Surface Reflectance Product Guide
Terminology
BOA - Bottom-of-atmosphere reflectance
TOA - Top-of-atmosphere reflectance
Surface Reflectance (SR) - Reflectance without atmospheric interference (i.e. BOA)NDVI - Normalised Difference Vegetation Index
LAI - Leaf Area Index
LUT - Look up table

Description
The GRUS Surface Reflectance product is generated for every GRUS tile on AxelGlobe.
The values contained in the surface reflectance product have been corrected withrespect to the atmospheric conditions at the time of acquisition. The process ofatmospheric correction converts top-of-atmosphere reflectance (TOA) tobottom-of-atmosphere reflectance (BOA).
We expect this product to improve radiometric consistency and produce more accurate
multispectral indices, especially when working with multi-temporal datasets andcalculating real-world attributes such as biomass or LAI. This is achieved by removing
atmospheric bias which can affect reflectance values according to the conditions at thetime the data is acquired.

Figure 1: Comparison of TOA (left) and BOA (right) image products captured by GRUS-1E
Zaragoza, Spain

How to Access
Surface reflectance data is available for all GRUS tiles and can be accessed by request tothe AxelGlobe Sales team.

Product Generation
The process of atmospheric correction and converting GRUS Analytic product into theSurface Reflectance product is done using the 6S radiative transfer code (6SV1.1
https://salsa.umd.edu/). The 6S model requires GRUS TOA values along with a number ofatmospheric parameters specific to the conditions when the acquisition took place. Hereis a detailed list of the input parameters including their origin and example values foreach.
These data are distributed alongside the Surface Reflectance product in a JSON filewhere the actual parameters used in the atmospheric correction process are listed.
Table 1: Input parameters used by the 6S model to generate bottom-of-atmosphere data
Input
Parameter

Description

Example
value

Unit

Product Codeor Version

SourceTop-of-atmospherereflectance

0.5(scaled by10,000from
Uint16)

N/A

N/A

GRUS Multispectral
product

AOT (Aerosol
OpticalThickness)

Thickness of aerosol at550nm

0.4

N/A

MOD08 & MOD04

MODIS1

Water Vapour

Total amount of water inatmosphere vertical path

3g/cm^2

MOD08 & MOD07

MODISOzone

Total amount of ozone inatmosphere vertical path0.3

cm-atmMOD08 & MOD07

MODISElevation

Average height above
sea level

0.84kmSRTMGL1Nv003

SRTM2

Satellite zenith

Angle of satellite
inclination

5.11

degrees

N/A

GRUS Multispectral
product

Satellite azimuth

Angle of satellite viewing

76.5

degrees

N/A

GRUS Multispectral
product

Solar zenith

Angle of sun inclination

58.0

degrees

N/A

GRUS Multispectral

TOAproductSolar azimuth

Angle of sun viewing

156.25degrees

N/A

Multispectral productAxelspace Implementation of 6S
Surface reflectance values are calculated for each multispectral GRUS product using the input
data shown in Table 1, however the values do slightly change depending on the time ofacquisition. All recently acquired GRUS acquisitions will use the MODIS NRT (Near Real Time)products, and captures being processed that are more than 5 days old use an archival MODIS
product.
Using 6S to estimate surface reflectance from TOA reflectance is a time consuming calculation
which would not be practical to apply to every pixel in a GRUS image tile. Lookup tables (LUT)
are generated for each new capture and contain a model for a range of conditions and TOAreflectances. The LUT model is used to rapidly convert every TOA pixel into a BOA reflectancevalue with a 2nd order polynomial.

Product Validation
To confirm the quality of the Surface Reflectance product, we compare GRUS data withreliable 3rd party data.
Because surface reflectance data has had atmospheric bias removed, it can be moreeasily compared with other datasets than the TOA data.
This also includes comparisons made between surface reflectance data from different
sensors, although in this case care must be taken to account for differences in spectral
response curves. For example, although two sensors may have “red” bands, the exact
width and centres of these bands may be different. The relative spectral responses foreach GRUS band can be found on the AxelGlobe Image Specification sheet(https://docs.axelglobe.com/en/image-specifications).
For validation of GRUS Surface Reflectance data we have chosen to compare it with
RadCalNet3 sites (https://www.radcalnet.org/), where reliable surface reflectancemeasurements are made on a daily basis. RadCalNet measurements are acquired bysensors on ground and are also taken for a large number of spectral wavelengths withgood overlap with GRUS bands; this ensures that the data is not affected by theatmosphere and can be more reliably compared with GRUS.

The figures below show the locations of different RadCalNet sites used for validation, as
well as comparisons between the spectral response of TOA and BOA measurements ateach site. The benefit of atmospheric correction is demonstrated here by illustrating theclose similarity between the GRUS and RadCalNet surface reflectance values.

Figure 2: Visualisation of RadCalNet site locationsFigure 3: Comparison of TOA & BOA reflectance values measured at different RadCalNet sites
with GRUS. Band ordering: 0=Pan; 1=Blue; 2=Green; 3=Red; 4=Red Edge; 5=Near Infrared.

In addition to comparisons between RadCalNet and GRUS surface reflectance data, we havemade similar comparisons with the Sentinel-2 surface reflectance product4. Sentinel-2 imagery
is widely used, but it must be taken into account when viewing that this is not a perfectcomparison due to the differences between GRUS and Sentinel-2 (L2A) spectral responses.

Figure 4: Comparison of GRUS (left) acquired at 2021-07-08 and Sentinel-2 acquired at2021-07-07 (right) RGB composites using surface reflectance.

Figure 5: Comparison of BOA (right) and TOA (left) reflectance values for GRUS-1C andSentinel-2. Atmospheric correction improves closeness between the two datasets with
R²=0.89 compared to before atmospheric correction when R²=0.57.

Specification Versioning
Table 2: Versioning history of the GRUS Surface Reflectance Product Guide
Version No.

DateRemarks

1.0.001-03-2023

Initial version

Acknowledgements
Axelspace would like to thank our collaborators at the National Institute of Advanced Industrial
Science and Technology (AIST) who were invaluable in the research, generation and testing of
this product.

References
1 https://www.earthdata.nasa.gov/learn/find-data/near-real-time/modis
2 https://lpdaac.usgs.gov/products/srtmgl1nv003/
3 https://www.radcalnet.org/#!/4 https://sentinel.esa.int/web/sentinel/missions/sentinel-2