Position error while using cell phone in Garmin apps

[AnnOMalleys]

Here’s the equipment I’m using
iPhone 8s
Older iPad mini
iPad Air

When I use my phone as a personal hotspot to get WiFi to my ipads, the position in Bliechart and A time Captain is consistently off by several hundred yards. Never always in one direction. Sometimes easy. Sometime west , etc.
But same setup for Navionics , it’s spot on. Position error is within the margin of error for the GPS signal.

Any ideas?

[Lepke]

Recorded track tied to a dock over several days/weeks. Open sky, OpenCPN using two different receivers. Boat symbol represents 83'.