Modifying Offset Stream Tag

[Adam Gorski]

Hello community,

 

I am currently using an out of tree module (gr-adsb) with GNU Radio 3.7 that captures received ADSB messages. I would like to change the timestamp resolution for each message received from microseconds to nanoseconds. The timestamp of each message is declared in the demod.py (attached) by adding the start time of the block to the offset stream tag (line 111 in demod.py).

 

The offset stream tag is not declared within the out of tree module, but somewhere in the GNU Radio source code. There are a handful of python files within the GNU Radio source code that include the offset tag, however I believe the declaration may lie in one of the following files (these have essentially the same code):

• ~/gnuradio/gnuradio-runtime/python/gnuradio/gr/packet_utils.py
• ~/gnuradio/gr-digital/python/digital/utils/tagged_streams.py

 

I would like to declare the offset tag with a nanosecond resolution. I have been able to accomplish this type of declaration with the demod block start time by calling a c function within a shared library, however I’m unsure how to do the same within GNU Radio source code as any sort of test print statements within the two files listed above never gets displayed in the flowgraph output.

 

My questions:

• Is one (or both) of the files indicated above the correct file to modify?
• How do I test modifications to GNU Radio source code files?
• Are there other methods of changing the offset tag resolution I should consider?

 

The c function and shared library code is as follows. I chose this way due to the flowgraph code builder in GNU Radio 3.7 using python2; if I used a python3 function I believe I’d have to rewrite the flowgraph code builder in python3.

 

Time_ns.c:

 

#include <stdio.h>

#include <time.h>

 

long long time_ns() {

    struct timespec t0;

    clock_gettime(CLOCK_REALTIME, &t0);

    long long ns = (t0.tv_sec * 1000000000) + t0.tv_nsec;

    return ns;

}

 

int main()  {

  return 0;

}

 

Next use this command to create a shared library:

cc -fPIC -shared -o time_ns.so time_ns.c

 

Ns.py:

 

from ctypes import *

so_file = '~/time_ns.so'

c_time_ns = CDLL(so_file)

c_time_ns.time_ns.restype = c_longlong

return c_time_ns.time_ns()

 

Thanks,

 

Adam Gorski

Virginia Tech Applied Research Corporation (VT-ARC)

Wireless Communications Systems Engineer

410-818-3188

 

demod.py
Description: demod.py