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