Thuraya ARFCN

historical

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: <http://lists.osmocom.org/pipermail/gmr/attachments/20200504/3e7e92c1/attachment.htm>