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Why Not See the Forest and the Trees? Print E-mail
Written by Dr. Steven E. Mitchell   
Friday, 05 June 2015

A 2.116Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

In stark contrast to the linear-mode LiDAR sensors available on the commercial market, single photon sensitive LiDAR sensors offer multiple advantages for customers seeking cost-effective collection of wide area elevation data. Conventional linearmode sensors require detection of hundreds to thousands of photons per range measurement. This approach dictates that the transmitted laser energy be concentrated into a single beam and limits the sensor to flight at low altitudes. Across a required collection area, the compromised aerial coverage due to inefficient use of aircraft time and employee resources results in significant costs incurred by the customer.

Alternatively, single photon sensitive LiDAR sensors represent the most efficient 3D imagers possible, requiring only one detected photon per range measurement. This technology enables the transmitted laser energy to be spread out across an array of laser beamlets and enables the sensor to be operated at high altitudes. The inherently high, cost-effective collection efficiency of single photon sensitive LiDAR technology enables orders of magnitude increases in data collection capability, providing increased spatial resolution and larger swath width, areal coverage, and point density than linear-mode systems.

Sigma Space Corporation recently unveiled its Single Photon LiDAR (SPL) technology and processing services to the commercial market. These SPL sensors combine the advantages of single photon sensitivity with nanosecond recovery times and a multistop timing capability, enabling entire counties to be mapped within hours. Single Photon LiDAR is a fully scalable, operational technology proven across multiple instruments flying today at altitudes from 2,000 to 60,000 feet (Degnan and Field, 2014). Sigma Space has refined its wide area elevation data technology over the past 15 years in support of government, academic, and private industry customers. The technology is revolutionizing the acquisition efficiency and affordability of high resolution 3D information. To highlight the groundbreaking nature of the technology, the National Aeronautics and Space Administration (NASA) is utilizing SPL for the ICESat-2 (Ice, Cloud, and land Elevation Satellite) space mission scheduled for launch in 2017 (Degnan, 2009). Based on this successful heritage, Sigma Space is excited to offer its SPL technology to business and civil government users.

How is SPL technology revolutionizing the commercial LiDAR market? Beyond the fundamental increase in mapping efficiency compared to conventional linear-mode sensors, two key factors distinguish SPL technology from other single photon sensitive LiDAR sensors and enable Sigma Space to shift the price point of LiDAR acquisitions.

Multistop Detection Capability
During operation, a 10x10 array of 100 individual beamlets is generated by passing the outgoing laser beam through a diffractive optical element prior to transmission through a common telescope and scanner shared by the transmitter and receiver. The high speed scanner can generate a variety of patterns, depending on customer mapping requirements. Ground returns from the individual laser beamlets are imaged onto a multi-element photomultiplier detector array, enabling wide area coverage in a single overpass. Each detector element output is input to one channel of Sigma Space's proprietary high resolution multistop timer, which operates with a RMS timing precision of 23 psec ( ± 3 mm in range).

The detector/receiver subsystem is a key distinction between SPL and other LiDAR systems on the commercial market, as it is capable of recording the arrival times of multiple, closely temporally-spaced photons per pixel channel. With an event recovery time of only 1.6 nsec (24 cm in range), Single Photon LiDAR systems operate at up to 3.2 Megapixels per second. This multistop detection capability is a significant upgrade from other single photon sensitive sensors such as Geigermode Avalanche Photodiode based LiDAR. Unlike systems using Single Photon Avalanche Photodiode (SPAD) detectors, the combination of ultra short recovery time (detector and electronics) and multistop event timers enables SPL systems to operate in full daylight and permits multiple returns per channel from volume scatterers such as tree canopies (Figure 2). With over 15 years of in-house, proprietary timer development at Sigma Space, we've always wondered: why not see the forest and the trees?

