## 2 Literature Review  
In order to design the access and handover decision strategy of heterogeneous satellite networks, we need to pay attention to the heterogeneous satellite network architecture and handover algorithm. The network architectures of GEO/LEO heterogeneous satellite systems have received wide attention from researchers. In [13], the LEO satellites work as the relay satellites to help communicate between GEO satellites and the ground users. In [14], the LEO satellite networks are set as access layers and the GEO satellites constitute the backbone network as the data center for data storage, processing, routing, and network management. In [15], the GEO satellite forwards the control packages sent by the controller to LEO satellites. However, these architectures do not consider that users have to connect GEO satellites to communicate normally when LEO constellations do not have continuous global coverage. GEO satellites can also help balance the load of LEO constellation to reduce the communication termination of delay-sensitive services that must connect to LEO satellites.  
MADA strategies, which are often used in handover decision, can be classified into several groups, including the Simple Additive Weighting (SAW), the Analytic Hierarchy Process (AHP), the Grey Rational Analysis (GRA), the TOPSIS, etc. The SAW method is used in [16] to make the handover decision, this method is simple to implement but poor in accuracy [17]. In [18], the AHP method is used to select the network. The AHP method makes decisions on complex problems by dividing complex problems into a simple and easy-to-analyze decision factor hierarchy. However, AHP requires pairwise comparisons between attributes and the scoring of decision makers, so it needs the expertise of human beings [17]. GRA is also used in the network handover, and the networks can be ranked by calculating the grey relational coefficient [19]. The best solution selected by using the TOPSIS method is the one closest to the ideal solution and furthest from the worst solution [7].  
The TOPSIS method needs to set the weight of each attribute. The work of [20] uses AHP to define the weights of the criteria, which needs the subjective judgment of the decision maker in the process. In [21], the analytic network process (ANP) is used to set the weight of attributes. The structure of ANP is more diverse than that of AHP, and it can better describe the structure of complex systems, but it also requires pairwise comparisons between attributes and the scoring of decision makers. The entropy weight method can also be utilized in weight setting [22], which uses the information entropy of attributes. The smaller the information entropy of an attribute is, the larger the weight will be. However, the entropy weight method is lacking in reflecting the contribution and importance of a specific attribute to the target.  
Therefore, inspired by the above literature, considering the practical limitations, the two-step access and handover strategy is proposed in this paper, which is based on the background of a heterogeneous satellite network of small-scale LEO satellite constellation combined with GEO satellites, a modified TOPSIS method is used to design a handover decision strategy.