Technotes
Flying Higher Working Faster
By Michael Cuddy

At first, the concept struck many observers as being impractical, even impossible: install a lidar (light detection and ranging) instrument in an aircraft for the purpose of gathering topographic elevation data. Now, less than a decade after the initial proposition, Optech, Inc., a Canadian-based company, has not only put this theory into practice, but they lead the industry by having the greatest number of airborne laser mapping systems operating in the world today.
  Though still considered by some as a new technology, Optech's Airborne Laser Terrain Mapper (ALTM) has already evolved significantly since its commercial debut. For example, the newest ALTM, model 1210, offers standard features that expand the capabilities of previous models: a more powerful laser, higher operating altitude, and faster pulse repetition frequency. Also offered are options for further extending altitude range, simultaneous first- and last-pulse elevation measurements, and their corresponding laser intensity readings.

Flying Higher
As impressive as the results from the original ALTM 1020 were, Optech was determined to push beyond design limits to continually increase capabilities with every new design. Research and development efforts led to the development of a new model, the ALTM 1210. The model name derives from its increased capabilities: the '12' refers to an extended operating altitude of 1,200 meters above ground level (AGL). In fact, the last two systems delivered included the option to further extend the operating altitude to 2,000m.
    What is the advantage of an extended altitude range? Simply put, flying higher saves money. Air time is expensive; aircraft rental, pilot, operator and surveyor fees, fuel and maintenance-it all adds up. Whether an aircraft is conducting a terrestrial survey or transporting cargo and passengers, the more time spent in the air, the more costly the flight. Therefore, any reduction in air time and person-hours results in lower costs. The table below shows how altitude directly affects survey time. This example is based on an aircraft surveying a 10x10 kilometer (km) area at a speed of 130 knots, using a 40 degrees field-of-view.
    One critical factor to consider when surveying at higher altitudes, however, is that the laser spot size is directly affected: the higher the altitude the wider the beam divergence. Excess beam divergence results in 'target mixing,' a situation where the ability to precisely locate a specific laser spot is impeded because each spot has become too large and diffused.
    Bill Gutelius, a product specialist with Optech, Inc., stresses the importance of controlling beam divergence, saying, "To effectively generate topographical mapping in a forested area with varied terrain and vegetation, it has been shown that optimal beam divergence should be 0.30 milliradians or less. Surveying at an altitude of 1km, a system with beam divergence of 0.30mrad will produce laser spot sizes of around 30 centimeters (cm) in diameter."
    In the ALTM 1210, the laser optics have been designed to limit beam divergence to within 0.30mrad. Such narrow divergence ensures that the laser minimizes 'target mixing,' increasing the likelihood of penetrating dense forest canopy, acquiring both treetop elevation data and ground elevation data.

Working Faster
In 1993, the first ALTM sold operated at 2kHz (i.e., the laser emitted pulses at a frequency of 2,000 times per second). This may seem fast, but subsequent models operated at 5kHz, and the ALTM 1210 has a default setting of 10kHz (hence the '10' in the model number). What is the advantage of such speed?
    The performance of the early systems varied from one application to another. Certain applications-power wire modeling and forest canopy penetration, for example-presented conditions that would benefit from an increased rate of transmitted laser pulses. The average girth of a power wire presents an extremely small surface area to the survey aircraft above. In order to produce sufficient power wire modeling data, the wires' minimal surface area must return an adequate first-pulse signal. As the aircraft flies over, tracking the power wire corridor, the scanning laser sweeps a pattern perpendicular to the line of flight. In this application, the difference between emitting 2,000 and 10,000 pulses per second may be the difference between detecting the power wire or not.
    Similarly, in forest surveys, a highly dense tree canopy can impede the laser from penetrating to the ground. However, if the pulse repetition frequency (PRF) increases from 2kHz to 10kHz, the net effect is that of emitting 8,000 additional laser pulses per second. Such an accelerated PRF increases the percentage of laser pulses that reach the few breaks in the dense canopy, thereby allowing greater penetration to the forest floor. Such penetration is necessary in order to acquire ground elevation data, which are needed to determine average tree height. (Estimates of tree height are calculated by subtracting treetop elevation data from ground surface elevation data.)
    In order to make such calculations, both treetop elevation data and ground elevation data are required. Acquisition of these two data sets is made possible by the time of flight measurement electronics. The TOF measurement electronics count the time elapsed between the transmission of a laser pulse and the pulse's return when it reflects off an object. Dividing the elapsed time interval by the constant 2c (speed of light), the range is determined. Originally, the TOF measurement electronics were designed to operate in one of two modes: first-pulse or last-pulse. When the system was collecting laser data in first-pulse mode the TOF measurement electronics measured the range between the ALTM sensor and the first target to return a signal. Similarly, in last-pulse mode, the measured range was between the sensor and the last target to return a signal. Thus, an area would have to be surveyed in first- and then last-pulse modes to obtain tree height estimates by differencing the two data sets.
    This original design meant that in order to acquire both first- and last-pulse data, a survey area had to be flown twice, once in each mode. While it is extremely useful to have two data sets produced from first- and last-pulse modes, the drawback was the increase in flight time caused by flying one pass in each mode. The ALTM 1210 solves this problem via a redesigned TOF measurement electronics module which acquires first- and last-pulse data simultaneously. Insofar as the acquisition of first- and last-pulse mode data is concerned, the practical effect is that of reducing air time by half. Again, the most impressive difference lies in the cost savings.

