Re: [Discuss-gnuradio] Help about using gr-air-modes

[Nick Foster]

On Mon, Jan 27, 2014 at 12:23 AM, Cheng Chi <address@hidden> wrote:

Hi Nick,

Thanks for your quick reply!

I have to use a 10M sampling rate in my case, but due to computer constrain, modes_rx will cause overflow when used directly with -r 10000000. I gauss it's because it's sampling data in float? I am using a GPSDO with USRP.

So I record data in short at 10M sampling rate, convert from short to float and then input to modes_rx. The output looks like this:

{{{

(27 0.00000000) Type 0 (short A-A surveillance) from dc2ed9 at 45900ft (speed 600-1200kt)

(23 0.00000000) Type 4 (short surveillance altitude reply) from 2443b0 at 59800ft (GROUND ALERT)

(24 0.00000000) Type 4 (short surveillance altitude reply) from 3b1fc3 at 3200ft (SPI)

(21 0.00000000) Type 21 link capability report from d5ca0e: ACS: 0xda21, BCS: 0xc923, ECS: 0xee, continues 8 ident 123e

(22 0.00000000) Type 5 (short surveillance ident reply) from 5ad8b4 with ident 728 (GROUND ALERT)

(37 0.00000000) Type 17 BDS0,9-1 (track report) from 7805dd with velocity 218kt heading 238 VS -1664

}}}

The timestamp are all zeros. 

This appears to be a bug. Can you paste the entire output of gr-air-modes?

I'm also concerned by the data you've shown. There is only one real reply in the above data -- the last one. The others are all spurious replies. Can you tell me the command line you're using as well as the equipment setup -- daughterboard, antenna, etc.?

 

Another question is that if I use modes_rx, how to save the sampled complex baseband signal? Is the data saved somewhere that I've missed?

This is not something modes_rx is designed to do. I suggest instead that you record samples to disk using uhd_rx_cfile, and then run modes_rx on the saved file using --source=<filename>.

--n

 

Best regards,

Cheng Chi 

On Mon, Jan 27, 2014 at 3:57 PM, Nick Foster <address@hidden> wrote:

On Sun, Jan 26, 2014 at 11:48 PM, Cheng Chi <address@hidden> wrote:

Hi,

I am using gr-air-modes for decoding the air plane signal with USRP. I've successfully used the "modes_rx" and "modes_gui" for decoding the mode-S packets.

However, it seems that the modes_rx or modes_gui can't provide the timestamp of the mode-S packets being decoded. Is there any option that I can set to timestamp the mode-S packet? The reason I want this timestamp function is that I want to know the decoded packet data correspond to which part of the raw data (complex baseband data samples). 

If you're using a USRP, you should be getting a timestamp. It's the second number printed, as in the following:

(-14 1.29258827811) Type 0 (short A-A surveillance) from ab2984 at 3000ft

If you are using a GPSDO with your USRP, the printed time will be in UTC seconds. Otherwise, it will be in seconds since the application started running.

--n

 

Thank you for any help you can provide in this situation.

I found that there's a file called "air_modes_preamble.cc" seems to provide the timestamp function. Does anyone know how to use this file separately? 

Best regards,

Cheng Chi

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