Reference data
===

In the field validation, we will use high-end radiometry to quantify the accuracy of the iSPEX 2 reflectance. This section summaries the relevant measurement systems, measurement protocols, data submission formats, and submission to the Open Science Framework.

What are the requirements for the reference systems? 
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Reference measurements for iSPEX 2 are to be conducted using `three-sensor' above-water reflectance systems, comprising of a downwelling irradiance sensor, water radiance sensor, and sky radiance sensor.  **We are interested in collecting input radiance and downwelling irradiance reference measurements processed to SI units** (commonly called Level 1 data) rather than processed reflectance (commonly called Level 2 data) as these will enable us to quantify uncertainties underlying the reflectance derived from iSPEX 2. 

The reference spectral measurements comprise of:

- **Ed** (sometimes called Es) – downwelling hyperspectral planar irradiance above the water surface 

- **Ls** (sometimes called Li) - hyperspectral sky radiance measured at 40 degrees to zenith (vertically upwards). 

- **Lt** – hyperspectral water radiance measured at 40 degrees to nadir (vertically downwards). 

We anticipate that many reference data submissions will be made with sensors and systems that have established usage in ocean colour satellite validation. For example, Satlantic radiometers with either manual HyperSAS or robotic PySAS acquisition systems, or TriOS-RAMSES radiometers with either manual or So-Rad acquisition systems.  

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<img src="https://mfr.de-1.osf.io/export?url=https://osf.io/download/ma7sq/?direct%26mode=render&format=2400x2400.jpeg" width="400" height="400" />



*The Solar-tracking Radiometry (So-Rad) system is an example of a three-sensor above-water reflectance system.
The TriOS-RAMSES sky and water radiance sensors are attached to a rotating platform in the left of the image. The TriOS-RAMSES irradiance sensor is in the right of image.*


The  iSPEX 2 validation is, however, not limited to these systems, and we welcome submission from other systems which meet the following requirements below:  

 - **Spectral range** of 380 – 850 nm. The spectral range of iSPEX 2 (which is based on the spectral response of smartphone pixels) is ~ [390,670] nm, but we request measurement data at longer NIR wavelengths for quality control. 
 
 - **Spectral resolution**. The target spectral resolution of iSPEX 2 is ~ 10 nm, so any reference system needs to have comparable or finer spectral resolution. Data should ideally be supplied at native spectral sample spacing (typically ~ 3 nm). 
 
 - **Recent calibration**.  The sensors should be recently calibrated, ideally with a known calibration history.
 
- **Sensor field of view**. We anticipate that most radiance sensors will have an angular field around 6-7 degrees as is the case for Satlantic and TriOS-RAMSES sensors. Any significant difference from this should be reported.


Measurement protocol for reference measurements 
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Reference measurements should follow a similar measurement geometry protocol to above-water satellite validation (Ruddick et al. 2019a, Ruddick et al. 2019b)  .

Of particular note: 

- Radiance measurements are to be made at +/- 40 degree viewing zenith, which matches the target angles in the iSPEX2 app. 

 - Relative azimuth (angle between the radiance sensors’ viewing direction and the sun) should be targeted at 135 degrees.  If the azimuth cannot be set, then the relative azimuth should be recorded at the time of each measurement. Typically, a relative azimuth between |90|-|150| degrees is acceptable. 
 
 - Data is to be collected when the solar elevation angle is > 20 degrees (or equivalently solar zenith angle is < 70 degrees).
 

Reference data format
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- **File format**. Data are to be submitted in CSV format, following the example for the PML So-Rad system in the OSF storage: OSF Storage/aquatic-rrs-validation-campaign  reference-data/PML_Quest2025  20250806
 PML_SR001_EdLsLt_2025-08-06.csv, https://osf.io/357bk/files/osfstorage.


- **Header**.  The header includes the following fields: investigators, affiliations, deployment, reference system, reference sensors, comments. 

- **Auxiliary data**. Necessary auxiliary fields are date, time, latitude, longitude, relative viewing azimuth. Desirable auxiliary fields are solar elevation, wind, aerosol optical thickness.			

- **Spectral units**. The measurements should be processed to SI units, but do not have to be in base units. For example, the TriOS-RAMSES defaults of mW/m^2/nm and mW/m^2/nm/sr and the Satlantic defaults of uW/cm^2/nm and uW/cm^2/nm/sr are suitable.

- **Time and wavelength sampling**. The spectral and auxiliary data should ideally be supplied at native spectral and temporal sample spacing (i.e. data that has not been interpolated or re-sampled to a common wavelength grid). 
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For the PML So-Rad example, we are able to combine in a single file as the data is pre-synchronised, and there are common timestamps for each system. For non-synchornised systems, we antcipate that there will be 4 separate CSV filse which have thir own time stamps (one for each sensor, and one for the auxillary data.

References
====
Ruddick KG, Voss K, Boss E, Castagna A, Frouin R, Gilerson A, Hieronymi M, Johnson BC, Kuusk J, Lee Z, et al. A Review of Protocols for Fiducial Reference Measurements of Water-Leaving Radiance for Validation of Satellite Remote-Sensing Data over Water. Remote Sensing. 2019; 11(19):2198. https://doi.org/10.3390/rs11192198.

Ruddick KG, Voss K, Banks AC, Boss E, Castagna A, Frouin R, Hieronymi M, Jamet C, Johnson BC, Kuusk J, et al. A Review of Protocols for Fiducial Reference Measurements of Downwelling Irradiance for the Validation of Satellite Remote Sensing Data over Water. Remote Sensing. 2019; 11(15):1742. https://doi.org/10.3390/rs1115174.