Sensing Our Earth from Above

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NASA’s  Langley Research Center  is renowned for the development and use of lidar remote sensing technologies and is recognized worldwide for its expertise in lidar techniques and atmospheric measurements (encompassing aerosols, clouds, winds, water vapor, and trace gases, including methane and ozone), algorithm development, geophysical data retrievals, satellite validation, and, more recently, ocean profiling. 

Lidar was originally a combination of the words “light” and “radar” but is now generally used as an acronym for “light detection and ranging,” or LiDAR.

Atmospheric scientists use both radars and lidars to study the atmosphere because different frequencies of energy reveal different atmospheric characteristics. Lidars use higher-frequency (shorter wavelength) pulses than radars. In general, the shorter the light source's wavelength, the smaller the particles the system can detect. 

In the same way a radar system uses radio waves, a lidar detection system sends short pulses of laser light towards a given object or area. The laser light strikes the lidar system’s target and reflects backwards to a detector in the instrument. For many of Langley’s lidar systems, the “targets” are air molecules, aerosol and cloud particles suspended in the atmosphere, or water molecules and particles suspended in the ocean.  By sending out tens to thousands of laser light pulses every second, lidar systems are able to provide information on things like the altitude of the suspended particles (or depth of ocean particles), optical properties of suspended particles, wind speed and direction, and concentrations of important gases in the atmosphere, such as ozone, water vapor, and methane. 

When lidars are deployed on moving platforms such as aircraft or spacecraft, they provide “curtains” of data showing cloud, wind, aerosol and even ocean composition at varying altitudes/depths and horizontal location. With multiple overpasses of a region, the lidar data can be used to understand the vertical and horizontal transport of particles and gases in the atmosphere. 

Langley has opened a new frontier in lidar remote sensing of oceans, spearheaded by the first-ever global ocean retrievals of CALIPSO data and advanced through the world's most accurate airborne ocean-profiling lidars (using the HSRL technique) to serve the next-generation spaceborne observing system. 

Lidar is a core capability at NASA’s Langley Research Center, which has led to new discoveries and the creation of ground-breaking knowledge providing major scientific contributions to climate, weather, air quality, and environmental health.

Langley's leadership in lidar is informed by decades of expertise and stems from a heritage rich in successful atmospheric measurements. Now that we have an understanding of what lidar is, continue reading below to learn more about how Langley has contributed to the ongoing success of lidar.

Lidar provides scientists information on the vertical dimension of the atmosphere.  Elastic backscatter lidars such as the one deployed on the CALIPSO satellite provide key information on the altitude and vertical thickness of aerosol and cloud layers as well as information on optical properties that impact climate. The High Spectral Resolution Lidar (HSRL) technique goes a significant step further in terms the amount and accuracy of the information that can be collected on aerosols and clouds.   

The HSRL technique relies on spectral differences in backscatter from molecules and particles in the atmosphere. The receiver is designed to separate aerosol and molecular backscatter into two optical channels that are measured separately. In so doing, aerosol backscatter and aerosol extinction can be measured independently, which is not possible with simpler elastic backscatter lidars.  Aerosol extinction is a particularly important quantity for climate studies, as it is a measure of the degree to which aerosols reflect incoming sunlight back to space, rather than allowing it to warm the Earth’s surface. The ratio of aerosol extinction and backscatter provides information on the type/source of aerosol in the atmosphere (e.g., smoke vs. sea-salt aerosol).  

Like elastic backscatter lidars, the Langley HSRLs also have extra channels to measure the degree to which aerosol and cloud particles depolarize the backscattered laser light. The greater accuracy in aerosol backscatter provided by the technique translates into greater accuracy in aerosol particle depolarization, which also provides information on aerosol shape and type. Taken altogether, this highly accurate suite of information on aerosol backscatter, extinction, and depolarization provides information on aerosol particle properties (e.g., shape and size), aerosol type (e.g., dust vs smoke vs urban pollution), and the radiative impact of aerosol layers.  Used in a down-looking mode from an aircraft platform, the HSRL technique provides a 'slice of the sky' style curtain image, highlighting atmospheric layers, layer composition, and key optical and microphysical properties of the aerosol and cloud particles in those layers. 

Langley’s first generation HSRL instrument (‘HSRL-1’) was first flown in early 2006 on a joint NCAR-DOE-NASA mission based in Veracruz, Mexico. It employs the HSRL technique at 532 nanometers (nm) and the elastic backscatter lidar technique at 1064 nm and is polarization sensitive at both wavelengths.  It has flown on over 20 scientific campaigns since then and was upgraded in 2012 to enable ocean profiling at 532 nm. 

