JOSEPH GULICK, W. COLEMAN HYATT, and OSCAR M. MARTIN, JR. THE TALOS GUIDANCE SYSTEM The Talos guidance system evolved from a series of development efforts within the Bumblebee and Talos programs - specifically, the first beamriding system and the first semiactive interferom­ eter homing system. The guidance system that evolved was virtually unjammable, and it provided the missile with capabilities against manned aircraft, antis hip missiles, surface ships and boats, and radar targets. INTRODUCTION Nutation angle = 0.85° When the Bumblebee Program was conceived, missile guidance technology was in its infancy. The early guidance development work was based on pulse radar technology since pulse radars were well devel­ Guidance beam oped by 1945. The first guidance concept was that a radar beam, following the target, could be used to guide the missile to the target. It was determined early, however, that the maximum allowable miss distance could not be achieved by such a "beamrid­ Desired missile course is ing" system at ranges beyond approximately 10 naut­ along nutation axis ical miles. When the target intercept range for the missile was increased, the guidance concept was Figure 1 - The guidance beam for the beam rider missile revised to use beamriding for the midcourse phase was conically scanned at 30 hertz with a clockwise rota­ with semiactive homing for the terminal phase. This tion, as viewed from behind the radar antenna. The radar resulted in a guidance system that produced very pulse rate, nominally 900 pulses per second, was varied by small miss distances, essentially independent of inter­ ±50 pulses per second in synchronism with the conical scan. cept range. During the later Talos years, the major guidance effort was focused on the homing system. This effort resulted in a monopulse homing system ular to the nutation axis of the guidance beam (Fig. that was virtually unjammable and an antiradiation 2). The missile was roll stabilized in flight by a free missile seeker that enabled the missile to home on gyro so that the error signals would be directed to the radar targets. proper aerodynamic steering surface. The guidance transmitter radiated pulse groups at MIDCOURSE GUIDANCE a nominal rate of 900 pulses per second. Each group In a beamriding system, departure of the missile consisted of three pulses that were coded by inter­ from the axis of a conically scanned radar beam pulse timing to identify the guidance beam to the mis­ causes deflection of the aerodynamic surfaces to re­ sile. Nutation position information was transmitted turn the missile to the beam axis. Basically, the mis­ by frequency modulating the pulse group rate at the sile must determine the vector that indicates the an­ nutation frequency (30 hertz) with a deviation of gular direction and distance to the scan axis. The dis­ ± 50 groups per second. The maximum pulse group tance is determined by observing the amplitude of the rate occurred for a nutation position that was up and modulation produced by the conically scanned beam, left. On board a ship, an inertial system was required and the angular direction is determined by the phase to compensate the pulse group modulation for roll of the amplitude modulation with respect to the scan and pitch. frequency reference signal. A simplified block diagram of the beamrider re­ The beamriding system for Talos was the result of ceiver is shown in Fig. 3. Microwave pulses were de­ a joint effort by APL and Farnsworth Television and tected and passed through a decoder to produce one Radio Corp. It employed a conically scanned radar pulse for each valid code group. The decoded pulses beam (Fig. 1) and, together with a sinusoidal varia­ were then applied to a 30-hertz amplitude detector, a tion of the pulse repetition frequency, provided the 30-hertz frequency modulation detector, and a missile-borne receiver with the signals needed to mea­ beacon transmitter. Outputs from the frequency sure missile angular distance and direction perpendic- modulation detector were used as references for the 142 Johns Hopkins A PL Technical Digest Beam center Viewed from behind radar almost the beginning of the development program. t The task was to devise a homing system that would /900 be compatible with the constraints imposed by the )( )( 900 )()( )()( ramjet diffuser and that would rapidly acquire the 0 target without requiring accurate missile-to-target ,, )( )( 180 270 0 )( 8508QO line-of-sight positioning information. The goal of in­ tercepting small targets at long range (about 100 nautical miles) placed a premium on high receiver sensitivity as well as on good target resolution. The interferometer homing system was chosen for terminal guidance for the following reasons: 1. Widely spaced body-fixed antennas were com­ patible with ramjet inlet constraints. 