GB2184911A

GB2184911A - Measurement of noise in a radar receiver

Info

Publication number: GB2184911A
Application number: GB08501112A
Authority: GB
Inventors: Guy Descodt; Daniel Seguin; Philippe Elleaume
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1984-01-27
Filing date: 1985-01-16
Publication date: 1987-07-01
Also published as: IT8567055A0; IT1182371B; GB8501112D0; DE3502398A1; FR2589586A1; IT8567055A1; FR2589586B1; DE3502398C2; NL8500212A; GB2184911B

Abstract

To measure the thermal noise signals of a coherent pulse radar receiver including a bank of Doppler filters, the device includes a first (10) and a second (11) measurement channel which are mutually coupled and include a common input which is connected to a central unit of the bank of Doppler filters (4.5): the first channel (10) includes means of calculating (12-14) the mean value mu c of noise signals, and the second channel (12) includes means of measuring (15-19) the jamming rate, L. If L exceeds a threshold Lo, then mu c is replaced by a predetermined level mu o. <IMAGE>

Description

SPECIFICATION Process and device for the measurement of the noise signals of a radar receiver The present invention relates to coherent pulse Doppler radar systems and it relates more particularly to a process and device for the measurement of the noise signals of the receiver of such a radar system.

The bases of Doppler radar systems enabling the display of moving targets, such as aircraft, are widely described in the corresponding technical literature. On this subject, however, it will be possible to usefully consult M.H. SKOLNIK's work, "Radar Handbook", 1970, published by McGraw Hill. Moving target display Doppler radar systems must be able to operate in a satisfactory manner in the presence of undesirable radar signals, which are mainiy formed by ground return signals and buildings, the echo signals coming from land vehicles, rain return signals, "angel" echos, etc... These undesirable radar signals, correspond with objects moving at low or moderate speeds, i.e. less than 30ms-1.In addition, these radar systems must remain operationai in a hostile environment, when a potential attacker puts passive counter-measures into operation, such as "chaffs", or electronic counter measures (ECM) consisting of transmitting continuous electromagnetic jamming signals (B.C.) or time chopped electromagnetic jamming signals (B.D.). Radar systems can also be subject to electromagnetic interference such as that produced by other equipments operating in the vicinity.

In radar receiver output signal processing units, the radial radar distance is divided into distance-cells, the unit width of which approximately corresponds to the distance discriminating power of the signals radiated by the antenna. The jamming rate, in a given antenna direction, can be defined as the ratio of the number of jammed distance-cells to the number of distancecells considered. This jamming rate tends towards unity in the case of high level continuous continuous jamming signals, i.e. at a level higher than the mean level of the thermal noise.

In a radar system, the moving echo detection device is located downstream of the receiver, it is therefore necessary to accurately know the mean value of the level of thermal noise signals generated by the receiver, since the magnitude of this parameter fixes the sensitivity limit of the target echo detection device. An exact knowledge of the mean value of the level of thermal noise signals also enables the checking of the system's false alarm rate (TFA) and therefore the problem arises of measuring these noise signals of thermal origin in the presence of interference contributed by undesirable radar signals on the one hand and possible jamming signals on the other hand.

The purpose of the invention is, in the absence of jamming signals, to provide an accurate measurement of the level of thermal noise signals in a radar receiver and, in the presence of jamming signals, to evaluate the characteristics of these jamming signals in order to indicate the jamming situation encountered.

In order to achieve the abovementioned purpose, the present invention proposes a process of measurement of the noise signals of a coherent pulse radar receiver which includes a bank of Doppler filters, in which a central unit provides sequences of M independent noise samples Am, the position m of a noise sample being within the limits zero and (M- 1) corresponding with M distance-cells; this process consists, during each of these sequences of M noise samples, in carrying out the following operations: a-successively entering the M samples of noise Am available at the output of a central unit of the bank of Doppler filters.

b-comparing the Am amplitudes with a threshold value Kl.tn 1 proportional to the output data 14 1 representative of the mean value of the Am amplitudes of the previous sequence (n- 1) c-retaining the Am amplitude if its value is less than the threshold value K1JC-1 d-substituting the n - 1 value for the A, amplitude and incrementing a counter by one unit if the Am amplitude exceeds the threshold value Kl./n l.

