DE3502398A1 - METHOD AND DEVICE FOR MEASURING NOISE AND INTERFERENCE SIGNALS WITH A RADAR RECEIVER - Google Patents

Description

The invention is concerned with coherent pulse Doppler Radar systems and relates in particular to a method and a device for measuring the noise and interference signals at the receiver of such a radar system.

The basics of Doppler radar systems for the visual representation of moving targets, for example airplanes, are described in detail in the relevant literature. Can be referred z. B. on the textbook by MH SKOLNIK "Radar Handbook", 1970, publisher McGraw Hill. The Doppler radar systems for the visual display of moving targets must work satisfactorily even in the event of interference from unwanted radar signals. Undesirable radar signals include, in particular, the signals reflected on the floor and on buildings, echo signals from ground vehicles, signals reflected on precipitation, the so-called "angel" signals and the like. These unwanted radar signals correspond to objects moving at low or moderate speeds, ie at a speed of less than 30 ms ^-1 . Radar systems of this type must remain operational even in hostile surroundings if a possible attacker uses passive countermeasures, for example dowels, or if electronic countermeasures are used which consist in the emission of continuous or time-chopped electromagnetic interference signals. Radar systems may also experience electromagnetic interference, such as interference caused by other devices operating nearby.

In the processing units for the output signals of the Radar receiver is the radial radar range in distance fields divided, whose unit width essentially the range resolving power for those of the signals emitted by the antenna. The interference ratio in a given direction the antenna can as a ratio of the number of disturbed distance fields defined for the number of distance fields considered will. This interference ratio strives in the case of continuous interference signals at high levels One, in the case of a medium thermal one Noise level exceeding noise level.

The device for detection is in a radar system of movable echoes arranged on the output side of the receiver. It is then necessary to determine the mean of the Noise signal level of the thermal generated by the receiver Knowing noise exactly because of the size of this Parameters sets the detection device's sensitivity limit for moving goals. The exact knowledge of the mean value of the noise signal level controlling the false alarm rate of the system; it  the problem thus arises, these noise signals are thermal Measuring origin in the presence of disturbances, caused by unwanted radar signals on the one hand and possible Interference signals are caused on the other hand.

The invention is based, in the absence of the task of interference signals an exact measurement of the level of the thermal To enable noise signals from a radar receiver and an assessment in the presence of interference signals and evaluation of the characteristic features of this To enable interference signals to the encountered interference situation specify.

To achieve this object, the invention proposes a method for measuring the noise signals of a coherent pulse radar receiver, in which use is made of a Doppler filter bank, the central element of which provides sequences of M independent samples A _{m of} the noise, where the ordinal number m of a noise sample lies between the limits 0 and ( M -1) corresponding to M distance fields, and the method consists in the following operations being carried out in the course of each sequence of M noise samples:

• a) the M noise samples A _m , which are available at the output of the central element of the Doppler filter bank, are input in succession;
• b) the quantities A _m are compared with a threshold value K ₁ · μ _{n -1} -, which is proportional to the output data quantity μ _{n -1} , which represents the mean of the quantities A _{m of} the preceding sequence ( n -1);
• c) the size A _m is preserved if its value is less than the threshold K ₁ · m _{n -1} ;
• d) The value A _m is replaced by the value n -1, and a counter is incremented by one if the quantity A _m exceeds the threshold value K ₁ · μ _{n -1} ;
• e) the size A _m is summed up with the sum of the m samples A _m already obtained;
• f) the subsequent sample A _{m +1} is entered if the atomic number m is less than M -1;
• g) if m = M -1, the value becomes calculated
  and comparing the value L of the number of overshoots with a predetermined reference variable Lo , which depends on the permissible interference ratio;
• h) if the value L is smaller than the reference value Lo , the data size μ _n = μ _{c is} output;
• i) if the value L of the number of overshoots is greater than the reference value Lo , the value μ c is replaced by a predetermined value μ o , the data size μ n = μ o and a complementary data size are output, this indicating complementary data size, that the value L is greater than the threshold value Lo and consequently a large number of samples A _m result from an interference signal.

