Limitations of High Precision Tidal Prediction

The most demanding applications are tidal gravity predictions. We examine if the nms precision can be reached. It correspond roughly to a 4.10 of the tidal range (TR) at mid latitude. High precision tidal prediction requires either tidal factors derived from tidal observations or modelled tidal factors based on the response of the Earth to tidal forces and on the ocean tides contribution. Both methods rely on a precise knowledge of the astronomical tides. The accuracy of the astronomical tides is very large and different tidal prediction programs agree within 10TR. A reduced tidal development (1200 terms in Tamura) still insures a precision of 2.10TR. For tidal predictions based on observations the calibration is the main limiting factor and 0.1% remains a target still difficult to reach. The records length limits the separation of the different tidal groups. If the tidal factors of different tidal waves within the same group are not the same, systematic errors are introduced. For example neglecting the resonance around 1 in the K1 group, can introduce an error at the level of 3.10TR. For tidal predictions based on modelled tidal factors the choice of the model for the response of the Earth to tidal forces is critical as differences between recent models are slightly larger than 0.1%. The best models seem to fit the observations within 5.10. The evaluation of the indirect effect of the ocean tides is critical and general conclusions are only valid at distances larger than 100km from the coast, where improved grid is not compulsory for tidal loading computations. In the best cases we can reach a precision of 5.10TR. Our conclusion is that the accuracy of 0.1% is generally difficult to reach and that 5.10 is nowadays the limit of accuracy using long series of observations of regularly calibrated instruments.