## 2 Methods
### Imagery Dataset  
Hyperspectral images were acquired for _P. australis_ interpretation in the study area, retrieving its AGB. _P. australis_ usually starts growing in March and reaches its maximum biomass in July [34] and undergoes a controlled harvest by the administration agency of the HWNR in late November. Thus, images of 1 November 2020, 17 June 2021, and 31 July 2021 were obtained from the Orbita hyperspectral satellite constellation (OHS) ([[https://www.obtdata.com/](https://www.obtdata.com/)]([https://www.obtdata.com/](https://www.obtdata.com/)), accessed on 12 October 2021) to reflect the conditions of _P. australis_ at the peak and the end of the growth cycle.  
Developed by Zhuhai Orbita Aerospace Science & Technology Co. Ltd. (Zhuhai, China), the OHS is the first hyperspectral satellite constellation in the world with surface coating technology for sensors [35]. The constellation consists of eight hyperspectral satellites, and the first batch of four satellites was successfully launched on 26 April 2018, followed by the second batch of four satellites launched on 19 September 2019. Satellites of OHS have an on-orbit life span of more than 5 years, and the revisit cycle is 5 days. A dual-frequency global navigation satellite system (GNSS) receiver (GPS and Beidou) is used to measure the position of OHS in its orbit. The sensor altitude of OHS is around 500 km above the ground, resulting in images of a spatial resolution of 10 m. Each satellite of the OHS has 256 bands with a wavelength ranging from 400 nm to 1000 nm at approximately 2.5 nm intervals. However, only 32 bands can be selected due to limitations in data transmission and storage. The main parameters of OHS are listed in Table 1[36].  
High-resolution images (pixel size \(<\) 10 m) such as SPOT images with a pixel size of 2.5 m and QuickBird images with a pixel size of 2.4 m have been widely used to form a dataset of training samples for _P. australis_ interpretation, as they contain abundant spatial information [37; 38]. In this study, a high-resolution BJ-2 image of June 2021 was used for selecting training samples for interpretation. The BJ-2 images are provided by the BJ-2 satellite constellation, which was developed by Surrey Satellite Technology Ltd. ([[https://www.sstl.co.uk/](https://www.sstl.co.uk/)]([https://www.sstl.co.uk/](https://www.sstl.co.uk/)), accessed on 2 August 2021) and launched in July 2015. The constellation has a revisit cycle of one day and provides sub-meter resolution images of red, green, and blue bands at a pixel size of 0.8 m and infrared images at a pixel size of 3.2 m. With a relatively high spatial resolution, BJ-2 images have been utilized for various applications such as monitoring landslides, extracting hydraulic engineering constructions, classifying land-cover types, etc. [39; 40; 41]. In this study, the 0.8 m resolution BJ-2 image that contained red, green, and blue bands was acquired, geo-rectified to the WGS 84 datum (50N), and further used as a reference image for the registration of the OHS images.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Spatial resolution & 10 m \\ Swath width & 150 \(\times\) 150 km \\ Satellite mass & 67 kg \\ Sensor height & 500 km \\ On-orbit life span & \textgreater{}5 years \\ Signal-to-noise ratio & \textgreater{}30 \\ Wavelength & 400 nm to 1000 nm \\ Spectral resolution & 2.5 nm \\ Bands & 256 (32 bands can be selected) \\ Revisit cycle & 5 days \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main parameters of Orbita hyperspectral satellite constellation (OHS).