HSRL (High Spectral Resolution Lidar)
HSRL is an airborne lidar instrument of NASA/LaRC. The objective of HSRL is to characterize clouds and small particles in the atmosphere, called aerosols. 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. It measures aerosol backscatter and depolarization at 532 and 1064 nm and aerosol extinction at 532 nm. 1) 2)
HSRL is compact and robust — designed to be flown on small aircraft like the NASA Langley King Air B200 or LearJet. HSRL is primarily flown on field missions used to validate measurements made by the CALIPSO spaceborne lidar as well as aerosol retrievals from satellite-based passive sensors. It is also used in campaigns focused on regional process studies and the validation of chemical transport models. Information on recent instrument deployments as well as data browse images can be found in the "Field Campaigns" section.
• In December 2004, first test flight on Lear Jet (Ref. 2)
• In December 2005, first test flight on the NASA Langley King Air B200 aircraft
• Since 2006, the HSRL was flown on the King Air B200 aircraft (up to 2010) providing support for:
- 15 field experiments/campaigns: 2006 (3), 2007 (3), 2008 (3), 2009 (3), 2010 (3).
- 330 flights; >1000 flight hours
• Fundamental measurements
- Aerosol extinction: 532 nm
- Aerosol backscatter: 532, 1064 nm
- Depolarization: 532, 1064 nm.
• HSRL provides quantitative information on extensive and intensive aerosol properties
• Intensive parameters useful for aerosol typing and partitioning AOT (Aerosol Optical Thickness) by type
• Intensive and extensive parameters useful for assessing satellite measurements and model predictions
• Aerosol vertical distribution useful for assessing potential errors in scaling column satellite observations to PM 2.5
• Water subsurface measurements useful for ecosystem studies
• Real-time downlink useful for guiding P3 altitude changes.
HSRL is operational in 2012. Measurements from the first-generation NASA airborne HSRL-1 (High Spectral Resolution Lidar-1), onboard the NASA/LaRC) B-200 aircraft, provide a diverse dataset for use in characterizing the spatial and temporal distribution of aerosols, as well as location and variability of the ML (Mixed Layer) height. The HSRL-1 data collected during these missions are used for computing ML heights and for determining the fraction of AOT within and above the ML, both of which are important for air quality assessments.
ML heights derived from HSRL-1 have been used to assess PBL (Planetary Boundary Layer) simulations produced using three models, including WRF-Chem (Weather Research and Forecasting - Chemistry), NASA GEOS-5 (Goddard Earth Observing System - version5), and ECMWF-MACC (European Centre for Medium-Range Weather Forecasts - Monitoring Atmospheric Composition and Climate). 3)
The LaRC instrument employs the HSRL technique at 532 nm and the standard backscatter lidar technique at 1064 nm. In addition, it is polarization sensitive at both wavelengths. The fundamental data products are aerosol backscatter and extinction coefficients at 532 nm, aerosol backscatter coefficient at 1064 nm, and degree of linear polarization at both wavelengths. These basic measurements are used to compute several aerosol intensive parameters, including Sa, backscatter coefficient wavelength dependence, and aerosol depolarization ratio. The extinction profile is integrated vertically to produce aerosol optical depth along the flight track. 4)
The basic architecture of HSRL is shown in Figure 1. The telescope light-shield provides the structural backbone of the instrument. It is essentially a cylindrical optical breadboard to which the pulsed laser, transmit optics, and the telescope aft-optics are rigidly mounted. In addition to providing a mechanically robust assembly, this design approach makes it straightforward to add new or replacement subsystems or to reconfigure the instrument for installation on different aircraft platforms. The control and detector modules are located in a separate rack on the right side of Figure 1.
Figure 1: Basic architecture of the HSRL instrument (image credit: NASA/LaRC)
Legend to Figure 1: The electronics rack (106 cm) which contains the PXI data acquisition computer, seed laser, all detectors, iodine cells for filtering and monitoring spectral purity, and power supplies for the lasers, is shown in the right-side photo.
The HSRL technique is based on the spectral discrimination of aerosol and molecular backscatter coefficients. This imposes requirements on the laser transmitter as well as the receiver. The laser must operate on a single longitudinal mode to ensure that the transmitted frequency distribution is significantly narrower than the molecular broadening of the backscattered signal. Also, because the LaRC system relies on the iodine vapor filter technique for spectral separation in the receiver, the transmitter must be capable of being tuned and locked to an iodine absorption feature.
