I sure you are aware of the ongoing developments of the European Ground Motion Service part of
Copernicus Land Monitoring Service. In the page of the EGMS there is a document entitled Specification and implementation plan. link - https://land.copernicus.eu/user-corner/technical-library/egms-specification-and-implementation-plan
In it on page 106 there is formula for total displacement which is like this:
Could you please comment on it.
In the same document there are also some nice definitions that could be of use.