Job Shop Hand Crafts Components For Hi-Tech TV Weather Satellite

by Mark Drzewiecki

When you watch TV and see an actual picture of the clouds and weather patterns, figure 1, the first reaction you have is "isn't" hi-tech fantastic. I can sit in my living room and watch TV and see a picture taken from a satellite in space." Not only can the marvel of modern science take a picture and transmit it to earth, it also takes many other readings that allow scientists to predict future changes in the earth's weather. However, this hi-tech marvel cannot be built without the skill and craftsmanship utilized in the hand finishing of the components that make up the satellite, some of which have geometric tolerances that cannot be achieved by any other method.

The next generation of Geostationary Operational Environmental Satellite, or as it is more commonly called GOES, is a three axis, body stabilized spacecraft, figure 2. The two main instrument packages on the satellite are the "Imager Sensor Assembly" and the "Sounder Sensor Assembly."

The Sounder is a nineteen channel, discrete filter radiometer. It has four spectral bands- long wave, midwave, short wave and visible, figure 4. The scan system is totally digitally controlled and can start and stop at any desired time or location. Location is calibrated by its star sensor. The device uses a telescope system similar to the Imager. It senses surface temperature, ozone, water vapor, visible clouds, and 4th magnitude stars.

A typical photograph is shown in figure 1. It shows the earth and the constantly changing weather patterns. Subsequent photographs show how these patterns change and allow meteorologists to predict, with some degree of accuracy, the future weather in "your neighborhood." In this case, figure 5, the Eastern U.S.

It is obvious that these devices and the rocket required to launch the GOES require the most sophisticated electronics available to make them operate and transmit the data back to earth. What is not obvious is the fact that these components and the base they are mounted on need to be made with super critical accuracy for the electronic output to be meaningful.

Figure 6 show several of the "base plates" on which the optical and electronic components are mounted. They were finished at Surface Finishes Co. Inc. Each plate measures 20" x 40" and is made of honeycombed aluminum 2 inches thick, figure 7. Two of them were required for the GOES, one for the Sensor Assembly and one for the Sounder Assembly. The base plate had to be held to a flatness on the optical mounting surfaces of 0.000050" (fifty millionths of and inch) with a parallelism of 0.0001". This is equivalent to an area 75 miles by 150 miles being level within less that a foot. This was accomplished by starting with the proper material having the appropriate weight to stiffness ratio. In this case, 6061-T6 aluminum alloy was used. The blank was machined on an NC machining center to remove the material from the "pockets" of the honeycomb in two steps. This resulted in a very strong, lightweight structure.

Special heat treatments were utilized after each machining operation to reduce the residual stresses that can cause warping during the final lapping operation. Next, the machined honeycomb is thermally cycled 3 times from -120 F to 350 F to further reduce the "micro stresses."

After this preliminary "artificial aging" process, comes the rough lapping operation producing a flatness of 0.001" which is done on an 84" diameter lapping machine using a thermal cycling, a hand lapping process combined with repeated optical inspection procedures is required to obtain the ultimate accuracy.

This final process requiring the patient use of knowledge obtained through years of experience with materials, abrasives, temperature effects, and sophisticated optical measuring techniques is the final and perhaps most important link in the success of this hi-tech electronic marvel.