A continuous-wave (CW) NIR carbon-dioxide monitoring system, incorporating Wavelength Modulation Spectroscopy (WMS), has been developed and was tested aboard the Spirit of Goodyear airship platform. The data shows sensitivities nearly identical to previous ground-based tests but with much higher information rates (100Hz). These tests were conducted over regions with varying ground albedo and included path lengths up to 1.5 km. The system utilized commercial-off-the-shelf (COTS) components including telecom laser diodes and amplifiers. Currently, the system is limited by Erbium Doped Fiber Amplifier (EDFA) spectral bandwdith, but the ever-increasing average power of quantum cascade lasers coupled with the development of midwave fiber technology could make this CW-based architecture a viable solution for future airborne sensors in the MWIR region.
We report standoff open path atmospheric CO2 monitoring with a field deployable, turn key system including a continuous wave (CW)distributed feedback (DFB) laser and an erbium doped fiber amplifier (EDFA) at 1.5-μm. A sensitivity of 28-ppm was achieved over 1.5-km of open air with 200-pW of received power, a 10s acquisition time, and a peak absorption cross section of 8x10-23. This sensitivity corresponds to an error in fractional absorbance of 8x10-3. Closed cell lab sensitivities are better than 3000ppm*m, an error in fractional absorbance of 5x10-4. These results have been achieved using space qualified laser components, un-cooled InGaAs detectors, off the shelf electronics in a rugged all fiber architecture.
A number of gases present in the atmosphere play roles of interest to various parties. These are CO2 for its impact on understanding of global sources and sinks of Carbon, CH4 and H2O and their importance for global climate change, HCl and its importance in chemical processes. A space-borne sensor using multiple-wavelength Laser Absorption Spectroscopy (LAS) and mature CW fiber telecom lasers can address the critical questions concerning present and future patterns in these gases. The sensor identified above was designed from the outset using Taguchi Robust design techniques because of the need to adjust to varying science measurement requirements and technology capability as well as achieving optimum performance for optimum cost. The results describe a sensor with a SNR of 150 with a power aperture product of 3.92 watts-m2 on the absorption line is sufficient to meet the science requirements of 0.5% accuracy for determining the column density of CO2.
Imaging LADAR is a hybrid technology that offers the ability to measure basic physical and morphological characteristics (topography, rotational state, and density) of a small body from a single fast flyby, without requiring months in orbit. In addition, the imaging LADAR provides key flight navigation information including range, altitude, hazard/target avoidance, and closed-loop landing/fly-by navigation information. The Near Laser Ranger demonstrated many of these capabilities as part of the NEAR mission. The imaging LADAR scales the concept of a laser ranger into a full 3D imager. Imaging LADAR systems combine laser illumination of the target (which means that imaging is independent of solar illumination and the image SNR is controlled by the observer), with laser ranging and imaging (producing high resolution 3D images in a fraction of the time necessary for a passive imager). The technical concept described below alters the traditional design space (dominated by pulsed LADAR systems) with the introduction of a pseudo-noise (PN) coded continuous wave (CW) laser system which allows for variable range resolution mapping and leverages enormous commercial investments in high power, long-life lasers for telecommunications.
Significant progress has been made in the performance, qualification and validation of Active Remote Sensing systems to address complex questions in climate science from satellites in low earth orbit. During the past year, ITT has completed the design, qualified the components, and validated the performance of sophisticated Tunable Diode Laser Absorption Spectroscopy systems for airborne and space missions. ITT has shown that measurement of total column CO2 to an accuracy of 0.5% can be readily achieved using a 5 watt laser, 1 meter telescope and digital signal processing techniques to reject sunlight and noise. Furthermore, the design exploits the proven high reliability of photonic components developed by the telecom industry. ITT testing validated that these components survive launch and multi-year operation in space without significant degradation. Using a scaled sensor, the ground based validation campaign demonstrated the ability to accurately retrieve the CO2 diurnal cycle as well the automotive induced variations in CO2 observed in urban settings. These data validate the end-end sensor performance model and retrieval algorithms, which have previously been used to design a space based CO2 sensor proposed to NASA. ITT will discuss the application of these technologies to other atmospheric constituents. Combined, these results serve to demonstrate that laser based remote sensing of key components of the atmosphere which address global climate change can be achieved from low earth orbit without further development.
