I was planning a short low level (maybe 50 foot altitude) video of a downtown building in a small city where I live.  I had not flown my Spark for about a month and had updated DJI Go 4 app on my iPad.  The flight plan was to ascent to about 50 feet, look at a building down the street, then rotate 180 degrees and discend while heading toward the building I wanted to video.

I tested out the first launch across the street from the building I wanted to video and, as usual, I launched and hovered at about 4 feet as the Spark does when it first takes off.  This launch spot was on a typical concrete (and yes steel) bridge.  My normal process is to hover and play a little with the controls the very first time I launch in a location.  GPS was fine (I think it was 11 satellites).  Every thing looked OK for about 10 seconds then I got an Aircraft Disconnected message on the iPad screen and the RC was not responsive.  After some seconds the Spark started to land from the 4 foot hover and I assume it was due to an automatic Return To Home but without being connected I had no messages.  No damage.  After it landed the RC automatically reconnected in a few seconds.

I then moved across the street closer to the building I wanted to video.  I again followed the same process and after about 15 seconds of hovering it all looked good so started the video camera and decided to go higher to about 50 feet straight up above the sidewalk where I launched.  I did a fairly slow ascent and when it hit about 50 feet I got another disconnect message.  The Spark hovered for some seconds and the RC was of course not responsive.  It then began, I assume, a Return To Home which was directly below the Spark.  It landed uneventfully but somewhere near landing it reconnected to the RC.  At some point after the landing it automatically reconnected to the RC.

At that point I tried a couple of more short very low level (about 8 foot) flights at the same location in front of the building and did a short video and everything worked OK.  I did not want to take any more risks of any higher level flights needed for what I wanted to capture so I packed up and did not try any more flights at that location.

I looked at the WiFi setting on my iPhone (not iPad) and I only saw one WiFi location listed and I know where it was located in the building where I was flying but it did not even indicate full strength signal outside.

I did have my iPhone in my pocket and the iPad (with cellular service) was controlling the Spark.  I also had on my Apple Watch Series 3 which has WiFi and Bluetooth, both enabled.

I came home and launched in my driveway, where I have flown many times, and it was totally normal.  The test in my driveway also had my iPhone and Apple Watch in the same active state as the downtown building.  When I check WiFi signals at my house there are at least 5 in varying signal strengths including my own house that usually indicates maximum bars on my iPhone from the driveway.

So how can I debug what happened?  I looked at the SD card but there was nothing in the MISC->LOG directory (should there be something here).  The second flight with the video to 50 feet, where the Spark disconnected, was on the card and looked normal.

One thought I had as I write this post is when I was downtown my iPhone, which has previously been paired with the Spark RC, was NOT connected to any WiFi signals.  When I am home my iPhone is normally connected to my home WiFi.  So maybe the RC got confused with two strong WiFi signals (the controlling iPad and my iPhone which was probably trying to find a WiFi signal it had previously seen) when I was downtown.  I did not check my iPhone at the point of getting disconnected but it was NOT running DJO Go 4 at the time.

I will return to the downtown location and try again after turning off my iPhone and Apple Watch.

Any thoughts would be appreciated.

Wifi is very directional.  Wifi band signals that interfere with your Spark are also very directional.  The channel could work for the Spark in one area and not when it flies to another area.  

Range to disconnect is related to interferrence on the channel the Spark is using.  It is not related to "I was on the channel first" or "the area looked clear".  I would check the channel page in the DJI Go app and see how busy that area is.  The phone's Apple/Android wifi on off tab will only show wifi routers that are transmitting a SSID, not all of the transmitters in the band.