COHERENT RADAR USING RECIRCULATING FIBER-OPTIC DELAY LINE

Abstract: A radar system in which a coupled portion of the transmit pulse (12) is circulated in a closed path delay line (15), each loop (14) of which provides a delay equal to the transmit pulse period. Coupled off signals from the delay line (15) are utilized as a local oscillator for the system and also can be used to make the system coherent-on-receive.

Description: COHERENT RADAR USING RECIRCULATING FIBER-OPTIC DELAY LINE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a radar system, and, more particularly, to a coherent radar system utiliz- ing a fiber-optic delay line in order to produce pulse-to- pulse coherency and serve as system local oscillator. 2. Description of Related Art A radar system in its primary major elements may include a radio frequency oscillator which is controlled by a pulse modulator, or pulser, so as to produce periodic pulses of high power but of relatively short duration. These pulses are applied to a highly directional antenna, typically adapted for rotation, and the antenna is con- trolled to transmit the pulses toward a target or towards a region in which a target is sought. A receiver, which may be interconnected with the same antenna as the trans¬ mitter, is controlled by a switching arrangement ("duplexer") to interconnect the antenna to the receiver during the interval between transmitted power pulses in order to receive relected energy from a target. By monitoring the antenna direction and timing of reflected pulse returns, location and range of a target can be obtained. There are other radar systems for detecting targets and their ranges which rely upon the radiation of unmodu¬ lated continuous wave energy. The present invention is not concerned with these continuous wave or "C " radar systems, but rather with pulse type systems. One of the earliest forms of oscillators for pro¬ ducing radar pulses was a device called a magnetron, which has an undesirable characteristic in that the phase of each transmitted pulse is randomly related to the phase of previous pulses. The phase ' problem requires special ad¬ justment and control when the radar is being used as a moving target indicator (MTI) . That is, a primary use of radar, for either domestic or military applications, is to detect a moving target amid echoes obtained from adja¬ cent fixed objects, the latter frequently being referred to as "ground clutter" or just "clutter". More particularly, it is necessary for a radar to act as a satisfactory MTI that the phase of a transmitted pulse be made coherent with the system local oscillator (LO) and to the phase of previous pulses, and it is this coherence that we are primarily concerned with here and in connection with which the described invention achieves its primary utility and advantages. For example, in air-to-surface missile guidance seekers, coherent processing is extrememly valuable for finding and tracking targets (e.g., tanks) amid ground clutter. However, conventional coherent radar systems have not been considered affordable for this class of weapons, and it is, therefore, highly desirable to find reduced cost alternatives for air-to-surf ce seekers. In the case of air-to-air missile seekers, they usually use medium pulse RF or high pulse RF, both of which are ambiguous in range necessitating maintaining pulse-to-pulse coherence. Also, air-to-air seekers require large power-aperture components in order to achieve sufficient acquisition range, and for that reason they usually operate in the X-band (i.e., center of frequency band is 9,000 MHz) . The early radar systems, generally operating at lower frequencies than used now, achieved coherency between the LO and the transmit pulse by locking the LO to the transmit phase and maintaining the locked phase during the .listen interval. This technique, however, did not provide pulse-to-pulse coherence. At the present time, fully satisfactory coherent radar seekers having pulse-to-pulse coherence require a relative¬ ly expensive frequency reference unit (FRU) that also must be stable over the operational temperature range. This necessity for an FRU is considered to make some missile radar seeker systems prohibitively expensive. It has been suggested for shipboard use to detect low flying targets over the sea surface that a high re¬ solution radar be provided with a noncoherent delay line canceller, the delay line being of the fiber optic variety. C. T. Chang, D. E. Altman, D. R. Wehner, D. J. Albares, "Noncoherent Radar Moving Target Indicator Using Fiber Optic Delay Lines," IEEE Trans, on Circuits and Systems, Vol. CAS-26, No. 12, Dec. 1979, pp. 1132-1135. SUMMARY OF THE INVENTION It is a primary aim and object of the present invention to provide an improved coherent-on-receive (COR) radar system utilizing a delay line which serves as a local oscillator for the system. Another object is the provision of a coherent radar system as in the previous object that eliminates the need for a frequency reference unit. A still further object is the provision of a co¬ herent radar system in which pulse-to-pulse coherency is obtained by locking each transmit pulse to the previous pulse delayed in time by a RF delay line. Yet another object is the provision of a coherent radar system as in the previous objects in which the delay line is a fiber optic delay line. In accordance with a first embodiment of this in¬ vention, a coherent-on-receive radar system is provided having an optical