locking

[ltilaser]

How long does it take for a radar gun or mobile unit to get a lock?

What does a speed radar look at when it is getting a lock? Is it looking for a steady signal or a consistent speed reading over a short span of time?

[kpatz]

Most modern guns can get a speed reading in under a second, or even a half second. Some older ones may take a little longer, maybe 1-2 seconds.

What the gun looks for is a reflected signal, which gets mixed with the signal being sent. The doppler shift of the reflected signal causes a tone to be generated. The radar calculates the speed based on the pitch of the tone.

Even though the gun may get a speed within a second, the officer may continue to track the car's speed for a few seconds, to provide a tracking history (see if the car is slowing down or speeding up, making sure that car is the one that's causing the speed reading on the gun, etc.)

[jimbonzzz]

http://www.radardetector.net/viewtop...ghlight=author

Consistent speed reading over a short span of time.

[ltilaser]

So if you constantly change your speed at a high they can not lock you?

I did get to do a test run in my TA (the '98 with the aluminum hood) against the ATR handheld, and it is not so fast. If I jammed on my brakes as soon as the V1 went off, the radar showed "RFI," instead of a speed. Of course I knew the instant on was coming, so I was ready LOL! I had the cruise control set and my foot right over the brake pedal. That does wonders for the reaction time. I couldn't get that trick to work with the Dual SL.

How fast does a radar detector respawn to instant on?

I was thinking, I should build something that triggers your abs when the detector goes off. That way the policemen can not get a lock on your speed. Note: DO NOT TAMPER WITH YOUR BRAKES . It's dangerous.

Or, you can play a tone which pulses on and off to confuse the locking radar.

Oh, I don't see why either one would be illegal but check the legality of these things before trying them.

[SmaartAasSaabr]

Having something that automatically triggered a panic brake from the RD would be extremely bad... Firstly imagine getting falses :shock:

Secondly you need to make sure that the traffic patterns behind you aren't going to cause a problem.

It's fine for you to manually initiate a panic brake from an RD alert but if you had it done automatically it would be very dangerous.

[ltilaser]

Im not talking about applying heavy braking, an automatic braking system, or a panic brake. Instead, when you apply the breaks while the rd is alerting the antilock system automatically goes off. You controll however much breaking you need. That way, when you are slowing down you aren't slowing down at a constant speed. Preferably, you can add a tiny bit of gas to your breaking. That way, your speed will be constantly changing at a 200hz, or whatever you abs system is set to, rate. Therefore, the instant on radar may not lock.

[SmaartAasSaabr]

Well you mean you are slowing down at a constant acceleration, the speed is continously changing regardless...

I don't know if a standard car's braking and engine system can really react in such a speed. 200hz that is 200 times a second fast/slow... Do ABS systems react that quickly now? Mine doesn't...

[ltilaser]

Let me clarify this.

New abs systems work at around 20hz. Extra zero, sorry.

I mean when you are reducing your average velocity down to psl you need to do it at a resonable rate to prevent accidents. At that rate your instantaneous velocity will not be changing fast enough and the radar will lock your speed by getting a relatively constant reading over a short period of time.

Instead of slowing down by reducing your average velocity through a relatively slow decrease in instantaneous velocity over time, you slow down by using sudden acceleration and sudden braking. This creates a vibration, sort of like a decreasing sharp sawtooth function. Your average velocity will be decreasing but the relative instanteous velocity would be constantly fluctuating at at a high rate. The radar will see its readings jump all over the place over its sampling period and will not lock.

[SmaartAasSaabr]

I know what you're saying now, but can you get enough acceleration in each cycle to make a change in velocity that is large enough? I mean if you are oscillating between 85,24 mi/h and 85,02 mi/h it'll probably just lock on 85. To change the velocity to any real degree in short time the acceleration must be very high, thus you need a lot of force. Brakes usually only put I dunno, 2 kN in a panic brake and the engine probably can't touch that or be able to put the power down fast enough (eg downshift)

[ltilaser]

I'm assuming that the radar unit rejects readings based on change in velocity not the actual difference in velocity. If measures the mi/hr^2 and if it is too high or low it will reject. As in GTO_04's example, the radar gun probably rejected the singal because it had a enormously low change in velocity. If somebody was braking at -15mi/h/s, the difference in velocity over .3 seconds will only be 4.5mph.

Now, you probably can slam on the brakes, but all your stuff will go flying forward and you'll probably get a dangerous driving ticket.

My solution to this problem is to only decelerate at a high rate in small increments. Im pretty sure that radars will take multiple samples over their sampling period. The DSPs run in MHz and from jimbonzzz's data it seems as if they sample for around .3 seconds. Therefore, in its sampeling period with a 20hz abs system you can get about 6 cycles. What I expect the radar unit to see is a good signal from the acceleration then a bad signal from the sudden deceleration. The signal will go from good to bad to good at least 6 times. Futhermore, the change in velocity will go from positive to an extreme negative then back to positive and so on. I'm do beleve that the constant fluctuation between multiple bad and good readings from the cycles of acclereation and deceleration will cause the dsp to reject the signal because it would not fit into a pre programmed algorithm. In addition, the "sawtooth" effect of going from positive to negative and back to positive may confuse the dsp.

I remember a video on this forum. It showed a scorpion radar jammer working on a radar unit with its doppler audio on. I remember the doppler signal was fluctuating. This system makes your car behave in a way which will make a fluctuating singal without any jammers or emitted signals.

The throttle is not used to keep a constant oscillation.

Sure, this system will shake you to death. But, its better than getting a ticket and its better than your dog getting thrown through the front windshield. And yes, it is dangerous to tamper with your brake system.