Is it legal to fly IFR with only a non-WAAS GPS?

A non-WAAS GPS is not sufficient as the sole means of navigation for IFR operations because it does not provide the required precision for approach procedures. The FAA mandates that for IFR flight plans, a pilot must have a reliable alternate navigation means such as VOR, NDB, or other approved systems to ensure that if there is a failure in the primary navigation system, the pilot can still safely navigate and execute the flight plan. This requirement is crucial in maintaining safety in instrument flight conditions where precision is necessary, particularly during approaches and departures. Therefore, having an alternate means of navigation is essential to comply with IFR regulations, making it illegal to rely solely on a non-WAAS GPS system when flying under IFR.