Effect of unmodelled tidal displacements in GPS and GLONASS coordinate time series

This study demonstrates the different effects of unmodelled (sub-)daily tidal displacement in
Global Positioning System (GPS) and GLObalnaya NAvigatsionnaya Sputnikovaya Sistema
(GLONASS) coordinate time-series. The results show that more than two propagated periodic
signals appear in GPS and GLONASS Precise Point Positioning (PPP) coordinate time-series
in the presence of an unmodelled M2 /O1 tidal displacements as a result of a non-overlapping
24-hr data sampling. To summarize the propagated periodic signals at the fortnightly period,
an unmodelled M2 tidal displacement propagates predominately into two long-period signals
at 13.6x (x is a positive integer) and 14.76 d for GPS, while only one significant propagated
periodic signal at 14.76 d is discernible for GLONASS. Similarly, significant propagated
periodic signals at 13.6x and 14.19 d for GPS and only at 14.19 d for GLONASS are evident
as a result of an unmodelled O1 tidal displacement. However, an unmodelled M f (long-
period) signal results in a strong power of similar magnitude at 13.6x d (∼13.66 d) for both
GPS and GLONASS solutions. The appearance of different periodic signals as a result of
the same unmodelled tidal displacement is attributed to the different ground repeat periods
of the constellations. The latter is likely to explain the reason why the 13.6x-d fortnightly
signal is present only in GPS solutions. Comparing the powers of the M2 propagated periodic
signals at 13.6x and 14.76 d on average from 32 globally distributed stations, the amplitude
of the former is larger than for the latter by an order of magnitude. The results of this
study demonstrate that the 13.6x-d periodic signal in GPS/GNSS (Global Navigation Satellite
System) derived products is a joint contribution of the propagation of unmodelled (sub-)daily
tidal displacements and errors at longer periods with the former appearing to contribute more.
Significant reduction of the propagated periodic signals is achieved from combined-system
solutions where including Galileo (the European GNSS) to the combined solution already
shows benefits by reducing the effect even before the system has reached its full constellation.
Combined GNSS solutions will benefit the applications of GNSS time-series for retrieving
tidal harmonic signals such as Mf as they reduce constellation specific propagation effects.