SEARCH RADAR SYSTEM

Abstract: A search radar system, provided with at least one search radar device (1) having a primary antenna (2) and at least one secondary radar device (3) having an SSR/IFF antenna (4). The radar devices (1, 2) independently supply plot messages from which signals are derived that are both suitable for presentation on a radar display. The antennas (3, 4) are oriented in opposite directions where the said signals are derived from individual plot messages and can be individually presented. Hence, the data rate of the radar system is doubled relative to a conventional radar system having a conventional secondary radar device and a conventional antenna system.

Description: The present invention relates to a search radar system provided with at least one search radar device having a primary antenna and at least one secondary radar device having an SSR/IFF antenna, which radar devices independently of each other provide plot messages from which signals are derived that are suitable for presentation on a radar display. In such search radar systems, provided with one search radar device having a primary antenna and one secondary radar device having an SSF/IFF antenna, it is common that the SSR/IFF antenna is fitted on top of the primary antenna, aligned in azimuth with and synchronously rotatable with the primary antenna. The secondary radar device only serves as a support for the search radar: it supplements the information obtained by the search radar apparatus with, for example, an identity code of a target. The plot messages obtained by the two radar devices are combined and presented as one picture on the radar display. If such a search radar system is used for Air Traffic Control purposes, and particularly for the guidance of air traffic approaching the airport, it is favourable to maintain a relatively high antenna rotation rate, thus achieving a high refreshment rate of the received data on a radar display. On the other hand, it is also favourable to keep the antenna rotation rate relatively low, because in that way the demands on the antenna drive mechanism of the search radar system are lower, and the number of hits per scan can become higher, resulting in a better MTI performance. The present invention has for its object to provide a search radar system, as described in the opening paragraphs, providing on the one hand a relatively low antenna rotation speed and on the other hand a sufficiently high refreshment rate of the received information to make the search radar system particularly suitable for control of air traffic approaching airports. According to the indication, a search radar system of the type known from document US-A-3 997 897 is provided, characterised in that the antennas have substantially opposite orientations in azimuth and that the system is provided with means for deriving said signals from individual plot messages for individual presentation. A special embodiment comprises only one search radar device having a primary antenna and only one secondary radar device having an SSR/IFF antenna; according to the invention this secondary radar device is not only used to support the search radar device, but independently thereof provides plot messages that, because the two antennas are mounted back-to-back facing opposite directions, increase the frequency with which a target can be indicated on a radar display by a factor of two, resulting in a high refreshment rate of the data while the antenna rotation rate can be maintained at a relatively low level. This effect becomes even more apparent when e.g. two search radar devices are present, each fitted with a primary antenna, and two secondary radar devices, each fitted with an SSR/IFF antenna, where both primary antennas and both SSR/IFF antennas are pointed in opposite directions, while the primary antennas on the one hand and the SSR/IFF antennas on the other hand are mounted in such a way that they are oriented perpendicular to each other in azimuth. The frequency at which target information is presented on a display is thus four times as high as the rotation frequency of the antenna combination. The invention will now be explained with reference to the accompanying figure 1, showing a block diagram of an embodiment of the search radar system according to the invention. The invention is in no way limited to the embodiments as described with reference to these figures; it should be regarded merely as an illustration of the invention. Fig. 1 shows a search radar apparatus 1 provided with a primary antenna 2. This search radar apparatus is in the customary way provided with a transmitter and receiver, a detector and a video processor (not shown), and supplies plot messages consisting of information on the target position and, if possible, additional data, such as target width, target strength, etc. The figure also shows a secondary radar device 3 having an SSR/IFF antenna 4. The SSR/IFF antenna can either be an SSR antenna, or an IFF antenna used for the same purpose but in the military sector. The secondary radar device is in a customary way provided with a transmitter, receiver and video extractor, and supplies plot messages comprising data on target position as well as an identity code and, if applicable, a target height code and possibly additional data such as target width, target strength, quality indication of the received code, etc. SSR/IFF antenna 4 has been mounted in a customary way on top of primary antenna 2. However, in accordance with the invention, oriented in a direction opposite to that of primary antenna 2. Fig. 1 is a schematic illustration of the antenna configuration seen in profile. The plot messages of both radar devices are supplied to a target track data processing unit 5. This data processing unit comprises means to: correct the plot messages from secondary radar device 3 for the angle difference of 180° in orientation of SSR/IFF antenna 4 with respect to that of primary antenna 2, in order to prevent that echoes from the same target are assigned azimuth angle values differing in turn by 180° and that they would be presented as such on a radar display to be connected; predict from the plot messages of the search radar apparatus the target position for subsequent revolutions of the primary antenna, upon which each time the target positions obtained during subsequent revolutions of this antenna by the search radar apparatus are compared with the predicted target positions and if these target positions are the same, observing certain tolerance limits, signals are produced that can be individually presented on a radar display; predict from the plot messages of the secondary radar device the target positions for subsequent revolutions of the SSR/IFF antenna, upon which each time the target positions obtained during subsequent revolutions of this antenna by the secondary radar device are compared with the predicted target positions and, if these target positions are the same, observing certain tolerance limits, signals are produced that can be individually presented on a radar display; predict from the plot messages of the search radar apparatus the target positions for the moments at which the SSR/IFF antenna will spot the target during subsequent revolutions, upon which each time the target positions predicted by the search radar apparatus during subsequent revolutions of the primary antenna for the moments at which the SSR/IFF antenna will spot the target during subsequent antenna revolutions, are compared with the target positions obtained by the secondary radar device during subsequent antenna revolutions and, if the target positions are the same, observing certain tolerance limits, it may be determined that the plot messages obtained by the two radar devices originate from the same target. In that case, for instance, the plots from the search radar apparatus presented on a radar display may be provided with information originating from the secondary radar device, such as identity and height codes. The procedure described above will result in a refreshment rate of the plots of a single target on a radar display which is twice as high as the rotation rate. The advantages thus obtained, as described in the introduction to this description, will be even greater if two search radar devices are used, each provided with a primary antenna, and two secondary radar devices, each provided with an SSR/IFF antenna. Both the two primary antennas and the two SSR/IFF antennas are then mounted back-to-back facing opposite directions while the primary antennas on the one hand and the SSR/IFF antennas on the other hand are mounted in such a way that they are oriented perpendicular to each other in azimuth. In principle, the target track data processing unit will be capable of plot processing according to the method described above. For this purpose, again the plot messages of a search radar apparatus are used as reference for the then 90°, 180° and 270° azimuth angle difference correction of the plot messages from the other radar devices and for correlation of the plots from the individual radar devices. It goes without saying that multiple correlation of plots from the four radar devices will provide a more reliable answer to the question whether the individual plots originate from one and the same target.