The transmitter in the LaRC system involves four subsystems: a pulsed Nd:YAG laser, a tunable dual-wavelength CW (Continuous-Wave) injection seed laser, an electro-optic feedback loop that locks the seed laser frequency to the center of the desired iodine line, and the transmitter optics module at the output of the pulsed laser.
As shown in Figure 2, the output of the 1064 nm CW Nd:YAG seed laser is coupled into a single mode polarization-maintaining fiber that is then directly coupled into the pulsed Nd:YAG laser for injection seeding. The 532 nm CW output is used as a reference to tune and lock the seed laser frequency.
Figure 2: Basic layout of the HSRL showing the optical and electronic subsystems (image credit: NASA/LaRC)
The transmit optics module, Figure 3, expands the pulsed output beam, linearizes the output polarization, provides a means to shutter the output, and automatically attenuates the output as dictated by the operational scenario, and provides a measurement of the pulsed laser spectral purity for each pulse using a separate iodine vapor cell.
The receiver involves five subsystems: a Newtonian telescope, an aft-optics module mounted near-kinematically to the telescope, detector modules for the standard backscatter and depolarization channels, an iodine filter module for molecular/aerosol discrimination at 532 nm, and a data acquisition and control computer. The aft-optics module optically separates wavelengths, polarization states, and, at 532 nm, the molecular and total backscatter channels. The various optical signals are fiber-optically coupled from the aft-optics module to the respective detectors-amplifier modules, as is the light to the iodine vapor filter module.
Figure 3: Schematic of the transmit beam optical layout (image credit: NASA/LaRC)
Legend to Figure 3: The Glan laser polarizer ensures linear polarization output. The output polarization for both wavelengths can be set relative to the receiver with the last half-wave plate before transmission in to the atmosphere. A fiber pickoff is shown for the energy and spectral purity monitor which are housed in a separate module.
The pulsed output laser is a custom-designed injection-seeded Nd:YAG system developed by Fibertek, Inc. of Herndon, VA. It provides output beams at the fundamental (1064 nm) and second harmonic wavelengths (532 nm). The injection seed laser is a tunable Prometheus Nd:YAG nonplanar ring oscillator laser built by Innolight GmbH. In addition to providing a CW source for seeding the pulsed laser at 1064 nm, the seed laser also provides a CW output at 532 nm. The Innolight laser can be tuned over ~90 GHz at 532 nm and has a continuous, mode-hop-free scanning range of ~12 GHz. The frequency agility of the seed laser allows the CW 532 nm output to be used in an electro-optic feedback loop designed to lock the seed laser frequency to an iodine absorption line and, in a calibration mode, to determine the transmission of the iodine vapor cell as a function of wavelength.
The basic parameters of the laser systems are provided in Table 1. Both laser systems are operated at ~25ºC, providing output wavelength control near 532.242 nm, the wavelength of the strongest absorption line in the iodine spectrum within the Nd:YAG tuning range. Of the available absorption lines in the tuning range of the transmitter, the strongest line was chosen to maintain high aerosol backscattered light rejection at the laser wavelength while minimizing the iodine density in the receiver filter. Lowering the iodine gas number density, and therefore the operational temperature and vapor pressure, minimizes both the collisional broadening of the absorption line and broadband continuum absorption, thereby providing the maximum transmission of the wings of the broadened molecular backscattered signal.
The pulsed laser is injection seeded using a modified ramp-and-fire technique to ensure that every laser shot fires with high spectral purity at the appropriate wavelength when operating in the challenging vibrational environment of an aircraft. A total of 350 mW of CW 1064 nm power is used for seeding and is coupled into the pulsed laser via the high reflector mirror in the pulsed laser cavity. After accounting for losses through the high reflector mirror, approximately 10 mW of seed light is injected into the pulsed laser cavity. The laser spectral lineshape has been measured using a confocal interferometer with a 600 MHz free spectral range and has an estimated full width at half-maximum (FWHM) of 38 MHz, as shown in Figure 4.
