A surveillance system comprising: a sensing unit configured to communicate over a communication link with a control unit; the sensing unit comprises a high resolution imaging assembly operatively connected to at least one processor; the high resolution imaging assembly is configured to capture a succession of high resolution images; wherein a bandwidth of the communication link is not sufficiently large to allow transmission of the high resolution images in a desired rate; the at least one processor is configured to: generate degraded images from respective images in the succession of high resolution images and transmit successively sensing-data to the control unit; the sensing-data comprising the degraded images and data indicative of one or more elements in the degraded images; wherein the degradation is adapted according to the available bandwidth of the communication link for enabling transmission of the captured images to the control unit over the communication link at a sufficient rate; the at least one processor is further configured, responsive to control-data received from the control unit, indicative of at least one selected element, to: identify a location of the selected element in recently captured high resolution images in the succession of high resolution images; extract respective high-resolution image-segments from images in the succession of high resolution images, the image-segment comprising at least the selected element; and transmit successively the respective high-resolution image-segments to the control unit; wherein the size of the high-resolution image-segments depends on the available bandwidth of the communication link and is selected to match a desired transmission rate. a sensing unit configured to communicate over a communication link with a control unit; the sensing unit comprises a high resolution imaging assembly operatively connected to at least one processor; the high resolution imaging assembly is configured to capture a succession of high resolution images; wherein a bandwidth of the communication link is not sufficiently large to allow transmission of the high resolution images in a desired rate; the at least one processor is configured to: generate degraded images from respective images in the succession of high resolution images and transmit successively sensing-data to the control unit; the sensing-data comprising the degraded images and data indicative of one or more elements in the degraded images; wherein the degradation is adapted according to the available bandwidth of the communication link for enabling transmission of the captured images to the control unit over the communication link at a sufficient rate; the at least one processor is further configured, responsive to control-data received from the control unit, indicative of at least one selected element, to: identify a location of the selected element in recently captured high resolution images in the succession of high resolution images; extract respective high-resolution image-segments from images in the succession of high resolution images, the image-segment comprising at least the selected element; and transmit successively the respective high-resolution image-segments to the control unit; identify a location of the selected element in recently captured high resolution images in the succession of high resolution images; extract respective high-resolution image-segments from images in the succession of high resolution images, the image-segment comprising at least the selected element; and transmit successively the respective high-resolution image-segments to the control unit; wherein the size of the high-resolution image-segments depends on the available bandwidth of the communication link and is selected to match a desired transmission rate.