HARLIE Hemisphere Scanning Stage

Time period

August 2001 to November 2002

Project description

The Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE) is an existing Lidar instrument at the NASA Goddard Space Flight Center. It uses a Holographic Optical Element (HOE) as a scanning telescope. The HOE views a column of light, which is oriented 45 degrees from normal to the HOE's face. The column of light transmits through the HOE and focuses onto a fiber optic coupled to a detector. Rotating the HOE around its center lets HARLIE view a 90-degree wide cone.

We designed a scan stage that enables HARLIE to scan a hemisphere in addition to its original conical scan. HARLIE is tipped 45 degrees from zenith. Its existing HOE scanner now views a cone extending from the horizon to zenith. The stage rotates about the zenith axis while the HOE continues to scan. Now HARLIE can collect aerosol and wind speed measurements through a hemisphere.

The scan stage consists of a large custom-designed bearing, which is approximately 12 inches (30.5 centimeters) in diameter. All moving components—belts, gears, and ball bearings—are completely enclosed to keep them clean and prevent operator injuries.

The scan stage's maximum rotation rate is approximately 60 RPM; the pointing resolution is 100 microradians.

The final system will be deployed in a mobile tow-behind trailer. Once the system is at a site, a hatch is opened in the trailer's roof, then HARLIE, which is sitting on the scan stage, is lifted through the trailer roof and begins collecting data.

Challenges and lessons learned

Designing the custom bearing was a challenge. The design required very large bearings, which are expensive when bought off-the-shelf; therefore, a custom design was more effective. The custom bearing reduced the number of parts in the assembly, improved stability, and reduced costs.

 
  Much of the system and its performance requirements were defined through trade-off analyses with the customer. This required studying and understanding the interactions between the mechanical, electronic, software, and optical components of the design. The scan stage could have been designed as a stand-alone component; however, the final design is better when created as part of an integrated system.