Both the U.S. and the Soviet Union were expected to launch satellites during the IGY, so it was necessary to develop methods to track them. This article from the IGY Bulletin describes the Minitrack system developed by the Naval Research Laboratory to do that. I'll also use some supplementary material from a very useful Wikipedia article. (Much as I try to use alternate sources to sample other interesting and authoritative websites, Wikipedia truly is an amazing resource.)
The U.S. Naval Research Laboratory began operations in 1923, seven years after Thomas Edison suggested that the Government establish “a great research laboratory.” NRL developed the first operational American radar, in time for use in World War II. It became a global leader in space science and development as a predecessor to the formation of NASA in 1958.
When plans for satellites emerged in the years preceding the IGY, the question naturally arose as to how to track them. Three approaches were considered: optical tracking, radar, and the NRL plan to measure angles using radio wave interferometry. The optical and radar approaches would work with a passive target, but had the major problem of finding the target in the first place, since they had very small fields of view. The NRL technique required a transmitter be placed on the target, but could easily measure a target anywhere in a wide field of view. The NRL proposal was accepted and turned into the basis of the Minitrack system.
The Minitrack transmitter that would send the signal from the satellite had a power level of about 0.05 watts. (A cell phone today emits on the order of one watt, 20 times greater.) The frequency of the signal was 108 cycles/second (hertz), or super/very low frequency.
The figure below from the Bulletin articles suggests how this system would work. Points A1 and A2 in figure 1(a) are two receiving antennas at the ground station. If the signals were arriving vertically, there would be no phase difference between the two sinusoidal waves; in other words, the distance P1-A2 would be zero. As the angle of the satellite tilts from the vertical (as shown in the figure), the length of this phase difference increases, and indicates the angle of the satellite from being straight overhead (the zenith). With two pairs of antennas perpendicular to each other, as shown in figure 1(b), two angles would determine completely the direction of the satellite from the antennas.
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| IGY Bulletin, #2, Fig. 1(a) and (b) -Phase measurement in the Minitrack system |
A so-called "picket fence" at longitude 75°W was to have eight stations spaced out in latitude so that the satellite would not be missed as it crossed that meridian. Two of the stations were in the U.S., two in the Caribbean, and four in South America. A photo of the station at Blossom Point, Maryland, is shown below.
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| Blossom Point Minitrack station (NRL) |
I mentioned in an earlier post that, during my first "real" job with Fairchild Space and Electronics Company in Germantown, MD, I was involved in a couple of subcontracted projects with the NRL analyzing satellite power requirements for their satellites. One of these was for TIMATION-3, a satellite with technology that was a stepping stone to the development of GPS.
I still have that report from 1973! It was the first really substantial professional thing I helped write. The cover page, slightly weathered, was signed by my excellent group lead Doug Rusta and by me. I generated hundreds of pages of tables and graphs for this report, many of which were hand written and plotted on graph paper, respectively.
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| Title page, report on the power subsystem for NRL's TIMATION III satellite |
Hope you enjoyed today's physics lesson!
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