Other Aircraft?

The statistics pages show ADS-B aircraft, MLAT aircraft, and Other aircraft. I have two receivers. One is a Raspberry Pi 2 with a ground plane antenna in the attic. The other is a Raspberry 3 with a FA antenna inside in the second floor of my house (probably about 12’ lower than the other one). The former picks up a lot of “Other” aircraft. The latter used to but isn’t anymore. What are these other aircraft?

I’m probably going to move the latter setup outside onto a mast I have for my weather station anemometer this weekend if I have time. But I’d like to get more information on this issue before I do.

Also, the RP 3 halted once for no apparent reason. I didn’t think of checking the logs when that happened, and those logs have probably aged off by now, so I don’t have a clue what may have happened.

Today, we had multiple power outages over a period of about 4 hours, due to the lightning storm that blew through. That probably obfuscates the logs, too, in case anyone thinks they might learn something by looking at them.

So all I really want at this point is to know what those other aircraft are and whether I should be expecting to pick them up.

Other are aircraft that aren’t ADSB-out equipped, and for which you don’t get MLAT positions, but you still get other mode S messages like altitude or airspeed. For example they could be flying at a low altitude and not being detected by enough other MLAT stations to calculate a position , or they could be very far away from you and outside your range of MLAT stations you sync with.

That’s kind of what I was thinking. But I was trying to figure out why one receiver was picking them up and the other wasn’t. The one that is not picking them up (actually picking them up in small quantities) used to pick them up. For some reason, the data dropped and stayed low starting a few days ago.

I figured it out. There was a period when MLAT was not working because I had set the location of the receiver incorrectly. When FA noticed this, the aircraft that would have shown up as MLAT showed up as Other. It just looks like the volume dropped off because for that period of time it is abnormally high.

I conclude also from this that Other aircraft would show up as MLAT aircraft, if MLAT is working properly and the signal is picked up be enough receivers.