A、The first GPS satellite was launched in 1947. B、Old satellites will be replaced after five years. C、It is operated by the Unit

　　GPS stands for the Global Positioning System. (16) It refers to a system of satellites and receivers that allows people and devices to detect their precise location on earth. The heart of the system relies on 24 satellites that orbit the earth twice per day. Devices that are equipped with GPS equipment receive transmissions from at least a few of the satellites and are able to collect very precise positioning data. The first GPS satellite was launched in 1974 and the 24th was launched in 1994. (17) The system is operated by the United States Department of Defense and the use of the system is free for anyone. New satellites are periodically launched to replace aging ones. As the technology has improved, the cost of GPS devices has greatly declined while the accuracy has increased. Small portable GPS receivers have become very affordable, and the accuracy is amazing. Accuracy varies based on various factors, but it can be as good as a few meters! Land-based supplementary devices can be used to improve accuracy if higher precision is required. (18) The application of the GPS is very broad, and as the prices come down the number of users is increasing. Portable GPS devices are used by fishermen to help them sail around the world. Many new cars are being equipped with GPS systems to help drivers find directions. The military uses GPS to guide cruise missiles to pre-specified targets.
　　16.  What does GPS refer to?
　　17.  What do we know about GPS according to the passage?
　　18.  Why is the use of GPS increasing?