# GPS-Based Indoor/Outdoor Detection Scheme Using Machine Learning Techniques
## 3 Data Analysis  
### Global Positioning System  
The GPS is a global navigation satellite system that geolocation information to GPS receivers [16]. GPS is a satellite-based radio navigation system; consequently, solid obstacles, such as mountain and buildings, weaken and can even block GPS signals. The GPS does not require any transmission data from users, and its operation is independent from telephonic or internet signal. Currently, the GPS is owned by the United States' government and operated by the United States Air Force. The United States' government is responsible for creating and maintaining the system and makes it freely accessible to any GPS receiver [17]. The GPS calculates the receiver's position using the time and position of the satellite system. The satellites use stable atomic clocks that are synchronized all satellites and with the ground receivers. Any offset from GPS time of ground receiver is corrected daily. Similarly, satellite locations are controlled with great accuracy. Each GPS satellite continuously transmits a radio signal to provide the current time and its position; therefore, GPS sensors can receive signals without issuing a request. The distance between a satellite and a GPS receiver is calculated based on the delay time of the transmitted and received signals due to the constant speed of radio waves. A GPS receiver updates the distance of multiple satellites and determines the accurate position of the receiver and its deviation from real time. At least four satellites must be used to compute four unknown quantities and solve the accuracy equation [5]. The GPS satellite system comprises 24-32 satellites in medium Earth orbit. Originally, the GPS had only 24 satellites. In that configuration, each set of eight satellites were in three approximately circular orbits [18]. However, the system was modified to six orbital planes with only four satellites each (Figure 1). The orbital period is one-half a day (11 h and 58 min) so that the satellites pass over the same locations every day. The satellite orbits are carefully arranged so that at least six satellites are in line of sight from anywhere on the Earth's surface [5]. As of February 2019, there were 32 GPS satellites in orbit, and 27 satellites are in use [19]. Increasing the number of satellites improves GPS sensor calculations by providing redundant measurements, which leads to better position estimation. With the addition of redundancy satellites, the constellation was changed to a non-uniform arrangement. Compared to a uniform arrangement, a non-uniform arrangement dramatically improves the reliability and availability of the GPS system if multiple satellites malfunction [19]. At any given time, at least nine satellites can be observed from a single point on the ground, which is significantly more than the minimum four satellites required for accurate positioning. Other satellite navigation systems, including GLONASS (GLObal Navigation Satellite System) developed by Russia and GALILEO developed by Europe [20], can be used in the I/O detection applications as an alternative option besides the GPS. The combination of GPS and other navigation systems can also be a promising approach, since it increases the reliability in both navigation [21] and I/O detection. However, the requirement of I/O detection is different from navigation systems since it prefers the unification of satellites over the number of it. Using only GPS signal guarantees a united system, signal, and the number of satellites, that can be adopted in all mobile phones nowadays. The other reason is that I/O detection does not require as much information as the navigation system, and the redundancy of GPS satellites is enough for the application. In this study, we use two characteristics of raw GPS signals for I/O detection. The first is GPS signal redundancy. Assuming no occluding obstacles, approximately nine satellites can be observed from any one point on the ground. Consequently, GPS signals can always be received with desirable signal strength.