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GDL90: traffic report convert GNSS altitude #755

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GDL90: traffic report convert GNSS altitude #755

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21 changes: 15 additions & 6 deletions main/traffic.go
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Original file line number Diff line number Diff line change
Expand Up @@ -315,15 +315,24 @@ func makeTrafficReportMsg(ti TrafficInfo) []byte {
//
// Algo example at: https://play.golang.org/p/VXCckSdsvT
//
var alt int16
if ti.Alt < -1000 || ti.Alt > 101350 {
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.

// 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.

} else {
baroAlt = ti.Alt
}
var encodedAlt int16
if baroAlt < -1000 || baroAlt > 101350 {
encodedAlt = 0x0FFF
} else {
// output guaranteed to be between 0x0000 and 0x0FFE
alt = int16((ti.Alt / 25) + 40)
encodedAlt = int16((baroAlt / 25) + 40)
}
msg[11] = byte((alt & 0xFF0) >> 4) // Altitude.
msg[12] = byte((alt & 0x00F) << 4)
msg[11] = byte((encodedAlt & 0xFF0) >> 4) // Altitude.
msg[12] = byte((encodedAlt & 0x00F) << 4)

// "m" field. Lower four bits define indicator bits:
// - - 0 0 "tt" (msg[17]) is not valid
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