## 3 AIS and Small-Satellite Constellation Configurations  
Small-satellite constellations allow for the further exploitation of space, and the vastness of space makes small-satellite constellations flexible in deployment. Currently, Starlink itself has more than 3000 satellites in orbit, which can provide network connectivity around the world, including ships operating at sea. This also provides the opportunity to perform ionospheric observation over sea areas. By using the AIS signals received by small-satellite constellations, we can calculate the STEC on the signal rays, and in the lanes with dense ships, we can reconstruct the electron density by applying the above STECs, which can provide data support for global electron density modeling and the design of maritime communication links. The global distribution of transient vessels is shown in Figure 2.  
Global ship distribution is shown in Figure 2, and it can be seen that in the Atlantic region, ships are most densely distributed between the Americas, Europe and Africa. This is followed by the shipping lanes of the Indian Ocean, which surround the Malaysian Islands and the vicinity of Australia. Because the Pacific Ocean is so vast, ships sailing in it exhibit a distinct distribution along shipping lanes. In addition to the offshore areas, there is also a denser distribution of ships in the shoreline harbors, which can provide better imaging opportunities. The region we chose for the electron density inversion is located in the Indian Ocean, with a latitude range of 1.1-5.1\({}^{\circ}\) N and a longitude range of 82.2-86.2\({}^{\circ}\) E. The resolutions in both latitude and longitude directions are 0.25\({}^{\circ}\). For small-satellite constellations, taking Starlink as an example, its real-time satellite latitude and longitude positions are shown in Figure 3. The black dots in the figure show the real-time positions of the satellites, from which it can be seen that Starlink is densely distributed between the north mid-latitude and south latitude, which can well cover the densely distributed areas of the ships in Figure 2, providing more effective rays for the layered 3D reconstruction of ionospheric electron density. In this paper, we use a small-satellite constellation containing four satellites to perform electron density inversion for the above maritime regions. The four satellites are distributed at different orbits to obtain more access rays, and the four satellites are distributed equidistantly for a more even distribution of signal rays. We set the orbit altitude of the small satellites to about 1170 km, referring to the orbit height of Starlink satellites, and the rays can pass through the whole ionospheric region below 1000 km. A small-satellite constellation is a distributed-satellite system, and we set the AIS receivers on a small-satellite constellation with four small satellites to perform CIT (Figure 4). Figure 3: Real-time latitude and longitude positions of the Starlink satellites (the black dots are the real-time positions of the satellites, and the green grids indicate the covering areas by internet service provided by the satellites) [(29)]. Figure 2: Global real-time vessel distribution map [(28)]. The electron density reconstruction altitude range is set to be 100-1000 km with an altitude resolution of 25 km because the ionosphere electron distributed mainly in this altitude range. We counted more than 100 ships in the 6\({}^{\circ}\times\) 6\({}^{\circ}\) latitude-longitude range in the dense ship lanes in the real-time ship distribution maps. Thus, we assume there are 100 shipborne AIS transmitters in the projection ocean area of the reconstruction area in the latitude-longitude plane. The STECs along the AIS four small-satellite signal rays are used to perform the electron density reconstruction. We simulated and reconstructed the electron density distributions at 00:00, 06:00, and 12:00 in this area with such a distribution of receivers and ships, and we analyzed the performance of CIT over the seas.