GB2093306A - Multiple Radar Combination System - Google Patents

Abstract

A system for combining signals from a plurality of radar sets 31, 41, that may operate at different frequencies and may have spaced- apart antennas 32, 42, located to ensure coverage of all areas despite the presence of obstructions, digitally converts (at 51) radar echo signals from separate sets into signals referenced to a common coordinate system and then correlates at 52 these converted signals to produce an output useful for operating a display 58 or a collision avoidance system 54. <IMAGE>

Description

SPECIFICATION Multiple Radar Combination System This invention relates to a multiple radar combination system.

Radar systems may suffer from a variety of limitations including masking of radar beams by physical obstructions, degraded operation in poor weather and poor resolution, particularly with low frequency radar. Radar problems and limitations become even more important when radar signals are employed to operate collision avoidance systems and thus the present invention is primarily described herein with respect to this application although the invention is not limited to any particular application.

Collision avoidance systems for automatically warning of possible collision between ships, for example, are known in the art and a highly successful system is described in my U.S. Patent specification No. 3 660 846. The general background of collision warning and avoidance systems and the purpose and advantages thereof are set forth in the above-noted patent.

One of the problems with shipboard radar employed with collision avoidance system is masking of radar search areas by obstruction on the ship, such as funnels, cranes and the like.

Many ships have structures which obstruct radar signals so that one or more sectors of the horizon are "hidden" from the radar antenna. This produces blind spots or areas from which no target information may be obtained and thus introduces an uncertainty into the overall system which is undesirable. Another problem is the poor performance of high frequency radar, i.e. poor target detection of small targets that are in areas of precipitation. Low frequency radar is affected less by precipitation, however, it has poorer short range resolution. With respect to common ship radar practice, the terms "high frequency" refers to an operating wavelength of approximately 3 centimeters and the term "low frequency" refers to an operating wavelength of approximately 10 centimeters.

Modern radar requirements for large ships may include the provision of a back-up radar set, signals from the main and back-up sets are not compatible and are not combined with each other. Theoretically, an analogue combination of signals from a pair of separate radar sets might be accomplished by the use of analogue delays, but only with great complexity of equipment which has effectively precluded such combination from practical application.

The present invention broadly comprises a system for combining information from two or more radar sets that may operate at different frequencies and may have the antennas thereof physically separated.

The present invention overcomes the problem of radar masking by employing signals from spaced-apart radar antennas connected to separate radar sets whose outputs are converted to numerical form for interchangeable utilization to thus ensure searching of the entire horizon for displays and/or warning of possible collision between a ship, for example, carrying the system and other objects.

The invention is directed to the solution of a practical problem arising particularly on ships equipped with collision warning and/or avoidance systems incorporating radar equipment, but not limited to shipboard use. Structural elements on shipboard are often necessarily located in positions to intercept radar signals over come part of the radar scan so as to block effective radar searching in one or more azimuth sectors.

According to one aspect of the invention, there is provided a multiple radar combining system adapted to receive the outputs of at least two radar set comprising; target extractor means receiving said radar outputs and separately converting target information of said radar outputs into digital signals, means receiving and converting said separate digital signals into converted digital signals referenced to a common coordinate system, and correlating means for correlating said converted digital signals into a single output.

The invention is hereinafter described with respect to a pair of radar sets although it will be appreciated that additional sets may be incorporated in the same manner as may be required for particular applications.

Radar echo signals or target signals from the separate radar sets are herein converted into numerical form identifying the boundaries of targets and such information may be interrogated to determine the target centers in terms of range and azimuth or employed directly. These target signals in numerical form are converted to a common coordinate system of the "addition" of alignments data determined by separation of radar antennas, rates of antennas, rotation and time delays in the circuitry of separate radar sets so that they may be correlated and employed interchangeably and compared to establish coincidences of targets. The correlated numerical target information either singly or in combination may be employed to operate a collision avoidance system that includes automatic warning means and may also include tracking and display means.

The invention provides the capability of electronically comparing targets from separate radar sets which are not otherwise comparable in order to additionally verify the existence of true targets. Thus, for example, detection of a target by one set only may be treated as a probable target while detection of the same target by two sets may be treated as a verified target.

Alternatively, the correlator may adjustably determine the level of target signals required for target identification by either or both inputs to the correlator and apply same in the correlation process. Additionally, the separate radar sets may be operated at quite different frequencies to obtain the combined advantage of high resolution and all weather operation.

