> It means exactly what the documentation says it means... that the
> elevation is unknown, which is what you expect AFTER the signal has
> been picked up, but BEFORE it has been able to calculate it.
a) No documentataion confirms that.
Yes it does, and very specifically. It says that the valid range is +/-90, and "unknown if out of range".
-91 is outside of the range [-90, 90], thus -91 is "out of range", meaning that the elevation is UNKNOWN.
b) makes no sense to know the Azimuth and not Elevation.
Right, so if the elevation OR azimuth is out of range, then they BOTH should be considered UNKNOWN.