How not to automatically open the gate

I read multiple articles on the internet about automatically opening/closing your gate for your car based on the GPS in your cellphone. When you enter/leave your home zone, the cellphone triggers the opening/closing. To make sure it works properly, stick a sensor on your gate to sense that the gate was properly closed. And use a tiny script to integrate it. A task for an hour, at worst a day, right?

I thought so as well. But I was wrong. Murphy's Law says that if something can fail, it will fail. In this case, everything initially failed. If you plan to automate your gate, I hope my observations will save you some time. The content is divided into 3 sections: location sensing, gate position sensing, and integration.

Location sensing: I use an iPhone. It has GPS and cellular internet. It should work, right? And it did. However, the iPhone checks whether you have entered/left a zone only once per 30 seconds (or a minute, I do not recall exactly). That means that if you enter the home zone, the gate can start opening immediately, after 30 seconds, or anything in between. One option for how to deal with that is to start opening the gate when I am still far away. However, what if I stop by a nearby shop or a restaurant when returning home? Then the gate will stay open, potentially for hours (or until a timeout). That is not ideal. Another option is to wait in front of the gate for up to 30 seconds until it opens. That is more irritating than pushing a button on the remote control. No thank you.

I researched how to increase the zone testing frequency. However, the iPhone does not allow you to touch the hardware. And it does not allow you to run a background process indefinitely. Hence, you can't just keep triggering the GPS in a wait loop without running the app in the foreground. Hence, I decided to use dedicated hardware: iBeacons. With iBeacon, the maximal delay in location sensing was less than 1 second. Awesome! However, the reach was ~50 meters. And I wanted 170 meters. After a bit more research, I ordered devices with 4× higher reach. But that was false advertisement. It had exactly the same reach. I concluded that this is a dead end.

After rereading the internet articles, I realized that all the authors were using Android. So I tested Android. It had a maximal delay of 5 seconds in sensing the zone changes. That was... acceptable. Unfortunately, it was draining the battery so much that by evening the Android was dead. But I wasn't giving up. The Android doesn't have to keep sensing the location frequently all the time. It is fine if it does so only when it is in my car. Commonly, it is done by sensing a Bluetooth connection between the cellphone and the car. But my car does not have Bluetooth. I could have sensed the iBeacon, which I had in the car from the previous experimentation. But I decided to mitigate the battery drain issue as well and I purchased a wireless charger for the Android. When the Android is charged by that specific wireless charger, it switches to high-frequency location sensing. Once it disconnects from the wireless charger, it switches to the normal refresh frequency. And that worked. Until it didn't.

When a cellphone attempts to get its location after a long pause (e.g., in the morning), it needs to update its GPS ephemeral data. It can download it directly from the satellites, but it is (usually) faster to get from the internet (i.e., it uses "assisted GPS"). Unfortunately, assisted GPS does not work well when you are in the middle of a Wi-Fi to cellular handover (a common occurrence when I am leaving my home). While the handover for normal processes is close to seamless, for assisted GPS, it isn't. The download process has to first exhaust memory and get killed by the kernel... And only once it is restarted, it successfully downloads the ephemeral data over cellular internet and I finally get my location. I decided to mitigate the issue by switching to unlimited internet on my phone and just always use cellular internet. Another nuisance was that GPS was sometimes providing location estimates that were kilometers off, just to correct itself in the next second. Fortunately, all that was needed was to reject measurements that had higher inaccuracy, as reported by GPS itself, than ~100 meters.

Gate position sensing: For sectional garage doors, it is customary to use a tilt sensor or two magnetic sensors to sense whether the door is truly closed/opened. After experiencing all the issues above, it was clear to me that this cannot be skipped. However, I didn't want to place a battery-powered magnetic sensor on my swing gate as the battery would not survive long in the winter. So, I got a bright idea to reuse my camera, which was already pointed at the gate. I got it working in the day, at night, at dusk (tough, as neither IR nor the visual spectrum is giving a good picture at this part of the day), in haze, with partial occlusion by people, cars, snowflakes, and flying bugs. I have dealt with sun glare by 3D printing a lens hood. And when I realized that there is still going to be a brief moment in the year when the sun blinds the camera even with the lens hood, I felt like Indiana Jones in Raiders of the Lost Ark. And I planned to install a second camera aimed at the gate, just at a different angle. That way, as long as there is at most one sun in the sky, I get a clear picture of the gate. For a while, it worked. Until it didn't. Somehow, dandelion seeds managed to attach to the glass lens and screw up my image processing. I had enough. I decided to install the magnetic sensors that I should have installed right away. After studying the exploded diagram of the engine that powers my gate, I found convenient places to place wired magnetic sensors. After debouncing the signal from the mechanical relays in software, it worked reliably. Success.

Integration: At this point, it would be suspicious if everything worked right away, wouldn't it? But all I had to deal with was race condition in the processing code.

TL;DR: For automatic gate opening:
    iPhone DIDN'T WORK
    iBeacon DIDN'T WORK
    Android WORKED
    Camera DIDN'T WORK
    Magnetic sensors WORKED