Coherent Doppler Wind Lidar Development for NASA Space-Based 3-D Winds Mission

The measurement of global wind velocities from earth orbit is both much desired and very difficult. The measurements would be highly useful to several fields such as atmospheric physics (weather prediction, severe weather, climate research, air quality, and trace gases), aviation, and wind power. The various scientific fields use different names to refer to wind such as advection, circulation, currents, dynamics, flows, fluxes, streams, transport, and waves. For active remote sensing instruments such as lidar, the mission is difficult for three primary reasons. First, the range to target is always large, from 400 km for the lowest reasonable orbit heights and nadir angles, to over 1,000 km for the highest reasonable LEO orbit heights and nadir angles. Second, the speed of the orbiting lidar will be about 7,000 m/s while the desired overall wind velocity accuracy is about 1 m/s (0.014%). Third, the pulsed laser must pulse continuously over the life of the mission. For example, a 10 Hz laser requires 315.4 million shots per mission year in addition to pre-launch shots. In the US, NASA and NOAA and DOD researchers have been working to enable this space mission since the 1970s.

The over 40 years of work towards the global winds mission in the US has included over 50 studies, theoretical development, computer simulation of the wind measurement technique and of the utility of the wind measurements, measurement requirements development, space mission design, lidar technology development, and ground and airborne validation.

NASA and NOAA scientists have worked with lidar scientists to formulate the wind measurement requirements appropriately stated for a lidar solution. These requirements are occasionally updated. There is a consensus among researchers that the final operational wind sensor should be a hybrid pulsed Doppler wind profiling lidar with scanning. The term hybrid refers to the complementary, simultaneous wind measurement by both a coherent-detection and direct-detection lidar. Conceptually, the coherent lidar uses aerosol particles for its signal and favors the lower altitudes, while the direct lidar uses molecules for its signal and favors higher altitudes. The US National Research Council's advice to NASA recently endorsed both the global winds mission and the hybrid lidar concept.

In this presentation, we will review the 20-plus years of pulsed transmit laser development at NASA Langley Research Center (LaRC) to enable a coherent Doppler wind lidar, as part of the hybrid wind lidar system, to measure global winds from earth orbit. We briefly also discuss the many other ingredients needed to prepare for this space mission.