DocID025978 Rev 1
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AN4450
Software
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//=feet/3.280839895
*m = *ft/3.280839895;
}
Terminology is very delicate for altitude. There is not one definition of altitude:
1.
Absolute pressure sensor measures the air pressure at the sensing point. The base
rule of thumb is that the pressure drops by 1 hPa every 8.3 meters.
2. The “sea level pressure average” is 1013.25 hPa (standard altitude measurement done
in most digital barometer wrist watches)
3. The “instant sea level pressure” is weather and location dependent and can vary by
more than 5 hPa (compare SFO and SIN airports on the web)
4. Also beware that pressure altitude may be different than GPS altitude, which may also
be different than density altitude.
Aircraft flying between airports use the ISA altitude which is based on yearly/earth-wide sea
level pressure average value (as shown above) to avoid plane collision; this is the altimeter
reference.
When aircraft approach an airport, local equivalent sea level and airfield temperature are
wirelessly shared from airport to plane to compute local instant airfield altitude. This is
known as QNH, using METAR data.
6.5
SW filtering & internal FIFO
If higher precision is required, or if the air pressure/flow is unstable, a SW filter could be
implemented on the sensor measurements.
The internal FIFO buffer and HW filter can also be used, as indicated in previous sections;
this is strongly suggested since it reduces the level of pressure noise and in the meantime
decreases the power consumption.
For indoor navigation when sub meter detection is required, a special recursive filter could
be used. Tailoring the SW filter for the sensor characteristic and the application requirement
is usually recommended. Sensor fusion is the most advanced filtering which uses all
available information and uses them to reduce the positioning noise.
6.6
Reset the device to power-on configuration
If there are any doubts regarding the register content, there is the possibility to reset the
configuration of the device at the power-on condition setting both BOOT bit and SWRESET
bit in CTRL_REG2 (0x21).
The bit[2] SWRESET resets all the non-trimming registers to their reset values and stops
any internal state machines.
The bit[7] BOOT can be used, when the device is enabled, to reinitialize the volatile
registers with the content of the internal non-volatile trimming memory. These values are
factory trimmed and are different for every device. They permit good behavior of the device
and normally they should not be changed. If, for any reason, the content of the trimming
registers is modified, it is sufficient to use the BOOT bit to restore the correct values.
