NASA Goddard Space Flight Center has developed an integrated-path, differential absorption (IPDA) lidar approach to measure atmospheric column-averaged CO2mole fraction (XCO2). This pulsed laser approach uses a step-locked laser diode source, a fiber laser amplifier, and a high-sensitivity detector. The approach allows measurements of CO2absorption and time-resolved laser backscatter profiles. These enable accurate estimates of XCO2and range to the Earth's surface and cloud tops under a wide variety of conditions, including darkness, at low sun angles, through broken cloud fields, and the presence of aerosols. The CO2Sounder lidar team has demonstrated airborne measurements during several campaigns over the past decade. Here we use the data from the summer 2017 airborne campaign in Alaska to further demonstrate the capabilities of this technique. We validated the XCO2retrievals from the lidar against in situ measurements made from the aircraft during spiral-down maneuvers. The results show that this lidar approach provides XCO2measurements with low bias and high precision under a wide variety of measurement conditions. The campaign showed the lidar's capability to resolve the vertical and horizontal gradients of CO2and to quantify CO2enhancements from wildfires. The results show it is an attractive candidate for future airborne science campaigns and, with more laser power, for a space mission like NASA's planned Active Sensing of CO2Emissions over Nights, Days and Seasons (ASCENDS) mission.

NASA's Goddard Space Flight Center has been developing lidar to remotely measure CO2and CH4 in the Earth's atmosphere. We have advanced the tunable laser technology to enable high-fidelity measurements from space. In this paper, we will report on the progress of a fiber-based, 1.57-micron wavelength, laser transmitter that has demonstrated the optical performance required for a low earth orbiting instrument. The Laser transmitter has been packaged and is undergoing environmental testing to demonstrate its technology readiness for space.

NASA's Ice, Cloud, and land Elevation Satellite (ICESat), which operated between 2003 and 2009, made the first satellite-based global lidar measurement of Earth's ice sheet elevations, sea-ice thickness and vegetation canopy structure. The primary instrument on ICESat was the Geoscience Laser Altimeter System (GLAS), which measured the distance from the spacecraft to Earth's surface via the roundtrip travel time of individual laser pulses. GLAS utilized pulsed lasers and a direct detection receiver consisting of a silicon avalanche photodiode (Si APD) and a waveform digitizer. Early in the mission, the peak power of the received signal from snow and ice surfaces was found to span a wider dynamic range than planned, often exceeding the linear dynamic range of the GLAS 1064-nm detector assembly. The resulting saturation of the receiver distorted the recorded signal and resulted in range biases as large as ~50 cm for ice and snow-covered surfaces. We developed a correction for this "saturation range bias" based on laboratory tests using a spare flight detector, and refined the correction by comparing GLAS elevation estimates to those derived from Global Positioning System (GPS) surveys over the calibration site at the salar de Uyuni, Bolivia. Applying the saturation correction largely eliminated the range bias due to receiver saturation for affected ICESat measurements over Uyuni and significantly reduced the discrepancies at orbit crossovers located on flat regions of the Antarctic ice sheet.

Work is underway to demonstrate a new linear mode photon counting (LMPC) detector in Earth orbit inside a 3U Cubesat. The detector is 16 pixel (2×8) HgCdTe electron Avalanche Photodiode (e-APD) array developed by DRS Technologies, C4ISR Group, Infrared Sensors and Systems (ISS). It has ~70% quantum efficiency and single photon level response at wavelengths from 0.4 to 4 microns. The detector will be mounted in a small integrated detector cooler assembly that operates the e-APD at 80K. The LMPC CubeSat is planned for launch into a 450 km × 820 km × 99 degree inclination orbit in July 2016. In space measurements are planned at 1.06, 1.57 and 2.06 microns using internal sources and with ground-to-space experiments.

Determining the$\hbox{CO}_{2}$column abundance from an integrated path differential absorption (IPDA) lidar requires accurate knowledge of the range to the scattering surface, i.e., the column height. We have adapted and tested a ranging algorithm for the airborne IPDA$\hbox{CO}_{2}$lidar designed at the NASA Goddard Space Flight Center, and have evaluated its accuracy and precision. We applied a quasi-maximum-likelihood method, using cross correlation, to estimate the range from the lidar's 1-$\mu\hbox{s}$-wide echo pulses. The objective was to show that the use of these temporally long laser pulses allows the determination of the optical path length with required precision. We analyzed airborne measurements made in August 2009 over the Chesapeake Bay near the Eastern Shore of Virginia. The results indicate a ranging precision of better than 3 m, which is sufficient for airborne and space-based retrievals of$ \hbox{CO}_{2}$column concentration.

