The Space Geodesy Project makes exact measurements of locations and distances on Earth from sites across the globe.
The scientists working on NASA’s Space Geodesy Project (SGP) are some of the only people in the world who can say they know exactly where they are. The scientists and engineers at sites like the Goddard Geophysical and Astronomical Observatory (GGAO) are part of an international endeavor to measure distances and locations with an astounding degree of precision; for example, tracking a satellite 25,000 km away to the nearest centimeter.
“Geodesy is the science of Earth’s shape, orientation, and gravity,” said Stephen Merkowitz, project manager for SGP. “We’re the people that help you determine where you’ve been, where you are, and where you’re going.”
The project achieves its objective using four distinct measurement techniques: Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and the Global Navigation Satellite System (GNSS).
Very Long Baseline Interferometry is a method of measuring the orientation of the Earth in space relative to the Celestial Reference Frame. Radio telescopes in one hemisphere point to distant quasars, which act as stationary points relative to the Earth, and monitor the time delay between each telescope by time-tagging the arrival of microwave signals from the quasars at each telescope. These time delay data provide a measure of the orientation of the Earth in space and also produce a three-dimensional grid of relative locations along the surface of the Earth. The orientation and position measurements contribute to the International Terrestrial Reference Frame (ITRF), the foundation for the geolocation of all Earth-observation measurements.
Satellite Laser Ranging works by sending short laser pulses from ground stations to satellites in orbit. Retroreflector arrays mounted on the satellites reflect the laser pulses back to the stations, where the round-trip time-of-flight is recorded and converted into precise measurements of the range between the satellites and the ground stations. The range measurements are used to precisely determine the location of the ground stations on the Earth’s surface and the location of the Earth’s center of mass, which is the origin point of the ITRF.
DORIS is a French system that acts as a beacon or lighthouse for orbiting satellites. The DORIS transmitters on the ground produce a constant radio frequency, which is shifted by the Doppler Effect as receiving satellites approach and pass the beacon. The frequency shift, when combined with timing data, can be used to precisely determine the position of both the satellites and the ground stations.
The Global Navigation Satellite System (GNSS) consists of several constellations of satellites including the Global Positioning System (GPS). The GNSS satellites transmit ranging codes to receivers on the ground to determine the position of the receivers in three-dimensional space relative to the constellation of satellites. A global network of GNSS ground stations fills in the geographic gaps between the other geodetic techniques, densifying the locations used for the ITRF.
At a core site of the Space Geodesy Project, all four of these techniques are applied and incorporated with data from the international space geodesy network to form a precise estimate of the location of the site. For this combination of data, not only do the locations of each instrument need to be surveyed but exactly where on each instrument measurements are received must be characterized at the millimeter level. This orchestration is done through the Robotic Total Station (RTS), which measures these distances and locations for conversion into a comprehensive framework of known vectors for the entire site.
The major product of the international space geodesy network is the terrestrial reference frame, which can be accessed using a GNSS receiver. A less well-known product of space geodesy is measurement of the Earth’s rotation compared to atomic clock time that is used to determine when to schedule a leap second. Within the Earth-observation and space-based science community, space geodesy has an even larger impact. For example, altimetry satellites like the Jason satellites and ICESat need to precisely determine their height and location using SLR, GPS, and DORIS measurements. VLBI measurements also have been used to study plate tectonics and crustal motion since the use of mobile VLBI stations in the 1980s. GNSS now provides very detailed measurements of crustal motion and is particularly useful for monitoring earthquakes. On a societal level, space geodesy aids in precision farming, where crops are grown as efficiently as possible by planting and treatment via augmented GPS, and in infrastructure by providing a common reference frame for large public works (particularly international ones where each country might be using a different local reference frame). For example, a bridge built using a common terrestrial reference frame can start from either end with assurance the bridge will connect in the middle.
The next goal for the Space Geodesy Project is accurate measurements to within a millimeter. “For subtleties like variations in sea level rise, you need the precision of 1 millimeter,” said Larry Hilliard, who leads the VLBI development team at Goddard.
Precision orbit determination also will benefit from millimeter accuracy, particularly altimetry satellites. “If [altimetry satellites are] trying to measure the height of sea to a millimeter, they need to know their own position to that kind of accuracy,” said Merkowitz.
This advancement requires even more exact measurements of the local ties within the sites. In addition, a more complete ITRF will involve a wider spread of core sites. DORIS and GNSS sites are small and relatively well distributed across the globe, but VLBI and SLR instruments are large and require a significant investment. Gaps in the worldwide net, particularly in Africa and South America, need to be addressed for a complete terrestrial reference frame. “(The millimeter goal) is going to require a global effort,” said Merkowitz. “It’s going to be incremental to get us there. It’s also going to require modern technology.”
To this end, VLBI stations have been pushed to become more effective through smaller, faster antennas. Modern SLR systems are moving toward high levels of automation and autonomy to bring down operational costs. With these advancements and modifications, these systems can be more easily be deployed and operated than previous generations of technology.
The future of NASA space geodesy is just now being implemented. The first operational next generation VLBI system is established in Kōkeʻe Park Geophysical Observatory in Hawaii, with plans for another new VLBI and next generation SLR system in Texas, which will make it the first core site of next generation space geodesy. The Texas VLBI system is projected to be operational in two years, with the new SLR system about a year later.
Joseph Dowling is a former summer intern at NASA’s Goddard Space Flight Center and a fire protection engineering student at the University of Maryland.