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On the evening of December 17, 2025, a strange incident occurred in Nanjing: citizens found that the Baidu, Gaode, and Tencent maps on their phones all "went on strike," and even Apple's built-in navigation was not spared. Some people were located "swimming" in Xuanwu Lake, while others riding shared bikes were prompted to be 57 kilometers away... The official announcement stated that this was due to temporary interference with satellite signals, though the military frequency bands of BeiDou were unaffected.

In a previous article Navigating Without GPS: Exploring Alternative Positioning Technologies - MalaGIS, we introduced several navigation and positioning methods besides GPS and BeiDou systems. However, those methods are either require professional equipment, need specific hardware support in phones, or are technologies that are not yet ready for large-scale commercial application. So, some friends have asked: how can I test if my GPS/BeiDou signal is being interfered with in such a situation, or how can I determine the credibility of my signal? Today, we recommend a method and tool for those who need it.

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It has been some time since the GPS and BeiDou signal anomaly incident in Nanjing. While the cause of the incident remains debated, an interesting question has been frequently discussed in the 'Malagis' GIS group recently: If one day, both our GPS and BeiDou signals are abnormal, and we don't have access to military-grade bands, how can we determine our location? The author has previously written a bit on this topic. Drawing from the group members' discussions and collected materials, here is a summary. Corrections are welcome if any inaccuracies are found.

Before delving into this topic, let's introduce a new concept: All-source Positioning. In this era of the Internet of Everything, the heavily relied-upon GPS is not omnipotent. Satellite signals cannot penetrate thick reinforced concrete or travel underwater. All-source positioning involves integrating various other sensors to provide alternative solutions when GPS fails. So, what are the current common solutions? Let's explore them.

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