GPS

(i) Atmospheric error 

• Due to different atmospheric conditions in earth’s different layer, the speed of GPS signal is affected which causes difference in time taken and hence fix will not be accurate.
• Each signal transmits its signal in two frequency (L1 & L2) and hence a dual frequency receiver receives both the frequency & error is calculated and compensated within the receiver which improve the accuracy of the fix.
• The atmospheric error is minimum when satellite is directly overhead and is maximum when satellite is near the horizon.

(ii) User clock error 

• If the receiver clock is not precisely synchronised with satellite clock, the range calculated will have some error. This is known as pseudo (false) range.
• This error is eliminated by obtaining range from minimum three satellite, which is done automatically within the receiver.

(iii) Satellite clock error 

• This error is caused due to difference in satellite clock with respect to GPS time.
• Error in satellite’s clock is monitored by ground based station which passes the information to MASTER control station who then upload the Clock data to the satellite as part of the navigation message (1st sub frame).

(iv) GDOP error 

• The angular separation between satellites determine the accuracy of fix. Wider the angular separation between satellites, the better is the accuracy of the fix.
• Knowing the position of the satellite from almanac data of navigation message (5th sub frame), the receiver automatically select the suitable satellite based on their geometry and elevation.
• GDOP values are displayed on the receiver unit.

(v) Multipath error 

• This error is caused when signal received indirectly from some obstruction like buildings, walls, beacons etc.
• When both direct and indirect signal receives at slightly different time, both the signal mixes, cause distortion of signal, thus leading to error.
• Positioning of antenna at the right place will eliminate this error.

(vi) Orbital error 

• The satellites are monitored by the ground station and their paths are predicted. However, some might deviate from their predicted path causes small error in position.

• Dilution of precision (DOP),or, Geometric dilution of precision (GDOP) is a term used in satellite navigation and geomatics engineering. It is a value of geometrical effect on GPS accuracy.
• The angular separation between satellites determine the accuracy of fix. Wider the angular separation between satellites, the better is the accuracy of fix.
• If the satellites are clustered together at the time of measurement, the sphere formed from different satellite is much close to as a single sphere. So the precision will be diluted and DOP value will be high. The higher the DOP, lesser the accuracy of the fix.
• If the satellites are widely spaced, the sphere formed from different satellite are separated by good distance, so the dilution of precision will be low. The lower the DOP, greater the accuracy of fix.
• Modern receivers automatically select the suitable satellite for position fixing.

• Horizontal DOP (HDOP) is the error in the horizontal plane or 2-D fix (Lat,long)

Precise Positioning Service (PPS):-

• It is positioning and timing service which have authorized access and is used by military.
• It modulates on both L1 and L2 frequencies.
• The L1 frequency transmitted by all satellites contain a C/A code ranging signal with navigational data message which are available for civil use; & P-code ranging signal with navigational data message which are reserved for US military use.
• The L2 frequency consist of any P-code and is reserved for military use.
• The positional accuracy is much higher for security point of view.

Standard Positioning Service (SPS):-

• It is positioning and timing service provided for civil use.
• It modulates on L1 frequency only.
• The L1 frequency transmitted by all satellites contain a C/A code ranging signal with navigational data message which are available for civil use.
• The positional accuracy is not higher in compare of PPS.
• All the civil users worldwide use SPS.

• A DGPS is used to enhance the accuracy of a normal GPS.
• The DGPS reference station is situated at a fixed location and from this position, the GPS receiver obtain data from all the satellite within its range.
• As the reference stations knows their precise geographical location, so they compare it with position received from GPS and computes the correction.
• These corrections are then broadcast to GPS users to improve their position accuracy.

• A GPS receiver must lock-on to four satellite to get a 3-D fixed (Lat, long, altitude) in which three satellites are used to get position and altitude, and the fourth satellite is used to synchronize the receiver clock and the satellite atomic clock.
• For navigation, we require a 2-D fix (Lat,Long) and for that minimum of three satellite in which two satellites are used to calculate the position and the third satellite is used to synchronize the receiver clock and the satellite atomic clock.
• If a receiver picks up a signal from a satellite and receiver knows that it is 20,000 km away, that means receiver must be somewhere on a sphere of radius 20,000 km, having that satellite as a center.
• With two signals from two different satellite, the receiver must be somewhere where to spheres meet.
• Three signals puts the receiver at one point and confirms the receiver position. So the receiver need minimum of three satellite to calculate its position.