Surveillance radar

Abstract: The invention relates to a surveillance sensor provided with at least one surveillance radar antenna 1 and at least one co-located and co-rotating electro-optical surveillance sensor 5 mechanically connected to said radar antenna. A combined panoramic picture is compiled by combining information from both sensors using a common track unit 30.

Description: The invention relates to a surveillance sensor provided with a surveillance radar antenna, first drive means for rotating the surveillance radar antenna about a substantially vertically disposed first axis of rotation, an electro-optical sensor and second drive means for rotating the electro-optical sensor about a second axis of rotation, disposed substantiallly parallel to the first axis of rotation. An embodiment of such a sensor can be found in the U.S. patent 3,010,102, where a thermal-energy detection system in conjunction with a radar system is disclosed. The radar antenna and the optical components of the thermal-energy detection system are mounted upon a rotatable mast or shaft in cooperation and direct alignment with each other. The radar is held normally inoperative, and the thermal optical units are operative and are moved towards particular sections of an area of scan, and collect thermal-energy from the section under observation. Whenever a temperature discontinuity source is encountered a variation occurs in the amount of thermal-energy received by the thermal energy detector. This variation is translated into electrical fluctuations which halt the rotation of the shaft upon which the scanning elements are mounted, thus placing the radar antenna and the thermal optical units at rest in alignment with the temperature discontinuity source. However, this embodiment has the disadvantage that the radar sensor and the infrared sensor are rigidly connected to each other. Thus it is never possible to use the radar sensor independently of the infrared sensor. The present invention has for its object to obviate this disadvantage and is characterised in that the electro-optical sensor is mounted on top of the surveillance radar antenna and the surveillance radar antenna is provided with the second drive means for rotating the electro-optical sensor with respect to the surveillance radar antenna. In this case the electro-optical sensor may rotate at a higher rate to obtain data at a higher frequency. On the other hand, a lower rate of rotation can increase the sensitivity of the electro-optical sensor. A further advantage of the present invention with respect to the prior art is that additional elevation data of a moving object to be tracked by the surveillance sensor, may be obtained by the electro-optical system, independently of the surveillance radar. The azimuth and elevation information from a relatively inexpensive electro-optical sensor can now be combined in a simple way with the azimuth and range information of a radar sensor. It should be noted that radar trackers exist which are assisted in detecting a target by, e.g. an infrared surveillance sensor. After detection of a target by an infrared surveillance sensor, the radar tracker takes over the target and keeps tracking it by continuously pointing a track antenna at the target. However, a radar tracker of this kind cannot be used to give an overall picture of the surroundings, contrary to a surveillance radar, which for this purpose periodically rotates an antenna about a usually vertical axis at a customary rotation time of at least several seconds required for sufficient illumination of a target. Moreover, infrared surveillance equipment exists which provides a panoramic picture of the surroundings by also rotating about a vertical axis. Rotation times up to approximately half a second are customary for this equipment. Owing to the usually high resolution of an optical sensor, lower rotation times may lead to blurring in case of positioning on a moving platform. A combination of both types of surveillance sensors is, for this reason amongst others, not obvious, because it would lead to a degradation in properties of one or both surveillance sensors. A low pulse momentum of the surveillance antenna is obtained in an embodiment characterised in that the first axis of rotation and the second axis of rotation are substantially coincident. An embodiment can be characterised in that the electro-optical sensor is provided with detector elements arranged in a row, which detector elements form a line, parallel to the first axis of rotation, of the field of view covered by the electro-optical sensor. Furthermore, an advantageous embodiment of the surveillance sensor is characterised in that the detector elements are sensitive to infrared radiation. The advantage of this embodiment is that the electro-optical sensor can also be used at night. Another embodiment of the surveillance sensor, comprising special advantages, is characterised in that the electro-optical sensor and surveillance radar antenna are adjustable with respect to each other across a limited angle about a third axis of rotation, the surveillance sensor is provided with adjusting means suitable for executing the said adjustment, and the third axis of rotation is oriented practically perpendicular to the first and second axes of rotation. A wide elevation range is thus obtained, where the electro-optical sensor and the radar antenna each illuminate a section of that elevation range. In case the radar antenna has a wider elevation range than the electro-optical