Re: [Discuss-gnuradio] Data Collection Across Multiple Machines and USRPs

[Marcus Müller]

Hey Jonathan,

when you cannot use GPSDOs, you should just sync your Laptops using NTP, and then set the laptop time as device time on the USRPs using set_time_now.

You can then agree on a specific point in time, use set_start_time on the USRP sources and try to estimate how well-coordinated you are by cross-correlating your measurements.

Greetings,
Marcus

On 03.06.2014 20:24, Jonathan Fox wrote:

Hey list, this is a question for anyone that has used the USRPs and GNU
Radio for synchronized data collection.

I need to collect data using USRP N210s across a wireless network for a
field test. Basically the USRPs would be connected to laptops running GNU
radio and the laptops themselves will be networked over WiFi.

My conceptualized collection would work as so: master computer that would
control the collection python scripts on remote machines and itself locally
(a simple USRP source to throttle to file sink). The caveat is these
scripts must start within a second of each other, so I am trying to avoid
delays and keep latency under a 1000 ms (preferably somewhere close to 500
ms). My initial testing of my idea at my work desk as been less than
spectacular. I was using a bash script that had two lines of code:

#!/bin/sh
./home/$USER/Documents/GNU_radio/data_collection.py
--uhd-addr=addr=10.2.8.104 &
ssh -t address@hidden python ~/Documents/GNU_radio/data_collection.py
--uhd-addr=addr=10.2.8.105 &

It is a very simple script executing a script on local and one remotely via
SSH, and according to the saved data files the file creation/modification
times are off. If the the save files are created from scratch, the timing
is extremely close and meets expectations when it is just two scripts. If
the files are already pre-existing then the modification times can range
from 1 to 5 seconds. When I add more scripts to the shell script. Like so:

#!/bin/sh
./home/$USER/Documents/GNU_radio/data_collection.py
--uhd-addr=addr=10.2.8.104 &
./home/$USER/Documents/GNU_radio/data_collection.py
--uhd-addr=addr=10.2.8.102 &
ssh -t address@hidden python ~/Documents/GNU_radio/data_collection.py
--uhd-addr=addr=10.2.8.105 &
ssh -t address@hidden python ~/Documents/GNU_radio/data_collection.py
--uhd-addr=addr=10.2.8.103 &

The times are even more off and can range from 3 to 10 seconds.
Alternatively, I can have dual USRP sources to file sink in the same script
to get back down to the original script and I have a 3 to 7 seconds gap
between the data files collected in the local script compared to the data
files collected by the remote script.

Is there a better way to go about collecting data quickly in synchronized
fashion? I thought about a timing function built in each GNU radio script
that should start the flow graph on any even second (based off a modulus
function of current system time in seconds) but I want to weigh all options
first.

Thanks for your time reading,

Jon

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