Flytec 6040 * - Manual (Page 51)

Operation Manual FLYTEC 6040 GPS
49
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.
Moreover there is a better chance to find 2 satellites at 90° near the horizon,
than to find 3 satellites at 90° to each other directly above your position.
Because the circumference area at the horizon is bigger than the vertical area above the position, and
because it is assumed that the satellites are about equally dispersed, there is a better chance to find
convenient satellites near to the horizon.
The GPS module will therefore prefer satellites near the horizon for the horizontal position finding. The
module will calculate all possible combinations of visible satellites and makes every second an average
of these combinations.
In order to perform the same with the vertical position, the receiver would need twice the calculation
power. Due to the fact however that automobiles and pedestrians normally need the horizontal position,
this complexity is economised and calculation of altitude is performed from the best combination and by
the average value of time. That is the explanation for the smallish accuracy and the slow follow behind
movement of the vertical position.
The GPS module in the FLYTEC instruments makes no time filtering over the horizontal position, but at
good reception it performs a filtering of around 5 seconds in the vertical position. The poorer the GPS
satellite reception, the longer this time constant.
Zeitunterschied
Satellit 1
45°
This surface is
bigger as the
surface of the calotte
above
Kalotte
Satellite 1
Satellite 2
Almost no time difference
in Satelite 2
Position A Position B
Calotte