WO2006072254A1

WO2006072254A1 - Target history and trails for digital radar system

Info

Publication number: WO2006072254A1
Application number: PCT/DK2006/000013
Authority: WO
Inventors: Karsten Biehl
Original assignee: Navico Danmark A/S
Priority date: 2005-01-10
Filing date: 2006-01-10
Publication date: 2006-07-13

Abstract

The present invention relates to a radar system and a method for detecting and storing target positions in a digital radar image and generating a trail for at least one target having a present target position and at least one previous target position. The radar system further comprises means for generating an output image based on the generated trail. The object of the invention is to provide a system or a method for saving previous target positions and for applying a trail to a detected target in a radar image where the trail duration and the display mode of the radar image can be changed on the fly without latency in the output radar image. This might be achieved if the apparatus further comprises means for storing the position data of detected targets of at least two radar images in a target history memory where the target history is maintained in true-motion, where the target history memory comprises of layers of target bitmaps, which bitmaps are formed of data elements, each representing a pixel in the bitmap, where the target history is organised as a 3 dimensional memory, where the bitmaps are represented in a first and a second direction, where a third direction represents history.

Description

Target history and trails for digital radar system

FIELD OF THE INVENTION

The present invention relates to a radar system comprising means for detecting and storing target positions in a digital radar image and means for generating a trail for at least one target having present target information and at least one previous target position. The radar system further comprises means for generating an output image based on the generated trail.

The present invention also relates to a method for generating a trail for at least one target in a radar image comprising at least the following steps: receiving a radar image, converting the radar image into a digital bitmap, storing information of a present scan into digital target history bitmaps and generating an output image containing trails based on present and previous target history bitmaps.

BACKGROUND OF THE INVENTION

It is known to use a radar system with a cathode ray scope where a glowing sweep shows the direction of the moving radar beam. As echoes are received from a target, a spot appears on the screen. The spot is illuminated by an incident ray on the phosphor coating on the cathode ray screen where the illuminated spot slowly fades out when the beam has passed the area of the target. When the echoes from the next scan are received, a new spot of the target is illuminated showing a new position of the target. Hereby, a frail of fading spots behind a moving target is produced showing the direc- tion of the target relative to a chosen fix point.

Modern technologies offer the possibility of digitally storing and manipulating graphical information within an affordable amount of money. Therefore nowadays radar systems often comprise means for image processing. In the document US6522289, a method and an apparatus for producing a pulse trail are disclosed. The invention relates to a radar system where amplitudes from received radar echoes are stored in a radar video memory. If the content of an individual radar video memory cell exceeds a first determined value, the content of the radar video memory cell is stored as a 1-bit information item in at least one trail memory. The targets that are stored in the trail memory are deleted by thinning out the pixels in a chronologically successive manner. It is suggested that the thinning out of pixels could be done by a pseudo random algorithm in such a way that the oldest part of the trail consists of relatively fewer pixels than the newer ones. The content of the radar video memory and the trail memory forms the basis of a radar image. The invention could comprise two sections of trail memory for storing the targets in true-motion and e.g. relative-motion, respectively. Hereby, it is possible for an operator to switch between display modes without latency.

In the following, the terms image, target bitmap and target history memory are often used. The term image refers to a digital radar image obtained from digitalisation of echoes received from one scan by the radar system. The target history bitmap refers to a two-dimensional array of elements, which elements contains information of detected targets obtained from an image. The term target history memory refers to a memory in which previous target positions are stored. Additionally, the word independent is used to distinguish between information received from separate scans.

DESCRIPTION / SUMMARY OF THE INVENTION

The object of the invention is to provide a system and a method for saving a number of previous target positions and applying trail information to moving targets in a radar image where the trail duration and the display mode of the radar image can be changed on the fly without latency in the output radar image.

