A METHOD TO PREVENT A RADAR BEACON RESPOND TO RECEIVE RADAR PULSES ORIGINATING FROM THE SIDE LOBES OF A TRANSMITTER AND AN ARRANGEMENT TO CARRY OUT THE METHOD

Abstract: A method to prevent a radar beacon respond to received pulses from a vessel which originate from the side lobes of the vessel antenna. The carrier frequency of the radar signal pulses to and received by the beacon is measured and a value of this frequency is stored in the case that the amplitudes of the signal pulses exceed a certain high threshold level (Th). If signals after that are received which exceed a low threshold level (Tl) (but not the high) the carrier frequency of these is measured and the corresponding value is stored. Measuring two subsequent and equal carrier frequencies is considered as a criterion that a responding signal from the beacon responding to the second received signal should not be transmitted. An arrangement to carry out the method is described.

Description: A METHOD TO PREVENT A RADAR BEACON RESPOND TO RECEIVED RADAR PULSES ORIGINATING FROM THE SIDE LOBES OF A TRANSMITTER AND AN ARRANGEMENT TO CARRY OUT THE METHOD. TECHNICAL FIELD The present invention relates to a method and an arrangement " in the receiver of a radar beacon which is intended to respond to radar pul¬ ses from vessels within a certain radial distance from the beacon. The carrier frequency of the transmitted radar pulses from a certain vessel differ from the carrier frequency of the radar pulses from another vessel. The radar beacon then transmits as a response to the received radar pulses a signal which consists of one or several pulses with the same carrier frequency as the received and with a certain duration back to the transmitting vessel, whereby this will obtain information about the position of the beacon. BACKGROUND ART Radar beacons of the so called frequency sweeping type are previously known which provide responding signals to a vessel after that the frequency of the sweeping responding signal coincides with the frequen¬ cy of the signal transmitted from the vessel. This implies a certain delay of the responding signal of the magnitude 1-2 minutes which, however, is not considered to be fully satisfying by, among others, the shipping authorities. Another disadvantage of previously known radar beacons is that they under " certain circumstances also respond to sig¬ nals originating from the side lobes of the vessel antenna. This im- plies ambiguity concerning the position of the beacon when this posi¬ tion is indicated on the indicator of the vessel. The useful range of the radar beacon will then be limited to the position of such vessels which are situated outside a critical area within which also radar pul¬ ses originating from the side lobes are so strong that the radar beacon respond. DISCLOSURE OF THE INVENTION The principles of the method and the arrangement of the present inven¬ tion are based on the observation that the probability that two (or more) vessels transmit radar pulses with the same carrier frequency i small. By detecting and storing the frequency of the received pulses the receiver of the radar beacon and, after that, comparing the fre¬ quencies and the signal strength, it can be determined whether or not a transmitted pulse originates from a side lobe of the antenna system of the vessel radar, and thus whether or not a responding signal shou be transmitted back. The arrangement also makes it possible for the radar beacon to give a response to a transmitted radar pulse from a certain vessel a short time after that the carrier frequency of the received radar pulse is detected by the receiver. The response then consists of a signal with the same frequency as the radar pulse trans mitted from the vessel. The object of the present invention is thus to provide a method and an arrangement included in the receiver of a radar beacon which re- ceives radar pulses transmitted from a vessel with a certain carrier frequency and which prevents that the beacon respond to radar pulses which originate from the side lobes of the antenna system of the vesse but transmits responding signals only for the radar pulses which ori¬ ginate from the main lobe of the antenna system. The invention is the characterized as it appears from the characterizing part of claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be closer described with reference to the acco panying drawing in which Figure 1 schematically shows the position of three vessels in relation to a radar beacon whose receiver contains t arrangement according to the present invention. Figure 2 shows a schematic diagram of the time variation of the recei signal strength for the vessels shown in Figure 1. Figure 3 shows a block diagram of the part of the radar beacon receiv in which the arrangement according to the invention is included. Figure 4 shows a block diagram of a comparator included in the arrsng ment. -gT3R OM Figure 5 shows a block diagram of the frequency memory included in the arrangement. Figures 6a-6b show diagrams of certain wave-forms which appear in the arrangement