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

[David I. Emery]

On Tue, Sep 12, 2006 at 08:01:51AM -0400, Marcus Leech wrote:
> David I. Emery wrote:
> >
> >     The transponders are 1090/1030 mhz and not 1350.   1350 is just
> >radar.
> >  
> 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.

        I very much doubt that the "other radar" is responsible for the
secondary pulses you see unless they arrive at a distinctly different
time in the 5 second (I think you said earlier) rotation cycle of the
radar.   I presume from your description that each main pulse is either
proceeded or followed (by some short (us) interval that is unclear from
what you have written) by a weaker secondary pulse.

        Almost certainly two independent radars would operate at
slightly different PRF's (depending on how old they are that might be by
a percent or two or for more modern gear by the difference in typical
crystal oscillators in the two time bases, perhaps 10-100 ppm in usual
cases).   This would result in a steady phase change between the pulses
each pulse - if you can measure it you'd presumably find it basically
linear in microseconds per pulse.

        But because their antenna rotations are unlikely to be
accurately synchronized and the chance of their beams both pointing at
you at the same time slim it would be unlikely to find the pulses of one
present at the same time in the five second antenna rotation cycle as
the other at least for any extended period.  And very unlikely that the
relative strength of the strong pulse and weak pulse would remain the
same as the two antennas swept past you from different distances.  
Likely one would peak before the other and this could obviously result
in one being stronger for a while and then the other becoming the strong
one.

        It does sound to me like one of two other explanations is at
work here. Either you are seeing actual echos (reflections off aircraft
or ground based scatterers) which would of course always  follow the
main pulse by some time interval that would depend on the additional
path length.  Or the other explanation is that the radar deliberately
radiates a secondary pulse before or after the primary one (sometimes
from a different feed on the antenna).

        Actual real live echoes from aircraft strike me as quite
possible to pretty probable at least unless you have a direct line of
sight to the radar and can actually see it in the distance and therefore
see such walloping huge signal from it that any echoes are too weak by
comparison to be visible within the dynamic range of your gear.

        Likely the "direct" path involves Fresnel scattering and lots of
attenuation by foliage and the like and is many tens of db more lossy
than the free space path loss equation would suggest.   And likely
reflections from passing planes and perhaps even certain fixed mutually
visible scatterers like towers or mountains involve direct unobstructed
paths from the radar to the target (airplane) and from the target
(airplane)  to you.   These would have much less attenuation by close to
the ground propagation effects than the nominally direct path, and
while a reflective target often scatters energy more or less in all
directions (thus the fourth power law of returned signal in the classic
radar equation) this energy can still be considerable relative to a
trans horizon direct path if little or no energy is lost by close to the
ground absorbers and scatterers in the reflected path.

        And if you see the delay between main pulse and secondary moving
with time, well flying airplanes do move with time too... 

        What you have of course is a simple bistatic radar system.

        The other possibility would be more likely if the delay between
the runt pulse and the main one was constant.   Some radar systems,
especially those intended to operate with active transponders on the
target,  transmit a secondary pulse using an antenna with a different
pattern.   Typically these secondary antennas have a broader beam
pattern  than the primary beam so the secondary pulse is stronger
relative to the primary pulse except when the primary antenna is pointed
right at the target.   Used with a transponder that can distinguish
between primary and secondary pulses this allows the transponder to only
reply when it sees the primary pulse stronger than the secondary pulse
indicating the antenna is pointed right at it.   This of course is
important if the transponder (eg aircraft) is near the antenna (with low
free space path loss) as it may see enough signal from antenna sidelobes
and reflections to reply when the antenna is at azimuths pointing well
away from it.  And obviously this appears to the radar to be a target at
another azimuth.

        The air traffic control transponders in use today on 1030/1090
mhz work this way with a secondary pulse from a secondary antenna used
to ensure the transponder only replies to the main lobe of the antenna.

> At 1350, the pulses are arriving 35-40dB out of the noise.  But I agree 
> that a simplistic
>  line-of-sight propagation model isn't ideal.  But even if this thing 
> were coming in at
>  -40dBm, that's still plenty-strong to screw me up!

        No question.

        Of course you could track the rotation of the radar and blank
your receiver during the time it sweeps past you (this of course might
not work for reflected signals from passing aircraft very well as they
could appear at other azimuths and times in the radar rotation cycle
depending on where the airplane is).

-- 
   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."