Seasonal Position Variations and Regional Reference Frame Realization

Many GPS sites show significant seasonal position variations (especially in the vertical), resulting from a combination of geophysical loading and systematic errors. It is common to realize a regional reference frame or align a time series with a model by transforming each epoch solution into ITRF (or a local or regional frame) using a regional set of sites, where these reference frame sites are assumed to move linearly with time. Seasonal variations in the frame sites can bias this frame realization and the seasonal variations are then aliased into the site coordinates. I estimate seasonal variations at a set of sites across northern North America and the Arctic using an iterative non-parameteric approach. Seasonal variations at most sites exceed 4-5 mm in the vertical. A test with synthetic time series shows that ignoring these variations when realizing a daily or weekly reference frame can cause apparent seasonal variations in other sites as large as 4 mm amplitude. We should work toward a linear + seasonal model as part of our reference frame definition.