according to Figure 3. MODES FOR CARRYING OUT THE INVENTION In Figure 1, the positions of three vessels F1, F2, F3 are schematically shown in relation to a radar beacon R. Each vessel transmits signals in the form of radar pulses with a certain carrier frequency within, for example, the S-band (2,9 - 3,1 GHz) designated f1, f2 and f3, respec¬ tively,which frequencies are assumed to be mutually different. The radar pulses from a vessel are intended to be captured by the receiver in the radar beacon R and as a response this will transmit a responding signal back to the vessel which consequently obtains information about the position of the beacon R. The range of the transmitted radar pulses and the responding signals is, however, limited so that the vessel F3 which is situated outside the area whose limit is designated A1 cannot expect any response from the beacon when pulses are transmitted. The vessel F1 which is situated within the area which is ' limited by the limits A1 and A2 obtains a responding signal from the beacon R only fsr the pulses transmitted (and received) by the main lobe of the vessel antenna. For the vessel F2, situated just within the area defined by the limit A2 it is valid that also pulses transmitted by the side lobes will be detected by the receiver of the beacon, for which reason responding signals are also transmitted on these pulses. In Figure 2, the signal strength received in the beacon R from the transmitters of the vessels F1 - F3 according to Figure 1 is sche¬ matically shown. The transmitters are assumed to have an antenna diagram with a dominating main lobe and a number of side lobes around the main lobe. The strength of the transmitted signals from the vessels F1 - F3 is assumed to be substantially equal. According to the proposed method, two threshold levels TI and Th e re selected to detect and to receive the signals from the vessels F1-F3 in the beacon R. The lower Threshold level TI is selected so that the signals from the main lobe hi exceed the threshold Tl, i.e. with respect to the lowest level which should b detected within a range determined by the ' limit A1. The signals from th main lobe h3 (vessel F3) are then too weak to exceed the threshold Tl. The range limit A2 corresponds to the distance when the signal strengt of the side lobe s2 exceeds the lower threshold level Tl. The higher threshold level Th is after that determined so that the signal strengt of the signals from the main lobe h2 exceeds the level Th when the mag nitude of the side lobe s2 exceeds the level Tl. When the radar antenna of the vessel rotates, the main lobe h2 will at a certain instant point towards the beacon R. The transmitted radar pulses are captured by the beacon R and its receiver detects the carri frequency f2 of the pulses from the transmitter of the vessel F2. If these received and detected pulses exceed the lower threshold level Tl the beacon responds by transmitting a signal with the same frequency f Somewhat later the antenna of the vessel has rotated so that the side lobe s2 points towards the beacon R. Under the assumption that the sig nal received by the beacon at this instant exceeds the threshold Tl, t carrier frequency will be detected by the receiver and a new respondin signal (with the frequency f2) will be transmitted if no measures in t receiver are taken. Thus, from the beacon to the vessel F2 responding signals are transmitted at two occations, the signals of which on the PPI of the vessel give two positions (two "marks") which indicate the position of the beacon. This will give a not desired ambiguity of the detected position of the beacon. According to the proposed method, the carrier frequency of the received signal pulses is measured and a valu of this frequency is stored for the signal which exceeds the higher threshold level Th, i.e. the carrier frequency of the signal pulses from the main lobe h2 is stored. After detection of the signal pulses from the side lobe s2, the carrier frequency is measured which is the same as for the signal pulses of the main lobe. The fact that two equal carrier frequencies have been measured immediately after one another is used as a criterion that a signal responding to the other received signal should not be transmitted whereby the above mentioned ambiguity can be avoided. In the block diagram according to Figure 3, the part of the radar OM?I beacon receiver is shown which contains the arrangement according to the invention. The duplexe'r of the receiver is designated 1 and is controlled via an input 1d in dependence on control pulses from the output 9b of a control unit 9 to two positions, on the one hand to receive an incoming signal via the connections 1a~1b, and on the other hand for transmitting a responding signal via the connections 1c - 1a. This will be closer described below. To the output of the duplexer 1a limiting amplifier 2 is connected which amplifies and limits the re¬ ceived signal, a signal having a well defined amplitude appearing across its output 2a. The amplifier has a further output 2b across which the