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 (Azusa, California) where the system was devised.


The AZUSA radar interferometer tracking system employs two receiving points on the ground and comparing the phases of the signals each of them separately receives from an airborne source (transponder). Radio and radar interferometry has the advantage of yielding highly accurate angles. It achieved practical form in 1948 when engineers with Consolidated Vultee Aircraft Corporation (Convair) created the AZUSA tracking system for the Army, using an interferometer. James Wesley Crooks (1921 February 11 - 1983 September 12) is credited as inventor of the Atlas radio guidance system and the Azusa and Glotrac launch vehicle tracking systems.

Principle of Operation

The AZUSA ground station determined the vehicle transponder's position by measuring range and two direction cosines with respect to the antenna baselines. The AZUSA system supplies continuous, nonambiguous trajectory data in real time. Cartesian coordinates X, Y, and Z of the vehicle being tracked can be derived from these data. Higher-precision incremental cosine and range data are available for computation of X, Y, and Z velocities. The absolute and incremental values of the tracking data are supplied in digital form to an IBM 704 or IBM 709 computer for real-time impact or orbit prediction. The data are also recorded on magnetic tape for post-flight analysis. Advanced AZUSA systems can be used for vehicle commands, telemetry data transmission, space-probe tracking, and space communications. Combined with digital computers, advanced AZUSA systems can perform guidance functions, supply navigational information, and determine space-probe orbits.

=Radar interferometry=Radio interferometry has the advantage of yielding very accuratetracking angles when the target cooperates by emitting a radio signal. Theangular precision of interferometry led to the development of the Azusa tracking system as part of the Army Air Corps NUL-774 Project, forerunner of the Atlas ICBM program, at the Vultee Field Division of Consolidated Vultee Aircraft Corporation in Downey, California. Two of the basic patents (2,972,047 and 3,025,520) in the field of interferometer tracking are shared by James Crooks, Jr., Robert C. Weaver, and Robert V. Werner, all members of the Azusa design team. By the spring of 1948, the Azusa team had built an interferometer operating at 148.58 MHz.

In a strange circle of history, the U.S. Naval Research Laboratory (NRL) was working on underwater sound interferometers at the time Convair was developing Azusa. Since the two groups were in close contact, therewas considerable interchange of ideas.* The circle was completed in the early 1950s when the Navy picked up the Azusa interferometer work for its Viking project at White Sands, New Mexico. The Navy wanted to expiore the possibility of converting the Viking or some derivative of it into a guided missile and it needed an accurate guidance system. In an early report from this programg, NRL's J. Carl Seddon explained how the Viking would determine its position: "The Missile will detect its position relative to the hyperbolic guidance path by phase comparison of modulation waveforms derived from signals received from two pairs of stations." In this scheme, the missile would guide itself using onboard electronics and navigational signals received from the ground. This seems a far cry from Minitrack and satellite tracking, but phase comparison, the essence of Minitrack, is there. Within a year, NRL reports from the Viking program were diagramming ground-based, tracking interferometers, which relieved the Vikingof the burden of signal-processing equipment by computing the missile's position from the ground. Two precursors of Minitrack are evident in the interferometer arrangement. First, only a tiny radio beacon needs to becarried on the Viking itself. This was to be an important feature of the Vanguard "Minitrack," in which the prefix 'Mini" applies to the minimum-weight satellite transmitter. The second precursor is the "Lff arrangement of the interferometer antennas which persisted in some early designs of Minitrack, although the final deployed version extended the bars of the "L" to make a cross. [3]

=USAF Atlantic Missile Range, Cape Canaveral, FL=While the fact that some scientific satellites "achieve orbit" is enough, vehicles carrying men or payloads that must be placed in precise positions, such as geosynchronous satellites, require improved trajectory position and velocity measurement systems.

In the early 1960's, the mainstay for obtaining this data at Canaveral was Convair's Azusa Mark I, a c-w cross baseline interferometer operating in the C band, requiring a transponder in the missile. Output data were digitized for use in the IBM 709 computer and measured parameters consist of range, coherent or fine range, and two direction cosines.

