iPhone (6 or 7) ever function 100% on its GPS Receiver?

Simple question. Does an iPhone (6 or 7) ever function 100% on its GPS receiver, using ONLY GPS satellites for geolocation? No other means to locate? If you shut off cell service, Wi Fi and everything else can you force it to use just GPS satellites like a dedicated handheld GPS device would work? I know it would be slow, I know you probably wouldn't want to program for this, but I just want a straight answer. I can't seem to find the answer searching on the internet. I have an old iPhone 6 with NO cellular service on it, I can shut off everything in the system settings that would help it geolocate and it still geolocates fast! Faster than I would think off of just GPS satellites and it does it in the house. So even with no cellular service plan, does a phone still use cell tower triangulation? Thanks. Gordon