Operating Hyperspectral Line-Scanners
There are two philosophies to operating hyperspectral sensors, and the differences revolve around how a user decides to set the frame period/framerate, or the frequency at which data frames are collected. We have the science-first approach and the operations-first approach. Both methods have validity, and which one you choose will greatly depend on the conditions of your operation.
Sensor Parameters
The sensor's frame period is a measure of how fast each frame (row of pixels) should be captured. The equation for calculating frame period is (1 ÷ Framerate - Frame Readout Time). The unit of measure is milliseconds. Because the unit is time, a fast frame period will be a lower value than a slow frame period.
Exposure, or more appropriately Exposure Time, is a measure of how long we expose the sensor array to light for each captured frame. This value will determine how "bright" our image is by controlling how much incoming energy we allow to the sensor array. The goal for maximizing data quality should be to set exposure to the maximum saturation level possible without "clipping" any incoming spectral information.
Differences in Practice
In the science-first approach, users will set their exposure (and gain mode) based on the available solar radiation, then set their frame period as close to the exposure as possible (as long as it is slower than than the maximum possible framerate of the sensor). They then update their flight plan's speed to cooperate with their frame period.
In the operations-first approach, users will plan and fix their flight speed to meet their operational requirements. Because flight speed is fixed, frame period will also be fixed. Users then set exposure time as high as needed to fully saturate the sensor, without changing the frame period.
Science-First
Set sensor exposure to the point where desired saturation is achieved
Set framerate/frame period as close to necessary exposure as possible
Set flight speed to match frame period
Operation-First
Set flight parameters as necessary (given all sensor constraints) and leave fixed
Set framerate/frame period to match planned flight speed, leaving fixed
Set exposure until desired saturation is achieved
If desired saturation is not achievable, operator must decide between adjusting flight plan or leaving data potentially underexposed
Considerations
Science-First
When atmospheric conditions are cooperative (peak solar noon, no clouds), the necessary exposure time tends to be quite low. This enables the aircraft to fly at relatively quick speeds.
Fast speeds can result in pushing sensor framerate close to its limits.
Fast speeds also have negative impacts on aircraft attitude (pitch) and can have negative consequences on orthorectification alignment.
Requires flight plans to be adjusted in the field. At the very least, speed needs to be updated based on the necessary exposure/frame period.
Users will not know how long their flight will be, nor whether the area of interest can be surveyed in a single flight, until after setting up the hyperspectral sensor. This may require splitting flight plans in the field, building new capture polygons, etc.
This method results in minimizing the gap between exposure time and capture time. Therefore, each spatial pixel more closely represents the underlying scenery.
When flying multi-modal systems like GRYFN Gobi, other integrated sensors may not be as agile in flight parameter changes while ensuring maximum data quality.
This method works best operationally when survey areas are small and estimated flight times are not close to the aircraft's maximum flight time, and when frame cameras are not involved. This method excels from a data quality standpoint in unknown lighting conditions, and because each pixel captured will more accurately relate to the scenery spatially.
This method suffers from decreased efficiency given that flight plans must also be adjusted in the field, especially when flight times grow enough to need to split one flight into multiple. This method also poses risks on multi-modal systems where other sensors are more reliant on fixed flight parameters for best processing results.
Operation-First
When operating hyperspectral sensors, operators typically will only want to fly in good atmospheric conditions, where minimal exposure is needed and therefore frame periods do not necessarily need to be adjusted.
Flight plans remain fixed. User can be more efficient with in-field setup. User knows exactly how long a flight will be, and can properly plan for multi-flight missions, have additional capture polygons prepared, etc.
This method does not minimize gaps between exposure and capture time. Therefore, each frame is artificially "stretched" to fit the full frame period.
Ex: 4ms exposure time, 6ms frame period time. 4ms of exposure is "stretched" to fit the 6ms frame time.
When flying multi-modal systems, users can more easily and reliably plan for flight parameters that work for all sensors at once.
This method works best when users may not have any additional flight time to extend flights should speed need to change for excess frame period, when multiple sensors are involved in concurrent capture, and when operational efficiency is highly important. This method ensures multiple sensors can have good data quality simultaneously with little to no user intervention.
This method suffers in poor lighting conditions where flight speed/frame period doesn't allow for sufficient saturation of DNs. Users need to decide whether to adjust their flight plan or undersaturate their data (not often a problem as hyperspectral sensors are mostly flown in bright and consistent sky conditions). This method also suffers in micro analysis where individual pixel spectra, rather than area averages, are of the highest importance.
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