Re: [Discuss-gnuradio] usrp_wxapt_rcv.py

[Bob McGwier]

I am the author.   The bandwidth of the signal transmitted from the APT 
satellites (NOAA) is 30 kHz but this does not allow for doppler.

The basic RX is almost wholly inappropriate for NOAA APT and for 
anything band pass sampling application without appropriate filtering.  
Operating a 60-ism MHz sample rate A/D means the Nyquist frequency is 
30-ish Mhz.  So all of the signals you are talking about are done with 
undersampling, using aliasing.  But,  FM transmitter are extremely 
strong.  APT are NOT.  All of the noise that is in every 30-ish MHz 
segment up and beyond are folded down on top of the signal of interest.  
This is less important in FM than in weak signal satellite work.

I never considered that anyone would do this or I would have tested for 
the presence of an appropriate tuner or an override be asserted.

Bob




Patrik Tast wrote:
> Hi All,
>  
> My name is Patrik Tast from Vasa, Finland.
>  
> I this week received the Ettus USRP with daughterboards BasicRX and 
> 800-2400 MHz RX from CA, US.
> I fired it up today to test the BasicRX side using the usrp_wfm_*.py 
> and nbfm and everything works as expected. I am able to hear what 
> ever FM channels with great quality.
>  
> Since I am a wxapt guy, http://www.poes-weather.com/, the 
> usrp_wxapt_rcv.py was my next test in the examples directory.
>  
> NOAA 17 @ 137.62 MHz was here just (usrp_wxapt_rcv.py -f 137.62e6 -V 
> 0) and all I could hear was German radio comming through. Extremly 
> faintly, I could hear the TICK-TOCK sound in the background (I also 
> could have been imagining, it was so unclear) and (just) some change 
> in the graph while the satellite was over me (45 degrees).
>  
> When I changed to the R2FX-receiver (dedicated APT-receiver) the 
> signal is there http://www.df2fq.de/english_version/r2fx_eng.html
>  
> I could not find an author to the usrp_wxapt_rcv.py nor could I find a 
> GRC block diagram for it.
> I can see that the author is aiming for 32 kHz bandwith but I suspect 
> there is something missing?
> APT spec is at http://www2.ncdc.noaa.gov/docs/klm/html/c4/sec4-2.htm
>  
> I'm using Fedora 9 and GnuRadio 3.1.3 (latest stable release, not from 
> svn)
>  
> Any help appreciated,
> Thanks,
> Patrik
>  
>  
>  
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