In addition, the ramp-and-fire implementation has a relative phase adjustment to fire the q switch based on the resonant fringe, which is set by simultaneously injecting the pulsed and seed laser outputs into the confocal interferometer to match the seed laser to the pulsed laser output to within <5 MHz. This confirms that the frequency of the seed laser, which is used for frequency stabilization, matches the pulsed laser output frequency.
Figure 4: Pulsed laser spectral lineshape measured at 1064 nm (image credit: NASA/LaRC)
Legend to Figure 4: The frequency scale is referenced to the center of the best fit of a Gaussian distribution lineshape shown as a dashed line. The FWHM of the Gaussian fit is 38 MHz.
Table 1: Basic operational parameters for the pulsed and seed lasers
Seed Laser Frequency Control:
The seed laser frequency is controlled by a high-speed, autonomous electro-optic control loop that is based on a phase modulation technique. Figure 5 shows a basic block diagram of the seed laser and the elements of the frequency control loop. The control system senses the wavelength of the CW 532 nm seed laser output with respect to the position of the iodine absorption line and tunes the seed laser to center the 532nm output on that line. This technique was chosen over alternate techniques that rely on the pulsed laser output for frequency sensing and control, as it is much more accurate and reliable. Of the total 30 mW of CW 532 nm light from the seed laser, approximately 5 mW is coupled to a multi-mode fiber for delivery to the receiver where it is used to measure the spectral transmission of the iodine vapor filter in the receiver.
Figure 5: Diagram providing the basic layout for the seed laser with dual-wavelength outputs (image credit: NASA/LaRC)
Legend to Figure 5: The 532 and 1064 nm output optical paths for the energy monitor, laser line locking to an iodine cell, and filter scan output are shown. There are two outputs for the 1064 nm laser light. Approximately 98% is directly coupled with a PM fiber to the pulsed laser used for seeding. The 1% output is used for diagnostics and as a frequency marker for the iodine filter scans using a 300 MHz confocal interferometer.
About 1 mW of the 532 nm light is directed to the frequency locking system, which consists of three main components: a 240 MHz phase modulator, an iodine cell (separate from the iodine cell in the receiver), and detection and control electronics. The phase modulator is a New Focus Model 4001 driven at 240 MHz by a resonant tank circuit. The custom locking cell was fabricated by Innovative Scientific Solutions with a fixed number density of iodine within the 25 mm diameter by 50 mm long Pyrex cell. The cell is temperature controlled at 35ºC ± 1ºC, which is sufficient to ensure the iodine remains completely in the gas phase. The transmission spectrum of the locking iodine cell has an online transmission of ~29% for the absorption line centered near 532.242 nm at which the transmitter is operated (Figure 6).
Figure 6: (a) Measured transmission function of the main science channel iodine vapor filter using the 532 nm output from the seed laser is shown as the solid thin line. The Cabannes-Brillouin backscattered signal spectra (275 K, 0.75 atm.) with Mie scattering included is plotted as the dashed line. The filtered transmitted backscattered spectrum is shown as the solid thick line.
The detection and control system consists of a model FND-100 silicon detector from Perkin-Elmer, compact custom-built amplifier, phase-detection circuits, and mixing circuits. The output of the amplifier consists of the sum of two beat signals: the beat signal between the ±240 MHz shifted optical signals with the unshifted optical signal. These two signals are both sinusoidal at 240 MHz, but 180° out of phase. The amplitude of the resulting sum is zero when the amplitudes of the two shifted optical signals are equal, i.e., when the unshifted optical signal is nearly centered in frequency on the iodine absorption feature. Note the locking frequency does not occur exactly at the center of the absorption line due the asymmetry induced from nearby absorption lines. Note also that the locking frequency is referenced to the filter transmission based on the scan data.
The error signal is recorded simultaneously with the scan of the filter transmission. The phase detection circuit closely follows that of Bjorklund, 1980 and shifts the phase of the resultant 240 MHz signal from the photodetector amplifier to match that of the local oscillator signal injected at the mixer. The output of the mixer is a DC error signal that is proportional to the energy in the resultant of the 240 MHz beat signals. The error signal, shown in Figure 6 (b), is directly coupled to a dual-loop PID (Proportional Integral Derivative) circuit to provide control feedback to the seed laser that effectively equalizes the ±240 MHz optical sidebands downstream of the iodine filter, thereby centering the 532 nm seed laser output on the center of the chosen iodine absorption line.