Future Developments
Along with flying higher and working faster, Optech's most recent shipments have included the option of laser intensity reading. Essentially, this option allows the data processing software to categorize detected laser pulses based on the reflectivity of targeted surface materials. High and low intensity readings vary depending upon the targeted objects on the surface. Some targets naturally reflect more of the laser's 1,064 nanometers (nm) wavelength than others. Several other factors contribute to a target's reflective properties: elevation, composition, density, and orientation to the sensor unit.
    When elevation data are acquired by an ALTM with the intensity option enabled, processed data are output to a visualization software package where differences among laser points are represented graphically as a high-resolution digital image. These images display the complexity and detail of high-resolution digital photographs, yet they can be acquired in the dark of night because they are not produced by a photographic process (i.e., passive, relying on sunlight). Instead, the images are an end-product of the 'active illumination' of the sensor (i.e., a laser at 1,064nm).
    In one test, for example, an ALTM 1210 equipped with the laser intensity option flew over an airport runway. A runway was selected to demonstrate how laser intensity data could provide valuable information in areas where elevation data are so undifferentiated as to be useless. The data were processed and output in two files: a grayscale elevation map, and a grayscale intensity map. With the grid pacing set to 30cm intervals, the grayscale elevation map consisted of little more than a dark smear; the only recognizable feature was the narrow drainage swale between the approach and take-off runways.
    The grayscale intensity map, however, reveals photograph-like details. Approach and take-off runways are clearly delineated from the surrounding grass. Runway markings stand out boldly, numbers are legible, and tire tracks are visible. Close scrutiny of the intensity map shows differences between concrete and macadam; even new patches of macadam can be distinguished from old.
    The intensity option is a valuable addition to the ALTM's capabilities because it provides important information when elevation data are less than ideal. Presently, a number of potential applications are being explored and other, as yet untested,x possibilities are generating interest. In Texas, a company involved in coastal mapping has expressed interest in using the intensity option to differentiate between sand dunes and scrub grass. Similarly, forest speciesization-determining distribution of tree species in a forested area-may be possible due to differences in reflectivity among various types of trees. Results from field tests also suggest that laser intensity readings will be useful for detecting evidence of 'disturbed earth': equipment and personnel movement in no-logging zones, clandestine disruption of conservation areas, and 'denied zones.'
    As more laser intensity data are accumulated, an index will be established to categorize data types by active reflective properties. Eventually, correlation may be made between laser intensity percentages and various target materials' characteristic reflectivity: trees, soil, grass, pavement, metallic surfaces, water, snow, etc. In this way, the ALTM will offer yet one more advantage as a reliable instrument for acquiring, processing and manipulating accurate digital terrain mapping data.

About the Author:
Michael Cuddy is a technical writer at Optech, Inc. He can be reached at [email protected]

Contributors:
Bill Gutelius is a product specialist at Optech, Inc., and can be reached at [email protected]
Don Carswell is Optech's vice president of Business Development. He can be reached at [email protected]

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