just built my first CoCo antenna and brought it out onto the balcony, and it’s performing great. Ranges have increased to 200NM+ and the message rate is getting close to 1000/s, which got me thinking… Does anybody know how many messages per second one antenna / receiver will be able to handle? Since it’s all on the same frequency, there certainly is a point when collisions are so common that the rate simply cannot go any higher?
I just wonder how close I got and how far i can push the setup.
things i intended to do:
extend the antenna to 16 segments (i stopped at 8 last night because it was tedious work and i wanted to see if it was worth it before going any further - the results however are very encouraging)
getting it up higher (maybe just 0.5m for now, but since i’m already close to the rooftop line this might actually be a lot)
getting it higher still (on top of the roof would be ideal, but that will at least require some coordination and might not be worth it)
If however i’m closing in on the physical limit, it might be more reasonable to change the plan and go for directional antennas and multiple receivers.
Google did not really help me so far… So what’s your experience? Any technical details?
I have seen a number of receiver that get 1-1.6 million reports per day. You get about 300,000 so I think you have plenty of room to spare.
I get up to about 900 frames per second(and my antenna is still just in the attic) and around 500,000 reports per day. There is one in my area that gets 750,000 reports per day.
Ok i see… I’m using a FA Pro Stick, which is basically an RTL device.
I was a little hesitant to compare the ranking information because i think it’s possible to link multiple receivers together (i read that dump1090 can do that) which will of course offset the numbers. also, the 300k number is still negatively affected by the old dipole antenna, the new antenna has been running for <24h now, i believe it’s still on the rise. with a little luck i might get close to 500k.
4000/s sounds like i can stick to the original plan without worrying too much about the message rate for now, though. I’ll probably run into CPU issues with the Raspberry before.
The main site is out of stock until the end of the month(I will order one when they are back in stock). Another site has them for twice the price but it is almost fully built.
Consolidating feeds is possible, however, you could lose MLAT from the other feeds. Not a big deal in Europe as that traffic is dropping off considerably. You could just do MLAT on the consolidating device and have that device use your current antenna. Directional antennas could be used on the other devices(that won’t do MLAT).
Also interested to see what the max is. I seem to be hitting a wall between 1000-1100 messages tracking around 200 planes for most of the day. This is with a RTL-SDR dongle + amplifier and 6 element CoCo.
Your ADS-B Aircraft Seen / Tracked graph looks a little odd.
I don’t see a black line for MLAT traffic, yet your FA stats page shows it is working.
Maybe it is just a graphing anomaly.
The message rate graph seems to agree with you.
The rate per aircraft is much higher at night than it is in the day.
With more aircraft you will be seeing more messages. TCAS will also add the the messages, especially when aircraft are close.
My setup is just touching 1000 messages per second. I expect it will exceed this significantly when I mount the antenna outside and about 3m/10ft higher(it is currently inside the attic).
I’m building one of the coaxial collinear antennas that show up every now and then when looking for ads-b stuff. construction material is rg58 coax cable, velocity factor .66 (at least according to google) so i end up with a convenient 9.08cm per segment. 16 segments are just shy of 1.5m in total. I soldered (yes, soldered… i somehow ended up with a flex core cable) 8 segments so far and attached them temporarily using pvc pipe. seeing very good results so far.
Those stats look really sweet. What tool did you use to create them? Or did you set up the collection chain yourself?
Also: Yes, looks very much capped there. Would be interesting to see if the line would maintain the same level over the course of a day with e.g. a lower gain setting. After all, the air traffic at your location might be pretty much constant during that time (even though i doubt it).
The messages per AC graph also seems to suggest some kind of saturation, I’ve seen something similiar elsewhere, made me think (and in consequence led to this thread).
The average signal level also suggests that weaker signals are getting phased out by stronger ones.
The practical message rate for the RTL dongles appears to be about 1600 per second from my tests. I’ve seen it at that rate with 200 aircraft, however with 250 it was only marginally higher. Even though there were more aircraft, the increased congestion from non-ADS-B messages such as TCAS means that fewer position messages are received per aircraft.
