Mistaken identity?

[Jantar]

Originally Posted by Jantar
...Laser, though, works on the difference in time taken from send to receive of succesive bursts of light.

The speed of light... 180,000km per second (from memory... allbeit booze and alzheimers affected now...... )

Imagine that number showing up on the screen. What would the fine be?

....

Come now Patrick, I thought you were above deliberately mis-interpreting technical aspects of the equipment you use. Laser is an acronym for Light Amplification by Stimulated Emission of Radiation; an optical device that produces an intense monochromatic beam of coherent light. It is not the speed of the coherent beam of light that is being measured, but difference in reception time of successive pulses. This difference in time between transmission and reception gives the distance, but all that's need ed is the difference in reception times compared to transmission times. This does not give a readout of 3*10^5 kmh as you have claimed. (The speed of light is 186,000 miles per second or 3*10^8 m/s)

Eg. If your device sends out pulses at 10.0000 ms apart, but receives them 9.9981 ms apart then the speed would be 100 kmh. A simplified explanation is that the time for each pulse to return gives the distance, and hence the difference in distances between successive pulses divided by the pulse frequency gives the speed. But in actuality that process is not required. All that is needed is a simple difference between the transmission pulse period and the reception pulse period calibrated to the pulse transmission period.

[Patrick]

Nah... I just shoot the trigger thingy.

[davereid]

All that is needed is a simple difference between the transmission pulse period and the reception pulse period calibrated to the pulse transmission period.

Hi Jantar - I have read all the published data I can find on police LIDAR, and I haven't been able to find one using this technique, although I am absolutely sure it could be in operation.

This technique would be just as liable to errors from slippage as sending two pulses and measuring distance.

For example if the beam slipped from the back of the headlight reflector to the front, it would have the same effect with either system.

With a system measuring pulse width (period) I would expect it could easily overestimate speed.

This is because (lets say) ;

At the start of the measurement the reflection could be strong, but then halfway through the pulse the beam slips off the reflective surface, thus reducing the returned signal to nil.

This would double registered vehicle speed.

[ynot slow]

Nah... I just shoot the trigger thingy.

Shit if I am coming towards you I sure as shit yope it is the right trigger thingy you pull,rather have a fine than a 9mm round from a glock.

[Jantar]

Hi Jantar - I have read all the published data I can find on police LIDAR, and I haven't been able to find one using this technique, although I am absolutely sure it could be in operation.

This technique would be just as liable to errors from slippage as sending two pulses and measuring distance.

For example if the beam slipped from the back of the headlight reflector to the front, it would have the same effect with either system.

With a system measuring pulse width (period) I would expect it could easily overestimate speed.

This is because (lets say) ;

At the start of the measurement the reflection could be strong, but then halfway through the pulse the beam slips off the reflective surface, thus reducing the returned signal to nil.

This would double registered vehicle speed.

That is exactly what can happen. Do the math by working out the time for a pulse to relect from say 500 m, then 10 ms later from 499.72 m (remember to double the distance for the return pulse), then again from 499.44 m etc.

Next just take the time difference for the return pulses and you will get exactly the same result, without the need for multiple registers to store, subtract and move each successive distance.

Slippage will give the same result either way, but just measuring the time between return pulses is more accurate because of fewer processing errors, and also enables better discrimination of errors.

[Ocean1]

all that's need ed is the difference in reception times compared to transmission times.

That's how metrology laser transducers work. You do, however need to be carefull with pulse frequency/target velocity ratios, you can get phase errors.

Most such equipment has fairly sophistocated control software to manage such discrepencies, including culling suspect input and replacing it with interpolated data from either side.

[NighthawkNZ:Me]

Come now Patrick, I thought you were above deliberately mis-interpreting technical aspects of the equipment you use. Laser is an acronym for Light Amplification by Stimulated Emission of Radiation; an optical device that produces an intense monochromatic beam of coherent light. It is not the speed of the coherent beam of light that is being measured, but difference in reception time of successive pulses. This difference in time between transmission and reception gives the distance, but all that's need ed is the difference in reception times compared to transmission times. This does not give a readout of 3*10^5 kmh as you have claimed. (The speed of light is 186,000 miles per second or 3*10^8 m/s)

don't get me started...

radar and laser targeting is a fun subject

[Jantar]

don't get me started...

radar and laser targeting is a fun subject

Yes, yes. Please do start.

NighthawkNZ is a specialist in this field.