The advantages that the HSRL technique brings to atmospheric measurements also apply to ocean profiling, enabling independent depth-resolved measurement of water attenuation and particulate backscatter.  HSRL-1 was the first HSRL instrument used for ocean profiling, immediately transitioning it to the world’s most capable and accurate airborne ocean profiling lidar.   

HSRL enables advances in ocean science by measuring chlorophyll and colored dissolved organic matter. Oceanic lidar measurements are derived and viewed in a similar manner to atmospheric measurements — with a vertical scan of the oceanic column. These scans reveal valuable information on phytoplankton abundance, variations in the ocean column, and vertical distributions of biomass. Oceanic measurements help us to better understand marine biogenic aerosols and marine-atmospheric interaction studies. The HSRL’s ocean, aerosol and cloud profiling capabilities make this lidar particularly applicable to the assessment of aerosol correction in passive ocean color retrievals as well as coupled ocean-atmosphere studies.

Langley’s second-generation HSRL (‘HSRL-2’) implements the HSRL technique at 355 nm in addition to 532 nm and the elastic backscatter lidar technique at 1064 nm. It is also polarization sensitive at all three wavelengths. The addition of the 355-nm HSRL capability enables the retrieval of important aerosol microphysical parameters such as aerosol concentration and size. It is the most capable airborne aerosol lidar in existence and has flown on numerous science campaigns starting with a joint DOE-NASA mission from Cape Cod in 2012. In 2019, an upgrade to HSRL-2 was completed to provide extensive ocean measurement capabilities, including HSRL ocean-profiling at both 355 and 532 nm, chlorophyll fluorescence, and colored dissolved organic matter (CDOM) fluorescence.  It is now the world’s most capable ocean lidar. 

Another distinction between the Langley HSRLs and other lidars, is that the Langley HSRLs are the world's only lidars designed for simultaneous measurement of the atmosphere and ocean, whereas past ocean lidars have focused exclusively on ocean profiling.  

Due to their accuracy and capabilities, the Langley HSRLs are the go-to facility-class lidars requested for aerosol, cloud, radiation, and ocean field missions , and have been flown on 40 science deployments including missions for DOE and EPA as well as for NASA. Over 110 papers using HSRL-1 and -2 data have been published since 2008. 

In addition to HSRL, Langley has developed several instruments and techniques to study the atmosphere. Among these are: 

• The Differential Absorption Lidar (DIAL). DIAL uses four lasers to make two simultaneous measurements – ozone and aerosol. 
• The High Altitude Lidar Observatory (HALO), a combination of DIAL and HSRL used to profile atmospheric aerosols, water vapor and methane.
• The Doppler Aerosol Wind Lidar (DAWN), an instrument incorporating a pulsed lidar system capable of profiling aerosols, wind direction and wind speed. 
• And the Langley Mobile Ozone Lidar (LMOL), a mobile ground-based instrument that measures the vertical distribution of ozone

Langley's water vapor, methane, wind, and ozone lidars are also expanding capabilities and pushing the science forward. Langley has been a world leader in airborne trace gas profiling for over four decades, and have designed and developed the highest precision/resolution airborne wind lidar profiling capabilities as well.

Langley’s scientists bring a variety of lidar expertise to the table, from ocean-profiling lidar expertise to the science, algorithms and retrievals.​ Given the strategic prioritization of generational leadership in lidar science, Langley is a proving ground for new lidar PI's and one of the few training grounds for new hires, young technologists, and scientists. 

Langley has a well-known track record of success and continues to demonstrate their scientific leadership, community engagement, and international coordination and collaboration for the benefit of all. Three generations of scientists have worked on Langley’s lidar science, with each decade bringing in new talent and expertise.

Langley excels in providing the scientific leadership, competence and skills needed to continue evolving lidar technology and capabilities. Langley is home to three generations of talented lidar experts, which also serves as a bridge to the broader science community. Langley actively partners with industry, universities and international organizations to optimize the science and continue driving the technology forward. 

Langley's end-to-end experience will be foundational to the design of a next-generation observing system.

One of the key differentiators between Langley and other Centers and university labs is our end-to-end science-driven capability in lidars for atmospheres and oceans. Langley possesses capabilities in lidar instrument implementation from concept development through hardware development, deployment, mission operations and data processing/archival. The breadth of expertise spans:

• Science: understanding the SATM and observations required
• Algorithms: converting measured signals into the required observations
• Engineering: designing, building and testing lidar instruments for ground, airborne, and space applications; developing unique lasers based on science requirements; laser component and system modeling; incorporating state-of-the-art technologies for transmitter and receiver; etc.
• Calibration/Validation: ensuring that the lidar design provides calibrated measurements with the required precision and accuracy

Resident expertise with an in-depth understanding of key lidar performance drivers positions Langley to serve as a bridge to the broader community in lasers, detection sub-systems, and optical filtering. 

