I mentioned earlier that I was running a test on gain settings and how it affected reception. That’s now finished and I’ve got some results:
I ran a script which cycled through 6 gain settings - 11, 13, 15, 17, 19 and 21. I chose those values to get a spread that was definitely too high and definitely too low for my receiver (autogain was typically staying around 17 when busy and 18 at night). The gain was changed every 10 minutes, so each cycle lasted an hour. The order was shuffled each time so that same gain was not used at the same time each hour. The script ran for a total of 48 hours to fill the graphs1090 1 minute resolution database. This test was run with 2.2-RC24.
Setting the gain to a known value allows comparisons which can’t be made with auto gain, since then the gain is dependent on the conditions. By using a known value, we can see the effect on reception.
Here is the signal to noise ratio:
The results are reasonably consistent from 13 through 19, with only small changes within those values. At 19 there is a slight decrease in the average SNR, but the change is small. 11 and 21 are definite outliers with noticeably worse performance.
Plotting the density of values for median SNR also shows clearly what is happening:
11 and 21 are clearly much worse, but it can also be seen that the middle values 15 and 17 provide the best performance.
Here is the range between the highest and lowest SNR - higher is better in this case:
How does this translate into what we are really interested in, successfully decoded messages?
Firstly range:
There are two noticeable effects - firstly that gain as low as 11 has a definite effect on maximum received range. Second, that once the gain is at 13 or above there is minimal change. The maximum reception range is limited by terrain only, not signal strength.
Messages:
The result here follows a similar pattern. A gain of 11 has a measurable effect on the number of messages received, with an average across the sample period nearly 9% worse than the best figure. The peak message rate was with gain at 17.
Looking more closely at the distribution:
From gain 15 and above, the average message rates are very similar. Gain 13 definitely produces slightly lower message rates than the higher values.
Aircraft seen:
Again the effect of lower gain is clear - fewer aircraft are detect with gains 11 and 13. The highest average was actually with the gain at 21, but the variation between 15 and 21 is really quite small.
Summary data:
Conclusions:
The most obvious conclusion is that having the gain set too low has a definite and obvious detrimental effect on the maximum range and number of aircraft seen, and consequently the number of messages received.
Second, that too much gain has a measurable negative effect on RF performance, but the impact on the ability to successfully decode messages appears to be minimal if any. I’d want to repeat this over a longer period to average out variability due to traffic conditions before saying that definitively, but it doesn’t really seem to be a problem. This test doesn’t look at data quality however, so it’s possible that at higher gains some close range messages are lost that otherwise wouldn’t be.
I think this shows that the current autogain is doing a nice job of selecting an appropriate value - For my receiver, it is generally at 17 or 18, which puts it near the highest SNR range. It’s high enough to be getting the maximum possible range, while giving some headroom for very strong close range signals.