Can we use RSSI displayed for aircraft to get an idea on best gain setting? I’m generally seeing in the range of -2.5 to -17 or so, the latter being for distant or very low traffic. For example, there is currently a balloon at 58,000 feet and 191 nm away that is generally between -16 and -17. I also noticed a plane on the ground at the airport about 10 miles way at about -15. 1500 feet elevation and 4 miles away is -2.2. This all seems reasonable to me, but I don’t know what the target ranges should be.

Interesting: I just saw a plane with two transponder codes. One showed up as about -6 to -7. The other was about -14.

Hey ExCalbr -

I can’t answer your question with any authority. I don’t get anywhere near the RSSI range you indicate. I have a range that’s about -1.5 dBFS to -12 dBFS, and that’s it. Distance from receiver does matter, but if the a/c is above the horizon, even at 200 nm, I have a receive signal of about -6. Once the a/c drops below the horizon (because of earth curvature or obstacles, like mountain ranges), my receive signal drops like a rock and it’s gone.

Right now I have a GA a/c 7.6 nm from the receiver and about 400’ above the receiver antenna. I’m measuring -2.0 or so. It’s doing patterns out in the desert south of Wickenburg.

Cheers - Jon N7UV

re “target ranges”, it’s all just a relative power level not an absolute one, so there’s no particularly right answer here. Anything above -3dBFS is in danger of clipping though (this may be unavoidable for nearby aircraft).

RSSI (in dB) should fall off at a logarithmic rate with distance, if my back-of-an-envelope maths is right.

Thanks. That’s the kind of information I was after.

Good day, OBJ -

Thanks for the response! You are correct, it’s a straight inverse square power law (6 dB reduction in received power for every doubling of distance), all other things being equal. Multipath can play a bit with that, as can NLOS.

First, my setup (site 13966) is an FA antenna at 12’ off the ground, atop a 4,000’ peak that is about 3,000’ AGL. There are a couple of ridges and other peaks right around me, but for the most part my LOS is unimpeded in all directions with a local horizon below 0 deg. It’s quite good for a/c monitoring, among other hobbies! The FA antenna connects via 20’ of quad-shielded RG6 to a custom-made cavity filter set tuned for +/-20 MHz BW at 1090 MHz, with about better than 80 dB (all my SA can do) ultimate rejection, with less than 1 dB passband loss. There’s a good LNA behind the filter, about 1.5 dB NF. Then there’s a RTL-SDR 1 ppm TCXO SDR stick. I’m running the plain-vanilla version of FA 3.0.4.

In my case, the site is 53 km LOS from KPHX, and there must be a dozen GA parks within 100 km. The area around the site is open desert (except for my little 4000’ peak) and a lot of GA do training sorties out this way. RSSIs for a/c at KPHX on the ground are anywhere from -1.4 to about -3.5 dBFS. The planes passing by the peak are also in that range. Haven’t seen anything yet stronger.

At the very limits of my range, where HeyWhatsThat shows that the a/c at the given altitude is very NLOS, my weakest signal reception is -12 dBFS, give or take. That range so far has been as much as 440 km (275 sm) for a/c at FL400 or so.

Based upon the above, I think the setup is working pretty decently. However, I think I’m also a little hot into the SDR, but haven’t seen a method yet to turn down the gain in the stock FA version. I believe that it can be done in mutability, but I don’t have that installed, and FA 3.0.4 is new to me. Finally, is there a way in stock FA to get the statistics for reception?

Cheers - Jon N7UV

There is a rltsdr-gain config setting in 3.0.4 accessible in piaware-config.txt or via the piaware-config utility

Stats are written as json to /run/dump1090-fa/

Thanks for the lead, OBJ.

In piaware-config.txt, I found

rtlsdr-device-index 0
rtlsdr-gain -10
rtlsdr-ppm 0

So, I changed gain to -12.

Rebooted, and suddenly instead of my hottest signals in the -1.5 dBFS range, the best signals were in the -35 dBFS range and I was hearing only the very closest a/c. Not good.

Changed it back to -10, and everything returned as normal with the very hot signals.

Changed it to -11. Again, deaf as a post.

Changed it to -9. Seemed to be about the same as -10, the peak levels (especially that one GA a/c only 4 nm from the receiver) were still in the -1.5 dBFS range.

