Method in a tracking radar to attain a large unambiguous range for detected targets by means of radar pulses with high repetition frequency

(57) Summary:

The invention relates to a method in a foal radar_z which out During radar pulses against a certain mSl whose measuring distance is known from the beginning. The carrier frequency of the transmitted radar pulses is varied in the tracking of the target from one pulse to the next, so that a pulse series containing a certain number of M carrier frequencies is formed. The number M is chosen with regard to the measured distance still measured and s3 as long as this is less than a certain value. If the target distance exceeds this value, the number of M carrier frequencies changes and a new pulse series containing this new number is transmitted. The unambiguous distance to the followed maie then becomes dependent on the frequency rope in the frequency of the transmitted pulse series instead of the PRF of the radar.

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1 / Zt (56) Cited publications: US 3,324,469 (343-17.2)

IN

79036S3-9

TECHNICAL FIELD

The invention relates to a method according to the preamble of claim 1, which is used in a tracking radar of a kind known per se. More particularly, the invention relates to a method in which transmitted radar pulses have a high PRE (pulse repetition frequency) and wherein the carrier frequency varies from pulse to pulse without therefore the range of the radar is limited to a certain unambiguous distance determined by the PRF.

PRIOR ART

The main task of a tracking radar is to, after the location of a certain moving target has been determined, follow this, for example with regard to its distance and give information about the target's continued movement, see for example Barton "Radar System Analysis" Prentice Hall Electrical Engineering Series 1964 Chap. 9, p. 263. In known tracking radar stations, radar pulses are transmitted with a certain determined carrier frequency and with such a value of the pulse repetition frequency (PRF) that a pulse reflected from the target returns within a time interval before the next pulse is transmitted, the so-called Listening IntervalLet. The condition that the reflected measuring pulse must be received within the Listening Interval is necessary, since otherwise the Position of the target cannot be unambiguously detected.

It is further known in the present invention to vary the carrier frequency from pulse to pulse of the target radar signals transmitted to the target. See, for example, U.S. Patents 3,413,634 and 3,372,391. The purpose of this frequency change is to avoid disturbing echoes affecting the reception, tuning the radar receiver's local oscillator in step with transmitted pulses and to the frequency of each of them, so that only desired echo signals are detected and become unaffected by unwanted interfering echoes. However, known tracking radar stations which utilize this principle are still bound by the condition that the pulse repetition frequency (PRF) must be such that the echo pulse returns within the Listening Interval.

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DESCRIPTION OF THE INVENTION

As briefly discussed above, the PRF of the radar, the repetition frequency of the transmitted pulses, is decisive for the unambiguous calculation of the target the radar is to follow, for example in terms of distance, angular position or speed. If the target is located within a certain short distance from the antenna, a relatively high value of the radar's PRF can be selected without the risk of ambiguity arising, while if the target distance is long, a sufficiently low value of PRF must be selected. More specifically, the so-called unambiguous distance Ro = c / 2fp_Rf:, where f_DD_ is the pulse repetition frequency of the transmitted radar pulses and c is the speed of light, so the condition f_DD_ - c / 2Ro must be met for unambiguous detection. rKr tering of the target distance.

In a tracking radar, however, it is desirable that the value of f_pRF can be kept at as high a value as possible so that the goal can be better followed.

A high value means that information about the target position is obtained more often than in the case that pulses with a low value of f are transmitted. R other si. r K r.

According to the above, a high PRF value results in a small unambiguous distance, which can mean that a reflected pulse cannot be detected and that ambiguity about the target distance can occur. These two conflicting requirements constitute the fundamental problem of the invention, which is intended to be solved by the proposed method. A prerequisite is that the target distance is known from the beginning with an accuracy equal to ± 1/2 (ct / 2), where. t is the length of the Listening Interval. .

According to the invention, radar pulses were transmitted, the carrier frequency of which in a manner known per se varies from pulse to pulse and according to a certain randomly selected pattern. The number of selected frequencies M out of a total of N available forms a pulse series with a certain duration corresponding to a certain current measuring distance which is known from the beginning. This results in a certain unambiguous distance greater than the target distance and defined as the time between two transmitted pulses with the same carrier frequency multiplied by the factor c / 2. The pulse series were transmitted one after the other with a certain frequency, the so-called frequency repetition frequency f_FRF- the target has moved so far that the time between pulses with the same σ

7903653-9 _t 3 carrier frequency is not sufficient to obtain unambiguous distance determination, a new pulse series is selected with a larger number of carrier frequencies M - N, whereby the frequency repetition frequency decreases, and a larger unambiguous distance Cc / 2 f_CDt) is obtained. r Kr

The method according to the invention is then characterized as it appears from the characterizing part of claim 1.

