Atmospheric methane (CH<sub>4</sub>) is the second most important anthropogenic greenhouse gas, with approximately 25 times the radiative forcing of carbon dioxide (CO<sub>2</sub>) per molecule. Yet, lack of understanding of the processes that control CH<sub>4</sub> sources and sinks and its potential release from stored carbon reservoirs contributes significant uncertainty to our knowledge of the interaction between carbon cycle and climate change. At Goddard Space Flight Center (GSFC) we have been developing the technology needed to remotely measure CH<sub>4</sub> from orbit. Our concept for a CH<sub>4</sub> lidar is a nadir viewing instrument that uses the strong laser echoes from the Earth’s surface to measure CH<sub>4</sub>. The instrument uses a tunable, narrow-frequency light source and photon-sensitive detector to make continuous measurements from orbit, in sunlight and darkness, at all latitudes and can be relatively immune to errors introduced by scattering from clouds and aerosols. Our measurement technique uses Integrated Path Differential Absorption (IPDA), which measures the absorption of laser pulses by a trace gas when tuned to a wavelength coincident with an absorption line. We have already demonstrated ground-based and airborne CH<sub>4</sub> detection using Optical Parametric Amplifiers (OPA) at 1651 nm using a laser with approximately 10 μJ/pulse at 5kHz with a narrow linewidth. Next, we will upgrade our OPO system to add several more wavelengths in preparation for our September 2015 airborne campaign, and expect that these upgrades will enable CH<sub>4</sub> measurements with 1% precision (10-20 ppb).
We demonstrate the airborne measurement of atmospheric methane using a pulsed lidar at 1650 nm using an integrated path differential absorption scheme. Our seeded nanosecond-pulsed optical parametric amplifier (OPA)-based instrument works up to the highest altitudes flown (<10 km). The obtained absorption profile is in good agreement with theoretical predictions based on the HITRAN database.
We report on ground and airborne atmospheric methane measurements with a differential absorption lidar using an
optical parametric amplifier (OPA). Methane is a strong greenhouse gas on Earth and its accurate global mapping is
urgently needed to understand climate change. We are developing a nanosecond-pulsed OPA for remote measurements
of methane from an Earth-orbiting satellite. We have successfully demonstrated the detection of methane on the ground
and from an airplane at ~11-km altitude.
This past spring a new for-credit course on illumination engineering was offered at the College of Optical Sciences at
The University of Arizona. This course was project based such that the students could take a concept to conclusion. The
main goal of the course was to learn how to use optical design and analysis software while applying principles of optics
to the design of their optical systems. Projects included source modeling, displays, daylighting, light pollution, faceted
reflectors, and stray light analysis. In conjunction with the course was a weekly lecture that provided information about
various aspects of the field of illumination, including units, étendue, optimization, solid-state lighting, tolerancing, litappearance
modeling, and fabrication of optics. These lectures harped on the important points of conservation of
étendue, source modeling and tolerancing, and that no optic can be made perfectly. Based on student reviews, future
versions of this course will include more hands-on demos of illumination components and assignments.
The Georgia Tech Research Institute and the University of New Mexico are developing a compact, rugged, eye safe lidar
(laser radar) to be used specifically for measuring atmospheric extinction in support of the second generation of the
CCD/Transit Instrument (CTI-II). The CTI-II is a 1.8 meter telescope that will be used to accomplish a precise timedomain
imaging photometric and astrometric survey at the McDonald Observatory in West Texas. The supporting lidar
will enable more precise photometry by providing real-time measurements of the amount of atmospheric extinction as
well as its cause, i.e. low-lying aerosols, dust or smoke in the free troposphere, or high cirrus. The goal of this project is
to develop reliable, cost-effective lidar technology for any observatory. The lidar data can be used to efficiently allocate
observatory time and to provide greater integrity for ground-based data. The design is described in this paper along with
estimates of the lidar's performance.
We are developing a new type of lidar for measuring range profiles of atmospheric optical turbulence. The lidar is based on a measurement concept that is immune to artifacts caused by effects such as vibration or defocus. Four different types of analysis and experiment have all shown that a turbulence lidar that can be built from commercially available components will attain a demanding set of performance goals. The lidar is currently being built, with testing scheduled for August 2006.
A new type of lidar is under development for measuring profiles of atmospheric optical turbulence. The principle of operation of the lidar is similar to the astronomical seeing instrument known as the Differential Image Motion Monitor, which views natural stars through two or more spatially separated apertures. A series of images is acquired, and the differential motion of the images (which is a measure of the difference in wavefront tilt between the two apertures) is analyzed statistically. The differential image motion variance is then used to find Fried's parameter r<sub>0</sub>. The lidar operates in a similar manner except that an artificial star is placed at a set of ranges, by focusing the laser beam and range-gating the imager. Sets of images are acquired at each range, and an inversion algorithm is then used to obtain the strength of optical turbulence as a function of range. In order to evaluate the technique in the field and to provide data for inversion algorithm development, a simplified version of the instrument was developed using a CW laser and a hard target carried to various altitudes by a tethered blimp. Truth data were simultaneously acquired with instruments suspended below the blimp. The tests were carried out on a test range at Eglin AFB in November 2004. Some of the resulting data have been analyzed to find the optimum frame rate for ground-based versions of the lidar instrument. Results are consistent with a theory that predicts a maximum rate for statistically independent samples of about 50 per second, for the instrument dimensions and winds speeds of the Eglin tests.