Set it back to -10. No different from before.

Set it to -7. Just like setting it to -11. Deaf as a post.

Seems that either -9 or -10 is the only spot in the gain profile that works.

I would think that it would be a little less on/off…

Cheers - Jon N7UV

The gain you specify is just passed to dump1090 directly, which interprets some special values.

-10 is a special value meaning “turn on AGC” (which as you are probably aware, doesn’t actually give you auto-gain, just a high gain)

For regular gains, you want a positive value in the 0…50 range (nominally in dB). It will pick the closest gain step to the value you give. If you specify -9, then the closest gain is 0dB: minimum gain! Try a value more like 45.

Hi Jon, valid gain levels for the rtl-sdr would be: (besides the -10)

0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 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

I too live in your area and have thought about a remote setup - sounds like you are perched up on the White Tanks, or thereabouts (seems like an appropriate area). How are you handling the data? Wifi through a direct antenna, or 3G/4G? Sorry for the derail - hope the gain values help - although dump1090 will automatically scale to the nearest setting.

Howdy Nitr0 -
Thanks for the feedback on gain settings. With OBJ’s lead, I have discovered that “42” is likely about the best setting I can get. From your info, it looks like my only other choices right around that apparent sweet spot are 40, 43 and 44. Since the pesky planes don’t stay still and change in number, it’s a bit hard to compare between reboots, but 42 is looking pretty good. I’m at 277 a/c and 954 messages /sec right now, which is about the best I’ve done in the past 30 minutes of gain setting.

Yes, the site is on White Tank. Connectivity is a 37 mi hop via 5 GHz.

Cheers - Jon N7UV

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Hi OBJ -

Thanks for the education! I seem to now remember somewhere someone was saying that on the forum. On your lead, I went to 45 for the gain setting then walked it down until I seem to be getting excellent range and the highest number of a/c and messages.

I do have the added challenge of near/far at the site, since there’s so many GA right around the peak. Here’s N423PA (local flight school) north of the site. Today, I think because of the gain changes, the path is much more solid with very few breaks, unlike before.

With a setting of 42 it seems that the total number of a/c that are greater than -3 have dropped to a dull roar, and the a/c at the very edge of coverage are now more like a -20 to -25 dBFS. That seems more like a linear receiver I’d expect.

Thanks again for your help.

Cheers - Jon N7UV

Hi Jon,
I live next to (I mean 300ft from) the Hudson river NYC VFR corridor.
I see heaps of traffic over -3db. I even get the NYPD and PAPD flying 100ft from my house.
I get about 1500-1700 ads-b/mode S message per second. ~ 5000-6000 Mode A/C per sec but it is not enabled at the moment.

Every setup is different. 38db gain worked for me with an FA 1.0 dongle with antenna <100Ft AMSL.
I would set it a little on the high side to get more late night/quiet time traffic.

I have a DPD antena, LMR400UF and a 9Mhz cavity filter. I think I had the gain at max for the nooelec dongle in the same setup.

The Dual band FA Antenna should go outside in a few days for dump978. I am currently using a directional but it is lower than the DPD antenna(on the same mast). I have another most for use with the dual band antenna.


Good day, Jon -

Long time no chat. Thanks for the insight on your setup. You are getting a lot more traffic than I would ever hope to :smiley:

What are you using for the processor? RPi or a full-on computer?

Cheers and 73 - Jon N7UV

I have 2 RPI3s, one RPI2 and an Odroid XU4.
The Odroid is really just needed for the airspy. It needs the CPU but mostly the USB Bandwidth.

My main feed is by a Radarcape.

Hi Jon -

My main concern was handling MLAT for the number of a/c seen at the new location. Sounds like it might be ok, given that you see so many more a/c than I do. However, I did get an RPi3 on order just in case.

Cheers and 73 - Jon N7UV

See what your CPU is like. My RPI3 seems to have more heat issues than the RPI2. It seems to throttle back the CPU more quickly than the RPI2(I do get occasional MLAT dropouts. 5-10 mins). I ended up getting a case with a fan for the RPI3 and most of the issues went away.