sensor, the latter may be adjusted in elevation, if necessary per revolution. When the sensor is mounted on a moving platform, orientation stabilisation will be advisable. A first embodiment is thereto characterised in that the surveillance radar antenna is provided with first orientation stabilisation means suitable for perpendicularly orienting the first axis of rotation with respect to an earth-oriented reference plane. In this construction the entire surveillance sensor is stabilised. A second embodiment is characterised in that the surveillance radar antenna is provided with beam-orientation adjusting means. In this embodiment, control of the beam orientation results in a stabilised sweep. Because the need for an expensive and heavy stabilisation unit for the surveillance radar antenna is now obviated, the advantage of this embodiment is that the surveillance sensor remains light-weight and inexpensive. A further advantage is that the stabilisation requires only a very low time constant, because mass inertia effects of the mass of the entire surveillance sensor do not play a part. The advantages of combination are increased in an embodiment where the surveillance sensor is provided with a radar plot extractor connected to the surveillance radar antenna for obtaining each antenna revolution at least azimuth, range and doppler speed information; with an electro-optical plot extractor connected to the electro-optical sensor for obtaining at least azimuth and elevation information each revolution of the electro-optical sensor; and with a common track unit connected to the radar plot extractor and the electro-optical plot extractor for combining the information obtained, for generating on the basis of the combined information a target track to control a weapon system to be connected. By combining the information originating from both sensors in the earliest stage possible, and subsequently supplying it to a common track unit, a considerable amount of time is gained in comparison with a configuration comprising a separate track unit for each sensor. In an embodiment where the track unit comprises a fast logic unit for promoting combined information to a target track within the time required for one revolution of the surveillance sensor, if this information complies with preset criteria, the normal track process may be reduced for urgent situations. The invention will now be explained with reference to the accompanying figures, of which Fig. 1 represents a surveillance radar antenna provided with an electro-optical sensor; Fig. 2 is a schematic diagram of a rotation-fixed attachment of the electro-optical sensor not in accordance with the invention; Fig. 3 is a schematic diagram of a rotatable embodiment of the electro-optical sensor; and Fig. 4 shows a signal processing unit for the combination of the signals from the surveillance radar antenna and the electro-optical sensor. In Fig. 1, a surveillance radar antenna 1 is rotatable with respect to an antenna support 2 about a first axis of rotation 3 by means of drive means 4. In this embodiment, surveillance radar antenna 1 is a well-known slotted waveguide. An electro-optical sensor 5 is fixed in line with the first axis of rotation 3 on surveillance radar antenna 1 by means of fixing means 6. The electro-optical sensor 5 is rotatable about a second axis of rotation 7 oriented perpendicular to the first axis of rotation 3. A fixing means 6 is shown in Fig. 2. The electro-optical sensor 5 is suspended in a supporting frame 8 which is fixed to a top side 9 of surveillance radar antenna 1. The electro-optical sensor 5 is rotatable about the second axis of rotation 7 and is thereto provided with suitable bearing 10, rotation drive means comprising a gear transmission 11 and 12 which are linked with servo motor 13. The optical line of sight of electro-optical sensor 5 in this embodiment crosses the first axis of rotation 3. A feed-through aperture 14 is provided for feeding through electrical and/or optical signal carriers to and from the electro-optical sensor 5. Fig. 3 shows an embodiment of the invention with a rotatable connection of the electro-optical sensor 5 to the surveillance radar antenna 1. Frame 8, in which the electro-optical sensor 5 is suspended, is fixed to a supporting part 15 which is supported by a bearing 16. This bearing 16 allows rotation of the supporting part 15 about an axis of rotation 17 with respect to top side 9 of surveillance radar antenna 1. In this embodiment, this axis of rotation 17 coincides with the first axis of rotation 3, but axis of rotation 17 may also run parallel but not coincidental with the first axis of rotation 3. Gear transmission 18 and servo motor 19 are applied for this rotation. To allow feed-through of electrical and/or optical signal carriers, feed-through aperture 14 is provided, as well as a well-known rotatable electrical and/or optical coupling 20. The electro-optical sensor is preferably an infrared-sensitive sensor which is also capable of functioning at night and under adverse atmospheric conditions. A more cost-effective embodiment is obtained with a daylight sensor such as a common TV camera or a image intensifier. A combination of the above-mentioned types of sensors has the advantage of increasing the detection probability because it enables selection of the sensor that offers the optimal target contrast. The electro-optical sensor is preferably a complete, autonomous camera unit provided with the necessary optics, radiation-sensitive elements and, if