Long Duration Surveys

Data Quality

Given that higher altitude likely means faster flight speeds, one consideration to take into account is that increasing flight speeds do have a negative correlation with hyperspectral alignment between flight lines. In general, but especially on smaller aircraft where more pitch is needed due to less thrust per motor, 8m/s is the fastest flight speed before having noticeable misalignment between transects. Depending on study size, maintaining this flight speed limitation may require further subdividing flight plans.

Operators should also plan for close to 50% side overlap on VNIR. Rotorcraft, especially smaller models, tend to be less stable at high speeds/altitudes than larger aircraft. The aggressive roll angle from this instability can cause data gaps without considerable sidelap.

As a rule of thumb, 25-30 minutes is generally the highest acceptable time for only having one transect above reflectance panels for hyperspectral collection. If you have multiple people in the field for the operation, you may consider having the additional person(s) move the reflectance panels from the first transect to the last transect after the aircraft has traversed over them. Thus allowing for multiple target selections in post-processing to get the most accurate reflectance data.

With the "overshoot" distance beyond the study area, our recommendation is at least twice the number of meters as you fly per second (8m/s = 16m + overshoot). The correlation probably shouldn't be perfectly linear as has been described. The faster you fly, the higher that multiplier should probably be. 2.5-3x would be safer at high speeds and shouldn't add a considerable amount of flight time. In our experience, most aircraft overshoot their acceleration for high speeds to get up to nominal velocity quickly, and end up over the nominal velocity before coming back down to nominal. Therefore, more overshoot will minimize missed frames in hyperspectral. We also recommend running as much oversampling as you can (while maintaining good saturation and not exceeding the sensor's framerate limits) to help with the pitch/speed instability smaller aircraft have at higher speeds.

Flight Dynamics

Take into account the extra time/battery required to get up to and down from high altitudes and update the flight plan length accordingly.

Cloud cover can become a bigger issue when flying long duration surveys as longer windows of clear skies are needed. Monitor forecasts appropriately, and ensure the survey will not be interrupted by clouds. Long flight times mean less extra time in the solar noon window to re-do flight lines or full flights, so make the surveys count.

Storage

Given the massive amount of data collected in long surveys, be sure to head to the field with all remotely sensed data on the system backed up and cleared. If performing multiple surveys, double check your available storage after the first flight to make sure you have space for the second. You may need to plan on having local storage (laptop, external drive, etc.) available to offload data between flights.