Global Positioning System
Accuracy of GPS?
There are four basic levels of accuracy - or types of solutions - you can obtain with your real-time GPS mining system:
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Autonomous
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Accuracy
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15 - 100 meters
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Differential GPS (DGPS)
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Accuracy
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0.5 - 5 meters
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Real-Time Kinematic Float (RTK Float)
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Accuracy
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20cm - 1 meter
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Real-Time Kinematic Fixed (RTK Fixed)
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Accuracy
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1cm - 5 cm
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GPS satellites broadcast on three different frequencies, and each frequency (or career wave) has some information or codes on it. You can think of it as three different radio stations broadcasting several different programs. The table below lists the signals and the contents:
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L1 Career
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L2 Career
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L3 Career
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19 cm wavelength
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24 cm wavelength
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Data not available |
| 1575.42 M Hz
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1227.6 M Hz
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C/A Code
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P Code
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Navigation
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Navigation Message
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- P Code : Reserved for direct use only by the military
- C/A Code : Used for rougher positioning
- For Single frequency use only L1 career is used
- For Double frequency, L1/L2/L3 career is used
- The navigation message (usually referred to as the ephemeris) tells us where the satellites are located, in a special coordinate system called WGS-84. If you know where the satellites are at any given time, then you can compute your location here on earth.
Some Interesting Links :
- GPS Accuracy
How is the accuracy of GPS Receiver Described ? - By Chuck Gilbert, Road Measurement Data Acquisition System
Different types of answers given by a GPS.
Autonomous Positions
Uses……….. C/A code only
Requires….. Only one receiver
Data from at least four satellites
Provides…… An accuracy range of about 15 - 100 meters
This solution is designed for people who just need an approximate location on the earth, such as a boat at sea or a hiker in the mountains.
Real-Time Differential GPS (DGPS) Positions
Uses……….. C/A code only
Requires….. Two receivers
A radio link between the two receivers
- Reference receiver at a known location broadcasts RTCM (Radio Technical Commission for Maritime Services) corrections.
- Rover receiver applies corrections for improved GPS positions
Data from at least four satellites - the same four at both the references and rover (common satellites)