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

[Marcus Leech]

David I. Emery wrote:

>         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.
Yes.  Not understanding that these "pulses" have underlying structure 
would lead
 me to simply call them "pulses" (which, from the point of view of someone
 who expects events to unfold over periods of minutes, they are :-) ).

>         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).
>
>  
>
>
>         They tend to filter them pretty well in the radar in order to
> keep them from causing problems out of band.
>  
Any idea what the regulations say about this?  For example, if 
out-of-band stuff is
 only 60dB down 100Mhz away, there's still something like +5 to +10dBm
 leaving the radar antenna.  Although, given a minimum of 130dB of 
path-loss
 between it and me, the "direct" signal seems decreasingly like a 
culprit, but rather
 some mixing product between the relatively-strong main signal at 
1350Mhz and
 something else, causing it to appear in my IF passband.

>         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.
>  
I'm easily able to detect sources down to 1000Jy (where 1Jy = 
1.0e-26W/M**2/Hz), and
 the Sun crossing the main beam creates a response equal to 140000Jy.  
There is diurnal
 variation in the baseline--caused both by the Sun, and the galactic 
plane.  Even when
 the radar pulses are a problem, I can detect, for example, Cygnus A, 
which peaks at
 about 1500Jy.

My main concern is that the strong radar pulses will disturb the 
baseline so much that
 there's no way I can reach my ultimate system sensitivity--which 
should be roughly
 20-30Jy.

>         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.
>
>  
Indeed, on another forum, somebody else noted that sometimes L-band radar
 propagates better at night, which might explain the 
statistical-preference for
 this stuff showing up at night.  It *has* shown up during the day, but 
only
 about 1/3rd as often.

--
Marcus Leech                            Mail:   Dept 1A12, M/S: 04352P16
Security Standards Advisor        Phone: (ESN) 393-9145  +1 613 763 9145
Strategic Standards
Nortel Networks                          address@hidden