This might be achieved if the radar system comprises means for storing the position data of detected targets of at least two independent radar scans in a separate part of a target history memory which target history memory is maintained in true-motion, and where the target history memory comprises a number of independent scan-related bit- maps which bitmaps are formed of data elements, each representing a pixel in the bitmap, and where the target history is organised as a three-dimensional memory, where the bitmaps are represented in a first and a second direction, where a third direction represent previous scan-related bitmaps.

Hereby, it is achieved, that a history of detected targets originating from a number of independent images is stored in a memory in true-motion. The target history contains the information necessary to apply a trail to a target where the maximum trail duration is set by the number of previous target positions stored in the memory. It would, hereby, be possible for an operator to change the display mode of the radar output image from e.g. true-motion to relative-motion without latency and, at the same time, decide the duration of the target trail shown in the output radar image. In this way, it is possible to achieve relatively long trails which facilitate deciding the tendency of a direction of a moving target for an operator. Furthermore, the history of the own ships position and heading is also stored and trails can hereby be transformed to different display modes such as:

1. True motion trails. One to one transformation.

2. Relative motion trails. Transformation based on target history and history of own ships position and heading.

3. Instant relative motion trails, where transformation is based on target history and own ships current position, speed and heading.

Hereby, it is achieved that in use on a ship the personal on the bridge has a total free- dom in switching between the different modes on a display.

In an embodiment of the invention, the target history of the pixels of the target bitmaps are organized in at least a first and a second target bitmap, where the content of the actual target bitmap is copied into the following target bitmap. Hereby is achieved that the update of the target history might be carried out by copying the target information stored in e.g. a first target bitmap into a second target bitmap. The update of the target history might be carried out by pushing all the target information stored in the target memory and, hereby, overwriting the oldest target information stored in the memory.

In an embodiment of the invention, the data elements originating from the same posi- tion of at least two chronologically successive independent target bitmaps are organized in at least one record. Hereby, it is achieved that each record contains a history of target elements at one given position relating to a number of previous scans.

In an embodiment of the invention, the record is divided into two or more sections which sections consist of at least one element. Hereby, each section might be updated with an individual frequency, which could lead to a target history, where the sections have different time resolution relative to each other. Instead of dividing the records into one or more sections, a similar number of records could be used.

Each successive section can be updated with a relatively slower frequency than the previous section. Hereby, it is possible to store a relatively long history of targets back in time with a relatively small amount of memory.

The target elements of at least one section of a record can be updated with a first-in- first-out manner. Hereby, it is achieved that the oldest element is deleted or overwritten when a new element is stored. The records or sections might be implemented as a FIFO.

Each target element in the target history memory can be stored by one bit of informa- tion. Each bit indicates if a target was present or not. In this way a relative small amount of memory is needed to store the entire target information.

The invention also relates to a method comprising a step of storing n target bitmaps in a target history memory in true-motion, and where an output image is generated based on information of the present targets, and the associated trail information of at least some of the n previous target bitmaps is stored in the target bitmap memory. Hereby, it is possible for an operator to adjust the duration of the trail in the output image. It is also possible to change the display mode from e.g. true-motion to relative- motion and preserve the trail information without latency in the output radar image. The relative-motion trails are calculated based on the target information and informa- tion of own vessel movement. hi a preferred embodiment of the invention, n is an integer having a number higher than 2. Hereby, .it is achieved that the history could contain target information from a long period of time.

In a preferred embodiment of the invention, the target bitmap memory comprises at least a first and a second section, which first section is updated by a high frequency, and which second section is updated by a second lower frequency. Hereby, it is achieved that a longer target history could be stored because the amount of data in the second section is representing a longer period of time compared to the information stored in the first section.

The target bitmap memory can comprise a number of sections, where the first section is updated with a high frequency, where the next section is updated with a relatively slower frequency than the previous section. Hereby, it is achieved that a section dou- bles the duration of the history compared to the previous section when the number of elements are the same. When one or more successive sections are appended to each other, a long target history could be obtained with a relative limited use of memory. The method could be used for successive records in the same way.