According to a second aspect of the present invention, there is provided an improved collision avoidance system comprising at least two radar sets separately connected to spaced-apart antennas located to scan all blind spots of each other, means connected to each of said radar sets converting radar output signals into numerical form, means correlating the numerical radar output signals including range and bearing information to produce a single target identification for target centers lying within a predetermined small distance of each other for subsequent processing as a single target, and means applying said bearing and range information in numerical form to a collision avoidance computer and display system.

The invention will now be described further by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic side elevational view of a container ship that may advantageously employ the present invention.

Figure 2 is a schematic plan view of the ship of Figure 1 illustrating the effect of obstructions in the "field of view" of a radar set on such ship; Figure 3 is a schematic perspective view of another vessel illustrating a common type of radar obstruction; Figure 4 is a plan view illustrating zones that are masked from the radar of the vessel of Figure 3; Figure 5 is a block diagram of a collision avoidance system; and Figure 6 is a block diagram of a multiple radar combining system in accordance with the present invention and having display and/or collision avoidance outputs.

The present invention is adapted to a wide variety of applications, however, the invention is particularly advantageous in connection with shipboard radar and the preferred embodiment there of is thus illustrated and described in connection with this application. It is first noted that a collision warning and/or avoidance system is adapted to cooperate with or incorporate a conventional radar set having an antenna from which radar signals are propagated and echo signals from targets are received. These radar echo signals or target signals are employed in the collision avoidance system to warn of possible collisions and to provide information to prevent same and thus it is clearly necessary for the radar set to accurately detect and locate objects in the field of scan.

Although the present invention has many applications, it is hereinafter described primarily with respect to the combination of signals from two or more physically separated radar antennas and utilization of the output in a collision avoidance system with notations of other applications.

Many ocean-going vessels have physical obstructions that interfere with the propogation and reception of radar signals. Thus reference is made to Figure 1 of the drawings schematically illustrating a container vessel 11 having a mast 12 atop the bridge with a rotating radar antenna 1 3 thereupon. This radar antenna is normally mounted upon a high point of the surface structures or ship, however, it will be appreciated that various structures or elements may also extend upwardly from the deck of the ship, such as, for example, a cargo of containers 1 4 stacked on the deck of the ship.

At least certain of the upwardly extending objects may well lie in the path of the radar beam as it is swept through an arc of 3800.This will then cause the radar beam to be blocked from some areas of the horizontal scan. Referring to Figure 2, there is schematically illustrated masking of the radar signals that may occur, for example, by cargo containers 14 loaded on the deck of the ship. It will be seen that the shaded area 1 6 is masked from the radar antenna 1 3 by portions of the cargo 1 4. Although fringing effects of the radar propagation may reduce the actual size of this area, it will be appreciated that there does exist an area ahead of the vessel which is not viewed by the radar set so that targets that are in this area may not be detected by the radar.

The presence of such blind spots or the masked areas in the horizontal scan of the radar system will introduce uncertainties, such as lost tracks or target swap in information supplied to a collision avoidance or warning system which is undesirable from the view point of protection against possible collision.

In Figure 3 there is illustrated another common physical obstruction to radar on a container ship in the form of a massive crane structure 1 8. A radar antenna 13 located, for example, on a mast atop the bridge of the ship would be masked from portions of the horizon by such structure and in Figure 4, there are illustrated shaded zones 1 8a and 1 8b lying behind vertical portions of the crane 18 from the antenna 13 so as to be masked from the radar beam. In these areas or zones, a target such as a small vessel would not be detected by a radar beam transmitted from the antenna 13.

The invention provides a solution to the foregoing problem by the incorporation of one or more additional radar sets in the system employed on a vessel 1 for example. The antenna of the additional set is physically spaced from antenna 13 and located so as to have an unobstructed scan of any or all masked of blind areas that are in the scan of the set connected to the antenna 1 3. Thus, for example, a further antenna 1 9 may be mounted adjacent to the bow of the vessel 11 so as to unobstructively sweep the area 1 6 with a radar beam therefrom so as to provide information as to the presence of targets therein. Quite clearly the field of view or scan of the antenna 1 9 will be obstructed rearwardly of the ship, however, these areas are covered without obstruction by the beam from the antenna 1 3. With this arrangement of the present invention, it will be seen that the entire 3600 of the horizon is unobstructively swept by a radar beam from one of the other of the sets connected to the antennas 13 and 19.

Individual radar sets connected to antennas 13 and 19, for example, individually produce coherent and highly useful information as to range and bearings of targers, however, the signals from these separate sets are not compatible nor are they related in any manner that will allow their normal combination or utilization in combination. The invention provides for combining signals from two or more radar sets to thus facilitate problems related to radar masking and the like.

