Verification and Validation of Hybridspectral Radiometry Obtained from an Unmanned Surface Vessel (USV) in the Open and Coastal Oceans
Abstract
:1. Introduction
- C1
- Data are obtained within exceptionally clear waters with horizontally homogeneous optical properties (over several kilometers and typically in the deep ocean);
- C2
- Properly characterized hyperspectral instruments, calibrated with radiometric traceability to the National Institute of Standards and Technology (NIST), are used so spectral response functions for the satellite instrument can be applied; and
- C3
- Sampling is under clear skies, stable illumination, and marine aerosols (verified with shadow band or sun photometer data) with extraordinary calibration maintenance.
2. Materials and Methods
2.1. COTS USV
2.2. Optical Observations
2.3. Modifications to the Standard SV3 Configuration
- The original cover plate was replaced with a strengthened version of the same dimension to allow the mounting of the above-water instrument suite described in Section 2.2;
- The cellular and AIS antennas plus the weather station mast were shortened to achieve a final height below the and PAR diffusers. Although all shortened components functioned as anticipated, the original AIS unit was subsequently replaced with a model made by Antcom (Torrance, CA, USA), the supplier of the cellular antenna, to ensure height compliance with no alterations to the original device (because AIS functionality is a critical safety requirement);
- Hard anodized aluminum brackets were added to the stern port and starboard stabilizing handle mounts to support the harness cabling attached to the sea cable used for towing the TOW-FISH with no loss in functionality of the handles;
- Stainless steel and insulated P-clips plus reinforcing metal plates were used together with longer screws securing the two aft solar panels to allow the sea cable to be held down along the edge of the solar panels without shading the panels;
- The center pick point on the mast plate was ultimately replaced with an extended pick-point assembly to facilitate SV3 recovery. The original center pick point was positioned rather low and close to the float deck, which was considered somewhat problematic due to the near proximity of the higher above-water instrument suite mounted on the cover plate.
2.4. Optical Data Products
- N1
- is derived () as a function (denoted g) of extrapolated to null depth (), to also define the extrapolation interval, and applicable corrections, e.g., dark currents, aperture offsets, temporal (solar transit) changes, self-shading, etc;
- N2
- The correction for the hypothetical water-leaving radiance that would be measured in the absence of any atmospheric loss with a zenith Sun at the mean Earth–Sun distance is accomplished by adjusting with the time-dependent mean extraterrestrial solar irradiance, , which is usually formulated to depend on the sequential day of the year (SDY), , and is derived from look-up tables [24];
- N3
- The f term is defined () as a function relating the irradiance reflectance () to the inherent optical properties (IOPs), the bidirectional Q-factor is defined () by and , i.e., , wherein and are defined for a zero solar zenith angle () with nadir viewing at null depth () for the latter, and look-up tables for both are based on the viewing and solar geometry plus the .
2.5. Water Sample Analyses
2.6. Verification and Validation Approach
- A validation comparison of hyperspectral C-PHIRE and multispectral microradiometers;
- A validation comparison of C-PHIRE and C-OPS multispectral microradiometers;
- A verification comparison of hyperspectral C-PHIRE and MOBY data products; and
- A verification comparison of the hyperspectral MOBY and multispectral microradiometers.
- PHY
- The C-PHIRE hyperspectral data;
- PAH
- The C-PHIRE hyperspectral data converted to averaged wavebands;
- PBH
- The C-PHIRE hyperspectral data converted to bandpass wavebands;
- PMS
- The C-PHIRE multispectral data;
- OMS
- The C-OPS multispectral detector data;
- MHY
- The MOBY hyperspectral data; and
- MBH
- The MOBY hyperspectral data converted to bandpass wavebands.
3. Results
3.1. Validation Using Calibration Data Products
3.2. Validation Using Lower-Level Data Products
3.3. Validation and Verification Using Higher-Level Data Products
- Data products based on the higher flux measured by an upward-pointed irradiance instrument () have smaller differences than those based principally on the lower flux of a downward-pointed radiance instrument ();
- The differences are only a little larger than the best possible differences determined on the optical bench (Table 1);
- Comparisons between multispectral data sources (e.g., PMS and OMS) have smaller differences than those involving hyperspectral data (e.g., PBH and MBH);
- The smallest differences are found for the VIS domain; the neighboring UV and NIR domains have larger differences, with the NIR having the largest;
- There is little evidence of a significant and systematic bias as a function of spectral domain or sensor type except for NIR comparisons involving a hyperspectral sensor.