DESCRIPTION OF FIGURES

The invention will be explained in more detail with reference to the accompanying drawings, where

Figure 1 shows a diagram of transmitted radar pulses at different times and with different frequencies according to the proposed method;

Figures 2-4 show schematically transmitted and received radar pulses according to the method;

Figure 5 shows a block diagram of the transmitter-receiver part of a tracking radar utilizing the method according to the invention;

Figure 6 shows at different target positions the transmitter and receiver pulses from the transmitter-receiver part according to Figure 5, respectively;

PREFERRED EMBODIMENTS

Figure 1 is a diagram showing a number of M transmitted radar pulses, P1, P2, ... PM. Each pulse has a length τ, a carrier frequency ί £, ... f ^, which are mutually different. The period time, i.e. the time between two pulses with the same carrier frequency is denoted T. The time between transmission of two pulses with different frequency, for example and f ^ is denoted t, whereby the listening interval becomes t- τ ~ t. In Figure 1 the pulse time τ has been exaggerated for better clarity. The listening interval is the time interval when a transmission pulse reflected from the target with a certain frequency can be received and correctly indicated for measuring, for example, the low distance. Furthermore, the pulse repetition frequency f_DDC = 1 / h and the so-called frequency repetition frequency f_CDC = 1 / T, which indicates the rKr

7903653-9 frequency with which pulses with the same carrier frequency return at the earliest.

It is previously known in pulse radar systems to vary the carrier frequency from one transmitted radar pulse to another. However, in such a system, the radar PRF is selected so that a sufficient listening interval t - τ is obtained so that the echo from, for example, the pulse P1 (frequency f1) should catch up from the target to the radar receiver before the next pulse P2 (frequency f₉) was broadcast. Pulse repetition rate f_DDC_ rKr is adapted to the current low level.

must therefore

According to the present invention, as with known systems, radar pulses were transmitted with mutually different frequencies but with such a high pulse repetition frequency fp_RR that only a certain section of the target distance the so-called distance window - c / 2 f_DD_ is indicated, which belongs to a known target position, ie the method is only applicable to a tracking radar.

First, a rough estimate of the target distance is made in the radar, for example by some form of instruction in a known manner. · A certain value = Ro is then obtained. Then a certain value of the frequency repetition frequency f is selected_CDC so that c / 2 f_CDC &gt; Ro ... ¢ 1) ie · - = T&gt; ^ 7 vi l—

FRF FRF 'FRF ^c ket indicates that the unambiguity distance shall be greater than the estimated value of the target distance Ro. The selection of M different frequencies, which are repeated periodically and in the same order, defines a number of pulse series, where each series contains M different frequencies and is repeated with the frequency f_CDC.

rKr

The time interval t between two consecutive pulses with different frequencies determines a certain PRF value fp_Rp, which, with the exception referred to below, shall be constant. The minimum time interval between two pulses having the same frequency defines the frequency repetition frequency fp_Rp ·

The method according to the invention is intended to be used in a tracking radar, ie a radar which, after a certain target has been discovered, ie the target distance is known, must follow the target. The radar must then transmit.

pulses with fixed pulse Length τ and with continuously variable PRF.

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Furthermore, N fixed frequencies shall be available in the radar, where M - N where M is the number of transmitted frequencies at the tracking. As will be described in more detail later, there is a control device for calculating certain conditions and for giving control signals to transmitters and receivers.

It is assumed that the radar captured a certain target and performed a rough estimate of the target distance Ro. A frequency repetition frequency f-_DC well. rKr jes thereby so that the condition c / 2 f__DC• rRr&gt; Ro - (1) is satisfied.

A certain minimum listening interval tm between two transmitted pulses is determined and the value of tm is selected with regard to a certain dead time such as frequency change time and recovery time for the receiver. The number of selected frequencies Μ - N is determined so that

Mt - 1 / f_CDC ... (2) wherein R &lt;c Mt / 2 - c / 2 ί__Β_ m FRr about rRr

The condition (1) says that for a certain selected value of f__DC obtained a rRr certain unambiguous distance c / 2 f_CDC (generally applies to pulse radar). For rRr a certain target distance, it therefore applies that f_CDC has such a value that detrKr take unambiguous distance is achieved for the measured target distance Ro. The condition (2) limits the value of the number of selected frequencies M. A third condition is that the measure must be in a distance window, ie for some integer n must apply (777 $ - + τ + ε) c = 2Ro ... (3),^MfFRF makes a measure of the time margin for an echo pulse until the value of M is to be selected.

where ε leaks new

Factor 1 / Mf_CDr is the time between two consecutive radar pulses.

looks with different frequency. The factor n indicates the number of pulses which were transmitted between a certain transmission pulse (this not counting) and until the target echo from the transmission pulse is received. The factor n / Mf_CDC is then the time for the sum of emitted Kr da and reflected pulses, to which time the factor (τ + ε) &lt;Listening interval must be added. Figure 2 shows schematically and on a reduced scale the transmission and reception of pulses with mutually different frequencies within a pulse series. the upper part in Fig. 2 shows the transmission moment when the transmission pulse with the frequency f ^ leaves the antenna A and the pulse with the frequency f ^ is assumed to have reached a target m. The lower part shows the reception of an echo pulse with the frequency fy during the time interval T1 - τ. -!

