Composed by about 130 small coral islands, rocky outcrops, atolls, sandbanks and reefs, the Paracel Islands are located in the disputed territory of the South China Sea. They are distributed over a maritime area of around 15,000 square kilometres, and claimed by China, Taiwan and Vietnam.
Although they are largely uninhabited, these islands have a very high geostrategic interest. They are located in a major shipping route, home to fishing grounds and there are estimations that there may be reserves of natural resources around them. Rival countries have wrangled over territory and sovereignty in the South China Sea for centuries, with tension steadily escalating in recent years. Dredging works have been carried in several reefs by the disputing countries, with some of them developing into more extensive constructions of both military and civil character.
Remote sensing to monitor distant and isolated areas
Remote sensing is a key tool for change detection and maritime surveillance, especially in locations that are very distant from each other and where having people on the ground is difficult. Thus, satellite imagery is the perfect source of information to monitor the evolution of the dispute in the South China Sea. It allows to capture reliable imagery and ensures accurate and timely monitoring over sites located in remote areas, where it’s difficult to get up to date and reliable information otherwise.
With the raise of tensions in the South China Sea in early 2016, Deimos Imaging, a subsidiary of the Canadian UrtheCast Corp., launched a monitoring campaign to acquire archive imagery over this area and to demonstrate the coordinate use of its two satellites for imagery intelligence applications: the wide-swath, medium-resolution Deimos-1 and the narrow-swath, very-high resolution Deimos-2. Both satellites are operated continuously through a 24/7/365 service and a global network of five ground station that ensure a contact every orbit with each of them, and therefore allow to command and download data every 90 minutes. Tipping and cueing proved very useful for military and surveilling applications and also for disaster management and emergency services, environmental monitoring and change detection in a wide range of fields.
Synergistic tipping and cueing to identify changes
Deimos-1 captures 22-m multi-spectral imagery, with a very wide swath of 650 km. The satellite has a collection capacity of more than 5,000,000 km2 with a 3-day revisit time worldwide. Very high revisit
frequency is crucial for monitoring applications and it guarantees the delivery of sufficient cloud-free images, one of the challenges when capturing satellite imagery of this area.
Deimos-2 is a very high resolution, agile satellite, providing 75 cm pan-sharpened images and an off nadir pointing capability of ± 45 degrees. The system capacity is more than 150,000 km2 a day with 2-day revisit time worldwide, allowing to monitor any area of interest accurately, timely and cost-effectively. Images are 12-km wide and up to 200 km long, and with maximum tilt, the Field of Regard (FOR) can be extended to more than 600 km from nadir.
In this study, the technique employed was a synergistic tipping and cueing that allowed to collect information by coordinating activities between Deimos Imaging’s sensors. This methodology consists in tasking one satellite with a wide swath – Deimos-1 in this case – to get the broad picture of a hotspot and detect possible changes, activities, and elements of interest. A careful analysis of the data collected provides a general overview and critical knowledge that can be used as a base map to then task other satellites with different spectral and spatial resolution. The very high spatial resolution of Deimos-2 is important to identify relevant targets in the image, such manmade constructions, military facilities, ships, planes, etc. Meanwhile, having different spectral resolutions and the use of different spectral bands is very useful to highlight and better discriminate different features such as manmade structures, asphalt and vegetation covered land.
Unveiling new constructions on disputed islands
Deimos Imaging’s campaign over the Paracel Islands started in March 2016 and continues ongoing. Multitemporal acquisitions with both satellites allowed to compare fresh imagery with archive data and it enabled to track and capture the most relevant developments of this disputed region, both in context and in detail.
Thanks to its wide swath and high revisit time, Deimos-1 allowed to monitor at a glance the large and widespread area that covers the Paracel Islands, scattered over an area of approximately 15,000 km2. The images were analyzed to identify areas where possible changes might have happened. Ideally automatic change detection algorithms can also be used to minimize analysis time and automate information extraction. The near infrared band of Deimos-1, which depicts vegetation in bright red, is the perfect tool to distinguish manmade constructions from plant-covered land. This was essential to identify new atolls, dredged reefs and land clearing for the construction of facilities. The data gathered by Deimos-1 was examined and compared over time and, when significant changes were identified, the very-high-resolution Deimos-2 was tasked to further investigate and recognize those developments in greater detail.