US3896442A

US3896442A - Correlation processor

Info

Publication number: US3896442A
Application number: US392914A
Authority: US
Inventors: John R Heminway; Herbert M Sanborn
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1973-09-04
Filing date: 1973-09-04
Publication date: 1975-07-22
Anticipated expiration: 1992-07-22

Abstract

Each jamming target transmits a broadband noise that is uncorrelated with any other jaming target. The noise that each target transmits is received by a monopulse radar antenna on a missile and is retransmitted to the ground processor by that missile. The ground processor also receives directly the energy from the jamming targets. Use is made of the different signal paths of each jamming target to the missile and to the processor to obtain a correlation function for each target, by delaying the direct signal to the processor and mixing it with the signal from the missile so as to maximize the mixer''s output for a selected target.

Description

Unite States ate Heminway et al.

CORRELATION PROCESSOR Inventors: John R. Heminway, Topsfield;

Herbert M. Sanborn, Framingham, both of Mass.

Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

Filed: Sept. 4, 1973 Appl. No.: 392,914

[52] US. Cl 343/18 E; 235/181; 343/100 CL [51] Int. Cl. G01s 7/36 [58] Field of Search 343/18 E, 100 CL; 235/181 [56] References Cited UNITED STATES PATENTS 3,134,896 5/1964 Briggs 343/100 CL X 3,195,130 7/1965 Adrian 343/100 CL X 3,212,091 10/1965 Bissett et al. 343/100 CL X 3,502,989 3/1970 Honciser .1 343/100 CL X Primary Examiner-T. H. Tubbesing Attorney, Agent, or Firm-Robert P. Gibson; Nathan Edelberg; Robert C. Sims [57] ABSTRACT Each jamming target transmits a broadband noise that is uncorrelated with any other jaming target. The noise that each target transmits is received by a monopulse radar antenna on a missile and is retransmitted to the ground processor by that missile. The ground processor also receives directly the energy from the jamming targets. Use is made of the different signal paths of each jamming target to the missile and to the processor to obtain a correlation function for each target, by delaying the direct signal to the processor and mixing it with the signal from the missile so as to maximize the mixers output for a selected target.

6 Claims, 8 Drawing Figures JAMMING TARGET "I l JAMMING '7 TARGET *2 STEERING COMMANDS TO MISSILE CORRELATOR PROCESSOR PATENTEDJUL 2 2 I975 SHEET JAMMING STEERING COMMANDS TO MISSILE TARGET t I JAMMING TARGET #2 coRRELAToR PRocEssoR COMPUTER FIG. I

I5 ARA FROM MIXER MISSILE T LOCAL 23 24 25 BAND OUTPUT OSCILLATOR PRODUCT PASS DETECTOR DETECTOR F|| TER l8 2| NE HZ MIXER VARIABLE FROM DELAY TARGETS LINE (T) VOLTAGE /|9 CONTROLLED CRYSTAL OSCILLATOR 3 FIG. 3 I: /-JAMMERH /-JAMMER#2 a OUTPUT OUTPUT 2 2 Q '3 .J LU O: a: L o

DELAY (T) FIG. 2

PATENTEDJUL 2 2 ms /l8 /2l QZEZ VARIABLE FROM-# MIXER DELAY v TARGETS LINE VOLTAGE CONTROLLED OSCILLATOR 3D 29 28 f2? I DETECTOR FILTER PRODUCT DELAY DETECTOR Td l V I DIFFERENTIAL/32 MISSILE FIER SUM I AMPU 30' 29' 28' S'GNAL f T y I Q DETECTOR Fl T R PRODUCT I L E DETEcToR RANGE LOOP FIG. 4

VOLTAGE AMPLITUDE (VOLTS) I Td/2 +Td 2 RANGE DELAY FIG. 5

PATENTEDJUL 2 2 I975 SHEET 3 33 f 34 f 35 f 36 SUM a J DELTA DELAY PRODUCT ggg g SIGNAL FRoM DETECTOR FILTER MISSILE 38 39 LOCAL PHASE D.C.

