FEATURE MAPPING ➤
Atmospheric applications of lidar use outgoing laser pulses that return via scattering from small particles suspended in the atmosphere – these aerosols are assumed to move with the wind. The return signal gives a measure of the distance and speed of the particles and hence, the wind speed, relative to the lidar. These systems measure wind speed along radial lines from the lidar to the particles as determined from the Doppler frequency shift of backscattered light. Doppler lidar comes in two flavours. Direct- detection systems measure light intensity to calculate Doppler shift – and require a relatively strong signal – whereas coherent-detection lidar systems directly estimate frequencies, requiring a weaker signal and less energy than the direct- detection system. While coherent detection measures light
scattered off aerosol particles, direct-detection measures light scattered off individual molecules. This results in a significant increase in measurement coverage, simply because of the fact that, while aerosol concentrations can be very low in the atmosphere, molecules are found everywhere. As a result, direct-detection always produces measurements, even in the upper reaches of the atmosphere. Conversely, molecules are much lighter than aerosol particles and have much higher random velocities (Brownian motion), producing a much wider backscatter signal spectrum that requires a stronger signal for accurate wind estimation. NASA’s Langley Research Centre is developing
coherent-detection aerosol pulsed Doppler lidar, with parameters tuned to match requirements of missions to outer space. NASA is designing it’s Doppler Aerosol WiNd lidar (DAWN) to work in tandem with a molecular wind lidar. DAWN could, in the future, complement the European Space Agency’s Atmospheric Dynamics Mission Aeolus satellite. With a launch date in
Doppler Aerosol WiNd Lidar in the cargo level of the DC-8 aircraft. Since the laser beam is directed 30 degrees off nadir, the optical wedge that tilts the beam must be near the skin of the airplane to avoid hitting the airplane body. The silver/blue cylinder houses the laser, detector, telescope, and scanner. The rack contains some electronics. Other electronics are in the cabin level with the DAWN operators.
2017, ESA’s ‘Aladin’ instrument will measure winds using a direct detection system. A future operational mission might combine Aladin with the DAWN aerosol lidar from NASA. ‘We envision the combination of the two to cover the atmosphere, from top to surface. That would share the cost. Also, with two systems you have potential of intercomparison and some redundancy,’ said NASA scientist Michael Kavaya. The large pulse energy of DAWN’s two
The future of lidar
NASA DC-8 aircraft. For the space mission, the team plans to deploy a 70cm (aperture) telescope to counteract range losses. ‘We’re already flying a laser with the pulse energy and repetition rate that we envision necessary for a space mission,’ added Kavaya.
offers the opportunity for automatically extracting critical information from data sets
micron wavelength laser uses a Ho:Tm:LuLiF lasing crystal developed by NASA, and a 15cm beam-expanding telescope for flights on board a
While DAWN’s primary purpose is to enhance the capabilities of a future weather satellite, it is already providing useful information back on the ground. DAWN measured
winds in NASA’s 2010 Genesis and Rapid Intensification Processes campaign to study tropical storms. Moreover, in 2015, DAWN flew on the NASA Polar Winds campaign to study polar warming as well as to determine the feasibility of future calibration/validation with the ESA’s Aladin instrument. These missions offer insights into the hardware’s sensitivity to temperature and vibrations, and atmospheric conditions.
Geiger-mode lidar image of an amusement park in North Carolina 22 ELECTRO OPTICS l OCTOBER 2016
Where next? The future of lidar offers the opportunity for automatically extracting critical information from data sets; fusing active and passive sensors for characterising environmental resources; processing data in real time; and mobilising instruments in unmanned aerial systems. Further developments will bring the most up-to-date mapping capabilities to the most challenging environments. l
@electrooptics |
www.electrooptics.com
Harris Corporation
NASA
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