Signal Power and RSSI #328
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Outputs from the program variously provide "signalpower" or RSSI. Are these just synonyms for the same value? |
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It is the same. All the samples are normalized to -1 to 1 (so for RTL-SDR this means / 127). And at sample rate of 48K/seconds, it takes 10 * log10 of the the average of the sum of squares of the IQ samples. |
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Thanks for the answer! So, if I take that value as an exponent for ten (10^RSSI) I'll get a value directly proportional to received signal strength then.... right? This will be helpful for me scaling signal strength by distance from receiver. |
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Yes, you get the sum of squares of the sample, but typically I thought taking the 10log of that is a better measure for comparison?
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Van: SFSailor ***@***.***>
Verzonden: vrijdag 22 november 2024 01:07
Aan: jvde-github/AIS-catcher ***@***.***>
CC: Jasper ***@***.***>; Comment ***@***.***>
Onderwerp: Re: [jvde-github/AIS-catcher] Signal Power and RSSI (Discussion #328)
Thanks for the answer!
So, if I take that value as an exponent for ten (10^RSSI) I'll get a value directly proportional to received signal strength then.... right? This will be helpful for me scaling signal strength by distance from receiver.
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The 10Log is just fine, and depending on how I calculate raw masthead signal strength may be quite convenient. If not, I'll do other math. Basically, the intent is to infer the radiated power at the ship's antenna based on RSSI and distance from receiver. I'd expect to further scale the value for atmospherics based on a known good fixed transmitter operated by the USCG. I have not particularly looked at coding the math yet. First step is capturing the data so I can manipulate it. This is not my strong suit! |
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Nice! At some point i was planning to use these RSSI and drift to detect spoofing (or just incorrect lat/lon). That is the nice thing about using the RTL-SDR. Never had time to look into it though. Let me know how it goes. |
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Yeah, a pal and I actually proposed a similar anti-spoofing system to the
Coast Guard. We debated TDOA vs RSSI at some length. My goal is actually
to detect poor installations on ocean-going boats, but with even two
receivers one could get a clue as to when a signal isn't quite kosher.
Of course, our local USCG has radar.....
…On Sat, Nov 23, 2024 at 10:04 AM Jasper ***@***.***> wrote:
Nice! At some point i was planning to use these RSSI and drift to detect
spoofing (or just incorrect lat/lon). That is the nice thing about using
the RTL-SDR. Never had time to look into it though. Let me know how it goes.
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It is the same. All the samples are normalized to -1 to 1 (so for RTL-SDR this means / 127). And at sample rate of 48K/seconds, it takes 10 * log10 of the the average of the sum of squares of the IQ samples.