YELLOW SPRINGS, Ohio – A research project intended to enable more precise imaging of space objects has moved from lab bench testing to field testing at the John Bryan State Park observatory, illuminating night skies with a green laser beam of light.
The project is a collaboration between the Electro-Optical Space Situational Awareness Team of the Air Force Research Laboratory’s Sensors Directorate and the Air Force Institute of Technology’s Department of Engineering Physics.
Inside a seemingly nondescript facility enclosed by a chain link fence with a locked gate at the end of a narrow gravel road leading through the forest sits a unique telescope that has been retrofitted with a laser and sensors designed to probe turbulence in the atmosphere. The roof above the telescope opens and provides a clear view of a starlit sky as the telescope is moved into position according to coordinates programmed by scientists and engineers.
“The Turbulence and Aerosol Research Dynamic Interrogation System, or TARDIS, is an easily seen pulsed green laser used for atmospheric research,” said Steven Zuraski, principal investigator on the project. “The green light goes up, scatters off of molecules and dust particles in the atmosphere, and the reflected light comes back down and is measured by the sensor system. This measurement can quantify the optical turbulence structure or blurriness of the atmosphere.”
Analyzing these measurements will provide scientists with valuable information when imaging objects in space.
“In order to collect enough light scattering off that dust, you need a relatively big telescope, so we’ve selected the 24-inch telescope at the John Bryan Observatory,” said 2nd Lt. Charles Carr, an astrophysicist with the research team. “Not all the light is going to reflect back to the telescope. The majority transmits out through the atmosphere or scatters off in different directions. This is why it can be seen by observers looking into the night sky at John Bryan State Park. We’re trying to measure pretty small amounts of light that actually scatter back from different levels in the atmosphere.”
“You can think of it like looking at a star. Turbulence is what causes the star to twinkle, so we are measuring the amount of twinkle in the stars. This helps us understand how to remove the cause of the twinkle and obtain clearer images of space objects,” Zuraski said.
Turbulence in the atmosphere is caused by mixing of air at different temperatures, pressures and densities, Carr explained.
“If you’ve ever seen the shimmering above a barbecue, imagine trying to take a picture of something thousands of feet away through that shimmering. Knowing how that shimmering affects your image will allow for correction of it after the images are taken,” he said.
The observatory, owned by the State of Ohio, is under a joint lease with the Miami Valley Astronomical Society, or MVAS – a group of local astronomers, and Applied Optimization, a local small business supporting AFRL’s efforts on the research project. The facility was built by the Air Force in the 1960s, when it was in operation as a satellite tracking facility. In fact, the facility has multiple telescopes, with the Air Force project occupying one side of the facility and the MVAS the other.
The facility is great for observation because of the dark skies available at the park.
The laser has been propagating into the atmosphere since April 12, 2018, a date the team refers to as “first light.” Prior to first light, the laser had been tested in a laboratory environment for two years.
Light leaves the laser, travels through an optical beam routing and reshaping system, then goes out into the atmosphere. The returned scattered light is collected by the telescope and is measured by the sensor system.
“Without turbulence, the returned wave front would be perfectly flat as seen by the telescope; with turbulence, it will look like a complex wavy surface and we have a sensor that measures that,” Zuraski said. “That sensor has a really small field of view so there are lot of challenges with getting this light beam to shine correctly onto it. Alignment tasks are always very time consuming in optical systems.”
Safety procedures have been built into all stages of operation to avoid disruption to any airborne object, including those in orbit.
“When laser energy is emitted above the horizon, it continues to travel into space and may inadvertently illuminate satellites,” said Adam Battle, a deep space observation engineer with Applied Optimization. Before each night of operations they perform rigorous deconfliction procedures with various authorities to ensure this does not happen.
One such authority is the Federal Aviation Administration. Applied Optimization engineers notify the FAA air traffic control centers at both Indianapolis and Columbus to let them know of scheduled lasing times. Authorities are notified before and after lasing activities each night and at least two trained aircraft spotters are also on hand to ensure the laser is not fired near any passing aircraft.
“We actually shut the laser off if an aircraft comes within 25 degrees of the laser,” Carr said.
The laser is eye safe to pilots, but it would create a green glow on the cockpit window that could be alarming to a pilot, according to Zuraski. “We chose green for the laser specifically because it scatters off the molecules and dust in the atmosphere really well at the propagation distances we are interested in researching.”
“This research will help to validate existing models of turbulence in the atmosphere. There are many different models of how turbulence changes with height in the atmosphere and other parameters like pressure, density, wind speeds, etc., so having those climatological parameters in addition to the TARDIS measurements, we’ll be able to validate the accuracy of those models,” said Zuraski.
The research helps the SSA Team paint a clearer picture of space, but this research isn’t the team’s first project studying imaging through turbulence.
"We've been developing and using models to understand the effects of turbulence on imaging from not just ground to space, but in other geometries such as air-to-air, and air-to-ground, and at various ranges. This additional data will support validation of our models and help discover ways to improve image clarity," said Alan Ratcliff, branch chief for the Electro-Optical Space Situational Awareness team. "This is the first time we've actually had the opportunity to collect high quality data from a ground-to-space perspective." The six-inch diameter laser was mounted on the telescope by Battle and Lucas Guliano, an electro-optical observation engineer with Applied Optimization.
“On average, the atmosphere changes about 25 to 35 times per second,” Guliano said. “Conditions such as wind, humidity, and general movement in the atmosphere cause changes on that level. The idea is that if you take images at a high enough frequency, you can get multiple images of a static atmosphere. If it’s changing at 25 times per second and you’re taking images at 200 times per second that means you’ll get eight images per frozen atmosphere. The idea is that we can control what portion of the atmosphere is measured from the returning laser light,” he said.
Neal Miller, a computer science intern at Applied Optimization, and a student at Wittenberg University in Springfield, helps write and maintain the programs used to run the laser system and also helps with the program that captures images from the sensor used with the laser and telescope.
He described his experiences supporting the research as “fantastic,” and said, “I never thought that I’d be involved with anything this cool. My previous intern experience was helping maintain call center management software. Going from that to a giant laser research project supporting the United States Air Force Research Laboratory is pretty awesome.”