The Azusa II, intended to replace Azusa I, was installed in 1961. It is nearly identical to the Mark I except that its circuit design was refined and cosine rate was added which provides better direction cosine information. Both Azusas have identical limitations: they will not track cross-polarized signals; missile antenna nulls deeper than 10db cause noisy data, ambiguities and, in severe cases, loss of data. [4]

=Used in Apollo Program=The AZUSA tracking radar was used to monitor initial phases of launch for the Saturn S-II by telemetry with transponder frequency of 5,060 MHz (receiver) and 5,000 MHz (Transmitter) with 2.5W of power. [5]

=Technology=AZUSA is a radar interferometer tracking system which determines the position of a vehicle-borne transponder. Two AZUSA stations were in operation during the Saturn-Apollo programs, one station (MK II) at Cape Kennedy and the other station (MK I) at Grand Bahama Island. The AZUSA system provided real-time tracking data used for range safety impact prediction and post flight trajectory analysis. AZUSA stations cover only a portion of the Saturn V powered flight trajectory. The position of the vehicle (transponder) is determined at the AZUSA ground stations by measuring range (R) and two direction cosines (1, m) with respect to the antenna baselines.

Antenna Layout

The antenna layout of the AZUSA MK II station consists of two crossed baselines (at right angles) with three antenna pairs each. The transmitter antenna (T) radiates a CW signal at 5 GHz to the vehicle. This signal is offset by 60 MHz in the transponder and retransmitted to the ground station receiving antennas. The direction cosine, with respect to a baseline, is obtained from the measurement of the phase difference between signals received at spaced antenna pairs along this baseline. The range to the transponder is found by measuring the phase difference between transmitted and received signal. For range ambiguity resolution, the transmitted carrier is modulated with several low frequencies. Direction cosine measurement is accomplished by using the antennas in pairs to provide baselines of 5 meters, 50 meters, and 500 meters. A conical scan antenna (DF) yields unambiguous direction measurement and furnishes ambiguity resolution for the 5-meter baselines. The 5-meter baselines resolve ambiguity for the more accurate 50-meter baselines, and the 500-meter baselines supply information for computing cosine rate data. The direction finder antenna DF (conical scan antenna) provides pointing information for all other antennas.

The signals received at the two spaced antennas of an antenna pair have a phase difference caused by the difference in range between the transponder and each of the antennas. This phase difference is measured and used to compute the direction cosine. The accuracy of the measurement increases with increasing baseline length, but data becomes ambiguous and coarse measurements are necessary for ambiguity resolution.

Range Determination

Range is determined at the ground station by comparing the instantaneous phase of the transmitted modulation signal to that of the received modulation signal. The resulting phase difference is directly proportional to the propagation time from the ground station to the transponder and back to the ground station. In other words, phase difference is proportional to range. Using a high range modulation frequency to obtain fine resolution, results in output data that are ambiguous, the number of ambiguities is proportional to the modulation frequency.

The ambiguities in range measurement are resolved by using three modulation frequencies that are obtained by the frequency division of a single, precise, frequency source at the ground station. The phase shift is measured for each one of these harmonically related frequencies. The lower frequency phase data is used to resolve the ambiguities in the next higher frequency phase data. This arrangement has, as advantages, the resolving power of the highest frequency signal and the extended unambiguous range data provided by the lower data frequency. The modulation signals used for range measurement are 157.4 Hz, 3.934 kHz, and 98.356 kHz. The fine range modulation signal, 98.356 kHz, remains ON at all times so the transponder can "lock-on" with the ground station. Higher resolution range data is obtained by coherent carrier phase comparison.

Coherent Transponder

The coherent transponder includes a phase-control loop in which the 98.356 kHz fine range modulation frequency is multiplied by a factor of 612 to establish the transponder offset frequency (60.194 MHz) and a frequency and phase-coherent response signal (5000 MHz).

