Photonics Scores a Touchdown for Space Exploration

HANK HOGAN, CONTRIBUTING EDITOR

As the Apollo 11 astronauts were initiating the first human-crewed landing on the moon, Neil Armstrong looked out of the module’s small window and, rather than observing a relatively flat landing zone on the lunar surface, he saw a boulder field. So he took control of the spacecraft and piloted it to a safe site, which became known as Tranquility Base.

A Navigation Doppler Lidar (NDL) engineering test unit is evaluated by its ability to determine the range of a moving truck. Courtesy of NASA.

When crewed spacecraft return to the moon in 2024 as part of NASA’s Artemis program, landings should be less eventful, even though the chosen destination is the potentially vital lunar south pole (see sidebar at end of article). In this region, deep shadows and steep sun angles that would likely have overwhelmed the Apollo crew or robotic landers pose challenges for modern landing operations. However, innovative photonics technology will aid upcoming touchdowns in the area, as well as future landings of crewed and uncrewed spacecraft targeting other locations on the moon, Mars, Jupiter’s moon Europa, and other celestial bodies.

These landings will be safer, in part, due to the deployment of photonics- powered terrain-relative navigation, NASA’s Navigation Doppler Lidar (NDL), hazard detection lidar, and application-specific algorithms and computational platforms. These technologies will enable the ability to shrink the size of the targeted touchdown zones from several kilometers wide across to the size of a football field.

These precision landings are the result of long-running research and development. “Precision landing technologies have been an investment area within NASA for about 15 years now,” said John Carson, space technology mission director and technical integration manager for precision landing at NASA’s Space Technology Mission Directorate. The directorate is funding the Safe and Precise Landing — Integrated Capabilities Evolution (SPLICE) project.

NDL provides information about distance and relative velocity from a spacecraft to the surface of the moon or other celestial body during the spacecraft’s descent. The technology is another tool that helps to enable more precise touchdowns during space missions. Courtesy of NASA.

To meet a specific mission requirement, NASA scientists and engineers often develop a solution to a level of maturity at which they can pass it off to companies for commercialization. NASA then licenses the technology to selected firms, which further develop it and find markets for products. Such is the case with SPLICE technologies that are being put to use in space missions and on Earth.

Terrain-relative navigation

The first SPLICE technology investment is the Lander Vision System, which implements terrain-relative navigation. “It uses a map that’s generated from orbital imagery,” Carson said. “It compares camera images taken during descent with that onboard map to help localize where you are in your descent relative to where you want to be.”

An illumination map of the lunar south pole. The shades of gray depict the amount of sunlight that will be received in 2024, the year scheduled for a crewed landing. The floors of many craters are permanently in shadow, while some higher elevations are constantly illuminated by the sun. Orbital measurements indicate that water or water ice may be present at the lunar south pole, which is one reason that both crewed and robotic landings are scheduled to visit the region over the next few years. Courtesy of NASA.

Deployed initially on the Mars 2020 Perseverance Rover, this technology helped the lander to touch down on Mars in February of this year within a few tens of meters of its intended target. For comparison, the landing uncertainty for the 2012 Mars Curiosity rover was measured in kilometers.

Terrain-relative navigation is activated when the lander is still several kilometers above the surface, and it tracks the vehicle’s position relative to the target landing site. But the technique only works at an altitude determined by orbital image resolution. Below this height, other sensor systems must help a lander to reach its target. This is where hazard detection lidar and Navigation Doppler Lidar come into play.

Navigation Doppler Lidar

With NDL, a laser beam bounces off a surface and creates a return signal that measures distance and — through the frequency shift caused by the Doppler effect — relative velocity. The latest generation of NASA’s NDL can accurately measure distance within 2 m at a range of more than 5 km. It can further gauge velocities up to 200 m/s with an error of 2 cm/s.

NASA’s systems use three laser beams, each transmitted in a fixed direction relative to the others. The three returning signals are combined to determine the three components (x, y, and z) of the vehicle’s velocity.

Terrain-relative navigation, first used for the Mars 2020 Perseverance Rover mission, helps to enable more- precise landings. Courtesy of NASA/JPL.

The technology is suitable for landing missions targeting the moon, Mars, and other celestial bodies. But it requires specific performance parameters from the lasers used, said Farzin Amzajerdian, principal investigator for NASA’s NDL technology. “We needed a very stable, single-frequency laser with a narrow linewidth.”

This requirement stems from the necessarily small margin of error for velocity measurements, which rely on the detection of frequency changes in the Doppler effect. A more stable outgoing frequency helps to minimize the measurable velocity error.

The lasers for the spaceflight-ready engineering demonstration units of the NDL system came from RIO, part of LUNA Innovations. According to Lew Stolpner, vice president of technology and product management at RIO, these planar external cavity lasers integrate an indium phosphide gain chip and a silica-on-silicon planar lightwave circuit with a waveguide Bragg grating. The grating acts as one of the reflective mirrors in the laser cavity and enables single-frequency coherent operation.

