DE3105140A1 - Fast-response adaptive interference filter - Google Patents

Abstract

Method and device for eliminating the effect of interference in a coherent pulsed Doppler radar system in which the mid-frequency of the interference is defined and the heterodyne oscillator frequency is periodically shifted in such a way that the interference from the Doppler bandpass filter is shifted out so that only targets can appear in this bandpass filter.

Description

description

Fast Response Adaptive Noise Filter The invention relates to an apparatus and a method for. Suppression of interference in the case of coherent pulsed on-board Doppler radar systems according to the preamble of claim 1 or 3.

The invention relates in particular to a pulsed M2I Doppler radar system, which is housed in a rocket and in search mode from a launch vehicle, about an airplane has to work out. The invention deals with problems by antennas with a relatively large beam width, by fixed targets, by appear mobile due to the launch vehicle speed, and through extremely large range and speed limits are caused.

Radars housed in missiles used to detect a The target and used to guide the missile are often of the coherent type designed pulsed Doppler radar system. You generally work in the search business, to establish a goal, to adjust to the goal and to approach it to. follow.

The search process is made more difficult by antennas with relatively large beam width, which is due to the limitation of the total diameter of Antennas on the cross-section of the missile results from the occurrence of interference signals from the background where a target appears between the missile and the ground or sea, due to the possible occurrence of an extremely rapid closing speed (Closing rate) between target and missile if the target and missile are in or near opposite Direction move, and by the high probability that target return frequencies are very close to large parasitic return signals, thus making the determination actual goals becomes even more difficult. Since the missile carrying the search radar moved, all ground targets appear movable and have a Doppler echo that is the same the relative speed between the point on which the moving radar device is directed, and the solid ground is. Since the beam width of the radar antenna is not zero, there are very different angles to different proportions of the disturbance even within the main ray. Thus the disturbance is related to the frequency distributed. Such a distribution is minimal if the radar antenna is along the is aligned with the missile's longitudinal axis. While the frequency spread can be reduced to a minimum under these conditions; maximum frequency or Doppler shift. Under this condition, the spectral width becomes of the interference return signal is a minimum, while the Doppler shift is maximum will.

If the antenna is aligned laterally, then the Doppler shift is smaller, but the spectral spread is increased.

Furthermore, the signal strength varies with the orientation of the ground too the radar. If the radar beam is aimed at the ground and falls on it at an angle of 90 degrees, i.e. perpendicular to, then the reflected energy generally be maximal. The radar energy hits under a little grazing Angle, then the retroreflective or reflection power is generally also reduced.

Another aspect of the trajectory is the reflected power influenced is the area. It is understandable that the performance reflected inversely to the area changes. There are many combinations of the above Conditions that lead to a stronger disturbance return from the ground than the signal from the destination is. The likelihood that this disorder is within of the bandpass occurring in the radar receiver circuits is high as the radar device himself moved. When the dynamic range of the radar receiver receives a signal from originates from a very small target, not at the same time as a signal from one can process very large interference radiation, then the interference occupies the radar receiver and drive the target signal below the threshold, making it unlikely that the radar is responding to the target. As the radar search filter groups in general Are developed in such a way that they are as wide as possible in order to be at the same time as possible Receiving a lot of target returns is the likelihood of large interference signals lie within the band pass, even greater.

Attempts to solve these problems in known radar systems have been limited focus on measuring the disturbance over a longer period of time and the result to average.

Such a period of time can be on the order of one to two Seconds lie. This averaging cannot be done in the search part of a floor guidance system used, because during this time, due to the extremely high closing speeds, those when the missile is moving in the opposite direction. and goal the target of the missile may already have escaped before a latch or setting could be done. Long-term mean educational systems often use an automatic one Post-voting loop (AFN), where the loop a bandstop area locked, this moved into the fault area and in this for a longer period There is time left to avoid an otherwise very disturbed system.

In the case of a targeting system for a missile or similar projectile, such a long period of time does not exist. Because the antenna beam width is large large frequency spreads are also determined, and a device which constantly blocks all interference frequencies, would also almost all possible target frequencies disguise.

The flight profile in which the missile radar searches for a target between the missile and the ground searches, comprises a small percentage of the total target volume, but without measures to reduce the disruptive effect, the attachment system is under unable to work under these conditions.

