## 2 Methodology
### Sequencing the Mission Priority  
#### 2.2.1 Impact Factors of Mission Priority  
Due to the limitation of satellite resources and different users' needs for remote sensing data, the priority level of emergency missions varies. By analyzing the application requirements of emergency missions, the following seven impact factors are considered in setting the mission priority (Table 1). (1) Imaging mission level. In the process of emergency response, different emergencies can be divided into four categories: natural disaster, accident disaster, public health incident and social security incident. According to the nature, severity, controllability and range of influence, it can be divided into four levels (I-IV), and the emergency response mechanism decreases from level I to level IV. Therefore, the corresponding imaging mission level is divided into four levels, and the missions with high level of this factor are given high priority. Figure 1: Flow chart of a multi-satellite dynamic mission scheduling model based on mission priority under emergency conditions. Type of observation image. Satellite scheduling for emergency missions involves multiple types of remote sensors, thus different images can be selected by users according to their needs. Therefore, the types of remote sensing images can be employed for calculating the priority level of mission. We take visible light, infrared and microwave images as examples in this study, and assume that the image types are ordered as follows: visible light image > microwave image > infrared image. 3. Visibility of target to satellite. During satellite scheduling, there are cases in which the same target is within multiple satellite observation ranges at the same time. For a mission, the visibility of target to satellite is related to the number of available satellites in the same time window, and the visibility between the satellite and target can be computed by calling satellite tool kit (STK) software. Therefore, a mission with less available satellite resources should be given high priority, thus to ensure the execution of the mission. Besides, when no satellite can serve the mission, the mission is declared invalid and removed from the list of missions that need to be scheduled. 4. Execution urgency degree. Emergency observations have different response time requirements. The start time, end time and remaining time of a mission determine whether it could be successfully carried out and completed. Therefore, the more urgent the mission is, the more likely it is to miss the execution period and fail, so it should be given priority for execution. 5. Type of mission. Due to the emergency missions with different characteristics, the types of missions should be classified and given different priorities. Especially, the position of maritime target has great uncertainty and always changes with time, so it needs to be observed first. Therefore, the importance of different mission types is defined as the following sequence: maritime moving target > maritime static target > land moving target > land static target. 6. Mission conflict degree. In some cases, there may be multiple missions requesting a certain satellite resource at the same time, resulting in resource contention. Therefore, we define the conflict degree of a mission as the number of missions competing for the same satellite resource. The mission with a high conflict degree should be given priority for execution in order to avoid the problem of deadlock during scheduling. 7. Revenue of imaging mission. A basic objective of mission scheduling is to maximize the total revenue gained from acquiring images. For imaging satellites, meteorological conditions, especially cloud cover, have a great influence on imaging effect. Therefore, the total mission revenue depends not only on the value of the mission, but also on the imaging quality. Therefore, we adopt the basic revenue which is related to the importance of the observation missions and cloud cover which is related to the imaging quality to measure the total benefit.