3.4. Algorithm Output Validation
4. Discussion
4.1. Autonomous Observations of Aquatic Ecosystems Using an SV3 Platform
4.2. Validation and Verification Comparisons
4.3. Advantages of the Hybridspectral Perspective
5. Conclusions
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
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UV | −1.1 (1.1) | −2.3 (2.3) | −0.4 (1.3) | −1.0 (1.0) | −2.1 (2.1) | −0.4 (1.0) |
Blue | 0.1 (0.2) | −2.0 (2.0) | −0.2 (0.2) | 0.0 (0.1) | −1.9 (1.9) | −0.2 (0.2) |
Green | 0.4 (0.5) | −1.6 (1.6) | 0.4 (0.4) | 0.3 (0.3) | −1.6 (1.6) | 0.4 (0.4) |
Red | 1.4 (1.4) | −1.0 (1.0) | 0.9 (0.9) | 1.2 (1.2) | −0.8 (0.8) | 0.7 (0.7) |
NIR | 1.2 (1.2) | −0.8 (0.8) | 0.6 (0.6) | 1.1 (1.2) | −0.7 (0.7) | 0.6 (0.6) |
0.4 (0.9) | −1.5 (1.5) | ;0.2 (0.7) | 0.3 (0.7) | −1.4 (1.4) | 0.2 (0.5) |
UV | 0.5 (1.2) | 1.4 (1.4) | 2.3 (2.3) | 1.0 (1.4) | −1.5 (2.3) | 4.3 (4.3) |
Blue | −0.9 (1.0) | 0.1 (1.2) | 1.5 (1.5) | 0.8 (1.2) | −1.2 (2.1) | 1.2 (2.5) |
Green | −0.5 (0.9) | −1.2 (1.3) | 1.4 (1.4) | −0.5 (1.1) | −1.5 (1.9) | −1.2 (1.5) |
Red | 1.1 (1.3) | −1.6 (1.6) | 0.7 (2.0) | 1.2 (1.7) | 0.3 (0.6) | 4.6 (4.9) |
NIR | 1.9 (1.9) | 1.8 (2.2) | −4.9 (6.5) § | 1.9 (2.4) | −4.5 (4.5) | 25.9 (25.9) † |
0.4 (1.3) | 0.1 (1.5) | 0.2 (2.7) § | 0.9 (1.6) | −1.7 (2.3) | 7.0 (7.8) † |
UV | 1.7 (1.8) | 0.4 (1.9) | 2.0 (6.3) † | 1.5 (2.0) | 3.1 (3.1) | −0.2 (6.7) † |
Blue | −1.5 (1.5) | 1.5 (1.5) | 0.2 (4.2) | 1.2 (1.4) | 0.2 (2.3) | 0.2 (4.0) |
Green | −0.5 (1.1) | 2.0 (2.0) | 0.6 (0.8) | −0.4 (1.2) | −1.8 (1.3) | 0.2 (0.7) |
Red | −0.5 (1.4) | −1.5 (3.6) | −1.1 (3.2) | 1.2 (2.5) | 4.5 (6.1) | 4.2 (4.9) |
NIR | 1.8 (2.0) | −4.4 (4.4) | 11.2 (11.2) ‡ | 2.6 (2.8) | 23.7 (23.7) § | −12.2 (12.2) ‡ |
0.2 (1.6) | −0.4 (2.7) | 2.6 (5.1) | 1.2 (2.0) | 5.9 (7.3) § | −1.6 (5.7) |
Input | |||||
---|---|---|---|---|---|
Data | |||||
PMS OMS | 1.4 (2.3) | −0.6 (2.0) | −1.8 (3.3) | 0.9 (3.4) | −0.8 (3.2) |
PBH PMS | 2.0 (2.9) | 2.1 (2.6) | −2.3 (3.7) | −2.8 (4.3) | 1.7 (3.9) |
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Hooker, S.B.; Houskeeper, H.F.; Lind, R.N.; Kudela, R.M.; Suzuki, K. Verification and Validation of Hybridspectral Radiometry Obtained from an Unmanned Surface Vessel (USV) in the Open and Coastal Oceans. Remote Sens. 2022, 14, 1084. https://doi.org/10.3390/rs14051084
Hooker SB, Houskeeper HF, Lind RN, Kudela RM, Suzuki K. Verification and Validation of Hybridspectral Radiometry Obtained from an Unmanned Surface Vessel (USV) in the Open and Coastal Oceans. Remote Sensing. 2022; 14(5):1084. https://doi.org/10.3390/rs14051084
Chicago/Turabian StyleHooker, Stanford B., Henry F. Houskeeper, Randall N. Lind, Raphael M. Kudela, and Koji Suzuki. 2022. "Verification and Validation of Hybridspectral Radiometry Obtained from an Unmanned Surface Vessel (USV) in the Open and Coastal Oceans" Remote Sensing 14, no. 5: 1084. https://doi.org/10.3390/rs14051084