Radar Gun Fundamentals

[JoeBloe]

They banned radar speed (not intersection) cameras here to get elected, so
the cops use laser and stand there with it.

Far more accurate and zero error. Radar guns are subject to
parallax error, let alone calibration error..

 

[[email protected]]

Please, tell us everything about your experiences with that - it sounds
fascinating - I had no idea such a thing could be so conceptually simple.

Thanks!
Rich

scrap a older superhetrodyne radar detector, you'll find plenty to play
with, including a weak gunn diode and a decent detector diode, and yes
can motionsense with it, up to about 5 feet, as that diode is set up
for only a milliwatt or two. With a good IF you can differntiate male
and female hip swing.

Steve Roberts

 

[[email protected]]

Are you claiming that SW Florida (local PDs) does not run speed "traps" on
urban streets,but only uses moving radar during ordinary patrol?
That they don't monitor school zones or local city streets?
That is very hard to believe.

In Central Florida,the local PD squad cars often sit in local street
medians clocking oncoming traffic,they sit motorcycles up on sidewalks
under shady trees in residential areas(25MPH zones),they hide cops with
radar guns in cardboard boxes next to roads,or dress them as construction
workers,panhandlers,etc.
Moving radar is rare unless you're on a Interstate or beltway.

That is pretty much true. The only time you will have speed traps is
when they are responding to a specific complaint and even that is
rare.
Central Florida has a horrible reputation for speed traps and
particularly little shit holes like Waldo that use the speed trap as a
major source of revenue.
The cops in Lee county are not "speeder maids" . Sometimes the FHP
will set up a speed trap on I-75 but that is usually the roving wolf
pack, not the regular troop F guys. They usually bring a plane.

 

[[email protected]]

Far more accurate and zero error. Radar guns are subject to
parallax error, let alone calibration error..

Parallax is exactly the same with radar or laser, it is simple trig.
The same with calibration. The only reason laser is seen as being more
accurate is that you are only shooting at one particular place on one
car. There is less chance of getting a return from a non-targeted
vehicle.

 

[Jim Thompson]

That is pretty much true. The only time you will have speed traps is
when they are responding to a specific complaint and even that is
rare.
Central Florida has a horrible reputation for speed traps and
particularly little shit holes like Waldo that use the speed trap as a
major source of revenue.
The cops in Lee county are not "speeder maids" . Sometimes the FHP
will set up a speed trap on I-75 but that is usually the roving wolf
pack, not the regular troop F guys. They usually bring a plane.

Florida is so notorious for speed traps that AAA trip maps are marked
with the usual locations ;-)

...Jim Thompson

 

[[email protected]]

Florida is so notorious for speed traps that AAA trip maps are marked
with the usual locations ;-)

Yup but they are up in Cebntral Florida, particularly along the 301
connector between I75 and I95. That is where Waldo and Starke are.
St Pete has a traffic unit and I imagine all of those big cities over
on the blue coast do a lot of traffic but it is not that prevelent
here in paradise. (Lee and Collier County)
The one exception is another little shithole called North Port. They
have a sliver of the city limits that crosses I75 and the city cops
will sit out there. That is Sarasota County.

 

[Homer J Simpson]

In Central Florida,the local PD squad cars often sit in local street
medians clocking oncoming traffic,they sit motorcycles up on sidewalks
under shady trees in residential areas (25MPH zones), they hide cops with
radar guns in cardboard boxes next to roads,or dress them as construction
workers, panhandlers, etc.

I hate the lazy bastards who sit at the bottom of a long hill.

 

[Homer J Simpson]

The one exception is another little shithole called North Port. They
have a sliver of the city limits that crosses I75 and the city cops
will sit out there. That is Sarasota County.

Isn't Louisiana notorious for this sort of thing?

 

[Jim Yanik]

Florida is so notorious for speed traps that AAA trip maps are marked
with the usual locations ;-)

...Jim Thompson

Florida loves speed traps because a percentage of the ticket fine gets
transferred to the police agency writing the tix.
It's far more profitable than writing other moving violations that require
more work on the officer's part.(but make driving FAR safer!)

Central Florida enforces red light running only maybe three times a
year,except for blatant incidents that startle the officers.

 

[Clifford Heath]

Parallax is exactly the same with radar or laser, it is simple trig.

Not if you fire it from further away, which is possible with laser.

 

[Richard Henry]

It's been done since at least 1948, with lousy klystrons and 1N21 diode
mixers.

All you do is take reasonable precautions to keep the transmitting horn
from leaking in to the receiving horn. If you can get 60db of
isolation that's plenty. The phase noise isnt very significant as
you're mixing with the same signal, so the noise mostly cancels out.

IIRC the inventor of Radar, Watson-Watt, was stopped for speeding by a
radar gun once. Hoisted by your own petard, as they say.

