The C-band CW interferometric AZUSA, in operation from 1950s, contains one transmitter and nine receivers located along two crossed baselines with the total lengths of about 500m. Intermediate receivers spaced at 5 and 50 m 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 3m in range and 20 ppm in direction cosine are obtainable. (1)
MISTRAM (Missile Trajectory Measurement) is a CW interferometric system with receiving stations situated along two mutually perpendicular baselines spaced at 3 and 30 km. This MSRS can measure range, four range differences, range rate and four range difference rates of a target. The range error is less than 0.8m. (1)
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 DOVAP, (Doppler Velocity and Position), was developed by NASA-KSC.
UDOP consists of three basic elements:
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 frequencies used. The standard is multiplied to 50 mc and broadcast as a reference signal to the receiver sites. The 50 mc is multiplied to 450 mc and transmitted to the transponder on board the vehicle as an interrogation signal. The transponder receives the 450 mc signal, doubles and re-transmits at 900 mc.
The ground stations simultaneously receive the 50 mc reference signal and the 900 mc transponder signal. The 50 mc signal is multiplied by 18 and compared to the 900 mc signal. The difference will be zero for a vehicle on the pad and there will be a doppler effect (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.
The existing system operates in an offset mode where the reference frequency is raised to 5 kc higher than 900 mc causing a 5 kc beat frequency as long as the vehicle is on the pad. When the vehicle moves, the doppler effect adds to the 5 kc frequency. The primary advantage is simplification of data handling as the frequency varies from 5 kc rather than zero. This offset frequency is derived using phased-locked loop techniques.
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 at 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. (3)