Radar Jamming (only Audi driver content.)

[Gross Scruggs <GScruggs@wposmtp.nps.navy.mil>]

Audi Fellows,

Whenever the topic of radar detection and jamming arises comments are often made
which indicate a misunderstanding of that process.  Although not an EE, I was an
avionics technician in the Navy for six years maintaining electronic
countermeasures systems and since the Aero engineering degree have been a
military hardware analyst for DoD for the past 15 years.  As you can imagine,
the topic of radar jamming does occasionally have my interest.  

There are two basic techniques of jamming a radar system: overwhelm them with
power, or give them back bad information.  The first is easy to do and easy to
detect when it is being done to you, and most contemporary systems can adjust
and compensate for this type of jamming.  To jam police radar using barrage
jamming will often get you the "jam" reading at the gun, and you're, "...in a
heap o'trouble now."  I will, however, admit to fantasizing about seeing a
trooper suddenly drop a smoking radar gun having been toasted by a burst of RF
energy.

The second way is virtually all that is used in the defense industry.  The ideal
application is when you give your target radar bad information, so he can't do
his job, and he never knows he is being spoofed.  The basic technique is to
capture his AGC (automatic gain control).  In your car radio, for instance, the
AGC is what allows the volume to stay the same whether you are near or far from
the transmitting antenna.  In a radar system the returned, or bounced, signal
arrives at the receiver input at a certain strength where it is then processed
and you are detected doing 90mph in a 65 zone.  But a good ECM system detects
the presence of a radar signal, through frequency and pulse shape, and will send
back a signal that is of the same shape and frequency but just a little bit
stronger than the 'true' reflected signal.  The AGC locks onto the ECM signal
and ignores the 'true' signal and you have captured his AGC.  Now rather than
the radar gun proccessing the doppler shift that your Audi is actually creating,
it is being fed a signal that will mislead the velocity processor to report
that, "... you're only doing 55," or "... can't get a lock," or whatever bogus
information the ECM designer has built into the system.  

The theory is easy but the execution is sometimes difficult when trying to
design a box that is cheap enough to attract an interested consumer.  Were cost
not a consideration then dead-flat perfect ECM would be on the market for your
car, and packaged in an undetectable manner.  But when cost is the design
driver, performance always suffers.  

As an aside, law enforcement radars usually contain a circuit that requires
several steady "hits" on a moving object before a speed readout is available. 
If the speed of the object varies much at all then the signal is tossed out as
being "spurious."  This means that an object that is accelerating, negative or
positive, will break lock and not be clocked.   Luckily that variation is quite
small, something on the order of 3 ft/sec/sec, so even a modest braking action
will cause a "no lock" condition.  I did this some years ago when a Daytona city
cop had me cold.  I saw him about the time he saw me, I braked, he shot and
begun to wave me over.  He did a double take at the readout and waved me on. 
The expression on his face is one of my fondest memories.

Last, and most enduring, principle of ECM: Any signal can be countered, and any
counter can be overcome.  It's largely a matter of how fast your side can
analyze the other guy's latest offering, design a counter, and get something
into production.

I own neither a jammer nor detector.  I do use the posted numbers as my "stall
speed" and strive to never fall below them.  When driving "in the zone" I use
ques from other traffic and hunches to avoid being ticketed.  For the amount of
time I spend above stall speed, it works quite well.

Refinements or corrections on the above are welcome.

Regards,  Gross