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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Replies: 10 comments 8 replies
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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?
…________________________________
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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Hi. Any further developments planned on this front? I see USCG and CBP MMSI's not transmitting position on my table of ships all the time. Would be nice to be able to "MLAT" them in some way with other stations. |
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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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I got interrupted with another related project, but I have several
receivers deployed and are feeding a database. The real issue is whether
that RSSI data is useful.
What is MLAT?
…On Thu, Oct 2, 2025 at 3:48 PM el-eternauta ***@***.***> wrote:
Hi. Any further developments planned on this front? I see USCG and CBP
MMSI's not transmitting position on my table of ships all the time. Would
be nice to be able to "MLAT" them in some way with other stations.
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MLAT (or multilateratiom) is the analogue of this concept but as used in the ADS-B world for tracking planes which are transmitting in position-less Mode-S. Basically it correlates the information from several overlapping receivers to calculate the airplane's position. I'm new to AIS and it kinda surprised me that the same capability doesn't seem to be implemented. |
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Well, that is one application of getting the RSSI. My original thought was
to use TDOA, but that's a bit more complex.
…On Thu, Oct 2, 2025 at 4:12 PM el-eternauta ***@***.***> wrote:
MLAT (or multilateratiom) is the analogue of this concept but as used in
the ADS-B world for tracking planes which are transmitting in position-less
Mode-S.
Basically it correlates the information from several overlapping receivers
to calculate the airplane's position.
I'm new to AIS and it kinda surprised me that the same capability doesn't
seem to be implemented.
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Right. Besides needing to scale the value for atmospherics, wouldn't reported RSSI values potentially vary across stations because of things like type and length of run of coax, the presence or absence of an LNA, type of antenna (both polarization and impedance matching)...? Or am I misunderstanding the concept? |
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My intent is to scale all of them based on the RSSI of a known good strong
station. Maybe that cancels out some of the factors. For evaluating ship
installation quality, a statistical cloud of data will do the trick. For
locating the transmitter, not so much,
A big factor is what way the boat is facing!
…On Thu, Oct 2, 2025 at 4:48 PM el-eternauta ***@***.***> wrote:
Right. Besides needing to scale the value for atmospherics, wouldn't
reported RSSI values potentially vary across stations because of things
like type and length of run of coax, the presence or absence of an LNA,
type of antenna (both polarization and impedance matching)...? Or am I
misunderstanding the concept?
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Ah, yes. that would work for me also. I'm in receiving range of a large, mountaintop, VTS station and several navigation aids... |
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The navaids seem the best.
…On Thu, Oct 2, 2025 at 4:58 PM el-eternauta ***@***.***> wrote:
Ah, yes.
that would work for me also. I'm in receiving range of a large,
mountaintop, VTS station and several navigation aids...
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I used the navaids to calculate my compensation for frequency shift and now
average about 0.2 ppm per hour, which is fine I guess.
…On Thu, Oct 2, 2025 at 8:18 PM SFSailor ***@***.***> wrote:
The navaids seem the best.
On Thu, Oct 2, 2025 at 4:58 PM el-eternauta ***@***.***>
wrote:
> Ah, yes.
>
> that would work for me also. I'm in receiving range of a large,
> mountaintop, VTS station and several navigation aids...
>
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I have totally been ignoring frequency shift. Is that gonna be a problem?
On Thu, Oct 2, 2025 at 5:22 PM el-eternauta ***@***.***>
wrote:
… I used the navaids to calculate my compensation for frequency shift and
now
average about 0.2 ppm per hour, which is fine I guess.
On Thu, Oct 2, 2025 at 8:18 PM SFSailor ***@***.***> wrote:
> The navaids seem the best.
>
> On Thu, Oct 2, 2025 at 4:58 PM el-eternauta ***@***.***>
> wrote:
>
> > Ah, yes.
> >
> > that would work for me also. I'm in receiving range of a large,
> > mountaintop, VTS station and several navigation aids...
> >
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Nope. This is not an issue any more for newer dongles. More for older or cheaper dongles without frequency stabilization. |
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Tbh, I have no idea.
…On Fri, Oct 3, 2025 at 11:17 AM SFSailor ***@***.***> wrote:
I have totally been ignoring frequency shift. Is that gonna be a problem?
On Thu, Oct 2, 2025 at 5:22 PM el-eternauta ***@***.***>
wrote:
> I used the navaids to calculate my compensation for frequency shift and
> now
> average about 0.2 ppm per hour, which is fine I guess.
>
> On Thu, Oct 2, 2025 at 8:18 PM SFSailor ***@***.***> wrote:
>
> > The navaids seem the best.
> >
> > On Thu, Oct 2, 2025 at 4:58 PM el-eternauta ***@***.***>
> > wrote:
> >
> > > Ah, yes.
> > >
> > > that would work for me also. I'm in receiving range of a large,
> > > mountaintop, VTS station and several navigation aids...
> > >
> > > —
> > > Reply to this email directly, view it on GitHub
> > > <
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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.