The invention also provides a device for measurement the interference or noise signals of a coherent pulse Radar receiver created in which the above Procedure applies. The invention Measuring device comprises two mutually coupled measuring channels, their common input to a Doppler filter is connected, which centers on a frequency which is substantially equal to half the repetition frequency of the radar system. The first measuring channel essentially contains means for calculating the mean value  of the noise signal level, while the second measuring channel Contains means for measuring the interference ratio. This measuring device works according to the above Method.

Further advantages and features of the invention result from the following description of exemplary embodiments and from the drawing to which reference is made. In the drawing shows:

Figure 1 shows the flat detection zone of a coherent pulse radar system.

Fig. 2 is a matrix of distance fields of this radar system;

Figure 3 is a matrix of Doppler-distance / speed fields at the output of the Doppler filter bank.

Fig. 4 is a block diagram of a radar receiver containing a Doppler filter bank;

Figure 5 shows a typical amplitude-frequency response curve of the central element of the Doppler filter bank.

Fig. 6 is a block diagram of an embodiment of a device for measuring the noise and interference signals according to the invention;

Fig. 7 is a timing diagram of the sequence control signals for the measuring device;

Fig. 8 is an illustration to illustrate the operation of the measuring device;

Fig. 9 is an alternative embodiment of the measuring device;

Fig. 10 shows a further embodiment of the measuring device; and

Fig. 11 is a flowchart which illustrates the different applied in the present process operations.

The planar detection zone of a radar system shown in Fig. 1 has a maximum detection _distance set to a value R _max such that R _{max is} equal to Tr · c / 2 or less than this value, where Tr is the repetition period of that from the antenna radiated pulse signals and c is the propagation speed of the electromagnetic waves. This distance R _max is divided into M distance fields or windows of the elementary duration R , which is equal to δ c / 2, where δ is the duration corresponding to the range resolution of the emitted signals. In a coherent radar system, the interval of coherent processing (CIP as an abbreviation for coherent interval processing) contains a number P of successively transmitted pulses with the same repetition period and the same wavelength. The number n of CIT in the flat detection zone is given by the following relationship:

where Fr = 1 / Tr
Ta the rotation period of the antenna
and M is the number of range fields, which in turn is equal to R max / Δ R.

The angular movement R of the antenna during an interval of coherent processing (CIT) is given by the following relationship:

and this angular movement R is generally smaller than the nominal width R _{o of} the antenna bundle. Depending on the radar system under consideration, it can continuously emit the same pulses or emit bursts of pulses of a number P of pulses, the repetition frequency of these pulses changing from one burst to the next, or the repetition frequency is maintained and the frequency Fo of the carrier oscillation from one burst to the next changed to at least partially avoid the occurrence of undetected speeds.

Figure 2 shows a matrix of range fields for an interval of coherent processing (CIT) to P pulses. Along the axis τ , which corresponds to the radar time axis, M columns of range windows are arranged, which are denoted by R to ( M -1), and along the t time axis, P rows of range windows are arranged, which are R to ( P - 1) are designated. An elementary distance window is denoted by the term a _{m · p} , where a is a complex quantity that is determined by its signal components I and Q. If the radar system works with coded bursts, i.e. if the product of the repetition frequency and the wavelength of the carrier oscillation changes from one burst to the next, a certain time gap occurs between two successive bursts n and ( n + 1).

Fig. 3 shows the matrix of the speed window for a salvo of the ordinal number n. In this figure, only the range windows are shown, which are centered on half the value of the repetition frequency Fr. The central Doppler filter FDc corresponds to these distance windows , in which the value of c is equal to N / 2 or approximately equal to this value if the Doppler filter bank comprises N elements. The bandwidth of the Doppler filter is limited by the number P of pulses that are taken into account in a CIP. By appropriate selection of the parameters of the radar system, in particular repetition frequency Fr , carrier frequency Fo of the emitted pulses and number P of pulses of a CIP, the output signal of the filter FDc can be freed from any undesired radar signal, and only the echo signals from moving targets with the Doppler frequency shift Fr / 2 and odd multiples of the same can pass through the filter, so that the magnitude of these echo signals from moving targets, when this Doppler filter is viewed over M distance windows, is negligible to the noise level of the receiver and, moreover, is reduced by the measurement method described below becomes.

