I have seen written else where that whilst AGC is good for a signal that is at a constant strength and length, ADSB signals are much briefer, more frequent and shorter and AGC does not work as well with them. By the time a signal is being analysed by the AGC, it drops off.
Yes…I spent a lot of time trying to accommodate for that, but I gave up. I’ll re-visit my current gain setting only after the antenna is moved. Waiting for the weather to improve.
I run master work sheet, with the different records for msg, position and AC, gained from different times of the day and days, put together then an average obtained of all entries, which may/hopefully cover that change of data per trf load, weather etc, in our message, position and AC data fields. Trick is to grab data from different times of the day etc and generally a certain gain setting will tend to be obvious, within a couple up or down, which might indicate a good average overall. The general trend that the message and position fields are pretty similar gain setting but the AC seems always to need a higher gain, but the figure that stands out from message and position, is pretty close to the AC figure, may be 2 or 3 aircraft lower. But then again, just to make it a bit harder, the AC gain settings might have say 6 db settings that all will give say 30 AC but the db figure will be a little different each one, not close together I.E. 22.9 28.1 17.2 etc
Could I just ask what is the measure of optimum gain?
I would have thought that there are three outcomes (there may be more) that you may want to maximize;
- Number of aircraft
- Observation distance
- Position reports
Each user will have their own requirements and there is obviously no right answer.
In general, 1 & 2 lead to the same and the further away you can see aircraft the more aircraft you will see and count.
I have read this thread a number of times and I cannot see a link between optimizing gain visually and maximizing one of the outcomes. Am I missing something?
Chris is 7 km away from https://flightaware.com/adsb/stats/user/tjeffer9999#stats-39121 and comparing their respective stats pages Chris has significantly less aircraft, less positions and less range.
This suggests that there is something else that is less optimum and preventing signal and/or noise getting to the receiver and possibly this should be resolved as it is most likely to change the need for or application of both filtering and amplification and this will change completely the search for optimum gain.
S.
Exactly!! This is the point I have tried to make in the past. The only caveat is that some of us feed the data to FA. FA needs all 3 items you listed to the maximum extent possible. This is not necessarily the case with you and I.
My focus has always been the number of aircrafts, followed by observation distance, the latter only because it increases the number of aircrafts. Number of position reports was never my interest. Whether I see a plane for one second, or 30 minutes, the longer tracking time adds no value to my aspect of the hobby.
To the extent that by visually seeing how different gain settings perform, one can see SNR, saturation, noise level, and low gain. This can determine if more positions are received, and whether close by or more distant aircrafts are received more efficiently.
There is more to an efficient receiving station than simply gain. Location, antenna type, and area RF pollution, just to name a few possible factors.
Actually what matters is the message rate. FA and everyone else in the air tracking business needs to know exactly WHERE a plane is. The more often you get reported it’s position, better you know it’s location. Better MLAT triangulation for non-ADS-B ones.
So FA doesn’t care primarily how many planes or how far you can see. There are plenty of other receivers to fill that in. They need reliable, accurate, real time, position data for the planes that get reported.
Sure some might care of how many planes he “saw”. But that’s really not that relevant.
If you saw a plane for 1 minute at 250 miles away it doesn’t help as much as if you reported it’s position reliably for 30 minutes.
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When I started this hobby, it was because of my interest in hardware: antenna, preamp, coax, filter, dongle. For me the software was a tool, and maximum range + number of planes were parameters to test the hardware performance.
For about an year at start of this hobby, I did NOT feed any site. My station was totally local, nothing to do with internet. DIY Antenna (1/2 WL Dipole, Franklin, Coco, Cantenna, Spider, J-Pole etc) >> Satellite Amplifier (L-band, 950 Mhz to 2050 Mhz) >> DIY Bias-T >> RG6 coax 45 ft >> Windows Desktop >> RTL1090 decoder >> adsbScop map.
I had no interest in feeding any site or getting free equipment from any site as my main interest was DIY.
First 2 years I did not have a Pi, and did not know any Linux
Well, that was 5 years ago. My interests and focus has gradually changed with passage of time. Life is change, change is life. 
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Interesting input ABCD. My next chapter Dxista, I had to whip home from work re a possible leak, (hasn’t rained for 3 months) but turns out a creaking roof in the autumn sun … lol. But then had the chance,as on the roof, to remove the rtl-sdr amp from main main system, so now just a naked pro stick plus onto the antennae. Will be interesting to see the change, already though I had to quickly up the gain, from my local 1090 map, there was a difference in distance detection, so I just up’d it to 25.4. Will test tonight, both the old gain test and this visual one, to note difference in the reduced physical gain in the system
This thread is saying nothing about the overall potential of a site’s physical installation. For example I am using an FA external antenna on a windowsill indoors. Obviously this hardware has much greater potential but I am not in a position to take advantage of that yet. For all intents and purposes this site is currently installed as well as it can be. That leaves configuration of the decoder to extract maximum performance from the site.
