Refer to figure.
The chart depicts two RNAV overlay SIDs (DKB 9B and DKB 4H).

An RNAV overlay SID, basically, overlays a conventional SID procedure by creating RNAV waypoints at significant points so that the procedure is followed by the FMS by following the same track.

The conventional Standard Instrument Departure procedures (SIDs) use ground navigational aids, while the RNAV SIDs use Fly-by and Fly-over waypoints and are followed by the FMS.

RNAV equipment tends to be more accurate than basic fixing and can be based on either VOR/DME information, satellite systems, inertial systems or combinations of these.

GNSS/GPS receiver is an essential part of the RNAV system, as many of the waypoints will rely upon GPS coordinates.

The FMS takes navigational inputs from GPS, IRS and ground navigational aids. If GPS signal is lost, then IRS and ground navigational aids still work, as a back-up option.

Thus, the aircraft's GPS receiver must be always available for the RNAV SID procedure and the others are required in case of problem with the GNSS.