Fig. 4 is a block diagram of an embodiment of the means for generating a sequence of M noise samples centered on half the value of the repetition frequency of the radar system. The output signal s ( t ) of the intermediate frequency amplifier of the radar receiver is applied to the input of a coherent detector 1 . The output of this coherent detector is connected to an analog / digital encoder 2 , which supplies the signal components I and Q of each of the signals am , p which are present in a corresponding distance field. In the course of the successive bursts of pulses, the coded signals supplied by the analog / digital encoder are alternately stored in two memories 3 a and 3 b , which can be addressed for writing and reading, via an electronic switch SW ₁ . The memories 3 a and 3 b , which can be designed as RAM memory (random access memory or working memory), each have a capacity of 2 MP words to K bits, where K is the number of bits that the analog / Digital encoders are supplied. The outputs of these memories 3 a and 3 b are connected to the inputs of an electronic switch SW 2 , which operates in the opposite phase to the input switch SW 1 .

The output of the electronic switch SW 2 is connected to the inputs of a Doppler filter bank 4 , which contains a number N of adjacent elements, of which only a central element is shown in this FIG. 4. These filters are complex filters, and their outputs are connected to an operator 5 , which enables the calculation of the amount Am of the noise component that has passed through the Doppler filter.

Figure 5 is a typical normalized amplitude / frequency response curve of a Doppler filter centered at half the repetition frequency Fr of the radar system in the event that the number P of pulses in a salvo is limited to 8 or 10 pulses. It should be noted that the echo signals from slow targets whose Doppler frequency shift is less than 0.35 Fr are completely suppressed. In the example shown, for a value P in the order of 10 pulses / CIP, the echo signals from objects whose radial speed V _{r is given} by the following relationship:

not passed to the noise signal measuring device, that on the output side of the central Doppler filter lies.

Fig. 6 is a block diagram of an embodiment of the device for measuring the receiver noise. This measuring device comprises a first measuring channel 10 and a second measuring channel 11 . These two measuring channels have a common input which is connected to the output of element 5 of Doppler filter bank 4 , as shown in FIG. 4. This element provides the sequences of M coded noise samples, which are digitally encoded at K levels. The first measuring channel 10 is used to calculate the mean value μ c of the M samples, while the second channel 11 compares the instantaneous quantities Am with a threshold value K ₁ · μ _{n -1} , as will be described later.

The first measuring channel 10 comprises the following elements in series connection:

• - A first switch 12 with a first input ( 1 ) to which the input signals Am are applied, a second input ( 2 ) and a control input ( c );
• - An operator 13 , which is the operation performed, this operator being formed by a digital integrator weighted by the factor M , which is provided with a clock input ( c · p ) and a zero reset input ( RZ );
• - A memory register 14 with a load input ( LD ) for the result μ c of the operator 13 ; and
• - A second switch 22 with a first input ( 1 ), which is connected to the output of the memory register 14 , a second input ( 2 ), to which a voltage signal of a predetermined value m o is applied, and a control input ( C ); the output of this change-over switch provides the output data size μ _n, the (n -1) on the previous output data size follows that (n-1) from the preceding Savle results of M noise samples, wherein the value of μ _{n is} equal to m c or μ o is, depending on the result provided by the second measuring channel.

The second measurement channel 11 essentially comprises a level comparator 15, whose reference input receives a threshold value signal from the value K ₁ · μ _{n -1.} This level comparator delivers a control signal at its output when the size Am of the input samples exceeds the value of the threshold signal; This control signal is applied on the one hand to the control input of the first changeover switch 12 , which is contained in the first signal channel, and on the other hand to a counter circuit which contains the following elements in series:

• a counter 16 with a counting capacity of approximately M bits or less;
• a decoder 17 for decoding the content of the counter 16 to determine whether the content L of the counter has exceeded a predetermined reference value Lo in the course of a sequence of M noise samples;
• a memory flip-flop 18 whose task is to store the result provided by decoder 17 at the end of a sequence of M samples; this flip-flop circuit provides at its output ( Q ) an output data variable E in two states, this output data variable also being fed to the control input ( c ) of the second switch 15 , which is contained in the first measuring channel. The state of the output data variable E allows the presence of interference signals to be displayed if the number of overshoots L of the M input samples is greater than the reference value Lo .

