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The International Height Reference System (IHRS) adopted by the IAG in 2015contains two novelties. First, the mean-tide system is adopted for handling thepermanent tide. Second, the reference level of height system is defined by theequipotential surface where the geopotential has a conventional valueW0=62636853.4 m2s-2. This value was first determined empirically to provide agood approximation to the global mean sea level and then adopted as a referencevalue by convention. I analyse the tidal aspects of the reference level basedon W0. The W0 is by definition independent of the tidal system adopted for theequipotential surface, but for different tidal systems, different functions areinvolved in the W of the equation W=W0. I find that the empirical determinationof the adopted estimate W0 is inconsistent from the viewpoint of tidal systems.However, the adopted estimate and the consistent estimate from the same dataround off to the same value. I propose a simplified definition of IHRFgeopotential numbers that would make it possible to transform between IHRF andzero-tide geopotential numbers using a simple datum-difference surface. IHRFshould adopt a conventional (best) estimate of the permanent tidal potentialsuch as is contained in the IERS Conventions and use it as a basis for otherconventional formulas. The tide-free coordinates of the ITRF, and tide-freeGlobal Geopotential Models are central in the modelling of geopotential for thepurposes of the IHRF. I present a set of correction formulas that can be usedto move to the zero-tide model before, in the middle, or after the processing,and finally to the mean-tide IHRF. To reduce the confusion around the multitudeof tidal systems I propose that modelling should primarily done in thezero-tide system, with the mean-tide potential as an add-on.

The permanent tide and the International Height Reference Frame IHRF