Taking topographic measurements with single photons in broad daylight means pushing the limits of precision, and this is what Sigma Space Corporation set out to do with its impossible-sounding single-photon lidar (SPL) Earth-imaging technology.
The result is an airborne device that splits a single laser beam into 100 low-power beamlets directed at different points on Earth’s surface, firing at a rate of 32,000 pulses per second and then detecting individual photons reflected back to it, distinguishing them from a storm of sun-generated photons, and determining how long each photon’s round trip took to an accuracy within about 20 picoseconds, or twenty-trillionths of a second. In this way, the lidar creates a 100-pixel, three-dimensional image with every pulse, which is then integrated into a contiguous, high-resolution image by an optical scanner matched to the aircraft’s ground speed. Furthermore, the lidar uses a fraction of the energy typical of conventional imaging lidars.
A system with this sort of capability, however, was not born overnight.
The technology draws heavily from a half century of satellite laser ranging, going back to 1964, when then-physics co-op student John Degnan, now the Lanham, Maryland-based company’s chief scientist, arrived at NASA’s Goddard Space Flight Center just in time to participate in the world’s first laser satellite ranging experiment. The project successfully timed the round trip of laser pulses fired from Earth and reflected back from a satellite in low-Earth orbit to determine its location to an unprecedented accuracy of 10 feet.
Laser ranging accuracy quickly increased by orders of magnitude and became common practice around the world. In 1994, Degnan, still at Goddard, proposed the SLR2000 system, a single-photon-sensitive, high repetition-rate, eye-safe, fully automated satellite laser ranging system. SLR2000 later evolved into the Next Generation Satellite Laser Ranging System, the prototype for NASA’s future international network of Space Geodesy Satellite Laser Ranging Systems. Both projects received support from Sigma and other private contractors.
Meanwhile, it occurred to Degnan to try reversing the direction of laser ranging. “If I can see the slope in that data for a satellite due to orbital time bias, why can’t I see a slope on the ground?” he asked.
His ensuing proposal, funded by NASA’s Instrument Incubator Program, bore fruit more quickly when NASA’s Airborne Multi-Kilohertz Photon-Counting Microlaser Altimeter flew over the Eastern Shore of Maryland and Virginia in 2001. The experimental system, which Sigma also worked on, demonstrated the ability to map Earth’s surface by catching individual reflected photons from a height of 23,000 feet and was even able to see forest floors through the canopy, as well as ocean depths down to a few meters.
“The fact that we could actually detect single photons in broad daylight—that had never been done before,” Degnan says. “With single photons, we could do things a lot faster, with less energy and smaller instruments.”
In 2003, Degnan retired from Goddard and started his work at Sigma, bringing four decades of NASA laser ranging experience with him. In the ensuing years, the company licensed two of his NASA patents—the Space Act Award-winning microaltimeter patent and a 3D-imaging lidar patent. Sigma was later awarded additional patents for improvements to the technology.
Under an SBIR contract with the U.S. Air Force, the company built on the NASA microaltimeter prototype to create the 100-beam Leafcutter, a low-altitude SPL imager capable of seeing underwater and through tree coverage for military reconnaissance. The instrument has a spatial resolution of 15 centimeters at an altitude of one kilometer.
With funding from follow-on contracts with the Department of Defense and NASA, as well as Sigma’s own money, the High-Altitude Lidar and High-Resolution Quantum Lidar System (HRQLS) followed. These were designed to operate at 25,000 and 7,500 feet, respectively.
All the imagers are based on a similar design: 100 streams of laser pulses are sent out from a 10-by-10 array of emitters, with a matching array of ultrasensitive photodetectors picking up the returning photons. A 100-channel timing system records photon departure and arrival times at laser pulse rates up to 32,000 per second, allowing 3.2 million measurements every second.
Photons reflected from the surface can be distinguished from sunlight because they arrive more or less all at once, and the more uniformly distributed solar noise can be removed in post-processing, explains Roman Machan, Sigma’s chief technology officer and creator of the systems’ timers. “You’re looking for structures, and all of those will return lots and lots of points next to each other.”
Because the aircraft barely moves in the time it takes the laser pulse to reach the surface and return, the same gaps in tree canopy that allowed the photons to reach the forest floor also allow their return to the system’s receivers, he adds.
The technology is able to penetrate water because it uses light in the green, 532-nanometer wavelength, rather than the near-infrared frequencies most other altimeters use.
The ability to cheaply and accurately map surface features across large areas has a multitude of applications, even without factoring in the ability to see through water and tree cover. While the Air Force was interested in foliage-penetrating reconnaissance, and other agencies have worked with Sigma on a system for spotting underwater mines and submarines, HRQLS can also serve such civilian purposes as rapid, wide-area surveying or seeing whether train tracks are warped or trees are infringing on power lines.