I have never understood why GPS service is free of charge.

From Baidu, let's take a brief look: GPS signals are broadcast, and the scientific name is passive positioning, just like our radio broadcast, except that the signals broadcast by his satellite are accurate time (using atomic clocks) and the position information of satellites, and GPS user machines also have very accurate time. Using the information broadcast by four satellites and the time when they received the signal, plus the distance in classical physics = time × speed (light speed), plus the deviation treatment of relativity, we can get our own position coordinates. But his signal is encrypted, the frequency of military and civilian is different, and the military encryption is stricter. The GPS user machine must decrypt this information. Not everyone can decrypt it, but solidified decryption software must be preset, so the price of GPS user machine is not only cost plus profit, but also a one-time payment for your access to the network. The manufacturer is authorized by the GPS operator and must pay the GPS operator. If you want to produce and sell it without paying the GPS operator, you will be sued for infringement of intellectual property rights. The other three are similar. Among them, the Beidou generation currently used in China is different from GPS positioning principle and has better encryption. For example, if the user's mobile phone is captured by the enemy, Beidou Center can disable that user's mobile phone, so that he can't solve the signal or even self-destruct the solidified software. This precise control ability is unique. Because the first generation of China used satellite radio measurement mode, although it is a bit backward, it is practical and safe. Beidou-2 will adopt GPS mode. You can trust me privately if you are interested. I just read three books on this subject during the winter vacation. In other words, A-GPS collects money from users and GPS collects money from A-GPS. Even companies that develop satellite positioning and navigation applications must pay GPS operators. The wool is on the sheep, and in the end, ordinary consumers have to pay the bill.