geometricverticalaccuracy

Hi,

I am working on application that uses the geometricverticalaccuracy parameter from impala's operational_status_data4 table.

I assume geometricverticalaccuracy corresponds to the GVA parameter in ADS-B v2, encoded on 2 bits as follows:

0 - Unknown or > 150 meters
1 - ≤ 150 meters
2 - ≤ 45 meters

It seems however that historical data systematically contains "-1" values in geometricverticalaccuracy column, at least for aircraft with a good position fix (positionnac, i.e. NACp = 10).

I have only seen once a different value, namely "150", for an aircraft with a slightly degraded position fix (NACp = .

Should I assume that "150" correctly represents GVA = 1?

Does "-1" mean GVA = 2? If yes, what's GVA = 0? Or is there a problem with that parameter in general?

Thanks