Volcanic eruptions, continental drift, rising sea levels, earthquakes, floods, and other natural processes are ceaselessly reshaping Earth’s surface. To measure these changes, scientists rely on a terrestrial reference frame—a collection of defined points around the globe that function as landmarks to help monitor the position of Earth’s features.
The International Terrestrial Reference Frame (ITRF) serves as a standard for researchers around the globe, and it is updated every few years with newer, more precise details on how the reference points themselves are expected to drift. Meanwhile, scientists develop alternative terrestrial reference frames to try out and learn from new approaches.
In a new study, Abbondanza et al. present an alternative terrestrial reference frame called JTRF2014 (the “J” is for the Jet Propulsion Laboratory in Pasadena, Calif.) and compare it with ITRF2014, the latest version of ITRF. To develop JTRF2014, the research team used the same four types of space- and ground-based observations that underpin ITRF2014.
These four observation types come from four sources. In very long baseline interferometry, ground-based stations use signals from distant quasars to monitor Earth’s orientation. Satellite laser ranging uses lasers to calculate the precise distance between a ground station and a satellite. The Doppler Orbitography and Radiopositioning Integrated by Satellite system makes similar measurements using radio signals. The Global Navigation Satellite Systems use ground receivers to determine location on the basis of signals from satellites, including GPS satellites.
The mathematical approach used to combine data from these four sources is what sets JTRF2014 apart from ITRF2014. ITRF2014 catalogs the initial positions of ground stations and the velocities at which they move from those points. In contrast, JTRF2014 provides successive measurements—time series—of ground station positions, as determined by combining data from the four sources on a weekly basis.
JTRF2014 relies on a new tool developed by the researchers, known as the Kalman Filter for Terrestrial Reference Frame (KALREF) realization. KALREF incorporates a Kalman filter, an algorithm often used to produce estimates based on time series data that contain some inaccuracies. Such an approach is well suited to the sometimes messy data provided by the four sources used here.
After developing JTRF2014, the team showed that its prediction estimates closely agree with ITRF2014. They also used JTRF2014 to predict how Earth’s center of mass changes position as other features change, showing that these predictions align very closely with those made using other methods and data sources. Now, the team is working to improve the performance of their KALREF tool. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB014360, 2017)
—Sarah Stanley, Freelance Writer