I think SNS3g has answered his own question. You can't fly an NDB approach to SFO in real life because there isn't one. Nor
afaik to just about any other major carrier airport in the US. Round here, Stockton (KSCK) still has one, I thought it had been taken out of service but I heard one of the JAL trainees out of Napa using it the other night. I flew it many times during my IR training and can still remember it off by heart. I used to practice it occasionally until my ADF went on the blink and I decided to use its panel slot for a backup AI instead of getting it fixed. That *could* save my life, which is fairly improbable for an ADF.
As for what do I do when the GPS fails... I use the other one. If that fails too I get out the handheld. At this point I've presumably lost ship's power anyway so the ADF wouldn't help me.
But anyway the o/p wasn't about flying NDB approaches, it was about finding your position with the ADF. The point isn't whether you CAN do it (I could, given a working ADF) but whether it's of any practical use or not. It isn't.
n5296s