applicable, cooling equipment and control electronics for sampling and filtering of the received signals. To reduce blurring due to movement to a minimum, the electro-optical sensor may be provided with mirrors or prisms adjustable in angle and positioned on the optical axis. In case of the embodiment comprising the infrared sensor, a favourable embodiment is obtained when use is made of a line array of infrared sensitive elements, where the line array has a vertical orientation in the field of view. The scanning movement perpendicular to the longitudinal direction of the line array, required for picture compilation, is obtained by means of the rotation of the electro-optical sensor about the first axis of rotation 3. Furthermore, the electro-optical sensor can be supplemented with a laser rangefinder for obtaining range information independent of the radar. Fig. 4 shows the signal processing unit relevant to the combination of signals. In this signal processing part, signals originating from the electro-optical sensor are supplied to a plot extractor 29. In this plot extractor 29 extraction of targets takes place by means of known image processing techniques. The azimuth, elevation and intensity values of possible targets obtained per full revolution of the rotating sensor, are subsequently supplied to a common track unit 30. The signals originating from surveillance radar antenna 1 are supplied to known radar plot extraction means 31, which provide at least range, azimuth, signal strength and doppler speed information of possible targets. This information is also supplied per revolution of the antenna 1 to the common track unit 30. In case of a 3D surveillance radar antenna 1, elevation information is also added. Track unit 30 comprises a fast decision unit 32 for the purpose of selecting threatning situations in which the usual track processing would take too much time. This is the case, for instance, when a helicopter, appearing above the edge of a wood during a short time to detect a target, subsequently disappears to assign a weapon and reappears to fire the weapon. The usual track processing would in that case take too much time to allow target engagement within the short time the helicopter is visible. Fast decision unit 32 therefore selects targets with a Doppler speed of practically zero and an elevation around the visible horizon to supply this information to a fast track unit 33 which, preferably within the time required for one revolution, generates a track for the purpose of control of weapon system 34. A track in this case is understood to be target position information related to a target within a fixed coordinate system and during several measurement, suitable to control a weapon system. It should be noted that in the case of surveillance radar, it is customary to only generate a track after several revolutions of the antenna, which would in this case take too much time. However, the available elevation information and simultaneous detection in another wavelength range as a result of the combination with an electro-optical sensor may produce a reasonably reliable track after only one revolution. In non-threatening situations, the information from both sensors is supplied to a normal track unit 35. Here combination takes place of the information from both sensors on the basis of common azimuth values. The combined information is then mathematically expressed in a vector of state characterising the target, which vector is adapted each measurement to new information from one or both sensors. Track unit 35 uses well-known track algorithms with preset parameters dependent on the expected target trajectory. For example, for a straight target trajectory other parameters should be selected than for a curved target trajectory. A target may have a sinusoidal trajectory or become temporarily invisible. The latter occurs in the case of a helicopter popping up every now and again from behind the edge of a wood. Well-known tracks algorithms are for instance α, β, γ algorithms, with parameters α, β, and γ for weighting respectively the target position, target speed and target acceleration. It is important that the track unit 35, at predetermined intervals, should generate track data to control weapon system 34. Processing of the information originating from extractors 29 and 31 should preferably be completed well within the time required for a next measurement. A process subject to strict time limits is therefore also characterised as a real-time process. However, because of the variety of available information originating from sensors operating in different wavelength ranges and the complex environment in the case of land-based applications, it is advisable not to limit the track processing to one algorithm only. For this reason, the track process is supported by a non-real-time support unit 36. Support unit 36 draws up hypotheses concerning the target and the target trajectory. With these hypotheses the parameters of the track algorithms can be determined. Within the support unit 36, a distinction can be made between functions for the generation of new hypotheses (37), for maintaining hypotheses (38), rejecting hypotheses (39) and for testing of hypotheses (40). New hypotheses may consist of the various target trajectories mentioned before. Maintaining hypotheses implies that it is assessed whether the tracks suit the hypotheses selected. Support unit 36 uses artificial intelligence techniques in contrast to the strict algorithmic track processes of track unit 30.

