From johann.tzt at gmail.com Fri May 1 15:25:37 2020 From: johann.tzt at gmail.com (Johann T.) Date: Fri, 1 May 2020 17:25:37 +0200 Subject: Thuraya ARFCN In-Reply-To: References: Message-ID: Hi Sylvain, Thank you for your quick reply. Much appreciated! I will try your suggestions and come back to you when I have more results. Best regards, Johann On Thu, Apr 30, 2020 at 6:53 PM Sylvain Munaut <246tnt at gmail.com> wrote: > Hi Johann, > > So, first wrt to different formula, the spec is definitely right ... > (I mean it's the spec ...). > However they are narrow channels and if your B210 is not GPS > disciplined, its oscillator could be out quite a bit. It's default > xtal is only rated to +- 20ppm and that's a 30 kHz error at 1.5 GHz > ... so you can be off by a full channel just with that. > > Second, the frequency you give to gmr1_rx_sdr.py is where to tune. In > reality you want to tune not to the channel directly but a bit to the > side so that the DC offset and 1/f noise at the center of the capture > don't mess up the data. The script will automatically filter and > recenter the proper frequency depending on the -a argument you passed > to it. > > Finally that python script only does the sample and channelization, > and saves to a cfile in /tmp it doesn't do any of the decoding at all. > For that you need to also run gmr1_rx binary ( or gmr1_rx_live , see > the live branch ) on the recording. And it's that one that will take > the raw samples do all the demod and decoding of the channels and send > the data to wireshark. > > Cheers, > > Sylvain > -------------- next part -------------- An HTML attachment was scrubbed... URL: From johann.tzt at gmail.com Mon May 4 14:09:17 2020 From: johann.tzt at gmail.com (Johann T.) Date: Mon, 4 May 2020 16:09:17 +0200 Subject: Thuraya ARFCN In-Reply-To: References: Message-ID: Hi Sylvain, Thanks again for your suggestions. I have performed some captures with some offsets w.r.t. to channel frequency. Then I ran the gmr1_rx binary on the recording. In your presentation video during CCCH, you mentioned that sps should be set to 4. The Wireshark output is shown below. GSM TAP Header, ARFCN: 0 (Downlink), TS: 0, Channel: CCCH (0) Version: 2 Header Length: 16 bytes Payload Type: GMR-1 air interfeace (MES-MS<->GTS) (10) Time Slot: 0 ..00 0000 0000 0000 = ARFCN: 0 .0.. .... .... .... = Uplink: 0 Signal Level (dBm): 0 Signal/Noise Ratio (dB): 0 GSM Frame Number: 30 Channel Type: CCCH (2) Antenna Number: 0 Sub-Slot: 0 Perhaps the reception of the Thuraya signal is rather bad on my side. When I used the osmocom_fft to check the broadcast channel, I could not identify it. Switching to a homemade LHCP antenna didn't seem to help either. I might try different locations next time. Currently I used a compass to roughly align my antenna direction towards the satellite (at 44deg E). What do you think? Thank you in advance. Best regards, Johann On Thu, Apr 30, 2020 at 6:53 PM Sylvain Munaut <246tnt at gmail.com> wrote: > Hi Johann, > > So, first wrt to different formula, the spec is definitely right ... > (I mean it's the spec ...). > However they are narrow channels and if your B210 is not GPS > disciplined, its oscillator could be out quite a bit. It's default > xtal is only rated to +- 20ppm and that's a 30 kHz error at 1.5 GHz > ... so you can be off by a full channel just with that. > > Second, the frequency you give to gmr1_rx_sdr.py is where to tune. In > reality you want to tune not to the channel directly but a bit to the > side so that the DC offset and 1/f noise at the center of the capture > don't mess up the data. The script will automatically filter and > recenter the proper frequency depending on the -a argument you passed > to it. > > Finally that python script only does the sample and channelization, > and saves to a cfile in /tmp it doesn't do any of the decoding at all. > For that you need to also run gmr1_rx binary ( or gmr1_rx_live , see > the live branch ) on the recording. And it's that one that will take > the raw samples do all the demod and decoding of the channels and send > the data to wireshark. > > Cheers, > > Sylvain > -------------- next part -------------- An HTML attachment was scrubbed... URL: