Under a bridge, that won’t end well 
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@Pirbright_Drone How many satelites did you have left once you flew under there? 
Imagine you are a GPS satellite in orbit with your GPS satellite mates. How many of you have direct line of sight to a drone under a bridge at any given time? One ir two might get a fleeting glimpse if they are aligned with the bridge, but it/they are going to be low to the horizon where it is most likely that LOS will be blocked by geography, trees, buildings, &c.
And this is a canal, where the drone is probably flying very close to or below ground level. No chance.
Rubbish
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Seriously? The drone is flying just above the water level in what is effectively a tunnel, a canal running beneath a wide main road, at the beginning, though it does come up a bit later. GPS works by trianglulation between the largest number possible of visible satellites, so that the postition of the drone and it’s height above the ground can be calculated by calculating in real time the differences between the exact distances and angles to each satellites, meaning those that can be ‘seen’ from the drone. The more satellites the drone can ‘see’, or the more that can ‘see’ the drone, whichever way you want to view it. The signal is LOS, and anything that interrupts this means that the signal is lost. As I said, in a situation like this, it would be very lucky if even one satellite was ‘visible’.
I repeat, no chance! The drone will fly, but not steadily or accurately, which is exactly what is shown in the video, despite the efforts of the gimbal’s attempts to stabliise the camera!
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This was a test I did in my garage under a steel up-and-over garage door, as you can see from the second picture the number of sats in the field of view and a good amount of locked ones give an accurate position hold but like any GPS system that can change at any time depending on a lot of variables but being below ground level shouldn’t be one
Flying under bridges can and does cause problems but not always…you take the risk!
I didn’t look down at the screen once it started going awry. It went from normal, to a GNSS warning and drifting all over the place.
Yeh that will do it. Glad you got it back at least. Thats one way to learn about flying under stuff that may block signals 
If you ever looked into air data you can connect it to your DJI account and play the flight back and see what your joysticks were doing, how many satelites you had and a load more stuff. Its good for working things out.
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Why not post a link to AirData?
Others can see the logs and give far more advice without having to guess and hypothesise on possible key factors?
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I did look at Airdata but not entirely sure how it all ties up and what can be done. Will have another look at some point. So you sign up to airdata then link the DJI account somehow to retrieve your data?
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Then each flight will have an entry in Airdata, which can be shared, and others can examine the flight logs.
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