SDR P25 trunking decoding

Enthusiast discussion on trunked radio networks, protocols including MPT1327, TETRA, EDACS, SMARTNET - as well as talk group ID's
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juzzbott
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SDR P25 trunking decoding

Post by juzzbott » Wed May 29, 2019 10:14 pm

Hey all,

I've been following the following tutorial on how to get my SDR dongle to work with trunking networks: https://www.hagensieker.com/wordpress/2 ... r-dummies/

I'm trying to scan for the CFA and SES channels, using the site closest to me, on the new RMR system.

I've got it working, following the guide, mostly. I've setup the trunking files and the op25 script *seems* to be working. However, I'm not able to hear anything, and the program is picking up an unknown "channel" with the id of 3210, instead of one of the channels in the list at: https://www.radioreference.com/apps/db/ ... all_tg#tgs.

Furthermore, it's able to pick up id 30516 - CFA Dispatch District 16, however I'm not able to hear any volume from this. Is this because the channel is encrypted? Or is there something else going on that I'm not sure about?

If anyone has had any experience with the op25 SDR trunking, and could give me a help out, it would be greatly appreciated.

Cheers,
Justin

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Phil
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Re: SDR P25 trunking decoding

Post by Phil » Thu May 30, 2019 9:53 am

Justin,

what software are you using for your operating system as there may be a much easier way to do this
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juzzbott
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Re: SDR P25 trunking decoding

Post by juzzbott » Thu May 30, 2019 11:03 am

Hi Phil,

I'm currently running Linux Mint 19.1.

I've just also discovered the SDRTrunk java based software and I have that up and running, and seeing the control frequencies and it's recognising Group Call, Patch Call, Encrypted Group Call etc. However, again, I'm still unable to hear anything on these channels.

I figure with the encrypted stuff I won't hear anything, but I should still be able to hear the rest?

Cheers,
Justin

juzzbott
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Re: SDR P25 trunking decoding

Post by juzzbott » Thu May 30, 2019 11:57 am

Further to above, I'm not sure I'm even on the correct control frequency.

I'm trying to listen via this site: https://www.radioreference.com/apps/db/?siteId=22630 as it's the closest to me.

The control frequencies for this site are: 166.5875 (primary) and 166.4375 (alternate) however I can't hear the digital tone that I should be hearing on these channels, there is nothing there. The closest I have, and what I have been using is 166.432. This appears to be a trunked signal, the op25 software from my original post picks this freq up, as does the SDRTrunk software, which shows it as a Control freq and lists call events (mostly data only). However, this 166.432MHz frequency is not listed anywhere reference for the RMR site https://www.radioreference.com/apps/db/?sid=7679. As a CFA vol, I have access to the CFA frequencies for the RMR in my district, but again, this 166.432 freq is not listed.

So I don't know if my SDR dongle is out of whack or something and I actually am on 166.4375 which is the alternate control freq for that site?

Cheers,
Justin

Comint
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Re: SDR P25 trunking decoding

Post by Comint » Thu May 30, 2019 6:21 pm

The frequencies are in 12.5 kHz steps, so 166.432 is NOT a valid frequency.

You should have some instructions in the SDR set-up procedure about a "correction factor".

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Comint

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VK3RX
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Re: SDR P25 trunking decoding

Post by VK3RX » Thu May 30, 2019 8:11 pm

I don't know how up to date any Australian data is on the RR site. It is dependant on people notifying them of new data and changes.

I suggest instead you look at the data here:

https://vicradiozone.com/government-net ... ork-sites/

juzzbott
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Re: SDR P25 trunking decoding

Post by juzzbott » Thu May 30, 2019 9:46 pm

@Comint:
I don't understand how I can be receiving signals on that frequency then? Unfortunately the dongle came off ebay, so instructions is not something that it has a lot of. However I know it seems to be reading actual frequencies because I can get pager messages from EAS 148.9125 and the FM radio stations 101.1FM and 101.9FM.

@VK3RX:
I've also crossed check that site, and still can't seem to find relevant frequencies. I feel like I'm missing something with the setup of the trunking perhaps?

Image

Image

Comint
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Re: SDR P25 trunking decoding

Post by Comint » Fri May 31, 2019 12:06 am

I am not familiar with the software you are using, but to receive Trunked Radio Audio, you usually need two receivers.
One to listen to the Control Channel, and one to listen to the relevant Traffic Channel (ie the Voice Channel).
When a Call is being made, the Control Channel tells the second receiver which Traffic Channel frequency to go to, and you would then hear the audio, assuming it is not encrypted.

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Comint

juzzbott
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Re: SDR P25 trunking decoding

Post by juzzbott » Fri May 31, 2019 8:53 am

OK, so I realised that my PPM correction was out by about 30. And now with that, I've managed to get it to the correct control freq of 166.4375.

When using the original OP25 software, it now seems to be working, and I can hear some audio, however it just sounds all jumbled. I'm assuming this is an encrypted channel? I'm still not sure where the talk group 3210 is where all this conversation is happening?

Finally, is it possible to listen to other talk groups on the network? For example, as part of the CFA, we can hear all traffic when tuned into VICFIRE 515, i.e. other stations responding. How would I go about setting this up?

Or is SDR trunking just not worth it, and should I look to get an actual hardware scanner such as UBCD436PT or UBCD536PT?

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VK3RX
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Re: SDR P25 trunking decoding

Post by VK3RX » Fri May 31, 2019 11:05 am

3210 is one of the encrypted police TGs. They do all seem to be in the 3nnn series.

juzzbott
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Re: SDR P25 trunking decoding

Post by juzzbott » Fri May 31, 2019 2:00 pm

That would be why it sounds so garbled then. I've managed to blacklist that TG and I can now hear District 16 CFA / SES Channels. Which is great as I now have this working sufficiently!

I'm just wondering though, is the system setup so that only certain CFA districts go through certain towers? How do the radios at the station hear multiple dispatch groups? Or is that some kind of "multi-channel" system?

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