1. Introduction

Water resource disasters are occurring frequently worldwide due to climate change, and the extent of such disastersis also expanding;According to the 2018 IPCC report, global temperatures have risen by about 1°Cdue to human industrial activity, it can rise to about 1.5°C between 2030 and 2052. As a result, disasters such as heavy rain, heat waves, and droughts can occur frequently and cause a lot of damage. South Korea has experienced a growing frequency and extent offlooding and drought due to changes of extreme climate events, as well as abnormal natural phenomena such as sea level rise (Lee, 2017). Such natural disasters occur on a vast scale and their sites can be difficult for people to access, so monitoring them using a satellite is necessary.

The Ministry of Environment and K-water are developing the next-generation medium satellite No. 5 (Water resource/water disaster satellite) with the goal of launching it in 2025, and the relevant satellite will be equipped with high resolution C-band Synthetic Aperture Radar (SAR). SAR is an active sensor using microwavesthat passesthrough clouds, and can capture a ground image regardless of weather or solar altitude (Curlander et al., 1991). It is used in various fields including ground,sea, polarregion and the atmosphere both domestically and internationally (Waske et al., 2009; Soh et al., 1999).

Overseas satellites equipped with C-band SAR include RADARSAT-1, RADARSAT-2 and Sentinel1, and these satellites are used in various environmental fields including the monitoring of forest change and fuel contamination spread in addition to their original purposes(water and moisture related monitoring, etc). Therefore, it is necessary to find a way to actively utilized the next-generation medium satellite No. 5 (Water resource/water disaster satellite) in different environmentalfieldswhile achieving its original purpose. To this end, thisstudy presented the characteristics and application ofthe waterresource/water disastersatellite equipped with C-band SAR.

2. Compare to characteristics of C-band SAR satellite

Key duties of the next-generation medium satellite No. 5 are the acquisition of information in the event of a disaster in East Asia, the establishment of a flood forecasting system, detection ofsigns of disaster (flooding, drought, green tide/red tide, earthquake, maritime disaster, etc.), national security in the event of a disaster, and accumulation of national land information in order to secure public safety. The specifications of the payload in the RADARSAT-1, RADARSAT-2, next-generation medium satellite are as follows (Table 1). The center frequency, temporal resolution and spatialresolution ofthe next- generation medium are 5.4 GHz, twice a day and less than 10 m, respectively. The target operation period is 4 years or more, and this satellite will create and provide a thematic map related to water resource and water disaster after obtaining the image data of the entire Korean Peninsula. This helps to ensure more efficient and scientific response and management of flood hazard areas.

Table 1. Specifications of C-band SAR satellite payload

Source: K-water (https://www.kwater.or.kr/news/repoView.do?brdId=KO26&s_mid=36& seq=104217

 https://en.wikipedia.org/wiki/Radarsat-1#cite_note-characteristics-1

https://directory.eoportal.org/web/eoportal/satellite-missions/r/radarsat-2

Compared to satellites of other institutions, the RADARSAT-1 andRADARSAT-2 satellites equipped withC-band SARhave a limit of 24 daysforthe revisit cycle, which cannot be monitored in real time in the event of a disaster. But, the next-generation medium satellite is characterized by a two-day revisit cycle that enablesrapid monitoring of disasters. Satellites are also classified aslarge satellites,mediumsatellites and small satellites depending on their weight, of which the nextgeneration medium satellite, though small in size and weight, has a C-band with similar performance and Operation period.

SAR combines and composes data obtained from each antenna position using sequential antenna position changes caused by themovements ofthe payload.Also, this sensor is an active sensor that is powered by the satellite’s own energy source, so it can obtain data with almost no weather effect. And SAR can obtain amplitude and phase data from an image, unlike a normal optical image, which can only obtain the brightness of an image. The physical characteristics of a target object can be extracted from amplitude and phase data; amplitude data are used to recognize targets, detect the changes, and create a stereo map; and phase data are used to extract precise altitude information and detect and classify according to changes in altitude value (Kwak, 2011).

