Why Does Litchi...?

When a POI is associated with a waypoint, Litchi sets the appropriate heading at that waypoint to point at the POI. In a mission where one POI is associated with two adjacent waypoints, the heading at each waypoint is set to point at the POI. This is easily seen in Litchi's Mission Hub. As the drone flies from the first waypoint to the next, the heading is linearly interpolated from the value defined at the first waypoint to the value defined at the next waypoint. This works well for cases where the POI is a similar distance from both waypoints. However, in certain cases, this method yields less than perfect results.

In extreme cases, where two waypoints are associated with the same POI and one waypoint is much closer to that POI than the other, the interpolated heading will cause the view of the drone to wander off of the POI as the drone travels from one waypoint to the next. In the photo to the right, the drone will correctly point at the POI at waypoints 1 and 2. However, midway between the two waypoints, the interpolated heading will not point at the POI. The solution is doing either of the following:

• Position waypoints pointing to the same POI roughly equal distances from that POI.
• Insert additional waypoints between waypoints 1 and 2 to help keep the focus on the POI

There is an exception to what is described above. "Hotpoint Mode" is used if all of the following conditions are met:

• The drone has native waypoints supported in the firmware (Mavic 2 Pro/Zoom and older).
• The mission has only one POI.
• All waypoints in the mission are associated with that one POI.

If the above three conditions are true, the DJI firmware employs something called "Hotpoint Mode" where the firmware does all of the heading calculations to keep the drone pointed at the POI. References:

• VLM mod for non-hotpoint mode
• Hotpoint mode
• Not pointing at POI

When a POI is associated with a waypoint and the Gimbal Pitch is set to "Focus POI", Litchi sets the appropriate gimbal pitch angles at that waypoint to point at the POI. In a mission where one POI is associated with two adjacent waypoints, the gimbal pitch angle at each waypoint is set to point at the POI. This is easily seen in Litchi's Mission Hub. As the drone flies from the first waypoint to the next, the pitch angle is linearly interpolated from the value defined at the first waypoint to the value defined at the next waypoint. This works well for most cases. However, in extreme cases where the path between waypoints goes over the POI being tracked, the POI will not be tracked properly.

In this case, where two waypoints are associated with the same POI and the path between these waypoints goes over the POI, the interpolated gimbal pitch will not point down while over the POI between the two waypoints. In the photo to the right, the drone will correctly point at the POI at waypoints 1 and 2 where, in this example the gimbal pitch angle at each waypoint is -36 degrees. However, midway between the two waypoints, the interpolated gimbal pitch angle will still be -36 degrees and not point down at the POI. The solution is to do the following:

• Insert an additional waypoint between waypoints 1 and 2 near the POI to keep the focus on the POI.

There is an exception to what is described above. "Hotpoint Mode" is used if all of the following conditions are met:

• The drone has native waypoints supported in the firmware (Mavic 2 Pro/Zoom and older).
• The mission has only one POI.
• All waypoints in the mission are associated with that one POI.

If the above three conditions are true, the DJI firmware employs something called "Hotpoint Mode" where the firmware does all of the gimbal pitch calculations to keep the drone pointed at the POI. References:

• VLM mod for non-hotpoint mode
• Hotpoint mode
• Not pointing at POI

There are three choices for "Path Mode" in Litchi's waypoint settings:

• Straight Lines
• Curved Turns
• Smooth Curves (Litchi Pilot or in Mission Hub v2)

In the original Litchi app, when "Path Mode" is set to "Straight Lines", the mission can be divided up into multiple straight-line segments from one waypoint to the next. As the drone executes a straight-line mission, it pauses momentarily at the end of each line segment. When using straight line segments, it is trivial to trigger the execution of "waypoint actions" at the beginning or end of each segment.

In the original Litchi app, when "Path Mode" is set to "Curved Turns", the flight path near each waypoint is modeled with a cubic curve. The flight path will not generally pass through the interior waypoints. As the drone executes a curved-turn mission, it does not pause at each waypoint.

In the original Litchi app, the use of curved turns did not provide an easy way to define a point on the path where an action could be executed. While it would have been possible to determine a "midpoint" on the curve, the original Litchi app did not do this. Therefore, actions were not performed on interior waypoints of a curved-turn mission. Even with a curve size of zero, a drone flown using the original Litchi app would not performa actions at any interior waypoint.

With the new Litchi Pilot app, this has all changed. When using the new Litchi Pilot app, the drone will perform any action at any waypoint. Also, the drone will not pause at the waypoints unless there is more than one action or there is a "Hover" action defined at that waypoint.

Waypoint missions are created by specifying the GPS location of two or more waypoints. While a GPS location can be specified with great precision, the accuracy of flying to that GPS location is something less than perfect. Simply stating the accuracy for GPS can be difficult because there are many different types of GPS and many things that affect its accuracy. This is probably why one finds so many different GPS accuracy values quoted on-line.

Smartphones tend to use assisted GPS (A-GPS) which combines traditional GPS signals with cellular network data to increase the location accuracy. Therefore, smartphone position accuracy is generally better than that of devices using standard consumer GPS.

Like many consumer devices, DJI drones use what is called "Consumer GPS". Depending on your source, the accuracy of consumer GPS implementations is anywhere between ± 3 to 8 meters (10 - 26 feet). However, DJI states the horizontal positioning accuracy of their older drones to be around ± 1.5 m (Mavic 2, Air 2s) and their newer drones to be ± 0.5 m (Mini 3, Mini 4, Mavic 3). I have flown literally 100s of waypoint missions and DJI's stated accuracy corresponds well to what I have experienced when flying and repeating my waypoint missions. I have never experienced a waypoint mission to be anywhere near 8 meters (26 feet) off of the drone's intended course.

Web sites with good explanations of GPS

• How Accurate is GPS?
• An in-depth exploration into how accurate is GPS