# AoI Analysis of Satellite-UAV Synergy Real-Time Remote Sensing System
## 1 Introduction
On the other hand, while the initial studies primarily concentrated on the AoI in single-source status update systems, the research has also started to pay attention to the AoI in multi-source update systems as the field has advanced. In Refs. [26; 27; 28; 29; 30], the authors analyzed the AoI in a multi-source and single-server system where each source \(i\) generates updates as a rate \(\lambda_{i}\) Poisson process and the expected value of the service time for updates is \(1/\mu\). The authors of [27] derived the expressions of the average AoI and the average peak AoI of each source in the M/G/1/1 queuing system. The authors of [26] proposed a modified SHS approach to analyze the AoI and derived the average AoI of status information from all sources in the multi-source _LCFS_ M/M/1 lossy systems. In Ref. [29], the authors analyzed the AoI of the status information from all sources in the multi-source FCFS M/M/1 lossless systems by employing the SHS approach. In Ref. [30], based on the SHS approach, the authors investigated the AoI of the prioritized stream in a multi-source _LCFS_ M/M/1 lossy system. In the studies described above, the correlation between multiple sources was not considered. Note that the information sources in the satellite-UAV \"multiscale explanation\" synergy sensing scenario are correlated. More specifically, most of the studies are based on the assumption that the statuses observed and updated by the sources are independent and without common observations. As the research progressed, the researchers increasingly focused on exploring the AoI in status update systems with multiple correlated sources. In Refs. [7; 31; 32; 33], the authors investigated the AoI in correlated-source status update systems. In the systems under consideration, multiple IoT devices are deployed as sources to monitor a common objective from multiple perspectives, revealing correlations among the sources. In Ref. [7], the authors assumed that sources transmit status information to the receiver over orthogonal frequency channels and then researched transmission scheduling policies for sources to optimize AoI in the fused status information. In Ref. [32], the authors contributed an AoI minimization algorithm applicable to the frequency division multiple access (FDMA) strategy. In Ref. [33], the authors investigated an AoI optimization problem under the orthogonal time slot assumption. The correlated sources update models considered in these studies are not suitable for satellite-UAV \"multiscale explanation\" synergy sensing scenario. Additionally, considering the spectrum constraints, the full frequency reuse (FFR) strategy is widely adopted in SAGIN. Therefore, the orthogonal channel assumptions in these studies are not suitable for the communication-resource-constrained SAGIN. The existing studies lack the performance analysis of AoI in a satellite-UAV \"multiscale explanation\" synergy remote sensing system. In this paper, the satellite and UAV \"multiscale explanation\" synergy sensing scenario is considered. Considering the freshness of information, accurately determining the AoI of fused status information in the scenario is significant. Unfortunately, the research into the AoI of fused status information in SAGIN is lacking in the previous works. Additionally, there is a research gap in the AoI analysis of fused status information in the correlated-sources status update system. Therefore, it is challenging to accurately determine the AoI of fused status information in the satellite-UAV \"multiscale explanation\" synergy remote sensing and monitoring scenario. To meet the above challenges, in this article, we derive the closed-form expressions for AoI and analyze the performance of AoI in the satellite-UAV synergy remote sensing scenario. The remaining part of the paper is organized as follows. In Section 2, the scenario, network model, and the AoI analysis model are described. Then, the queuing disciplines considered in this paper are presented.