delay line including a fiber-optic transmitter which converts a microwave radar signal into an optical signal, an optical fiber of predetermined length for carrying and delaying the optical signal, and a fiber-optic receiver for converting the optical signal once again into a microwave signal. A microwave switch or directional coupler couples a part of the transmitted signal during a transmitted radar pulse into the fiber¬ optic delay line. At the end of the radar pulse, the microwave switch is transferred to another connective mode forming a closed loop to cause recirculation of the stored signal. In addition, part of the recirculating signal is continuously coupled off the delay line to serve as the radar local oscillator (LO) . Each cycle in the delay line loop, the microwave signal is converted into an RF mod¬ ulated optical signal which is delayed in accordance with the physical characteristics of the fiber loop, the optical signal is converted back to microwave form, amplified, and the sequence is repeated. The amplification for the delay line recovers signal loss incurred by the signal trans¬ mission through the delay line. The optical fiber of the delay line iε cut to a predetermined length so as to delay a pulse making one passage around the delay line by an amount precisely equal to the duration of a radar transmitted pulse. In this manner, repeated replicas of the radar transmitted pulse are continuously coupled off, amplified and mixed with the radar echoes or received signals. At the end of the listen interval, the microwave switches return to the original connective mode directing the recirculated pulse to the radar transmitter for injection locking of a new pulse. As before, a part of the new pulse is coupled into the delay line and the operation continues as already described. An alternate form of the invention uses an all optical recirculating delay line thereby avoiding the requirement of the first embodiment of converting the delayed transmit pulse from a microwave signal to an optical signal and then once again back to a microwave signal, each time the pulse circulates around the delay line loop. Specifically, this eliminates conversion noise buildup associated with the microwave and optoelectronic components inside the delay loop. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a schematic representation of the radar system of this invention using a recirculated pulse in a delay line as system local oscillator (LO) . FIG. 2 is a function block circuit schematic of a first embodiment of this invention for producing a system LO. FIG. 3 depicts in block form a homodyne radar system with a recirculating delay incorporated therein. FIG. 4 shows a recirculating delay line incorp- orated into a heterodyne radar system in accordance with the present invention. FIG. 5 is a circuit schematic of a coherent-on- receive radar system according to this invention which also serves as system local oscillator. FIG. 6 is a timing graph of certain signal waveforms in the circuit of FIG. 5. FIG. 7 is an alternative embodiment employing purely optical recirculating delay line. FIG. 8 is a graph of an optical delay signal output wave form compared with conventional radar waveforms. DESCRIPTION OF THE PREFERRED EMBODIMENTS For the ensuing description of preferred embodi¬ ments of this invention, reference is now made to the drawings and particularly to FIG. 1 thereof. Since the general operation of a radar system is well-known in the art, a detailed description of the more generalized as¬ pects of a radar system will not be given. In a way well- known in the radar art, a high-frequency oscillator 10 iε modulated (arrow) to provide a serieε of tranεmit pulεeε 11 and 12 of predetermined constant duration. A portion of, say, the transmit pulse 11 is coupled at 13 into a recirculating delay line 14 including a delay medium or line 15 which is selectively arranged into a closed loop by closing a switch 16. More particularly, on closing the switch the signal is "stored" and circulates in the delay line closed loop. The loop delay is set to preciεely equal the transmit pulse duration, i.e., the closed loop is "filled" when the trailing edge of the transmit is reached. The stored signal circulates around the closed loop and is coupled off at 17 to provide the system local oscillator (LO) . With the loop delay equaling the trans¬ mit pulse duration, then a continuous LO signal is obtain¬ ed. On arrival of the next transmit pulse 12, the switch is opened allowing the new pulse to be stored and the pre- viously stored pulse 11 to be dumped. FIG 2 shows that a portion of the transmit pulse iε inεerted into the recirculating delay line 14 via a suitable coupling means 18 (e.g., switch or directional coupler) . The delay medium or delay line 15 can option- ally be a coaxial delay line, SAW delay, or fiber-optic delay line. Also, optionally an amplifier/limiter 19 and band pasε filter 20 can be incorporated into the re¬ circulating delay line 14 for replacing losses and re¬ moving noise, respectively. Coupling of the delayed re- circulated pulses to the radar receiver is accomplished via directional coupler 17. FIG. 3 depicts the incorporation of the recirculat¬ ing delay line 14 (RDL) in a homodyne radar system. Thus, a radar transmitter 21 is modulated by pulse modu- lator 22 to provide a pulse output which in conventional manner uses the same antenna 23 for both the transmitter 21 and phase detector 24 through the action of duplexer 25. As