The most well-known use of GPS satellites is to help people know their location, whether they are navigating in a car, ship or plane, or hiking through remote areas. Another important but little-known use is sending information to other Earth observation satellites, helping them make precise measurements of the Earth. Laser retroreflective arrays (LRA) are advancing the capabilities of Global Positioning System (GPS) satellites, which are critical for accurate measurements of the Earth in geodesy. This technology enables precise tracking of the Earth's shape, rotation and environmental changes.
GPS satellites support Earth measurements by relaying information to other satellites. Laser retroreflective arrays (LRA) are improving this process and increasing the accuracy of geodesy - the study of the Earth's shape and changes. Laser retroreflective arrays enable precise distance measurements, which are critical for monitoring global phenomena such as rising sea levels and tectonic shifts.
NASA and other federal agencies, including the U.S. Space Force, U.S. Space Command, the U.S. Naval Research Laboratory and the National Geospatial-Intelligence Agency, are using a new laser retroreflective array (LRA) to improve the positioning accuracy of these measurements to the millimeter level.
"The primary benefit of laser ranging and LRA is improved geolocation for all of our Earth observations," said Stephen Merkowitz, NASA Space Geodesy Program Manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
The team of project scientists and engineers tested the arrays earlier this year to ensure they were up to the task and could withstand the harsh space environment. The first of these new laser retroreflector arrays were recently shipped to the U.S. Space Force and Lockheed Martin in Littleton, Colorado, to be added to the next generation of GPS satellites.
Laser retroreflector arrays make laser ranging possible - using small beams of laser light to detect the distance between objects. Laser pulses emitted from the ground station are directed toward orbiting satellites, reflected off the array, and returned to the ground station. The time it takes for light to travel from the ground to the satellite and back can be used to calculate the distance between the satellite and the ground.
Laser ranging and laser retroreflective arrays have been part of space missions for decades. They are currently installed on Earth observation satellites such as ICESat-2 (Ice, Cloud and Land Elevation Satellite 2), SWOT (Surface Water and Ocean Topography) and GRACE-FO (Gravity Recovery and Climate Experiment Follow-up) and are critical to the operation of these satellites. LIDAR for laser ranging was even deployed on the lunar surface during the Apollo missions.
"Laser rangefinders are special mirrors. They are different from ordinary mirrors in that they bounce light directly back to the original source," Melkowitz said.
For laser ranging, scientists want to direct the beam back to the original light source. To do this, they placed three mirrors at right angles, essentially forming the interior corners of a cube. The laser retroreflector array consists of 48 such mirror corner arrays.
"When light enters the array, because of these 90-degree angles, the light bounces around and has a series of reflections, but the output angle is always the same as the angle at which it entered," said Zach Denny, an optical engineer with the Goddard Space Geodesy Program.
Geodesy is the study of the Earth's shape, gravity, and rotation and how they change over time. Laser ranging on laser retroreflective arrays is a key technology in this research.
The Earth's surface is constantly undergoing subtle changes due to shifting tectonic plates, melting ice and snow, and other natural phenomena. Because of these constant changes, and the fact that the Earth is not a perfect sphere, there must be a way to determine measurements of the Earth's surface. Scientists call this a frame of reference.
These arrays and laser ranging not only help pinpoint satellites in orbit but also provide accurate positioning information to ground stations on Earth. With this information, scientists can even find the Earth's center of mass, which is the origin or zero point of the reference frame.
Geodesy - Laser ranging reference satellites (such as the LAGEOS Laser Geodynamic Satellite) - are used to continuously determine the position of the Earth's center of mass, accurate to millimeters. These measurements are critical for scientists to assign longitude and latitude to satellite measurements and plot them on maps.
Major events such as tsunamis and earthquakes can cause small 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 U.S. Naval Research Laboratory in Washington.
Satellite measurements of subtle but important Earth phenomena such as sea level rise rely on precise frames of reference. The global long-term trend in sea level rise and its seasonal and regional variations are only a few millimeters per year. If scientists want to accurately measure these changes, the frame of reference must be more precise than these changes.
"Geometry is a fundamental part of our daily lives because it tells us where we are and how the world is changing," said Frank Lemoine, NASA Space Geodesy Program Scientist.
Compiled source: ScitechDaily