My value for money recommendation is still an RPI3(RPI2 in quiet areas) with an FA dongle/filter, decent coax and a high as possible antenna. Cavity filter is a good add-on for really busy areas like NYC, LAX and maybe DC(where there are multiple radars). Lightning protection as required(I have too many tall buildings and bridges nearby to need it. I use the cavity filter for static protection).

Hi Jon -

I was going to say that my RPi on White Tank can’t run in MLAT, as (at least yesterday early morning) it showed 100% CPU usage until I turned off MLAT. However, I turned on MLAT again an hour or so ago and the CPU usage is only 35% or so. Not sure what was going on.

Setup on White Tank is in the photo below:

I agree - filter is extremely important, especially in a high noise environment. I have a very nice custom filter in front of a preamp and the 1ppm SDR dongle. I spec’ed out a cavity filter, found a Shenzen-based filter manufacturer, and ordered 5 prototypes. +/-15 MHz BW, probably ~100 dB ultimate rejection (my little SA can’t measure that far down). Picture of filter response below:

Next I will put up another receive site on an 8,000’ mountain east of PHX. Should see better to the north, over the Mogollon Rim.

Cheers and 73 - Jon N7UV

Hi Jon, I’m thinking your 100% usage may have been attributed to the -10 gain setting you previously had. When you lower gain, you lower the amount of noise that needs to be processed - you can literally watch processor usage go down as you lower RTL gain. Your lower CPU usage today is most likely the by-product of lower RTL gain/noise.

That’s a very nice looking filter setup ~.52 insertion loss it looks like on center. I’ve been messing with a Mini-circuits CBP-1090C ceramic, but I’m not overly impressed as I’m seeing a ~2db dip on 1090Mhz center for some reason. I’m currently working with our local Mini-circuits rep to get that particular filter “connectorized” for further testing. I’d definitely have an interest in looking into your cavity setup for some testing.

Good morning, nitr0 -

Your observation echoes what I had been thinging. Yes, before I was attempting to run MLAT at -10 gain, and it was 100% CPU utilization. Now, I’m at a gain of 42, and the CPU is cruising along at a much more reasonable number, even with MLAT enabled.

However, I just ran a TOP command, and the CPU is back to 100%. According to the log,

[2016-08-31 06:33 MST] 223670 msgs recv’d from dump1090-fa (2452 in last 5m); 207788 msgs sent to FlightAware
[2016-08-31 06:36 MST] multilateration data requested
[2016-08-31 06:36 MST] Starting multilateration client: /usr/lib/piaware/helpers/fa-mlat-client --input-connect localhost:30005 --input-type dump1090 --results beast,connect,localhost:30104 --results beast,listen,30105 --results ext_basestation,listen,30106 --udp-transport nn.nn.nn.nn:6508:yyyyyyyyyy
[2016-08-31 06:36 MST] mlat-client(2464): fa-mlat-client 0.2.6 starting up
[2016-08-31 06:36 MST] mlat-client(2464): Using UDP transport to nn.nn.nn.nn port 6508
[2016-08-31 06:36 MST] mlat-client(2464): Listening for Beast-format results connection on port 30105
[2016-08-31 06:36 MST] mlat-client(2464): Listening for Extended Basestation-format results connection on port 30106
[2016-08-31 06:36 MST] mlat-client(2464): Input connected to localhost:30005
[2016-08-31 06:36 MST] mlat-client(2464): Input format changed to BEAST, 12MHz clock
[2016-08-31 06:36 MST] mlat-client(2464): Beast-format results connection with connection established

it’s cycling through this sequence every 5 minutes or so. Unfortunately, last evening it wasn’t doing this.

I’m going to leave it this way for the rest of the day, I’ve got to get to work, and it seems like it’s doing valid MLATs according to the FA website… The Pi3 shows up later today as well, but I can’t get back to the site until the weekend.

On the filter: I’d ordered a prototype batch of 6. I have several remaining even after my various projects. I could be persuaded to offer one at the price I paid so long as it’s going to a good home.

Cheers - Jon N7UV

Jon can you PM me the filter company? I would like to get one made for 978Mhz UAT as I can’t find any on the market. I do have the hab/nevis amp/filter but would also like to try a cavity filter. I have cavity filters in front of my radarcape, FA dongles and airspy.

I use the jetvision cavity filters for 1090Mhz … ucts/71010