described, the RDL produces the LO signal for re¬ ceiver use. ' Also, as will be more particularly described the pulses in the RDL can be re-injected along line 26 into the transmitter to achieve coherency. FIG. 4 shows the incorporation of the RDL 14 of FIG. 3 into a heterodyne radar system. FIG. 5 depicts in somewhat more detail a schematic block diagram of a coherent-on-receive radar system of the present invention in which radar pulses are maintained in a prescribed phaseRelationship and stored signals from the LO. An optical delay line 27 is provided having as its general purpose to delay received or echo pulses from a target, the delayed pulses are then subsequently mixed with other pulses to achieve increased noise- cancellation which is a desirable and advantageous aspect of the inven¬ tion. The delay line includes a prescribed length of optical fiber 28 cut to a length for delaying a signal pulse making one full traversal of the delay line a pre- determined amount of time, which will be discussed in some detail later. A fiber-optic transmitter 29 converts a microwave signal, namely, a radar pulse, into a corresponding microwave modulated optical signal which is injected into the optical fiber 28. A complementary set of apparatus is a fiber-optic receiver and limiter 30 interconnected with the opposite end of the delay line fiber 28 and which converts optical signals traversing the delay line into a corresponding microwave signal pulse. First and second microwave switches 31 and 32 interconnect with the respective ends of the delay line and have two connective aspects identified as A and B, which, in a way well known in the electrical art, are simultaneously switched to either A cr B. More particu- larly, during the transmission of radar pulse, the micro¬ wave switches are both located in position B at which time microwave energy is coupled via a microwave direc¬ tional coupler 33 and through the microwave switch 32 to the fiber-optic transmitter 29 where a corresponding optical pulse is generated and inserted into the delay line fiber 28. At the end of the transmitted pulse, the microwave switches are switched to connective aspect A which forms a closed loop in the delay line allowing the transmitted signal inserted into the line to recirculate. For each cycle or recirculation around the closed loop, the microwave pulse is converted into an optical εignal, the optical signal is delayed in the optical fiber, the optical signal is converted back to a microwave signal in the fiber optic receiver which is amplified, and the cycle is repeated. The amplifier 34 in the recirculation loop recovers signal loss which is incurred in the fiber optic delay line. The amplifier may limit the recircu¬ lating signal to suppress noise buildup. The optical fiber 28 of the delay line is cut to provide a length for delaying the circulating pulse an amount precisely equal to the duration of the transmitted radar pulse. In this manner, repeated replicas of the transmitted signal are continuously coupled off by the microwave directional coupler 35 and amplified at 36 prior to mixing with the radar signals, as will be more particularly described. That is, part of the recirculat¬ ing signal is continuously coupled off and can serve as the local oscillator for the radar system. At the term¬ ination of the listen interval, the switches 31 and 32 once again return to position B directing the recirculat- ing pulse to the transmitter for injection locking of a new pulse. Meanwhile, a part of the new transmitted pulse is coupled off into the optical delay line in the same manner as has been described. A high Q oscillator 37 serially interconnected with the B output of switch 31 in the transmit line pre¬ vents the buildup of feedback noise in the transmitted pulεe, and, in this way, cleans up phase noise which may be present in the signal. Aε already indicated, the amp¬ lifier 34 suppresses amplitude noise, and also the limiter in the fiber optic receiver 30 aids this same purpose. The local oscillator signal (LO) which is coupled at 35 after amplification at 36 is split into two parts by a 90° hybrid 38, namely, an in phase or 0° video signal and a quadrature or 90° video signal. These sig¬ nals are mixed with similar in phaεe and quadrature εignalε from radar returnε in a conventional manner at 39 and 40, reεpectively, to provide the neceεεary radar pro- ceεεing information identified in the drawing as VIDEO-I and VIDEO-Q. If desired, the system could be readily ex¬ tended to multiple channels to accommodate sum and dif- ference signals from the antenna processor. FIG. 6 shows in graphical form the local oscillator waveform available at 35 and its time relation to the radar transmitted pulseε. In recapitulation, the described coherent-on- receive radar system mixes radar return signals with de¬ layed transmit signals producing video baseband signals at the target relative Doppler frequency. Pulse-to-pulεe coherency iε achieved by locking each transmit pulse to the preceding transmit pulse delayed in time. The described radar system is particularly advan¬ tageous in not requiring a stable continuous wave oscil¬ lator which is a relatively expensive item. Magnetrons and IMPATT (silicon or gallium arsenide semiconductors) oscillators, although sufficiently modestly priced, are not used in conventional radar systems as a stable fre¬ quency source because both have a short coherence inter¬ val (i.e., time during which the reference oscillator iε