Pressure altitude in CAN datagrams missing

[Simsys]

There is no CAN datagram that contains the pressure altitude

[degeflija]

Hi, pressure altitude depends on setting a reference pressure. Normally thats QNH, in exceptions QFE. The sensor only knows ambient pressure, but - at start up - does not have any reference.

The way I do that in the AD57 SW is (to save the pilot from doing this) :

• I keep a push down silo of values of amb pressure. Every second I push a value, the oldest one drops out.

• I observe GPS speed and air speed. When both are sufficiently high, I decide on take off.

• In that moment I pick the oldest amb pressure value from the stack (which is presumably not deteriorated by take off), and calculate

    g_ConfigData.QNH = QNH_at ( g_SensorData.GPS_Altitude,
                                g_SensorData.AmbPressure,
                                g_SensorData.GPS_Latitude
                       );
  

Formula can be derived from any textbook. Special caution has to be applied for cases of power outage in the air, which would create a fake takeoff. I maintain the last QNH and elelvation in EEPROM and I detect cases of fake takeoff. Then I start with EEPROM values. My little morcel of AI.
As a fallback, my AD57 SW allows for correcting QNH and start point elevation, yet only in flight, not on ground.

As far as I know, Klaus has not implemented anything like this in the sensor, for good reason, when you think about it.

BTW : Thats currently an issue with Uwe's OpenSoar. In OpenSoar you have to set QNH manually.

[Simsys]

Hi Horst, it is clear that the sensor box does not recognise reference pressures such as QNH or QFE. Nevertheless, the sensor box can output a barometric altitude with reference to a fixed pressure (1013.2 hPa, for example).

• The Flarm devices, which contain a pressure sensor, do something similar (see $PGRMZ, Flarm dataport protocol).
• Klaus also outputs a pressure-related altitude on the NMEA interface (see $PLARV, Larus NMEA protocol)

This data point is missing on the CAN bus interface