At the ground station, the 98.356 kHz frequency is also multiplied by 612 to equal the transponder offset frequency (60.194 MHz). Both, the 5000 MHz signal received from the transponder and the data signal, are heterodyned with a local oscillator signal to obtain approximately 5 MHz. Data and reference signals are then fed into a discriminator which provides one pulse (count) output for each 360-degree phase difference (1 cycle) in the input circuit. Plus and minus pulses are fed on separate lines to a bidirectional counter. The coherent carrier range data is then fed to an IBM 7090 Computer with the direction cosine data (l, m) and modulation-derived slant range data. The computer, using 20 input samples per second, solves the equations for the position of the vehicle.

Automatic Frequency Control Loop

The AZUSA Type-C Transponder used in the Saturn Vehicles is a part of the overall Automatic Frequency Control (AFC) loop in the AZUSA system. Upon transponder activation, the klystron transmits a 5060.194 MHz signal which is swept ±2 MHz by a 15 Hz internal sweep generator. This enables the ground station and the transponder to find a common frequency so that lock-on can occur. When frequency lock-on occurs, the 5060.194 MHz signal transmitted by the ground station will be shifted slightly by AFC action so that, after the transponder receives the signal and retransmits R, the frequency received by the ground station receivers will be 5000 MHz. The 5060.194 MHz (plus Doppler) signal entering the transponder receiver is mixed with a fraction of the retransmitted signal to produce a 60.194 MHz offset frequency. A frequency discriminator is used to maintain the offset frequency within 30 Hz.

Phase lock between the transponder and ground station is established by multiplying the fine range modulation frequency (98.356 kHz) received from the ground station and using it as a reference for a phase discriminator. This phase discriminator has higher output gain than the frequency loop discriminator and holds the transponder offset frequency (60.194 MHz) correct to within a fraction of a cycle. The Type-C Transponder uses this combination frequency-phase discriminator to obtain maximum stability in both AFC and APC loops. This coherent condition in the transponder enables phase measurements to be made at the ground station between the received 5000 MHz signal and the 5060. 194 MHz transmitted signal (heterodyned to 5000 MHz) to obtain high-resolution incremental range data. Similarly, phase comparison of the transmitted and received 98.356 kHz modulation signals produces non-ambiguous range data. Before lock-on, the transmitter local AFC loop and the local over-all AFC loop are used to keep the ground station transmitter frequency at 5060.194 MHz. When the ground station and the transponder are frequency locked, a 5 MHz signal from the ground station range receiver IF amplifier controls the overall AFC loop. This high-gain loop overrides both the local over-all AFC loop and the transmitter AFC loop Should the return signal received from the transponder deviate from 5000 MHz at the ground station receiver, the ground station receiver will detect the error and cause the ground station transmitter frequency to shift slightly in a direction to correct the error (through AFC action). The transponder receiver detects the frequency or phase change and corrects the return frequency so that the offset frequency remains exactly 60.194 MHz (through the transponder AFC and APC loops). Thus, the vehicle-to-ground link maintains a constant frequency (5000 MHz) at the ground station receivers despite systematic frequency drift and Doppler shift.

Transponder Operation

In the transponder, the 5060.194 MHz interrogation signal and the 5000 MHz klystron local oscillator signal produce a 60.194 MHz IF, or offset frequency, in the crystal mixer. This signal is mixed with a 55.2 MHz second local oscillator signal. The second IF (4.994 MHz) is amplified and fed to the receiver frequency discriminator where the range signals which modulate the ground station carrier are detected. This range modulation (98.356 kHz) is fed to the compensation network. The 4.994 MHz IF signal is also fed to the phase network and to one side of the frequency-phase discriminator.

