ORMs vs. Object Models
RDS are used to establish a relationship between measurements in the world and (ultimately) the parameters of a CS, mediated by an ORM.
An ORM is sufficient to establish the basis for a CS in relation to the celestial body.
An ORM may be useful for other purposes; in particular, it may be used as a model of the celestial object itself.
- E.g., an OBS RD as a model of the shape of the Earth.
Models of celestial objects (e.g., their surface) are expressed in terms of SRFs (and therefore ORM and RDS).