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Researchers Work to Improve Global Reference Frame Used in GPS Navigation, Earth Monitoring

Project Is Supported by $1.2M Grant from NASA

GPS satellite Photo by Wikimedia Commons

GPS uses radio signals from a constellation of 31 orbiting Global Positioning Satellites to tell people – as well as trains, planes and ships – exactly where they are on the surface of the Earth in real time. With next-generation smartphone technology, it is possible to obtain positions to within one foot of accuracy.

By Edwin L. Aguirre

For this year’s Thanksgiving, nearly 49 million Americans are expected to hit the road for the holiday, according to the AAA. And many of those travelers will be using their cars’ onboard GPS navigation systems or a smartphone app to guide them to their destinations.

Those motorists may one day have even more precise directions to follow, thanks to a grant awarded to a team of researchers from UMass Lowell and NASA’s Goddard Space Flight Center in Greenbelt, Md.

The researchers received a two-year, $1.2 million grant from NASA’s Earth Science Division to develop an instrument – called the Geodetic Reference Instrument Transponder for Small Satellites, or GRITSS – that would significantly improve the accuracy of the International Terrestrial Reference Frame (ITRF), which is the basis of GPS positioning and navigation. UML is the lead institution for the project.

The ITRF is a virtual map of the Earth that pinpoints not only specific geographic positions but also describes Earth’s precise shape, physical topography, orientation and rotation with time based on a stationary, Earth-centered coordinate system. 

“The location of each GPS satellite is defined within the ITRF. To obtain an accurate location with your handheld GPS receiver, accurate knowledge of the position of these satellites is crucial,” says Christopher Beaudoin ’00, ’02, ’06, who works at the university’s Submillimeter-Wave Technology Laboratory as a senior radar engineer.

Christopher Beaudoin Photo by Edwin L. Aguirre
Christopher Beaudoin tests the system for the GRITSS X-band transmitter at UMass Lowell’s Submillimeter-Wave Technology Laboratory. He received his bachelor’s, master’s and doctoral degrees in electrical engineering from the university in 2000, 2002 and 2006, respectively.

The ITRF is the foundation for virtually all airborne, space-based and ground-based Earth observations, says Beaudoin, who is leading the effort at UML. Aside from GPS, the ITRF is also used in studying and monitoring global changes in the Earth’s oceans, ice sheets, tectonic plates and atmosphere. It also serves as a fundamental reference for interplanetary spacecraft navigation.

According to Beaudoin, an improved and refined ITRF would help scientists measure more precisely the effects of human activity, climate change and rising sea levels. It will also lead to enhanced weather forecasting and better prevention and mitigation of the impacts of natural disasters such as floods, earthquakes, landslides, tsunamis and volcanic eruptions, which are constantly reshaping Earth’s surface.

An Incredibly Complex System

“The Earth is an incredibly complex system,” notes Beaudoin. “It is far from a simple spherical model and, much like ocean tides, even the ground that we stand on undulates locally at the level of a meter on daily scales.” Scientists need to measure how the ground is moving and the rate and direction in which it is moving to study its evolution and make accurate forecasts.

Beaudoin says the ITRF is actually a combination of reference frames derived by four independent space-based surveying techniques: Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) and Global Navigation Satellite System (GNSS), which is a network that includes GPS and other navigation satellites. Instrumental errors, however, have resulted in mapping inconsistencies between the reference frames. 

“Our goal is to reconcile the differences in data used to construct these reference frames so we can achieve an enhanced level of mapping consistency that can be measured in millimeters, instead of centimeters, which is the case right now,” he explains. 

To achieve this, Beaudoin and his co-investigators will use GRITSS to continually measure these instrumental errors between the GNSS and VLBI reference frames and eventually, the SLR as well. (There are no plans for GRITSS to support DORIS at the moment.)

“GRITSS is unique because our instrument will receive GPS transmissions onboard a planned fleet of Cubesats or other small satellites in low Earth orbit, which will in turn relay the GPS signals to the ground at a frequency that can be received by the VLBI radio telescope array,” says Beaudoin. “In this way, we anticipate that we should be able to reconcile differences in the GNSS and VLBI measurement standards to the level of one millimeter.”