Expanding with heat and contracting with cold are both natural phenomena. Surface of the Earth is no exception. In fact, variations of surface temperature is an important contributor to surface deformation. To assure the precise positions of GPS stations is the key to provide various verified GPS observation parameters to users accurately. Recent attention to surface temperature variations has focused on their contributions to seasonal displacements of GPS stations.

Dr. XU Xueqing's team at Shanghai Astronomical Observatory of Chinese Academy of Sciences calculated the displacements of GPS stations due to surface temperature variations based on a global thermoelastic model. They found that the seasonal GPS observations, the transverse displacements in particular, can be better explained after considering the thermoelastic effects caused by temperature variations. This study has been published in GPS Solution. 

Previous studies often assume that despite of surface temperature variations, the transverse displacements is always zero, which is not true in reality. Utilizing a global thermoelastic model based on the surface temperature observed by National Oceanic and Atmospheric Administration, researchers found that the largest annual radial deformation is found to be about 3 mm, while the transverse displacements are generally smaller than those of the radial component, with the largest amplitude being about 1.5 mm. 

They further calculated the annual thermoelastic surface deformation at the 234 global GPS sites, which were then compared with GPS observations. The findings indicated that with the annual thermoelastic effects included, the reduction ratio for vertical and transverse components would improve about 7–9% respectively.

The study revealed that the deformation induced by land surface temperature variations is large enough to affect the stability of the terrestrial reference frame, which is also an important source for the remaining seasonal variations in GPS position, especially for the horizontal components.

“In the future, we will focus on finding other subtler factors, such as poroelastic deformation.” XU said.