Reference is now made to Figure 5 of the drawings illustrating in block form, a collision avoidance system employing a single radar set.

Radar set 21 propagates a narrow beam of high frequency electromagnetic radiation from an antenna 22 which is rotated to scan the horizon and this antenna 22 receives radar signals reflected from objects as echo signals or target signals. The radar output, which is herein taken to include video, trigger and azimuth signals, is applied from the radar set 21 to electronic circuitry herein termed a target extractor 23, wherein a variety of functions could be conventionally performed, such as verifying the size and existence of an actual target. It is recognized that various conditions may produce false target indications, such as reflections caused by atmospheric conditions.In all circumstances, so called "sea clutter" results from the reflection of some radar signals from the sea surface so as to produce a noise level or the like which may be sufficiently high to mask physical objects comprising targets of interest. Thus the target extractor separates target identification information from sea clutter as by counting the number of reflected pulses out of some predetermined number of transmitted pulses to distinguish between random reflections and target reflections. Additionally, the circuitry may count the number of scans in which a particular target is identified and compare this to a predetermined number as a further verification of target existence. Numerous manners of target verification are known in the art including the "m" out of "n" detection and at least many of these are applicable to the present invention.Also the amplitude of reflected pulses may be compared to a predetermined amplitude level as a further manner of verifying target existence.

The extractor 23 additionally and primarily converts the data from the radar set into numerical form. This may conveniently comprise the generation of four binary words identifying the azimuth and range limits of the detected targets.

This conversion is normally carried out in collision avoidance system of the type herein contemplated as being connected to the output of the present invention. This data in numerical form may then be operated upon by a center finder 24 to determine the range and bearing of the center of the target and target information as to target limits or center is commonly fed to a collision avoidance computer 26 which may include tracking means for producing a variety of information such as closest point of approach time to closest point of approach. In addition to the foregoing, the collision avoidance system includes an alarm 27 and one or more displays 28 which visually indicate protected areas established by the system and the location of targets therein an analogue form and also in digital form if desired.At least a part of the visual display is to be provided on the display of the radar set, as indicated by the dashed line in Fig. 5.

The invention may incorporate a collision avoidance system, such as that illustrated in Figure 5 and briefly described above. Referring to Figure 6, there will be seen to be illustrated a pair of radar sets 31 and 41 separately connected to radar antennas 32 and 42, respectively, which may be physically separated and located as noted above, so as to unobstructedly scan any and all blind spots of the other antenna. Each of these radar sets 31 and 41 generate data as to the bearing, range and extent of objects causing reflected signals. The radar output data from the radar sets 31 and 41, as defined above, are separately fed into separate target extractors 33 and 43, respectively, wherein such data is converted into numerical or binary form and target information may be verified as noted above.Each of the target extractors 33 and 43 may, if desired, feed center finders 34 and 44, respectively, wherein there is produced a numerical signal indicative of the center of the target information or data from the separate radar sets. It is noted that the target data, i.e., range and bearing limits, may be directly employed in the remainder of the system without locating target centers, however, sophisticated systems normally include identification of target centers.

The preferred embodiment provides an alignment data unit 51 which contains information for converting numerical data from the radar sets into data related to a common coordinate system. The foregoing may either comprise information for converting the data of one radar set into the coordinate system of the other or for converting the data of both radar sets into a convenient separate coordinate system.

The term coordinate system is herein defined as a reference from which range and bearing are measured, so that correction is made for physical separation of antennas, and it is noted that different time delays in separate radar sets produce an apparent shift of coordinate systems.

The foregoing may be best understood by considering an example of a target A located at relative bearing of 0 or 3600, i.e. dead ahead of the vessel 11 in Figure 2. With the radar antennas 32 and 42 of Figure 4 located at the points 13 and 19 of Figure 2, it will be seen that both radar sets 31 and 41 will identify the target at bearing 00, however, each set will identify the target at a different range, with difference in range being the distance between the points 1 3 and 1 9 on the vessel.Assuming that the data from the radar sets is to be converted into information relative to the position of antenna 32, the alignment data unit 51 would then add a constant to the range from radar set 41 with this constant being equal to the distance between the points 1 3 and 1 9 in the vessel.