7903653-9 between two transmission pulses. In Fig. 2, n is ~ 6, which corresponds to the number of pulses during the time from the pulse with the frequency f ^ being transmitted until the pulse with the frequency f ^ is to be received.

Figure 3 shows a time diagram of transmitter and receiver pulses from the radar antenna system, respectively. The diagram indicates the layer of transmission pulses and received echo pulses at the times to, tö + t, to + 2t, etc., where tr constitutes the reference time. (indicated by a dashed line) and is the time when the transmission pulse and receiver pulse must not coincide in order for detection to be possible in the receiver. At time t0 (Figure 3a), the timing of the transmit pulses is such that the pulse with the frequency f ^ is to be transmitted after the pulses with the frequencies f₂/ f ^ ... f $ etc. sent. It is assumed that at the same instant of time (to), the receiver pulse with the frequency f ^ should almost reach the antenna and be detected in the receiver before the receiver pulses with the frequencies ff ^ etc. are received. The transmission pulses with the frequencies fj ... f ^ etc. have a constant PRF, ie the time interval t is constant. For the receiver pulses with the frequency f ^, - ·· it has been assumed that the target moves at a constant speed, so that if the time interval between, for example, the pulse f₂ is t + At, the corresponding time interval between the pulses f ^ and f ^ becomes t + 2At, and so on. When detected during the time interval t - T, the receiver is tuned to the frequency f1 which is then detected and a value of the target distance R1 can be determined since the time of transmission of the pulse f1 is known. At time to + t (Figure 3b) the pulse f ^ j has just been transmitted and the receiver has been tuned so that the pulse f ^ j can be detected during a new time interval t - t, whereby a new value R2 of the target distance is obtained. At the time to + 2t (Figure 3c) the pulse f1 is transmitted, but at the same time the receiver pulse f_z vi Iket. means that reception and transmission take place simultaneously. This breaks the transmission schedule and a new determination of f_CD_ and M are made on the basis of the actual value of the target distance and according to the conditions (1) - (3). Alternatively, a change of the pulse repetition frequency may be performed. The change if ^ p and M will in practice occur with low frequency if the distance window is not too small (for example at least 100 m), since current target speeds are below, for example, 1000 m / s. With the exemplified values, the frequency for updating f__DC and M less than 10 Hz.

ΓΚγ

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This can be considered to mean a negligible disturbance of the radar function if ^fPRF ^is greater than, for example, 1 kHz.

When the measured low distance approaches a value R = Rk such that the difference (1) is no longer fulfilled, an unambiguous detection of the receiver pulses no longer takes place. Figure 4 is intended to illustrate this case. In Figure 4a, the transmission pulse f ^ has just been transmitted and the receiver pulse f ^ corresponding to the transmission pulse f ^ within the same pulse series is to be received and detected. This detection is correct, because the receiver pulse f1 clearly corresponds to the low state, since it is assumed that all frequencies in a series of pulses f1, ..., f2 are transmitted and reflected. In Figure 4b, which shows the position at time to + t, a transmission pulse f1 is again transmitted in a new series of pulses ··· '^fi6 and the receiver pulse f ^ is received. The previous receiver pulse f ^ then gives an unambiguous and correct value of the target distance R. At time to + 2t, however, Figure 4c shows that the transmit pulse f ^ in the new series has time to be transmitted before the receiver pulse f ^ in the previous series has been received and given a detection of the target distance. The receiver pulse therefore gives an incorrect value of the target distance which becomes too small. The transmission pulse f ^ must therefore be abolished and a new series with a larger number of frequencies must be started, for example a series f ^ z ... Also in this case the transmission schedule is broken and a new determination of f_pRp and M are performed as above.

Figure 5 shows a block diagram of the transmitter-receiver part of a tracking radar utilizing the method according to the invention. The antenna unit A of the radar may consist of, for example, a fixed radiating antenna, which is connected to an SM exchanger SM, which consists of a circulator. A transmitter unit S and a mixer B are connected to the SM exchanger in a known manner, the output of which is connected to the intermediate frequency amplifier MF. A locator oscillator LO is connected to the mixer B and emits a signal, the frequency of which is the sum of a selected intermediate frequency f ^ p and a high frequency f [&lt;. A control unit SE is connected to the transmitting unit S and to the local oscillator LO to emit certain determined voltage levels v1, v2, ..., vM as control signals. Furthermore, a signal processing unit SB is connected to the control unit SE and to the output of the intermediate frequency amplifier MF for calculating the target distance R from the incoming

7903653-9 measuring pulses. The units shown in Figure 5 correspond to those in a conventional non-coherent pulse radar with the exception of the control unit SE and the transmitting unit S together with the local oscillator LO, which two latter units are designed differently but have basically the same function as in a conventional pulse radar, see for example Barton, pp. 383-384.

