Can an iPhone track a hike with no SIM card?

The short answer

A phone's GPS receiver and its cellular radio are two separate pieces of hardware doing two unrelated jobs. The SIM identifies the phone to a carrier so calls, texts and cellular data can be billed and routed correctly — a job that has nothing to do with satellites. The GPS chip listens to signals broadcast by satellites overhead and works out a position from them on its own, with no carrier, no SIM and no cellular data involved anywhere in that process. Recording a route logs the same points, at the same rate and the same accuracy, with a SIM installed, with no SIM installed, or with a SIM present but no plan on it at all.

What a SIM card is actually for

A SIM, physical or eSIM, is a small chip that tells a specific carrier's network which account a phone belongs to, so that network knows where to route a call, a text or a byte of cellular data, and who to bill for it. That is the entire job. Pulling it out, or never activating one, switches off access to that one cellular network and nothing else. Wi-Fi keeps working if there is a network to join. Bluetooth keeps pairing headphones and a watch. The camera keeps taking photos. And the GPS receiver, which was never wired into the cellular radio to begin with, keeps doing exactly what it always did.

Why GPS was never on the other side of that switch

GPS satellites broadcast a continuous, one-way signal carrying their own position and a precise timestamp, and they do this whether or not a single phone on the ground is listening. A GPS receiver only listens — it never transmits anything back up, and it never needs to ask a cellular network for permission to tune in. Comparing the arrival time of signals from several satellites at once is enough for the chip to work out where it is, entirely inside the phone. There is no step in that process that a missing SIM could interrupt, because the SIM was never part of the process.

The one place a network helps — and why its absence just means a slower start

A phone with a working cellular connection or a joined Wi-Fi network can download current satellite orbit data over that network in a couple of seconds, rather than waiting to pick the same data up directly from the satellites, which takes longer. That shortcut only speeds up the very first fix of a session — a cold start, covered in why GPS takes a few minutes to settle at the start of a hike. With no SIM and no Wi-Fi in reach either, that first fix has to build itself from the sky alone, which can add a little to the normal settling-in time at the very start of a walk. Once a proper fix is established, tracking is exactly as accurate either way, because the position itself always comes from the satellites, network present or not.

What still genuinely needs a network, and what does not

Recording the route itself is entirely local, start to finish — the points, timestamps and elevation readings all get written straight to the phone with nothing sent out and nothing waited on. What does need a network is anything that has to reach somewhere else: syncing a finished hike to the cloud, or sharing a live location with someone who is not there, the same connectivity a phone with a SIM but no signal would also be missing, covered in tracking a hike without cell service. A SIM-less iPhone is that same situation, just permanently rather than only in patches of bad coverage. The one thing worth planning for is getting the app itself onto the phone in the first place, which needs some connection — Wi-Fi works exactly as well as cellular data for that one-time download, and nothing about it needs to happen anywhere near a trail.

Where this comes up in practice

• An old iPhone handed down or kept as a spare, with the SIM moved into a newer phone, repurposed purely as a GPS recorder and camera for the trail.
• Deliberately leaving a SIM at home, or not activating an eSIM, on a trip abroad to avoid roaming charges, while still wanting a recorded route from the walk.
• A Wi-Fi-only device that was never sold with a cellular radio at all — the same reasoning applies, since the GPS chip and the cellular modem were always independent of each other, when a phone has both.

Things that trip people up

• Assuming no SIM means no GPS — the confusion is understandable, since a SIM-less phone genuinely can't call or text, but positioning was never one of the things a SIM enabled.
• Expecting a slower first fix without a SIM to keep happening for the rest of a hike — it is a one-time startup cost at the very start of a recording, not a running penalty on accuracy.
• Not realizing Location Services, in Settings, Privacy & Security, is a completely separate toggle from having a SIM at all — an iPhone with no SIM can still have full location access granted, and that permission is what actually matters.
• Forgetting that a SIM-less iPhone still needs Wi-Fi once, before or after a hike, to install the app or grab an update — not for anything that happens while a route is actually being recorded.

Where TrailCam fits in

TrailCam records a route straight from the iPhone's GPS chip in the background, without needing a cellular connection while it runs, so a SIM-less iPhone kept purely for the trail records exactly the same track as one with an active plan. The only network a SIM-less phone genuinely needs at any point is Wi-Fi, once, to get the app installed in the first place. TrailCam is free to download; GPX 1.1 and CSV export are part of the optional TrailCam Pro upgrade.