## 4 Analysis of Position-Holding Strategy of Constellation Stations  
The strategy of constellation station holding can be divided into two types: absolute station holding and relative station holding. 1. Absolute control strategy. Absolute control means that each satellite in the constellation is controlled independently to keep it within a control range centered on the control reference satellite. The absolute control method is relatively mature and reliable. It is the same as the control method of a single satellite. It has a certain engineering application basis and is highly reliable for the constellation control system. The launch of multiple satellites with one launcher is considered an example. After the satellite is in orbit, the control of the initial orbit acquisition is preliminarily completed based on the nominal orbit. The absolute control strategy is based on the orbit of the nominal constellation as a reference. By controlling the semi-major axis, inclination, and right ascension of a single satellite one by one, the constellation configuration can be established within a specified time limit. Due to the influence of various perturbations, the satellite will gradually deviate from the originally designed orbit during orbit operation, the safe positions between the satellites in the constellation will change, and the overall structure of the constellation will be destroyed. It is necessary to maintain the long-term and periodic orbit of the constellation to ensure the integrity of the constellation's performance. Considering the measurement errors and onboard control errors in the ground measurement and control system, the absolute control strategy is to control each satellite one by one. By improving the measurement accuracy of the semi-major axis, the average orbit-control period can be increased, and the number of orbit-control tasks during the lifetime can be reduced, thereby reducing the configuration maintenance frequency. 2. Relative control strategy. Relative control is to control the constellation globally, rather than independently controlling each satellite. The control target also changes from the absolute position of each satellite to the relative position between satellites to keep the configuration of the whole constellation unchanged within certain precision requirements. Different from the absolute control strategy, the relative position-holding strategy requires a ground measurement and control system to conduct precise orbit determination and orbit prediction for the whole satellite network. Then, the reference orbit and control strategy of each satellite are calculated and combined with the relative control strategy. The relative control strategy occupies more ground satellite tracking telemetry and command (TT&C) resources, and the scheduling algorithm is more complex. At the same time, a relative control strategy requires high on-board autonomy. Considering Internet Constellation does not have the function of directly measuring the orbital parameters of adjacent satellites, it can only rely on the GNSS of the satellites to obtain precise orbital data and transmit it back to the ground control center through inter-satellite links and satellite-Earth mode. A relative control strategy synthesizes the orbital perturbation deviation of the whole network satellite and only corrects the parts that affect the relative geometric configuration of the constellation. The propellant consumption, orbital control frequency, and control period are more advantageous than those of the absolute control strategy, but the control process is relatively complicated and the reliability is low. 3. Comparison of absolute and relative control strategies. The absolute holding strategy has the advantages of simple control, high reliability, and mature control methods. Due to the pressure on the ground measurement and controlsystem, the absolute holding strategy requires fewer measurements and control resources.