FlightAware Discussions

Message rate vs. aircraft seen

How does the Message rate correlate with the number of aircraft seen at this time?

My setup sometimes shows 1250 Messages/second but only 120 aircraft seen, an hour later it’s 150 aircrafts, but the message rate is below 1000

What exactly can i see from the number of messages/second over time?
Is there a time difference between the two values? So the message rate is showing “earlier” the traffic before it’s counted as registered aircraft?

Or are they simply not connected in any case?

The message rate does correlate to the number of aircraft, however it’s not a linear relationship due to the practicalities of the ADS-B protocol and how it’s used in different regions. It’s a non-cooperative system, where each aircraft transmits without consideration for any other transmission taking place on the frequency. This isn’t problem usually because the transmissions are very short.

For few aircraft, this means the relationship is somewhat linear - more aircraft means more messages. What happens when it is busier however, is that these messages start to overlap with each other. This can mean that some get garbled or lost completely, but since there are many transmitted the information does usually get through. ATC call this FRUIT (false replies from unsynchronised interrogator transmissions), and refers to the fact that many messages are transmitted in response to a request from an ATC radar site. If there are many planes visible from many radars, as is the case in Europe, it results in a lot of transmissions which inevitably talk over each other.

In addition, 1090MHz carries a lot of other protocols as well as ADS-B. Mode S, which is a response to a radar request that gives all sorts of data - Mode A/C which is the older system giving altitude and transponder code. ACAS/TCAS which is the collision avoidance system where aircraft talk to each other all use the same frequency. Some of those systems also regulate their own transmission rate when it is busy.

The effect is that as the number of aircraft increases, the number of messages received per aircraft reduces even though the overall message rate might still increase. You can see it here:

At night, the number of messages per aircraft is a lot higher than during the day when it’s busy.

Some regions such as the USA don’t interrogate aircraft quite so aggressively, so they might have a lower message rate for the same number of aircraft compared to a receiver in Europe.

This graph shows the correlation of messages and aircraft:

It also shows the difference between an rtlsdr receiver (most of the blue dots) and the airspy mini, which has better dynamic range and sample rate, so is able to distinguish overlapping signals much better.

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wow, I didn’t expect that much details in explanation.

Many thanks, now i understand the system a bit better and explains why i see these differencies mentioned above from time to time.

Yes, the airspy, i’ve read about it. But i have somebody at home which would not agree buying one :wink:

What i also realized is the right position, especially if used indoor.
Just one meter back and forth can mean 25% difference in message rate, either one or the other way

I was going to make a separate post about that because I’ve just been experimenting with indoor antenna position, but I might as well post it here. I moved the antenna in the loft space by about 1m horizontally and kept it at the same height. The message rate is not changed much, but the difference in coverage is noticeable:

Here is the old position:

Here is the new position, moved towards the centre of the house by roughly a metre. No other changes were made:

Ignore the difference in brightness, one plot just has more samples than the other, but the new position is obviously getting aircraft from positions it wasn’t before. In particular, the very far south east and south west have better coverage, as well as the north-west. The shadow to the direct south is narrower because the antenna is further from the solid wall at that end.There is a bit more shadowing to the north east in the new position, but the loss is smaller than the gains elsewhere. There are some shadows that don’t change at all which are likely due to some other buildings

What is odd is that the antenna is now very close to a fluorescent light fitting (which is usually switched off, but has a metal reflector) and that doesn’t seem to have had much effect at all.

Yes, this is something i have identified as well based on the environment.

The first thought was “bring it close to the window”.
This was incorrect, i better moved it away by 2 meters from that window to the inner part of the room.
After walking around, monitoring the message rate together with the range, i did find the best position while i have to keep it inside.

I am ok with my range, it’s close to the gegraphical maximum shown on heywhatsthat.
This is the average coverage from the last seven days.



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Could you share the gnu plot script…?!

Yep, here you go:

https://raw.githubusercontent.com/caiusseverus/adsbcompare/master/daily.sh

There’s no options, just run the script. It does assume you have wiedehopf’s graph installation as that exports data every day to text files that this uses.

The coloured dots are from the preceding week with colours by range, and the blue dots are the month prior to that. The box plots on the right don’t overlap, so they are yesterday, then the week before that, then the month before that etc.

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“/dev/stdin” line 227: could not open log-file /tmp/fit

Probably a permissions thing. Does /tmp/fit exist already? If so try deleting it. If another script created it with root permissions running this without root won’t allow it to overwrite it. Running it with sudo should make it work.

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Indeed permissions

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My experience after 3 years of experimentation, is that an indoor antenna should not have anything close to it for around 1m (3 ft). Also, it’s not just metal objects, anything really.

Does enabling these other protocols increase the number of planes decoded, or they originate from the same ADS-B planes?

It all originates from the aircraft. dump1090 decodes most of it anyway, with the only option being whether to enable mode a/c decoding (not really worth it unless you are trying to do beamfinder stuff in planeplotter). All of those protocols are transmitted by the aircraft regardless, depending on their particular avionics fit. You will probably see more messages originating from a modern airliner than you will a mode-s equipped GA aircraft for example. Most of the messages aren’t really useful for our purposes, only the the ADS-B messages and mode S (can be used for mlat and can give other data such as aircraft status).

The uplink and interrogation stuff is all done on 1030MHz, which you probably won’t be able to get unless you are within sight of a radar head, and even then it’s directional, aligned with the radar antenna rather than a general broadcast. I’m not sure there’s much interesting to be seen in that data anyway.

Well you could be sure what the aircraft are responding to.
That would mean much less guessing when decoding 1090 MHz :wink:

You can’t enable those other protocols (except for Mode A/C which is useless).

You are already receiving altitude from ACAS/TCAS transmissions.
(They are a type of ModeS transmission so to say)
It’s all transmitted on 1090 MHz.

I think I enabled mode A/C once. If I recall correctly the number of messages increase a lot, but no noticeable increase in the number of planes decoded.

Thank you all for the answers.

You can look at the individual messages:

view1090-fa --show-only AF1234

AF1234 is the hex id of the aircraft.
You’ll probably need to Ctrl-C and then scroll up to look at the individual messages, they come in rather fast.

You can find the hex-id in the webinterface next to the callsign or in the first column of the table.

Sure, but I don’t think many people are actually in a position that they will be able to receive it given the directional nature of the interrogation - even then you are going to see only those interrogations transmitted as the beam sweeps over your location. Perhaps there are some ACAS interrogations that you can decode, but I don’t know what the message content is for those.

That’s because mode A/C doesn’t contain any data to identify aircraft - it just returns a transponder code (the squawk) and the altitude. That means you can’t tell one response from another, which can lead to confusion if several aircraft respond to the radar at the same time, and false data due to garbling. It’s one of the reasons for introducing mode S, which has the hex identifier and CRC code to make it more reliable.

Mode A/C came from IFF technology used in WW2, and was first used in the 50s, so is pretty outdated by now.

Agreed,

it took a while to find the position. Now i mounted the antenna at a table in the middle of the room which gives the best results in my environment.

My wife does not like it, but it’s only my office room.