Alternatively, a target located at point B of Figure 2 lying on a perpendicular bisector of a chord between points 13 and 19 would cause each of the radar sets 31 and 32 to produce equal range signals, but different bearing signals. These azimuth signals might, for example, comprise 1500 for radar set 31 and a bearing indication of 300 for radar set 41. These readings, of course, depend not only upon the bearing, but also range, because of the physical separation of the radar antennas and thus the alignment data unit 51 contains a substantial number of constants related to range and bearing of target data from radar set 31 ,for example, which, when added to target data from radar 41 would compensate the latter for the physical separation of the radar antennas.The alignment data unit 51 also contains correction data for different time delays in the circuitry of the separate radar sets 31 and 41 in order to fully convert radar output data into a common coordinate system. The radar output data from radar set 31 is thus fed into the alignment data 51 wherein the appropriate conversion constants are chosen on the basis of this data and applied to a combiner 46 connected to the output of the center finder 44 for the radar set 41. Under this circumstance, the data of radar set 41 is modified to correct for the physical displacement of the radar antennas 32 and 42 and time differences.

Under the circumstances wherein it is desired to modify the data from each of the radar sets to correct for displacement of the antennas thereof from some central point such as the center of the vessel 11, data from radar set 41 is also applied to the alignment data unit 51, so that the output thereof comprises a correction or modification for the data from each of the radar sets, and this is applied not only to the combiner 46, but also to a combiner 36 connected to the center finder 34 for the radar set 31.

As illustrated in Figure 6, the outputs of the combiners 36 and 46 are applied to a correlator 52 wherein the data from the two sets are compared to determine, for example, whether or not both radar sets have identified the same target. Under certain circumstances of operation, the present invention may be employed to produce an output from the correlator 52 either upon a coincidence or target identification by both radar sets or upon target identification by either radar sets. The separate target signals applied to the correlator 52 from radar sets 31 and 41 are wholly compatible and thus a variety of operations may be performed by the correlator which may include control means 53 for selecting different modes of operation to accomplish different functions.The correlator determines whether or not simultaneous targets of separate radar sets are within a predetermined small range and bearing limits so as to comprise the same target or whether the single output of the correlator should identify two targets. The correlator may also compare target quality information such as pulse amplitude carried by digital signals for each and/or both sets to some predetermined level for identifying or not identifying a target. The target information from the correlator 52 may be applied to collision avoidance computer 54 which may include a tracker as in a conventional collision avoidance system wherein information as to range, bearings, speed and course of targets appearing within the operating range of the systems are then applied to a display 56.It will, of course, be appreciated that collision avoidance and warning systems often incorporate the capability of establishing areas or guard zones of determinable range, range depth and azimuth extent for the automatic production of warning signals upon the verified intrusion of objects within such zones.

Additionally, the limits of such areas may be visually displayed together with the identification of the location of targets therein for ready utilization by operators or the like.

The system of the preferred embodiment has a range of capabilities from the simple exclusion of blind spots or areas in radar scanning to the verification of targets in a number of different modes. Thus the output of the correlator 52 may comprise the identification of the range and bearing of a target or targets detected by either radar set and may additionally contain information as to whether or not the target is identified by both radar sets. This information is, as noted above, employed in the collision avoidance computer either in most simple form, i.e. target identification, or in more complicated form as to whether it is a target identified by a single set or a target identified by both sets.Some collision avoidance systems provide a display bracket about each target being tracked and the correlator output may also identify the size of targets for assistance in establishing this bracket and for additional information to the operator of the system.

It is also noted that at least certain functions of the present invention are capable of being performed in whole or in part by a computer and a collision avoidance computer may be employed for this puspose when the present invention feeds a collision avoidance system having such a computer.

The multiple radar combining system described produces output signals, as noted above, that may be employed for a variety of purposes. Thus these signals, may be converted into "analoge" signals suitable for operating a conventional radar display, for example. In Figure 6 there is shown a converter 57 receiving output signals from the correlator 52 and converting these signals into suitable form for operating a display 58. Although the original radar signals need not be fully recreated, the conversion is herein termed as being to analogue form as distinguished from straight digital which is normally unsuited for operating most analogue display systems. This display of the output of the present invenion may be quite advantageous for situations, as aboard a large seagoing vessel, wherein separate radar sets and antennas are provided, for example, on opposite wings of the bridge.

The invention provides for the combination of radar outputs from separate radar sets and the situation of spaced apart radar antennas has been described above as an example. Many other examples also exist and note is particularly made of circumstances wherein scanning is carried out with two radar sets operating one at high frequency and one at low frequency. The antennas of such sets may be mounted on the same mast so that there is no appreciable physical separation, however, differencies in rate of rotation of the antennas, heading of antennas and time delays in circuitry of the separate sets will cause the same target to be located at different times by separate sets. This has the effect of different coordinate systems which is equivalent to physical separation of antennas.

The invention is equally applicable to and advantageous in the combination of radar outputs under the above noted circumstances even though no physical separation of antennas exist.