Satellite observations of atmospheric CO2 are the key to answering important questions regarding spatial and temporal variabilities of carbon sources and sinks. Global measurements sampling the air above land and oceans allow oceanic flux to be distinguished from terrestrial flux. Continuous sampling on frequent basis allows seasonal variations to be distinguished. This study quantifies the potential value of satellite-based measurements of column- integrated CO2 concentrations in terms of the carbon source/sink information that can be derived from these concentrations via inverse modeling. We discuss the utility of the carbon flux inversions in terms of both spatial and temporal resolution, compare capabilities of active and passive approaches to the measurements, and demonstrate the feasibility of high precision CO2 column concentration retrievals.
A unifying theme throughout the ESE science objectives is the identification of regions with large temporal and spatial gradients. Severe storm formation occurs in the boundary regions between airmasses with very different temperatures, pressures, water content, aerosol loading. Severe storm tracking and forecasting utilizes the discontinuities in observed fields and gradient fields to diagnose and forecast the formation, evolution, and motion of severe storms. In a similar fashion, heat islands, super-regional pollution, and rain shower formation are each the result of temporal and spatial gradients present in the atmosphere. Diagnosing and forecasting these events requires an ability to map atmospheric gradients and discontinuities in real-time on micro to meso-scales in the atmosphere (0.5 - 500 km). A new measurement concept, the Imaging Fourier Transform Spectrometer (IFTS) is capable of demonstrating a class of autonomous event identification, monitoring and tracking sensors. In order to provide this capability a sensor with the ability to combine high spatial resolution (0.5 - 1 km) imaging with high spectral resolution (0.25 cm - 1 across the mid infrared 3 -10 microns) in time intervals of a few seconds is required. An electronically programmable infrared camera that combines a large-format focal plane array with a Fourier transform spectrometer can deliver this capability. It also builds on currently fielded airborne demonstration systems and an instrument concept in development for the Next Generation Space Telescope (NGST). The IFTS concept is revolutionary in several aspects. It can produce 2 - 10 fold increase in spatial resolution, 2 fold increases in spectral resolution, and 30 fold increases in temporal resolution. In combination the measurement concept would require a 100 - 600 fold increase in telemetry bandwidth without a new approach to imaging. IFTS breaks this paradigm with a new approach to hyperspectral imaging. Severe storm forecasting requires gradient fields (i.e., first and second derivatives of atmospheric observations). Hence, this measurement concept for IFTS is enabled by four innovations: (1) directly observe the derivative fields, (2) Nyquist sample the image plane to enable full utilization of the telescope performance, (3) have multi-channel detection of gradient regions, and (4) provide an autonomous targeting and tracking system that identifies, subsets, and follows regions with significant discontinuities (i.e., regions where severe storms, toxic pollution, heat islands, or rain/thunderstorms will form).
The technique of integral projections is used to perform co- registration of data from a wedge spectrometer instrument that has been developed by NASA Goddard Space Flight Center. The spectrometer is currently being flown on a plane and operates in the 1 - 2.5 micron range. The technique involves a number of steps. First, an algorithm was developed to calculate the absorption bands that occur within the spectral region. At this point the method of Integral Projections is used to vectorize the image. The Integral Projections method performs a number of key functions in the registration process: increases SNR, reduces affects of spatial non-uniformities within the data, and results in a much faster algorithm since the operation is on vectors. The final step is to register the zero crossing of the second derivative of the vectors. Two issues encountered with co-registration is dealing with the absorption bands that occur within the spectral region of interest and the multiple problem of recognizing features that are not only shifting in x and y but also appear different at different wavelengths. Results will be presented in which the application of our algorithm obtains the appropriate x,y shifts necessary to reconstruct a registered data cube.
As part of an ongoing investigation of airglow emissions of the upper atmosphere, an intensified CCD imaging spectrograph has been developed for a sounding rocket project called GEMINI (general excitation mechanisms in nightglow). The instrument, known as LISA (limb-imaging spectrograph for airglow), will be used to measure the limb profiles of some important nighttime airglow emission features. The observed limb profiles will be analyzed to provide atmospheric temperatures and density profiles of excited atomic and molecular species of interest to specific modelling problems in the mesopause and lower thermosphere. The GEMINI rocket is to be launched from White Sands Missile Range, New Mexico, in late 1993 or early 1994. The payload will be three-axis stabilized and absolute pointing will be derived from a star video camera. We describe the design capabilities of the LISA instrument, which include a spectral range of 310 to 390 nm, a wavelength resolution of ~0.3 nm, a height resolution of 1 km, and a theoretical count rate of 0.04 count R-1 s-1, where R represents rayleighs. The imager design is discussed and we present the results of some laboratory tests performed by means of an artificial source of the oxygen nightglow emission.