Shakespeare said "For 'tis the sport to have the engineer
Hoist with his own petar" (Hamlet III iv). But that was a long time
ago 1604?).

 

[JoeBloe]

Parallax is exactly the same with radar or laser

Bullshit. You do not even know what the term refers to.

With the laser, the target pointed at IS the target hit, and the
returned light has absofuckinglutely zero error. With the radar, the
field-of-view is the target and the returned result is that of the
fastest moving element in the field of view. THAT MEANS that the cop
can pull you over for a speed that another vehicle was traveling at.
With automotive radars, THAT is what the term "parallax error" refers
to.

In California, radar is not even used on freeways for this very
reason. The only thing that holds up in courts out here is moving
clock. They don't even use planes for speed determination. They use
planes to spot asshole idiot drivers, and state troopers to nail their
asses.

Radar gets used on surface/residential streets in the speed
indicator machines. The cops use lasers, and they flag you over if
you are there ticket target al la mode.

 

[JoeBloe]

The only reason laser is seen as being more
accurate is that you are only shooting at one particular place on one
car.

Nope. It is because the light is coherent.

There is less chance of getting a return from a non-targeted
vehicle.

There is NO chance of getting such a "signal".

Try again.

 

[JoeBloe]

Not if you fire it from further away, which is possible with laser.

You guys are both on the wrong track. Radar has an FOV. A laser
resolver has a single target, and it ONLY ALWAYS has a single target.

 

[Clifford Heath]

With automotive radars, THAT is what the term "parallax error" refers
to.

Complete crap. Parallax error is error in measured speed due to the
radar looking out from the side of the road at an angle. In my city,
cops are taught to align the radar to aim 26 degrees from the line
of the road, because that's the *parallax* error for which the units
are calibrated.

Your points about FOV are perfectly valid - and parallax is unavoidable
with radar because it has a wide FOV - but wide FOV != parallax.
Parallax error is possible with lasers too, but can be avoided.

See <http://en.wikipedia.org/wiki/Parallax> for more details.

 

[Clifford Heath]

Nope. It is because the light is coherent.

Crap again. Coherent light does not have to be collimated,
and in fact laser diodes do *not* emit collimated beams,
but divergent ones which are *approximately* collimated by
a lense. The actual IR beam isn't used for sighting, either.
A second visible beam is used for that, which hopefully
points at the same target. And even really good lasers have
a non-zero divergence, so that when we first used radar
reflectors to measure the distance to the moon, the beam
that hit the target was already *miles* wide.

Granted that this is not a problem with actual laser guns,
but that doesn't hide the fact that your understanding is
*fundamentally flawed*.

Plus, who can tell whether someone's using an optical chaff
generator. It isn't hard to flash an LED or uncollimated LD
of the right wavelength, flash length, and period, to
completely baffle the laser gun so it either can't get a
reading at all, or gets a wrong one. The receiver in these
guns is nowhere near as directional as the transmitter, so
this could come from another vehicle.

 

[[email protected]]

Not if you fire it from further away, which is possible with laser.

It still depends on the number of targets on the road. A K band radar
will clock just about anything you can see on a long flat road.

 

[[email protected]]

With the laser, the target pointed at IS the target hit, and the
returned light has absofuckinglutely zero error. With the radar, the
field-of-view is the target and the returned result is that of the
fastest moving element in the field of view. THAT MEANS that the cop
can pull you over for a speed that another vehicle was traveling at.
With automotive radars, THAT is what the term "parallax error" refers
to.

Perhaps you should go to fire control school and find out a bit about
what these terms mean. Parallax in the fire control problem represents
the differennce between line of sight and line of fire so it really
doesn't apply here at all but the closest thing to it is the
difference between the angle of the beam vs the line of travel. The
farther the radar is off the road the more error introduced.
Unfortunately for the argument the error is in the favor of the
speeder. This is the hypotenuse of a triangle. When the error angle is
very low the speed indicated is fairly accurate but as range closes
the angle increases and the indicated speed drops. When the angle is
90 degrees the indicaterd speed will be zero. You are shooting at the
side of the car.

The range of the reflected signal will vary greatly depending on the
reflectivity of the target. Some cars don't reflect radar or laser
well. They will have a very short effective range. If your state has a
front plate or you have exposed headlights that becones a good target.
There are cars with low radar or lidar profiles. Lidar can be somewhat
jammed with very hot driving lights. It overloads the preamp..
Eventually when you get close they can get a clean target.
The various car magazines do test this occasionally and report on
different cars and deyection range.

 

[[email protected]]

There is NO chance of getting such a "signal".

These are the same cops who shot 81 rounds and only hit the suspect 27
times at point blank range. How do you think they can hit a license
plate at half a mile and never miss?.

 

[[email protected]]

You guys are both on the wrong track. Radar has an FOV. A laser
resolver has a single target, and it ONLY ALWAYS has a single target.

They can miss the intended target and score a direct hit on the wrong
one?