**5. Conclusions**

This research attempted to investigate the potential of satellite broadband vegetation indices for crop canopy chlorophyll content estimation with minimum effects from leaf inclination angle distribution. The broadband vegetation indices of four satellites (Sentinel-2, RapidEye, WorldView-2 and GaoFen-6) were resampled from canopy airborne imaging spectroscopy data of six crop species with various canopy structures. To obtain generic and robust crop CCC indices, both field-measured datasets and model simulations were used in this study. The best vegetation indices identified in this study are the soil-adjusted index type index SAI (B6, B7) for Sentinel-2, Verrelts's three-band spectral index type index BSI-V (NIR1, Red, Red Edge) for WorldView-2, Tian's three-band spectral index type index BSI-T (Red Edge, Green, NIR) for RapidEye and difference index type index DI (B6, B4) for GaoFen-6. The recommended indices produced strong correlations with CCC (*R*<sup>2</sup> CCC = 0.76–0.80 in field-measured data and *R*<sup>2</sup> CCC = 0.84–0.95 in model simulations) and no correlation with MTA (*R*<sup>2</sup> MTA = 0.00 for field-measured data and *R*<sup>2</sup> MTA = 0.00–0.04 for model simulations) and maintained consistent performance in both the field-measured dataset and model simulations. Thus, it is anticipated that more generic vegetation indices for crop CCC estimation can be derived from satellite broadband data. However, this is only a case study, and further studies are required to examine the suitability across more crop species and growth stages using real satellite imagery.

**Author Contributions:** X.Z. and J.J. conceived the research and implemented the data analysis. X.Z. prepared the original draft. M.M. revised the manuscript and supervised the research. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the National Science Foundation of China (grant No. 41801243) and the Academy of Finland (grant No. 317387).

**Data Availability Statement:** All data are presented within the article.

**Acknowledgments:** The authors would like to thank Priit Tammeorg, Clara Lizarazo Torres, Piia Kekkonen, F.L. Stoddard and Pirjo Mäkelä from the University of Helsinki, who kindly provided the SunScan and SPAD measurement data, and Petri Pellikka from the University of Helsinki for the hyperspectral acquisitions.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix A**

**Table A1.** The central wavelength, bandwidth and spatial resolution and *R*<sup>2</sup> values from field measured dataset between CCC, MTA and individual band reflectance of four satellite sensors.


**Figure A1.** Spectral response functions of satellite sensors used for simulation of broadband reflectance.

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**Figure A2.** Map of the coefficient of determination between the CCC (*R*<sup>2</sup> CCC) and vegetation indices using all two band combinations based on the RI, NDVI, DI, SAI, MSR and MSAI formulations. The color indicates different *R*<sup>2</sup> values.

**Figure A3.** Map of the coefficient of determination between MTA (*R*<sup>2</sup> MTA) and vegetation indices using all two band combinations based on RI, NDVI, DI, SAI, MSR and MSAI formulations. The color indicates different *R*<sup>2</sup> values.
