# Dynamic Routings in Satellite Networks: An Overview
## 3 System Model and Characteristics
### Key Challenges for Satellite Network Routing  
Routing technology has been the focus of research in satellite networks. The earlier satellite networks generally used GEO satellites as relays and bent tube transponders for data forwarding between two stations on the ground. Such a data transfer form is fixed, and no routes are available. In a constellation network with many satellites, the satellite network must select the optimal path among multiple reachable paths between the source and destination satellite nodes. Therefore, several challenges are still faced for satellite routing, mainly in the following aspects. Link Switching on Routing: Due to the rapid movement of satellites, specific switching mechanisms are needed to maintain communication continuity. However, inappropriate link switching may cause re-routing issues in the satellite networks. Re-routing can generally be separated into partial re-routing and complete re-routing.The partial re-route scheme refers to re-routing only the links on which the switchover occurs based on the current network state and routing situation. Although such routing is relatively simple, the route selected utilizing the routing mechanism is not guaranteed to be the optimal route. Complete re-routing means redistributing new routing paths for communicating with users after the link has already completed switching. For such routing strategies, the optimal routing of the network is ensured but at the expense of a certain computational overhead. Therefore, to resolve the re-routing problem and improve the routing performance of satellite networks, more profound studies on link switching strategies are needed in academia and industry. High-Transmission Delay and Channel BER:In satellite networks, owing to the long-distance transmission between satellites of different layers, the latency of transmission across inter-satellite links will be considerably higher than the nodes in terrestrial networks. The link transmission delay between LEO satellites is 15-25 ms, while the transmission delay between MEO satellites is 40 to 60 ms. As the physical conditions of space networks are affected, noise greatly interferes with the modulation and error correction of communication signals during inter-satellite links, resulting in a much higher channel BER of satellite networks than terrestrial networks. Although such noise is less disruptive to the laser link, the link still requires a high level of satellite attitude control. In the communication process, the satellite attitude control will affect the communication quality of the satellite network. However, when calculating satellite routes with the dynamic routing algorithm onboard, it is extremely challenging to fully reflect the real-time state between satellite links through network state updates alone. Furthermore, the approach is prone to making each satellite node calculate routing tables with inaccurate network state information, leading to the waste of on-star computing resources. Restricted on-star Processing Power and Storage Resources: Constraints include the spatial environment and satellite loading technology, such as high ionizing radiation and low power consumption. The processing power and storage resources on the satellite are extremely limited. Hence, storing all the status information of the whole network on the star is impractical. Owing to the highly dynamic topology of the satellite network, obtaining link status information from the ground network system alone cannot guarantee accurate results. With the limited on-star computing power, the on-star routing algorithm must have low implementation complexity (namely, low computational complexity), communication costs, and storage costs. In addition, owing to the unique launch mode of the satellite, updating and upgrading its functions is hardly possible. Highly Dynamic Network Topology: LEO and MEO satellites operate at high speeds, resulting in a highly dynamic network topology.