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Scientists of the National Bureau of Standards' Boulder Laboratories are analyzing errors in radio position measurements and fluctuations in the radio refractive index of the troposphere to determine their correlation. Long-term fluctuations were studied by analysis of punched-card data available in the Radio Refractive Index Data Center 1 of the NBS Central Radio Propagation Laboratory. Shortterm fluctuations are being studied by correlating variations in radio transmission time with variations in radio refractive index, using measurements made with an operating model baseline missile tracking system. The correction factors obtained in these studies will be programmed into the computer of the M ISTRAM baseline missile tracking system, now being built by the General Electric Company for operation near Patrick Air Force Base, in the Cape Canaveral, Fla., area.
MISTRAM determines the missile's position by measuring the times required for radio signals to travel from each of several antennas to the missile and back. These antennas are arranged on an orthogonal set of baselines.
Translating these transit times to distances (and, hence, to position) requires a knowledge of the speed with which the radio signals travel through the atmosphere. This speed, usually expressed in terms of the refractive index, is a function of the composition of the atmosphere along the signal paths. Tracking inaccuracies result from variations in the earth's atmosphere along these paths. The variations consist of both large-scale changes, caused by air mass movements, and short-term changes, resulting from turbulence.
Such tracking errors, introduced by variations in atmospheric refractive index, may become the limiting factor in obtaining the accuracy demanded of new tracking systems by advancing space technology. The development of "second generation" missile tracking systems, such as M ISTRAM, near Patrick Air Force Base, Fla., has necessitated a study of the problem of obtaining correction factors to be programmed into the system's computer.
The radio propagation engineering staff of the NBS Central Radio Propagation Laboratory (CRPL) was requested to provide estimates of the nature and extent of the atmospheric effects. The problem was attacked by means of two separate studies.
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