Receiver saturating?

While looking at the dump1090-mutability web page on my RPi I noticed that aircraft that pass within about 5 miles actually stop updating. The signal level is usually about -1.5 dBFS when this happens. When the plane passes over and moves away the updates start again.
I have a DIY collinear with a 30dB LNA and Flight Aware bandpass filter inline.
Is the receiver front end saturating?

 

It seems that it could be the case. Try lowering the gain in the receiver and see if you can get very close planes below -2.0 dBFS and still receive them.

 

Here is a script that does some gain tests resets dump1090-mutability with that gain

Code:

#!/usr/bin/python2
import time, socket, subprocess, fileinput, os

measure_duration = 62  #duration of each pass, seconds
ntests = 5   #number of tests
gains = "20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6".split()
#gains = "36.4 38.6 40.2 42.1 44.5 48.0 49.6".split()
#gains = "22.9 25.4 28.0 29.7 32.8 33.8 36.4".split()
gains.reverse()
results = {}

for i in range(ntests):
  print "test", i+1, "of", ntests 
  for g in gains:
   if g not in results:
      results[g] = [0,0,{}] #msgs, positions, aircraft

   for line in fileinput.input('/etc/default/dump1090-mutability', inplace=1):
      if line.startswith('GAIN'):
         print 'GAIN='+g
      else:
         print line,
   os.system("sudo systemctl restart dump1090-mutability")
   time.sleep(2)
   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.connect(('localhost',30003))

 

Another possible reason is that almost all antennae used for ADSB have good all-round reception in the horizontal plane but not in the vertical plane. In fact, if an aircraft is genuinely directly overhead the antenna then it could be almost too weak to process.

 

Old thread but can be spot on.

I have a site now with a very close by cell site. The noise floor was -10 even with the Blue FA SDR. I added the inline FA 978/1090 filter and the floor dropped to -28.

I am torn now between letting AGC work or manually setting gain in the script. I am going to wait until max load tomorrow and tweak with the gain.

Right now with manual gain at 40.2 I am getting -1/-13/-13 on the Grafana Signal Strength graph.

 

On kind of a related issue... My 978 ADS-B receiver has a FlightAware 6 dBi 978 antenna (collinear I assume), and an AirSpy 978 Filtered/Preamp. I have the SDR gain set to 36 right now. I've adjusted the gain up and down and that seems to be about the right spot.

When I watch the aircraft tracks on SkyAware 978 I see aircraft coming toward me and as they get near I seem to loose their track. As the fly past and get a ways out the track comes back with a dashed line connecting the segments. I have two theories about this. 1) The signal gets so strong, something is getting saturated (clipping) and the distortion is making decoding impossible. 2) The gain antenna has a very pinched vertical plane and there is a consequent "cone of silence" in the immediate proximity of my receiver. If I could watch the noise and the signal as separate values in real time it would be easy to figure this out.

I assume that there is no way to watch the signal and noise separately in real time?

 

Oh yeah... A picture is worth...

Where does that strip come from? Can you give me a pointer to it? Is it part of a separate package? Thanks, Bill

 

Yeah omni can saturate .. the more expensive FPGA based SDR can compensate but it is what it is.

It's careful balance ... turn up gain to get range ... but risk close stuff getting to amped.

 

sorry for being a noob.......
I just got my feeder online and I'm a bit lost about the desired signal strength.
In attached pic you see three levels. The left side (before the big gap) was with my wide band antenna (25-1300MHz) and de gain on the default setting of 36.2. Right of that big gap is with my 1090MHz antenna from Aliexpress. First part with the gain on 36.2, then set to 32.8.
Both antennas are in the same hight and with the Aliexpress antenna I see many more planes and the horizon is much further away, but my problem is that I almost don't see low flying planes around an airfield nearby (just over a mile away)
Now I wonder if I went the correct way of should I increase the gain?

I will monitor the next days to see if can track the local airplanes.



Another 'problem' that I have is that I used the Google Map coordinates from https://www.mapcoordinates.net/en
Now when I look at http://localhost/tar1090/, my position is about a quarter of a mile off. Is that a problem?

 

Thanks, I changed it to 38.6 (as that seems to be a valid value).

I think the high flying planes are easy to catch and they will appear on the map anyway. Local, low flying planes seem to be more difficult to catch so my thoughts are that there is value in trying to feed data about them.

But, now I read a bit more about mlat, I must have a problem.
Planes are only tracked when received by 4 or more stations. I'm closer to the airport than any the 5 circles and some local planes that appear on the global map are not seen by my feeder.......... (or maybe not all feeders are visible on this map as I'm not there).
My feeder name is the same as my forum name.

Is there a way to see which feeders are tracking a particular plane on the global map?

 

As a good neighbor i've looked up, if i can see you on the map. I can't. But i can find you on the same page in the search window. There it says, that your info is private. That could be the reason.
In the region stats i can see, that you are online and my receiver is connected with yours.
I hope, this helps you out a bit...

 

@BebeMischaNL : Also in the Hilversum area?

I tried to figure out why my info is private...... But no luck.......
The only place I see to set info is the adsb-config.txt, but I don't see an option for private there:

Code:

# location
LATITUDE=52.abc
LONGITUDE=5.xyz

# meters 1050m
# feet   10050ft
ALTITUDE=12m

# mlat feeder name
USER="NL-Toontje"

# dump1090
DUMP1090=yes
GAIN=38.6

# dump978
DUMP978=no

# zerotier secure
ZEROTIER=no

 

Amsterdam - SouthEast City here. You can see me on the map.

I don't know anymore, how it works with the private settings :-(

 

Yep, I found you. Also a lot of mlat peers!

I noticed that from the current 375 feeders in 4A, only some percent has over 90 peers.
Which parameters of a feeder influence the number of peers? Especially because I didn't do any effort to create a good feeder (I assume many peers is good).
I have an old Pi (Model B) with a cheap Aliexpress antenna, mounted to the chimney at only 12m.