An FPGA based receiver could probably improve on that figure by being able to resolve some of the colliding packets. There was a thread discussing this recently.
I thought that might be the case. You might want to read the articles comparing number of segments to improved signal. I’ve seen several that state more segments is worse, with a sweet spot from 5 to 12 (depends on the tester).
Indeed the the scripts mentioned by Jon. Makes setting things up a lot easier.
Already tried different gain settings, did not have a big impact on number of received messages. When lowering gain AC count goes down a bit and number of messages per AC goes up. Will do some more experimenting over the weekend to see if there is room for improvement.
Also ordered a FA dongle to see if that makes any difference.
I didn’t like the range maps, however… VRS looks much better (but does require a windows machine, I don’t keep many of those around). Also, the python scripts do consume quite some cpu… so much it actually made my message rate drop by half, and that was before the improved antenna. Did take me some time to figure that out… Will switch to the “regular” mutability variant sooner or later, but since most of the modifications are limited to the web frontend I don’t expect much of a change in performance from that.
I will check out the scripts, I’m a little concerned about the CPU usage… I’m running on an old RasPi B, will see how that works out. But the stats seem very useful.
I just finished the 16 segments and mounted it on the balcony. It’s not permanent yet. So far it really seems that the ranges have decreased, but I will continue to measure and compare for a day or two, don’t want to jump to conclusions. If what you say is true, i can still chop off a few segments without too much of a hassle, and finish the setup once I found the best configuration. Thanks for the information though, i really did not expect that. It’s easily fixed though
The FA dongle did not make a huge difference for me, was using a Nooelec R820T2 one before, and that seemed to perform pretty well already. What I don’t know if it would be able to handle the new antenna though.
About the gain: That again really suggests that the message rate is limited. Less gain of course means lower range, that’s expected, lower range should also mean less messages.
The interesting thing is, that one can still keep track of most AC even with some (probably even many) messages dropped. So I guess message rate does not really matter that much after all.
As you add more segments to a co-co it becomes more directional i.e. the gain towards the horizon increases at the expense of gain in other directions. Most aircraft are going to be near the horizon but not all, so you can take it too far. With an untuned co-co that’s not quite the right length, the lobes of the gain pattern at 1090MHz might be angled away from the horizon, so you might end up “missing” the horizon with the main lobe if it’s very narrow.
On message rate, some of the mode S traffic is self-limiting e.g. TCAS will dial back its transmit power when it sees lots of messages, IIRC. For a passive listener, collisions are indeed the main problem. The theoretical limit if everything is perfectly coordinated is somewhere around 8000 messages/second, but there is no coordination or backoff/retransmission of messages (unlike e,g. ethernet) so at 1k messages/second there’s something like a 12% chance that any particular message is going to collide with another. The rtl dongles don’t have the sample rate to be able to do much with colliding messages.
Sectorized directional antennas are one way you can reduce the problems with collisions.
I moved my Antenna(DPD) today, not to the Chimney but from the middle to the front(North) of the attic at around 2pm US EDT (Lowish traffic time).
I now use a longer coax cable (35ft LMR400UF) in stead of a 3ft RG316 patch.
I actually dropped the gain and may do it a little more. Peak is -0.5db but noise is -18db average.
Dongle is the FA Pro with the FA filter.
We live on a street with all of the houses lined up, almost identical. Moving the antenna from the front meant that it had less roof obstruction(can be 5-10db).
This should give me a wider view north
My frames went from 700/sec(Cloudy day so not a lot of traffic) to 1100/sec average and 1250/sec instantaneous.
Average max range went from 120NM(6 hour graph) to 150NM (1 hour graph). I saw QF11 at 150NMs out.
I wish the weather and my health (just a stomach bug) would cooperate so I could move the antenna to the Chimney. It would be above the house ridge lines and 5-10ft AGL higher.
I’ve found very small changes in antenna position can affect performance a lot indoors. I had to move things around a bit last week and now I can’t get things back to how they were before - message rate and range are both decreased a bit and I don’t know why. It’s frustrating because there is nothing obvious that’s causing it.