Errors and biasis
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The document discusses various sources of errors and biases in the Global Positioning System (GPS), including ephemeris errors, clock stability, and multipath effects. It explains the mechanisms behind these inaccuracies, such as atmospheric delays and receiver noise. Additionally, it covers historical aspects like selective availability and anti-spoofing measures that impact GPS accuracy and user experience.
Errors and biasis
- 1. ERRORS AND BIASIS PRESENTED BY AGLAIA
- 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 Selective availability Anti spoofing Receiver noise Dilution of precision Satellite Orbits
- 5. 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
- 6. 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.
- 8. 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.
- 10. 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 .
- 11. ● density affects the severity of the delay of the GPS signal as it travels through the troposphere.
- 12. 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.
- 13. • 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.
- 14. 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
- 15. 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.
- 16. 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.
- 17. 9.Receiver noise ● It includes variety of errors in the associated with ability of gps receivers to measure a finite time difference .
- 18. 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) originated with users of the Loran-C navigation system. The idea of Geometric DOP is to state how errors in the measurement will affect the final state estimation.
- 19. ● Navigation satellites with poor geometry for Geometric Dilution of Precision (GDOP).
- 20. 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
- 21. Ideal satellite geometry Poor satellite geometry
- 22. Thank you For more…. Mail to aglaiaconnect2018@gmail.com For more…. Mail to aglaiaconnect2018@gmail.com