The above and other problems with known systems will be addressed solved according to the invention by a rapidly responding circuit that is at a is looking for the reciprocal of the range of orders given (typically 1 to 10 milliseconds), reducing the frequency dispersion of the return signal limited and the effective beam width of the antenna is narrowed and thereby determined will determine whether a target is within the scope, and if not, it is determined whether a threshold value is exceeded within the area section has taken place or not, in the latter case the next area segment is called, or it is found that within the area section Faults have exceeded the threshold. Since in the latter case there is no possibility is to determine whether the fault or the faults a target mask or not, the superposition oscillatorfreguenæ by an amount shifted, which is adapted to the frequency point of the disturbance return, see above that the latter is now brought out of the Doppler bandpass in the special area section will. Then the area is clipped again during a new search process checked for the presence of a target.

The disturbance was placed in a stop band so that the receiver sensitivity can be increased for the possible goal. This makes it possible to increase the dynamic range of the recipient considerably.

If no target is found on the second attempt, it will be ruSt System to the next area section, and the process is repeated. at When a target is established, the radar system adjusts itself to the target and tracks it it. At one of the outputs of the system's digital filter, information with respect to the bandwidth of the interference emitted, so that depending on the radar system the disturbance in one of two different restricted areas (in this exemplary embodiment) can shift, depending on whether the disorder meets the criterion of a narrow or wide bandwidth met.

The object of the invention is therefore to provide a section within a very short period of time (within a radar search time) relative to the radar system and seek missile operating speed and determine if there is a malfunction and / or a target is present in this section and if a fault occurs to effect filtering of the disturbance so that the target can be determined.

Furthermore, either searched for a destination or a disturbance and after finding a disturbance made a rapid change in the local oscillator frequency, the interference with a blocking filter before the start of the next search period of the radar system 'thereby reducing the sensitivity of the radar system to a neighboring target is improved.

The invention also brings extremely rapid response to the radar system on a moving target that is between a projectile and a due to the Background noise generated by the projectile velocity.

Further features and advantages of the invention emerge from the patent claims and the following description with reference to the drawings. 1 shows a Representation of a missile carrying the radar system and its relationship to one possible target and the ground causing the background interference Fig. 2 a graphical representation of the M2I transmission spectrum (at baseband) of a coherent pulsed Doppler radar system Fig. 3 is a graphic representation of the M2I spectrum FIG. 2, however, expanded with respect to the frequency base. FIG. 4 shows a block diagram of a typical radar detection channel Fig. 5 is a block diagram of a fast response Noise filter of the invention Figure 6 is a more detailed block diagram of a part of FIG. 5; FIG. 7 shows a time diagram for parts of FIG. 5 and FIG. 8 is a more detailed block diagram of other parts of FIG.

Reference is first made to FIG. 1 which shows a missile 10 shows that is the carrier of a search / tracking radar system, not shown. Of the Missile 10 has a speed vector 12 and 12 directed in the direction of flight a radar search angle 14. Since the housing for the rocket radar antenna has a small. Diameter, the radar beam width, as indicated at 16, is proportionate great. The beam falls on the ground or a surface of water 18. The reflections from the ground 18 result due to the speed of the missile 10 in the direction 12 disruptive reflections.

A maximum Doppler shift is caused by floor reflections of of the surface 20 while there is a minimal Doppler shift from the bottom surface 22 results.

Thus, the Doppler echo from the floor 18 varies clearly within the Beam width with the doubler frequency. Search area excerpts are given in Figure 1, how they appear on the ground to the radar system. The extent of each search area segment depends on the number of range gates in a given radar system and on the width of each gate. Typically, each section would be made up of several at the same time area gates to be searched exist.

Figure 2 graphically depicts the relationship between echo and frequency the Doppler reflections from targets.

Radar passbands are indicated at 20, 22 and 24.

Areas 26, 28 and 30 referring to the pulse repetition rate (PRF) FP, 2 (FP), 3 (FP), represent areas that fall at blind speeds or ambiguous speeds. In each case, of course, are Doppler frequencies the result of the difference between the speed of the radar system and the Target.

This spectrum represents the frequency content produced by the radar system is received in each range gate. Since the radar system is related to the target moved, and especially with respect to background disturbance, appear towards the ground non-moving objects as moving with respect to the radar system. The area 32, f is an area with the Doppler value O and represents a blind area with respect to Targets moving at the same speed as the radar system.

