NASA's laser reflector instrument helps to accurately locate Earth measurements

The most famous use of GPS satellites is to help people understand their location, whether it is driving cars, ships or planes, or hiking in remote areas. Another important but little-known use is to distribute information to other Earth observation satellites to help them accurately locate measurements of our planet.

NASA and several other federal agencies, including the US Space Force, the US Space Command, the US Navy Research Laboratory, and the National Geospatial Intelligence Agency, are improving the positioning accuracy of these measurements to the millimeter level through a new laser retroreflector array.

"The main benefit of laser ranging and LRA is improving the geographic location of all our Earth observations," said Stephen Merkowitz, project manager of NASA's Space Geodesy program at the Goddard Space Flight Center in Greenbelt, Maryland.

Earlier this year, a team of scientists and engineers from the project tested these arrays to ensure they were capable of carrying out missions and withstanding harsh space environments. Recently, the first batch of new laser reflector arrays were shipped to the US Space Force and Lockheed Martin in Littleton, Colorado to be added to the next generation of GPS satellites.

Laser reflector arrays make laser ranging possible - using small pulse lasers to detect the distance between objects. The laser pulse from the ground station is guided onto the orbiting satellite, which is then reflected from the array and returned to the space station. The time required for light to propagate from the ground to the satellite and then return can be used to calculate the distance between the satellite and the ground.

For decades, laser ranging and laser retroreflector arrays have been part of space missions, and they are currently installed on Earth observation satellites such as ICESat-2, SWOT, and GRACE-FO and are crucial for their operation. During the Apollo mission, LRA for laser ranging was even deployed on the surface of the moon.

"The Lord's Resistance Army is a special mirror," Mercowitz said. They are different from ordinary mirrors because they reflect light directly onto their original light source.

For laser ranging, scientists hope to guide the beam back to the original light source. They achieved this by placing three mirrors at right angles, essentially forming the inner corners of the cube. The laser reflector array consists of 48 mirror angle arrays.

"When light enters the array, due to these 90 degree angles, the light will reflect and produce a series of reflections, but the output angle will always be the same as the angle of entry," said Zach Denny, an optical engineer for the Goddard Space Geodetic Survey project.

Geodesy is the study of the shape of the Earth, its gravity and rotation, and how they change over time. The laser ranging to laser retroreflector array is a key technology in this study.

Due to the movement of tectonic plates, melting of ice layers, and other natural phenomena, the Earth's surface constantly undergoes minor changes. With these constant changes and the fact that the Earth is not a perfect sphere, there must be a method to define the measurements of the Earth's surface. Scientists refer to it as a reference frame.

These arrays and laser ranging not only help to accurately locate satellites in orbit, but also provide accurate positioning information for ground stations on Earth. With this information, scientists can even locate the center of Earth's mass, which is the origin or zero point of the reference frame.

Geodesy - laser ranging to reference satellites, such as LAGEOS - used to continuously determine the position of the Earth's center of mass, accurate to one millimeter. These measurement results are crucial for scientists to assign longitude and latitude to satellite measurements and place them on maps.

Major events such as tsunamis and earthquakes can cause minor changes in the Earth's center of mass. Scientists need precise laser ranging measurements to quantify and understand these changes, said Linda Thomas, a research engineer at the US Naval Research Laboratory in Washington.

Satellite measurements of subtle but important Earth phenomena rely on precise reference frames. The long-term trend of global sea level rise and its seasonal and regional variations occur at a rate of only a few millimeters per year. If scientists want to accurately measure a reference frame, the reference frame needs to be more accurate than these changes.

"Geodesy is a fundamental component of our daily lives because it tells us where we are and how the world is changing," said Frank Lemoyne, a project scientist for NASA's Space Geodesy program.

Source: Laser Net