We have developed a pulsed lidar technique for measuring the tropospheric CO2concentrations as a candidate for NASA's planned ASCENDS space mission. We have developed an airborne lidar to demonstrate the measurements from aircraft. In 2010 and 2011we measured atmospheric CO2and O2column absorptions and line shapes on a series of flights at altitudes from 3-13 km.

NASA Goddard Space Flight Center has developed a series of space lidars over the past 15 years and successfully mapped Mars, Earth, Mercury and Moon. Each of these space lidars had its special challenges in measurement requirements and operation environments. Each made a major advancement in space lidar technologies. This paper gives a brief overview of these space lidar systems.

We report on progress in developing a new multi-beam non- scanning, swath mapping laser altimeter measurement approach for future spaceflight missions using a high repetition rate, short-pulse laser transmitter. The instrument contains multi pixel photon counting detectors, high bandwidth, sixteen-channel 8-bit digitizer and a high- throughput data system.

Accurate global measurements of tropospheric Carbon Dioxide abundance to the “parts per million” (ppm) level is required to better quantify processes that regulate CO2exchange between atmosphere, land and ocean. To measure CO2globally NASA has planned the ASCENDS mission, Active Sensing of CO2Emissions over Nights, Days and Seasons. We report on an airborne laser-based remote sensing instrument we have developed at NASA-GSFC as a candidate for the ACENDS mission. Preliminary analyses of data from initial flight testing to 13 km altitude indicates a CO2mixing ratio of 396 ppm from CO2column abundance where in situ sensors determine 390 ppm mixing ratio.

The Slope Imaging Multi-polarization Photon-counting Lidar is an airborne instrument developed to demonstrate laser altimetry measurement methods that will enable more efficient observations of topography and surface properties from space. The instrument was developed through the NASA Earth Science Technology Office Instrument Incubator Program with a focus on cryosphere remote sensing. The SIMPL transmitter is an 11 KHz, 1064 nm, plane-polarized micropulse laser transmitter that is frequency doubled to 532 nm and split into four push-broom beams. The receiver employs single-photon, polarimetric ranging at 532 and 1064 nm using Single Photon Counting Modules in order to achieve simultaneous sampling of surface elevation, slope, roughness and depolarizing scattering properties, the latter used to differentiate surface types. Data acquired over ice-covered Lake Erie in February, 2009 are documenting SIMPL's measurement performance and capabilities, demonstrating differentiation of open water and several ice cover types. ICESat-2 will employ several of the technologies advanced by SIMPL, including micropulse, single photon ranging in a multi-beam, push-broom configuration operating at 532 nm.

NASA Goddard Space Flight Center is developing an active laser approach for global atmospheric CO2concentration measurement from space as a candidate for NASA's future mission ASCENDS - the Active Sensing of CO2Emissions over Nights, Days, and Seasons. This pulsed laser approach provides several advantages with respect to passive approaches and other laser techniques toward high-precision CO2concentration measurement from space. Sensitivity studies conducted for this development identify the best line and ideal wavelengths for the lower atmospheric CO2and O2abundance measurement that allow the estimate of CO2concentration in the lower atmosphere associated with CO2sources and sinks. Studies also identify the ancillary data requirements, laser system and other measurement optimizations in order to achieve the high-precision measurement.

We report progress in assessing the feasibility of a new satellite-based laser-sounding instrument to measure CO2concentrations in the lower troposphere from space.

GLAS is a space lidar on NASA's ICESat Mission. GLAS was qualified and delivered and ICEsat is scheduled for launch in December 2002. This talk summarizes the as-built characteristics of GLAS and its predicted measurement performance.

A ground-based, autonomous, low-power atmospheric lidar instrument developed at NASA Goddard Space Flight Center has been returned from an extended period on the Antarctic polar plateau. We report on the pre- and post-deployment performance of the AGO Lidar, compare the instrument performance with lidar models, and compare results with the anticipated performance of the new Automatic Weather Station (AWS) Lidar.

This correspondence compares OOK and low-order PPM signaling formats in terms of bit error probabilities versus required signal counts per bit. The results show that QPPM requires 3 dB less signal than OOK, while BPPM requires the same or slightly more than OOK for the same performance. Optimum APD gain values are from 200 to 400. When using QPPM,k_{eff} = 0.006, and optimum gain, 60 signal counts/bit are required at 500 Mbits/s for a 10-6bit error probability.