3. Application of C-band SAR satellite in the environmental fields

1) Application of C-band radar images in other countries

Typicalsatellites equipped withC-band SARinclude RADARSAT-1, RADARSAT-2 and Sentinel-1. The purpose and fields of application ofthese three satellites are as follows. RADARSAT-1 was developed by the Canadian Space Agency (CSA) in 1995 (CSA, 2014), and its operation was suspended in 2013. Its main duties were monitoring of local damage spread in the event of a disaster, monitoring of crop conditions, map production and glacierstudy, and it was used in a range of fields. It was also used in South Korea for studying ocean current movement and the effects of sea surface wind (Yoon et al., 2006; Kang et al., 2007).

RADARSAT-2 is a successor of the RADARSAT1, and this satellite was launched in 2007 by CSA. It was developed for monitoring of environments, such as monitoring of oceans and thawing around the world, disaster management and vegetation status analysis, resource management and map production (CSA, 2017). In addition, it enablesthe detection of the forest succession structure in the foreststhat cover more than 30% of the world’stotal land area using RADARSAT2, helping improving mapping by forest type, and is also effective formapping flooded areasin forests when flooding occurs (Townsend, 2001; CSA, 2017).

Finally, Sentinel-1 consists of Sentinel-1Alaunched in 2014 and Sentinel-1B launched in 2016 and was launched from the European Space Agency (ESA). And it is developed for continuous ground andmaritime monitoring using C-band SAR (ESA, 2014). Data collected by this satellite are utilized in various fields including emergency responsesto disasters(monitoring earthquake and volcano, etc.) and short-term and long-term water-related monitoring such as flooding and river inundation caused by localized torrential downpours, and soil drought(measuring soil moisture, etc.). Recently, various utilizations of Sentinel-1 in environmental fields have been attracting attention, as many papers have been presented indicating that it was helpful for evaluating soil moisture (Hornacek et al., 2012; Paloscia et al., 2013; Gao et al., 2017).

In summary, other countries have used satellites equipped withC-band radarin forest and environmental fields,such asthe analysis of ocean current movement and forest verticalstructure using SARimages, based on its water-related utilization such as flooding, overflow and drought. Accordingly, before the water resource/ water disastersatellite equipped with SARislaunched, the applicability of the satellite in environmental fields was reviewed and utilization measures were explored without limiting its application to waterresource/water disaster

2) Status of environmental spatial information in South Korea, and C-band radar image convergence

South Korea’s environmental spatial information is established based on the thematic map for each environment-relatedmedium(water quality, atmosphere, ecology, etc.). Various government agencies establish information for each medium according to their own purposes, and while such information may have similar contents, the form and structure of established information vary. The government agencies that establish and provide environmentalspatial information include the Ministry of Environment, NationalInstitute of Environmental Research, National Institute of Ecology, Korea National Park Service and Korea Environment Institute. Nine government agencies indirectly establish environmental spatial information, including the Korea Forest Service, Ministry of Land, Infrastructure, and Transport and Statistics Korea (Lee, 2017). The systems that provide investigated environmentalspatialinformation are summarized based on the final results (Table 2). Direct environmental spatial information consists ofland cover map, land use control area/zone map and ecological zoning map. Indirect environmentalspatial information includessoil map, forest type map and geological map, etc.; aerial photographs and digital maps are used as basic data. Table 2 is a list of the current status of Environmental spatial information, Agency, URL in South Korea. Although various institutions provide a lot of spatial information like this, if the C-band image is used to generate spatial information, more precise and accurate theme maps can be generated. In addition, InSAR (SARInterferometry) and PolSAR(Polarimetric SAR) are possible through the use of phase and amplitude. InSARhasthe advantage of obtaining precise DEM or surface microdisplacement using phase information of two ormore SARdata obtained fromthe same scatterer. PolSAR has the advantage of being able to classify the properties of scatterers using polarization of electromagnetic waves. Therefore, these technologies will increase the accuracy offorest vegetation structures, surface changes and coastline monitoring.