INTEGRATOR oScILLAToR DETEcT c uTPu' SUM 33' 34' 35' 3e' SIGNAL DELAY PRODUCT Egg? FRoM MISSILE T 2 DETECTOR FILTER voLTAGE CONTROLLED oscILLAToR 2] l8 DIRECT SIGNAL VARIABLE FROM DELAY TARGETS FIG. 6

ouTPuT 33' 34' 35' 4o SUM SIGNAL DELAY PRODUCT Q 39 FRoM MISSILE H 2 DETECTOR FILTER LocAL oscILLAToR 2| DIREcT SIGNAL VARIABLE FROM DELAY TARGET LINE voLTAGE CONTROLLED INTEGRAToR A FREQUENCY oScILLAToR DIScRIMINAToR FIG. 7

CORRELATION PROCESSOR SUMMARY OF THE INVENTION The correlator processor has two major inputs. One input comes directly from the jamming targets the other input comes from a missile which is to home in on one of the jamming targets. These inputs are reduced in frequency by mixers and oscillators and fed through a product detector where they are combined. Since the target path from the jamming target directly to the correlator is shorter than the path of the jamming target to the missile and then to the correlator path, a variable delay line is inserted between the direct signals to the product detector. By varying this delay line the output of the product detector can be enhanced to a peak value for a single jammers output. The computer or operator will initially set the variable delay line so that one ofthe jammers output is selected. In order to keep this variable delay line adjusted on the selected jammer as it moves in range relative to the correlator and the missile, a range loop is provided to adjust the variable delay line. The range loop consists essentially of two product detectors with one of the detectors being fed the output of the variable delay line directly and the other detector being fed the output of variable delay line through a further delay means. The sum signals from the antenna of the missile are also fed to the product detector. The outputs of the product detector are fed to a differential amplifier which compares the two outputs and generates an error signal to control the variable delay line.

The mixer for the direct signal from the target is fed a voltage from a voltage controlled crystal oscillator. The frequency output of this oscillator is controlled in order to compensate for the differential doppler frequency difference between the direct and indirect jamming target signal paths. This is done by introducing a delay in the output of the direct signals and sending the output of the product detectors through a narrow bandpass filter to a frequency discriminator whose output voltage will be proportional to the frequency tracking error. This output is fed to the voltage control oscillator to control the frequency output thereof.

Now the boresight error can be generated by the correlator processor by feeding both the sum and the delta signals from the monopulse antenna of the missile through two channels each comprising the series circuit of a mixer, delay line, and product detector. Each product detector is also fed the direct signal from the target. The outputs of the two channels are fed to a phase detector. Difference in the phase will indicate a boresight error with respect to the jamming target being tracked. This error is sent to the computer which sends a signal to a steering antenna which transmits a signal to the missile to cause the missile to align upon the selected jamming target.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the broad overall configuration of the present invention;

FIG. 2 is a waveform showing the correlator output for two targets;

FIG.'3 is a block diagram showing one of the basic fundamental of the invention;

FIG. 4 is a block diagram showing how the invention corrects for range errors;

FIG. 5 is a waveform showing the output of the differential amplifier of FIG. 4;

FIG. 6 is a block diagram showing the basic system for obtaining boresight error;

FIG. 7 is a block diagram showing the basic system for compensation for differential doppler effect; and

FIG. 8 is a block diagram showing the complete correlator system.

DESCRIPTION OF THE PREFERRED EMBODIMENT The missile-target-processor geometry for the present invention is shown in FIG. 1, for two jamming targets. Each target transmits a broadband noise that is not correlated with the noise radiated by any other jamming target or targets. Two input signals are applied to the signal processor 5. One of the input signals to the processor is the jamming noise received by the missiles seeker monopulse antenna 7 from each target. This signal is retransmitted by the missile to the ground receiver antenna 9. This input signal is called the indirect input signal to the processor and consists of two signals: the sum and the sum plus J Delta, The other input signal to the processor is the energy received directly from each jamming target received by antenna 11 and is called the direct input signal to the processor.

From FIG. 1, it can be seen that the indirect input signals to the processor undergo a larger propagation delay than the direct input signal. That is, for target I, the indirect path signal'undergoes a propagation delay proportional to the sum of the missile-to-target path length L and the missile-to-receiver path length L while the direct path signal undergoes a propagation delay proportional to the target-to-receiver path length L The sum of the indirect signal path lengths (1 L is greater than the direct signal path length (L Correspondingly for target 2, the sum of the missile-to-target path length L, and the missile-to-receiver path length L is greater than the target-to-receiver path length L3 By placing a variable delay line in the signal processing of the direct path signal and cross-correlating that delayed direct path signal with the indirect path signal, an output signal voltage vs delay time T in the direct path signal similar to that shown in FIG. 2 is obtained. The details are set forth later. The peak of the crosscorrelation function obtained for target I, would occur when the delay placed in the direct path signal T was equal to The peak of the cross-correlation function obtained for target 2, would occur when the delay time placed in the direct path signal, T was equal to C speed of light. A plurality of targets can be discriminated using this technique for the signal processing through their differences in differential propagation delay time.

