GDL90: traffic report convert GNSS altitude#755
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GPL90 traffic reports expect barometric altitude. Convert GNSS altitudes in traffic information to barometric altitude. Estimate barometric altitude from reported GPS geoid height, own geoid separation and barometric pressure. Signed-off-by: PepperJo <pepperjo@japf.ch>
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ping |
| alt = 0x0FFF | ||
| // GDL90 expects barometric altitude in traffic reports | ||
| var baroAlt int32 | ||
| if ti.AltIsGNSS { |
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&& isGPSValid() && isTempPressValid().
Not all Stratux builds have GPS units and/or pressure sensors.
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Right. Should be discussed what to do then since the reported traffic altitude could be quite a bit off if we fall back to not adjusting it at all.
| if ti.AltIsGNSS { | ||
| // Convert from GPS geoid height to barometric altitude | ||
| baroAlt = ti.Alt - int32(mySituation.GPSGeoidSep) | ||
| baroAlt = baroAlt - int32(mySituation.GPSAltitudeMSL) + int32(mySituation.BaroPressureAltitude) |
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baroAlt += -mySituation.GPSGeoidSep - mySituation.GPSAltitudeMSL + mySituation.BaroPressureAltitude
simplifies to
baroAlt += -mySituation.GPSHeightAboveEllipsoid + mySituation.BaroPressureAltitude
Seems like an interesting estimation to convert HAE to pressure altitude using local data.
One thing that is missed here is how data is passed to stratux from dump1090:
https://github.com/stratux/dump1090/blob/master/net_io.c#L569-L582
HAE delta is received and passed over as GnssDiffFromBaroAlt.
I think this PR could be rolled up into a one line change on stratux/dump1090:
https://github.com/stratux/dump1090/blob/13d61a9a3276de26322b88c99a7426a5625f592b/net_io.c#L575
Removing the Modes.use_hae condition (seems unexpected to only pass the GNSS-derived altitude when the delta is available).
Have you seen many aircraft that come through with AltIsGNSS being true?
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Haven't seen any ADS-B traffic where this is set however the FLARM forks use this since FLARM reports GPS altitude. The flag was already there but was not handled. I propose we either handle this in the FLARM code and remove the flag and make clear that altitude is barometric here or we convert it.
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What's the verdict on this pull request? Do we want it or do we rely on every module feeding data to do their own conversion? If so we should at least clarify that the AltIsGNSS does not do any conversion but sends the data as is via GDL90 which expects baro alt. Or remove the flag as suggested above. |
2 participants
GPL90 traffic reports expect barometric altitude. Convert
GNSS altitudes in traffic information to barometric altitude.
Estimate barometric altitude from reported GPS geoid height,
own geoid separation and barometric pressure.
Signed-off-by: PepperJo pepperjo@japf.ch