In this Fish Lidar System, a laser emits a short pulse (~15ns) of green (532nm) light toward the ocean surface. Some of the light is reflected from the surface and some continues down to be reflected back from a school of fish. A 20-cm telescope is pointed in the same direction as the laser and collects the reflected light. This telescope is a simple refractor that uses a condensor lens. A 10-nm wide interference filter filters out most of the background light and lets only the laser light through to the photomultiplier tube. The photomultiplier tube (PMT) detects the reflected laser light and converts it into an electrical signal. A logarithmic amplifier compresses the dynamic range of the signal. The signal is routed to an 8-bit analog-to-digital converter (ADC) in a personal computer where it is digitized and saved to the hard disk. In 2006, a second telescope and receiver were added to collect the co-polarized return (Churnside, 2008).
Two different displays are available on the aircraft computer. The first is a line plot of the return signal as a function of time for each shot. Since time can be related to distance through the speed of light, this is equivalent to a plot of the return energy as a function of depth for each laser pulse. At 30 pulses per second, this display produces 30 plots per second, which can be hard to follow. Therefore, we have another display that shows depth in the vertical axis, shot number in the horizontal axis, and encodes return power as a gray level. This is similar to the display in a conventional echo sounder. Fish schools show up a regions of more return power. We have the option to identify the surface in each shot and relate the vertical axis to actual depth. Another option is to estimate the background water return and subtract it from each shot to make fish schools easier to see.
|Optics Package||Detector||Electronics Rack|
The photo on the left shows the optics package. This is the part that mounts over the port in the aircraft. A different mounting plate is made to fit each plane. The black box on the left is the laser. The white tube on the right is the telescope. Both of these are mounted to the main supporting plate that is fixed to the base plate in such a way that the incidence angle of the lidar can be adjusted. At the top of the telescope are the detector housing and the instruments to measure aricraft attitude. These are shown more clearly in the center photo, which was taken from the telescope side of the optics package. The photo on the right is the electronics rack as it was used in the King Air. The touch-screen computer monitor is on the top; next is the digital delay generator used to control the timing between the laser pulse and the digitizer; next is the computer; next is the laser power supply; on the bottom is the laser cooling unit. Photos of the lidar installed on the various airplanes are on the pages for the Partenavia Observer, Beechcraft King Air, Casa, and Cessna Cardinal.
|Pulse length||12 nsec|
|Pulse energy||100 mJ|
|Pulse repetition rate||30 Hz|
|Beam divergence||17 mrad|
|Aperture diameter||co - 5 cm, X - 15 cm|
|Field of view||17 mrad|
|Optical bandwidth||1 nm|
|Electronic bandwidth||100 MHz|
|Sample rate||1 GHz|
|Polarization||linear, co and X|
An intensified CCD camera has also been used with the system for some applications. This camera has a very short exposure time (20 ns) that can be triggered to take a picture when the laser pulse is at a preselected distance below the surface of the water. The picture that results is an image of the laser spot scattered by particles in the water at that depth. If there is a large fish or other object above the illuminated particles, its shadow will show up in the image. This camera has been used to detect salmon and floating debris.