FIG. 3 shows an enlarged view of the curve according to FIG. The signal frequency curve 40 is the interference curve as received by the radar system would if it didn't move. The perturbation curve 42 is typical Result of the moving radar system. An actual goal could be a typical one Generate Doppler echo 44. The basic problem that inevitably arises for a scoundrel of this kind adheres is that the dynamic range of the system is not large enough linearly to both the disturbance 42 and at the same time to target signals 44 to adjust. The object of the present invention is that the presence of a Detect fault 42 and then move it to one of the restricted areas, which are present in the radar filter system so that only the target 40 is within the Doppler pass band 46 appears. By eliminating the disturbance 42 from the Passband shows that the dynamic range of the radar system is no longer both the interference signal 42 as on the target signal 44 must include, but only needs to be adjusted to the target signal. If the fault signal 42 in the If the zero frequency cut-off range is shifted at 48, the target signal 44 also becomes frequency-wise moved downward, but appears within the Doppler passband 46.

FIG. 4 shows a block diagram of a typical radar search channel as is generally known to the person skilled in the art.

Figure 5 shows a more detailed block diagram of the quick response Noise filter according to the invention It can be seen that part of the in Figure 5 system shown is the same or very similar to the known system according to FIG 4 is. It should be noted, however, that the Doppler band pass filter 50 of FIG 4 in Figure 5 has been replaced by the Doppler band pass filter 52 with a variable Broad. Although a filter for two modes (wide or narrow) in this embodiment has been disclosed for simplicity, it will be clear to those skilled in the art that filters with many additional operating modes, also continuously changeable in the trade are available and can be used.

The threshold value device 54 and parts of the failure / destination decision device 56 are known and, for example, US Pat. No. 4,119,966 "Clutter Discriminating Appartus" For Use With Pulsed Doppler Radar Systems and the Linke "by Bouvier et al. An interference frequency measuring device 58 is used to measure both the center frequency as well as the bandwidth of the disturbance. The interference frequency-width arbitrator 60 receives a signal 62 from interference frequency measuring device 58 and determines whether the interference bandwidth is relatively wide or narrow. The interference, frequency-width device 60 then sets the Doppler passband Variable width filter 52 under control on control line 64 to the wide or narrow one configuration a. The interference frequency measuring device 58 also generates the center frequency of the Disturbance which is passed to a frequency offset calculation unit 66. These serves to control the output signal 68 from the superimposition or local oscillator 70, which in turn is fed to a down converter 72. Another output signal 74 is fed from the local oscillator 70 to the transmitter 76 in order to maintain the phase coherence the local oscillator 70 and the transmitter 76 to ensure. The output signal 74 from local oscillator 70 is not used by the frequency offset calculator 66 influences, but always remains on a fixed frequency.

Figure 6 illustrates the failure / destination decision device 56 in more detail. For the sake of simplicity, an essential gimmick of the failure / destination decision device becomes used in U.S. Patent 4,119,966. In this embodiment there is an additional NUND element is provided, which also activates said device so that this outputs a disturbance timing pulse to the disturbance frequency measuring device 58. The temporal relationship of the interference / timing pulse to the multiplexer clock (in the processing unit of FIG. 6) is shown in FIG. The interference / timing pulse (Figure 7c) begins; when the failure / destination decider 56 is failures while they are being sampled, those representing the target echoes at different frequencies recognizes bundled data. This all happens during the multiplexer scanning period (Figure 7b) i.e. if data relating to the target or disturbance amplitudes at different Frequencies are read out from the processing unit. The interference / timing pulse ends when the failure / destination decider 56 is not a failure more scans. In this way a timing pulse is generated that is unique to a Frequency point for the disturbance is correlated and its duration is the frequency width the disorder represents.

FIG. 8 shows a more detailed block diagram of the interference frequency measuring device , 58, the interference frequency width device 60 and the frequency offset arithmetic unit 66. The operation of the interference frequency measuring device 58 is as follows: The Multiplexer clock drives two counters 85 and 86 and both counters start from the beginning each multiplexer sampling period, i.e. at the beginning of each readout time of the processing unit.

(This is an example of a digital realization.

Other digital or analogue implementations are possible). Of the first counter 85 for low interference frequency listens at the beginning, i.e. on the rising edge of the fault timing pulse to count and the state of the counter 85 sets then the frequency point 9 represents where the frequency of the disturbance begins. The second payer 86 for high disturbance frequencies stops counting at the end of the disturbance timing pulse. The state of this counter 86 represents the frequency point a where the disturbance stops.

The operation of the interference frequency width device 60 is like follows: The frequency point for the low frequency of the disturbance is taken from the one the high frequency is subtracted in a summing device 88, so that the frequency width the disturbance SH-fL results. This value is then with preset limits in a comparator 89 is compared for a still permissible disturbance width and it is decided whether the disturbance is "narrow" or "early". The corresponding noise filter is in that Variable width Doppler bandpass filter selected.