Claims: Surveillance sensor provided with a surveillance radar antenna, first drive means for rotating the surveillance radar antenna about a substantially vertically disposed first axis of rotation, an electro-optical sensor and second drive means for rotating the electro-optical sensor about a second axis of rotation, disposed substantiallly parallel to the first axis of rotation, characterised in that the electro-optical sensor is mounted on top of the surveillance radar antenna and the surveillance radar antenna is provided with the second drive means for rotating the electro-optical sensor with respect to the surveillance radar antenna. Surveillance sensor as claimed in claim 1, characterised in that the first axis of rotation and the second axis of rotation are substantially coincident. Surveillance sensor as claimed in one of the above claims, characterised in that the electro-optical sensor is provided with detector elements arranged in a row, which detector elements form a line, parallel to the first axis of rotation, of the field of view covered by the electro-optical sensor. Surveillance sensor as claimed in claim 3, characterised in that the detector elements are sensitive to infrared radiation. Surveillance sensor as claimed in one of the above claims, characterised in that the electro-optical sensor and the surveillance radar antenna are adjustable with respect to each other across a limited angle about a third axis of rotation, the surveillance sensor is provided with adjusting means suitable for executing the said adjustment and the third axis of rotation is oriented practically perpendicular to the first and second axes of rotation. Surveillance sensor as claimed in one of the above claims, characterised in that the surveillance radar antenna is provided with first orientation stabilisation means suitable for perpendicularly orienting the first axis of rotation with respect to an earth-oriented reference plane. Surveillance sensor as claimed in one of the above claims, characterised in that the electro-optical sensor is provided with second orientation stabilisation means. Surveillance sensor as claimed in one of the above claims, characterised in that the surveillance sensor is provided with a radar plot extractor connected to the surveillance radar antenna for obtaining each antenna revolution at least azimuth, range and doppler speed information; with an electro-optical plot extractor connected to the electro-optical sensor for obtaining at least azimuth and elevation information each revolution of the electro-optical sensor; and with a common track unit connected to the radar plot extractor and the electro-optical plot extractor for combining the information obtained, for generating on the basis of the combined information a target track to control a weapon system to be connected. Surveillance sensor as claimed in claim 8, characterised in that the information originating from the surveillance radar antenna and the electro-optical sensor is combined on the basis of corresponding azimuth information. Surveillance sensor as claimed in claims 8 or 9, characterised in that the track unit comprises a fast logic unit for promoting combined information to a target track within the time required for one revolution of the surveillance sensor, if this information complies with preset criteria.

Inventors: DE GROOT GERRIT, DE GROOT, GERRIT
Applicants: HOLLANDSE SIGNAALAPPARATEN BV, HOLLANDSE SIGNAALAPPARATEN B.V.
Application year: 1989
Main classification: G01S13
IPC codes: G01S13, G01S7, F41G5, G01S3