T&T: Radar: Sweep Timing and Main Bang suppression

[Ken Bloomfield]

Hello Rich and all,
I was in radar/air for many years but many years ago.  However, I suspect 
that a lot of the basic theory is constant, and just a lot better automation 
is now involved.  I am well aware that the signals are now digitized and 
then digitally filtered with complex algorithms, but in principle the 
functions have remained the same, just with a LOT more sophistication. 
Here is a bit of primer:

1.  SWEEP TIME/ADJUSTMENT --- To be honest, I am astonished that sweep 
timing is a user adjustable factor, and am not really sure why.  A "radar 
mile" is 12.4 microseconds which is the time it takes the pulse to go out 
from  your antenna and get back.  This is an absolute, so the distance out 
on the screen from your location (let's presume it is centered) should be 
absolutely fixed in delay, and assuming that the sweep generator is well 
controlled (let's say crystal controlled) then there should be no need for 
calibration.  Maybe the futzing around now done to raster-ize this signal 
may bring potential new errors in, but I am astonished if so.  Of course, 
there is the normal different selectable sweep times corresponding to the 
range settings.  A 16 mile range needs a sweep time of 16 x 12.4 = 198.4 
usecs and if accurately adjusted at the factory to this, a target at 8 miles 
would come in exactly half way out from the center to the outer range ring, 
i.e. at 6.2 usec where the 8 mile range ring would be generated.

2.  SENSITIVITY TIME CONTROL --- STC has been around for many years now, and 
is easy to understand the need for.  Needless to say, when the main bang 
occurs, the returns that come from close-in objects are much more powerful 
than from distant objects.  To accomodate this and not overwhelm the 
receiver there usually is a form of sawtooth ramp generated that initially 
decreases the sensitivity of the receiver and then ramps the sensitivity up 
over time.  This happens once for each main bang.  Shaping this ramp (say 
from linear to logarithmic, etc.) is part of the fine art of image 
enhancement for targets.

3.  MAIN BANG DURATION ---  As is fairly obvious, if the main bang has a 
significant duration (let's say 1 microsecond) then returns from an object 
1/12th of a mile away are already back at the receiver before the main bang 
is done!  Needless to say, they are not going to be seen.  In addition, as 
is easy to understand, the receiver needs to be "blocked" during the main 
bang.  Remember that it is tuned to received the same frequency as the bang 
(needless to say) and during the actual bang, would be totally swamped and 
take a long time to recover if its input was not protected from the main 
bang RF.  This protection used to be called the ATR (anti-transmit/receive) 
gate and in the old radars was a ruby rod that ionized in the main bang 
pulse and effectively blocked the input to the receiver.  No doubt the 
modern units use a more sexy approach but the principle must remain the 
same, i.e. block the main-bang from the reciever.  After a given main bang 
is finished, the reciever then must recover sensitivity, and this can take 
some finite (albeit small) time.  So, if you think of time zero of a 
main-bang, and then add the bang duration, and then add the ATR recovery 
time, you have the period of time where it is not possible to see targets. 
The normal solution is to reduce the duration of the main bang, but in any 
case it has some duration and thus there will be effectively some "hole" 
near you that is blind.

Now, having said all the above, think about the 1/8 mile range you have 
discussed.  That means that the furthest targets on this range will be 1/8 
mile away and thus their returns will be arriving back at your antenna in 
12.4 div.by 8 = 1.55 micro-seconds (usec).  If you want to try to divide 
that range into four concentric rings, then each ring represents 1.55 div.by 
4 =  0.3875 usec which is rather short, and if you wanted to see anything 
very meaningfull very close.  I would hazard a guess that marine radars have 
main bang lengths in the 0.05 to 0.15 range, and I don't know what your 
units minimum pulse length is, but assuming perhaps about 0.1 usec bang, 
then the blind area will be 0.1 div.by 12.4 = .00806 mile or in other words 
.00806 x 5280 = 42 feet and this area is blind no matter how you stretch it.

Hope your are still awake after this!  Also, hope it is of some help.

All the best,
Ken

Ken Bloomfield
Cell# 865-293-2174
MTOA# 2062
AGLCA# 3529
M/V Tellico Lady, 50' Marine Trader-Walkaround
Maryville, TN


>
> Explanations and comments on timing, main bang, and the trade-offs are 
> appreciated.
>
> Thanks in advance,
>
> Rich Losch
> RIO LINDA  KK48-37
> Salpare Bay, OR