tailieunhanh - Mobile Robots Navigation 2008 Part 13

Tham khảo tài liệu 'mobile robots navigation 2008 part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 468 Mobile Robots Navigation al. 2001 if the error is significant enough to be of concern. Once calibration or estimation has removed the above errors 25 simplifies to 26 . Bb R T Bn w 26 GPS The Global Positioning System GPS is a global navigation satellite system GNSS which consists of an array of satellites that is used by a receiver to triangulate the receiver s position anywhere in the world depending on satellite availability Tsui 2005 . GPS operates on the principle that given a common time reference a range to a satellite can be calculated by the time difference between transmission from the satellite and reception by the receiver of a radio signal multiplied the speed of light. This measurement is called a pseudo-range PR as the measurement is a range calculated from a time difference rather than a direct range measurement. Using three satellite range measurements and positions pf. the three values that constitute a position in 3D space pe can be solved using simultaneous equations 27 . A common time reference does not exist however as the receiver only has a quartz crystal oscillator whereas each of the satellites have atomic clocks that are accurate to a few nanoseconds being periodically corrected by the GPS control segment. This problem can be solved by including a receiver clock offset Str into the triangulation problem 27 . Since four unknowns now exist a minimum number of four satellites are required for a position fix Tsui 2005 . pRi str i pe-pfiii PR2 Str pe p . 27 PRn Str pe -pfjl- The main sources of error in this system are Clock Inaccuracies Str - as mentioned previously there exists a clock bias due to clock drift that is compensated for in the positioning algorithm. Remaining inaccuracies in the clock can create ranging errors of up to two meters. Satellite Geometry - the apparent angle between satellites as perceived by the receiver can have a significant effect on the triangulation problem. If the angles are small any error in the .