received signal before limitation appears. The output 2b is connected to a detector 10 which, after detection of the signal from the ampli¬ fier 2, across its output 10a delivers a signal (U) whose level corre ¬ sponds to the strength (level) of the received signal. The output of the detector is connected to a comparator 11 in which the' incoming signal level is compared with two threshold levels UI and Uh corre¬ sponding to the threshold Tl and a second higher threshold Th. According to Figure 4, the comparator 11 can as previously known con¬ sist of two differential amplifiers 111, 112 which across their plus inputs receive a signal level from the detector 10 and whose minus in¬ puts are connected to two reference voltages UI and Uh, respectively, as shown in Figure 4. If the level U > UL, a logic "one" is obtained across the output 11a. If U > U.H (and then also U > UL), a "one" is obtained across the output 11b. To the control unit 9 thus logic con- ditions are delivered via the outputs 11a, 11b in dependence on the level of the incoming and received signal. The output 2a of the amplifier 2 is connected to a frequency measuring unit 3. From this unit an analog signal in the form of a DC-voltage is obtained across the output 3a, which constitutes a measure of the fre- queπcy of the received signal. Across the output 3b a continuous AC- voltage appears having the same frequency as the frequency of the re¬ ceived pulse. The frequency measuring unit 3 can be designed as de¬ scribed in, for example, the Swedish patent application 79099S4-2. The output 3a of the frequency measuring unit 3 is connected to a frequency memory 4. This consists according to Figure 4 of an analog- digital converter 41 whose input is connected to the output 3a of the frequency measuring unit 3. The converter 41 is, for example, an 8-bit analog/digital converter. The eight outputs 41a are connected to the adress inputs of a static RAM-memory 42 (for example "Monolitic Memo ¬ ries" type 6531) with 256 different memory cells in which each a "one" or a "zero" is stored. The memory 42 thus has 256 different adresses corresponding to the 256 different memory cells which can be adressed by means of an 8-bits digital word from the output of the analog/digi- tal converter. A certain adress in the RAM-memory 42 thus corresponds to a certain frequency value measured by the frequency measuring unit A "one" in a certain memory cell thus indicates that this frequency ha been measured. Across the input 4b of the frequency memory a signal is obtained from the output' 9b of the control unit 9 which indicates when the AD-con- version should be initiated. From the AD-converter, across the output 41 b which is connected to the input 42 a of the memory, a pulse is obtained when the conversion is terminated. During this pulse, the memory cell corresponding to the adress transferred to the memory is connected to the output 4a and, during the pulse duration, also the contents of the memory cell can be changed by a writing pulse obtained from the output 9d of a control unit via the input 4c of the frequency memory. The output 4a of the memory is connected to the control unit 9 The control unit 9 receives signals on the one hand from the two out- puts 11a, 11b of the comparator 11, on the other hand from the output 4a of the frequency memory. The signals are: a ) A logic signal from the output 11a in the case that an incoming signal exceeds the threshold value UI, b ) A logic signal from the output lib in the case that the magnitude of a received signal exceeds the threshold value Uh determined in the comparator 11 and c ) A logic signal from the frequency memory 4 across the output 4a which indicates if a certain frequency is stored in the memory or not._ Across the output 9a of the control unit connected to the control in¬ put 3c of the block 3, a pulse is obtained which activates the frequency measuring as described in the above mentioned Swedish patent application, the received carrier frequency being measured. The pulse across the out- put 9a is obtained when the threshold Tl is exceeded. After a determined time Δ , the pulse across 9a ceases and, instead, across the output 9b a pulse is obtained. The output 9b is connected to the control input 3d of the block 3 where the pulse implies that the block 3 is controlled so that it delivers a continuous signal having an accurately stabilized frequency identical with the measured carrier frequency. The output 9b is, in addition, connected to the control in¬ put 4b of the AD-converter 41, the pulse across the output 9b causing that the AD-converter 41 starts the conversion. Furthermore, the switch 1 changes from the receiving - to the transmitting position, i.e. a con- nection 1a - 1c is obtained when a pulse across the output 9b and thus across the input 1d is obtained. The switch 6 is then in an open position. The third output 9c of the control unit 9 is connected to the control input 6a of a controllable RF-switch 6. This is connected between the output 3b of the frequency measuring unit 3 and the input of a trans- mitting amplifier 7. When activating the switch by means of a control signal across the input 6a the amplifier 7 is connected to the output 3b. The condition to obtain a control signal across the output 9c is: a ) The threshold Tl should have been exceeded, i.e. a "one" should be present across the output 11b of the comparator 11, or b ) The threshold Tl should have been exceeded, i.e. a "one" should be present across the comparator output 11 a and a logic "zero" is obtained form the output 4a of the frequency memory. Across the fourth output 9d the control unit delivers a writing pulse to the frequency memory 4. The writing pulse implies that a "one" is written into the memory cell which is deter.