I had launched from a narrow canyon where I had no phone reception. Luckily, I was able to force it to land in the canyon about a mile from where I launched, and I eventually tracked it down.
Does anyone know why this happened and how I can be sure to avoid this in the future?
Without data, we can only guess.
It would be good if you can upload your flight data so we can see if there is a real issue or work out what happened.
To do that, go to
DJI Flight Log Viewer
Follow the instructions to upload your flight record.
Come back and post a link to the report it provides.
Now for some guessing .... having no phone reception has no effect on flying ... but the fact that you were in a narrow canyon was probably a key point in the incident.
Your Phantom can only receive GPS sats when it has a clear view of most of the sky.
In most flying, this isn't a problem but if you launched from a narrow canyon, your Phantom wouldn't have been able to get a good sat fix and record the home point.
You would have had to launch in atti mode.
What you should do in that situation, is to go straight up out of the canyon and get GPS reception and let the Phantom record it's home point where you would want the Phantom to descend if it was to RTH.
It sounds like you may have flown along the canyon and the Phantom didn't get GPS until quite a distance away.
It will record it's home point when it first gets GPS reception.
Because you were in the canyon without GPS, you couldn't have reset home to your location so it's important to get a good home point up above before flying away.