e-adding the Am amplitude to the sum of mAm samples already obtained f-entering the next Am+i sample, if the position m is less than M-l g-if m=M-1, calculating the value

and comparing the vaue L of the number of overshoots with a predetermined reference magnitude Lo, depending on the acceptable jamming rate.

h-if the value L is less than the reference magnitude Lo, outputting the data ,xtn=tc.

i-if the value L of the number of overshoots is greater than the reference magnitude Lo, substituting a predetermined value o for the value ,c, output the data ,un=po and an additional item of data indicating that the value L is greater than the threshold value Lo and that in consequence a large number of samples Am result from a jamming signal.

The invention also proposes a device for the measurement of the noise signals of a coherent pulse radar receiver implementing the measurement process described above. The measurement device according to the invention includes a first and a second measurement channel, mutually coupled, and having a common input which is connected to a Doppler filter centered on a frequency approximately equal to half the recurrence frequency of the radar system; the first measurement channel essentially includes means of computing the mean value of the noise signals level, and the second measurement channel includes means of measuring the jamming rate. This measurement device operates according to the previously described measurement princess.

Other additional characteristics will appear in the detailed description of the invention, given with reference to the appended drawings; in these drawings: Figure 1 is a representation of the plane cover of a coherent pulse radar system.

Figure 2 represents the matrix of distance-cells of this radar system.

Figure 3 represents the matrix of the Doppler speed distance-cells considered at the output of the bank of Doppler filters.

Figure 4 is a functional block diagram of a radar receiver including a bank of Doppler filters.

Figure 5 is a typical amplitude-frequency response curve of the central unit of the bank of Doppler filters.

Figure 6 shows, in the form of a functional block diagram, a form of embodiment of the device for the measurement of noise signals according to the invention.

Figure 7 is a timing diagram of the sequencing signals of the device for the measurement of noise signals.

Figure 8 illustrates the functioning of the device for the measurement of noise signals.

Figure 9 is a variant of embodiment of the device for the measurement of noise signals.

Figure 10 is an additional variant of embodiment of the measuring device.

Figure 71 is a flowchart representing the various operations implemented in the process of measurement according to the invention.

Fig. 1 represents the plane cover of a radar system the maximum detection distance of which is fixed at a value R,l,;,X such that Rm,X is equal to or less than Tr.c, where T, is the period of recurrence of pulse signals radiated by the antenna, and c is the velocity of propagation of electromagnetic waves. This distance R,,,ax is divided into M distance-cells of unit duration R equal to a.c 2 where dis the duration corresponding to the discriminating power in distance of transmitted signals. In a coherent radar system, the Coherent Interval Processing (CIP) includes a number P of consecutively transmitted pulses with the same repetition period and the same wavelength.

The number n of CIPs in the plane cover is given by the following expression: Fr.Ta Ne M P where: Fr 1 /Tr.

Ta is the period of rotation of the antenna and M is the number of distance-cells equal to RmaxaR The angular displacement 0 of the antenna during one CIP is given by the following expression: 2.P 6= Fr.Ta and this angular displacement 6 is generally less than the nominal width 0 of the antenna beam.

According to the radar system concerned, this system can continuously transmit identical pulses or can transmit a number of pulses P in bursts, the frequency of recurrence of these pulses varying from one burst to the next, or in a different way, by maintaining the recurrence frequency fixed and varying the carrier wave frequency Fo from one burst to the next for the purpose of eliminating, at least partially, the blind velocities.

Fig. 2 represents the matrix of distance-cells for a CIP of P pulses. Along the axis corresponding to radar time are arranged M columns of distance-cells which are referenced from 6 to (Ml) and, along the time axis t are arranged P rows of distance-cells which are referenced from 6 to (P-1). A unit distance-cell is identified by the term amp, where a is a complex value formed by its signal components I and 0. When the radar system operates by coded bursts, i.e.

when the product of the frequency of recurrence and the carrier wavelength differ from one burst to the next, a certain space of time exists between two consecutive salvos n and (n+ 1).