The complete correlation signal processor is made up of three functions; namely, range (delay) tracking,

doppler tracking and boresight angle processing. These three functions will be explained individually using FIGS. 3-7. Then an explanation of how the functions are combined will be given using FIG. 8.

A functional block diagram for the signal processing needed to obtain the correlation output signal shown in FIG. 2 is shown in FIG. 3. FIG. 3 shows the signal received from the missile is applied to a mixer where it is frequency translated to a lower IF frequency determined by the local oscillator 16. The signal received directly from the targets is also applied to a mixer 18 and frequency translated to a lower IF frequency determined by the voltage controlled crystal oscillator (VCXO) l9 and then applied to variable delay line 21. The output signal from the variable delay line is multiplied or mixed by product detector 23 with the frequency translated signal from the missile. The mixer output is applied to a narrow band pass filter 24 whose output is applied to detector 25. By observing the detector output voltage as a function of delay time in the processor, a waveform like that shown in FIG. 2 will be produced.

After target discrimination is accomplished, one of the targets may be selected for the missile to home on. The selection process is completed by setting the variable delay line in the processor such that the crosscorrelation function of the selected target is peaked at the processor output. To maintain this correlation peak at the processor output, the differential time delay between the direct and indirect path signals for the selected target is tracked within the signal processor using the range tracking mechanization as shown in FIG. 4.

The variable delay line 21 plus the blocks shown outlined in FIG. 4 make up a range (delay) tracking loop. This loop automatically positions variable delay line 21 to the proper position to maximize the selected target signal. One side of the loop is composed of a short delay line 27 whose delay is Td seconds. The delay line is followed by a product detector 28, bandpass filter 29, and an envelope detector 30. The other side is identical except delay Td is omitted. Therefore, at the differential amplifier input, two correlations displaced by Td seconds are available. These are subtracted in differential amplifier 32 forming an early-late range (delay) error signal as shown in FIG. 5. When the output voltage from the differential amplifier 32 is zero voltage, the two cross-correlation signals obtained for the selected target are identically displaced from the correlation peak of the selected target. That is, assuming the correlation peak occurs at T seconds, one correlation signal obtained in the range tracking loop would be at T Td/2 signals, and the other correlation signal obtained in the range tracking loop would be at T Td/Z seconds. When the setting of the variable delay line in the signal processor is incorrect, so that the delay time was not set at T seconds, the output voltage from differential amplifier 32 in the range tracking loop would not be zero voltage but would be a voltage proportional to the error in the delay time setting in the processor. The error voltage developed in the range tracking loop as a function of the delay time setting error is shown in FIG. 5. This error voltage developed is then used to command variable delay line 21 in the processor to its proper setting.

Guidance information for the missile about a particular jamming target in the presence of several jamming targets is developed by processing the monopulse signals developed in the missiles seeker antenna as shown in FIG. 6. Two identical channels are shown each having a mixer 33, small delay means 34, product detector 35, and bandpass filter 36. The input to one channel is the missile sum (2) signal, while the other channel receives the sum J delta (2 JA) signal. Both product detectors 35 and 35' receive input from the variable delay line 21. Phase detector 38 in FIG. 6 compares the phase of the E to the 2 .IA correlation signal and applies this to integrator 39. The monopulse signals developed in the missiles seeker antenna are the E and 2 JA signals. For each jamming signal received by the missiles seeker antenna, the boresight heading error to the jamming target is proportional to the phase difference between the E and the Z .IA signals. The phase detector output voltage is a voltage proportional to the targets boresight error.