coherent) . However, in the present invention either of these oscillators can be used (i.e., oscillator 37) in the pulse mode to provide both the reference frequency as well as the high power transmit pulses. Reference iε now made to FIG. 7 εhowing a circuit εchematic of a further embodiment primarily differing from the previouεly deεcribed embodiment in utilizing a purely optical delay line. An advantage of thiε pure optical delay is that since the delayed transmit pulse iε not converted from a microwave signal to an optical signal, and then back again, this eliminates noise buildup asso¬ ciated with the microwave components inεide the delay loop. As before, the basic optical fiber delay line 41 includes a length of optical fiber 42 which is precisely cut to provide a delay corresponding to the length of a transmitted radar pulse. First and second optical switches 43 and 44 have A and B connective aspectε, the εwitches being simultaneously both set to A or B. A por- tion of the transmitted signal is coupled at 45 and con¬ verted in a fiber-optic transmitter 46 into an optical signal, which on the switch 44 being set to B condition is inserted into the optical delay line. The opposite end, or switch 43 end, of the delay line when the switch is set to B, feeds the delayed optical signal to a fiber¬ optic receiver and limiter 47 for conversion to a wave signal. On adding the delayed pulse to the microwave oscillator 48, a coherent transmit signal is then sent to the antenna or transmitter of the radar along line 49. Similarly, a portion of the optical signal is coupled at 50 where it is converted in a further fiber-optic re¬ ceiver 51, amplified at 52 and inserted into a 90° hybrid for producing the 0° and 90° video processing signals. Processing is as in the first described embodiment. It is highly desirable in an air-to-ground missile to be able to function a radar system at millimeter wave frequencies (MMW) which provides the seeker with the ability to resolve rather small targets such as a tank with a relatively small diameter antenna. The essential modification from the FIG. 7 version for this purpose would be to use the coupled radar pulse to actuate a laser beam, the optical output of which is then treated by a modulator to produce the desired pulse optical signal for insertion into the delay line. Otherwise, the system would operate in the same manner and include the εame componentε aε shown in FIG. 7 and, therefore, will not be described in detail. In explanation of operation, there is a fundamental limitation on the maximum direct modulation that may be applied to a laser in view of the laεer relaxation resonance. Accordingly, indirect amp¬ litude modulation pf the laser beam must at the present time be relied upon. A number of optical modulators provide a satis¬ factory external modulator for a laser beam, such as Mach-Zehnder interferometric and directional coupler modulators in Ti * Linb0 3 and GaAs, and electro-absorption modulators in GaAs. Increased performance has been obtained in certain sophiεticated air-to-ground missileε (e.g., the AMRAAM miεεile) by employing pulεe coding techniques. As al¬ ready alluded to earlier herein, a difficulty arises in use of the described coherent radar when high resolution is required along with relatively large range capability (e.g., 7.5m for 6.5km range) since this would normally require an excesεively large number of circulations in the delay line to obtain the necessary delay. This dif¬ ficulty can be obviated by phase coding the transmit pulse which reduces the requisite number of circulations in the delay line. This phase coding may be accomplished here by serially inserting a digital phase shifter 53 in the pulse transmit line as shown in dotted line repre¬ sentation in FIG. 5. It is to be noted that pulses to be delayed are coupled off before the phase shifter so as not to include phase coding. This arrangement also results in phase coded information being extracted from the video baseband signal. Furthermore, other types of coding can be used achieving the same advantageous resultε. For example, linear frequency modulation can be uεed to increaεe the pulεe length while maintaining high resolution. Still further, standard pulse compresεion techniqueε can be employed with the same beneficial results. Although the different embodiments of the inven¬ tion have been described using an optical fiber delay line, it is contemplated that other delay media can be employed with advantage, such aε, for example, coaxial delay lineε and SAW delay deviceε. Review of the coherent radar system described herein establishes not only that it has the ability to utilize less expensive oscillatory power sources (e.g., magnetron, IMPATT oscillator) , but also shows a greater degree of noise correlation, or outright cancellation, in the receiver. The following are commercially available items for certain of the more unusual radar system components deεcribed herein: 1. Fiber-optic tranεmitter 29— SL-1000, Ortel Corporation; 2. Fiber-optic receiver 30—RSL-25, Ortel Corporation; 3. Coupler 50—2020-6621-IO, Omni-Spectra; 4. External laser beam modulator— MZ313P, Crystal Technology. The remaining system components are all deemed well-known to those skilled in the appertaining art. Although the invention has been described in cer¬ tain preferred embodiments, it is to be understood that one skilled in appertaining art could effect further modifications without departing from the spirit of the invention.

