EESC 3750 - Geographic Information Systems: GPS

Are these GPS, or GPS receivers?

Finding your Location with the Global Positioning System

A GPS is composed of three segments:

1. THE SPACE SEGMENT which are satellites arranged in a constellation around the Earth to provide coverage of the Earth’s surface.  The satellites broadcast signals to Earth via high frequency radio waves with information about where the satellite is located and the time of transmission.
    
2. THE CONTROL SEGMENT is a the network of ground stations worldwide that track and monitor, and ensure accuracy of the satellite signals.
    
3. THE USER SEGMENT is the GPS receiver that receives the signal, such as what you find in a car or mobile phone. How the location is calculated. You need at least information from at least 3 satellites to determine a position on the Earth’s surface. The process of finding a location on a sphere (the Earth) using three reference points is called 3D trilateration.

Accuracy of GPS

Why aren’t GPS measurements always accurate?

Before 2000, the US military intentionally degraded the timing and position information transmitted by satellites as a missile defense precaution, reducing accuracy to of GPS receivers to 100m, however, that is no longer the case. Handheld receivers still average 10m-15m horizontal error. What is the source of this error?

Some errors occur in the space segment, such as ephemeris errors. These are error in reporting the location of the satellite. This kind of data can be identified and corrected by a ground control station.  This can account for about 2 m of horizontal error.

The arrangement of satellites can also have an effect on the accuracy of a signal. Error incurred by a poor arrangement of satellites is known as Position Dilution of Precision, or PDOP.  As a rule of thumb, the further the satellites are from each other, and the wider the angle between them, the more accurate the measurement can be.

Atmospheric interference is another source of error.  Interference in the ionosphere and troposphere can cause delays in signal transmission to the ground.  Ionosphere can add 5 or more m of error, and water vapor interference in the troposphere can add another .5m of error.

In addition, anything that blocks the signal from the sky, (like a building or a tree canopy) can interfere with signal reception.  And sometimes signals bounce or are reflected off of objects, instead of making a direct path to the receiver.  This is called the multipath effect- the error caused by a delay in the signal due to reflecting from surfaces before reaching the receiver.

The Control Segment plays a role in improving the accuracy of GPS position measurements. DGPS- Differential GPS is a method using a ground based correction in addition to the satellite signals in position determination. It works this way: since the base station has known coordinates, it can calculate a correction whenever it receives an inaccurate position from the satellite data. 

With DGPS, error can be reduced to 5m or less.

QUESTION: Which is more accurate? A $100 field GPS, or the GPS in your mobile phone?