The sequence control of the measuring device is ensured by the output signals of a clock signal generator 20 , the operation of which takes place synchronously with the clock signal generator of the transmitter of the radar system.

Fig. 7 is a timing chart of the clock signals supplied from the clock signal generator 20. The sequence or sequence with the ordinal number n of the M samples Ao to A _{M -1} is shown in relation to the clock signals CK 1 , CK 2 and CK 3 . The clock signal CK 1 enables the operator 13 to successively sum up the size Am of a sample under consideration with the accumulated sum of previous m samples. The clock signal CK 2 controls the loading of the result of the operator 13 into the memory register 14 and the setting of the memory flip-flop 18 . The clock signal CK 3 controls the zero reset of the operator 13 and the counter 16 before the arrival of the M noise samples of the subsequent sequence ( n +1).

Fig. 8 illustrates the operation of the measuring device shown in Fig. 6. The M noise samples Ao to A _{M -1} are quantified quantities whose mean value is μ c . If the size of the threshold value K 1 · μ _n on one sample exceeds _-1, this size is replaced by the value Am μ _{n -1,} which was obtained during the previous sequence (n -1). The sizes of the signal μ o considerably exceed the threshold value K 1 · m _{n -1} , which is generated by the device. In this Fig. 8, the content of the counter L is shown 16 for the case where the value L is smaller than the threshold value Lo. In the event that the value L exceeds the reference value Lo , the threshold value K 1 · m _{n -1 is} replaced by the threshold value K ₁ · μ o , whereby - if the level of the interference and interference signals decreases - the mean value of the thermal quickly again Receiver noise can be found.

According to a preferred embodiment, a linear / logarithmic encoder 21 is arranged between the output of the central Doppler filter and the common input of the measuring device, as shown in FIG. 9. It follows from this that the multiplier operator 19 shown in FIG. 6 must be replaced by an add operator and that the size K 1 must be adapted accordingly.

On the other hand, it can be useful to distinguish between the three interference situations that are encountered in a radar system: absence of interference sources, continuous interference sources or chopped interference. The corresponding operation is easily realized by the circuit shown in Fig. 10, or also by an equivalent circuit. The input of the decoder 17 a is connected to the corresponding outputs of the overshoot counter 16 , and this decoder supplies an output data size of 2 bits, which is stored in the memory register 18 a . The output data E are defined as follows: ( N. B ) for the absence of interference sources; ( B. D ) in the event of the presence of time-chopped disturbances; (B. C) for the case of continuous interference. The output data size E = ( N. B ) and E = ( B. D ) or E = ( B. C ) can then be combined with one another in order to obtain the three disturbance situations specified above.

In the described embodiment of a device for measuring the noise signals or interference signals of a Radar receiver is the device through a specialized Processor formed; but there are also others Embodiments of such a processor possible, in particular an embodiment with a programmable Processor.

Fig. 11 is a flowchart showing the sequential operations of a radar receiver are performed in the method for measuring the noise signals and interference signals, to calculate the data size of μ c, which represents the mean value of the noise signals or noise of the receiver, and the data size E to determine which indicates the fault situation. At the end of a sequence ( n -1) of M samples, one has the preceding value m _{n -1} , which represents the mean value μ _c , which was calculated during this sequence ( n -1), or a fixed quantity μ o , which is always larger than the value μ _c . In addition, at the beginning of each sequence of M samples, the results of the noise signal level averaging operations and the noise level exceedance count were reset to zero. During the subsequent sequence with the atomic number n , a new value μ _{c is} calculated by accumulating the values of the input signals and replacing this value with the value μ _{n -1} whenever the size Am of a noise sample is greater than the threshold value K 1 · μ _{n -1} . In parallel, the number of crossings of the threshold value K is 1 · μ _{n -1} evaluated to then classify the interference situations which have been encountered in the course of the sequence considered. As shown in the flow chart, the number of overshoots L can be classified according to the following three malfunction situations encountered:

- not disturbed L ≦ ωτ L ₀ ≦ ωτ L ₁ = N. B - chopped perturbation L ₀ ≦ ωτ L ≦ ωτ L ₁ = B. D - continuous disturbance L _{1 1} ωτ L = B. C.

