What is GPS?(3 major segments)
The Global positioning system (GPS) is a space-based satellite navigation system that provides location and time information in all weather, anywhere on or near the earth, where there is an unobstructed line of sight to 4 or more GPS satellites. It is maintained my US government and accessible to anyone with a GPS receiver.
1) Space segment (空间部分）: 24 orbiting GPS satellites or Space Vehicles (SV), 6 orbital planes with 4 satellites each. About 6 satellites are visible from any point on the ground at any one time, ensuring considerable redundancy over the minimum four satellites needed for a position.
2) Control segment (控制部分）: consists of a master control station (MCS), an alternate master control station, four dedicated ground antennas, and six dedicated monitor stations.
3) User segment （用户部分）: GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock. They may also include a display for providing location and speed information to the user.**
How GPS works？
Trilateration: determine position or “fix”
Fourth satellite: solve the 4 unknowns (X,Y,Z and receiver clock error)
Disadvantages of GPS
Not apply for indoor environments.
The largest problem is multi-path effect!
Main localization system
- GPS
- cellular based
- Wi-Fi positioning
- Bluetooth positioning
- RFID chips embedded in the environment
2 basic steps of localization technologies
3 typical methods of localization
- ToA (Time of Arrival)
- TDoA (Time Difference of Arrival): it doesn’t need time synchronization (时间同步)
- Signal feature based localization: Not rely on special hardwares, but leverage（利用） the signal feature directly.
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