NASA’s Langley Research Center has developed a waveform processing technique to eliminate signal noise resulting from sources of interference (scatterers) that can degrade continuous wave (CW) lidar return data. The algorithm was developed to enable CW lidar measurement of atmospheric gas concentrations as part of NASA’s Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) program, but can be used to test any chemical species, such as poison gas or other trace elements in the atmosphere. The algorithm demonstrated reduction in interference resulting from thin cloud layers and other scatterers. The improvement holds the potential for significant advancement of CW lidar systems that are less expensive, of simpler design, and can be operated at higher average power than pulsed lidar systems.
The NASA algorithm was developed to support the ASCENDS mission Laser Absorption Spectrometer (LAS) for carbon dioxide measurements in the mid-to-lower troposphere. The LAS is a satellite-based CW lidar system capable of monitoring global variability of carbon dioxide concentration in the troposphere from space, with a measurement range of up to ~500 km (low earth orbit). The modulation algorithm (a filtered pseudo-noise code algorithm) is capable of eliminating cross-channel noise and interference by modulating the lidar return signal using a time shifting approach (see the figure).
The technology builds on a strong remote sensing and lidar technology heritage at Langley Research Center. The algorithms are complete and have been verified as error-free by independent third parties. Lidar system specifications for the test bed include three 17.7-cm telescopes, a CW lidar system powered by three 10-W erbium-doped fiber amplifiers to provide 30-W average laser power, and a low-noise, high-gain HgCdTe detector and cryocooler.
The algorithms are mission ready and are available for licensure and implementation in a wide range of continuous wave lidar applications. These applications include gas sensing and detection, meteorology and atmospheric studies, and mapping and range finding.