SPL's multistop detection capability enables our sensors to provide 24/7 data collection and requires only a single overpass during airborne operations. This collection efficiency optimizes aircraft time and enables an array of industries to utilize SPL's 3D modeling technology for stakeholder communication, design, and planning. For example, customers can now leverage Sigma Space's detailed elevation data to streamline ongoing and future infrastructure activities--such as evaluation of power lines, landscaping, and roadways--with centimeter-level accuracy (Figure 3).

Complementing our multistop timers is a high speed, conical scanner which provides two looks (fore and aft) of the scene during a single overpass. With no limitations on daytime operation, SPL systems routinely fly on a variety of airborne platforms to collect elevation data and digital imagery, build point densities of 8 pts/m2 or greater, and produce manageable data sets in the standard LAS format for analysts to process. This capability was recently utilized by a customer in which one of our SPL systems acquired a high resolution map of Garrett County, MD (Figure 4). Conventional linear-mode LiDAR systems would have required weeks to complete the collect, while Geiger-mode LiDAR systems would have been limited to nighttime operations. Over the course of 12 daytime flight hours, however, Sigma Space's SPL system mapped over 1,700 km2 and provided our customer with continuous elevation data and digital imagery.

Continuous Topobathymetric Coverage
While several naturally-occurring properties such as atmospheric transmission, natural surface reflectivity, and solar background favor the use of infrared laser wavelengths, the green (532 nm) operating wavelength utilized by SPL sensors was selected on the basis of technology benefits. These include higher efficiency COTS detectors with nanosecond recovery times, high transmission narrowband filters, as well as is high transmission through water, enabling water bathymetry and 3D imaging of underwater scenes. As a result, SPL systems provide customers with seamless topobathymetric coverage in a single overpass, streamlining development activities in coastal areas including road and highway construction.

The combination of multistop, single photon detection capability with the green laser wavelength represents a paradigm shift in the LiDAR bathymetry community. Unlike existing bathymetric LiDAR sensors available on the commercial market which are limited to flight altitudes of less than 3,000 feet above ground (Quadros, 2013), efficient measurements of both topographic and bathymetric elevations are made concurrently by SPL sensors, typically at 7,500 feet above ground and 175 knot air speed. Furthermore, Geiger-mode LiDAR sensors use infrared laser wavelengths and as such offer negligible water penetration.

Thus, the advantages of SPL technology in terms of high resolution, wide area topographic mapping are also applicable to the commercial bathymetry market and enable Sigma Space to provide customers with a single data product for inundation mapping and a host of other applications. For example, SPL's continuous topobathymetric coverage can accurately outline coastline erosion, water elevation, and flood hazard areas. 24/7 operational capability and wide area coverage can rapidly assist in the assessment and determination of losses in disaster areas, providing quick and reliable data to stakeholders and local/state government agencies.

Unique Customer Solutions
The collection speed, resolution, and seamless coverage enabled by SPL technology have shifted the price point of LiDAR acquisitions. For customers seeking cost-effective acquisition of wide area elevation data, the continuous, high resolution topobathymetric mapping provided by SPL systems enables Sigma Space to provide customers with unique data solutions at a lower price per square kilometer. Industries utilize SPL's high resolution 3D data to improve industry processes, project modeling, and enhancing industry solutions without regards to project specifications. With the unveiling of SPL technology, commercial data acquisitions have now become affordable, timely, and repeatable.

For additional information on SPL solutions, please visit www.sigmaspace.com.

Degnan, John J. "Present and future space applications of photon-counting lidars." SPIE Defense, Security, and Sensing. International Society for Optics and Photonics, 2009.
Degnan, John J., and Christopher T. Field. "Moderate to high altitude, single photon sensitive, 3D imaging lidars." SPIE Sensing Technology+ Applications. International Society for Optics and Photonics, 2014.
Quadros, Nathan D. "Unlocking the characteristics of bathymetric lidar sensors." LiDAR News 3.6 (2013): 62-67.

Dr. Steven E. Mitchell is Director of Mechanical Engineering at Sigma Space Corporation. Dr. Mitchell has supported the development of single photon LiDAR technology programs to enable remote measurements of clouds and aerosols, surface topography and water depths, and wind velocities.

A 2.116Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

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