Fig. 3 represents the matrix of velocity-cells for a salvo of position n. On this figure only the velocity-cells centered on half the value of the frequency of recurrence Fr have been represented.

These velocity-cells correspond with the central Doppler filter FDc where the value of C is equal to or close to N/2, if the bank of Doppler filters contains N units. The bandwidth of the Doppler filter is limited by the number P of pulses taken into account in a CIP. By careful choice of the radar system parameters; particularly the frequency of recurrence Fr, the carrier frequency Fo of the transmitted pulses and the number P of pulses in a CIP, the output of the filter FDc can be free from any undesirable radar signal and only the echo signals of moving targets having a Doppler shift frequency Fr/2 and its odd multiples can pass through this filter such that, if this Doppler filter is considered over M distance-cells, the contribution of these moving target echo signals to the noise level of the receiver is negligible and, in addition, it will be attenuated by the process of measurement as will be described subsequently.

Fig. 4 is a functional block diagram which illustrates, as an example, means enabling the generation of a sequence of M noise samples centered on the half value of the frequency of recurrence of the radar system. The output signal s(t) of the amplifier F1 of the radar receiver is applied to the input of a coherent detector 1. The output of this detector is connected to an analog-digital (A/D) coder 2 which supplies the signal components I and Q of each of the am,p signals present in a corresponding distance-cell. During the successive salvos of pulses, the coded signals provided by the A/D coder are alternately stored in two memories 3a and 3b which are addressable in read and write by means of an electronic switch SW,.Memories 3a and 3b, which can be formed from RAM (Random Access Memory) devices, each have a capacity of 2.MP words of K bits, where K is the number of digits supplied by the A/D coder.

The outputs of these memories 3a and 3b are connected to the inputs of an electronic switch SW2 which operates in opposite phase to the input switch SW1. The output of electronic switch SW2 is connected to the inputs of a bank of 4 adjacent Doppler filters containing a number N of units, of which only one central unit has been represented in this Fig. 4. These filters are complex filters and their outputs are connected to an operator 5 which enables the calculation of the modulus Am of the noise component having passed through the Doppler filter.

Fig. 5 is the typical standardized Amplitude/Frequency response curve of a Doppler filter centered on the half-value of the frequency of recurrence Fr of the radar system in the case in which the number P of pulses in a salvo is limited to 8 or 10 pulses. It can be seen that the echo signals of slow targets, of which the Doppler shift frequency is less than 0.35 Fr, are perfectly rejected. In the example illustrated here, for a value of P in the order of 10 pulses/CIP, the echo signals coming from objects having a radial velocity V, given by the following expression: Fr.c V, < ;0,35 2 Fo are not transmitted to the noise signal measuring device located downstream of the central Doppler filter.

Fig. 6 is a functional block diagram which represents a form of embodiment of the device for the measurement of the noise of the receiver. This measuring device includes a first measurement channel 10 and a second measurement channel 11. These two measurement channels have a common input which is connected to the output of the unit of the bank of Doppler filters 4.5 as described in Fig. 4. This unit provides the sequences of M noise samples digitally coded over K levels. The first measurement channel 10 enables the calculation of the mean value c of the M samples while the second channel 11 enables the comparison of the instantaneous amplitudes Am with a threshold value K n t as will be described later.

The first measurement channel 10 includes, connected in series, the following items: -a first switch 12, having a first input (1) which is connected to the input signals Am, a second input (2) and a control input (c).

-an operator 13 which performs the operation

this operator can be formed from a digital integrator weighted by the factor M and it is supplied with a clock input (c.p) and a reset to zero input (RZ).

-a memory register 14 having an input (LD) for loading the result ,ttc of the operator 13, and -a second switch 22 having a first input (1) which is connected to the output of the memory register 14, a second input (2) which receives a voltage signal of predetermined value ,two, and a control input (C); the output of this switch supplies the output data fin which follows the previous output data (n- 1) resulting from the previous salvo (n- 1) of M noise samples, the value of Cm is equal to fic or ,Ito, depending on the result supplied by the second measurement channel.

The second measurement channel 11 essentially includes a level comparator 15 the reference input of which receives a threshold signal of value K,.,Ltn~1. This level comparator supplies at its output a control signal when the amplitude 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 switch 12 included in the first signal channel and, on the other hand, to a counting circuit which includes, connected in series, the following items: -a counter 16 having a counting capacity of about M bits, or less.

