ESTAR Calibration Scanner

Time period

Early 2002 to Summer 2002

Project description

The ESTAR instrument is a microwave radiometer that flies in the bomb bay of the P-3B. (More can be seen about the ESTAR instrument at http://neptune.gsfc.nasa.gov/microwave.) The instrument is calibrated periodically at the anechoic radar test chamber at the NASA Goddard Space Flight Center. The instrument is located at one end of the anechoic test chamber while a noise source is located at the other end.

During these calibrations the instrument is scanned about the vertical axis (azimuth). The response of the instrument to the emitter (noise source) at the other end of the chamber is recorded as a function of the angle. The instrument is then rotated about an axis normal to the face of the instrument (roll) and again scanned in azimuth. This provides a basic calibration map of the impulse response of the instrument in two planes.

In a previous version of this instrument the rotation about the axis normal to the instrument was only done at one angle (90 degrees). In the current version of the instrument, it was necessary to do this at more angles so the customer wanted to increase the number of roll adjustments. Instead of the two calibration scans, the new calibrations needed to occur at least every 2 degrees for a total of 46 scans. This was necessary to make images with from the measurements taken by the new instrument.

Welch Mechanical Designs (WMD) was asked to design a stand to hold the instrument and provide the additional roll axis. This stand sits on top of the existing azimuth scanner in the antenna chamber. The roll angle is adjusted and locked before each scan about the azimuth axis. A 7.5-foot (2.3-meter) diameter ring is built around the 4-foot (1.2-meter) square instrument. The round outer edge of this ring rides on ball bearings in the cradle. The design allows any angle to be chosen. Currently the ring position is locked to the cradle with a clamp. A design has been made that will allow the system to be motorized. This will greatly speed the calibration times once the motorized parts are implemented.

Challenges and lessons learned

To stay within the limited budget for this project, WMD implemented certain design features. Instead of making the large ring from one solid piece, it was divided into four pieces that were bolted together rather than welded. The smaller pieces were significantly cheaper to manufacture and took much less space to store. A similar design was created by a different group for another instrument shortly after this calibration stand was made. That design used monolithic pieces, cost approximately three times more to manufacture, and weighed significantly more than our design. The techniques we learned here to minimize manufacturing costs can be applied to most future design projects.