Use of RNAV started in the 1970s. Early equipment consisted of (long range) mechanical triple Inertial Reference Systems (then called Inertial Sensor, or Inertial Navigation System dependant on the manufacturer) combined with a position update system based on VOR and DME.
The IRS is the reference position system, which is self contained without the need for ground reference, but unfortunately system accuracy decays in time.
Because the RNAV system uses VORs, DMEs and/or GPS for position updating, the IRS accuracy decay has no influence on the RNAV position accuracy, unless the aircraft flies out of range of the ground based beacons or if position updates are not acceptable to the system (e.g. due to signal strength or position relative to the aircraft).
As in all current RNAV systems, a (simple) database was part of the system. Apart from the beacons for position updating, the database contained routes to be flown. In separate sections en-route and terminal area information and even approaches could be stored.
The ‘translation’ of the existing conventional routes into (RNAV) computer code was based on simple, point to point navigation. Those points were marked as ‘waypoints’ (a geographic point in space, initially in two dimensions) and the route could only be flown laterally from one waypoint to another in straight lines. Compared to current possibilities, coding was limited and simple. Complex procedures and complex prescribed turns could not be coded.
There was no vertical navigation capability, as the system was not fed with engine data, so performance was ‘unknown’. Vertical navigation was only presented via path deviations on the (mechanical) HSI.
As Electronic Flight Information Systems were not available at the time and all data were presented on the CDU only, separate charts were needed to monitor the flight and correct coding.
In later generation aircraft, Flight Management Systems were introduced. FMSs ‘manage’ the flight in more than just the navigational field and a manageable third dimension was added to the waypoint.
Engine and airframe data became part of the system and coding of conventionally (i.e. not specifically for RNAV) designed routes was made more flexible but also became more complex.