Same aircraft, same time, difference in altitude - PiAware/FlightFeeder

FlightAware ADS-B Flight Tracking
1

Curiosity - another project LOON flight HBAL0177 wandered nearby early this morning. In looking at the data from the local SkyView plots, I noticed the FlightFeeder was reporting a significantly different altitude from my PiAware feeder. Why would there be such a significant difference in reported altitudes (57,500 vs 39,200 feet)?

The two feeder antennae are located withing 2 feet of each other at the same elevation.

The screenshots below were taken within 10 seconds of each other, there is a tiny difference in position.

Both data sources are ADS-B

FlightFeeder Orange box, FlightAware antenna+cable, gain set at 40
image

PiAware - Spider antenna, FlightAware ProPlus, gain set at -10
image

2

dump1090 with piaware is set to correct 1 bit CRC errors. This can result in bad altitudes among other things.

Not a problem for flightaware as they filter out bad data i guess.
Still i was annoyed by strange altitudes so i turned the --fix option into a --no-fix option.

According to my stats the amount of messages that were fixed was about 1 percent and i was happy to live without those.