Errors and biasis
- 2. Global Positioning System • The global positioning system(gps), originally navstar GPS. • It is a satellite-based radio navigation system owned by the united states government and operated by the united states air force. • It is a global navigation satellite system that provides geolocation and time information to a gps receiver any where on or near the earth where there is an unobstructed line of sight to four or more gps satellites
- 3. Error for the Global Positioning System… Ephemeris error Clock stability ionospheric delay Tropospheric delay Multipath Satellite and receiver
- 4. Selective availability Anti spoofing Receiver noise Dilution of precision Satellite Orbits
- 6. 1. Ephemeris error • While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old. • Variability in solar radiation pressure has an indirect effect on GPS accuracy due to its effect on ephemeris errors. • they are the errors in the prediction of position • It is satellite dependant and it is difficult to correct and compensate
- 7. 2. clock errors • Time standards are very accurate to 3 nano seconds • The satellites' atomic clocks experience noise and clock drift errors. • The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock.. • These problems tend to be very small, but may add up to a few meters (tens of feet) of inaccuracy.
- 9. 3. Ionospheric delay • Ionospehre is the first part of the atmosphere which extends up to 50-100 kms from the earth surface which is highly charged environment. • When gps signals which are electromagnetic radiations passes through ionosphere it get reflected and refracted.
- 11. 4. Tropospheric delay • The troposphere is that part of the atmosphere closest to the earth. It extends from the surface to about 50 km. • The troposphere is part of the electrically neutral layer of the earth’s atmosphere • The troposphere is also non dispersive for frequencies below 30 GHz or so. Therefore L1, L2, and L5 are equally refracted. This means that the range between a receiver and a satellite will be shown to be a bit longer than it actually .
- 12. • density affects the severity of the delay of the GPS signal as it travels through the troposphere.
- 13. 5. Multi-path error • As the GPS signal finally arrives at the earth’s surface, it may be reflected by local obstructions before it gets to the receiver’s antenna. This is called multi-path error • In this signal is reaching the antenna in single line path as well as delayed path. The effect is similar to a double image on a tv set. • The multipath effect is caused by reflection of satellite signals (radio waves) on objects. • For GPS signals this effect mainly appears in the neighborhood of large buildings or other elevations.
- 14. • The reflected signal takes more time to reach the receiver than the direct signal. • The resulting error typically lies in the range of a few meters.
- 15. 6.Satellite and receiver • Gps receiver used ordinary quartz crystal clock since it is economical. • Sometimes draft slightly and casus small errors affects the accuracy of the position
- 16. 7.Selective Availability• Selective availability (SA) was the intentional error introduced by DoD to make sure that no hostile forces used the accuracy of GPS against the US or its allies. • On May 1st, 2000, the White House announced a decision to discontinue the intentional degradation of the GPS signals to the public. • Civilian users of GPS will be able to pinpoint locations up to ten times more accurately.
- 17. 8.Anti-spoofing • This encrypts the P-code so that it cannot be mimicked by a transmitter sending false information. • Few civilian receivers have ever used the P-code, and the accuracy attainable with the public C/A code was much better than originally expected. • Anti spoof policy has relatively little effect on most civilian users. • Turning off anti spoof would primarily benefit surveyors and some scientists who need extremely precise positions for experiments such as tracking tectonic plate motion.
- 18. 9.Receiver noise • It includes variety of errors in the associated with ability of gps receivers to measure a finite time difference .
- 19. 10.Dilution of precision • DOP, or geometric dilution of precision (GDOP), is a term used in satellite navigation and geomatics engineering to specify the additional multiplicative effect of navigation satellite geometry on positional measurement precision. • The concept of dilution of precision (DOP)
- 20. • Navigation satellites with poor geometry for Geometric Dilution of Precision (GDOP).
- 21. 11.Satellite Orbits • Slight shifts of the orbits are possible due to gravitation forces • Sun and moon have a weak influence on the orbits • The resulting error being not more than 2 m
- 22. Ideal satellite geometry Poor satellite geometry
- 23. Thank you