Claims: CLAIMS What is Claimed is: 1. In a radar system having a high-frequency trans¬ mit pulse generator, an antenna for radiating transmit pulses toward a target, means for receiving reflected transmit pulses, and means for processing the received pulses, the improvement comprising: means for coupling a portion of a tranεmit pulse; means for converting the coupled transmit pulse portion to a corresponding optical signal; means for delaying the optical signal a period of time equal to that of a transmit pulse duration; eanε for converting the delayed optical pulεe to a high-frequency pulεe; first means for interconnecting one terminal of the delaying means to the transmit pulse generator; and second interconnecting means for interconnecting the meanε for coupling a portion of a transmit εignal to the meanε for converting the coupled tranεmit signal to an optical signal. 2. A radar system as in claim 1, in which the first and second interconnecting means include switches actuat- able to interconnect the means for converting the transmit pulse to an optical εignal, the delaying meanε and the meanε for converting the optical εignal to a high-frequency pulse into a closed path. 3. A radar system as in claim 1, in which the de¬ laying means includes a predetermined length of an optical fiber. 4. A radar system as in claim 2, in which the switches are actuated at the end of a transmit pulse to maintain the closed loop path until the beginning of the next subsequent transmit pulse. 5. A radar system aε in claim 2, in which a low- noiεe amplifier iε serially related in the closed loop. 6. A radar system as in claim 1, in which the means for coupling a portion of a transmit pulse includes a high-frequency directional coupler. 7. A radar system as in claim 1, in which a digital phase shifter is serially interconnected between the high-frequency tranεmit pulεe generator and the antenna. S. A radar system aε in claim 7, in which the meanε for coupling a portion of a transmit pulse is coupled between the pulse generator and the phase shifter. 9. A radar system aε in claim 2, in which further meanε are provided for coupling off pulεeε between the means for converting the coupled transmit portion and the second interconnecting means. 10. In a radar system having a high-frequency radio wave oscillator for producing a train of transmit pulses to an antenna, and a receiver for receiving a reflection of the pulses from a target, the improvement comprising: a high-frequency radio wave directional coupler for coupling off a portion of a transmit pulse; means for converting the coupled off portion of the tranεmit signal to a corresponding optical signal; an optical delay line having an imput and an output; a first optical switch interconnecting the con¬ verting means and the delay line input; means for converting an optical signal to a corresponding high-frequency radio wave signal fed to the oscillator; a second optical switch interconnecting the delay line output and the means for converting an optical signal to a corresponding high-frequency radio wave signal; said first and second switches being simultaneouεly and periodically actuable to form the delay into a cloεed loop recirculating path; an optical directional coupler for coupling off the optical εignalε in said delay line; and . meanε responsive to the coupled off optical signalε for providing correεponding high-frequency radio wave εignalε. 11. A radar εystem as in claim 10, in which a delayed optical pulse from the delay iε converted to a high-frequency radio wave signal and locked to the next in time transmit pulse. 12. A radar system as in claim 10, in the delay line has a delay equal to that of one transmit pulse. 13. A radar system as in claim 10, in which the means for converting the coupled off portion of the trans¬ mit signal to an optical signal includes a laser and an indirect modulator for modulating the laser beam to form a corresponding optical pulse. 14. A local oscillator generator for a radar sys¬ tem having a transmit pulse generator, a reflected pulse receiver, and received pulse processing apparatus, compriεing: a delay medium; a recirculating delay line interconnected with said delay medium and selectively actuatable to form a closed path; means for coupling a portion of a transmit pulse into the recirculating delay line; and means for coupling the recirculating delayed signal off the delay line to form a pulse local oscillator signal. 15. A local oscillator generator aε in claim 14, in which the delay medium is an optical fiber and the re¬ circulating delay line with delay medium provide a one cycle delay equal to the length of a transmit pulse. 16. A local oscillator generator as in claim 14, in which the delay medium includeε a coaxial tranεmiεεion line. 17. A local oεcillator generator aε in claim 14, in which the delay medium iε SAW delay meanε. 18. A local oscillator generator as in claim 14, in which a switch is interconnected in said recirculating delay line and is selectively actuatable to form a closed path.

Inventors: BRENNEISE CLARK D, BRYAN RICHARD E, DE LA CHAPELLE MICHAEL, BRENNEISE, CLARK, D., BRYAN, RICHARD, E., DE LA CHAPELLE, MICHAEL
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1989
Main classification: G01S7
IPC codes: G01S7