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.
What is GPS Interference?
GPS interference refers to the act of affecting the normal reception of GPS satellite signals by transmitting specific radio signals. Simply put, GPS satellites are about 20,200 kilometers away from the Earth's surface. When their signals reach the ground, the intensity is extremely low, only -158.5 dBW, which is akin to trying to hear a whisper from 100 meters away in a noisy square. Such a weak signal is easily disturbed by "noise."
Currently, common GPS interference is mainly divided into two categories:
- Jamming: This is the most direct and crude method. It involves emitting strong interference signals that cover the GPS operating frequency bands (L1: 1575.42MHz, L2: 1227.60MHz), preventing the receiver from identifying genuine satellite signals. It's like someone shouting in your ear, making it impossible to hear others speak. Jamming power can be as high as tens or even hundreds of watts, capable of disabling GPS devices within a range of tens of kilometers.
- Spoofing: This is a more covert and dangerous method. It transmits fake signals similar to GPS signals, misleading the receiver into generating incorrect position, velocity, and time information. It's like someone impersonating your friend to give you directions, leading you the wrong way. This interference technique is more complex but requires less power while being highly effective.
Technical Principles of Detecting GPS Interference
So, understanding the principles, how do we detect it? Generally, it can be achieved by looking at two key indicators: AGC (Automatic Gain Control) and C/N0 (Carrier-to-Noise Density Ratio).
Let's start with AGC. It is an important parameter in the GNSS radio module of a phone. When background environmental noise is low, the chip increases the gain to capture weak satellite signals. When there is strong interference (high energy) in the environment, the chip is forced to lower the gain to prevent circuit saturation. By monitoring the decibel (dB) value of the AGC level, if the AGC value drops suddenly and significantly, falling clearly below the device's baseline, the software may determine that interference is likely present.
C/N0 reflects the ratio of signal strength to noise. Normal satellite signals reaching the ground typically have a C/N0 within a reasonable range. If it's a spoofing signal emitted from a short distance, it often has excessive power to override the real signal, which can cause the C/N0 to increase instead.
Another commonly used method is data consistency checking, for example:
- Doppler Shift: The frequency shift caused by the satellite's high-speed motion must match the satellite's orbital information.
- Clock Drift: Changes in the receiver's clock bias should be continuous.
- Pseudorange Rate: If the position jumps drastically in an instant (e.g., from Beijing one second to New York the next), or if the calculated speed defies physical laws, the system will flag it as potential spoofing.
Tool Download and Usage
Currently, we have found a software tool developed by Google: GNSS Logger. It is recommended to install it via the Google Play Store. For users in China, you can try other app stores.
After installation, open the software, and there is a "Spoof/Jam" page.
Image Above: When the phone is placed close to a Wi-Fi router (red ellipse), both AGC and C/N0 drop. The average of the top 3 signals for each constellation and frequency band is shown with dashed lines. The AGC value (a single number) for each constellation and frequency band is shown with solid lines. The red circle in the chart illustrates the simultaneous drop in AGC and C/N0 when the phone is near a Wi-Fi router, indicating observable interference.
In the interference detection section, the application checks if the average C/N0 and AGC over the last 10 epochs have changed compared to the previous 50 epochs. If both C/N0 and AGC drop simultaneously, it may indicate GNSS interference. If this condition is detected, the card will display a FAIL message along with more detailed information.
Summary
Currently, we have only found one tool based on Android. It is relatively simple to use, but it involves a lot of fundamental mathematical principles related to GPS, which can be challenging to understand thoroughly. Even when looking at the raw data, someone like me without deep expertise might not immediately discern much; it's a process of searching and learning simultaneously.
On the iOS platform, it seems APIs for accessing raw, low-level GPS signal information are not currently exposed. Therefore, detection of GPS interference can currently only be done through some third-party tools, such as GPS Status, GNSS Monitor, etc. Due to time constraints, we have not tested these; interested friends can try them out.