THE COMPUTER AID FOR SEARCH OBJECT’S
DETECTION PROCESS DURING SAR ACTION
AT SEA
Michna P.
Przemysłowy Instytut Telekomunikacji, Oddział Gdańsk, Gdańsk, Poland
Abstract: The paper presents the problem of search object’s detection using thermo – night vision
cameras. The negative influence of hydro meteorological conditions on time to search is the most important factor during SAR action. Therefore it’s necessary to use computer aid to increase the effectiveness of detection. The computer aided system requires special procedures of analysis of thermo vision images to expand search object from the background.
1. Introduction
The international conventions SAR (1979 r) and SOLAS (1974 r) determine that the rescue operations at sea are obligatory for each country which has a sea border. Yet there are no guidelines for night search actions indeed in IMO documents. The search of survivors at night is left to the search team decision. Essential for the improvement of the safety in the sea transport is the increase of probability of detection by use of thermo and night vision cameras, also at night.
At night the systems which work using the visible waves generate inaccurate results, this makes them almost useless at the rescue actions. Therefore it is justified to use apparatus which work using the infrared (IR) waves. Those systems record IR waves emitted by every single object which temperature is higher than 0K, what is more they do not require additional lighting. One of the basic factors determining the success of rescue actions is the determination of search patterns of the search area e.g. search pattern and sweep width.
Sweep width is an area which is marked as searched with the assumed probability after one pass of a search rescue unit. Standard search area’s width is a width in which the
probability of detection outside the area is equal the probability of not detecting the object inside the area. Graphical illustration of the search area is shown at the fig 1.
Fig.1. Graphical illustration of the search sweep, W – search sweep width
In figure 2 different patterns of searching sweep width the particular area are shown.
Fig. 2. Paths of searching the particular area.
Usage of the systems which work using the visible waves generates very narrow search area, and, in consequence the time of search rises making the chances of finding alive
survivors very little. Using the system based on the IR waves enables the rescue action to be continued at night widening the area and shortening the time of search. To realize such tasks a system of search objects for surface units applications has been created.
2. Working of the system
The Optical Survivors Search System (System Optycznego Poszukiwania Rozbitków -SOPR) consists of a thermo camera and a night-vision device. They are fastened on a special platform with stabilization. Data from the camera and the night-vision device are transferred to computer to process and visualize. The method of data process in the SOPR system is shown at the fig 3.
Fig. 3. Simplified pattern of visual data process in the system.
The system display contains the image from thermo camera and nightvision camera, the presentation of image processing results, the elements of system performance control (image processing parameters, control elements for visualisation, control of driving system). The history of registered and processed images (selection is possible) in the automatic mode of work of the system - contains one sector observation - is also presented. In this mode the drive is adjusted about a particular angle in azimuth and registered and analyzed in each of those positions. The example of an operation display of the system performance is presented in figure 4
Fig. 4. Example of system dispaly
The first step of computer signal process is correction of the image – the improvement of the image from the thermo camera. Those defects results from the disadvantages of the CCD matrix. An example of the picture before and after correction is shown in fig 5.
Fig. 5. The example of the image correction algorithm a) original picture, b) picture after the correction
The next step is to process the image with the digital effects to show some aspects of the picture more clearly. After this the observer is able to watch the picture on the screen. The following algorithms are used:
the regulation of contrast and brightness,
the colouring of the picture – the image from the camera is monochromatic, it’s brightness is a function of temperature. The colouring is done by software,
the inversion – negative of the picture
Simultaneously with the picture processing the analysis is performed. It’s goal is to detect and stress objects or areas which temperature is different than the backgrounds temperature.The results of image analysis are presented on system display. It is being done by one of three algorithms:
edge detection [2] – in order to detect edges a high-pass filter with a Sobel mask is used. The effect of this algorithm is shown at the fig 4. The searched object has been singled out.
Adaptive threshold [3] (CFAR – Constant False Alarm Ratio
algorithms) - the value of each pixel is being compared with the
detection threshold, according to this the decision is being made if
there is a detection. To establish the detection threshold statistical
parameters of surrounding pixels are being used. The effect o this
algorithm is shown at the fig 5 – an example of thermal picture of two
life rafts on the seaside.
Fig. 6. An example of the edge detection algorithm a) – original picture – a swan on the sea surface, b) the negative – after the usage of the algorithm
Fig. 7. An example of the adaptive threshold algorithm – a) original picture – life rafts on the sea, b) after the usage of the algorithm
3. Conclusions
The usage of thermo vision system which is combined with image analysis algorithms and signal processing could increase the safety in sea transport by:
the possibility of continuing the action at night, increasing the sweep width of the search area.
Moreover the decrease of time to search is possible which could result in increasing the chances of finding alive survivors.
The paper presents the result of reserach conducted according to the research project No 6T12 083 2001C/5686 sponsored by Polish Ministry of Science and Education
References
1. Mission co – ordination International Aeronautical and Maritime Search and Rescue Manual Volume II IMO/ICA London/Montreal, 1999. Adoption of amendments to the International Aeronautical and Maritime Search and Rescue Manual, MSC/Circ.999-11, June 2001.
2. Tadeusiewicz R., Kohoroda P.: Komputerowa analiza i przetwarzanie obrazów. Wydawnictwo Fundacji Postępu Telekomunikacji, Kraków, 1997.
3. Gandhi, P.P., Kassam S.A.: Analysis of CFAR Processors in Nonhomogeneous Backrgound, IEEE Transactions on Aerospace and Electronic Systems, 1998.
4. Michna P., et.al.: Detection of survivors using thermo & night vision system. Technologies for Search, Assistance and Rescue Seminar Le Quartz Brest, France 18-19-20 October 2004.