The operation of the frequency offset arithmetic unit 66 is as follows: The measured value for the lower frequency of the disturbance is added in a summing device 91 to half the width of the disturbance, so that the center frequency of the disturbance is obtained. this means Disturbance = fL = Ii ~ This disturbance frequency is then converted into an analog voltage in a D / A converter 93 in this embodiment and summed in a summing device 95 with the normal voltage containing no disturbance which controls the local oscillator. If there is a disturbance and a new area is to be searched, then the oscillator is offset by the disturbance frequency, whereby the center of the disturbance spectrum is shifted into the Doppler filter.

When operating the circuit according to FIG. 5, radar reflections are generated fed to a plurality of range gates 78.

After processing this signal, through range gates 78 and the associated circuit determines the failure / destination decision unit 56 whether either a disturbance or a target is present within the gate area 78.

If there is a target, the radar system positions itself on the target and tracks it without any further measures for the search processing of the signal. If the device 56 does not determine a target (output signal 80), but one Disturbance, then the disturbance frequency measuring device 58 becomes the disturbance frequency width device 60 and the frequency offset arithmetic unit 66 used to convert the interference into a Move the restricted area, the in bandwidth through the device 60 is checked. This is achieved by shifting the output signal 68 of local oscillator 70, whereby the center frequency of the disturbance in the selected Restricted area is relocated. This shift occurs from the next radar search period. The area detail is now once again made by the disruption / destination decision device 56 checked for the presence of a target.

If a target occurs at output 80, the Ra radar system switches enters a tracking mode and continues tracking the target. Will If no target echo is received at output 80, then the radar system goes to the next one Section over and repeat the entire procedure. It is not a frequency locking loop or AFN loop, which is required in known devices, and thus the response time is close to the reciprocal of the locking bandwidth, namely typically one to ten milliseconds.

Also, the filtered interference bandwidth is only so large that the interference is locked in every area, and not in all areas, as would be necessary with the known long-term tracking methods. The Bachmane realizes that minimizing the frequency range at any point in time must be filtered out, the frequency range brings to a maximum where goals may occur, so that the likelihood of improved targeting is improved in a disturbance environment.

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Claims (4)

P a t e n t a n s p r ü c h e 5 procedures for suppressing interference, around neighboring desired goals within a useful dynamic range of a coherent one to bring pulsed on-board Doppler radar system, one with a local oscillator equipped receiver has no sign by a) Establishing the presence of disturbances in a specific area section b) Measure the center frequency of the interference during the radar search time it c) changing the output frequency of the local oscillator by a value that is equal to the center frequency of the interference before starting the next radar search d) Establishing the Presence of a target (44) within the area segment, and e) repeating of steps a), b), c) and d) in successive area sections to a target (44) has been established, each slice having a different adaptive Has fault lockout section. 2. The method according to claim 1, g e k e n n z e i c h n e t by the the following steps: f) measuring the bandwidth of the disturbances (42); g) Setting the Bandwidth of a notch filter (52) with a variable bandwidth corresponding to the measured Bandwidth; h) Sending the interference (42) through the blocking filter (52) for masking the effect of the interference on the radar receiver. 3. Coherent pulsed M2I-30rd Doppler radar system, g e -k e n n z e i c h n e t by a circuit for determining frequency properties of a Disturbance signal in a section of the area during the normal search time of the radar, a circuit for shifting the frequency of a local oscillator (70) depending on the determination of the frequency properties of the disturbances (42), one Sampling circuit for detecting a target signal (44) in the area segment after shifting the frequency of the local oscillator (70) to the interference signal To bring (42) into a filter area (52), a circuit for successive shifting of the scanning circuit (78) to at least one subsequent one Area section in which no fault signal (42) was detected, and determining of the target signal (44) in the first area segment and a tracking circuit (56) for tracking an established target (44) and blocking further Operation of the sampling circuit (78) and the shifting circuit (66) dependent on the detection of the target signal (44). 4. Radar system according to claim 3, characterized in that g e k e n n -z e i c h n e t that the sampling circuit (78) further comprises a circuit (58) for measuring a bandwidth of the interference signal (42) and a circuit for adjusting the bandwidth of a Notch filter (52) depending on the measured bandwidth of the interference signal (42) wherein the interference signal 42) is sent through the notch filter (52), insensitive to the response of a radar receiver to the interference signal (42) close.