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MISTRAM (''MISsile TRAjectory Measurement'') was a high-resolution tracking system used by the United States Air Force (and later NASA) to provide highly detailed trajectory analysis of rocket launches. A "classic" ranging system used since the 1960s uses radar to time a radio signal's travel to a target (in this case, the rocket) and back. This technique is accurate to approximately 1%. The accuracy of this technique is limited by the need to create a sharp "pulse" of radio so that the start of the signal can be accurately defined. There are both practical and theoretical limits to the sharpness of the pulse. In addition, the timing of the signals often introduced inaccuracies of its own until the introduction of high precision clocks. In MISTRAM, this was avoided by broadcasting a continuous signal. The basic system used a ground station located down range from the launch site (at Valkaria, Florida and Eleuthera Island, Bahamas) and a transponder on the vehicle. The tracking station transmitted an X-band carrier signal which the transponder responded to by re-broadcasting it on another (shifted) frequency. By slowly changing the frequency of the carrier broadcast from the station and comparing this with the phase of the signal being returned, ground control could measure the distance to the vehicle very accurately. Even with the analog circuitry used, MISTRAM was accurate to less than 1 km at the distance of the moon. To meet more stringent ballistic missile test requirements, several systems were designed, procured and added to the US Air Force Eastern Range's instrumentation in the 1950s and 1960s. The AZUSA continuous wave tracking system was added to the Cape in the mid-1950s and Grand Bahama in the early 1960s. The AN/FPS-16 radar system was introduced at the Cape, Grand Bahama, San Salvador, Ascension and East Grand Bahama Island between 1958 and 1961. In the early 1960s, the MISTRAM (Missile Trajectory Measurement) system was installed at Valkaria, Florida and Eleuthera island in the Bahamas to support Minuteman missile flights. ==Principles of operation== MISTRAM is a sophisticated interferometer system consisting of a group of five receiving stations arranged in an L shape. Baselines are . and . The central stations contains a simple tracking antenna. The distance from the central station to the furthest remote station is approximately . Antennas at the central station and the four remote stations follow the flight of a missile and receive signals from its radio beacon. In the MISTRAM system, the ground station transmits a carrier to the spacecraft and the spacecraft returns this carrier on another frequency. The ground station sweeps the uplink carrier and the phase shift of the downlink carrier is measured (counted) while it is being swept. The round trip delay time can be shown to be T=(delta-phi)/(delta-f) ; where delta-f is the frequency shift (~4000 Hz for example) and delta-phi the measured phase shift in radians. Suppose T=2 sec (~lunar distance) then delta-phi=8000 radians, i.e. (8000 *180)/Pi. Assume also that the phase can be measured with an accuracy of 1 deg, i.e. means that the range can be determined with a precision of (600000 *1 *Pi)/(2 *8000 *180)=0.33 km. An additional carrier quite near the one described above that remained fixed in frequency and used as a phase reference. That carrier and the two frequencies (that the sweep changed between) were generated as multiples of the same basic oscillator frequency. In this way, all signals would have a fixed phase relationship, as was done in MISTRAM. A similar technique was used in the Soviet Luna 20 spacecraft at 183.54 MHz to survey the moon's surface.〔(【引用サイトリンク】url=http://www.svengrahn.pp.se/trackind/luna20/LUNA20.htm )〕 MISTRAM was a multistatic long baseline radar interferometer developed for precision measurements of missile trajectories at the US Air Force Eastern Test Range. Multistatic radar systems have a higher complexity with multiple transmitter and receiver subsystems employed in a coordinated manner at more than two sites. All of the geographically dispersed units contribute to the collective target acquisition, detection, position finding and resolution, with simultaneous reception at the receiver sites. In a simpler sense, multistatic radars are systems which have two or more receiving sites with a common spatial coverage area, and data from these coverage areas are combined and processed at a central location. These systems are considered to be multiple bistatic pairs. Multistatic radar systems have various uses, including prevention of jamming and anti-radar munitions. Although this method of measurement is not new, either in theory or in practice, the unique manner in which the techniques were implemented in the MISTRAM system permit measurement of vehicle flight parameters with a degree of precision and accuracy not previously obtainable in other long baseline trajectory measurement systems. To a large extent, this was accomplished by a unique method of transferring intact the phase information in the signals from outlying stations to the central station. A two-way transmission path on each baseline was used to cancel out uncertainties due to variance in ground geometry and temperature. The transmitter at the master or central station generates two CW X-band frequencies, nominally 8148 MHz and 7884 to 7892 MHz. The higher frequency (the range signal) is very stable, whereas the lower frequency (the calibrated signal) is swept periodically over the indicated range. The airborne transponder receives the signals, amplifies & frequency shifts them by 68 MHz, and retransmits back to earth. The Doppler shift is used to determine velocity. The Florida MISTRAM system had baselines (~18.9 mi.) with design performance as follows: 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「MISTRAM」の詳細全文を読む スポンサード リンク
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