Lidar Technology Breakthrough Yields 25% Increase in Collection Efficiency

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The application of lidar for highdensity topographic mapping in high-relief environments has long been a challenge to professional surveyors, compared to mapping in relatively flatter terrain environments. Such challenges were historically a result of the time-of-flight constraints associated with using higher laser pulse repetition frequencies (PRFs) and the requirement to fly lower to the ground, thereby inhibiting the ability to collect higher-density data at altitude.

With the advent of multipulse operation, a new era in lidar collection efficiency was realized and high laser PRFs at extended altitudes became possible. Sensors are now limited only by the detection limits associated with available laser power and receiver sensitivity.

However, while multipulse operation was an important step forward for surveying, it did not resolve the problem of receiver electronics confusion: At certain altitudes and PRFs, laser pulses would return to the sensor at the same time the next pulse was fired. No ranges could be recorded when the sensor was in these altitudes, earning them the name "blind" zones. Sensors could reduce the size or number of these zones somewhat, depending on their design, but no multipulse-equipped sensor could completely avoid them.

In the end, operators needed careful planning to avoid places where variations in altitude or terrain elevation caused the sensor to go into a blind zone. Various multipulse solutions have since been developed to try and address the blind zone issue and fill in the data gaps, but none thus far have solved the density reduction that occurs when trying to cross the multipulse transition zone.

The release of Teledyne Optech’s latest high-performance airborne lidar system, the Optech Galaxy, has not only solved the entire blind zone data gap challenge, it has gone one step further by eliminating density loss across the pulse-in-air (PIA), multiple-time-around (MTA), or multipulse transition zone. This unique technology solution, known as PulseTRAK, forms the foundation of the innovations and patent-pending feature sets available only in the Optech Galaxy sensor (Figures 1 and 2).

One of the features of PulseTRAK expected to realize significant value to surveyors is a unique lidar swath tracking capability, known as SwathTRAK. Unlike flatter terrain types, complex terrain creates immense challenges with respect to point distribution, density and collection efficiency. As the fixed FOV of the scanner moves across the terrain, changes in terrain elevation cause the swath width to vary: When the terrain rises up to the aircraft the swath becomes narrower, and when the terrain falls away from the aircraft the swath widens.

This causes two problems for surveyors. First, the data suffers from oversampling in the mountain peaks and under-sampling in the valleys. Second, to ensure that there are no data gaps between flightlines, the planner must accommodate the necessary minimum flightline sidelap based on the narrower swath width at the terrain peaks. As a result, the surveyor must plan for the worst-case scenario by decreasing the spacing between flightlines to prevent data gaps when swath widths are narrow. This necessitates much higher collection costs compared to flat, low-relief areas.

Galaxy’s fully programmable galvonometric scanner provides a unique opportunity to address a number of these core challenges simultaneously. Of importance to surveyors, SwathTRAK:
1. Maintains lidar point density, even in variable terrain conditions;
2. Produces constant-width flightlines;
3. Reduces the overall number of flightlines;
4. Increases area coverage rates;
5. Simultaneously solves for platform roll effects;
6. Produces significant cost savings over fixed-FOV sensors

SwathTRAK adapts to changing terrain elevation by adjusting the galvonometer’s FOV dynamically during flight. This is only possible if the sensor knows its altitude above ground level, so Galaxy must first solve the multipulse problem in real time. Galaxy’s sophisticated real-time sensor protocol then computes high-resolution XYZI point clouds on the fly and monitors the above-ground altitude and swath extent. Finally, SwathTRAK leverages this real-time sensor feedback to constrict or widen the scanner FOV from its nominal value, enabling fixed-width swaths on the ground irrespective of terrain variation or changes in flight altitude.

The benefits of doing so are enormous, especially since it requires no operator input. Point density becomes more predictable, the number of flightlines required in variable terrain decreases, and point density is normalized given the fixed swath width over ground, terrain slope aside. Furthermore, overall data accuracies are increased: in the valleys, long-range measurements are compensated by smaller incident angles, while in the mountain peaks wide incident angles are compensated by shorter ranges.

SwathTRAK is a two-step process. The first involves mission pre-planning using an underlying DEM. Flightline spacing is automatically adjusted as a function of the preferred flight altitude, desired side lap and nominal scan FOV selected. A lookup table identifies the minimum and maximum scan excursion from the nominal value given the range of terrain variation expected. The dynamics of the variable FOV in this mode are reflected as swath-extent overlays on the DEM. The flight plan and flightline coordinates with sensor parameters are then passed to the flight management system.

The second step depends on the real-time sensor protocol to dynamically adjust the scan FOV during flight given terrain feedback. This is imperative to ensure that violations or deviations from the plan or flightlines do not impact the data. In other words, dynamic scan excursion parameters are determined during flight based on terrain feedback and are not dependent on the scanner planning parameters determined during mission planning (Figure 3).

To examine how SwathTRAK improves collection efficiency in variable terrain, we created the following flight plan example for Galaxy using Teledyne Optech’s proprietary flight management system, Optech FMS. The study area is located in a mountainous region about 40 km north of Queensland, New Zealand, where terrain variation ranges from 2801970 meters above sea level (ASL) for a total variation in excess of 1600 m. The nominal flight altitude was planned at 1600 m AGL (2670 m ASL), given a mean terrain elevation of 1070 m ASL.

Plan A was made with SwathTRAK turned off, while Plan B was made with SwathTRAK turned on. All other parameters were assumed to be the same. Both were assigned the same 425 km2 survey area polygon and were designed to deliver Quality Level 1 of the USGS 3D Elevation Program (3DEP QL1), which requires 8 pts/m2 and an RMSEZ of 10 cm. Both plans used a 40 FOV with a minimum side lap requirement of 15%, an aircraft speed of 110 knots, and Galaxy’s maximum effective PRF of 550 kHz. 20% target reflectivity was assumed.

The planning scenarios yielded the following results:
With SwathTRAK turned OFF, Plan A required 24 flightlines to meet plan requirements. Flightlines were spaced relatively close together in areas that had mountain peaks to maintain the minimum side lap when swath widths narrowed. The total laser-on time to collect the data was 6:00:00, or 360 minutes.
With SwathTRAK turned ON, Plan B required only 18 flightlines for the survey area. The widening of the scan FOV over the mountain peaks enabled flightlines to be spaced further apart while still achieving the same minimum side lap requirement. The total laser-on time was 4:30:00, or 270 minutes.

This translates to a 25% improvement in collection efficiency over a fixed-FOV sensor, in addition to the benefits of improved point distribution, density and accuracy identified above (Figure 4).

Such an efficiency improvement is a huge leap in sensor capability and area coverage rates, so much so that it is conceivable to realize a return on invesent in significantly less time than what was previously possible with fixed-FOV sensors, particularly in high-relief environments. In lower-relief environments, changes to flight altitudes and resultant point densities are automatically solved, yielding more predictable data sets that meet the planned requirements. Teledyne Optech’s new Galaxy lidar sensor with PulseTRAK and SwathTRAK is a tremendous leap forward in innovation and efficiency that equates to real dollars in surveyors’ pockets.

Michael Sitar, M.E.S., BSc., is the Business Manager of Teledyne Optech’s Airborne Mapping Solutions Division. He has over fifteen years of lidar experience within the airborne products technical management team, and is responsible for the development, promotion and sale of Teledyne Optech’s airborne lidar and imaging solutions.

A 5.248Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE

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