[scumdog]

[QUOTE=davereid;1872015
For example if the beam slipped from the back of the headlight reflector to the front, it would have the same effect with either system.

With a system measuring pulse width (period) I would expect it could easily overestimate speed.

This is because (lets say) ;

At the start of the measurement the reflection could be strong, but then halfway through the pulse the beam slips off the reflective surface, thus reducing the returned signal to nil.

This would double registered vehicle speed.[/QUOTE]

It don't happen...and the reading I take is not a split second one, often have two goes just to make sure.

Having said that, there are so few motorbikes on the road down here I don't think I've pinged one yet.

[gunrunner]

But isnt he spose to show you the speed you were doing to prove that he clocked you at that speed

[scumdog]

But isnt he spose to show you the speed you were doing to prove that he clocked you at that speed

Doesn't HAVE to but most do.

[davereid]

It don't happen...and the reading I take is not a split second one, often have two goes just to make sure.

From the stuff I have read the LIDAR uses the first technique - that is to say it sends a pulse, and calculates your distance.

A known time later it sends another pulse and it knows your distance again, thus it can calculate your speed.

Then the LIDAR sends a third pulse. This must give the same result or the software discards the test and starts again.

This reduces the chance of a incorrect reading, as slipage would have to occur twice, and be the same. This is, of course entirely possible, just much less likely.

But the technique Jantar suggested would make a fine device for measuring speed, excepting the defect of lost signal causing an inflation in vehicle speed.

Indeed, I am sure that it would be possible to send a series of pulses, modulating the beam, and using doppler shift measurement of the modulated signal to determine speed.

I doubt this technique would be used on its own, as it adds considerable complexity, without actually overcoming any of the problems.

The LIDAR manufacturer faces a dilemma...

a) He can just take a simple 2 pulse measurement, and get a reading of speed easily. But it has a very high chance of being wrong.

b) He can take a 3 pulse measurement, and significantly reduce the chance of an error being processed and accepted as correct. But it may take a little time to get a lock, as potentially faulty readings are discarded. And even then the reading may be incorrect.

c) He can take a large number of readings, or combine modulated doppler shift techniques, to ensure that slippage and multiple path problems arent occuring.

This would reduce the chance of error to virtually nil. But the operator will be frustrated with the device as it may take a long time to lock, or may not lock at all.

[scumdog]

This would reduce the chance of error to virtually nil. But the operator will be frustrated with the device as it may take a long time to lock, or may not lock at all.

That is what happens - it's not as simple as 'point,pull-trigger,get reading' just as quick as that, especially at longer distances.

The good thing is you can use it in place you can't use radar and KNOW which vehicle the signal is coming from.

[Jantar]

....
But the technique Jantar suggested would make a fine device for measuring speed, excepting the defect of lost signal causing an inflation in vehicle speed....l.

Again, I ask you to do the math. The two methods ARE in fact the same. The first is just a simplistic way of describing what happens, the second is the mathematical way of dealing with the task. Calculating the distance is a byproduct of the return pulse time and so is easily displayed. A missed return pulse is obvious and will cause an error display as should a return from slippage.

[davereid]

Eg. If your device sends out pulses at 10.0000 ms apart, but receives them 9.9981 ms apart then the speed would be 100 kmh. A simplified explanation is that the time for each pulse to return gives the distance, and hence the difference in distances between successive pulses divided by the pulse frequency gives the speed.

But in actuality that process is not required. All that is needed is a simple difference between the transmission pulse period and the reception pulse period calibrated to the pulse transmission period.

I would have thought the two processes are slightly different. If you measure the interval between two pulses you will know the speed of the target. No problem there.

But comparing the period (duration) of the transmitted pulse with the period (duration) of the received pulse is not quite the same.

Loss of the reflector half way through the pulse would change its recieved duration, thus giving you a different reading.

Lets look at an example, using long time perods to make it easier to follow.

I send a 1 second long pulse. I measure the distance to an object. 100 seconds later, I send another pulse. The object has moved by 2700m it is therefore travelling at 27 m per sec. If either of my pulses slips off the reflector half way through, it won't give me a false reading. As long as I detect the pulse, I have my measurement.

As you point out, I can skip the measurement of distance bit if I want, I can just time the arrival of the second pulse at something less than 100 seconds, and that will provide a speed reading.

Example 2 - Period measurement.
I send a 1 second long pulse. I compare the received pulse. It is only 1/2 a second long. This could be because the target is travelling towards me. Or it could be because it went over a bump and my LASER drifted off the reflector.