Re: [Discuss-gnuradio] Homing in on the mystery of Pulsy McGrooder

[David I. Emery]

On Wed, Sep 13, 2006 at 10:05:56AM -0400, Marcus Leech wrote:
> >>There are double pulses that I'm seeing, with variable timing between 
> >>the main pulse
> >>and the sub-pulse.  The other 1350Mhz radar is much further away from 
> >>me, but
> >>perhaps the "sub pulses" I'm seeing are coming from the other radar 
> >>station, and
> >>they're drifting in and out of phase with respect to one another.  The 
> >>sub pulses
> >>are weaker than the main pulses by quite a bit.
> >
> The timing difference between the "main" and "sub pulse" varies between 
> a fraction of
>  a second, and about 1.5 seconds.

        OK, you confused me (awfully easy to do these days as I sink
into senility) because I think of radar related events on a microsecond
and not a second scale. Obviously more or less the only thing happening
on a second scale is antenna rotation.   And I believe when you speak of
"pulses" you mean BURSTS of pulses.  PRF of radars tends to be in
100-500 hz area typically in this kind of application so almost
certainly what you are seeing is clusters or bursts of tens or more
probably hundreds of pulses - not individual couple of microsecond or so
duration typical pulsed radar pulses.

        And now it is obvious to me you were referring to the inter
burst interval and not the inter pulse interval.

        All of which leads me to comment that either the two radars you
think might be responsible are remarkably well synchronized in rotation
speed (which is possible I guess but I tend to think unlikely), or your
observations are not over a long enough period to see them drift more
randomly out of phase, or you are seeing something in the antenna
pattern of one SINGLE  radar that results in multiple lobes pointing
toward you.   This could be features in its antenna pattern (such a
broad secondary beam and a narrow primary beam) or due to reflections
off some distant but highly reflective object not exactly between you
and the radar but capable of reflecting  a significant amount of energy
your way when illuminated by the radar at peak intensity.

        Remember that the reflections off aircraft I mentioned in my
last comments would be expected to show up not only at the azimuth of
the radar when the radar is pointed right at you but when the radar is
pointed in other directions  - eg at the aircraft, illuminating that
particular aircraft well with its beam.   This would imply that aircraft
echoes would appear on other azimuths as seen by you than the main beam
of the radar and more importantly at DIFFERENT times in the seconds
timescale rotation of the radar than when the main lobe sweeps directly
by you.   And yes, moving aircraft would result in the time interval
between the peak of the reflected energy from the aircraft and the main
lobe of beam sweeping past you changing over seconds or minutes.

        But because they are reflections from the beam of that one radar
they would always relate in timing to the rotation of that antenna and
the timing of the main direct path lobe as observed by you.   This is
not of course true of a second radar.

        One possible way to distinguish between energy from a second
radar on approximately the same frequency and reflections off aircraft
or other reflectors not directly between you and the radar is to
determine if the PRF of the secondary burst energy is exactly the same
as the PRF of the primary lobe as it sweeps by you.   There is a pretty
good chance that the "other" radar would not use exactly the same PRF
(no obvious reason to)  and if it didn't and that is what you were
seeing you'd see two distinctly different "buzz" frequencies which ought
to be obvious with a suitable measurement (eg a FFT of the low pass
filtered detected video).

> 
> I plan to acquire a narrower filter than I'm currently using.  I 
> estimate that the
>  new filter has about 55dB of rejection at 1350.  That should help 
> some, although the
>  sideband components from such a radar are also something to think about.

        They tend to filter them pretty well in the radar in order to
keep them from causing problems out of band.

> 
> Another odd feature of this pulse noise is that it prefers to appear at 
> night, although
>  it also appears during the day sometimes, too.  My theory is that 
> during the day,
>  my sidelobe noise is dominated by the Sun, with the radar pulses not 
> being able
>  to compete.  Whereas at night, there's no Sun noise for the radar 
> pulses to
>  compete with.

        Sun noise is pretty damn weak compared to your estimate of -40
dbm for the radar.    But obviously such a phenomenon as you describe
(sun noise in the sidelobes) should show up as huge diurnal variation in
background directly correlated with the movement of the sun across the
sky.   This should be pretty obvious from your data independent of the
radars.

> My other theory is that perhaps the ERP of the radar is pumped up at 
> night for some
>  reason, or simply that propagation is better at night.

        Propagation of radar energy trans horizon undergoes lots of
weather and time related changes due to tropospheric ducting and bending
phenomena - which can cause spurious "angel" echoes on weather radars...

        And indeed this is sometimes worse at night.

-- 
   Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."