GPS answers 5 questions simultaneously:
Where am I?
Where am I going?
Where are you?
What’s the best way to get there?
When will I get there?

In essence, the GPS system is made up of a constellation of 24 satellites orbiting at a standard of 20,000 km. spotted on 6 orbital planes equally spaced. In order to determine the position of a GPS satellite three coordinates are needed: latitude, longitude, and altitude. This type of measurement is known as “pseudo-range” measurement in GPS terminology. But as in everything there is a problem when the information is coming to the GPS receiver.

Phenomenon affecting phase measurements precision

Ionospheric refraction
As stated by its name this takes place in the ionosphere which is an envelope around the world made up of ions which orbit at an altitude higher than20,000 km.

Tropospheric refraction 
As the ionosphere, the vapor covering the earth known as troposphere slows the process in which the information from the satellite get to the receiver.

GPS satellite orbit precision
Obviously if an error occurs in the satellite, this error will be mapped into the station position, GPS orbits can be very precise with a margin of error of only 200m.

Error correction
As is expected a series of errors occur in the satellite first the Ionosphere, then the troposphere and finally man-source errors, this last one was created by the US department of defense to avoid the rest of the people to be able to know the exact location

• Differential GPS: used by coast guard reduces the effects caused by selected Availability, this uses the signals of two different satellites, however if this satellites are close by about hundreds of kilometers the information will be wrong.
• Carrier-Phase GPS: the carrier can eliminate the problem better than the other forms of GPS but the frequency of this is much higher which causes different problems

The cost of having your own GPS satellite is about $30,000 USD. But the receivers are only in the range of $300 USD.