A 4.994 MHz reference signal for phase lock is provided as follows: The 98.356 kHz range signal is directed through the compensation network, an amplifier, and two crystal filters, to a frequency multiplier circuit. Here the signal is multiplied to 60.194 MHz. (One crystal filter is in the compensation network subassembly and one is in the sweep oscillator subassembly.) The 60.194 MHz signal enters the phase network and is mixed with 55.2 MHz from the receiver second local oscillator to obtain the 4.994 MHz reference for the phase discriminator. In the compensation network, the phase and frequency discriminator error voltage from the phase network overrides the output from the sweep oscillator. It is then combined with the 98.356 kHz range modulation and fed to the modulator. In the modulator these signals are superimposed on the klystron anode voltage. The dc error signal maintains phase lock between the 5060.194 MHz received signal and the 5000 MHz response signal. The 98.356 kHz is used to modulate the response signal. The 98.356 kHz phase shift in the transponder is held to an absolute minimum so that phase comparison of the received signal and transmitted signal at the ground station will indicate actual range to the transponder.

The waveguide subassembly is designed with two bandpass filters internally connected to a symmetrical Y section. This arrangement permits the use of a single receiving-transmitting antenna. The klystron output is carried to the 5000 MHz filter by coaxial cable. The signal passes through a 3-section filter, the symmetrical Y, and a waveguide-to-coaxial transition and is carried to the antenna by coaxial cable. A 5060.194 MHz signal entering the antenna passes through the same coaxial cable into the waveguide. A 2-section filter passes the incoming 5060.194 MHz signal and a small amount of the 5000 MHz signal present in the symmetrical Y to a crystal mixer which produces a 60.194 MHz offset frequency.

The Type-C Transponder uses transistors in all circuitry except for the klystron. The Type-C Transponder operates also with GLOTRAC stations.

=GLOTRAC=GLOTRAC (GLObal TRACking) was originally planned as a global tracking system, but changes in programs restricted the number of ground stations. GLOTRAC uses the AZUSA Type-C Transponder in the vehicle. GLOTRAC ground stations are equipped with either a transmitter or a receiver or both. Both existing AZUSA stations may be considered as part of GLOTRAC. The transponder in the vehicle is interrogated by an AZUSA ground station or by a GLOTRAC transmitter site. The transponder offsets the received frequency and retransmits the signal to GLOTRAC receiving sites where the Doppler shift is measured by comparing the received signal with the transmitter signal (if receiver is located near the transmitter) or with a local frequency source. The measured Doppler shift provides the range sum similar to ODOP. At GLOTRAC stations equipped with both a transmitter and receiver, the range to the transponder is measured by phase comparison between the transmitted and received signals. The AZUSA Type-C Transponder can also be interrogated by C-band radars for range and angle determination.

The range rates measured at three receiving stations yield the vehicle velocity, and by integrating this velocity, the position is obtained. Initial conditions for integration are obtained from radar range measurements. Data measured at all stations is transmitted to the computer at Cape Kennedy. Accuracy of GLOTRAC measurements is 30 meters (98.4 feet) in position and 0.15 meters/second (0.49 feet/second) velocity.

AZUSA Transponder Type-C Characteristics

Receiver frequency ................. 5060.194 MHzTransmitter frequency….............. 5000.000 MHzRF power output ......................... 2.5 wattsInput voltage ............................ 28 VdcInput current ............................ -5 amperesReceiver input signal…................... -12 to -90 dbmWeight ................................. 8.74 kg (19.3 lbs)Size .................................. 0.006 meters3 (372 inches3)

AZUSA Ground Station Mark II

Transmitted power …...,,,,,,,,,,,,,,,,,,,... 2 kwTransmitter frequenc................... 5060.2 MHz +0.75 MHzRanging modulation ….......... 157.4 Hz, 3.934 kHz, 98.351 kHzReceiver frequency …..................... 5000 MHz Receiver sensitivity……........... -145 to -147 dbmReceiver antenna gain ….................... 33 db (MK II)Accuracies: Range ............................ 3.05 meters Angle ........................ 1 x 10-5 in cosine data

=Tribute to Robert Weaver=Robert Christian Weaver Sr., co-inventor of the AZUSA radar, had a post-World War II career in the aerospace industry that influenced the future of guided missiles and other space vehicles. In the early 1950s, Weaver and a colleague invented the AZUSA continuous wave tracking system, implemented at the Air Force Missile Test Center, Cape Canaveral. This system was designed to measure the trajectory of missiles, and was instrumental in pioneering military missile tests and the Project Mercury manned space program.

