Title: Development and Demonstration of an Optical Autocovariance Direct Detection Wind Lidar
Primary Author: Grund, Christian
Organization: Ball Aerospace & Technologies Corp.
Co-Author(s): Sara Tucker, Robert Pierce, Miroslaw Ostasziewski, Kelly Kanizay, Dina Demara, Jim Howell
Winds are key dynamic drivers of the atmospheric mass field. Insufficiencies and inaccuracies in current wind observations, particularly over the oceans, in the southern hemisphere, and in the tropics, lead to uncertainty in the modeling of global atmospheric circulations, limiting weather forecasting accuracy and diminishing our understanding of water, chemical species, and energy transport. The Decadal Survey specifically states that: Tropospheric winds are the number one unmet measurement objective for improving weather forecasts, and defines a 3-D Winds mission that identifies Doppler Wind Lidar (DWL) in low earth orbit (LEO) as the key technology needed to meet the global wind profiling objectives. To profile winds, DWLís measure the line of sight optical Doppler frequency shift from aerosols and molecules as a function of range. Backscatter from molecules is ever-present but spectrally broadened by thermal motions diminishing the achievable wind measurement precision. Aerosols produces insignificant backscatter spectral broadening providing the best wind information, but are not always present, A hybrid lidar measuring winds from both aerosol and molecular backscatter therefore maximizes the availability of wind profiles. The Optical Autocovariance Wind Lidar (OAWL) is a direct detection DWL approach that uses a unique high resolution (10^9) interferometer to measure aerosol backscatter winds to <0.5 m/s precision. When operated at near UV wavelengths (~355nm), OAWL can simultaneously measure molecular and aerosol backscatter winds; however, a more photon-efficient Integrated Direct Detection (IDD) architecture employs a dual-edge etalon pre-filter to separately measure the Doppler offset of the molecular fraction of the backscatter return, while leaving the aerosol-backscatter-dominated center of the Doppler shifted spectrum for analysis by the high resolution OAWL. The IDD hybrid approach uses a single transmit laser and receiving system with relaxed 1-wave telescope optical requirements, making it much simpler, lower mass, lower cost, and more power efficient than the alternative hybrid approach that uses a combination of a 2 m Coherent Detection DWL for aerosol wind profiling combined with a separate 355nm dual-edge etalon direct detection DWL for measuring the molecular component. This paper discusses the development and demonstration of the OAWL DWL system under NASA ESTO IIP funding. The primary objective of the IIP are to raise the OAWL DWL technology from TRL3 to TRL5 by taking the multi-wavelength field-widened OAWL receiver built under Ball internal funds, integrating it into a complete lidar system (telescope, laser, data system, framework) configured to fit in a WB-57 aircraft palate, ground validate the OAWL system against a coherent Doppler lidar, and then validating the system performance from the WB-57 against wind profilers and a ground-based coherent DWL system. To date, the integrated wind lidar system has been built and aligned. We are awaiting delivery of the final laser, but have temporarily substituted an available but much smaller laser so system shake out can proceed. First light was just achieved, and we expect first wind measurements within days. We will report on the IIP project objectives, performance expectations, the design, fabrication, and vibration testing of the OAWL system, and on ground measurement results.