Appreciate this has been covered in depth and apologies if this has been answered already
but would like to enquire; the default setting of -10 automatic gain control, why might someone deviate from that setting?
I’ve seen it mentioned that if very close to an airport then reducing gain may be an advantage. Would AGC not deal with very close aircraft?
I am around 12 miles from Heathrow, many planes pass over at around 8000 feet, when directly above the RSSI can drop to around -2/ but this generally shows between -15 & -20 all which appears fine but I want to visit a local airfield (with portable PiAware) where I will be yards from the runway. Should automatic setting be OK for this?
Just to confirm if I do alter; on the config settings are low numbers low gain?
Manual gain settings range from 0 to around 49dB. Larger numbers mean more amplification.
AGC is fairly confusing in the context of ADS-B, the AGC actually doesn’t work well with the type of signal that ADS-B is using, so the feedback loop ramps up to maximum gain and stays there. Due to an unfortunate (fortunate?) quirk of how the rtlsdr libraries configure the tuner, in AGC mode the maximum total gain is actually a little higher than the maximum gain you can set in manual mode.
So think of “-10” as effectively being “a bit more than 49dB”. (Yeah, it’s confusing)
If you’re right by the runway then you probably will want to set a lower manual gain, not AGC, maybe try 30dB as a starting point.
Since those RSSI numbers are negative, “higher” numbers mean smaller signal; -2dB is a stronger signal than -20dB.
So it is not correct to say “drop to -2dB”, it is correct to say “raise to -2dB”.
If you have lots of high-level signals (as in -2dB), they will overwhelm and saturate the receiver input, blocking the reception of weaker signals (far away). That’s when you want to lower the receiver gain, to allow more signals to be processed. AGC can’t work, because it is designed for fixed signal sources, with a low frequency signal intensity variation. ADS-B are nothing like that, because the signal sources are moving.
It’s an iterative process, because it depends of the actual number of planes and their location at a given time. So it will be a compromise.
PS: The “-10” is just a convention used to enable AGC in dump1090-fa , is not a real number. I think that dump1090-mutability uses “0” for that same effect.
also see comments from obj from may 2015 for another great explanation of how gain is set.
it would be great if there were a “knowledge base” tab on this site that contained certain topics (not available for comment or editing on that tab) such as gain, antenna styles etc. there are certain topics that keep coming up and some great explanations but they are hard to find sometimes. just a suggestion
I remember reading some time ago that -10 was something like 52 or 53. I still think using this nomenclature, -10, is confusing. In any case, with some experimentation, most sites will likely see better performance if using another setting. Go to the ‘standard’ maximum, 49.6, if you must, but I doubt -10 is the best setting in general.
Of course they are not. dB is a ratio.
RSSI is the signal “measured” level (where 0dB=1mW). Measurement is not very accurate i will say.
If you want an analogy is like the water flow/debit from a faucet.
Gain is the amplification of the signal before the measurement of the above level. It is like the setting of the faucet knob for that flow.
dBm and RSSI are different units of measurement that both represent the same thing: signal strength. The difference is that RSSI is a relative index, while dBm is an absolute number representing power levels in mW (milliwatts).
RSSI is a term used to measure the relative quality of a received signal to a client device, but has no absolute value. The IEEE 802.11 standard (a big book of documentation for manufacturing WiFi equipment) specifies that RSSI can be on a scale of 0 to up to 255 and that each chipset manufacturer can define their own “RSSI_Max” value. Cisco, for example, uses a 0-100 scale, while Atheros uses 0-60. It’s all up to the manufacturer (which is why RSSI is a relative index), but you can infer that the higher the RSSI value is, the better the signal is.
Since RSSI varies greatly between chipset manufacturers, MetaGeek software uses a more standardized, absolute measure of signal strength: received signal power, which is measured in decibels, or dBm on a logarithmic scale. There’s a lot of math we could get into, but basically, the closer to 0 dBm, the better the signal is .
FlightAware has a FAQ and newsletter for different topics.
If there is a bunch of questions about a specific topic we usually have a newsletter on it.
The newsletter you want is “Dynamic Range” for RSSI.
For reference, the dynamic range is based on the number of bits the receiver can measure. The prostick and the cheap RTL dongles have 8-bits of dynamic range. The newer consumer grade receivers coming out are around 11-12 bits of dynamic range (a bit more expensive). There are also specialized receivers with 16-24 bits of dynamic range but are very expensive (used mostly for military applications).
Wikipedia on Dynamic range
Also, if you have seen modern Television sets in the stores they are marketing HDR (High Dynamic Range) very heavily. This is because the way to differentiate TV is now on image quality (Dynamic Range) instead of the number of pixels (1090p vs 4k). Once the price drops for the 11-12 bit radio receivers you will see much more talk about having a higher dynamic range radio vs other radios.
Dynamic range (number of bits for radios) is nice to have but not required for a great ADSB system. If you set the gain correctly with a 8bit receiver (RTL dongles) you can see both far away and close by planes.
The main thing to remember is that the software and hardware matters a lot in radio systems. A correctly adjust 8bit radio can beat a badly configured 12bit radio. Also the difference between a correctly configured 8 bit and 12 bit system isn’t much. Something around 10% more messages for 12 bits. This difference isn’t due only to the extra bits but also other components used in 12 bit radios which are usually better.
Dxista, thanks for posting the prices and links to the radios. The prices going from 8bit to 12bit radios are in the 4x-5x the cost area. The prices are dropping with the air spy originally going for around $200 and the new mini around $100. These higher end radio can easily be the most expensive part of your system while also not giving the best performance per money spent.
Not as dump1090 measures it; it’s just power relative to the ADC full range (i.e. 0dB RSSI = max measurable power, -3dB is ~half that, etc) and is not related to any particular power level at either the ADC or the antenna. To generate actual power levels requires more knowledge of the receiver frontend than is easily available.
Gains should be roughly correct in terms of dB, but I have found that the values that the rtlsdr driver uses are a bit off from reality. They’re not really calibrated or anything and so much of these chipsets is undocumented, it can be hard to say what the nominal gain steps are meant to be.
But yes, in general, if you increase the gain by 5dB then you’d expect the RSSI for a particular message to go up by about 5dB too.
While the radios I referenced are ‘modestly’ priced, I agree with you, they are overkill for ADS-B.
I have an RSP1A, it’s a great radio for the price, but there is no way I would dedicate it to ADS-B reception only. The radio is too frequency agile and versatile to be used that way.
My current ‘top of the line’ SDR is an Icom IC-7300. The ‘beauty’ of the IC-7300 is that no computer is needed to use it, but you can and must, if you are going to ‘process’ the signal further.
seems odd to say signal went up from -20 to -2 even though it did.