The purpose of this thread is to present a means by which it may be possible to intuitively select an optimum tuner gain by visual inspection of heatmaps across a range of manually set gain values, as opposed to generating numerical statistics and trying to interpret them.
His coverage graph implies he has a high-gain antenna high up with a good view of the horizon.
Some sites will be in that situation and you’re right, it makes sense to take care of those factors before fiddling around with gain settings. Despite its wasted potential my site doesn’t have any problems which need to be resolved because it’s as good as it’s going to be for now. This thread isn’t making any judgement on a site’s potential or possible sub-optimal installation, it’s simply presenting a visual approach to finding an optimal gain, all other things being equal.
Interesting requirement. Is that documented by FA anywhere?
This is a Sunday evening view of what I can see. The circled areas in RED are occluded by neighbouring buildings and there are plans in place to overcome those.
The South East approach has an unknown occlusion which has yet to be investigated.
As for the rest I can see reliably and consistently out to 200 miles and anything coming or going that gets into range generally stays there for 30 minutes or more. At the time this image was taken there were no MLAT aircraft in range but there are usually quite a few when the weather is good and the light planes and trainers are out and about.
I have not seen a passenger carrying aircraft without ADS-B reporting positions for quite some years.
That is not necessarily so. I can’t tell who reports what aircraft to FA and whose information is used by them. If my system is running then FA has my information. I also feed Planefinder and FlightRadar24 and I have established that at times if I am not reporting to them then some planes aren’t visible on their radar. I turn the feed on and some planes appear.
There is another station near me that gets about 15 more aircraft a day but about 40% more positions. I think they can see a tiny bit further and are seeing aircraft on the Adelaide-Sydney, Adelaide-Hobart and Hobart-Sydney much better that I can. Hence he sees a few more aircraft for a bit longer and therefore gets a better message rate.
If I can increase my range reliably then, in my case, I will report more planes more reliably for more than 30 minutes and my message rate will increase.
The only way I can otherwise increase my message count is to change my antenna system and ensure I get a better view of aircraft on the ground at Melbourne YMML, Moorabbin YMMB, Essendon YMEN and Avalon YMAV. My message rate will go up really significantly and I will still see almost the same number of planes but my range will drop below 100miles as all the close in ground traffic will drown out the weaker signals. I’ll also have high gain directional antenna pointing at noise sources such as cell sites.
Not sure who wins then.
S
Fair enough.
I tried an FA antenna in a window before I put it on the roof.
There were so many local factors that changed its performance.
The window is near a desk with two computers and three monitors which all effectively raised the noise floor.
Depending where I was in the room also had an observable adverse affect on the antenna’s performance.
I am still not understanding what the optimum tuner gain gives in your situation.
I suggested three properties that I would try to maximise in my situation and altering gain is one parameter available to me.
Are you simply trying to get a spectrum scan that looks cleanest at 1090 MHz?
Cheers,
S.
Then will really matter the art of gain adjusting and filtering your signal (I have the FA filter plus a preamp with two ceramic filters in front of my receiver). The more you can clean the signal, (eliminate foreign signals, eliminate clipping of 1090 signals), the more numbers you will have for message rate.
And you will see that distance come along with “maximizing” the number of messages, because now the weakest messages get to be decoded too.
If you are alone in your area, then sure, you won’t be able to MLAT (needs at least 4 stations feeding cleanly timed signal).
An example for my area (even if is still early in the morning, traffic didn’t picked up yet). I can “see” planes far and close by, without the signals interfering. If I raise the gain, suddenly the far away plane start to drop. I can see a equal drop in the decoded message rate. The same would happen if I lower the gain too much - I will loose the far away planes, again with a drop in message rate (maybe less, but still instantly visible).
PS: the “holes” in coverage might not always be because of obstacles. It might be also because… there are no planes flying in that area, due to ATC rules. Like in my case, there are not too many planes to the east. Not because I have obstructions there but because… planes don’t really fly over there. This is from the website tracking:
In your case, it might be a combination of both. Exclusion flight areas (military for example) and obstruction. I took a quick snap of flights over AU, note how the planes going to middle east have their flights “curved” around some areas, going all on the same pre-determined course (and the lack of coverage over Nullarbor)?
Thanks for the picture.
Can you do it again with Show all Tracks turned on and zoomed out a bit to show your range.
It seems that there are no aircraft under about 8000ft. Is that normal or just because it is Sunday morning?
That hole in the coverage to the North East is caused by the house next door adding a second story last year and blocking the view. That track is the second busiest in the world by aircraft movements flights. At times there can be 30 planes in the air between Melbourne and Sydney.
As you can see even on a quiet and fine Sunday evening they have to circle before landing.
S.
I lost TV signal too.