The seed laser frequency is adjusted with a fast tuning piezo transducer custom-mounted in the seed laser, and the second slower PID loop maintains the transducer voltage near zero by adjusting the seed laser temperature. The error signal is recorded during scans of the iodine filter transmission. All data are taken simultaneously during the scan and then scaled to frequency, thereby creating a transfer function of the error signal from voltage to frequency. Using this transfer function, this configuration has been shown to provide long-term frequency stability to within 0.1 MHz (rms) during flights, which far exceeds the requirements imposed by the HSRL technique.
Figure 5 also shows the 1064 nm optical path, which incorporates a high-power isolator to prevent backreflected light from affecting the performance of the frequency locking loop. The 1064 nm beam is coupled into a PM (Polarization-Maintaining) single-mode fiber and is subsequently split into three channels using a 1 x 3 PM fiber splitter made by Canadian Instrumentation and Research Ltd. The split ratios of the 1 x 3 splitter are 98%, 1%, and 1%. The 98% output is coupled into a single-mode PM fiber which is then directly coupled into the pulsed Nd:YAG laser for injection seeding, while the 1% legs are used for diagnostics and for frequency scaling the spectral scans of the iodine cell using a confocal interferometer.
Transmit Optics and Spectral Purity Monitor:
The transmit optics assembly is mounted directly to the pulsed laser housing and provides several important functions for the lidar system. A layout of the optical components is provided in Figure 3. The two coaligned beam-expanded outputs of the pulsed laser are first sent through a 1064 nm half-waveplate (full wave at 532 nm) and a then a 532 nm half-waveplate (1064 nm full-waveplate) to coalign the two laser output polarizations to a Glan-laser polarizer, which is used for two functions: (1) insuring high polarization purity of the laser output, and (2) providing an attenuation mechanism to reduce the energy of the 532 nm output when required for eye safety.
The attenuation function is implemented by rotating the 532 nm half-wave plate to detune the axes of polarization of the laser output at 532 nm from the pass axis of the polarizer, and is automatically adjusted as a function of aircraft altitude. The laser divergences of the two beams are adjustable using a telescope mounted inside the laser housing and are nominally set to 0.8 mrad. A small fraction of the pulsed laser light is picked off into a 1mm multi-mode fiber for diagnostics that include measuring the spectral purity of the 532 nm light and the energy at both wavelengths. A coarse mirror before the polarizer is used to align the output beam in the lab before integration on the aircraft. A high resolution (<2 µrad) piezo mirror (Mad City Labs, model Nano-MTA2) is used to provide accurate, quasi real-time boresighting of the laser output to the receiver FOV (Field of View). A final dual-wavelength half-waveplate on a motor stage is used to align the polarization axis of the transmitted output with that of the receiver polarization analyzers.
A unique and important feature of the airborne HSRL system is the spectral purity monitoring subsystem. In this subsystem, pulsed 532 nm laser output from the transmitter is directed via fiber to a separate iodine cell (the “spectral purity cell”) to determine the spectral purity of each laser shot. The spectral purity cell is built using a 25 mm diameter by 50 mm long quartz cell that has a fixed density of iodine and is temperature controlled to 65ºC, at which point the iodine is completely in the gas phase. While the spectral purity cell is two times shorter than the main iodine cell, it contains twice the iodine density as that of the main iodine cell and has a centerline transmission at 532.242 nm less than 10-6. The spectral widths, ~2.0 GHz at the 50% transmission points, of the two cells are comparable since they are operated at the same temperature. While pressure broadening is higher for the spectral purity cell, for the densities and temperatures at which the cells are operated, the relative increase in spectral width is negligible.
The pulsed laser light passes through the spectral purity cell, and the transmitted pulse amplitude is determined. Single longitudinal mode laser shots centered in frequency on the iodine absorption line suffer high attenuation in the cell. Note, measurements of the temporal shape of the weak transmitted unseeded component through the spectral purity cell clearly show the mode beating that highlights the broadband component is observed. Shots that are not spectrally pure, i.e., that have energy in other longitudinal modes, result in a much higher signal at the detector, as any other modes are well outside the iodine absorption line. Before integration onto the aircraft, a separate calibration is performed with the laser frequency tuned off of the iodine absorption line to measure the overall electro-optical calibration constant of the subsystem for normalization.
