The U.S. National Research Council recently identified the need for a near-term space mission of Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS). The primary objective of the ASCENDS mission is to make CO2 column measurements across the troposphere during the day and night over all latitudes and all seasons, and in the presence of scattered clouds. These measurements would be used to significantly reduce the uncertainties in global estimates of CO2 sources and sinks, provide an increased understanding of the connection between climate and CO2 exchange, improve climate models, and close the carbon budget for improved forecasting and policy decisions.
A system using a nonlinear multi-swept sine-wave system employs multi-channel, multi-swept orthogonal waves to separate channels and make multiple, simultaneous, online/offline CO2 measurements. An analytic expression and systematic method for determining the orthogonal frequencies for the unswept, linear swept, and nonlinear swept cases is presented. It is shown that one may reduce sidelobes of the autocorrelation function while preserving cross-channel orthogonality.
This is an improvement to a system currently used to measure CO2 using a differential measurement technique where online and offline laser wavelength are modulated by orthogonal linear frequency sweeps. The main problem with that technique was that as implemented, it required a 1,000-sweep frame length with 200,000 point frames in order to achieve orthogonality between channels. In addition, the autocorrelation function had –13 dB sidelobes that makes it difficult to achieve an accurate measurement in the presence of thin clouds due to sidelobe interference.
Modulated online and offline lasers are combined, then amplified and transmitted through the beam expander. A portion of this signal is routed to a reference detector and digitized simultaneously with the data received through the science detector. The science detector data comes from ground, and other reflections are further processed to discriminate returns (see figure).
A simple technique was used that allows one to use a very small frame size with as little as two sweeps, while preserving orthogonality using a very simple algorithm. Sidelobes were reduced below –80 dB in a lossless fashion with a nonlinear sweep in such a way that the channel orthogonality is preserved. Orthogonality between channels is required for the differential absorption measurement. Reducing the number of sweeps makes this technique more viable in a space application, and sidelobe reduction is necessary to do thin cloud rejection. These results were published in Applied Optics (Vol. 52 Issue 13, pp. 3100-3107 (2013).
This work was done by Joel F. Campbell of Langley Research Center. LAR-18326-1