Those areas you circled are great big empties. Pretty much nothing there at all. I travel in that big red circle and I have to be completely self sufficient for everything. Last trip out there we were out of cell range for 15 continuous days.
Alice Springs in the middle and Ayes Rock just to the South West of it are the only commercial airports in that circle. Alice has 11 arrivals and 11 departures scheduled for tomorrow So not really busy.
If you look again in about 5 hours you will see international flights crossing Central Australia from SE Asia but even then it is not hugely busy.
QF1 which you highlighted flew SYD to SIN and will try to fly great circle but will fly one of the five tracks across Australia mainly dependent on weather. Yesterday it took a completely different track further north https://flightaware.com/live/flight/QFA1/history/20180427/0610Z/YSSY/WSSS
"The Nullarbor Plain … is part of the area of flat, almost treeless, arid or semi-arid country of southern Australia, located on the Great Australian Bight coast with the Great Victoria Desert to its north. "
It is well below your red circle and along the coast.
S.
Sunday at 8AM there are only 90 planes in my “radar” area, with 400 messages/sec.
At 9AM I have around 140 planes with more than 650 msg/sec.
That is my optimization target, number of messages/sec. Less “dotted” lines.
Distance it is what it is (antenna height and free line of sight), I have some obstructions too (trees with leafs).
I agree…I’m a hardware rather than a software guy. While not as prolific as you, I tried a few things on my own as well. I smiled a couple of times when you described some of your ideas in your early days, and attempts to do them in an affordable way. One such case was when you described using an in-line satellite TV amplifier. I had thought the same independently. I purchased one from eBay years ago. Then there was the Digiwave satellite cable power inserter sold by Best Buy at one point.
I started out with the $8 generic eBay dongle with the small mag mount antenna, well, they were not that cheap in the beginning. The E4000s cost me $25 each, and they were very susceptible to eletrostatic discharge. The price dropped later. The first ones with the E4000 chip were not good for ADS-B if I recall correctly, then switched to the R820T, followed by the R820T2. The software was ADSB#, then RTL1090. All Windows versions, running on a PC. VRS was ‘discovered’ much later, and more recently this ‘community’, and other tracking sites. The RPi is the platform of choice now.
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the majority of airports and hi-enroute airways are to the West (EUR/MES/ASI traffic) and to the East (PAC traffic) with very few in the center of Australia as seen in Skyvector chart.
To see the route the plane will fly go to the link for that flight eg https://flightaware.com/live/flight/QFA1/history/20180427/0610Z/YSSY/WSSS
and find the bit that gives the route for this flight
Route
DCT KADOM A576 PKS DCT MACLA/M081F340 DCT APOMA/M084F380 DCT POLEV DCT DERAK DCT BOYDI DCT 15S130E DCT KIKEM/M084F400 M774 OBDOS/M077F340
The names I have highlighted are waypoints on the map and each one can be located on the Skyvector chart by using the search function in the top left.
The Skyvector chart is available at Skyvector
With my Pro Stick Plus, Uputronics filtered preamp and FA antenna I’ve been testing a few different gains and got the best results so far with it set to maximum manual gain 49.6 despite what appears to be a lot of in-band noise on the heatmap.
It performed way better than 20.7 which visually looks like the best signal to noise, so clearly interpreting these scans requires some appreciation of how well the receiver is able to cut through what looks like noise. It also performed better than 38.6 which looked to have more noise. It also performed better than 48 despite this seeming to be one of the noisiest levels.
With that appreciation in mind a gain of 49.6 also outperformed AGC and so I was curious as to what that actually looks like on the heatmap. So I ran the rtl_power command with gain set to -10 which I believe sets the tuner gain to an even higher value around 53.
Scan at maximum manual gain 49.6
Scan at Automatic Gain Control -10
It’s any wonder that AGC performs worse than 49.6; it introduces loads more in-band noise. Despite this, adding the preamp on AGC still outperformed the non-preamp setup on AGC.
Conclusion
It looks like AGC is good for Pro Stick Plus + FA antenna setups as long as there is no nearby strong interference, in which case a manual gain at or near maximum may offer an edge and a filter will help further.
For the filtered amplified setup it looks like AGC can be too much and doesn’t turn the gain down to a better value as its name implies. However in setting a manual gain it’s clear that the receiver does a great job of extracting signals despite what looks like noise on the heatmap, and so it’s best not to turn down the gain too much, and in my case having at 49.6 is giving the best result.
In otherwords, when using the results of gainmap to try and find an optimum gain visually, what looks like smooth in-band preamp noise drowing out the signal is in fact quite well tolerated by the receiver so don’t needlessly drop the gain too low as you’ll throw away signal that the receiver can actually use.
I will continue to test and see how stats pan out over the next week.
I’ve edited the gainmap script in the original post to add auto gain (-10) to the start of the testing.
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