MISTRAM
MISTRAM was a high-resolution tracking system used by the United States Air Force 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 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 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 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.
Image:AFG-060829-002.jpg|thumb|right|US Air Force Eastern Test Range.
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 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 while it is being swept. The round trip delay time can be shown to be T=/; where delta-f is the frequency shift and delta-phi the measured phase shift in radians. Suppose T=2 sec then delta-phi=8000 radians, i.e. /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 /=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 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.
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.
Image:MISTRAM-diagram.jpg|thumb|right|MISTRAM block diagram shows ground-based components and airborne transponder.
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 is very stable, whereas the lower frequency 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 with design performance as follows:
| 360 deg |
| 5 to 85 deg |
| 20 to |
MISTRAM transponder
The Transponder receives the two phase-coherent X-band cw signals transmitted from the ground equipment. A klystron with a 68 MHz coherent frequency offset is phase locked to each of the received signals. These klystrons provide the phase coherent return transmission. There are two separate phase locked loops, continuous and calibrate.;MISTRAM "A" Model Transponder Specifications
M-236 computer
MISTRAM was used on a number of projects, including the ATLAS missile system, other large military radar projects in the 1960s, and Project Apollo, and the Air Force required a data-collection computer to be installed in a tracking station downrange from Cape Canaveral. The data would eventually be shared with the 36-bit IBM 7094 machine at the Cape, so the computer would likely have to be 36-bits as well. General Electric built a machine called the M236 for the task.According to Dr. Neelands, certain military people involved in the project were adamant about not relying on "computers", therefore this "information processor" was developed. This high speed 36-bit minicomputer was developed by the GE Heavy Military Electronics Department in Syracuse, New York. The M236 was designed for real-time processing in a radar-based missile flight measurement system and lacked some general purpose features, such as overlapped instruction processing, the floating point operations needed for Fortran, and operating system support features, such as base and bounds registers. The M-236 computer was developed for the US Air Force Cape Canaveral Missile Range, and installed at Eleuthera. The 36-bit computer word length was needed for radar tracking computations and for the required exchange of data with an IBM 7094 located at the Cape. The chief architect of the M236 was John Couleur.
The M236 being a component of a military project, rather than a general-purpose computer, conformed to the desires of Ralph J. Cordiner, Chairman & CEO of General Electric from 1958 to 1963, not to go into competition with IBM by selling general-purpose computers. In addition, development expenditures would be paid for by the U.S. government. However, the GE-225, developed as a process control computer, had become a profitable general-purpose computer; GE's 1950 business plan included development of more computer systems.
The debate in favor or against subsequent development of an M236-derived general purpose computer took more than one year and concluded finally with the victory of the M2360 project proponents in February 1963. The GE upper management was impressed by the opportunity to save the rental fees from IBM leased equipment used internally by GE. The other GE departments were not very impressed and were reluctant to jettison their IBM machines. The M2360 project became the GE-600 series, developed by a team led by John Couleur.
Applications
MISTRAM was used in the development and testing of inertial guidance system for the Minuteman ballistic missile, and subsequently was used for testing the Gemini spacecraft and the Saturn V launch system. With the decommissioning of the MISTRAM X-band interferometer at the Air Force Eastern Test Range in 1971, the flight-test community did not have a conventional ground-based range-instrumentation system better than, or comparable to, the inertial guidance systems whose performance was being assessed. This was true in the intervening years preceding GPS development and deployment.Minuteman Inertial Guidance System testing
The first Minuteman missiles were launched in the early 1960s from the Air Force Eastern Test Range and were tracked with the AZUSA CW tracking system. The comparatively low quality of the AZUSA tracking data, combined with the rudimentary stage of evaluation techniques, allowed only estimation of the total error; no isolation of individual inertial measurement unit error sources was possible.Subsequent development of improved tracking systems, UDOP and MISTRAM, at AFETR yielded much higher quality velocity tracking profiles. During the Minuteman II flight test program, significant improvements were made in the post-flight evaluation of the IMU accuracy. The most important of these improvement was the introduction of maximum likelihood error estimation using the Kalman algorithm to filter the velocity error profile. Continued improvement of the UDOP and MISTRAM tracking systems and refinement of the evaluation techniques during the Minuteman III flight test program made it possible to gain considerable insight into NS-20A1 IMU error sources.