Originally Posted by Dan B
My airplane is equipped with dual GNC-420Ws (no ILS or VOR). For the last 6 1/2 years, I most often file IFR. 99% of the runways I land on have an RNAV approach, but no ILS. I have encountered GPS interference testing from White Sands at higher altitudes. First thing ATC did when I reported the interference was put me on a vector. RADAR is always a viable backup to loss of a nav system. I have yet to encounter a need for ILS or VOR capability in my airplane.
During my time as an AF initial flight training instructor (only a couple years ago), we flew almost exclusively into civilian fields for training. Virtually all had both RNAV & VOR/ILS/TACAN approaches.
Dan is certainly correct that radar is a viable backup, but the question you have to ask yourself is how much do you want to put your fate in the hands of someone not in the plane with you?
Is the space weather good for that day? What about some other interference? I can't count how many days students planned to do approaches somewhere the day before only to find that a NAVAID was out the next day or RAIM wasn't available for an RNAV approach minutes before we arrived. Granted the RAIM was a tiny fraction of those events, it speaks to an idea. As mentioned by others above, IFR is about redundancy and the worst time to have none is when you're actually facing IMC and there just happens to be interference in the vicinity of the airport at which you want to land or a system is degraded for some reason. How many airports have PAR approaches these days? If they don't and you're in IMC with a GPS problem, you're in a pickle.
Reference the link below for GPS "interference" instances.
PLEASE NOTE THIS IS AN ILLEGAL PRODUCT IN THE US, but this thing is less than $400 bucks and there are others out there for less than that. They also advertise to a US market. You can make one for less than $100 according to mainstream news outlets here in the US.
So, my $.02 is what I liked to tell my students when asked about why we still use NAVAIDs instead of just GPS, and what I've always heard. "Always have an out." That applies to VFR, IFR/IMC, driving down the highway, heck...walking around somewhere you're not familiar with. If you can afford a VOR/ILS Nav/Comm, it's cheap(??) insurance in my book if you end up in actual IMC.