The transmitting unit S can consist of, for example, a voltage-controlled oscillator, so-called VCO of a type known per se, which receives the control signals v1, v2, ... from the control unit SE. The transmitting unit S emits pulse-shaped transmission signals with the pulse repetition frequency f_ODC and the pulse length t, 10 wherein the carrier frequency of each of the pulses is different from each other and is determined by the incoming levels v1, v2, ... vM. Synchronization and correct pulse Length is achieved in a manner known per se by means of sync pulses from the control unit SE. The SM exchanger is thus supplied with a series of pulses containing the M frequencies fn (n = 1, 2, ..., M), the carrier frequency changing from pulse to pulse according to a selected pattern as described above, cf. Figure 2. For the synchronization, the transmitting unit may, for example, contain a gate circuit (not shown) which is controlled by pulses of length τ and the pulse repetition frequency f_DD_. rKr

The locator LO, like the transmitter unit S, consists of, for example, a voltage-controlled oscillator, VCO, which emits to the mixer pulse-shaped signals with frequencies f ^ + f ^, where f ^ is the selected intermediate frequency and where k is an integer of each pulse series is selected in the control unit SE in dependence on the number M selected frequencies fn (n - 1, 2, ..., M) and the calculated target distance R.

Figures 6a-6c show at different times to, to + t, to + 2t the order numbers n and k for the carrier frequency (fn and fk, respectively) of the transmitted and received pulses. At time to, according to Figure 6a, the frequency f £ (n = 2) was transmitted and the pulse with the frequency f $ is to be reflected towards the target (dashed line). The pulses with the frequencies f $ - f ^ constitute echo pulses 30 and the pulse with the frequency f ^ is to be next received ?, and detected.

The locator LO according to Figure 5 must thus emit one signal with the I / frequency + fq 2 (k = 14). At the time to + TI a pulse with the frequency Cn = 1) was emitted and an echo pulse with the frequency f

7903653-9 is received, why the loca loscillator frequency should be f „„ + f ™ At MF 13 time to + 2t (figure 6c) a new series of transmission pulses is started by the transmission pulse with the carrier frequency f ^ CM = 16), while the tokaloscillator gives a signal f ^ + f ^ corresponding to the transmission pulse with the frequency f ^ from the previous series. In the example according to Figures 6a-6c it is assumed that the target speed is so low in relation to the pulse repetition frequency fpRF that the target does not have time to move outside a listening interval.^1/fpRF during the time a series of pulses f ^f16 sent and received.

The measured distance Ro calculated from the beginning is fed to the control unit SE. Based on this value and according to conditions (1), (2) and (3) above, this calculates a value first of the frequency repetition frequency fpPF and then the number of carrier frequencies Μ. A number of tables are stored in the control unit, where each table occupies a number of frequencies, say M1, ..., Mj - N, the number Mj being dependent on a certain target distance Rj. After selecting a certain table containing the frequencies f ^, ..., f ^ j corresponding to the measuring distance Rj (j = 0 from the beginning), the control unit SE transmits the voltage levels v1, ..., vHj and the levels u1_z uMj to the transmitting unit S or to the local oscillator LO. Then the change AR in the measuring distance from the original value Ro is calculated. At the end of each pulse series, it is determined whether the new value Rj is such that the same pulse series can be retransmitted, whereby the calculation of a new Rj is performed. If this new value is too large with respect to the selected value of 1 f_CD- a new pulse series is selected from a table, which contains a larger number of carrier frequencies f „, ..., f_M. than previous 1 Mj series.

The proposed procedure offers the following benefits:

High resolution can be obtained with a short pulse length (τ), which combined with a high PRF gives a high data rate and a high work factor. This is an advantage especially for semiconductor transmitters.

The frequency shift in the carrier frequencies (the so-called frequency agility) results in glitter reduction in fire control radars and improved interference resistance.

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Narrowband interference from objects other than the target is virtually impossible, since the current receiver frequency for these objects is unknown.

Low pulse power is possible, because the working factor is high. Furthermore, a wide frequency spectrum is obtained due to the frequency shift. These properties provide a relatively "quiet" radar. In addition, due to the frequency shift, as in the case of the initially known radar systems, no disturbances are obtained from nearby reflectors, such as, for example, ground graffiti.

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