Originally posted by g7ruh
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The code linked is very much exploratory/experimental in nature because I don't yet know how best to characterise the quality of ADS-B messages, so check back periodically and look at the commit log where I explain what changes I've made and why in some detail.
I only know that, when I look at the dump1090 web page, some tracks are mostly unbroken and some tracks are mostly broken up linked by dotted estimation lines. It's the latter I want to characterise, hence looking at mean and standard deviations. They won't yet mean much on their own, not until we have a better idea of what to expect from a "good" installation (which mine is not because the antenna is in the loft, not on the roof).
So far, I've found that while the s.d. of displacement ratio (against displacement at last known airspeed) seemed plausible, the inter-arrival s.d. was wildly variable and very difficult to make any sense of. I could speculate why that might be but it would be pure guesswork.
Yesterday afternoon, I pushed a change to adsb_msg_dist that significantly changes the way that intervals are calculated. Whereas before I used to look consider intervals between any type 3 or 4 message (position and velocity respectively), I now consider only intervals between position messages.
Since then, the interval s.d. has dropped to a much more sensible level and in fact, mean interval ≈ mean displacement and interval s.d. ≈ displacement s.d. This is not surprising; they are, after all, distributions of the same data (position messages), but congruence validates the displacement ratio as a metric (as opposed to raw displacement in n.m. or feet) is working as intended.
The next step is probably to try binning a/c by range from receiver on the basis that longer range messages are likely to experience a higher degree of loss than closer range messages and, currently, there is no distinction made between them. If the bins are small enough (maybe 25 nm might be a reasonable size), then that gives a parametric profile of any given receiver from which receiver pattern might be deduced.
Question is whether the bins ought to be linearly spaced or whether they should be proportional to the inverse square of range on the basis that signal strength falls off in proportion to the square of distance. In terms of comparing like with like, that would seem sensible to me.
I elaborate further on interpretation of the mean and s.d. in the README.
Originally posted by g7ruh
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What gain are you using? I noticed that my SNR went down a bit afterward but I seem to get a substantially higher peak (valid) message rate and my range and count statistics improved by between 50–100%.
Originally posted by g7ruh
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Originally posted by g7ruh
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Originally posted by g7ruh
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IIRC, ADS-B is pulse-position modulated which is essentially still amplitude modulated binary bits. Because there's no active symbol for a binary 0, (only the position of the 1 relative in time), it's not straight forward to discern the difference between noise of a '0' and '1' from a more remote transmitter.
Dump1090 currently records the mean and peak signal strength in dBFS (dB Full-Scale) and at least -mutability reports noise floor, but it doesn't attempt to detect collisions. One possible way to do that might be to look at the amplitude of where a '0' ought to be relative to the noise floor, but this is complicated by phase differences between different a/c.
Originally posted by g7ruh
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