-inεd by the a 'rεss obtained from the AD-converter. A writing pulse is delivered from the control unit provided that the threshold Th has been exceeded. The written and stored "one" in the memory cell can remain during at least the time interval which corresponds to the distance between two subsequent in- coming main lobes from the same vessel. The "one" can also remain in the memory cell during a longer time, in certain cases up to 1 minute. The stored "one" then blocks the storing of carrier frequency values from vessels whose transmitted main lobes, when received in the beacon do not exceed the threshold Th (but exceed the threshold Tl ) . The operation of the arrangement will now be closer described by describing the events at three different vessel positions. In the first case it is assumed that the vessel is situated outside the distance A1 (vessel F3 in Figure 1). The signals from the vessel radar are at the radar beacon R too weak to be detected and any respon from the beacon is not obtained. In the second case, the vessel is sit ated in the area between A2 and A1 ( vessel F1 in Figure 1). The signal from the vessel are in this case so strong at the radar beacon that th signals which originate from the main lobe of the antenna exceed the threshold Tl. The threshold Tl is, however, not exceeded and signals from the side lobes do not exceed the threshold Tl. The process is the the following ( see Figure 6a): At the time tO, a pulse appears with the carrier frequency f1 (from F1 across the output 2a of the amplifier-li iter stage 2. The pulse is assumed to originate from a main lobe. The comparator 11 will then de- liver a signal ( a "one") across the output 11a but no signal across the output 11b. At the time tO, also an activating pulse is delivered across the output 9a of the control unit. The pulse across 9a activate the frequency measuring in the block 3. The carrier frequency f1 is measured and a signal in the form of a DC-level corresponding to the measured value is delivered to the frequency memory 4. At the time t1 = tO + ^ , where 4 is a certain determined delay, a pulse across the output 9b is delivered which, as described above, implies that the block 3 is controlled so that it across the output 3b delivers a con¬ tinuous signal with an accurately stabilized frequency εcual to the OMP . WIP f requency 11 . The AD-converter will start the conversion and, at the time t2, a sig¬ nal is delivered (via 41b) to the memory indicating that a conversion is terminated. Since, however, the threshold Th has not been exceeded, nothing is written into the memory. The output signal 4a from the memory at the time t3 thus is a "zero". By this, the condition b for the control signal across the output 9c of the control unit is ful¬ filled which implies that a control signal is delivered and that the switch 6 is closed. Since previously (by the signal across the output 9b), the switch 1 has switched a carrier wave pulse with the frequency f1 will be transmitted. After a certain time the antenna of the vessel has turned so that signals from the side lobes reach the beacon. The signals are, as assumed above, too weak in order to exceed the threshold Tl. Thus any signal across the output 11a of the comparator is not obtained and any frequency measuring will not be performed. This implies that the side lobe signal will not be responded. In the third case the vessel is situated within the distance A2 (vessel F2 in Figure 1). A signal from the main lobe then exceeds the thres- hold Th and the signals from the side lobes the threshold Tl. The process will be the following (see Figure 6b): At the time tO, a pulse appears with the carrier frequency f2 across the output 2a of the amplifier-limiter stage 2. The pulse is assumed to originate from a main lobe. Due to the strength of the signal, the threshold Th is exceeded and then of course also the lower threshold Tl. Thereby the condition a) for delivering the control signal across the output 9c is fulfilled and, after the frequency measuring and ad¬ justing the frequency according to the above, a pulse will be trans¬ mitted with the carrier frequency f2. In addition to this the following happens: By that t he th res ho ld Th i s exc eeded, t h e cont ro l uni t wi l l ac ros s i t s output 9d de li ver a w ri ti ng pu ls e to the f requency rr. rr.cry 4. The writing