To minimize angle measurement noise caused by the presence of other jamming targets, the narrowband filters 36 and 36' before the phase detector must be kept as narrow as possible. To accomplish this, the differential doppler frequency between the direct and indirect path signals is tracked within the doppler tracking loop of the processor. The mechanization for tracking the differential doppler frequency is shown in FIG. 7. As before, the indirect path signal from the missile is frequency translated to an IF frequency by mixing it with the crystal oscillator. The mixer output is delayed by Td/2 seconds by delay means 34. The direct path input signal to the processor is frequency shifted by mixing it with a voltage controlled crystal oscillator 19. Mixer 18 output is applied to the variable delay line whose delay is set such that the correlation function obtained for the particular target is peaked at the processor output. The delayed direct path signal is then mixed with the indirect path signal and applied to a bandpass filter 40 as shown in FIG. 7. The filter output is then applied to a frequency discriminator 41 whose output voltage is proportional to the doppler frequency tracking error. The discriminator output is integrated by integrator 43 and applied to the voltage controlled oscillator 19 to correct same.

FIG. 8 indicates how the sub-systems, previously described in detail in FIGS. 1-7 are connected together to form the complete correlation guidance system. The desired output is the boresight error output. However, before reliable information is present here, the doppler and range loop must be used to select one of the targets to be designated as the tracked target. Once the doppler and range loop are locked on that target, the signal out of the boresight channel is proportional to angle of the missile seeker off boresight.

A step by step operational description of the overall correlator mechanism shown in FIG. 8 is given:

1. Initially, the correlation processor is in a standby mode with no target information being processed.

2. Upon detection by the radar of enemy ECM activity, the SAM-D weapons system computer 47 supplies the correlator with

estimates

45 and 46 of the jammers range and doppler position. These inputs are shown in FIG. 8 to the delay line 21 (T) and to the voltage controlled crystal oscillator 19.

3. The doppler estimate 46 to the voltage controlled crystal oscillator (VCXO) 19 positions the jamming signal within the pass-band of the correlators bandpass filters.

4. The range estimate 45 positions the variable delay line 21 such that the jamming signal is within the range of acquisition of the range track loop.

5. The doppler loop explained previously is shown outlined in FIG. 8. The input frequency from product detector is filtered by bandpass filter 40 and applied to discriminator 41. If the frequency estimate had error, discriminator 41 forms an error voltage into integrator 43 which re-positions VCXO 19 to exactly place the target frequency for maximum signal. Within 0.2 to 0.3 seconds the doppler loop has achieved final position and the computer supplied estimate 46 is no longer used.

6. At the same time, the range loop, outlined in FIG. 8, is receiving signal from the variable delay line and correlating it with signal from the missile in the pair of product detectors 28 and 28' and filters 29 and 29'. If the range estimate was slightly in error, one of these channels of the pair will have a stronger output and will develop a signal to variable delay line 21 to correct the error. Within 0.2 to 0.3 seconds the error is negligible, and the computer supplied estimate 45 is not needed.

7. Should the jammer now move in range or doppler, the loops develop an error signal to re-position the delay line and VCXO to always maximize the signal of the jammer being tracked.

8. Once steady track has been established (0.2 to 0.3 seconds) delay line 21 and VCXO 19 are positioned such that the signals from the

product detector

35 and 35 in the boresight angle section are maximized. These signals are filtered in band pass filters 36 and 36' and then applied to the two inputs of phase detector 38.

9. The phase detector output of the boresight angle section is integrated by integrator 39. The phase detector output is a voltage proportional to the phase difference in the signals which (as is well known in the art) is in turn proportional to boresight angle, as the signals originate in a monopulse antenna 7 (FIG. 1 Integrator 39 output is the desired signal, since it contains information showing whether the missile is aiming at the jamming target.

10. The boresight error signal is fed to the weapon system computer 47 shown in FIG. 1. Any error in aiming produces a voltage at the boresight channel output which is used by the weapon system computer to generate new steering commands to the missile by way of antenna 49 to correct the error.

We claim:

1. Correlator system comprising at least one signal generator means; first, second, and third receiving means; said first receiving means being located spacially from said second and third receiving means; transmitting means on said first receiving means; said first receiving means receiving signals from said signal generating means and transmitting them to said second receiving means; said third receiving means receiving the signals generated by said signal generating means; a variable delay line; first mixer means having first and second inputs and an output; an output of said third receiving means connected through said variable delay line to one of the inputs of said mixer means; an output of said second receiving means being connected to the other input of said mixer means; said variable delay line having a controlled input; further comprising a range control loop connected between the output of said variable delay line and said controlled input; said range control loop comprising first and second channels;

each channel having a product detector which has first and second inputs; each product detector having the signal from said second receiving means connected to the first input; a delay line connected between the output of the variable delay line and the other input of said product detector in said first channel; the other input of the product detector in said second channel being connected directly to the output of said variable delay line; a differential amplifer having first and second inputs and an output; the outputs of said product detectors being connected to different inputs of said differential amplifier; and the output of the differential amplifier being connected to the controlled input of said variable delay line.

