I was flying in a park today when I noticed my Spark wasn't responding to my commands. I looked down at the screen and noticed it said something like "Lost Transmission" or something like that. It was about to RTH and then I got control back and landed it. I looked at the flight record in the DJI Go 4 app for some kind of info to help me figure out what happened, but it didn't show anything. Is this something I need to use the Assistant app to help troubleshoot?

Here are the conditions:
1. No buildings or metal around - in a good size open park.
2. Drone was about 50 feet away and maybe 15 feet high.
3. This happened about 30% into my 3rd battery of the day.
4. Signal was recovered after about 10-15 seconds.
5. Antennas were pointing correctly according the the manual.

A similar thing happened in another park but I just figured it was because I was near a metal park bench and my antennas were pointi straight at the Spark...

Anyone else experience this? Will DJI Assistant shed some clues to help me?

Constantly experience this. There are so many scenarios that can affect a good transmission from RC to Spark. Turning off bluetooth and putting your phone to airplane mode can help, also changing frequencies. A OTG cable can also help. The spark has a failsafe inbuilt feature that if you lose transmission it 'should' return to home if transmission has been lost for a certain amount of time.

Another thing I noticed when I played back the flight log, the blue dot that shows where I'm standing with the controller was moving around to areas where I did not move to, and then it disappeared for 15 seconds. It then reappeared right next to the home point.

There was also a blue area around the blue dot and the diameter of this blue area changed a couple times. What does that mean?

Looking at the log file from the DJI GO 4 app I noticed that the IMU and VPS altitudes were typically within 2-3 feet of each other, but then before the disconnect I saw the VPS altitude go to 0 for several seconds, come back for a couple seconds, then back to 0 for a few more seconds, and then it says Downlink Restored after 4.7s.

Downlink restored at 4M 56s in the log:
https://www.dropbox.com/s/fywx1q ... 2-27-53%5D.txt?dl=0

Just to verify, do you mean the flight record doesn't show the message when it might be "lost transmission" or there is no record about the issue? If the app doesn't show the record of the issue, it could be the drone's disconnected or app disconnected, these two reasons might interrupt the record. According to your description at the very beginning, if the drone did not respond to your control, then it could be the aircraft disconnected.
For the blue dot, it uses the GPS module of your mobile device, if the accuracy of the device is deficient, it might jump, and the blue area is also the accuracy allowance of the GPS of the mobile device.
For your situation, I would suggest flying the drone in another place with fine GPS signal, shut down your Bluetooth device, make sure there is no other WiFi interference around.

In your message you said:  "my antennas were pointi straight at the Spark"

Can you please clarify this?  Do you mean you literally had them pointed at the Spark like you would point with your finger so that they are parallel to an imaginary line between yourself and the Spark?  Or were they configured so that they are perpendicular to that imaginary line?

For the best signal, you should have your antenna perpendicular to that imaginary line.

This video from DJI does a good job:  

I lost my Drone and DJI is only giving me %15 off. Becarful!

DJI Thor Posted at 2017-10-11 00:10
Just to verify, do you mean the flight record doesn't show the message when it might be "lost transmission" or there is no record about the issue? If the app doesn't show the record of the issue, it could be the drone's disconnected or app disconnected, these two reasons might interrupt the record. According to your description at the very beginning, if the drone did not respond to your control, then it could be the aircraft disconnected.
For the blue dot, it uses the GPS module of your mobile device, if the accuracy of the device is deficient, it might jump, and the blue area is also the accuracy allowance of the GPS of the mobile device.
For your situation, I would suggest flying the drone in another place with fine GPS signal, shut down your Bluetooth device, make sure there is no other WiFi interference around.

DJI Thor, the flight record I was first talking about was in the DJI GO 4 app when you click 'Me -> Flight Records' and then swipe up from the bottom to see your flight list. When I hit the play button and watched it I didn't see any indication that anything went wrong.

Since then I've learned how to view the flight record on that Phanton website and saw the error.

I will try going to different parks in my area and testing it out.

Thanks

Kloo Gee Posted at 2017-10-11 07:05
In your message you said:  "my antennas were pointi straight at the Spark"

Can you please clarify this?  Do you mean you literally had them pointed at the Spark like you would point with your finger so that they are parallel to an imaginary line between yourself and the Spark?  Or were they configured so that they are perpendicular to that imaginary line?

Kloo Gee,  Yeah, when I had my first disconnect several weeks ago I had the antennas pointing at it the wrong way. Since then I've watched the video you linked and learned how to do it correctly. My last disconnect had the antennas perpendicular to the craft.

I came from the days when the TT Raptor30 was king of the RC Heli world and there were no 2.4GHz controllers and your UAV never went very far from you so it didn't matter where you pointed that big long antenna.