SDR/dump general

I’ve been tracking surface a/c for a few many months & have a 98% hit rate with that I’m trying to achieve.
But, every once & a while the specific a/c I’m interested in is not detected. I’ve done plenty of experimenting , gain changes, aerial location etc, but still get this odd frustrating issue.

As I was watching it just now, the a/c even disappeared from flight radar at a similar time as to what my sdr lost it.

I’ve been at a loss on this, but after seeing the a/c disappear from flight radar at a similar time almost makes me think that it is something to do with the a/c itself.
My understanding is that the a/c stays transmitting until shutdown, or standby at least.

Does anyone have any experience or knowledge with similar scenarios.?

There are a few possible scenarios:

  1. The aircraft is operating exclusively with UAT on 978 MHz so you are not seeing it on 1080 MHz.
  2. The aircraft may not be transmitting an ADS-B signal. It is only required in certain airspaces according to the FAA and if the aircraft is not in that airspace then it is optional.
  3. It might be getting detected with MLAT which is why it is invisible to a single device.

I run dump1090 mutability and allow inbound MLAT traffic to it so I do see otherwise “invisible” aircraft. I am not far from KFRG, which is mostly private aircraft. There are flight schools there and various aircraft pass over my office that do not show up until they are in an area where multiple receivers see them. There is one specific Robison helicopter that flies overhead quite on training flights quite often that is invisible to me.

Thanks for the reply.

I am reasonably sure that it is 1090 as the a/c was detected by 3x separate receivers (albeit within 3m of each other) at almost exactly the same GPS point.

Down under, it is now mandatory. Also, it was detected when on-ground.

Do you mean Flight Aware/Radar? MLAT is where it uses multiple feeders, whereas what I have is direct receival via the SDR. That the a/c ‘disappeared’ from FA/FL & my SDR at almost the same exact point surely indicates something with the a/c…?

My application does not have ability for MLAT nor do we really need that right not.

Again, thanks for the response. I will continue to monitor/research & see if any more pilots can shed any light…

As a GA pilot, my transmission on ADS-B starts in ACS mode when I start taxiing.
As soon as I vacate the runway the ADS-B transponder will be set in stand-by mode.
During stand-by the positions are not broadcasted.
So taxing from the runway to parking wouldn’t be visible.

For commercial aircraft the ADS-B is activated on pushback until parked at the designated gate or parking spot.

Obstructions between your reciever and the aircraft could cause the signal to fail reaching your reciever. This could lead to a temporary loss of the postion.
If you are the only reciever at that point in time you would lose the aircraft on the radarscreens as well.


this is what I understand also, but seems we do not always see this. This is my current challenge to work out why.
What is strange, is that the SDR’s that I have with line of sight lose the a/c, at exactly the same time as the FlightAware/Flight Radar tracking.

I dont think this is case for this location & the location of the SDR/aerials that we have.

If the antenna is close to the aicraft it could also lead to a very strong signal that is recieved.
Then you might overload the SDR and that way blinding the reciever for the signal.
I can’t judge that without more information on the location and the settings of the SDR and antenna.

It could be that the radar picture is depending on your SDR for the position of the aircraft. If you lose the aicraft it will drop from the radar view at the same time.

Not for this instance as the SDRs I have are not feeding FA/FR .

This was a thought, so have dialled down the sdr gain significantly.
What distance would you call close.?
And the other thing to consider, is why it is so intermittent. These instances are working fine 98% of the time

Close would be anything within 3 NM from you antenna postion ( in my opinion). To see it you would have to look at the RSSI values. Anything close to -2 or -1 is a close (and strong) signal. If you have lots of these values you might overload the SDR in signal strength. It could be intermittent due to the fact that the transmitting power of the ADS-B reciever can vary in signal strength

My site (1090 with AirSpy mini SDR) is urban and I have traffic fly by within a few hundred meters (helicopters) fairly frequently. I find that their RSSI goes to 0 (top of scale) but the tracks appear to be continuous until the aircraft is very close indeed.
Here’s a sample of one:

ok, will look at that aspect as well.
Current design is using DUMP1090, do you have much experience with that?

All my recievers use dump1090-fa. You might want to install graphs1090 in order to do some more analysis. Install the package and let it run for 24 hrs to get a view of your performance during the day

Graphs1090 connects to a socket output of dump1090?

Graphs1090 analyses the datastream from the decoding program, in this case dump1090 indeed.

My application is already logging the output streams as that is how I captured the surface positions etc.

It looks like graphs also gives the signal strength in db, so that could be what is the next step…

So back to this, as is still an issue for us.
What are the chances, if any that it is the aircraft?
The reason for asking, is that the 3x decoders I am using all lose the signal at exactly the same point, AS WELL as the flight radar track.

I’m going to assume, that FA/FR would have a couple of feeders in this location & that they are all similar in concept ( SDR & a version of dump1090 )
So the challenge is, why or what could be causing them all to be lost at the same time…

I have an update to install that has the latest version of dump1090-fa (hopefully) as well as the graphs1090 application.
hopefully the graphs will give me some data around the timeframes that the current setups are missing…

Given that everything loses it at the same time - fairly likely.

A burst of broadband interference is another possibility.

If this was the case, then it would be pilot initiated.?

any advice on how to determine this?
Could it be by identifying any close by Cell transmitters?
Would it come up on graphs1090
Lastly, if that is the case is there anything that can be done to resolve it?
Would an external filter help.?

Thanks for the feedback !!

Give us a hint where “Down under” you are located.

I am located about midway between Moorabin and Essendon airports and at least 10% of aircraft flying within audible range have no 1090 at all and about 10% overload my receivers when they fly within 1-2 km. I have the gain wound up so I can detect aircraft far away and I don’t care what I loose near in.

If you run Piaware and register one of your receivers you will be able to see what other receivers are nearby.