Flight Planning
Basic Requirements
Ensure the aircraft is physically and legally capable of what it is being asked to do.
Ensure the flight plan will not put the aircraft in hazard's way.
Satisfy necessary characteristics for optimal collection of each sensor modality, fully covering the area of interest.
Nuance of step 3: Operators must understand the requirements of the collect, and how to maximize data quality from each sensor, particularly with hyperspectral line-scan cameras.
Reference our Flight Planning Calculator with appropriate selections of sensors, lenses, frame periods, etc. Adjust flight altitude, speed, and spacing until ground sampling distance, overlap, sidelap, and frame periods meet the requirements for optimal data processing quality and your research.
Flight Time
Most aircraft manufacturers provide a graph displaying flight time with various payload weights to give operators a sense of expected flight times. But manufacturers do everything they can to increase this number as a competitive selling point. Many caveats must be taken into account for real world applications. Flight time estimates are:
Often derived at sea level altitude, improving rotor and propeller efficiency with higher air density.
Using the most power efficient stable forward flight speed possible.
Often taking batteries from full charge to dead, ignoring typical safety limitations.
Some manufacturers transparently provide curves estimating flight time to dead, to critical battery voltage, and to safe landing voltage.
Extrapolated from a small sample of payload weights.
On calm days with optimal air temperature and humidity.
Using brand new batteries with no health loss.
Flight planning software also provides rough mission duration estimates, sometimes over or underestimating true times. All of this to say, when planning flights, conservatively plan flight times to ensure safe returns home. We recommend keeping track of flight time estimates, true flight times, and landing voltages as you get comfortable with an aircraft and payload combination. Over time as you gain experience with a setup, you will learn the practical flight duration and can slowly push the limits within your own comfort level.
Flight Planning
Pictured below is a study area we want to collect data over. The agricultural field in the center of the image is the area of interest. In this example, the autonomous data collection mission will be performed at 60m AGL, 5m/s flight speed. This mission will capture LiDAR, RGB, and VNIR data.
Area of interest (AOI) is highlighted in red in the figure below. This is the absolute minimum required area that must be covered by the collected data to identify a flight as successful.
When approaching turns, the aircraft slows down and banks to make its turn. As it completes the turn, it will quickly accelerate, often exceeding the planned flight speed then backing off speed slowly. To guarantee high quality collection of the AOI, we prefer to make these deceleration, turn, and acceleration phases well outside the AOI.
A great rule of thumb is that the survey boundary should be wider than the area of interest, in the flight line direction, by ~2m per m/s of flight speed.
We must also make sure our sensor field of view fully covers the area of interest. We typically recommend increasing the survey boundary, perpendicular to the flight line direction, by several meters as well.
Hyperspectral Collection
For most hyperspectral sensors that capture data based on a GPS polygon, called the Capture Polygon, this boundary can usually be created using corner coordinates of the survey boundary, ensuring the flight lines are not too close to the perpendicular edge of the capture polygon.
If flight lines are too close to the perpendicular edge of the capture polygon, edit the capture polygon to increase distance from flight lines in that direction. At least 5m of buffer.
With hyperspectral line-scan sensors, we like to give additional time beyond the AOI for speed and heading to fully resolve after turns to the nominally planned rates. In this case, we can increase our recommendation from 2m per m/s flight speed to 4m+ per m/s flight speed.
Many flight planning software have built in features that automatically extend flight lines past the survey boundary. QGroundControl calls it "Turnaround Distance," UgCS calls it "Overshoot," DJI Pilot 2 has a similar setting called "Margin" that increases flight coverage in all four directions.
In the example flight at 5m/s, the survey boundary is set 10m beyond the edge of the AOI. Turnaround Distance/Overshoot is set to 10m, allowing flight lines to extend beyond the survey boundary.
Continued Hyperspectral Collection Details
From the takeoff location, the UAV/sensor should not enter the polygon as it navigates to the first survey waypoint.
For sensors with vertical components of the polygon, set the minimum altitude of the polygon such that any alignment and navigation to the first waypoint can be done below the polygon
For sensors without vertical polygon limits, ensure the aircraft can easily navigate to the first waypoint without laterally entering the polygon.
Reflectance panels should be placed in the capture area.
Preferably in the first or last flight line to find them easier in post-processing software.
When reflectance panels cannot be placed in the survey area due to restrictions or lack of access, users may build conjoining segments to an additional polygon area, and add waypoints the the flight plan that traverses over the panels.
Additional Notes
Terrain Following
When relief in a scene is noticeable, greater than a few meters, we highly recommend using terrain following in your flight plan. This is especially important to hyperspectral line-scanners as the spatial sampling rate is determined by distance to ground (or more accurately, to scene). If terrain elevation increases but aircraft elevation is constant, line-scanners may end up missing measurements on the ground.
Distance to Scene
Keep in mind that when calculating resolution, overlap, sidelap, and other collection characteristics, the important metric is distance to scene, not distance to ground. When surveying tall objects, such as in forestry, canopy tops would lose a considerable amount of overlap by using calculations to the ground. Factor in the height of objects in the scene when calculating sensor parameters and in flight planning.
Ex: Surveying 10m tall trees in an arboretum. If I set my flight plan to 60m AGL, I would need to set the altitude in the flight calculator to 50m to accurately calculate sensor behavior above the scene.
Terminology
Flight Time Total time UAV is capable of flight, manufacturer calculations typically performed at MSL in a hover.
Mission Time Total time UAV is capable of carrying out an autonomous flight plan. This number is less than flight time due to accounting for takeoff/landing time, GNSS alignment procedure time, and time to travel to the start point of an autonomous mission.
Area of Interest The area intended for data collection, minimum area required for capture/analysis
Survey Boundary Area extended beyond AoI to ensure full data product coverage from post-processing
Turnaround Distance/Overshoot/Margin Parameter in many flight planning software that extends flight lines beyond the survey boundary. If this parameter does not exist in your flight planning software, simply extend the survey boundary further.
Capture Polygon GPS coordinates used in a polygon file (KML/txt) to trigger Headwall hyperspectral data collection
Hyperspectral Flight Lines GPS flight line coordinates used in KML file to trigger Specim hyperspectral data collection
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