The Light-Based Speed Adjustment setting will make it so the aircraft will potentially be slowed down to the fastest speed the aircraft can travel while maintaining a Motion Blur of less than 2 times the GSD.
Yes. This sounds complicated but is set by default to make it so you are not taking bad data. These calculations are almost impossible to do by hand ahead of the flight so we do it for you as long as this setting it checked.
How it is Calculated
From the time the aircraft reaches the point directly over the takeoff location to the time it gets 70% of the way to the beginning of the programmed flight path (green dot) it will monitor the camera's exposure time and calculate an average value. The Motion Blur for this average value is calculated and is then compared to the GSD/Altitude. If this calculated Motion Blur value is greater than 2 times the GSD we deem that to be too much ground smear and will slow the aircraft down to make it less than 2X the GSD.
Limited: If the camera is seeing an average exposure value of 1/100, is flying at 60 meters and is supposed to be travelling at 10 m/s the travel will be limited. 10 m/s * 1/100s gives a Motion Blur (ground smear) of .1 meters (10 cm). At a 60 meter altitude using the Phantom 4, the GSD is 2.6cm/px (as shown on the Altitude selection slider). This shows that the currently defined speed is too fast for the current exposure and the aircraft will be slowed down accordingly.
The new light conditions based flight speed will be (2 * 2.6cm/px) / (1/100s) to give a new speed of 5.2 m/s.
Not Limited: For the exact same setup with an average exposure time of 1/1000s, the motion blur would be 1 cm which is within the 2X GSD limit of 5.2 cm and the speed would continue on at 10 m/s.
The crossover point usually occurs around 1/320s for most conditions.
This value only gets calculated once and will hold true for the entire mission (even across subsequent batteries). Our goal is to make sure that the images are both usable and all taken while travelling the same speed. If you are concerned that this will adversely affect your ability to collect your data properly please disable the feature.
Depending on the camera mode settings, it is possible to manually adjust the exposure time using the top right click wheel on the remote. If you know that the area being flown over while this calculation is occurring is not representative of the rest of the survey area (or just plain dark) you can knock the exposure down a bit to accommodate it.