Radar warning correlator.

0001The present invention relates generally to radar warning systems for aircraft and, more particularly, to a radar warning correlator connected in the signal processing chain of an aircraft radar warning system ahead of the final processing computer of the system.

0002To be able quickly and exactly to determine the position and type of a radar transmitter sweeping over an aircraft with its radar pulses, the radar warning system of the aircraft preferably should be capable of processing measured radar pulses in real time. In view of the pulse frequencies that may occur, some type of compressing pulse processing ahead of the final processing of the computer is then required.

0003It is the object of the present invention to facilitate such real time processing of radar pulses by providing a radar warning correlator reducing the amount of data to be processed by a subsequent computer.

0004According to the invention, this object is achieved by means of the said correlator which is characterised by an address forming unit to form addresses of records in a first file of a memory unit on the basis of the contents of pulse descriptors each comprising measured values of a plurality of parameters characteristic of a pulse in a radar pulse burst sweeping over the aircraft, and to form addresses of records in a second file of said memory unit on the basis of the contents of each individual record in said first file, a correlating unit for comparing the contents of each pulse descriptor with the contents of the corresponding record in said first file and comparing the contents of each record in said first file with the contents of the corresponding record in said second file, a logic unit for updating each of said last-mentioned contents with each of said first-mentioned contents upon the occur- currence of correlation therebetween, and a transfer unit for transferring the contents of the records of said second file to the computer for final processing, said units being adapted to carry out, as a first step, the processing of said pulse descriptors and the records of said first file and, as a second step, the processing of the records of said first file and the records of said second file.

0005Preferred embodiments of the correlator according to the present invention are characterised by the features stated in the subsequent nonindependent claims.

0006The input date to the correlator thus are digital pulse descriptors which are the result of a preprocessing of the electromagnetic radar pulses received by the radar antenna of the aircraft. These pulse descriptors are processed in the correlator in two steps, i.e. a pulse sorting step and a burst sorting step.

0007In the pulse sorting step, it is endeavoured to build up, by means of the incoming pulse descriptors, signal type descriptions of the radar transmitters which at the moment are sweeping over the aircraft. These transmitter descriptions which are termed bursts, are organised in a burst file in which it is endeavoured to find the correct record (burst) to be updated with the pulse values by means of a hashing procedure in which the pulse parameter values constitute the address of the probable record for updating.

0008In the subsequent burst sorting, it is endeavoured, when a burst has ceased, to build up, by means of said burst records, signal type descriptions of active transmitters which at the moment are not sweeping over the aircraft. These transmitter descriptions which are termed transmitters, are organised in a corresponding transmitter file. Searching and updating are effected essentially in the same manner as during pulse sorting.

0009By the thus successive build-up of the transmitter descriptions, rapid processing is obtainable, the transmitter records of the transmitter file being dealt with, directly record by record, by the final processing computer for further identification.

0010A preferred embodiment of a radar warning correlator according to the present invention will now be described in more detail below, reference being had to the accompanying drawings in which Fig. 1 is a block diagram of a microprogrammed correlator according to the present invention. Fig. 2 is a functional diagram illustrating the hashing and correlating mechanism in the correlator according to the present invention. Fig. 3 is a process and data structure diagram for the correlator according to the present invention.

0011A radar warning correlator 1 illustrated in Fig. 1 comprises a communication memory unit 2, a main memory unit 3, an address forming unit consisting of an address logic unit 4 and an address table unit 5, a register unit 6 and an arithmetic logic unit 7, all of which units are connected to a common data bus 8. The correlator further comprises a correlating unit 9 and a control unit 10, said control unit 10, said correlating unit 9 and said arithmetic logic unit 7 being connected to one another and to the data bus 8 by means of a first separate bus 11, while said correlating unit 9, said arithmetic logic unit 7 and said register unit 6 are connected to one another by means of a second separate data bus 12. The correlator 1 is connected, via the communication memory unit 2, to a data bus 13 forming part of the final processing computer of the aircraft radar warning system.

0012The correlator 1 receives its input data, via an interface 14, from a preprocessing unit connected to a radar antenna. More particularly, the preprocessing unit generates, from the measured radar pulses, digital pulse descriptors comprising measured values of a plurality of parameters characteristic of a pulse in a radar pulse burst, such as the pulse width, the pulse amplitude and the direction to the radar transmitter.

0013The communication memory unit 2 is a write and read memory (RAM) which constitutes the communication area between the external computer and the correlator 1. The function of the memory unit 2 comprises receiving from the computer a message which the correlator 1 can read and interpret. In reply to this message, the correlator can supply data or status information, or open a communication channel to units positioned farther out in the signal processing chain.

0014The main memory unit 3 is a mass memory of the write-read type logically divided into three data areas or files which are termed burst file, transmitter file and logger file. The burst file comprises a number of records containing signal type descriptions of pulses from radar transmitters which.-at the moment, are sweeping over the aircraft. The transmitter file, in its turn, comprises a number of records containing signal type descriptions of active transmitters which, at the moment, are not sweeping over the aircraft. The records of the transmitter file are updated by records from the burst file for which sweeping has ceased. The logger file, finally, comprises a number of logger records, each of which is a copy of the burst record which was the last to update a specific transmitter record. Each active transmitter record has, in itself, a pointer to that location in the logger file where the burst record which was the last to update the transmitter post, can be found. Those burst record copies of the logger file which pertain to one and the same transmitter post, are organised in the form of a linked list.