Mr. Weaver began his civilian career with what was then Consolidated Vultee Aircraft in 1946. "After the war he had a choice of engineering jobs – one in Los Angeles, the other in San Diego with Consolidated," his son said. Mr. Weaver, whose engineering career with the Convair Division of General Dynamics spanned 35 years, died in September 2003 at the White Sands of La Jolla retirement community. He was 87. He died of natural causes, said his son, Robert Weaver Jr.

The technology that Mr. Weaver and his colleague, Jim Crooks, devised flourished during the Cold War. It was applied to the Navy's Polaris program and the Air Force's Thor, Atlas and Titan programs. It also was used by NASA in the Saturn IV (Apollo) program. One of the advantages of the Azusa system over its predecessors was that it required fewer radar sites and operating crews. The increased emphasis on ballistic missiles and efforts to improve their accuracy spurred the development of the Azusa system and Mistram, a competing technology developed by GE in the early 1960s.

Choosing the San Diego assignment, Mr. Weaver made his home in La Jolla. He was familiar with the area from a 1933 visit to La Jolla Shores, where he had camped out with friends in cow fields, his son said. Mr. Weaver, a Fresno native, graduated from the University of California Berkeley in 1938 with a bachelor's degree in electrical engineering. He joined the Army Air Corps in 1940, rising to major during World War II. He was responsible for radio and radar equipment in the China-Burma-India theater. Later, with Convair, Mr. Weaver's assignments took him to Cape Canaveral and several other test sites. His specialty became designing radar, guidance and tracking systems for guided missiles and space vehicles. Computers, electronic gadgets and photography occupied much of his leisure time. He also enjoyed sports cars, as well as "the green flashes of the La Jolla sunsets and the breathtaking beauty of Yosemite Valley," his son said. [6]

#'AZUSA. A Precision, Operational, Automatic Tracking System'. General Dynamics/Astronautics, San Diego, CA, NTIS Report No. AD0832153, MAR 1959. 52 pp.
#Corliss, W. R. 'Evolution of Satellite Tracking and Data Acquisition Network STADAN from pre-IGY AND Minitrack facilities'. NASA Goddard Space Flight Center. Report Number: GHN-3, NASA-TM-X-55658, X-202-67-26, 1967.
#Corliss, William R., 'The Evolution of the Satellite Tracking and Data Acquisition Network (STADAN)'. Goddard Historical Note No. 3. Greenbelt, MD.: Goddard Space Flight Center, 1967.
#'Earth-based Electronics' in Electronics, vol. 34, no. 46, pp. 108-118, 17 November 1961.
#F.O. Vonbun, 'Ground Tracking of the Apollo', NASA Report N66-22219, 1966.
#Jack Williams. 'Robert Weaver; Inventor of missile-tracking system'. San Diego, CA UNION-TRIBUNE, September 25, 2003. []
#Astrionics System Handbook, revised ed., NASA MSFC No. IV-4-401-1, NTIS Doc. N70-70002, 1 November 1968, 418 pp. (International Business Machines Corporation working under NASA Contract NAS8-14000).

*Robert V. Werner, Robert C. Weaver, and James W. Crooks, Jr. Tramsitter-Receiver. Patent number: 2972047, Filing date: Nov 21, 1955, Issue date: Feb 1961.
*Robert V. Werner, Robert C. Weaver, and James W. Crooks, Jr. Positioning Determining Device. Patent number: 3025520. Filing date: Nov 21, 1955. Issue date: Mar 1962.

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