2. Correlator system comprising at least one signal generator means; first, second, and third receiving means; said first receiving means being located spacially from said second and third receiving means; transmitting means on said first receiving means; said first receiving means receiving signals from said signal generating means and transmitting them to said second receiving means; said third receiving means receiving the signals generated by said signal generating means; a variable delay line; first mixer means having first and second inputs and an output; an output of said third receiving means connected through said variable delay line to one of the inputs of said mixer means; an output of said second receiving means being connected to the other input of said mixer means; said first receiving means containing a monopulse radar receiver; said first receiving means having first and second outputs which are transmitted to said second receiving means; first and second channels; said second receiving means sending the first and second outputs of said first receiving means to said first and second channels respectively; said first channel being the connection to said first mixer means; a second mixer means having two inputs and an output; the first input of said second mixer means being connected to the second output of said second receiving means; said variable delay line being connected to the second input of said second mixer means; a phase detector having two inputs and an output; and the output of said first and second mixer means being connected to different inputs of said phase detector.

3. A system as set forth in claim 2 wherein first and second delay lines are connected between said first and second outputs of said second receiving means and the mixer means.

4. A system as set forth in claim 3 wherein said variable delay line has a controlled input; and further comprising a range control loop connected between the output of said variable delay line and its controlled input.

5. A system as set forth in claim 4 further comprising third, fourth, and fifth mixer means; a local oscillator; said third, fourth, and fifth mixer means each having two inputs and an output; the first output of said second receiving means being connected to one input of said third mixer means; the second output of said second receiving means being connected to one input of said fourth mixer means; said local oscillator being connected to the other input of each of the said third and fourth mixer means; the output of said third mixer means being connected to an input of said first mixer means; the output of said fourth mixer means being connected to an input of said second mixer means; a

voltage control oscillator having an output and a con- 6. A system as set forth in claim wherein said signal trolled input; abandpass filter; a discriminator; an mtegenerators Consist f a plurality f jamming target grator; said bandpass filter, discriminator, and integrator being connected in a series circuit between the output of said first mixer means and the controlled input 5 of said voltage control oscillator whereby the output of 531d target meanssaid first mixer is maximized.

means; and said first receiving means is a missile means having a monopulse antenna to receive the signals from

Claims

1. Correlator system comprising at least one signal generator means; first, second, and third receiving means; said first receiving means being located spacially from said second and third receiving means; transmitting means on said first receiving means; said first receiving means receiving signals from said signal generating means and transmitting them to said second receiving means; said third receiving means receiving the signals generated by said signal generating means; a variable delay line; first mixer means having first and second inputs and an output; an output of said third receiving means connected through said variable delay line to one of the inputs of said mixer means; an output of said second receiving means being connected to the other input of said mixer means; said variable delay line having a controlled input; further comprising a range control loop connected between the output of said variable delay line and said controlled input; said range control loop comprising first and second channels; each channel having a product detector which has first and second inputs; each product detector having the signal from said second receiving means connected to the first input; a delay line connected between the output of the variable delay line and the other input of said product detector in said first channel; the other input of the product detector in said second channel being connected directly to the output of said variable delay line; a differential amplifer having first and second inputs and an output; the outputs of said product detectors being connected to different inputs of said differential amplifier; and the output of the differential amplifier being connected to the controlled input of said variable delay line.

5. A system as set forth in claim 4 further comprising third, fourth, and fifth mixer means; a local oscillator; said third, fourth, and fifth mixer means each having two inputs and an output; the first output of said second receiving means being connected to one input of said third mixer means; the second output of said second receiving means being connected to one input of said fourth mixer means; said local oscillator being connected to the other input of each of the said third and fourth mixer means; the output of said third mixer means being connected to an input of said first mixer means; the output of said fourth mixer means being connected to an input of said second mixer means; a voltage control oscillator having an output and a controlled input; a bandpass filter; a discriminator; an integrator; said bandpass filter, discriminator, and integrator being connected in a series circuit between the output of said first mixer means and the controlled input of said voltage control oscillator whereby the output of said first mixer is maximized.

6. A system as set forth in claim 5 wherein said signal generators consist of a plurality of jamming target means; and said first receiving means is a missile means having a monopulse antenna to receive the signals from said target means.