It's demonstrated fact that GPS is not 100% reliable, which would be required to meet the "on its own" requirement. See the article you're commenting under, and

It's demonstrated fact that GPS is not 100% reliable, which would be required to meet the "on its own" requirement. See the article you're commenting under, and documentation about it from the FAA [1]. [1] https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afx/afs/afs400/afs410/GNSS/GPS_GNSS_Interference_Resource_Guide.pdf https://www.faa.gov/about/office_org/headquarters_offices/av...