Geodetic observations are not yet adequately described by observation-level models. This leads to propagation of systematic errors into time series of coordinates, scale, geocenter, earth orientation and gravity, influencing time series noise and biasing mean and rate-of-change estimates. Geophysical studies are hampered as a result, as is reference frame realisation. Mechanisms for this propagation are not always well understood. Carrier phase multipath (including the effects of antenna monuments), tropospheric mapping functions, higher order ionospheric effects, antenna thermal expansion, tidal effects, satellite antenna phase centre variations and centre-of-mass corrections give only a limited list. This session welcomes reports on advances in geodetic observation-level models for geometric (GPS, VLBI, SLR, DORIS) and gravimetric techniques alike and examination of their effect in terms of time series bias, noise and agreement with estimates from independent geodetic techniques.