the 'double edge' technique and use high spectral resolution Fabry-Perot etalons to measure the Doppler shift. A 45 cm aperture azimuth-overelevation scanner is mounted on the roof of the van to allow full sky access and a variety of scanning options. GLOW is intended to be used as a deployable field system for studying atmospheric dynamics and transport and can also serve as a testbed to evaluate candidate technologies developed for use in future spaceborne systems. In addition, it can be used for calibration/validation activities following launch of spaceborne wind lidar systems. A description of the mobile system is presented along with the examples of lidar wind profiles obtained with the system.
We report the results of the experimental studies on the fabrication of Fabry-Perot filters using holographic methods. High finesse holographic Fabry-Perot (HFP) filters can be fabricated using low quality substrates and commercially available volume holographic recording materials. Several large aperture samples of HFP filters were successfully fabricated using glass and plastic (mylar) spacers of thickness ranging from 5 micrometers to 6.6 cm. The finesse of these HFP samples was sufficiently high (approximately equals 30) and limited only by the absorption of the presently available holographic materials.
The edge technique can provide high accuracy spaceborne wind measurements as well as high spatial resolution, high accuracy ground and airborne measurements. Global wind measurements can be made with the edge technique from space with an accuracy of 1 m/s and a vertical resolution as high as 150 m in the boundary layer and 1 km through the troposphere. The edge technique can also be used for ground and airborne measurements with a spatial resolution and accuracy as high as 15 m and 20 cm/s. We have recently demonstrated this capability and present these measurements in this paper.
The edge technique is a powerful new method for the measurement of small frequency shifts which allow high accuracy measurement of atmospheric winds (0.2 to 1 m/sec) with high vertical resolution (10 m) using currently available technology.
National attention has focused on the critical problem of detecting and avoiding windshear since the crash on August 2, 1985, of a Lockheed L-1011 at Dallas/Fort Worth International Airport. As part of The NASA/FAA National Integrated Windshear Program, we have defined a measurable windshear hazard index that can be remotely sensed from an aircraft, to give the pilot information about the wind conditions he will experience at some later time if he continues along the present flight path. Our technology analysis and end-to-end performance simulation, which measured signal-to-noise ratios and resulting wind velocity errors for competing coherent lidar systems, showed that a Ho:YAG lidar at a wavelength of 2.1 μm and a CO2 lidar at 10.6 m can give the pilot information about the line-of-sight component of a windshear threat in a region extending from his present position to 2 to 4 km in front of the aircraft. This constitutes a warning time of 20 to 40 s, even under conditions of moderately heavy precipitation. Using these results, a Coherent Lidar Airborne Shear Sensor (CLASS), using a Q-switched CO2 laser at 10.6 μm, is being designed and developed for flight evaluation in early 1992. The edge technique is a powerful new method for the measurement of small frequency shifts which allows high accuracy measurement of atmospheric winds (0.2 to 1 m/sec) with high vertical resolution (10 meters) using currently available technology.
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