VNP43NRTAlbedo.jl

Near-Real-Time Implementation of the VNP43 VIIRS BRDF Correction Algorithm for VNP09GA Surface Reflectance in Julia

The VNP43NRTAlbedo.jl package provides a Julia implementation of the Near-Real-Time (NRT) VIIRS Bidirectional Reflectance Distribution Function (BRDF) correction algorithm, specifically designed for VNP09GA Surface Reflectance data. This package allows for the calculation of key land surface albedo products, including White-Sky Albedo (WSA), Black-Sky Albedo (BSA), and Nadir BRDF Adjusted Reflectance (NBAR), by inverting a BRDF model from observed reflectances and associated solar-view geometry.

Contact

Margaret Johnson (she/her)
[email protected]
Algorithm implementation
NASA Jet Propulsion Laboratory

Gregory H. Halverson (they/them)
[email protected]
Algorithm re-implementation
NASA Jet Propulsion Laboratory

Crystal Schaaf (she/her)
[email protected]
Algorithm inventor
Boston University

Kerry Cawse-Nicholson (she/her)
[email protected]
Concept development and project management
NASA Jet Propulsion Laboratory

Features

• BRDF Model Inversion: Implements a linear least squares approach to invert the kernel-driven BRDF model (RossThick/LiSparseR).
• Albedo and Nadir Reflectance Calculation: Derives WSA, BSA, and NBAR from the fitted BRDF parameters.
• Uncertainty Propagation: Provides standard errors for the calculated albedo and nadir products based on the BRDF fit uncertainty.
• Weighted Least Squares: Supports weighted least squares regression with an exponential decay scheme, prioritizing more recent observations for NRT applications.
• Solar Zenith Angle Calculation: Includes functions to compute solar zenith angles from time and location.
• Sinusoidal Tile Dimension Utility: Provides a helper function to calculate spatial dimensions for MODIS/VIIRS sinusoidal tiles.
• Compatibility: Designed to work with Rasters.jl and DimensionalData.jl for robust geospatial data handling.

Installation

To install VNP43NRTAlbedo.jl, open the Julia Pkg REPL by pressing ] and run:

pkg> add VNP43NRTAlbedo

Usage

The core functionality of the package is exposed through functions like NRT_BRDF_all, which takes observed reflectances and angular information to produce albedo and nadir products.

Module Structure

The package is organized into several key components:

• Constants: SINUSOIDAL_CRS defines the Well-Known Text (WKT) for the Sinusoidal projection.
• Coordinate Utilities:
  • sinusoidal_tile_dims: Calculates X and Y dimensions for a specific sinusoidal tile, useful for Rasters.jl integration.
• BRDF Model Kernels:
  • Kvol, Kvol_vec, Kvol_sc: Implement the RossThick volumetric scattering kernel for matrix, vector, and scalar inputs.
  • Kgeo, Kgeo_vec, Kgeo_sc: Implement the LiSparseR geometric-optical scattering kernel for matrix, vector, and scalar inputs.
• Solar Geometry:
  • zenith_from_solarnoon_time, zenith_from_solarnoon_vec: Functions to calculate solar zenith angle from time and location.
  • calculate_SZA: A simplified SZA calculation based on day of year, hour, and latitude.
• BRDF Inversion and Product Generation:
  • BRDFParameters: A struct to hold the results of BRDF model inversion (coefficients, standard errors, R-squared, covariance).
  • NRT_BRDF: Performs the core BRDF model inversion for a given set of observations and kernels.
  • NRT_BRDF_albedo: Calculates White-Sky and Black-Sky Albedo.
  • NRT_BRDF_nadir: Calculates Nadir BRDF Adjusted Reflectance.
  • NRT_BRDF_all: A consolidated function to compute WSA, BSA, and Nadir reflectance along with their uncertainties and fit statistics.
• Date Utilities:
  • date_range: Generates a vector of Date objects within a specified range.
• Internal Modules:
  • Modland.jl (external package) and VIIRS.jl: Modland.jl provides utilities and constants for MODIS/VIIRS satellite sensors. VIIRS.jl remains as an internal module for VIIRS-specific functionality.

Contributing

Contributions to VNP43NRTAlbedo.jl are welcome! Please refer to the guidelines for contributing to Julia packages.

References

1. Schaaf, C. B., Gao, F., Strahler, A. H., Lucht, W., Li, X., Tsang, T., Strugnell, N. C., Zhang, X., Jin, Y., Muller, J.-P., Lewis, P., Barnsley, M. J., Hobson, P., Disney, M., Roberts, G., Dunderdale, M., Doll, C., d’Entremont, R. P., Hu, B., Liang, S., Privette, J. L., & Roy, D. (2002).
   First operational BRDF, albedo nadir reflectance products from MODIS. Remote Sensing of Environment, 83(1-2), 135-148.
   https://doi.org/10.1016/S0034-4257(02)00091-3

2. Lucht, W., Schaaf, C. B., & Strahler, A. H. (2000).
   An algorithm for the retrieval of albedo from space using semiempirical BRDF models. IEEE Transactions on Geoscience and Remote Sensing, 38(2), 977-998.
   https://doi.org/10.1109/36.841980

3. Schaaf, C. B., Liu, J., Gao, F., Strahler, A. H., & Jin, Y. (2011).
   VIIRS Bidirectional Reflectance Distribution Function and Albedo Climate Data Record (BRDF/Albedo CDR) Algorithm Theoretical Basis Document (ATBD).
   NOAA/NCEI CDR Documentation

4. USGS MODIS & VIIRS Land Products:
   MODIS BRDF/Albedo Product Overview
   VIIRS BRDF/Albedo Product Overview (VNP43)