UDOP
The UDOP (UHF Doppler) multistatic radar and multiradar system (MSRS) utilizes
The UDOP used an AN/DRN-11 transponder installed in the Saturn (rocket family) launch vehicle for Project Gemini missions.
The
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).[1]
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 John F. Kennedy Space Center (NASA-KSC).[3]
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.
Operation
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 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 (measured in cycles per second) if the vehicle is in motion. This effect will be proportional to a loop velocity 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,
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 techniques.
Data reduction
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 midpoint, 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.[3]
References
- ^ a b c V S Chernyak. Fundamentals of Multisite Radar Systems: Multistatic Radars and Multiradar Systems. (Translated from Russian). CRC Press: New York, 1998. pp. 26–27.
- ^ Schneid, Daniel L. The UDOP Handbook. National Technical Information Service document no. AD0609038, July 1964, 214 pp.
- ^ a b Instrumentation Systems Analysis Branch (K-ED2) and Tracking Branch (K-EF4). Saturn early launch phase tracking by CW Doppler. John F. Kennedy Space Center, SP-79, April 13, 1964, NASA doc. no. N65-19700, 52 pp.