Thor is an airborne lidar instrument designed to measure the thickness and internal structure of highly scattering media such as thick clouds, snow, or sea ice. It is a multi-view lidar system: It emits short laser pulses and observes both the spatial and temporal characteristics of the pulses spreading inside the probed media.

The main components of the THOR system are:

• Data system: controls operations and archives the measured photon count values 
• Laser: emits 1000 short pulses of green light per second
• Beam expander: tightly focuses the laser beam
• Telescope: collects the photons reaching its ~20 cm diameter front lens 
• Optical fiber bundle: channels photons from each concentric circle of the image at the telescope focal plane to a separate detector
• Filters: eliminate most photons due to background illumination by allowing through only photons in a narrow wavelength interval
• Detectors: count incoming photons individually

THOR's most unique component is a custom-made optical fiber bundle that consists of approximately 250,000 fibers. Each fiber is 66 cm long and 50 µm thick, except for the 200 µm thick central fiber that captures photons from the central field-of-view. The remaining fibers are organized into seven concentric annular rings, each collecting photons from a corresponding ring in the focal plane image. In order to compensate for the outward weakening of the observed signal, each ring is twice as thick as its inner neighbor, and hence collects photons from a four times larger area. Because of this arrangement, the outermost ring contains so many fibers that they cannot all be connected to a single detector. Therefore fibers in this ring lead to three separate detectors, each covering a 120° azimuthal sector.

 

Setup of typical THOR cloud observations

THOR sends short green laser pulses into the cloud and counts the photons that return directly from the illuminated spot and from seven concentric circles surrounding it. 

Applications:

• Cloud thickness retrievals have been validated during flights over stratus clouds in Oklahoma.
• Snow and sea ice thickness retrievals are expected once planned upgrades will allow THOR to resolve the small bright halos that surround the illuminated spot in snow and sea ice.