A subway alarm that keeps working underground

Why GPS alarms fail in the metro

A location alarm works by comparing your current GPS position against the destination you picked. Underground, there is no satellite fix to compare against. Your phone does not report a wrong position — it reports no position at all, sometimes for fifteen or twenty minutes at a stretch on a deep line.

The straightforward response is to wait for the signal to return. If the tunnel ends one stop after yours, that means the alarm fires late, which for a sleeping passenger is the same as not firing. Falling back to cell-tower positioning is the other common approach, and underground it is typically accurate to within several hundred metres — wider than the gap between two city-centre stations.

How Subway Watchdog works

Nap Alarm treats sustained signal loss as information rather than as a failure. Losing GPS in the middle of a commute is itself a strong signal that you have gone underground.

1. Detection. After two continuous minutes without a GPS fix, Subway Watchdog activates on its own. There is nothing to press, which matters because you are usually asleep by then.
2. Prediction. Instead of waiting for satellites, the app estimates where you are from your saved route and the average interval between stops on it. It keeps advancing that estimate for as long as you stay underground.
3. Handoff. The moment the train resurfaces and a real fix returns, tracking snaps back to live GPS and the prediction is discarded.

The practical effect is that the alarm keeps counting your stops through the tunnel rather than pausing until the far end of it.

Setting it up before you sleep

• Search for your destination station and select it.
• Set a proximity buffer — anywhere from 200 m to 5 km. On an underground route, a larger buffer gives the prediction more room to be slightly early rather than slightly late.
• Lock the phone. Tracking runs in an Android foreground service with Doze-aware scheduling, so it survives the screen going off and the app being swiped out of recents.

The alarm plays through your headphones if they are connected, and keeps working with the screen off and the phone locked.

Honest limitations

Predictive tracking is an estimate, not a measurement, and it is worth being clear about what that means:

• It leans on your saved route. On a line you have never travelled with the app, it has less history to work from and the estimate is correspondingly rougher.
• An unusual, long hold in a tunnel — a signal failure, a held train — pushes the estimate ahead of where you actually are. A wider buffer absorbs some of this.
• Accuracy improves the more you travel a given route, because the timing history for it gets better.

For a deeper walkthrough of the underground behaviour, the FAQ covers the common questions about signal loss, battery use, and locked-screen operation.

Where it works

Subway Watchdog is not tied to a particular city or operator. It keys off GPS availability, so it applies on any underground system where the signal drops — the London Underground, the Delhi Metro, the New York City Subway, or a suburban line that spends ten minutes in a cutting. Nap Alarm is available for Android on Google Play.