Abstract:
The invention relates to a method for producing a pulse trail for at least one target whose actual position is represented in a continuous manner in a radar image of a radar device. In order to improve said method, wherein the pulse trails are represented on the radar screen in a highly visible manner for a viewer without any overcrowding of the radar screen when a plurality of targets that should be displayed simultaneously are present, the actual target position can be seen by means of a plurality of pixels that cover a continuous surface of the radar image and prior target positions are represented by a thinning out of said pixels that increases according to the chronological status of the respective target positions in the area.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method and an apparatus for producing a persistence trail for at least one target whose respective current position is represented in a continuous manner in a radar image of a radar instrument. 
     Such persistence trails which are produced on the radar screen in a manner assigned to individual targets describe the tracks of located targets on the radar image, from which an observer can distinguish the movement behavior of the targets and also recognize collision risks evident from the directions of movement and eliminate them by suitable maneuvers instigated by said observer. 
     In a known method of this type (DE 29 24 176 C2), the radar echoes of the current panorama sample and of a plurality of preceding panorama samples, after storage in a single image memory, are displayed on the screen of the radar instrument. The image memory used is a read/write memory which is oriented in x,y coordinates and in which storage is performed by storing the intensities of the radar echoes currently received in each case in successive samples of the panorama, with the memory addresses corresponding to their spatial coordinates. The intensities obtained from previous samples are preserved in this case. The image content of the image memory is read out independently of the chronological order of reception of the radar echoes and represented on the radar image. As a result, the intensities stored for successive panorama samples appear strung together on the screen and describe the tracks or the persistence trails of the targets. 
     In a further configuration of the known method, the stored intensities are reduced by one or more increments after a predeterminable number of panorama samples, as a result of which the preceding target positions are represented on the screen with an intensity that decreases with age. The length of the persistence trails is thus dependent not only on the target speed but also on the speed with which a complete panorama sample is effected, and thus allows the time interval for which the persistence trail is represented to be assigned to the length of the persistence trail only in the case of a constant rotational speed of the radar antenna. If the rotational speed is changed, the time interval must be redefined. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is based on the object of improving a method of the type mentioned in the introduction to the effect that the persistence trails, with little outlay, appear in the radar image in a manner that is highly visible to the observer without overburdening the radar image in the case of a plurality of simultaneously represented targets. 
     The object is achieved according to the invention by means of the features in patent claim  1 . 
     The method according to the invention has the advantage that it can be carried out using standard graphics cards in a standard screen or video device and, as a result, can be implemented practically without any additional outlay. The persistence trails visible on the screen are very clear and do not overburden or overload the radar image even in the case of a multiplicity of targets appearing on the radar image, since, by virtue of the thinning out of pixels that increases with increasing age, their luminosity diminishes towards that end of the persistence trail which faces away from the current target position. 
     Expedient embodiments of the method according to the invention with advantageous developments and configurations of the invention emerge from the further claims. 
     The method according to the invention can be realized in a particularly advantageous manner in that, in accordance with a preferred embodiment of the method, the amplitudes of the radar echoes received by the radar instrument are stored in a radar video memory, whose memory cells are oriented in x,y coordinates, in that the memory content of the radar video memory is written as 1-bit information memory cell by memory cell to at least one trail memory, whose memory cells are oriented in x,y coordinates, in so far as the memory content of the individual memory cells exceeds a first predetermined value, in that the memory content of the trail memory is continuously erased according to an e.g. pseudo-random method for thinning out pixels, in that firstly the memory content of the trail memory is written to an image buffer memory, whose memory cells are oriented in x,y coordinates, and, chronologically succeeding that, the memory content of the radar video memory is written, in so far as the memory contents of the individual memory cells of the radar video memory exceed a second predetermined value, and in that the memory content of the image buffer memory is represented in a continuous manner on the radar screen—oriented in x,y coordinates—of the radar instrument as a pixel per memory cell. 
     Storing the persistence trails in a special trail memory not only enables the pixels to be thinned out in a particularly simple manner, but also enables, without difficulty, the changeover of the representation mode true-motion to the representation modes relative-motion and relative-motion true trails (center display), in which one&#39;s own location, that is to say one&#39;s own ship carrying the radar instrument, is fixed in the radar image, preferably in the center thereof, and vice versa, since only the memory content of the trail memory has to be cyclically shifted in accordance with the movement of one&#39;s own ship or the addressing of the memory cells of the trail memory has to be cyclically changed in accordance with the movement of one&#39;s own ship. If two trail memories are used, no loss of the persistence trails is associated with the changeover, with the result that, unlike in the case of known radar instruments, it is not necessary to accept a waiting time, which lasts at least a plurality of antenna rotations, until the persistence trails reappear on the radar image in the representation mode respectively chosen. 