pulse- is delivered during the pulse across the output 41b of the converter and implies that a "one^. at the time t3, is written into the memory cell which is indicated by the adress from the converter 41 After a certain time (at the time t4 ) the beacon is reached by signals from the side lobes of the vessel antenna. These signals have suffi¬ cient strength to exceed the threshold Tl but not Th. This implies, analoguous to what has been described above, that the frequency measuring and — adjustment and AD-conversion is performed. The side lobe signals have the same carrier frequency as the main lobe signals. The AD-converter will thereby adress the same memory cell as before. In this, now a "one" is written and is available across the output 4a. The control unit then finds that the condition b for the control signa across the output 9c is not fulfilled and any control signal will not be transmitted and consequently no responding pulse. By the described arrangement it is thus possible that after * measuring the strength and the frequency of the received signals make such logic decisions that the radar beacon will not respond to signals which ori¬ ginate from the side lobes of the vessel antenna. OM V/IP

Claims: C L A I S 1 Method to prevent a radar beacon respond to received rada r pul¬ ses which from one of a number of transmitters are transferred with a determined carrier frequency and which originates from the side lobes of the transmitting antenna, characterized in that the strength of the signal received in the radar beacon is compared with two threshold levels (Tl, Th), the first threshold level (τl) corresponding to the lowest signal strength which should cause the radar beacon to respond, and that the proportion between the second (Th) and the first (Tl) threshold level is chosen equal to or less than the proportion between the signal strength of the signals from the main lobe and the side lobes of a normal transmitter antenna (Figure 2), the carrier frequency of the received signal is measured and stored for the signal which exceeds the second threshold level (Th) during at least such a long time which corresponds to the time interval between the radar pulses originating from two subsequent main lobes of the signal from a determined transmitter, the radar beacon is permitted to respond only to those signals which exceed the second threshold level (Th) and to signals which exceed the first threshold level (Tl) provided that the carrier frequency of the signal has not been stored. 2 An arrangement in a radar beacon which receives radar pulses from a number of transmitters each of which transmits pulses with a certain carrier frequency to carry out the method according to claim 1, characterized in a detector unit (10) to detect the amplitude of an incoming radar pulse, and a comparator circuit (11) to compare the amplitude with a first and a second threshold level (UI and Uh), respectively. a frequency measuring unit (3) to detect the carrier freαuency (f1, f2 and so on) of the pulse, an analog value corresponding to this frequency and a continuous signal with a frequency equal to the measured carrier frequency being delivered, a switching circuit (6) to receive said continuous signal and, in an activated position, to conduct this signal to the antenna of the rada beacon, a memory unit (4) including a number of memory cells to store a value in a certain determined store corresponding to a certain carrier fre¬ quency which indicates that the carrier frequency has been measured, a control unit (9) connected between said comparator circuits ( 11 ) and said memory unit (4 ) to deliver logic control signals of which a ) a first control signal (9a)is delivered to the frequency measuring unit (3) to store the carrier frequency (f1, f2 ...) of the incoming radar pulse, b) a second control signal (9b) is delivered to the frequency measuri unit (3) a certain time interval ( Δ _) after the first control signal to initiate the frequency measuring unit (3) to deliver said continuou signal, c ) a third control signal (9c) to activate the switching circuit (6) when the comparator circuit- (11) has indicated that either the signal from the detector circuit (10) has exceeded said second threshold leve ( Th) or that the signal has exceeded the first threshold level (Tl) pr vided that any value of the measured frequency has not been stored in the memory unit (4). 3 An arrangement according to claim 2, characterized in that sai memory unit ( 4 ) contains an analog/digital converter (41) to convert t analog signal obtained from the frequency measuring unit and which corresponds to the measured frequency value to a digital signal which determines the adress to the ne-ory store in which said value should be stored, and that the conversion of the ar.alog/d gi al converter is initiated by said second control signal from the control unit (9). OM WI

Inventors: CHARAS P, GUSTAVSSON P, MORVING B
Applicants: CHARAS P, ERICSSON TELEFON AB L M, GUSTAVSSON P, MORVING B
Application year: 1980
Main classification: G01S13
IPC codes: G01S13