[The troublesome thing about inaccurate positioning series] Part 1: Talking about the position interference of satellite positioning

The life of modern people is inseparable from mobile phones and maps. Once encountering the problem of inaccurate positioning, many problems will arise, and some of them will be very serious. The sad thing is that it is possible to run in circles at the high speed while driving, and it is possible to run an extra 100 kilometers.

Everyone takes accurate positioning as a matter of course. Now that the positioning technology is so developed, and the Beidou 3rd generation network has been successfully established, how can the positioning be inaccurate? It is true that with the advancement of time, the accuracy of positioning is getting higher and higher, but there is a potential risk, which will lead to the decline of positioning accuracy.

This risk comes from the proliferation of satellite positioning spoofing techniques . Satellite positioning spoofing, or position jamming, is to broadcast positioning signals through satellite pseudo base stations on the ground, so that surrounding positioning devices mistakenly believe that they have received real positioning signals, so that the positioning result is set to a position preset by the interferer. Before, we heard more and more that some people use pseudo communication base stations to send spam text messages in groups, but today, location interference is becoming more and more common, such as:

• the driving test : Driving school learning will require students to practice in the car, and there are some devices in the car that will record the students' practice tracks as evidence that the students have completed their studies, but some students do not want to spend time learning to drive, so some people will use fake The base station broadcasts a fake signal, allowing the vehicle recorder's positioning point to automatically drive around the driving school road, even if the device is far away from the driving school. In this way, students do not need to spend 1 minute learning to drive to achieve the required learning time.
• Remote : Some companies will count employees’ working hours based on their trajectories, and a few employees may fake their own locations.
• Taxi swiping : Some drivers use cheating methods to modify their positioning points to make the platform think that they are at a specified location or have completed a certain order in order to obtain a reward for receiving orders or picking orders.
• UAV countermeasures : Some companies do not want UAVs to be close to their premises, so they interfere with the location of UAVs, making them mistakenly think that they enter the no-fly zone and automatically land, or deviate from the route.
• Anti-tracking : Beidou positioning devices are installed on operating vehicles. Some enterprises or individuals do not want their real location to be discovered, so they conduct location interference.

There are many methods of positioning forgery, some only affect a single device (such as directly modifying the positioning result on the device), while the method of satellite pseudo base station is a relatively high-tech method. Interference, the specific impact range is related to the transmit power of the base station. A low-power base station only affects a range of a few meters nearby, while a high-power base station can affect the surrounding tens of kilometers, even when the satellite signal cannot be received indoors. be disturbed.

This interference can locate the anchor point to any position, and the movement speed and direction can be specified. The user may then find that their positioning point is thousands of kilometers away, or the trajectory becomes a strange shape, such as drawing a circle or driving along a certain road.

What is the principle of location jamming technology? Simply put, satellite positioning is to receive the satellite signal, read the signal message, obtain the time stamp, then subtract the time stamp and the local time to calculate the distance to the satellite, and then solve a 4-element equation system (x, y) , the z coordinate and the local clock offset).

Since the specifications of the satellite signal are public, we can fill any time into the signal message, and then transmit it through a specific carrier signal, so that other devices can obtain the wrong positioning signal. The easiest way is to directly record the signal at one location and play it back at another location. To locate a specified location, you only need to use the location to calculate the timestamp broadcast by each star. And now someone has encapsulated the entire series of steps of message writing, carrier modulation, and signal transmission, and made them into shelf products. Anyone who buys the device can input the target latitude, longitude, speed direction, or even the entire trajectory, and then position interference.

For example, HackRF is a device that can transmit custom signals. There are some open source software that can generate GNSS soft signals, and then transmit them through HackRF, which becomes a pseudo base station. Moreover, one device can transmit multiple signals simultaneously, simulating multiple satellites.

Why are satellite signals so easily jammed? Because GPS was not very strict in security at the beginning of its design, the packets are not encrypted at all, and the user side is not authenticated, so it is completely open.

The subsequent Galileo, GLONAZ, and Beidou also follow the same design concept, and the message and modulation process are similar and will be disturbed. The better security method is the military code. This kind of message is transmitted at a special frequency, and its message specification and encoding method are kept secret. To read the message, a special key is required, so it is difficult to crack or interfere. , but this method cannot solve the security problem of civilian equipment.

At present, some chip manufacturers, as well as AutoNavi, are doing some work to find this kind of interference, so as to avoid affecting users. Specific methods include:

1. Carrier check : When the true signal and the false signal appear at the same time, you can check whether the signal strength is abnormal (the false signal is usually strong enough to cover the true signal).

2. Signal direction check : Detect the source direction of the signal, if it is not from the sky, it is abnormal (requires the antenna).

3. Message consistency check : Check whether each field of the message is consistent with the real signal.

4. Solving verification : After solving, it is found that the distance of some satellites is abnormal, such as receiving true signals and false signals at the same time.

5. Multi-source verification : Combine inertial navigation, network positioning and other positioning methods to verify whether the results of satellite positioning are abnormal.

The above methods can reduce the probability of being interfered to a certain extent, but they cannot be completely eliminated. For ordinary users, when finds that their positioning results are abnormal, how to find out that it is caused by satellite signal interference?

1. First of all, see if your positioning results appear in places such as airports, oil depots, driving schools, or if there are oil depots or some sensitive facilities near your location.

2. Then, confirm that the positioning result of your device comes from satellite positioning instead of network positioning. If it is an Android user, you can download a AndroiTS GPS Test software. If Fix or ON is displayed, it means that the satellite positioning is successful, otherwise it is network positioning.

For iPhone, download phyphox , enter the GPS page, if the speed is negative, it means network positioning, otherwise it is satellite positioning.

3. In addition, when it is clear that the mobile phone positioning method is satellite positioning, if multiple mobile phones around are located in the wrong position (at least 200 meters away, if the deviation is not far away, the satellite positioning point may drift. ), there is a high probability that this place is interfered by a surrounding pseudolite base station.

Some interference may not be the original intention of the interferer. He may only want to control a small range of equipment, but the antenna power setting is unreasonable, and the signal is spread to a long distance.

It is clear that there is interference, how to solve it? You can only seek help from the government department, can call the local radio management department and ask for investigation. Radio management departments have special equipment that can detect fake base station signals, determine the location, and shut down the equipment. Everyone is welcome to report illegal radio interference.

Please refer to the specific telephone number

https://www.miit.gov.cn/gzcy/cydh/art/2020/art_ba58d28194884d66b0032861a11e0bd2.html