Usrp_wxapt_rcv.py


#1

Hi All,

My name is Patrik T. 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


#2

2009/3/21 Patrik T. removed_email_address@domain.invalid:

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.

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).

I suspect you’re just seeing the effect of vastly different signal
strengths and a receive daughterboard with no filtering or gain.

Broadcast band FM station power is measured in kilowatts or tens of
kilowatts, and result in high signal levels at the BasicRX input.
These are easy to demodulate. However, the APT transmissions are
likely several orders of magnitude lower in level at the antenna
input, and without any filtering or gain, probably don’t rise above
the noise floor of the ADC.

At 137 MHz, you should be using the TVRX daughterboard which is based
on a VHF/UHF cable TV tuner, and provides the right sort of
prefiltering and receiver gain to successfully decode APT.

Johnathan


#3

Hi Bob,

I’m sorry if this is a cross post.

(NOAA) is 30 kHz but this does not allow for doppler.
Doppler shift at 137 MHz is < 10 kHz and will (should) not be an issue.

I added an pre-amplifier at the antenna and could hear sumthing, though
not
enough signal to produce an image.
I found the NB-FM tuner to work best for the BasicRX (after I fiddled
with
it some).

A tuned NBFM py, screenshot and recorded audio is at
http://www.poes-weather.com/~patrik/usrp/BasicRX-APT-Test/
I ordered the TVRX daughterboard a few days ago from Holland…

Now my next question:
I got the DBSRX and my intention is to aim a yagi stack at at NOAA-POES
and
receive HRPT at 1.7 GHz.
Do you think this is possible?

Thanks,
Patrik

----- Original Message -----
From: “Bob McGwier” removed_email_address@domain.invalid
To: “Patrik T.” removed_email_address@domain.invalid
Cc: removed_email_address@domain.invalid; “Jerry” removed_email_address@domain.invalid
Sent: Thursday, March 26, 2009 11:47 PM
Subject: Re: [Discuss-gnuradio] usrp_wxapt_rcv.py


#4

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 T. wrote:

Since I am a wxapt guy, http://www.poes-weather.com/, the

Thanks,


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