Home
Presence detection without GPS
Why phone GPS is the wrong tool for knowing who is home, and how the network you already run answers the question better.
Almost every home automation decision rests on one question: is anyone home, and who. Get it right and the house feels like it is paying attention. Get it wrong and it locks you out, arms the alarm on you, or cheerfully announces you have arrived while you are still at work. The default tool for answering it is your phone's GPS and a geofence drawn around the house. It is the obvious choice, and it is the wrong one.
Why GPS is a poor fit for one fixed location
GPS is built to tell you where you are anywhere on earth. You are asking a much smaller question, a yes or no about a single point, and the general-purpose tool answers it badly:
- It is late. Geofences fire on a delay, or not until the phone next feels like reporting. You are inside for five minutes before the house notices, which is exactly when you wanted the lights and the disarm.
- The phone fights you. Background location is the first thing an operating system throttles to save battery. The tracker goes stale precisely when you rely on it.
- It is a privacy tax. To answer a question about one address, your home platform, and whatever cloud it leans on, tracks your exact location everywhere, all day.
- Its failure mode is the worst one. Some setups stop getting updates off the home network and keep reporting you home while you are away. For anything touching an alarm, "confidently wrong, in the unsafe direction" is the failure you least want.
Ask the network, not the phone
You do not actually need coordinates. You need to know whether a known device is on your network and, ideally, roughly where. Your Wi-Fi already holds that answer. Instead of a satellite fix, track which access point each device is associated with. A phone associated to the bedroom access point is home, and roughly in the bedroom. Associated to nothing for a few minutes is away.
That one change fixes every complaint above at once. Association and disassociation are close to real time, so no geofence lag. It leans on the radio the phone keeps alive anyway rather than the background GPS it kills. The signal never leaves your network, so there is no location history and no cloud to trust. And as a bonus the general-purpose tool could never offer, per-access-point association gives you approximate in-home zones for free, which room, something GPS cannot do indoors at all.
You were reaching for a satellite network to answer a question your own router already knew the answer to.
The honest caveats
It is not magic. Phones roam between access points and sometimes drop Wi-Fi briefly to save power, so you debounce it: require a device to be gone for a few minutes before you call it away, and a short blip never unlocks the house. You track known devices, not guests. And it genuinely only answers "on my Wi-Fi," not "where on earth," which for running a house is the question you actually have.
The pattern generalises past presence. Most home automation gets simpler the moment you stop reaching for the powerful, general answer and use the small, local one already sitting on your network. The router knows who is home. Let it tell you.