The benefits of lidar for planetary science don’t end on Earth. Lidar has many applications in planetary science, such as surface topography, and global profiles of major atmospheric parameters, such as winds, density, temperature, and aerosols, literally shining a light on new aspects of these fields.  Spectroscopic methods being developed at Langley may aid in investigation of gases in planetary atmospheres.

Entry, Descent, & Landing (EDL) and Navigation Doppler Lidar (NDL)

To land safely on various types of planetary surfaces, NASA has developed several types of advanced technologies. Flash lidar, which produces a large beam that takes photos, helps to determine risks in landings. In earnest development at NASA Langley is the Navigation Doppler Lidar (NDL), a high-performance, compact, and cost-effective velocity and altitude sensor which can accurately measure the velocity and position of a landing vehicle. NDL sends three laser beams towards the ground and measures range and velocity. The measurements from the three beams are then combined, proving the velocity and position information with ultra-high precision. Learn more  here .

Surface Topography, Exploration, and Characterization

Understanding the patterns on a land surface using lidar surface topography capabilities can have many benefits, among them identifying potential areas at risk of landslide hazards and water runoff. Langley is also developing lidars that have the capabilities necessary to study other planetary bodies, such as Mars and the Moon.

Investigations of Planetary Atmospheres and Materials

The benefits of lidar for planetary science don’t end on Earth. Spectroscopic methods being developed at Langley may aid in investigation of minerals, organics, and biogenic materials, as well as atmospheric studies of Mars and Europa, among other celestial bodies.

Searching for Evidence of Life on Other Planets

Langley’s Engineering Directorate has designed, fabricated and built a Raman spectrometer in-house for remote and in-situ detection under the PICASSO Program and successfully demonstrated its performance by acquiring Raman spectra from minerals, organic and biogenic materials or biomarkers (e.g. amino acids). The flourescents produced from the Raman spectrometer can aid in the search for minerals, water and life on other planets such as Mars and Europa as well. 

The research campaigns and experiments that have been conducted over the years use lidar technology on various spacecraft, aircraft, ships and aircraft to learn more about our atmosphere, what’s in it, and how it’s changing. Hundreds of lidar campaigns have been flown over the years culminating countless hours of flight and collections of vital data records. You can browse some of the various campaigns  here .

Three of the most impactful Langley-supported Earth Venture Suborbital (EVS) missions that have been flown in previous years include DISCOVER-AQ, ORACLES, and NAAMES. These campaigns focused on air quality, aerosol-cloud interactions and their impact on the radiation budget, and ocean profiling.

Explore the Cornerstone Lidar Campaigns

The future of lidar looks bright. Langley will continue developing and applying advanced lidar systems to a broad range of investigations, atmospheric, oceanic and elsewhere. Keep reading for a preview of what the future holds.

Langley's HSRL capabilities will enable transformational science beyond CALIPSO and the A-Train satellite constellation. The HSRL system is a significant step forward in acquiring the data we'll need to understand Earth's climate system in the late 2020s and beyond. It will do this with:

• Higher accuracy – societal impact of aerosols at the Earth’s surface where we live and breathe ​

• Higher information content – aerosol/cloud type and microphysical properties needed to improve transport and process models ​

• Optimized ocean profiling capability – the next frontier in ocean remote sensing ​

The  2017 National Academies of Sciences, Engineering and Medicine Earth Science Decadal Survey  selected the atmospheric Planetary Boundary Layer (PBL) as an Incubation Targeted Observable. Improved understanding and prediction accuracy of the atmospheric PBL and the ability to make significant advances in several PBL application areas (e.g. air quality and human health, improved forecasting of severe storms, improved climate projections, renewable energies) are currently constrained by the lack of global PBL observations at sufficient spatial and temporal resolution and sampling. Improved observations of the PBL and its interactions with the ocean, land, and ice surfaces have the potential to advance science on a number of fronts, including improvements to both short-term weather and air quality forecasts, climate modeling, and estimates of trace gas emissions and transport.

Several scientists at Langley now serve on the NASA PBL incubation Study Team, whose goals are to identify methods and activities for improving the understanding of and advancing the maturity of the technologies applicable to the PBL Targeted Observable and their associated science and applications priorities. PBL Incubation science goals call for exploring next-generation measurement approaches that could be ready for spaceborne implementation in 10+ years. New observing technologies and approaches, including in situ as well as ground-based, airborne, and satellite remote sensing, have the potential to increase significantly the quality, amount and types of observations collected within the PBL.