     The provision of the additional memories having the same memory size has hardly any appreciable effect on the production costs of a screen device operating according to the method, since memory cards of the required size are extremely inexpensive. Furthermore, a color coding can be achieved in a simple manner using the trail memory, with the result that the persistence trails can also additionally be represented in different colors. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention is described in more detail below using an exemplary embodiment illustrated in the drawing. In the figures, in a diagrammatic illustration in each case: 
     FIG. 1 shows a block diagram of a radar instrument according to an embodiment of the claimed invention, 
     FIG. 2 shows an enlarged illustration of the radar image that can be seen on the screen of the radar instrument in FIG. 1, and 
     FIG. 3 shows an illustration of the memory content of a trail memory in the radar instrument in accordance with FIG. 1 at three successive instants. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The radar instrument illustrated as a block diagram in FIG. 1 has a rotating radar antenna  10  in a known manner, which antenna can be used to detect targets according to the principle of radio detection and ranging. Instead of mechanical rotation of the antenna  10 , the antenna can also be rotated electronically. Each radio detection and ranging signal, or radar echo, received by the antenna  10  is fed to the receiving device of a transmitting and receiving device  11 , is correspondingly conditioned there and converted into digital signals in an analog-to-digital converter  12 . The digital signals present in o,φ coordinates (distance and direction-finding) are transformed by a coordinate transformer  13  into an x,y coordinate system and fed to an image generator  14 , which represents the position of the targets identified by the radar echoes in a radar image  15  which appears on the radar screen  16  of a screen or video device  17 . 
     A radar image  15  for an arbitrarily chosen scenario is shown enlarged in FIG.  2 . The radar image  15  is illustrated in the “relative-motion mode”, i.e. the positions of the detected targets are represented in a reference system referring to one&#39;s own ship. Therefore, one&#39;s own ship, designated by  20  in FIG. 2, is always situated at a fixed point on the radar image  15 , in this case in the center of the radar image  15 . In the radar image  15  of FIG. 2, the radar instrument detects three different targets  21  to  23 , firstly the respective current position of the targets  21  to  23  being represented and, secondly, chronologically preceding target positions of the targets  21  to  23  in each case being represented as a persistence trail  24  identifying the chronological status, so that these persistence trails  24  can provide an overview of the positions of the targets  21  to  23  changing with time. The representation of the persistence trails  24  for the individual targets  21  to  23  is effected in such a way that the current target position of the targets  21  to  23  is made visible by a number of pixels covering a contiguous area in the radar image  15  and the chronologically preceding target positions are represented by a thinning out of pixels in the area, said thinning out increasing with the chronological status of the respective target position, so that the number of pixels activated within the area decreases as the age of the target position increases. This is illustrated in the radar image  15  of FIG. 2 by the fact that the current target positions are in each case occupied by a black area which is increasingly interrupted in a dotted manner at the preceding older target positions. The older the represented target position, the larger the gaps in the closed area on account of the thinning out of pixels, thereby producing for the preceding target positions a dot pattern which becomes increasingly sparse as the age of the respective target position increases. 
     In order to produce the radar image  15  with the targets  21  to  23  and the persistence trails  24  assigned to each target  21  to  23 , the image generator  14  has a radar video memory  25 , a trail memory  26 , an image buffer memory  27  and a control unit  28  controlling the read-in and read-out processes of the memories  25  to  27 . The three memories  25  to  27  are each oriented in x,y coordinates and have an identical memory size with regard to the coordinate range. The radar video memory  25  is connected, on the input side, to the output of the coordinate transformer  13  and, on the output side, to a respective comparison circuit or a comparator  29  and  30 . The trail memory  26  is connected, on the input side, to the output of the first comparator  29  and, on the output side, via a second multiplexer  35 , whose function will be explained later, to one input of the multiplexer  31 , to whose other input the output of the second comparator  30  is connected. The memory input of the image buffer memory  27  is connected to the output of the multiplexer  31  and the memory output of the image buffer memory  27  is connected to the video device  17 . A first comparator threshold  32  is applied to the first comparator  29  and a second comparator threshold  33  is applied to the second comparator  30 , the second comparator threshold  33  preferably being made equal to zero. The first comparator  29  is designed in such a way that a 1-bit information item is present at its output if a value present at its input, in this case the memory value of the radar video memory  25 , exceeds the first comparator threshold  32 . The second comparator  30  is designed in such a way that a value present at its input, in this case the memory value of the radar video memory  25 , is present at its output connected to the multiplexer  31  if said value is greater than the second comparator threshold  33 . The multiplexer  31 , which is controlled by the control unit  28  and has merely the function of a data selector, switches its output periodically and in a chronologically successive manner firstly to its input connected to the trail memory  26  and then to its input connected to the radar video memory  25 . 
