GA jet instructor reported GPS jamming while on initial approach in CONUS airspace. The instructor notified ATC and returned to the departure airport.

Narrative

Flying at 11000 MSL we encountered a loss of GPS signal as a result of jamming. We advised LA Center and inquired about the jamming. Initially they checked on the status; indicated they had also lost our ADS-B; and stated there was not military testing. We had just transitioned to VFR Flight Following after an IFR flight plan; with the intention to evaluate flying the RNAV GPS approach into ZZZ.ATC then advised he had lost ADS-B from various aircraft. There was a GPS NOTAM for testing; which covered an extremely large area.Other aircraft then reported issues with GPS location. With the prevalence of GPS RNAV approaches; and the increasing reliance on them for approaches; it creates a significant navigation and safety issue - something that has happened to me on numerous occasions.We then had to determine our intentions. We decided to cancel our flight and return to Montgomery Airport MYF initially by pilotage. The Julian JLI VOR was out of service; which is a frequent occurrence; which made VOR navigation to the San Diego airport unavailable.Upon contacting SOCAL TRACON; we advised we had lost GPS and requested an ILS into MYF under visual conditions. With the resulting GPS jamming SOCAL also advised all aircraft that RNAV GPS approaches were unavailable - impacting a large number of flights.In my case the disruption to navigation lasted 30 minutes.This extremely frequent jamming of critical GPS navigation is a significant threat to aviation safety. This affects all aircraft and could result in critical altitude changes for aircraft that are required to descend to lower altitude when losing a GPS signal. Ifcan aircraft has to descend to a lower altitude it can have a significant impact on its range and could result in a diversion.Most modern aircraft rely solely on GPS not only for navigation signals in cruise; but also to determine range; fuel remaining; and other critical aspects of flight. In addition this disruption also impact instrument approach procedures. Since the FAA isdeprecating VORs; NDBs; and associated approaches; we are becoming dependent upon an unreliable navigation system.Imagine an aircraft on an RNAV approach in IMC and it loses the GPS signal. Not only could it not continue to descend on the approach - the pilot would have no method to execute a missed approach.