[USRP-users] rx_time for radar application

Martin Braun martin.braun at ettus.com
Tue Mar 22 18:55:33 EDT 2016


Aaron,

to quickly add to this thread, I did some radar stuff with N210 a while
back and it worked fine. For a fixed setup the tx/rx lag is
deterministic and you can calibrate it out. I was doing measurements on
the order of meters with great success. You need to sync the clocks,
obviously, and then calibrate your sample offset (which is caused by the
DDC/DUC stages, as someone pointed out, as well as other delays in the
analog bits).

For calibration I used a device called an "undergraduate student"; in my
case it was much simpler though because we calibrated against a target
that was something like 20m from the USRPs.

Hope this is of any use,

Martin

On 03/22/2016 08:46 AM, Aaron Smith via USRP-users wrote:
> Hello,
> 
> I was hoping someone with experience could help me with the situation below.
> 
> Background:
> 
> My research group is trying to use a N210 USRP to sense the altitude of
> the ionosphere. Originally we were going to transmit our own signals, so
> to test this, I connected my TX daughterboard directly to my RX
> daughterboard.
> 
> I then used the function set_start_time() on my tx and rx to build up a
> little test case. I had a vector source -> repeat block -> usrp sink ->
> usrp source -> fir dec low pass filter -> file sink and this caused a
> discrepancy in samples of +50 samples (which would translate into
> kilometers--big deal). I attributed this to buffer loading and several
> other forms of non-zero lag between blocks. 
> 
> To simplify the issue, I'm going to focus on precise receiving. I am
> going to attempt to capture the WWV atomic clock signal out of Colorado,
> but I still need to be able to convince myself the the time I think I
> received the first sample, is actually the time I received my first sample. 
> 
> I am going to abandon using the set_start_time() as an absolute and
> instead hope to use the file meta sink to extract the rx_time tag. To
> handle clock synchronization, I am using the set_clock_source() and
> set_time_unknown_pps() functions with a GPS pps signal.
> 
> Questions:
> 
> Is this as simple as connecting a USRP source block up to a meta file
> sink block? If I add a block in between the USRP source and the meta
> file sink (like a decimator), will that impact my rx_time tag? Can I
> trust the the rx_time tag is the time that the receiver received it's
> first sample? Would you recommend this approach, or is there a better way?
> 
> Thank you for your time,
> 
> 
> 
> Aaron Smith
> 
> 
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