[USRP-users] rx_time for radar application

Thomas Wagner twagner at kurma.com
Tue Mar 22 12:40:46 EDT 2016

Hi Aaron,
I am working on a similar timing  problem and need very accurate sample timing between several receivers.  Any information on Rx timing will be helpful.  
Are you trying to measure the time delay between the direct WWV signal and the ionospheric bounce, or transmit a signal to measure how long it takes to get back?  Do you want the electron density profile?


Tom Wagner
twagner at kurma.com
Tom.Wagner at tautechnologies.com
310-387-4929 cell

> On Mar 22, 2016, at 09:46, Aaron Smith via USRP-users <usrp-users at lists.ettus.com> 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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