GPS alarm: an alarm that watches where you are, not what time it is
A GPS alarm — also called a location alarm or destination alarm — sounds when you reach a place rather than a time. That one difference is what makes it usable on a journey whose timing you can't predict. This page covers how it decides when to fire, how to set it, and where it falls short.
What a GPS alarm actually does
A normal alarm counts down to a clock time you chose in advance. A GPS alarm ignores the clock. It reads your phone's position, compares it against a destination you picked, and sounds when the gap between the two closes to a distance you set.
That distance is the part people underestimate. You are not asking to be woken at your stop — by then you are already standing up in a hurry. You are asking to be woken far enough out to gather your things and reach the door. On Nap Alarm that gap is called the proximity buffer, and it is adjustable from 200 m to 5 km.
The two ways it can decide to fire
Both are useful, and which one suits you depends far more on your mode of transport than on personal preference.
Distance mode
The alarm fires when you cross into a radius around the destination — say, 800 m out. It is predictable and it doesn't care whether your service is running late, because distance to the destination is the same regardless of how long it took you to cover it. This is the mode that suits closely spaced stops.
Time-based (ETA) mode
The alarm fires a set number of minutes before you are projected to arrive, and that projection is recalculated as traffic and delays change. On a road journey, where the last two kilometres can take two minutes or fifteen, a fixed radius gives you an unpredictable amount of warning and a recalculated ETA gives you a consistent one.
How to set one up
- Pick the destination. Search for the station or stop, or use a saved Home/Work location for a one-tap setup on a route you take daily.
- Choose the buffer. Anywhere from 200 m to 5 km, or a number of minutes in ETA mode. More on picking a number below.
- Lock the phone and sleep. Tracking runs in an Android foreground service, so it keeps going with the screen off and survives the app being swiped out of recents. If headphones are connected, the alarm plays through them.
Choosing a buffer
There is no single right number, because the useful buffer is the one that gives you enough time — and the same distance buys very different amounts of time at different speeds.
- Intercity or suburban rail. Stations sit kilometres apart and trains move fast, so a small radius can give you well under a minute. A larger buffer is usually the right call.
- City buses and trams. Stops can be 300 m apart, so a buffer wider than the gap between stops puts the trigger point a stop or two before the one you want.
- Underground routes. Where position is being estimated rather than measured, a wider buffer absorbs the error and makes the alarm slightly early rather than slightly late.
The journey-specific pages below go into the numbers for each mode of transport.
What a GPS alarm can't do
Worth knowing before you rely on one for something that matters:
- It needs a position fix. In a tunnel there is no satellite signal to read. Nap Alarm handles this by switching to a prediction built from your saved route — covered on the subway alarm page — but a prediction is an estimate, not a measurement.
- Consumer GPS has limits. Between tall buildings, signals bounce off surfaces before reaching your phone and the reported position wanders. A buffer of a few hundred metres absorbs this; a buffer of fifty metres does not.
- Continuous tracking uses battery. Any app watching your position for a whole journey draws power. Nap Alarm paces its polling against your speed and remaining distance to keep this down, but it cannot be zero.
- Some Android skins fight background work. Several manufacturers apply battery restrictions beyond stock Android. If yours does, exempting the app from battery optimisation in system settings is the fix.
When a normal alarm is still the better tool
If your journey has a genuinely fixed duration — a route you have timed a hundred times that never varies — a clock alarm is simpler and uses no location permission at all. The case for a location alarm starts the moment the timing becomes unreliable, which on public transport is most of the time. That trade-off is worked through in GPS alarm vs a normal alarm.
Getting Nap Alarm
Nap Alarm is a GPS location alarm for Android, free to download. Setting a destination, choosing a buffer and running the alarm are all part of the free, ad-supported app; an optional Pro tier adds extras like Gentle Wake and the Anti-Theft Shield. The FAQ covers permissions, locked-screen behaviour and battery in more detail.