Another "What's going on here" question


#1

Here are the Aircraft Reported statistics of two sites located just east of my location (KTPA), 56247 and 56400. The sites have different owners and are 6 miles apart. Both sites show Plant City (FL) as their nearest airport.

Site 56247 seems to have an exorbitant number of OTHER aircraft reported. Site 56400 is shown only as a reference as to what one would expect in the ratio of ADS-B + MLAT to OTHER.

What might be an explanation for such a large number of OTHER signals seen?

Site 56247
https://drive.google.com/uc?export=download&id=0B5eBCoqedqaxMjNidmtsZFJvbzg

Site 56400
https://drive.google.com/uc?export=download&id=0B5eBCoqedqaxMHZPVmprMWFOYVE


#2

I have two sites. Two different antennas are mounted about 10 feet apart on the roof, difference in total altitude is a couple of feet at max. Signal chains are identical – FlightAware filter, FlightAware blue dongle with LNA and filter, Raspberry Pi 3. Feed lines are roughly the same.

In the last 24 hour period, here are the Reports summaries for both systems:



ADS-B      319874     229263
MLAT       145504      70725
Other      136644      94664


Significant difference between the different antennas – the first one is a commercial gain antenna, and the second is a wideband discone.

Numbers aren’t the only difference – when I look at the birds the two systems see, the commercial antenna sees more birds at longer ranges, and the discone picks up more close-in and on the ground birds (and they both see a lot of the same birds, not surprising).

Add to that local topology – take a look at your two sites on Google Earth. From my location, up on the roof, I have a clear path to the local SJC airport (although Moffet Field, KNUQ, is closer to my place). I could site a system a half mile from my place and it wouldn’t have a clear view of SJC; I imagine the numbers would be quite different. If I could get one up at a friend’s place in the local foothills, I’d have line of sight to SJC, SFO, KNUQ, and OAK – and I’m not sure my numbers would be higher, as I’d have a whole lot more transmission collisions to deal with. I’d also have zippo for signals to the West, as they’d be occluded by those foothills. (Also, my buddy up the hill has a lousy internet connection – they’ve been told the best way to get higher speeds is to have a telecoms executive buy a place near them.)

Large differences in reports from nearby systems aren’t that surprising.

cheers–

bob k6rtm


#3

The query in the post was not to point out a difference between these two particular sites but to question why site 56247 (which is MLAT Supported and Enabled and shows about the same ratio of ADB-S aircraft to MLAT aircraft as most if not all other similarly configured sites) has so many OTHER aircraft (or whatever) shown in the stats.

The stats consistently show an OTHER count greater than both ADS-B and MLAT aircraft combined.

TD


#4

In my case, the “other” signals diminished a lot when I adjusted the gain to have less clipped signals. This was consistent with an increase in the ADS-B reported numbers.
The change was from 120-130k to 40-70k, so not hugely dramatic. And it might be still a fluke, need more time.