Low and Slow: Helicopter-based Lidar for Snow & Ice Observations

Editors Note: The original article is posted in LiDAR News, we hope you enjoy this fascinating story below involving USACE CRREL and NCALM as they work together to develop a small footprint helicopter-based LiDAR pod system utilizing RIEGL airborne instrumentation.

CRREL’s LiDAR pod mounted on a Robinson R44 Raven II helicopter during
surveys of Wolverine Glacier, Alaska in collaboration with the USGS.

Where there are glaciers and deep, persistent snow, you’ll almost always find equally impressive topography: high-relief mountainous terrain, glacial carved valleys, dense forest. Navigating and working in these environments, let alone conducting technical operations like high-accuracy topographic surveys, is challenging at best and downright dangerous at worst. The US Army Corps of Engineers Cold Regions Research and Engineering Laboratory’s (CRREL) Lidar Team specializes in the use of lidar sensors across all platforms (airborne, mobile, terrestrial, unmanned), ranging from focusing on sensor development and fusion, through data acquisition, advanced processing and data dissemination. In particular, the CRREL team participates in both snow and glacial sciences research utilizing airborne lidar scanning, aimed at better estimating snowpack volumes and densities and understanding glacier dynamics. Oftentimes the areas of interest are small, widely dispersed, lay within high relief terrain with dense vegetation coverage, and are covered in wet, low reflectivity snow and ice. At that, airborne lidar surveys typically need to be flown low (sub-500m AGL) and slow (50-knots) to penetrate dense canopy sufficiently and achieve the required point densities of snow and ice surfaces.

CRREL, in collaboration with the National Center for Airborne Laser Mapping (NCALM) at the University of Houston, has spent the past 6-years iterating on versions of a small footprint helicopter-based lidar pod system, targeting the Robinson R44 Raven II aircraft as the base platform. The initial concept was to utilize a RIEGL VZ-series terrestrial sensor (VZ-400 or VZ-1000) for the laser scanner, allowing a remote field team to conduct both terrestrial and airborne surveys while keeping the cost of both the system and operations (the R44 costs $500-700/hour) to a minimum. The team aimed to reduce the complexity and time required for system installation, providing a solution for rapid deployment on any R44 Raven II worldwide. Starting in 2014, CRREL and NCALM developed an integrated lidar system designed around the Simplex Top Loader II pod for the R44. Paired with an iXBlue ATLANS-C inertial measurement unit, tail-mounted GNSS antenna, and custom power and communication hardware, the system can be installed in under 3 hours and operated by a single user via laptop inside the aircraft. Used throughout Alaska during the development process, the pod was successfully utilized to capture glacier centerline swaths of hard to reach small alpine glaciers where traditional aircraft could not operate, and contributed to the successful measurement of mass balance for these small glaciers.

With the purchase of a RIEGL VQ-480i in 2015, CRREL and NCALM integrated the Lead’Air NexTrack2 flight management system (FMS) into the existing pod, providing precision pilot guidance and automated flight-line triggering, while RIEGL’s RiACQUIRE software controlled data acquisition. In replacing the terrestrial scanner with a fixed head sensor, the boresight calibration process was greatly simplified and stabilized. In following years as project requirements necessitated, CRREL, NCALM, and the United States Geological Survey (USGS) integrated a Hasselblad A6-D aerial mapping camera, FLIR SC-8343 medium wave infrared camera, and Headwall Nano hyperspectral camera, all of which could be operated coincidently within the pod. By designing the pod to include multiple 12 and 24-VDC power output ports and an expandable network switch, along with multiple camera ports, the system is flexible and modular to support numerous project needs. The addition of a RIEGL VQ-580 to the CRREL sensor inventory in 2017 further increased the performance of the pod on snow and ice surfaces with its 1064nm laser. In addition, the sensor plate mounted within the Simplex pod can be installed in any fixed-wing aircraft with a camera port, further increasing the utility of the design. This system has been deployed across various projects, including glacier, fault scarp, and remote river monitoring surveys with the USGS, landslide hazard mapping for the Federal Emergency Management Agency (FEMA), waterway projects for the US Army Corps of Engineers and in a multi-agency response to the 2018 eruption of Kilauea volcano in Hawaii.

Schematic of the CRREL/NCALM HeliPod airborne lidar system, showing
the components within the pod (right) and the components
on the exterior and interior of the aircraft. 

While the R44 pod has proven extremely useful and modular, able to be reconfigured quickly to meet project needs and installed within hours and be run by a single operator, the pod is restricted for use on just the R44. In 2019, in an effort to design a system compatible with multiple aircraft, CRREL designed and manufactured a lighter and smaller pod that can be mounted on any aircraft equipped with a Meeker Aviation dovetail quick release plate. In addition to expanding the number of aircraft compatible with the system, it also greatly reduced installation time (down from hours to minutes), improving the utility of the system in rapid deployment or disaster relief efforts.

Basing off the payload size and weight restrictions of the AirFilm/Meeker Aviation AF-R44NM nose mount for Robinson R44/R66 helicopters, the new pod had to stay below 50 lbs., half the weight capacity of the R44 Simplex pod, and with a frontal surface area of no more than 1.5 ft2. The combination of laser scanner, either the VQ-480i (21.8 lbs.) or VQ-580 (28.6 lbs.), and ATLANS-C IMU (6.4 lbs.) uses a majority of the 50 lbs. payload limit on their own. Therefore, it was necessary to reduce the number of components in the suspended payload to the bare minimum (laser, IMU, vibration dampening, shell) by moving components (power supply module, network switch, laser safety switch, FMS) to a location inside the aircraft. Components removed from the pod were built into an enclosure that could be placed anywhere inside the aircraft, with power and communication cabling exiting the aircraft (e.g. through the nose vent of a Robinson R66 helicopter) to the sensor payload. CRREL designed an aluminum skeleton with a carbon fiber shell to protect the payload and reduce drag. The shell was decoupled from the payload’s vibration isolators, a series of wire rope isolators in compression, reducing movement/vibration of the sensor payload due to wind forces.

Top) CRREL’s lidar pod mounted on the nose of a Robinson R66
helicopter outside of Boise, Idaho during a snowpack survey
in collaboration with Boise State University and NASA. Bottom left) CAD model
of the CRREL pod, showing interior components within the carbon fiber shell. Bottom right)
bare earth DEM of Dry Creek basin, Idaho derived from lidar data collected by CRREL

Looking to the future and expanding the systems’ utility for snow and ice observations, sensor fusion is the focus going into 2021. With weight restrictions imposed on the Meeker mount, additional sensors beyond the laser scanner are not possible. CRREL is currently modifying the design to integrate the lighter and more performant RIEGL VQ-580 II and Applanix AP60-AV IMU, designing smaller vibration isolators and reducing the shell weight through the use of thermoformed carbon fiber. While the extra payload capacity available from these changes will allow for integration of more traditional mapping sensors (EO, IR, hyperspectral), CRREL is focused on the integration of a GAMMA Remote Sensing L-Band synthetic aperture radar into the pod to allow for coincident snow surface topography and snowpack density for snow water equivalent measurements. Able to operate from a number of aircraft, as well as from ground-based vehicles such as snowmobiles, this fusion of sensors will provide a valuable tool to the cryospheric and hydrology communities, and will help CRREL continue to push the envelope of cold regions research and engineering, even in the most austere environments.

To view the original article feel free to click the link below.

Low and Slow: Helicopter-based Lidar for Snow & Ice Observations


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