A threshold is then set on the detection circuit to prevent laser shots that are above a preset spectral purity level from being recorded in the data acquisition system. For unseeded laser shots, the increased throughput is orders of magnitude more than wellseeded laser shots. Typically, spectral purity ratios (i.e., the ratio of energy inside the iodine line to that outside the line) are greater than 5000:1 for this transmitter and, under certain conditions, have exceeded 10, 000:1. The spectral purity monitoring system also provides a quantitative measurement of the percent of laser shots seeded during aircraft operations. This system has successfully flown on two different aircraft, and the percent of unseeded shots is extremely low, with those few unseeded shots occurring mainly during takeoff and landing.
Telescope Receiver and Aft-optics:
The telescope for the instrument is a 40 cm diameter f /2.3 Newtonian telescope designed, assembled, and aligned by Welch Mechanical Designs, LLC. The collected light is sent through a field stop that can be varied between 0.25 and 1.00 mrad and is then collimated through the aft-optics module. A diagram of the aft-optics module is provided in Figure 7, and the main specifications of the receiver are given in Table 2. A dichroic beam splitter separates the 1064 and 532 nm optical signal channels. Another beam splitter in the 532 nm path directs 2% of the 532 return to a boresighting subsystem, with the remaining 98% going to the science channels.
Downstream of the dichroic beam splitter, the 1064 nm return passes through a 0.4 nm FWHM interference and solar blocking filter manufactured by Barr Associates. This filter has a peak transmission of 79%. A PBS (Polarization Beam Splitting) cube is used to separate the two orthogonal polarizations of the backscattered light, denoted as 1064 nm para and 1064 nm perp in Figure 7 for the parallel and perpendicular components, respectively. A second cube is used in the perpendicular channel to remove the residual ~5% parallel polarized signal in that optical channel.
Figure 7: Schematic view of the receiver optics connected to the output of the telescope (image credit: NASA/LaRC)
Legend to Figure 7: There are three different receiver legs consisting of the 1064 nm channels, 532 nm channels, and the quadfiber boresighting channels. All output channels are fiber-coupled to the detectors and iodine vapor filter for the 532 nm molecular backscatter channel.
The 532nm light passes through a solid Fabry–Perot etalon (FWHM=60 pm, free spectral range =0.75 nm) and an interference filter (FWHM=0.75 nm) that rejects out-of-band fringes from the etalon. The etalon is temperature tuned to match the laser wavelength and has a peak optical transmission of 82%. The interference and solar blocking filter have a combined optical transmission of 89%. A small fraction (~2%) of the 532 nm light is split off to the boresighting subsystem. The remaining ~98% of the 532 nm light is separated into the two orthogonal polarizations using a single PBS cube that is custom designed to provide high contrast (10000:1) and low cross talk between both polarization channels labeled as 532 nm para and 532 nm perp in Figure 7. The parallel polarized light is split into two channels with a 90:10 beam splitter: the larger of the optical signals, denoted the 532 nm molecular, is directed to the iodine filter assembly and the smaller to a detector assembly identical to that of the perpendicular channel.
Except for the 95% parallel polarization channel at 532 nm, all science channels are coupled via 1mm diameter solid-core fibers to detector modules shown in Figure 8. In the detector modules, the returns are collimated, passed through a wedge/Lyot depolarizer, further split by a 95:5 beam splitter, and finally focused onto detectors. Detection at 532 and 1064 nm is accomplished with photomultiplier tubes (PMTs) and avalanche photodiodes (APDs), respectively. The optical split into 5% and 95% channels is implemented to increase the dynamic range of the instrument.
Figure 8: Detector modules that incorporate fiber-coupled inputs and 5%–95% optical splits for high signal and low signal detectors (image credit: NASA/LaRC)
The 532 nm molecular backscatter channel is fiber-coupled into the iodine filter module that contains the iodine vapor filter, additional inputs and mechanisms for calibration, and science and calibration detectors. The science detector is a PMT identical to that in the 532 parallel and perpendicular channels. A diagram of the 532 nm molecular backscatter channel detector module is shown in Figure 9.