-a decoder 17 of the content of the counter 16, which detects if the content L of the counter has exceeded a defined reference value Lo during a sequence of M noise samples.

-a memory flip-flop 18 the function of which is to store the result provided by the decoder 17 at the end of a sequence of M samples; this memory flip-flop supplies at its output (Q) a two-state item of output data E, this output data is also supplied to the control input (c) of the second switch 15 included in the first measurement channel. The state of output data E indicates the presence of jamming signals when the number of overshoots L of the M input samples is greater than the reference value Lo.

The sequencing of the measurement device is provided by the output signals of a clock signal generator 20, the functioning of which is synchronous with the clock signal generator of the transmitter of the radar system.

Fig. 7 is a timing diagram of the clock signals provided by the clock signal generator 20. The sequence of position n of the M samples Ao to AM 1 is represented in respect to clock signals CKl, CK2 and CK3. Clock signal CKl enables the operator 13 to successively accumulate the amplitude Am of a considered sample with the cumulative sum of the previous m samples.

Clock signal CK2 enables the result of operator 13 to be loaded into memory register 14 and to set the memory flip-flop 18. The clock signal CK3 enables the resetting to zero of operator 13 and of counter 16 prior to the arrival of the M noise samples of the next sequence (n+ 1).

Fig. 8 illustrates the functioning of the measurement device described in Fig. 6. The M noise samples Ao to AM-1 are quantified values the mean value of which is fic. When the value Am of a sample exceeds the threshold value K1. Xtn " the value n t obtained during the previous sequence (n- 1) is substituted for this value Am. The amplitude of signal uo is considerably higher than the threshold value K1 fin 1 produced by the device. On this Fig. 8 is also represented the content L of counter 16 in the case in which the value L is less than the threshold value Lo.In the case in which the value L exceeds the reference value Lo, the threshold value K1.Xzo is substituted for the threshold value Kl.un-1, which enables, when the interference and jamming signals level diminishes, the rapid recovery of the mean value of the thermal noise signals of the receiver.

According to a variant preferred of embodiment a linear-logarithmic coder 21 is inserted between the output of the central Doppler filter and the common input of the measuring device, as represented in Fig. 9. The result of this is that an addition operator must be substituted by a multiplication operator 19 shown in Fig. 6 and that the value K1 must be adapted as a consequence.

It can also prove useful to differentiate the three jamming situations encountered by the radar system: absence of jammers (N.B), and continuous jammers (B.C) or chopped jammers (B.D).

The corresponding operation is easily achieved by the circuit shown in Fig. 10, or an equivalent circuit. The input of the decoder 1 7a is connected respectively to the corresponding outputs of the overshoot counter 16 and this decoder provides output data in two digits which is stored in memory register 18a. The output data E=(N.B) and E=(B.D or B.C) can then be combined in order to obtain the three previously mentioned jamming situations.

A form of embodiment of a device for the measurement of the noise signals of a radar receiver has been described above, this device being formed from a dedicated processor.

however, other variants of the construction of such a processor can be envisaged and in particular that using a programmable processor. Fig. 11 details in the form of a flowchart the successive operations of the process of measuring the noise signals of a radar receiver enabling the calculation of the fic data representing the mean value of the receiver's noise signals and the indicating data E representing the jamming situation. At the end of a sequence of M samples of a sequence (n- 1) the previous value jiln-t is available which represents the mean value fic calculated during that sequence (n- 1), or a fixed value flO which is always greater than the value fic. Furthermore at the start of each sequence of M samples, the results of the noise signal level averaging operation and the counting operation of the overshoots of jamming signals are reset to zero.During the next sequence in position n, a new value fic is calculated by cumulating the values of the input signals and substituting the value fin- 1 for this each time the amplitude Am of a noise sample is greater than the threshold value K1. jUn-1* Similarly, the number of overshoots of the threshold value K1. fin-i is taken into account in order to subsequently classify the jamming situations encountered during the current sequence.As shown on this flowchart, the number of overshoots L can be classified according to the three jamming situations encountered: -Not jammed L < LO < L, =N.B - chopped jammer L0 < L < L1 =B.D -Continuous jammer L, < L =B.C.