In support of this effort, previously mentioned Langley scientist Amin Nehrir has been awarded funding for an Instrument Incubator Program (IIP-19) project entitled “Atmospheric Boundary-Layer Lidar PathfindEr (ABLE)”. IIP-19 proposals selected by NASA's Science Mission Directorate will provide instruments and instrument subsystems technology developments that will enable future Earth science measurements and visionary Earth-observing concepts.

Langley is well on the path to develop technology that will significantly reduce the cost and risk of the future space-based lidar missions. HSRL accuracy is preserved through profile, enables accurate aerosol retrievals below optically thin cirrus clouds and ensures accurate aerosol profiles through multiple aerosol layers ​. A key differentiator is HSRL's ability to quantify ubiquitous tenuous aerosols that are not captured by CALIPSO, yet have a significant direct aerosol forcing contribution​. Langley does this collaboratively involving multiple NASA centers, industry, and academia in supporting what we do.   ​

Langley is also developing the world’s most advanced airborne wind profiling instrument, called the Aerosol Wind Profiler (AWP), leveraging laser and lidar system technology development from the IIP-16 project entitled “Wind – Space Pathfinder (Wind-SP)”. The AWP will transmit hundreds of laser pulses per second off-nadir through a scanning mechanism to measure extremely high-precision vertical profiles of winds in the horizontal direction up to every 0.5 km along aircraft track, a factor of 10 improvement in detail over what is provided by the current Langley Doppler Aerosol WiNd (DAWN) lidar instrument. It will also be one of the first airborne instruments to also transmit pulses in the nadir direction to measure profiles of vertical winds in clear sky and within cloud tops. 

Such high detail and precision are critical for studying atmospheric processes where subtle changes in winds can have a big impact on clouds and the weather at the ground. In addition to providing validation for space-based winds based on cloud and moisture feature tracking, 3-D wind measurement from the AWP will provide new insights into PBL processes, aerosol and pollution transport, cloud formation and decay, and extreme weather events such as severe thunderstorms and tropical cyclones, especially when paired with other measurements from the HALO and ABLE instruments. Langley will also be applying lessons learned with DAWN and AWP to develop space-based Doppler wind lidar mission concepts.

One of the most well-known atmospheric science missions in the world is Langley’s CALIPSO. However, Langley has made several other key contributions to the understanding of atmospheric parameters. Langley pioneered ozone lidar as well as water vapor lidar.  Ozone lidar was a critical component in the findings that led to the implementation of the Montreal Protocol. Ozone research continues, including Differential Absorption Lidar (DIAL) techniques to provide global chemistry climate models with information on stratospheric ozone input. 

In addition to pioneering the first airborne ozone DIAL, NASA Langley pioneered the first ever airborne water vapor DIAL instrument in the early 80s which provided new insight to the weather and dynamics community on the highly variable nature of water vapor within the lower troposphere. Recognizing the importance of water vapor across almost all science disciplines and also the need for more global measurements, the LASE instrument was developed for autonomous operation on the high flying ER-2 as an airborne prototype for a space-based water vapor DIAL mission. In subsequent years LASE was adapted to fly as facility class instrument on the suite of NASA aircraft including the NASA DC-8 and P3 and contributed to breakthrough scientific findings in tropical and terrestrial weather and dynamic system. 

All of the knowledge gained from the heritage Langley systems led to the development of the high altitude lidar observatory (HALO) which is a more affordable, operationally flexible and multi-functional replacement for the LASE lidar. The intention of this effort was twofold, to develop a facility class lidar instrument to support R&A airborne science as well as to serve as an airborne demonstrator and technology testbed for important targeted observables that were likely to be identified in the decadal survey, namely water vapor profiling for PBL incubation and high resolution methane columns for the explorer class. 

The multi-function aspect of the airborne HALO instrument ties into the underlying theme of the DS to address multiple targeted observables with one instrument. The breakthroughs in laser and receiver technologies have allowed Langley to pursue a cross-cutting water vapor profiling and methane mapping space mission concept which is aligned to the PBL incubation and explorer class lines within the DS. This concept is called Atmospheric Boundary-Layer Lidar Pathfinder (ABLE) and consists of a cross-bread team of top scientists and engineers from government, academia, and industry to enable an affordable SmallSat pathfinder mission to demonstrate the utility of DIAL in space.

Langley pushes the boundaries – technologies, algorithms, and science to do what is needed. Repackaging a proven technique is good, but will not take our scientific understanding any further. The science that is needed for aerosols, winds and clouds clearly needs more, and Langley is here to help.   ​