Is your IFR GPS still legal for use?

Roy Smith wrote:

But it's legal to fly those very same approaches with a 30 year old ADF
which points vaguely in the direction of either 1) the radio beacon, 2) the
nearest T-storm, or 3) some other random propagation anomaly, and an
equally ancient DME which is doing good if it's correct to within 1/4 mile.
Gotta love the FAA.

I always wondered the same thing.

You'd think a 196 on the yoke would outperform the ADF is some situations.