0015The address logic unit 4 is specially designed to follow the logic structure of the main memory unit 3. Thus, it is possible with the address logic unit 4 to directly address different fields within a record or the same element within different records.

0016Addressing of the address table units 5 also is effected by means of the address logic unit 4. More particularly, the table unit 5 contains pointer tables for determining addresses of, respectively, the burst file and the transmitter file of the main memory unit 3. The table unit 5 also has an equalisation function on the contents of a pulse descriptor, before this is utilised, by a hashing function, for looking up in a pointer table the address of a record in the burst record file of the memory unit 3. The equalisation function can be modified from outside via the communication memory unit 2 by means of the external computer.

0017The register unit 6 is a notation memory for data and system variables, whereas the arithmetic logic unit 7 contains a conventional arithmetic logic unit and a mean value forming unit (digital lowpass filter), a logarithmising unit (table finding in read memory) and maximum and minimum comparators.

0018The correlating units 9 is adapted to carry out its correlating function by comparing input data with window or interval limits tabulated in a read-write memory. The size of the correlating windows can be modified from outside by the external computer via the communication unit 2.

0019The control unit 10 contains the microprogram memory, the program control logic, condition decoders for conditional jumps, and a separate read memory, and entirely controls the activity of the correlator on the basis of the microprogram.

0020The mode of operation of the correlator according to the present invention will be described below and is divided into a pulse handling process, a burst handling process and an aging process.

0021As regards the pulse handling process, i.e. the updating of the burst file records with received pulse descriptors, pulse descriptors are read from a queue into the register unit 6, simultaneously as the parameter values of the pulse descriptors are equalised in the table unit 5, whereupon the equalised pulse descriptor is utilised in the same unit as look-up data in an address table for the burst file in the memory unit 3. These sequences occur in parallel for each component in the read-inpulse descriptor which comprises eight character sets or bytes of data.

0022The pointer found in the address table unit 5 is used by the address logic unit 4 as an address of a record in the burst file of the main memory unit 3.

0023In the next step, the data of the pulse descriptor, as stored in the register unit 6, are compared with the contents of the pointed-out record in the burst file with the aid of the correlating unit 9 which receives these data via the two separate data buses. The correlating unit 9 also comprises the correlating window blocks indicated in Fig. 2. If·the descriptor and the addressed burst record are correlated, the latter will be updated with the values of the descriptor. If no correlation exists, a new record is looked up in the burst file by the address-forming unit 4, 5, and this is repeated until a predetermined maximum number of attempts have been made. After that, a new record is established in the burst file, and the record address concerned is written in the address table unit 5.

0024If, on the other hand, correlation exists, then the addressed record is updated in the memory unit 3 by the arithmetic logic unit 7 which for this purpose receives on the one hand the contents of the record concerned and, on the other hand, the contents of the processed descriptor stored in the register unit 6. As will appear from Fig. 2, burst handling, i.e. the updating of the records in the transmitter file with the records in the burst file, is carried out entirely in analogy with the pulse handling and follows similar algorithms, by utilising the same hardware. Instead of correlating pulses relative to burst descriptions, it is here attempted to refer burst records to existing transmitter descriptions.

0025The aging process indicated in Fig. 3 has for its object to maintain real time synchronism in the correlator. To activate the process, a real time clock (hardware) causes an interruption at intervals of the predetermined number of clock cycles, and it is checked whether a sweep is ready, i.e. that no burst record has been updated within a given period of time, and furthermore aging of transmitter records is effected so that nonactive transmitter posts are removed and room is made for new ones. If a burst record has not been updated within a given period of time, the aging initiates the burst handling process which attempts to refer the burst to a transmitter description. In this manner, the record in the burst file can be vacated, and the pointer in the address'table unit 5 may be removed.

0026When the external computer calls for attention from the correlator 1 by introducing a message into the communication memory unit 2, the correlator 1 abandons its normal work of sorting pulses and bursts and changes its mode of operation. Reading and interpreting the message from the external computer occurs in the control unit 10 which also sees to it that the correlator 1 supplies the information to the external computer which this computer may need for its identification work.

0027Finally, it should be mentioned that the external computer, via the communication memory unit 2, can gain access to the memory unit 3, the address table unit 5 and the register unit. For instance, system constants may be supplied from the outer computer via the communication memory unit 2 into the register unit 6, irrelevant descriptions may be deleted from the memory unit 3, new equalisation functions may be introduced into the address table unit 5 and new correlating window values may be introduced into the correlating unit 9.

0028In order quickly to look up a record against which a correlating attempt can be made, the present invention thus utilises a hashing procedure. First, relevant parameter values are transformed via an equalisation function which may be used in order to dampen large variations in a parameter, such as amplitude variations during a sweep, whereupon a table input value is devised via a hashing algorithm. This input value is then used to look up in an address table a pointer to a probable burst or transmitter record. If the input is "vacant", i.e. if the looked-up pointer does not point to any record, record pointers in the adjacent inputs in the address table are investigated.