The advantages created by the invention are now easier to see. The precision in measuring the Average value of the thermal generated by the radar system Noise is high, because in the course of each of them consecutive measurements will be a large number of noise samples taken into account and those Noise samples, which are likely to or interference signal are replaced by the most likely mean of noise replaced. Furthermore, the disturbances resulting from unwanted Radar signals already result before the Doppler filter eliminated, and the echoes of moving targets which have crossed this Doppler filter, affect on the measurement result in a negligible way.  

The invention is applicable to radar systems using be operated at a constant repetition frequency or those with coded pulse salvos, with coherent pulses or related impulses as well as impulses work for a short period of time, or with impulses of large duration Duration encoded for later compression in the receiver are.

A particular application of the invention is radar systems, where an optimal detection sensitivity for moving Goals are striven for.

Claims (6)

1. A method for measuring noise signals or interference signals of a coherent pulse radar receiver, in which a Doppler filter bank is used, characterized in that it consists in performing the following operations:

• a) a sequence of M noise samples Am available at the output of a central element of the Doppler filter bank is entered;
• b) the quantities Am are compared with a threshold value K 1 · μ _{n -1} , which is proportional to the output data quantity μ _{n -1} , which represents the mean value of the quantities Am of the preceding sequence ( n -1);
• c) The size Am is retained if its value is less than the threshold value K 1 · μ _{n -1} ;
• d) the value Am is replaced by the value m _{n -1} and a counter is incremented by one if the value of the quantity Am is greater than the threshold value K 1 · m _{n -1} ;
• e) The size Am is summed up with the sum obtained since the beginning of the sequence:
• f) the subsequent sample Am + 1 is entered if the atomic number m is less than M ;
• g) if m = M -1, then a value becomes is calculated, and the content of the counter counting the overshoots is compared with a predetermined reference value Lo which depends on the admissible interference ratio;
• h) if the content L of the counter is smaller than the reference variable Lo , the data variable μ _n = m _{c is} output;
• i) if the count value L for the number of overshoots is greater than the reference value Lo , the value m _{c is} replaced by a predetermined value μ _o , the data output variable μ _c = m _o is output, and a signaling data quantity is output, which indicates that the value L is greater than the threshold value Lo and that consequently a large number of samples Am result from an interference signal.

2. The method according to claim 1, characterized in that that an additional operation is performed, the is to determine the amplitude of the noise samples supplied by the central element of the Doppler filter bank be encoded in logarithmic form.   3. The method according to claim 1 or 2, characterized in that an additional further operation is carried out, which consists in classifying the count value L for the number of overshoots based on a plurality of predetermined reference variables L _o to L _n . 4. Device for measuring the noise signals or interference signals of a coherent pulse radar receiver, with a Doppler filter bank, characterized in that it comprises a first ( 10 ) and a second ( 11 ) measuring channel, which are coupled to one another and a common input have, which is connected to a central element of the Doppler filter bank ( 4, 5 ), that the first measuring channel contains means for calculating the mean value ( μ _c ) of these noise signals and the second channel comprises means for measuring the interference ratio, and that a predetermined one Signal ( μ _o ) replaces the calculated mean ( μ _c ) if the interference ratio ( L ) has a value that is greater than a predetermined reference value ( L _o ). 5. The device according to claim 4, characterized in that it additionally comprises a linear / logarithmic encoder ( 21 ) between the central element of the Doppler filter bank ( 4, 5 ) and the common input of the two measuring channels ( 10, 11 ) is arranged. 6. The device according to claim 4, characterized in that the first measuring channel ( 10 ) in series connection contains the following elements: a first switch ( 12 ), an integrator ( 13 ) for weighted integration, a memory register ( 14 ) and a second switch ( 22 ); and that the second measuring channel also contains the following elements in series: a level comparator ( 15 ); a digital counter ( 16 ); a decoder ( 17 ) and a memory register ( 18 ); and that the outputs of the level comparator ( 15 ) and the memory register ( 18 ) are connected to the control inputs of the first ( 12 ) and the second switch ( 22 ), a predetermined voltage signal ( μ _o ) is applied to the second input of the second switch and the output of this second switch is connected to the second input of the first switch on the one hand and via a multiplier operator ( 19 ) to the reference input of the level comparator ( 15 ) on the other.