Due to this construction, the laser has the superior wavelength stability, low-frequency noise, and narrow-linewidth emission needed for Doppler lidar, Stolpner said. “We combined the key performance attributes of the legacy low-noise single-frequency laser with the compactness, reliability, and stability of the semiconductor laser.”

This proprietary structure, when combined with active temperature control and mission-suitable packaging, enabled a navigation system to perform as NASA required. The technology has found use in distributed fiber optic sensing applications and has attracted further interest from engineers who develop lidar systems for the defense, aerospace, and automotive industries, Stolpner said.

Hazard detection lidar

NASA’s SPLICE program is also investing in the development of advanced hazard detection lidar to detect surface features and objects large and small that could damage a spacecraft as it lands on the moon.

The technology could also play a role in potential missions to Mars and Europa, the ice-covered moon of Jupiter that scientists say may harbor life. Any Europa lander would likely arrive without the aid of advanced orbital reconnaissance. So, the navigation system would be almost completely dependent on landing site data generated as the spacecraft descended. This places high performance demands on the hazard detection lidar to rapidly and accurately capture three-dimensional data about the surface and its features.

“You’re coming in fast. You need to map the surface really quickly,” said Bryan Blair, NASA’s principal investigator for the SPLICE hazard detection lidar. “Then you need to analyze that data really quickly. With the hazards being so small, you need high resolution.”

The hazard detection lidar system designed for such landings must capture data with 5-cm resolution for an area measuring 100 m on a side, and do so within a second or two — from a rapidly descending, vibrating platform that is possibly rotating in the horizontal plane. The solution, which is close to being deployable in space, uses a fiber laser and two prisms spinning at about 6000 rpm. This approach enables rapid beam scanning over the target to produce a highly accurate 3D map, according to testing done at NASA.

Ready for launch

The advanced hazard detection lidar technology is a year or more away from commercial partnerships, and its potential spaceborne applications range from navigation to scientific studies of celestial surfaces. NASA’s SPLICE Navigation Doppler Lidar technology, on the other hand, is already being sold as a commercial product by technology licensee Psionic. The first customers will be commercial space ventures, such as space robotics company Astrobotic Technology, based in Pittsburgh.

Astrobotic’s Griffin lander will carry NASA’s VIPER (Volatiles Investigating Polar Exploration Rover) to the lunar south pole in 2023, a year before the crewed Artemis spacecraft lands in the same area. The company also has other landers planned over the next few years, all of which have a mission-critical need for NDL.

“You need a sensor that can provide a full velocity vector all the way down to the surface, or else the spacecraft could tumble,” said Andrew Horchler, principal research scientist at Astrobotic.

NDL could have more down-to-earth applications, according to Stephen Sandford, Psionic’s chief technology officer. Helicopters, for instance, touch down much like a lander and often must do so where visibility is poor, such as in dusty or snowy locations. Landings at sea can also be challenging because both the ship and helicopter are moving.

Sandford said other scenarios for possible uses involve transportation, where lidar’s ability to measure distance and velocity could benefit self-driving buses, trucks, mining equipment, or other large and relatively expensive autonomous vehicles.

The demands of these applications all differ from one another and from the space exploration setting for which NASA developed the initial technology. Consequently, Psionic is refining the technology into its own designs, some of which already provide significant improvements to system size, power, and other features, according to Sandford. So, while commercialization is underway and lidar products will soon take flight in a number of platforms, more work must be done to meet diverse application needs.

“[The potential application markets] all have price points and performance points that are different than what was developed at NASA,” Sandford said. “Our job at Psionic is to take that technology and meet those price and performance targets for those markets.”

Why the Moon’s South Pole?

The Apollo landings all took place near the moon’s equator, but future robotic and crewed landers slated to visit Earth’s only natural satellite are targeting its south pole. Noah Petro, project scientist for NASA’s Lunar Reconnaissance Orbiter, said the poles are located far from other previously explored areas and therefore offer new scientific opportunities. The polar region could also harbor strategic resources that are critical to the establishment of permanent human lunar habitation.

Volatiles, such as water or ice, may be found at the pole and could become a resource for inhabitants and also a target of scientific investigation to understand its origins, Petro said.

Some measurements taken from the moon’s orbit indicate the presence of hydrogen — thought to be the result of H₂O or hydroxyl (OH) — while others do not. Data collected by people and robotic instruments on the surface could settle this question. Comparing the data to information simultaneously collected from orbit may also make it possible to map out likely water-bearing areas located elsewhere on the surface.

In addition to these incentives, parts of the lunar south pole experience longer periods of sunlight, a potentially useful power resource. And some craters near the poles never see the sun at all, leading them to have the lowest measured temperatures in the solar system. Access to such sites could be useful to science and industry.

“There are multiple reasons why the south pole is a place we want to explore,” Petro said.