The advantages procured by the invention are now more clearly seen: the accuracy in the measurement of the mean value of the thermal noise generated by the radar system is high since during each of the successive measurements a large number of noise samples are taken into consideration, and since the mean value of the most probable noise is substituted for noise samples which probably correspond to an interference signal or a jamming signal. Likewise, interference resulting from undesirable radar signals is eliminated upstream of the device by the Doppler filter and the echoes of moving targets, having passed through this filter have a negligible effect on the result of the measurement.

The invention is applicable in radar systems operating with a constant frequency of recurrence or in bursts of coded pulses, with coherent or cohered pulses and with pulses of short duration, or of long duration which are coded with a view to subsequent compression in the receiver.

The invention finds its application in radar systems in which an optimum sensitivity of detection of moving targets is sought.

Claims

1. Process for the measurement of the noise signals of a coherent pulse radar receiver in which is included a bank of Doppler filters, characterized in that it consists of the following operations: -a. Successively enter a sequence of M samples of noise Am, available at the output of a central unit of the bank of Doppler filters.

-b. Compare the Am amplitudes with a threshold value K1. An-tw proportional to the output data fin--i representative of the mean value of the Am amplitudes of the previous sequence (n-1).

-c. Retain the Am amplitude if its value is less than the threshold value K1. An-1.

-d. Substitute the fin- 1 value for the Am amplitude and increment a counter by one unit if the value of the Am amplitude is greater than the threshold value K1. fin -e. Cumulate the Am amplitude with the sum obtained since the start of the sequence (M-1) Am m=o -f. Enter the next Am+1 sample, if the position m is less than M.

-g. If m=M-1, calculate a value 1 M-i pc= Z Am M and compare the content of the overshoot counter with a predetermined reference value Lo, depending on the acceptable jamming rate.

-h. If the content L of the counter is less than the reference value L0, output the data 14=flc -i. If the count L of the number of overshoots is greater than the reference value Lo, substitute a predetermined value wO for the value c, output the data juC=,uO and an item of data indicating that the value L is greater than the threshold value Lo and consequently that a large number of Am samples result from a jamming signal.

2. Process according to Claim 1, characterized in that it includes an additional operation consisting in coding in a logarithmic form the amplitude of noise samples provided by the central unit of the bank of Doppler filters.

3. Process according to Claims 1 or 2, characterized in that it includes another additional operation consisting in ciassifying the count L of the number of overshoots with respect to several predetermined reference values LOLn.

4. Device for the measurement of the noise signals of.a coherent pulse radar receiver in which is included a bank of Doppler filters, characterized in that it includes a first (10) and a second (11) measurement channel, mutually coupled, having a common input which is connected to a central unit of the bank of Doppler filters (4.5) : the first measurement channel includes means of calculating the mean value (uc) of these noise signals and the second channel includes means of measuring the jamming rate, and in that a predetermined signal Wo) is substituted for the calculated mean value vic) when the jamming rate (L) has a value greater than a predetermined reference value (LJe

5.Device according to Claim 4, characterized in that it includes in addition a linear-logarithmic coder (21), which is inserted between the central unit of the bank of Doppler filters (4.5) and the common input of the first (10) and second (11) measurement channels.

6. Device according to Claim 4, characterized in that the first measurement channel (10) includes, connected in series, the following items, a first switch (12), a weighted integrator (13), a memory register (14) and a second switch (22) and in that the second measurement channel includes, also connected in series, the following items: a level comparator (15), a digital counter (16), a decoder (17) and a memory register (18) and in which device the outputs of the level comparator (15) and of the memory register (18) are connected to the control inputs of the first (12) and second (13) switches respectively, the second input of the second switch is connected to a predetermined voltage signal ( O) and the output of this second switch is connected to the second input of the first switch on the one hand, and via a multipiication operator (19) to the reference input of the level comparator (15) on the other hand.

7. A process for the measurement of noise signals of a radar receiver substantially as hereinbefore described with reference to and as illustrated in Figs. 6 to 11 of the accompanying drawings.