GRACE's spatial aliasing error

The GRACE satellite mission provides a near-continuous sequence of approximately 30-d gravity field solutions in the form of spherical harmonics (SH). Because SH functions are global while GRACE measurements are sensitive mainly to variations along the ground-track, undersampling (alias contamination) occurs. Here we investigate how geophysical signals are likely to cause alias error in GRACE gravity fields. We use actual GRACE orbits and systematically sample several types of time-varying signals that might represent either errors in geophysical models such as tide models, or unmodelled geophysical signals. We show how error in semi-diurnal tides like S-2 can alias into long period variations in particular harmonics, particularly as a possible error source in the degree 2, order 0 term (C-20) of GRACE fields. We also show that aliasing associated with non-tidal geophysical model errors is significant at order 15 or multiples of 15, due to the GRACE ground track spacing in longitude. This can be predicted from Kaula's resonance formula and might be reduced by suppressing amplitudes of affected harmonics.