2 accuracy of gps altitude, Accuracy of gps altitude, Operation manual flytec 6040 gps – Flytec 6040 * User Manual
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Operation Manual FLYTEC 6040 GPS
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11.2.2 Accuracy of GPS Altitude
A good explanation on the GPS Altitude accuracy is found on the following website:
First of all the word accuracy has to be defined. On the website mentioned above you can read: “The
declaration of the accuracy used by Garmin for their GPS receivers leads sometimes to confusion.
What does it mean if the receiver states an accuracy of 4 m? This readout refers
to the so-called 50 % CEP (Circular Error Probable). This means that 50 % of all measurements are
expected to be within a radius of 4 m. On the other hand, it also means that 50 % of all measured
positions are outside of this radius.
Assuming a norm repartition, 95 % of all measured positions are within a circle of twice this
radius, i.e. 8m and 98.9 % are within a circle of 2.55-fold of the radius, that is in this case within a
radius of 10 m.
The calculation of GPS position is derived from a triangulation. The GPS receiver measures the time
needed by the signals and calculates over the speed of light the distance to the respective satellite.
With three satellites the horizontal position finding is determined, and with 4 satellites
the sterical position with altitude.
You may find a good explanation of the involved calculations on Wikipedia.
For the sterical position finding (3D position) 4 satellites are required. One of the satellites is used for
synchronisation.
In case of a 3D position finding, best accuracy is achieved if 3 satellites are in 90° to each other.
In this case the volume of the defined pyramid is the biggest. Again the DOP (Dilution Of Position) is
the inverse value of the volume of this pyramid.
Earth Radius
=6360km
Satellite Orbit Radius
=26560km
90°
4th satellite for time
synchronisation