Flytec 6020 Operating Manual page 50

Almost no time difference
Satellite 2
Position A
The chance to find 2 satellites near the horizon being at 90° apart, is better than
to find 3 satellites with 90° to each other directly above the actual position.
Because the circumference area above the horizon is larger than the area perpendicular
above the position, and assuming that the satellites are nearly equally dispersed, there
is a bigger chance to find convenient satellites near the horizon. The GPS module will
therefore prefer satellites near the horizon. For determination of the horizontal position,
the module will calculate any combination of the visible satellites to make an average
of these combinations every second.
To do the same with the vertical position, the receiver would need twice the calculation
power. Due to the fact that motor vehicles and pedestrians normally require the horizontal
position, one saves the effort by computing altitude from the best combination and generates
the average by time. This is an explanation of the lower accuracy and the run after or delay
of the vertical position.
The GPS module of the Flytec instruments does not perform a timed filtering over the
horizontal position, but in case of good reception quality it does a filtering over approx. 5 s
for the vertical position. The poorer the GPS-reception, the longer this time constant.
Operating Manual Flytec 6020
Satellite 2
Position B
Calotte
45°
- 50 -
Time difference
Satellit 1
This circumference
area is larger than
the area of the
calotte above
Satellite 1