SHORT DESCPLIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, where fig. 1 shows a general system overview of the radar system, fig. 2 shows the update of the target history memory, fig. 3 shows three independent images with a moving target, fig. 4 shows how the detected targets are stored in the target history memory, and fig. 5 shows a memory organization of the target history. DETAILED DESCRIPTION / PREFERRED EMBODIMENT

Fig. 1 shows a general overview of the radar system comprising a radar antenna 110 connected to an analogue transceiver unit 120 from which the echoes are received and delivered to an analog to digital converter 140. The A/D converter 140 is connected to processor means 150 which makes digital signal processing of the echoes possible. The radar system is supplied with a memory unit 160 for storing the images, the target history and e.g. clutter and sensitivity maps, which maps store information about targets etc. The images are stored in the scan image memory 162, and the target positions of the previous targets detected are stored in the target history memory 164, which memory is updated for each scan. Clutter and sensitivity maps are stored in the memory 166 which is updated continuously for each scan. The radar system also comprises a display 156 for presenting a radar image and an interface 154 for manual gain and clutter adjustment etc. The system could further comprise connection means to an ex- ternal position system 152 and external heading system 158 e.g. a system comprising a GPS receiver and gyro compass. The processor means 150 could transform the target information stored in true-motion in the target history memory 164 into relative motion for presenting trail, information on a radar screen in relative-motion. The system can also store the history of own position and heading in the memory unit 160 in such a way that the trails can be transformed into different display modes.

Fig. 2 shows the update of the target history memory 250 comprising a latest image 210 which scan-related image 210 is processed by target detection 220 where target detection might be based on clutter information and sensitivity levels combined to- gether in a sensitivity map 222. Thereafter, the target history memory 250 is updated by the target history update. The target history memory 250 is maintained in true motion by the target history update 240.

Fig. 3 shows three independent radar images 310,320,330 as they would appear on a radar display with target history information, and where the images contain a target 315,325,335. The radar image 310 is the oldest image followed by the image 320 and finally the latest image 330. In the oldest image 310 a target 315 is shown. In the image 320, the target 325 has moved relative to its position of the previous image 310. A trail 326 illustrates the movement. In the latest image 330, the target 335 has moved even further compared to the position of the targets 315,325 of the previous scan- related radar images 310,320. The movement of the target 315 from the previous image 310 to the latest image 330 is illustrated by a trail 336 from which it is possible to determine the direction of movement of the target 335 relative to the display mode. The target history and trails could be deleted by a clear trails command available from an interface. The target history memory is stored and maintained in true motion from which it is possible to calculate the trails of the targets in relative motion.

Fig. 4 shows three radar image 410,420,430 in a chronologically manner illustrating the detected targets. The bitmap 430 is the latest image followed by the previous images 420,410. The target history of each pixel is stored in a record 460 comprising 1- bit elements where each element indicates if a target was present at that position at that given time. The pixel 450 of the latest radar image 430 is stored in the first element in the front end of the record 460 and hereby pushing all the elements in the record one place back in the record 460. The duration of the record depends on the amount of memory allocated for the target history memory. In the figure, the duration is illustrated as containing 2ⁿ elements, where n is an integer.

The target history could be seen as a three-dimensional memory space 440 comprised of layers of target bitmaps each indicating detected targets at a given time.

Fig. 5 shows the organization of one single pixel as apart of the target history memory stored in a record as the record 460 shown in figure 4. The record 460 on figure 4 is divided into five sections 510,520,525,530,535. Each section of elements is updated with different frequencies. As illustrated, the first section 510 is updated at a high frequency and the following sections 520,525,530,535 are updated with a frequency, which is half of the updating frequency of the previous sections by deleting half of the target bitmaps. This means that the second memory section 520 is updated every other period compared to the first section 510, and the third section 525 is updated every fourth period of the first section 510. A section is updated by pushing all its elements to the right. When the third section 525 is updated within the same update period as the first section 510 then the last element bit 47 of the second section is pushed into the first bit, bit 48, of the third section 525. However, if a following section is not updated within the same update period the last element of the section is accumulated.

