Main Sources of Errors in GPS
List and describe the main sources of errors in the determined position of GPS.
The main sources of errors in GPS:
- ionosphere and troposphere delay
- users clock error
- satellite clock error
- Satellite geometry
- multipath error
- Satellite orbits
- SA(selective availability)
Ionospheric and tropospheric delay:
When radio waves radiated from satellite passes through different layers of the ionosphere and
troposphere the velocity of wave very which affects the time difference measurement and the fix will
not be accurate. For this a dual frequency receiver receives both of The frequencies and correction is calculated and
compensated with the receiver. For low elevation of satellite, radio waves is affected by refraction. So, receiver is designed to select
satellites whose elevation is more than 9.5 degrees.
Users clock error:
If the users clock is not perfectly synchronized with the satellite clock the range measurements will not
be accurate. This error can be eliminated within the receiver by obtaining the pseudo range from 3
satellites automatically.
Satellite clock error:
This error is caused due to error in the satellite clock with respect to GPS time. This is monitored from
ground based control segments and eliminated.
Satellite geometry:
The geometry of the position of the satellites determine the angle of cut of the position lines. As the
angular separation between the satellites are higher, more accurate is the fix.
To indicate the satellite geometry, the DOP (dilution of precision ) values are used.In DOP calculations
the following different varients are used.
-GDOP(geometric dilution of precision)
-PDOP(positional dilution of precision)
-HDOP(horizontal dilution of precision)
-VDOP(vertical dilution of precision)
-TDOP (time dilution of precision)
GPS receiver is promulgated to select the best available satellites, considering their elevation and
geometry.
Multipath error:
This error is caused by satellite signal arriving at the ships antenna both directly from the satellite and
having being reflected by some other object causing in accuracy in range measurement, sitting the
antenna at a suitable place can minimise this error.
Satellite orbits:
Although satellites are very precisely placed in orbits, slight shifts are possible due to gravitation
forces.Sun & moon have waek influence over the satellite orbits.how ever this is monitored from the
ground control segment of the satelliye & corrections applied as necessary. The error resulting is never
more than 2 meters
SA(selective availability):
SA if the international degradation of SPS(satellite positioning system)SA is controlled by the United
States department of Defence to limit accuracy for non us military and government users. The potential
accuracy of C/A code around 10M-30 M is reduced to 100M-150 M.Mean while it is permanently
deactivated due to broad distribution & worldwide use of GPS.