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Thread: Different gain for day and night

  1. #1
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    Different gain for day and night

    Has anyone else tried using different gain settings for daytime and nighttime?

    I've been trying this for the last couple of weeks with good results.

    A bit of background first:
    Raspberry Pi, RTL-SDR dongle, LNA, homemade coax collinear antenna, all in a loft.
    House is between Heathrow and Luton, under fairly busy airspace.

    I had noticed that overnight my signal level and noise levels were well below what they were in the day. I had set gain so that the max level was around -3dBFS, so the setup was about as as good as a newbie like me could do. I realised that the high levels were caused by aircraft in stacks or approach/departure, and that as noise considerations meant this didn't happen overnight, I might be able to increase my sensitivity during that time.

    Looking at the signal level plots, I reckoned a good time for switching was 2330 and 0530. I made copies of the dump1090-mutability config file with different gain settings, and created cron jobs to copy them into /etc/default then restart dump1090-mutability.

    Timings and gain settings will obviously vary by location, but I can increase the gain by around 10 dB overnight. This brings max signal level and noise back to what they are in the day. Message rate and coverage/max range are now better at night than in the day. YMMV!

    I noticed that the stats do odd things at the switchover (e.g. the message rate spikes). I assume this is related to some buffering during the restart. I've got a small concern about how the restart might manipulate the stats, but haven't noticed anything else yet.

    Would love to hear people's thoughts and experiences of this.
    JFreddieF
    T-EGWU97

  2. #2
    Captain abcd567's Avatar
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    Good idea to change gain according to traffic.
    Are you using dump1090-mutability ver 1.14 or ver 1.15~dev ?

    You can find version installed by following command:

    Code:
    apt-cache policy dump1090-mutability

  3. #3
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    Quote Originally Posted by JFreddieF View Post

    Would love to hear people's thoughts and experiences of this.
    In addition, other RF noise sources such as cell phone base stations and other transmitters may be less of an issue in the small hours.

  4. #4
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    I'm running 1.15~dev. (As an aside; if you have it serving maps the version is shown above the aircraft list).

    Good point re: RF noise. I have a phone mast a few tens of metres from my site.

    My next thoughts on this are whether the optimum settings might vary by day of week. Figuring this out will take a long time, and after only a couple of weeks I may be seeing patterns where there aren't any. The variation might just be changes in runway in use or stack location.
    Last edited by JFreddieF; 2018-10-06 at 13:06. Reason: Fixed typos
    JFreddieF
    T-EGWU97

  5. #5
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    Quote Originally Posted by JFreddieF View Post
    I'm running 1.15~dev. (As an aside; if you have it serving maps the version is shown above the aircraft list).

    Good point re: RF noise. I phone mast a few tens of metres from my site.

    My next thoughts on this are whether the optimum settings might vary by day of week. Figuring this out will take a long time, and after only a couple of weeks I may be seeing patterns where there aren't any. The variation might just be changes in runway in use or stack location.
    There are so many factors that can affect received rates; sadly I am located in an area where 300 a/c a day is a very good day. 'Stacks' won't affect you greatly as those at Heathrow don't change with runway configuration (unless its changed since my days there), although runway in use will affect the actual flight paths to landing. For example aircraft that approach and possibly hold at Bovingdon will either head off SE if EGLL are landing 27 L/R or head WSW if landing 09 L/R. Departures will actually vary much more for your location. Weather may also have an affect, but it is more pronounced with longer ranges, less than 120 nm or so probably won't change much.

  6. #6
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    Thanks for your insights. I thought that the stacks moved to spread the noise footprint around, but I've no idea where I got that from. I may have just made it up myself!
    JFreddieF
    T-EGWU97

  7. #7
    Purser
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    Quote Originally Posted by JFreddieF View Post
    Thanks for your insights. I thought that the stacks moved to spread the noise footprint around, but I've no idea where I got that from. I may have just made it up myself!
    They (still?) operate a preferential runway system, whereby all landings were conducted on the 27 L/R runways unless the tailwind exceeded a certain figure and they changed to 09 L/R. This was to keep noise down over central London (landings are quieter than take-offs). They also alternated the use of 27L and 27R by switching landings and take-offs between them at 1400 each day (this may well have changed). The four main 'stacks' at Biggin Hill, Ockham, Bovingdon and Lambourne I believe still exist after almost 50 years with just minor changes of name etc.

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