The advantages of good resolution with high frequency radar and good long range and all weather performance of low frequency radar may thus be obtained with the present invention. Such a combination of high and low frequency radar may also be employed with spaced antennas.

It will be appreciated that no attempt is made in the block diagram of Figure 6 to illustrate all electronic connections or components that may be employed in an overall system, but instead, the blocks are intended as function blocks rather than circuit blocks. Convention electronic circuitry may be employed for the individual functions of the present invention, and one skilled in the art may design circuitry for accomplishing the functions of the separate blocks of the present invention.

In one example set forth above, the radar sets were stated to be operated at a relatively high frequency for one, and a relatively low frequency for the other, although it will be appreciated that this is not necessary. There are, however, certain advantages to employing substantially different frequencies for the sets inasmuch as a low frequency radar set provides poor resolution, but does provide superior operation in rain and for distant targets. A high frequency radar set, on the other hand, provides good resolution but degraded operation in bad weather. Thus the invention may be employed not only to overcome the problems of obstructions to the propagation and receipt of radar signals at radar antennas, but also to overcome problems of atmospheric conditions.

The components and operation of collision avoidance and warning system in general are only briefly described above, inasmuch as same are known in the art. It is, however, desired to emphasize the overall purpose and function of collision avoidance systems which are designed to automatically activate audio-visual alarms upon the location by radar or a target or object that may present a threat of collision for the purpose of requiring an operator to take some positive action either to turn off the alarm or possibly to change the course or speed of the vessel to prevent collision. Reference is again made to my above noted prior patent for a further discussion of the purposes, objectives and advantages of collision warning and collision avoidance system. The various capabilities of a collision avoidance system may also be employed for other purposes such as harbor control of vessels with radar antennas widely separated but feeding a central display, oil drilling platforms with radar antennas on separate corners, surveilance monitcring, and many others.

The invention will be seen to provide a relatively simply, but highly advantageous system that may, for example, be employed to achieve a marked improvement in collision avoidance systems. Although the invention has been described with respect to particular preferred embodiments thereof, it is not intended to limit the invention to the precise terms of description or details of illustration inasmuch as it will be apparent to those skilled in the art that numerous modifications and variations may be made within the spirit and scope of the invention.

Claims (13)

Claims

1. A multiple radar combining system adapted to receive the outputs of at least two radar sets comprising target extractor means receiving said radar outputs and separately converting target information of said radar outputs into digital signals, means receiving and converting said separate digital signals into converted digital signals referenced to a common coordinate system, and correlating means for correlating said converted digital signals into a single output.

2. The system of Claim 1, further defined by means connected to said correlating means for converting said output to analogue signals and applying same to a radar display.

3. The system of Claim 1, further defined by said correlator comparing converted digital target signals from separate radar sets to produce a single target identification signal for signals representing target locations within predetermined small range and bearing limits.

4. The system of Claim 1 , further defined by at least two radar sets operating one at a high frequency and one at a low frequency.

5. The system of Claim 1 , further defined by said radar outputs being derived from radar sets having physically separated radar antennas.

6. The system of Claim 1 , further defined by a collision avoidance system connected to receive the output of said correlating means for warning of possible collision with targets identified by said system.

7. The system of Claim 1, further defined by said correlator operating upon converted digital target information of each of said radar outputs to produce an output only for certain predetermined signal information from either one or both radar outputs.

8. The system of Claim 1, further defined by said system also providing target quality information to said correlation means for producing said single output frdm predetermined and adjustable target quality of either one or a combination of targets from separate radar outputs.

9. An improved collision avoidance system comprising at least two radar sets separately connected to spaced-apart antennas located to scan all blind spots of each other, means connected to each of said radar sets converting radar output signals into numerical form, means correlating the numerical radar output signals including range and bearing information to produce a single target identification for target centers lying within a predetermined small distance of each other for subsequent processing as a single target, and means applying said bearing and range information in numerical form to a collision avoidance computer and display system.

1 0. The system of Claim 9 further defined by means producing signals identifying each target whether or not such target was detected by more than one radar set.

11. The system of Claim 9 further defined by said radar antennas being mounted in extension upward from ship with one adjacent the bow and one adjacent the stern of the ship.

12. The improvement of Claim 9 further comprising means for comparing the target data of radar output signals in numerical form from each of said radar sets including means for compensating radar target data signals for possible physical separation of radar antennas and timing differencies and a correlator comparing compensated signals to identify the coincidence and non-coincidence of radar targets from separate radar sets.

13. A radar system constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.