Table 2. Current status of environment spatial information in South Korea

The existing environmental spatial information has been built up in the form of specific thematic maps on the environment, and remote sensing data such as satellite images and aerial photographs are used asraw data. For example, land cover maps were produced, mainly taking advantage of Landsat 30 m for large category, SPOT 5 m for intermediate category and Korean Multi-Purpose Satellite (KOMPSAT) 1 m for detailed category, which are the most frequently used of the environmental spatial information types. Recently, 25 cm aerial photographs are mainly being used for land cover maps. Spatial resolution-based application is not limited to land cover maps, and it is also used in the field survey for forest type maps and ecological zoning maps. Since remote sensing in environmentalfields has been developed mainly based on spatial resolution, the classification was mainly focused on an object’s morphological characteristics rather than its physicochemical characteristics. Today, aircraft hyperspectral data are applied to the monitoring of green tide in the basin of Korea’s four main rivers.

C-band radar images can be utilized in various environmental fields by extending their usage. For example, LULUCF (Land Use, Land-Use Change and Forestry) is state-certified data which the Ministry of Environment should submit to UNFCCC (United Nations Framework Convention on Climate Change). Such data are currently submitted after needle-leaftree, broad-leaf tree, mixed forest and agricultural area (fields and paddies), which are among the classification items of the land cover map, are adjusted. However, if C-band image radar is utilized, the forest’s vertical structure can be analyzed more clearly by applying the characteristics of reflectance. This type of application could improve the classification accuracy and precision of thematic maps, such as land cover maps and LULUCF.

Beyond this, ridgelines and ecological axes are very important areas, as areas worth preserving are selected in environment-related field worksfor these areas, and these serve as an ecological corridor for animals and plants. For this reason, it is important to accurately select ridgelines and ecological axes and extract their accurate spatial position. However, currently analysis is carried out using a fragmented combination offorest distribution status, relevant past drawings and DEM. Using C-band radar images enables a more accurate analysis of height values, leading to better analysis of ridgelines.

Eventually, C-band radar images will be used as a priority for improving the accuracy and precision of existing environmental spatial information in environmental field works. They can also be used for items (landslide, slope stabilization, prevention of soil disasters, etc.) in environmental fields that require an accurate analysis of height difference (Jung et al., 2004; Alessandra et al., 2019). In addition, C-band radar can be applied for observing land cover changes including the foreststatus and wildfire in North Korea where field survey cannot be conducted; and monitoring flooding and drought in the Korean Peninsula intensively; and is considered to be helpful for the prevention and restoration of relevant damage and the formation of related policies.

4. Summary and Conclusion

Recently,water-related disasters have been increasing worldwide due to climate change, and Korea is also experiencing a rise in the occurrence of flooding and drought. Forthisreason, the importance of accurate and continuous monitoring is being emphasized. Natural disasters occur extensively and often are in sites that are difficult for people to access, so monitoring them using a satellite isrequired.TheMinistry ofEnvironment and K-Water have been preparing the next-generation medium satellite No. 5 (Water resource/water disaster satellite) which will be equipped with high resolution C-band SAR. Other countries’ C-band radar images are used in water-related fields as well as in various environmental fields, both directly and indirectly. This study analyzed the characteristics of the C-band radar images application in environmental fields.

Water resource/water disaster satellite base on C-band is a high resolution satellite with a temporal resolution of twice a day and a spatial resolution of 10 m. Unlike an opticalsatellite,C-band SARcan observe flooding in real time and has an advantage when it comes to estimating urban and agricultural damages incurred by stream and river flooding. It can also observe the water resource information required for water management twice a day regardless of weather conditions, both during the day and at night. To utilize such information in on-site environments, it can first be applied to improve the classification accuracy and precision of existing environmentalspatial information. It can also be used to analyze environmental status in inaccessible areas that require an analysis of precise physical height change, among various environmental issues (landslide, slope stability, etc.). Using the water resource/water disaster satellite will enable the monitoring of flooding and drought in the Korean Peninsula as well as in various environmental areas, and will be helpful for prevention and restoration of relevant damages and policy establishment.​​​​​​​

Acknowledgements

Thisresearch was conducted at Korea Environment Institute (KEI) with support from ‘The Application technology and system of satellite image radar in the environmental’ by Korea Environment Industry & Technology Institute (KEITI), and funded by Korea Ministry of Environment(MOE) (2019002650001).​​​​​​​