multistatic radar and multiradar system (MSRS) utilizes Doppler radar
A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been...
for missile tracking and trajectory measurement. A target is illuminated at 450 MHz. Five receiving stations, located along the baselines with the lengths from 40 to 120 km (24.9 to 74.6 mi), receive signals from the target's transponder at 900 MHz. These five stations yield slant-range
In radio electronics especially radar terminology, slant range is the line-of-sight distance between two points which are not at the same level relative to a specific datum....
rate. To compute the range or position, an initial position is required from some other tracking system. The random error is 6 cm (2.4 in), but total error includes the systematic error of 2.7 m (8.9 ft) plus the initial error. UDOP had relatively low cost compared with other high-accuracy systems. In the US, MSRS has found important application in the precision measurement of missile trajectories at the Air Force Eastern Test Range
The Eastern Range is an American rocket range that supports missile and rocket launches from the two major launch heads located at Cape Canaveral Air Force Station and the Kennedy Space Center. The range has also supported Ariane launches from the Guiana Space Centre as well as providing support...
, which extends from the Florida mainland to the Indian Ocean. These MSRSs include the AZUSA
AZUSA refers to a ground-based radar tracking system installed at Cape Canaveral, Florida and the NASA Kennedy Space Center. AZUSA dated back to the early 1950s and was named after the southern California town where the system was devised....
, the MISTRAM
MISTRAM was a high-resolution tracking system used by the United States Air Force to provide highly detailed trajectory analysis of rocket launches....
, and the UDOP. All systems employ a cooperative beacon transponder on the observed target and a ground-based transmitting station with several receiving stations at separate, precisely located sites.
The UDOP used an AN/DRN-11 transponder installed in the Saturn (rocket family)
The Saturn family of American rocket boosters was developed by a team of mostly German rocket scientists led by Wernher von Braun to launch heavy payloads to Earth orbit and beyond. Originally proposed as a military satellite launcher, they were adopted as the launch vehicles for the Apollo moon...
launch vehicle for Project Gemini
Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966....
The C band is a name given to certain portions of the electromagnetic spectrum, including wavelengths of microwaves that are used for long-distance radio telecommunications. The IEEE C-band - and its slight variations - contains frequency ranges that are used for many satellite communications...
CW interferometric AZUSA, in operation from the 1950s, has one transmitter and nine receivers located along two crossed baselines with the total lengths of about 500 metres (1,640.4 ft). Intermediate receivers spaced at 5 to 50 m (16.4 to 164 ft) are used for phase ambiguity resolution. The AZUSA system measures range by phase measurement of sideband frequencies modulating the carrier, coherent range by Doppler count, two direction cosines, and two cosine rates. Errors of less than 3 m (9.8 ft) in range and 20 ppm in direction cosine are obtainable.
Missile Trajectory Measurement (MISTRAM) is a CW interferometric system with receiving stations situated along two mutually perpendicular baselines spaced at 3 to 30 km (1.9 to 18.6 mi). This MSRS can measure range, four range differences, range rate and four range difference rates of a target. The range error is less than 0.8 m (2.6 ft).
Principles of Operation
There is nothing new in using a CW tracking system to obtain metric data. The system was augmented in 1965 by short baselines of a few meters to a few hundred meters in contrast to the conventional UDOP system with baselines of several kilometers and longer. The UDOP system was used extensively for the Saturn program at the NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
John F. Kennedy Space Center
The John F. Kennedy Space Center is the NASA installation that has been the launch site for every United States human space flight since 1968. Although such flights are currently on hiatus, KSC continues to manage and operate unmanned rocket launch facilities for America's civilian space program...
UDOP is a 2-way, coherent, continuous-wave, tracking system. It is a highly reliable data source providing very accurate velocity measurements. The UDOP system, a descendant of Doppler Velocity and Position (DOVAP) was developed by NASA-KSC.
UDOP consists of three basic elements:
- The ground transmitters
- The airborne transponder
- The ground receiver
In practice, a central recording station and data handling system are also used.
A simplified, functional block diagram of the close-in UDOP tracking system is shown in the figure. The transmitters use a primary frequency standard to derive the frequencies used. The standard is multiplied to 50 MHz and broadcast as a reference signal to the receiver sites. The 50 MHz is multiplied to 450 MHz and transmitted to the transponder
In telecommunication, the term transponder has the following meanings:...
on board the vehicle as an interrogation signal. The transponder receives the 450 MHz signal, doubles and re-transmits at 900 MHz.
The ground stations simultaneously receive the 50 MHz reference signal and the 900 MHz transponder signal. The 50 MHz signal is multiplied by 18 and compared to the 900 MHz signal. The difference will be zero for a vehicle on the pad and there will be a doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
(measured in cycles per second) if the vehicle is in motion. This effect will be proportional to a loop veiocity with amount depending on the location of the transmitter site, receiver sites, as well as vehicle position and velocity.
The UDOP ground receivers are double, superheterodyne, dual-channel units with common local oscillators. All resulting frequencies after mixing are related to the frequency standard except those experiencing doppler shift. Consequently, the doppler effects are measurable.
Interim-Offset UDOP Operation
The existing system operates in an offset mode where the reference frequency is raised to 5 kHz higher than 900 MHz causing a 5 kHz beat frequency as long as the vehicle is on the pad. When the vehicle moves, the doppler effect adds to the 5 kHz frequency. The primary advantage is simplification of data handling as the frequency varies from 5 kHz rather than zero. This offset frequency is derived using phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
The UDOP digitized data recorded from each receiver station was fed to a computer which calculated positions X, Y, and Z. These positions were then fitted to a second degree polynomial using mid-point, moving arc smoothing over a one second interval. From this process, smoothed position, velocity, and acceleration were obtained.
The data presented were reduced to an earth-fixed, right-handed, rectangular cartesian coordinate system. The Y axis is normal to the Clarke Spheroid of 1866 and positive upward. The X axis is positive in the direction of the flight azimuth. The origin for the UDOP system is at the vehicle transmitting antenna at vehicle launch position.