A view from a Navy measurement station, and a typical spectrometer read out.

A view from a Navy measurement station, and a typical spectrometer read out.


The Navy is running what it calls the Water Vapor Millimeter-Wave Spectrometer Project, which monitors water vapor in the middle atmosphere between 30 and 80-km altitude. There is an interest in water vapor at these altitudes because it is the primary source of the OH radical and other hydrogen compounds that help form the earth's protective ozone layer. Tracking the water vapor also gives an excellent indication of how the atmosphere moves.

The apparatus for measuring water vapor uses a motor supplied by Empire Magnetics Inc., Rohnert Park, Calif., (empiremagnetics.com) to spin a 14-in. aluminum mirror mounted on a 22-GHz microwave receiver. This instrumentation is able to measure the vibration of molecules 80 km away.

The Navy uses three measurement stations located in geographically diverse locations, each presenting its own set of harsh environments. One of these is 11,800 ft high on Mauna Loa, Hawaii. The weather there can get rather nasty, ranging from sideways-blowing hail to continuous condensation and solid icing.

Another is at 3,500 ft in a pasture for sheep, cows, and hogs in New Zealand. The winds there can also be quite severe and, in addition, there is a weather phenomenon called hoar frost. The third is on top of Table Mountain at 8,500 ft in the San Gabriel Mountains just north of Los Angeles where 10 ft of snow is typical in the winter. In each case, the motors must point the mirror at a variety of angles, quickly, and then prevent the mirrors from moving during measurement, even in high winds.

The Empire Magnetics motors replaced a previous type that was not waterproof and required additional protection against the environment. As an example of the maintenance headaches encountered, the project manager frequently had to fly from Washington, D.C., to Hawaii just to repair the motor encoder that had frozen or otherwise become clogged. The three Empire Magnetics motors were deployed between March and December 2002 and have been running continuously since.