A review of spaceborne synthetic aperture radar for invasive alien plant research

Recently, a strong international focus has been placed on invasive species and their ecological, economic, and social impacts. Satellite remote sensing (SRS) for the detection of invasive alien plants (IAPs) is a promising and actively researched application of satellite-derived earth observation data. Despite its all-day, all-weather detection and mapping capability, synthetic aperture radar (SAR) data is underrepresented in these efforts. This review discussed the foundational elements and capabilities of spaceborne SAR for IAP monitoring and investigated the current state of the scientific literature concerning the detection and monitoring of IAPs by spaceborne SAR. Twenty-six published articles were discovered and analysed for trends.

The analysis revealed several key findings regarding the current state of SAR in the detection and monitoring of IAPs. Data fusion techniques, especially those combining SAR with multispectral data, are gaining popularity due to their improved performance compared to single-sensor approaches. However, the full potential of SAR imagery, particularly polarimetric SAR (PolSAR), remains underutilised in multi-sensor studies. SAR analyses demonstrated strong performance in scenarios where the IAP structure exhibited distinct characteristics compared to its surroundings, such as plants isolated on water surfaces or palms displacing mangroves, due to the unique interactions of microwave radiation with the structural characteristics of targets.

Several key principles in the deployment of SAR were identified, including band and polarisation selection, basic techniques such as grey-level thresholding, and more advanced analyses such as polarimetry. Also noted are the capabilities of SAR in enabling indirect methods, such as inundation mapping and soil modelling. Suggestions are made for future directions in consideration of recently launched and forthcoming spaceborne SAR sensors. Significant among these are fully polarimetric systems which will provide freely accessible data, offering huge opportunities for sophisticated PolSAR analyses. This data will need to be fully exploited to advance species-level IAP detection and monitoring. Examples of IAPs which may benefit from SAR approaches are given, with special attention paid to the Australian Weeds of National Significance (WoNS).