Atmospheric refraction

 Atmospheric refraction error is usually dealt with in two parts namely ionospheric and tropospheric 

The effects are substantially reduced by DGPS compared with single point positioning 

Computer figures are :

If refraction were identical at each end of a small baseline then the total effect would be cancelled when using DGPS 
The ionosphere is the region of the atmosphere from 50 to 1000 km in altitude in which ultraviolet radiation has ionized a fraction of the gas molecules thereby releasing free electrons 
GPS signals are slowed down and refracted from their true path when passing through this medium 
The effect on range measurement can vary from 5 to 150 m 
As the ionospheric effect is frequency dependent carrier wave measurement using the different L band frequencies i.e L l and L2 can be processed to eliminate sub stantially  the ionosphere error 
If the ionosphere were of constant thickness and electron density then remain 
Positional and temporal variation in the electron density makes complete elimination over longer baselines impossible and may require complex software modelling  
Ionosphere refraction effects vary with the solar cycle 
The cycle lasts about 11 years and was at a minimum in 2005 and will be at a maximum in 2011 therefore users will find increasing difficulty with GPS work during that period 
After 2011 of course the situation will improve until about 2016 when the cycle will start again 
Observations by night when the observer is not facing the sun are less affected 
The troposphere is even more variable than the ionosphere and is not frequency dependent 
However being closer to the ground the temperature pressure and humidity at the receiver can be easily measured and the integrated effect along the line of sight through the troposphere to the satellite modelled 
If conditions are identical at each of a baseline then its effect is completely eliminated 
Over longer baselines measurements can be taken and used in an appropriate model to reduce the error by as much as 95% 


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