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AIRCRAFT LASER BEACH MAPPING

Laser Mapping Process



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Laser beach mapping uses a pulsed laser ranging system mounted onboard an aircraft to measure ground elevation and coastal topography. The laser emits laser beams at high frequency and is directed downward at the earth's surface through a port opening in the bottom of the aircraft's fuselage. The laser system records the time difference between emission of the laser beam and the reception of the reflected laser signal in the aircraft. The aircraft travels over the beach at approximately 60 meters per second while surveying from the low water line to the landward base of the sand dunes.

One such laser ranging instrument developed by the National Aeronautics and Space Administration (NASA), the Airborne Topographic Mapper (ATM) (front, side, and perspective view), measures ground elevation with a vertical resolution of 10 centimeters. The ATM instrument is carried onboard a NOAA DeHavilland Twin Otter aircraft located at Wallops Flight Facility in Virginia.

The ATM II, the latest version, is operated with a Spectra Physics laser transmitter, which provides a 7 nanoseconds wide, 250 microjoules pulse at a frequency-doubled wavelength of 523 nanometers in the blue-green spectral region. The laser transmitter can function at pulse rates from 2 to 10 kilohertz (kHz). The laser system with a separate cooling unit weighs approximately 45 kilograms (kg) and requires approximately 15 amperes of power at 115 volts. The transmitted laser pulse is reflected to the earth's surface with the aid of a small folding mirror mounted on the back of a secondary mirror of a Newtonian reflector telescope. The transmitted laser pulse and receiver field-of-view (FOV) are directed earthward by a rotating scan mirror assembly mounted directly in front of the telescope. The scan mirror, which is rotated at 20 hertz, is comprised of a section of round aluminum stock, machined to a specific off-nadir angle. A scan mirror with the off-nadir angle of 15 degrees was utilized, producing an elliptical scan pattern with a laser mapping scan swath width equal to 50 percent of the approximately 700-meter aircraft altitude. The reflected laser pulse is transmitted to a photo-multiplier assembly that consists of a lens, a narrow bandpass filter, and a single photomultiplier tube.

The ATM II incorporates a Laser Ring-Gyro Inertial Navigation Unit to record aircraft pitch and roll attitudes along with the aircraft's heading during flight. The high-precision position of the global positioning system (GPS) antenna on the upper aircraft fuselage is determined during post-flight processing of the aircraft GPS data and simultaneous ATM II data recorded by a fixed GPS receiver located at a predetermined staging airport using differential kinematic GPS methodology.

Sampling at a rate of 3 kHz or higher results in an extremely dense spatial elevation data set. For example, within a 10 square meter ground area, the number of laser pulses can vary from 11 to 35, depending on scan orientation. Over 100 kilometers of coastline can be easily surveyed within a 3 to 4 hour mission time period.

After the flight, the data is downloaded and processed using specially designed computer software that combines all the information to produce accurate, geographically-registered "x,y,z" positions for every data point. These "x,y,z" data points allow the generation of a digital elevation model (DEM) of the ground surface.


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