     The image generator  14  constructed in this way operates according to the following method: 
     The amplitudes of the electrical reception signals of the transmitting and receiving device  11 , the “video signals or video data”, which represent the radar echoes received by the antenna  10  and have been digitized and subjected to signal conditioning, are stored in accordance with their associated x,y coordinates in the correspondingly addressed memory cells of the radar video memory  25 . The memory content of the radar video memory  25  is read out cyclically and written as 1-bit information by the first comparator  29  memory cell by memory cell to the trail memory  26 , if the memory content of the individual memory cell exceeds the first comparator threshold  32 . As a result, all the memory cells of the radar video memory  25  in which the stored amplitude is greater than the first comparator threshold are cyclically copied as 1-bit information to the trail memory  26 . For a color representation of the persistence trails on the radar image  15 , during the writing process the 1-bit information is additionally coded in such a way that its coding can be clearly distinguished from the coding of the memory content of the radar video memory  25 . During the copying or writing operation, the content of the trail memory  26  is not erased but only overwritten. If the amplitude in a memory cell of the radar video memory  25  is less than the first comparator threshold  32 , then there is no occupancy of the identically addressed memory cell in the trail memory  26 . The memory content of the memory cells of the trail memory  26  is continually erased by a method for thinning out pixels, e.g. a pseudo-random method. In this case, the erasure frequency is independent of the frequency of writing to the trail memory  26 , in which case, by altering the erasure frequency, the length of the persistence trail produced can be chosen independent of the rotational speed of the radar antenna. 
     FIG. 3 illustrates the construction of such a persistence trail  24  in three phases. FIG. 3 a  shows the trail after the initiation phase as copied from the radar video memory  25 . In FIG. 3 b , the target has moved on somewhat. The trail image at the current position is still closed, while the trail image at the position from the initiation phase has been thinned out by approximately 20% by the erasure method employed. In FIG. 3 c , the target has once again moved on somewhat, so that now three regions can be discerned. The trail is closed in the front region of the trail, that is to say the direct, current target position. The memory content is thinned out by approximately 20% in the middle region of the trail and by approximately 40% in the rear region. 
     The memory contents of trail memory  26  and radar video memory  25  are copied into the image buffer memory  27  cyclically and chronologically successively in the order mentioned, the second comparator  30  performing sorting such that the memory content of the radar video memory  25  is written, and if appropriate overwrites pixels previously written from the trail memory  26 , only if the memory contents of the individual memory cells of the radar video memory  25  exceed the second predetermined value. In this case, the multiplexer  31  controlled by the control unit  28  connects the respective output of trail memory  26  and radar video memory  25  to the memory input of the image buffer memory  27  in a chronologically correct manner. The memory content now copied in this way into the image buffer memory  27  is a mapping of the radar image  15  which is to be presently represented on the radar screen  16  and is cyclically renewed and is represented in a continuous manner on the radar screen  16 . 
     The multiplexer  31  can be dispensed with and the outputs of trail memory  26  and comparator  30  can be connected directly to the memory input of the image buffer memory  27  if a corresponding control routine is provided in the control unit  28 , which control routine effects the read-out of trail memory  26  and radar video memory  25  in the manner described above. 
     As already mentioned, the image representation is effected in relative motion. If it is to be performed in relative-motion true trails, then the content of the trail memory  26  has to be cyclically shifted or a readdressing of the memory cells of the trail memory  26  has to be performed, said readdressing compensating the intrinsic movement of the ship carrying the radar instrument. 
     In order, in the event of changeover between the representation modes, which can be performed arbitrarily by the operator, to avoid the loss of the persistence trails, which are built up again after a changeover operation only after a relatively long time lasting a plurality of antenna revolutions, a second trail memory  34  is provided, whose memory input is likewise connected to the output of the first comparator  29 . As a result, the 1-bit information items are simultaneously copied into both trail memories  36 ,  34 , in which case, however, when the 1-bit information items are written to the second trail memory  36 , the control unit  28  carries out a readdressing of the memory cells, said readdressing being important for the true-motion representation, in such a way that the movement of one&#39;s own ship is spatially compensated in the x,y coordinate system. As a result, the trail is built up in relative motion in the first trail memory  26  and in true motion in the second trail memory  34 , to be precise at the same time. The second trail memory  34  is processed in parallel by the control unit  28  in the same way, so that its memory content is continuously erased routinely according to the same method and the above-described thinning out of pixels is effected in the memory content representing the persistence trails  24 . In the event of a changeover of the representation mode on the radar screen  16 , it is now optionally possible to have recourse to the first trail memory  26  and to the second trail memory  34 , and the persistence trails  24  can thus be represented in relative motion or true motion on the radar screen  16 . 
     For optional access to the two memory outputs of the trail memories  26  and  34 , the latter are connected to the two inputs of a second multiplexer  35 , whose output is connected to one input of the first multiplexer  31 . The second multiplexer  35  is controlled by means of a changeover switch  36  that can be operated manually. In this case, if the changeover switch  36  assumes the switching position indicated diagrammatically in FIG. 1, then in the multiplexer  35 , the output thereof is connected to the input connected to the first trail memory  26 . If the changeover switch  36  is closed, then the output of the multiplexer  35  is connected to the input connected to the second trail memory  34 .