The boresighting subsystem provides near real-time control of the alignment between the transmitter and receiver. In the aft optics, the boresighting channel reimages the telescope field stop onto a quad-fiber bundle. Each quadrant consists of hundreds of 25 µm diameter fibers that are terminated in a SMA fiber connector and coupled to separate PMT detector modules, which are similar to the science channels but with smaller electronic bandwidths set to obtain higher signal-to-noise and a vertical resolution of ~0.5km. The signals from the four PMTs, corresponding to the four quadrants of the field stop image, are combined in a way to provide a feedback signal to an encoded piezo-electric actuated turning mirror (Mad City Labs) used to align the output laser beams to the center of the field stop.
Figure 9: Block diagram of the iodine vapor filter and the optical layout to provide periodic measurements of the filter transmission. The science channel input and the input to measure transmission spectra are both fiber-coupled (image credit: NASA/LaRC)
Data Acquisition and Control Electronics:
The data acquisition system is designed to provide complete control, diagnostics, and calibration of the instrument without additional ground support hardware. All parameters and control commands are input to a master laptop computer; no manual adjustments are required for instrument operation. An Iridium satellite modem provides real-time communication and data downlink between the aircraft operator and the scientists on the ground.
The laptop computer interfaces to a National Instruments PXI chassis with a real-time PX-8176RT controller that controls all instrument function and manages data acquisition. The PXI chassis contains four PXI-6115 12 bit digitizers that have four channels per digitizer providing a total of 16 available channels. The HSRL system utilizes ten of these channels for the science data. Additionally, four channels are used to measure the quad PMT outputs from the boresight system. All 14 channels are typically averaged over 100 shots (0.5 s) and then transferred to the laptop computer for recording, analysis, and display.
Before averaging, each channel is preprocessed to evaluate potential digitizer saturation, which is then denoted in the data stream. In postprocessing, saturated signals on the 90% optical channels (e.g., strong cloud signals) are substituted with gain-corrected signals from the 5% channels. Digital outputs from the PXI-6115 digitizer modules control the custom-built variable gain and offset amplifiers in the APD and PMT detector modules.
The PXI chassis also includes a multifunction PXI-6052 data acquisition card for AD/DA input and output for recording and control of various system parameters such as the pulsed laser energies and position control of the calibration mechanisms. A PXI-4351 card is used to monitor various temperatures in the system for health and status. A PXI-6602 counter/timer card with an 80 MHz time base is used to provide synchronization of the digitizer triggers, laser energy monitors, and spectral purity monitor to within 12.5 ns of each output laser pulse.
The half-wave plates that control the orientation of the output laser polarization and the laser attenuation are mounted in encoded Newport (model PR50) rotation stages that are controlled via a GPIB interface. These stages provide robust, relatively fast, and accurate rotational adjustment for both calibration operations and attenuation of the laser for maintaining eye-safe operations.
Table 2: Receiver optical specifications and measured optical efficiencies
2) Chris A. Hostetler, Richard A. Ferrare, John W. Hair, Raymond R. Rogers, Mike Obland, Sharon P. Burton, Anthony L. Cook, David B. Harper, Amy Jo Swanson, “Airborne High Spectral Resolution Lidar (HSRL),” URL: http://discover-aq.larc.nasa.gov/pdf/2010STM/Hostetler_HSRL_brief_to_DISCOVER-AQ_20101012_v6.pdf
3) Amy Jo Scarino, Sharon Burton, Richard Ferrare, Chris Hostetler, John Hair, Michael D. Obland, Ray Rogers, Anthony Cook, David Harper, Jerome Fast, Arlindo daSilva, Angela Benedetti, “Mixed Layer Heights Derived from the NASA Langley Research Center Airborne High Spectral Resolution Lidar,” AGU Fall Meeting, San Francisco, CA, Dec. 2012, paper: A23C-0240, URL: http://fallmeeting.agu.org/2012/files/2012/11/Scarino_2012_AGU_Poster_v2.pdf
4) Johnathan W. Hair, Chris A. Hostetler, Anthony L. Cook, David B. Harper, Richard A. Ferrare, Terry L. Mack, Wayne Welch, Luis Ramos Izquierdo, Floyd E. Hovis, “Airborne High Spectral Resolution Lidar for profiling aerosol optical properties,” Applied Optics, Vol. 47, No. 36, 20 December 2008, pp. 6734-6753, URL: http://www.patarnott.com/atms360/pdf_atms360/LidarMexicoCity.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.