Wednesday, 17 January 2007
Development and Demonstration of a Fiber Laser Sounder for
Exhibit Hall C (Henry B. Gonzalez Convention Center)
Understanding today's regional CO2 sources and sinks is a central theme in carbon cycle science and necessary for reliable projections of future atmospheric CO2 concentrations. As nations seek to develop strategies to manage their carbon emissions, the capabilities of quantifying regional carbon sources and sinks and understanding the mechanisms for them are necessary for informed policy decisions. It is an operational necessity; however, currently scientists cannot well account for the growth rate and inter-annual variations of atmospheric CO2 concentrations, let alone confidently localize in space and time important carbon sources and sinks. The apparent variability in carbon sources and sinks has an important climate connection, and this issue is directly tied to estimating future atmospheric CO2 concentrations. To improve the understanding of carbon sources and sinks, it is urgent to expand significantly the set of global atmospheric CO2 observations. A space-based mission would provide measurements of the spatial gradient and temporal evolution of CO2 mixing ratio, which can be used to determine surface fluxes. While the NASA Orbiting Carbon Observatory (OCO) will make a major step forward in our understanding of CO2 distributions, the reliance on a passive technique is a significant limitation. In contrast, an active mission, specifically a laser sounder, can produce global atmospheric CO2 observations at all seasons and all latitudes, with day/night coverage, and under both clear and broken cloud conditions. As a result, the international science community has recommended the development of an active CO2 mission for the Integrated Global Observing Strategy (IGOS) as an important next step after OCO. In addition, this mission will provide high-precision CO2 measurements that can also be used by the meteorological community to further improve passive temperature measurements that can lead to improved weather forecasts. The ideal laser sounder would have the following characteristics: a) make simultaneous laser remote sensing measurements of CO2 and O2, which is required to correct for atmosphere pressure to surface and cloud tops, b) make measurements through all seasons and latitudes, which is required to remove seasonal and latitudinal bias, c) operate equally well in day/night, which is required to remove diurnal bias, and d) have the ability to resolve/weight altitude distribution of the CO2 column. Such a system, deployed in low earth orbit, would provide precision, global, measurements of the spatial and temporal gradients of CO2 in the atmosphere without the coverage and bias limitations imposed by passive methods. ITT has developed, patented and is operating a prototype of such a laser sounder. The measurement method is Laser Absorption Spectroscopy (similar to DIAL) which is enhanced by an ITT patented method/apparatus which achieves true simultaneous illumination and measurement of the on, off, sideline wavelengths. This method of true simultaneous measurement has multiple advantages over previous lidar systems; 1) the system achieves significant improvement in the precision of the measurement by converting changes in reflectivity, transmission to common mode noise, c) the system has significantly improved long term stability because the on, off, sideline returns travel the same optics, detector and signal processing, 3) the method used to achieve true simultaneity significantly reduces the effects of speckle. The prototype is based on robust, mass produced, telecom components, including fiber amplifiers/lasers operate continuously for many 100,000's of hours under environmental conditions which are harsher than those encountered on a satellite. A Technology Readiness Assessment has been lead by the Photonics section of the NASA Electronics Parts and Packaging (NEEP) Program Office. The assessment confirms that the fiber laser transmitter subsystem has achieved TRL 5 and will achieve TRL 6 without delay. In addition to their inherent robustness, fiber amplifiers/lasers offer critical advantages to a space-based mission; a) they are mass produced with stable, proven manufacturing processes, which provides statistically accurate estimates of reliability, shorter lead times, which lowers program cost, and lower unit cost, b) at least 2X higher wall-plug efficiency, and lower mass, which reduces payload power, thermal, and mass by more than 4X. In short, a fiber laser sounder provides unique capabilities at considerable and proven lower risk and cost than conventional lidar systems. The prototype has been operated under a wide range of atmospheric and meteorological conditions, on ground at significant ranges, and from an aircraft. The test program, which continues today, has confirmed that the instrument has the precision, accuracy and stability required to resolve surface flux levels of CO2. The ITT prototype has demonstrated; a) precision/accuracy of 0.1% ppm , b) stability measured in hours, c) repeatability measured in days/weeks and d) excellent agreement (within 2%) with in-situ sensors on the aircraft. A high-fidelity, physics-based end-end instrument performance model has been developed, and validated. This model allows ITT to reliably predict the performance of the prototype under different scenarios, and to specify the power-aperture-integration time required for a space-based mission. The model is comprehensive, and based measured data for all components. The radiometry, signal/noise, and retrieval algorithms have been validated. Complimenting the instrument model, NASA LaRC, and LSCE - CEA de Saclay have lead the development of a Level One Mission Error Budget. Together these models, along with detailed cost models support the Missions & Systems Engineering Trades and Cost-As-Independent-Variable (CAIV) studies needed to support a space-based implementation. Mission implementation studies conducted by NASA LaRC, ITT, UNH and AER have confirmed that the proposed laser sounder mission has a (uniquely) high technical readiness level (system and components), is compatible with readily available bus/launch vehicles, and can be achieved within the schedule and cost constraints of an ESSP class mission.