If the update rate of the first section is 1.875 seconds then the bits 0 to 31 of the first section 510 are shifted to the right every 1.875 seconds. Bit 32 accumulates targets from bit 31 and the bits 32 to 47 are shifted one position the right. Similarly, the bit 48 accumulates targets from bit 47, and all the bits 48 to 63 are shifted to the right. The bits 64 to 19 are updated and shifted one place to the right every 15 seconds. The bits

80 to 95 are shifted one position to the right every 30 seconds. By using this organiza- tion of the elements in the target history memory, it is possible to store a complete history of targets 16 minutes back in time.

Claims

1. Radar system comprising: a) means for detecting (150,220) and storing (150,240) target positions in a bit- map radar image in a memory unit (160), b) means for generating a trail (326,336) for at least one target (315,325,335), c) means for generating an output image (310,320,330) based on the generated trail (326,336), characterized in that the radar system further comprises: d) means for storing (150,240) the position data of detected targets (315,325,335) of at least two radar images in a target history memory (164,250), e) where the target history (164,250) is maintained in true-motion, f) where the target history memory (164,250) comprises a number of independent layers of target bitmaps (440), g) which bitmaps (440) are formed of data elements (460), each representing a pixel h) where the target history (164,250) is organised as a 3 dimensional memory

(440), i) where the bitmaps (440) are represented in a first (441) and a second direction (442), where a third direction (443) represents target detections of previous radar images (410, 420).

2. Radar system according to claim 1 characterized in that the target history (164,250,440) of the pixels of the target bitmaps is organized in at least a first and a second target bitmap where the content of the actual target bitmap is copied into the following target bitmap.

3. Radar system according to claims 1-2 characterized in that the data elements (450) originating from the same position (460) of at least two chronologically successive independent target bitmaps are organized in at least one record.

4. Radar system according to at least one of the claims 1-3 characterized in that the record (460) is divided into two or more sections (510,520,525,530,535), which sections consist of at least one element.

5. Radar system according to at least one of the claims 1 - 4, characterized in that each successive section (520,525,530,535) is updated with a relatively slower frequency than the previous section (510, 520, 525, 530).

6. Radar system according to at least one of the claims 1—5 characterized in that the target elements of at least one section (510,520,525,530,535) of a record are updated by a first-in-first-out manner.

7. Radar system according to at least one of the claims 1-6 characterized in that each target element in the target history memory (164,250,440) is stored by one bit of in- formation.

8. Radar system according to at least one of the claims 1-7 characterized in that the target history and history of ones own ships position and heading is presented in relative motion.

9. Radar system according to at least one of the claims 1-7 characterized in that the target history and history of ones own ships position and heading is presented in instant relative motion.

10. Method for generating a trail (326,336) for at least one target (315,325,335) in a radar image (310,320,330) comprising at least the following steps: a) receiving a radar image, b) converting the radar image into a digital bitmap (310, 320, 330), c) storing information of a present scan into a digital target bitmap (410,420,430), d) generating an output image (320,330) containing trails (326,336) based on present (430) and previous target bitmaps (420,410), characterized in that the method further comprises step of: a) storing n target bitmaps (440) in a target history memory (164,250) in true- motion, b) generating an output image (320,330), c) which image (320,330) contains information of the present targets (410,420), d) which image contains trails (326,336), e) where the trails (326,336) are based on at least some of the n previous target bitmaps stored in the target bitmap memory (164,250,440).

11. Method according to claim 10, characterized in that n is an integer having a number higher than 2.

12. Method according to claims 10 or 11, characterized in that the target bitmap memory (164,250,440) comprises at least a first (510) and a second (520) section, which, first section (510) is updated by a high frequency, and which second section (510) is updated by a second lower frequency.

13. Method according to claims 10-12, characterized in that the target bitmap memory (164,250,440) comprises a number of sections (510,520,525,530,535) where the first section (510) is updated with a high frequency, and where the next section (520) is updated with a relatively slower frequency than the previous section (510).