I took my foldable quad to Mexico last week to get in some non VLOS flight time. The first two days were extremely windy, so I didn't even try to put any 'up'.
But on Wednesday, the conditions were perfect, so I got up early and went to a vacant soccer field for my launch point. I programmed in a path that took it far out over the ocean (4 miles) and back. I wanted to take some pictures of a rock outcropping that existed out there. I sat the quad down on the sand of the soccer field and waited for the GPS to get a lock, which took only about 20 seconds. I lifted it off manually, and when the copter was flying nicely above 'ground effect' range, I switched to AUTO. It went straight up to its normal flying altitude 100Meters = 328', and headed out over the ocean.
It hadn't gone more than a mile when I got the signal on the telemetry channel "radio signal lost". That was OK, and I expected it. This had happened dozens of times before. I almost always fly further than the range of my control radio (2.4GHz). The telemetry channel is at 915MHz, and has approximately 2.5 mile range.
But none of that matters much - once the aircraft is in AUTO mode, it doesn't need any radio communication to/from the ground at all. It is guided only by GPS, compass, gyroscope and altimeter.
So even though the telemetry signals was fading in and out, I wasn't concerned. It was a normal occurrence.
I got a signal from the craft just as it was turning around (about 4 miles away), and was surprised that I got any signal at all, but a signal is always comforting - you know it is doing what it was programmed to do.
About 15 seconds after it turned around, I got another message from it. "LANDING MODE".
But it was over the ocean! What was going on? I took my laptop (which has a telemetry antenna attached) and held it higher in an attempt to get a better signal to the aircraft. But it wasn't responding. I got one more message "70 Meters altitude". That meant it was coming down. It was actually landing in the ocean! And there was nothing I could do. 20 or 25 seconds later, I got the message "no telemetry packets received for 10 seconds". It had landed. And promptly sunk.
I examined the log files which were stored on my laptop, but not totally complete because of the bad signal quality. It appears that for some reason, the GPS malfunctioned. And when a device that is GPS guided has a GPS malfunction, it simply doesn't know where it is. The flight controller gives the user two options when there is a GPS failure 1. Try to hold your current position as much as possible or 2. LAND. Unfortunately, I had mine set to number 2. That is a good option when flying over land, but a very bad one when flying over water.
If I had used setting number 1 instead, I could have moved my antenna around in an attempt to get a signal to the quad. It would take only one short command to get it to climb, a good thing to do since higher almost always gives better reception. And once I had a reliable radio connection I could have brought it home (and down to the ground) manually.
So.. I'm already planning on a rebuild. And it will have a better and lighter airframe this time. And it will have 2 (redundant) GPSs.