# Potential of a SAR Small-Satellite Constellation for Rapid Monitoring of Flood Extent
## 1 Introduction  
Swift assessment of the location and extent of damage caused by disasters is vital to real-time situational awareness and effective measures in the response phase of disaster management [1, 2]. In the case of flood disasters, adverse weather conditions and surface inundation typically obstruct field surveys, especially at night. Remote sensing technologies can offer useful solutions in such situations; in particular, satellite-based synthetic aperture radar (SAR) has great potential for flood detection [3]. Unlike optical sensors, SAR can penetrate clouds and rain and collect observations independently from sunlight [4]. Moreover, satellite as a platform facilitates observation over wide swathes and global coverage of remote areas, which cannot be achieved with arcerts or unmanned aerial vehicles (UAVs) [5]. SAR satellites thus play an important role as powerful tools for flood detection across vast regions under unfavorable weather conditions by day or night. Most SAR satellites operate in a sun-synchronous orbit (SSO), a type of near-polar orbit. The orbit's SSO geometry is kept nearly fixed with respect to the sun [6], so satellites in SSO are synchronized to consistently remain in the same position relative to the sun. That is, the satellite visits the same spot at almost the same time twice daily, for example, at 12:00/00:00 every day. Thanks to these features, SSO provides global coverage at alllatitudes (except for just a few degrees from the poles) [6], and enables constant observation at the same local time. Each SSO can be identified by its mean local time (MLT) [7]. However, the observation opportunities offered by current SAR satellite systems are insufficient from the perspective of rapid flood monitoring in the context of disaster management. Currently, most civilian SAR satellites operate in SSO with 06:00/18:00 MLT (the so-called dawn-dusk orbit) [8] due to its efficiency regarding solar power generation, whereas few other SAR satellites travel in SSO with other MLTs (e.g., ALOS-2 with 12:00/00:00 and NovaSAR with 10:30/22:30). Therefore, the observation opportunities currently provided by SAR satellite systems are biased, even though any location worldwide may be observed twice daily by each satellite. In the context of flood monitoring, the disaster management community requires flood extent information with little latency and frequent updating, but inadequate satellite revisit time is one issue preventing the collection of this information [5; 9]. The dynamic flood process, which may both expand and shrink, is difficult to track within the limited time available. The development of small satellites equipped with SAR could be key to resolving this issue. Small satellites have attracted considerable attention due to their wide-ranging applicability, supported by technological advancements in space engineering over the last decade [10]. These satellites cost significantly less than conventional large satellites, and this enables 'constellation' operation involving several satellites; by working in concert, groups of satellites have an excellent capacity for high revisit rates and short revisit time [10]. One example is Planet, which operates more than 200 optical small satellites. However, the miniaturization of SAR satellites has been slow compared to that of optical satellites due to the challenging design requirements of small SAR satellites, such as larger antennae and higher power throughputs [11]. In recent years, however, the miniaturization of electronic components and recent technological advances have finally ensured the compatibility of SAR with small platforms [12]. Constellations of SAR-equipped small satellites are in the early stages of development, and application-driven satellite design is favored in this early period [12]. In other words, each space mission can be optimized through the adoption of the appropriate constellation design and SAR device [13].