Claims: Search radar system, provided with a primary radar device having a primary antenna and a secondary radar device having an SSR/IFF antenna, both antennas mounted for rotation around a substantially vertical search axis, both radar devices supplying plot messages from which signals are derived suitable for presentation on a radar display, characterised in that the antennas have substantially opposite orientations in azimuth and that the system is provided with means for deriving said signals from individual plot messages for individual presentation. Search radar system as claimed in claim 1, characterised in that the means comprise a target track data processor, provided with means for: a) correction of plot messages of the radar devices for the different azimuth orientations of the antennas. b) predictions from the plot messages of the primary radar device of target positions measured with the primary radar device and presentation of target positions on the radar display if measured positions correspond, observing certain tolerance limits, with the predictions. c) predictions from the plot messages of the secondary radar device of target position measurements with the secondary radar device and presentation of target positions on the radar display if measured positions correspond, observing certain tolerance limits, with the predictions. d) predictions from the plot messages of the primary radar device of target positions measured with the secondary radar device and the generation of a same target conclusion if a measured position corresponds, observing certain tolerance limits, with a prediction.

Inventors: GARSIDE, PETER FREDERICK
Applicants: HOLLANDSE SIGNAALAPPARATEN B.V.
Application year: 1988
Main classification: G01S13
IPC codes: G01S7, G01S13