Does using Personal Hotspot affect GPS tracking on iPhone?

The short answer

No. Personal Hotspot and GPS positioning run on separate hardware doing unrelated jobs — one relays a cellular data connection to another device, the other listens for signals from satellites overhead — and turning one on has no path to interfere with the other. A route recording in the background keeps logging position at the same rate and with the same accuracy whether Personal Hotspot is off, sitting idle, or actively relaying data to a connected device.

Why the two systems do not overlap

GPS positioning on an iPhone is receive-only: the GPS receiver listens to signals broadcast by satellites and works out a position from them, without sending anything back or needing any other radio's help to do it. Personal Hotspot is a completely different job — it takes the cellular data connection the phone already has and shares it with another device, over Wi-Fi by default, or over Bluetooth or a USB cable if chosen instead. One is about finding out where the phone is; the other is about handing a connection to somebody else. Neither one asks the other for anything, so there is no shared resource for one to hog and starve the other of.

What actually changes: battery, not accuracy

The real cost of Personal Hotspot is power, and it stacks on top of whatever GPS recording is already using rather than competing with it. Sharing over Wi-Fi keeps the phone's Wi-Fi radio broadcasting continuously so another device can find and stay connected to it, and the cellular radio stays busy relaying data back and forth the whole time it is in use. Add that to a phone already running GPS in the background for hours, and the combined draw is noticeably heavier than GPS recording on its own — the same kind of stacking effect covered in more detail in the guide to GPS battery drain on iPhone, just with a second radio added to the pile instead of, say, a bright screen.

The sharing method changes how much that costs. Wi-Fi is the heaviest of the three because the radio has to stay broadcasting and listening continuously. Bluetooth uses noticeably less power for the same relayed connection, at the cost of a slower link. A wired connection over a USB cable is lightest of all on battery — and since the cable is plugged in, it is usually charging the phone at the same time rather than only draining it.

It costs something even with nothing connected

Turning Personal Hotspot on and then not connecting anything to it is not free either. With the toggle on, the phone keeps its Wi-Fi radio broadcasting and ready to accept a connection whether or not a device ever joins, which draws power the same way leaving the toggle on out of habit after a trip does. And a hotspot has nothing to share in the first place if the phone itself has no cellular signal — deep in the backcountry with no service, switching it on relays an empty connection to whatever tries to join, which is a separate problem from GPS tracking, since positioning never needed that signal to begin with.

Things that trip people up

• Assuming a hotspot pulls bandwidth away from the app recording the route — a GPS fix is not data moving over Wi-Fi or cellular, so there is nothing for a hotspot to compete with.
• Turning off Personal Hotspot mid-hike to try to fix a track that looks off — it will not, because the setting was never touching the recording in the first place.
• Leaving Personal Hotspot switched on for the rest of a hike after only needing it for a few minutes at the trailhead — the radios keep drawing power the whole time it stays on, connected device or not.
• Expecting Personal Hotspot to work with nothing to share when there is no cellular signal at all — it relays a cellular connection that has to exist first; GPS tracking, unlike this, never depended on that signal in the first place.

Where TrailCam fits in

TrailCam records a route from GPS in the background and does not need a data connection to do it, so a hike keeps tracking exactly the same whether Personal Hotspot is off, idle, or actively sharing a connection with another device. TrailCam is free to download.