## 1 Introduction  
Earth observation has experienced unprecedented growth through the use of satellite data [1; 2]. Space-based, spatio-temporal data is now regularly used to remotely measure fresh water elevation [3], explore potential mineral deposits [4], monitor changes in land-cover and land-use [5; 6], to name a few practical applications. In particular, Earth observation for situational awareness often involves mobile targets, such as hurricanes [7; 8] or emergency areas whose locations cannot be determined a priori [9; 10]. For example, crop classification and growth monitoring may be routinely performed on pre-designated areas [11; 12], whereas the time and location of flooding or drought cannot be accurately predicted and requires contingent responses. As a way of incorporating responsiveness and agility into Earth observation, the concept of reconfigurable satellite constellation (ReCon) has been proposed [13; 14]. The operation of a ReCon comprises the following two modes: Global observation mode (GOM) for normal operations and regional observation mode (ROM) for contingent responses. In GOM, satellites in a ReCon evenly scan the entire region within a latitude band without any bias (Figure 1a). In ROM, satellites move to repeat ground tracks (RGT) and periodically fly more often over certain regions along their fixed paths (Figure 1b) while ignoring the remainder in between. The satellites in ROM have 2.5 times more access time than those in GOM on average [15]. Near the equator where the satellites struggle in terms of GOM access, ROM achieves up to 10 times more access, which is achieved at the intersections of ascending trajectories and descending trajectories depicted in Figure 1b. There are a few examples that may be regarded as precursors of a ReCon. Two European Remote Sensing satellites, ERS-1 and ERS-2, were lunched into a single orbit and could reconfigure the revisit time of their repeat ground tracks from 3 to 35 days [16; 17]. The KH(keyhole)-11 satellites consisted of five satellites allocated to two planes, all of which were highly maneuverable for reconnaissance purposes [18]. Further scale-up of a ReCon would require optimal planning of multiple satellite maneuvers. The ReCon just started gaining interests from industries, out of theory, as its operational complexity is being overcome via onboard algorithms and ground computing resources [19; 20; 21]. Satellite orbits and view orientation can thus be planned in time to maximize the value of distributed satellite missions (DSMs) [22; 23; 24; 25]. Walker Delta and Walker Star are well-known constellation types that symmetrically distribute satellites in the inertial reference frame to provide global coverage [26; 27]. The former distributes orbit planes over the full longitudes of 360 degrees while the latter distributes over the longitude range of 180 degrees because of near-polar orbital inclination. Flower Constellations (FCs) distributes satellites in a rotating reference frame instead of an inertial reference frame [28]. Sub-global coverage may also be provided by squeezing orbital plane distribution onto a confined longitude band of interest [5; 29]. The above-mentioned methodology has been implemented in a number of DSM tools. For example, Operational Network of Individual Observation Nodes (ONION) aims at simultaneously meeting Earth observation requirements and data download requirements, whose optimal solution is a hybrid Walker constellation of small satellites (CubeSats) and larger satellites [30]. Trade-space Analysis Tool for Constellations (TAT-C) conducts performance-cost analyses on uniform Walker constellations, non-uniform Walker constellations, and ad-hoc constellations [31]. The proposed ReCon framework may supplement these tools by adding reconfigurability to static Earth observing constellations operating in various wavelengths.