2. The body-fixed antennas were simple compared with a gimballed dish antenna. 3. Rapid target acquisition was possible without the need for missile-to-target angle data. 4. It was desired to have the largest possible aper­ 850 ~------~--------L--------L------~ ture and, hence, the most accurate measure­ o 90 180 270 360 Beam center position (degrees) ment of the line-of-sight angular rate. Figure 2 - Signal modulations detected by the beam rid­ The basic principle of the interferometer system is ing receiver were processed to determine missile position illustrated in Fig. 4. Two widely spaced antenna ele­ with respect to the guidance beam. Angular direction and ments of an interferometer have a composite antenna distance from the scan axis were determined by comparing the phase and amplitude of the 30-hertz amplitude­ pattern consisting of a series of peaks and nulls. The modulated signal with the frequency-modulated reference peaks and nulls are moved by a phase shifter, result­ signals. ing in amplitude modulation of the target signal. A discriminator tuned to this modulation frequency steering channel phase comparators. The phase com­ provides an output that is proportional to the angular parators were used to resolve the in-phase component rate of the target line of sight. If the target line-of­ of the amplitude modulation detector output to ob­ sight rate with respect to an inertial reference is main­ tain steering-error signals for each wing plane. tained at zero, a proportional navigation homing tra­ As a flight progressed, the direction of the guid­ jectory to the target is executed. It is therefore only ance beam was programmed by the fire-control com­ necessary to control the missile turning rate so that puter to cause the missile to fly the desired midcourse the line-of-sight rate is maintained at zero to effect an trajectory. Missile range was determined by automat­ intercept. ically range tracking the missile-borne beacon pulses. THE FIRST T ALOS HOMING SYSTEM That range was used by the fire-control computers to control the beam program and compute the time at Guidance concepts consisting of body-fixed, wide­ which a homing enable pulse code was transmitted to ly spaced antennas to be used as a radar interferom­ the missile, permitting target acquisition by the hom­ eter were proposed almost simultaneously by the ing system. Defense Research Laboratory (DRL) of the Univer­ Subsonic beamriding along a fixed beam was ac­ sity of Texas and the Massachusetts Institute of complished by a guidance test vehicle in January Technology. Both concepts used widely spaced 1947. The first supersonic beamriding Talos was antennas but different methods of signal processing. demonstrated in 1950. Because the basic system proposed by DRL (which employed an independent interferometer channel for TERMINAL GUIDANCE each wing-control plane) was judged to be more con­ A terminal-guidance phase following the mid­ sistent with the state of the art, the first Talos homing course beamriding phase had been envisioned from system was based on that concept. _ ~,-_M_ic_r_owa_ve~ receiver III III III Figure 3 - The beam rider "A" receiver detected and processed steering guidance beam signals to error generate commands for the mis­ sile control planes and the radar - "B" beacon responses. steering error Vo lume 3, Number 2, 1982 143 sin {3 Figure 4 - Interferometer parameters and symbols used in the article are defined in this figure. 'Y = sin-1 ~ d The receiver (Fig. 5) used a scanning interferom­ X 4 = Al sin wI eter system. Target signals at the antenna were: + A 2 sin [(w + Ws + 2 7r: (3 cos (3) I + <I> J . (4) XI = Al sin wt X 4 can be seen to be a carrier signal at wand is amplitude modulated at X 2 = A 2 sin (wt + 0) (1) 7rd . w 5 + 2 - {3 cos {3 . (5) = A 2 sin (wt + 2 7r: sin (3 ) " The heterodyne process in the receiver changed the where w is the microwave frequency and 0 is the elec­ carrier from microwave to a lower frequency but did trical phase difference at the antennas. A I and A 2 are not affect the modulation and, therefore, did not the amplitudes, which may be different, but the scan­ change the basic (3 information. It is also apparent ning process was essentially insensitive to that dif­ that because the desired information was the fre­ ference. The scanning phase shifter advanced the quency of the modulation, changes in amplitude of phase of X 2 by <I> (t) as follows: the signal had no direct effect upon the measurement of (3. Decoupling of body motion to provide the line­ (2) of-sight rate measurement (0) was accomplished by use of a body-mounted rate gyroscope where the gyro output frequency modulated an oscillator to produce where Ws is the scan frequency and <I> is the initial a carrier frequency (w ) with a deviation proportional phase shift. The X 3 can be written as o to the missile body turning rate (.J;).