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
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.
8.
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.
10.
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)
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
