Greetings Marius,
RNP is a required performance that your aircraft has to be able to match, it is lets say a tube, with a diameter, in our case the diameter is equal to 0.3, so if the sensors plus the other errors of your aircraft display 0.3 or less, then the aircraft, that we assimilate to a ball, can enter the tube and go through it.
sensor budget is call ANP (Actual Navigation Performance) in Seattle, and EPU Estimated Position Incertainty in Toulouse, in fact say are both the same.
Now if due to a system degradation, loss of GPS, bad GPS geometry inducing a high DOP, then the ball diameter will grow, and when it becomes greater than the RNP it will not be able to go through the tube anymore.
SO the RNP doesnot change, only your ANP/EPU does.
THe RNP has different values depending on where you are, for exemple it is RNP 1 until the FAF, then RNP 0.3 from your FAF to your MAP, then it is RNP 1 from the MAP for the Go Around segment, so as long as you have an ANP/EPU smaller than the RNP you can continue to fly the procedure, if the ANP/EPU becomes bigger than the RNP, you Go Around or, you vacate the RNP airspace.
Systems dont lie, but human do
ANP/EPU are mesured values using very sophisticated algorythms, the most popular on is referenced as a Kalman filter, Google it, and read.
The most probable case where the EPU could be, lets say optimistic, is during reception of a GPS solution that has a Bad Geometry, inducing a DOP, Dilution Of Precision, in that case the FMS/FMC will use the DOP to correct the precision, just by multiplying the precision by the DOP.
