measuring distance between two cars using infrared circuits

Are you talking about on/off modulation of a 2GHz carrier at a 1KHz rate? How long is the "on" time?

Please elaborate. What is done? Are you saying that there is an IR diode based system that can give precise range information? I am very interested in this system.

I admit that the transponder is not essential. It just makes it easier to detect the signal, and increases the range over which the system would work.

--Mac

easier

I disagree. In the grand scheme of a signal that's usually taking tens of mS to make its journey, a few uS of turnaround overhead is not, IMO, significant nor does it have to be an unknown.

of

useless.

It's not a useless idea, theoretically it could be put to fairly practical use provided the obvious problems of using sound to communicate on the freeway. Provided that you could transmit/receive a signal approx 500' I could envision a system whereby:

1) the car in front could know fairly accurately how far the car behind it was 2) it could also know how fast the car is going (i.e. how fast it is encroaching)

If the front transponder was appropriately calibrated to a specific frequency, the rear transponder of the leading car could use the frequency of a received signal (and it's own speed) to calculate the speed of the vehicle in behind it.

To calculate the distance, the onboard GPS's (all new vehicles will eventually have one) 1 second pulsed output can be used to determine when pings should be sent from the front transponder. By analyzing the time shift of the start of the received signal, the rear transponder of the leading vehicle can determine the distance to following vehicle up to about 500'. A single bit of data could be embedded into the signal to indicate that it was transmitted on odd vs. even numbered second to extend range to about 1000'.

To distinguish transmitted return signals from reflections, a different frequency is used. Standard compensation techniques for atmospheric variations would be applied of course.

We're talking abotu cars a few tens of feet apart. Thus it's nanoseconds, and *any* turn-around in the transponder is *significant*.

--
  Keith

Sure. I'm looking at launching a ~2GHz (wherever the FCC allows) CW pulse and measuring its time in flight. At a ns/ft that's 6"/ns round-trip. Some tricks should be able to get this down significantly less than this. A ns is a long time these days.

Sure. I don't see a few kHz on either side of 2GHz to be a big deal though. It might be a challenge to gate an uwave tranmsitter on in a millisecond, but...

RADAR was my primary interest here. Measuring ns delays is rather trivial these days. ...and that gets us to 6" distance resolution. Put enough of these together with a (very) little computation and we get velocity. I don't see how the mechanics of a couple of cars will exceed the physics or computational needs.

Ok. We can measure more points of the envelope. The question is where is the bandwidth limitation. I suspect it will be in the transmitter, though I don't know. Again, a few kHz isn't a lot of bandwidth.

How? The PLL has to capture the signal and then re-launch the "answer". That's time. If we're measuring the round-trip delay of two cars ten meters apart on the Autobahn, the capture/retransmit time is an error I'd rather not make.

--
  Keith
> --Mac

Doppler or direct time of flight, is doing it the hard way.

Chirp the transmitter at some number of MHz/uS The reflection, even at a few nS delay, will be your carrier frequency of that number of nS ago (can't be anything else!) so, the mix frequency product at the receiver, will be proportional the the distance.

Smooth the output a bit to eliminate jitter, and you're there.

It looks like it doesn't really matter, anyway. The Fourier transform is just a sum of two sinc() functions, one shifted right and one shifted left by the carrier frequency. The pulse duration controls the magnitude of the FT.

I believe the total bandwidth is infinite, but any finite signal has infinite bandwidth, so that doesn't really help us.

Unfortunately, I'm not sure I know how to answer the question myself.

I'll try to remember to ask some people who might know tomorrow and get back to you. (It also might pay to ask in the radar/sonar newsgroup.)

But the more you constrain the bandwidth, the more difficult it will be to identify exactly where the pulse starts or stops. So for precise ranging, you need more BW, regardless of pulse duration.

[snip]

Depending on exactly how the system is set up, the delay could be completely neutralized by using a PLL.

--Mac

"keith" wrote

tens

IMO,

nanoseconds,

This portion of the thread is about using ultrasound transponders, not radar or IR. Sound doesn't get very far in a nS.

anyone ever thought of a "visual measurement" and using a moire-effect ?

Say the leading cars has a pattern of concentric circles or parallel lines, and the trailing cars' visual system looks to first through also the same kind of pattern, then a moire pattern will be seen (same principle as looking through curtains, or on a computer monitor if pixelsize gets close to dot-pitch)

interpretation of the moire is related to the effective distance

if defining the system well, the moire may even be a "growing shape" related to the actual distance

Mount it on your car so it fires directly forward. Hack the output section so it gives a signal which can be fed into your cruise control...

;)

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

It is not difficult to turn on a low power transmitter in a millisecond. But I don't think a few kHz of bandwidth is anywhere near enough. I was too busy today to talk this over with people who would know.

Well, measuring a ns delay can be somewhat challenging in a digital circuit. It is easy for a good oscilloscope, of course. But even if you use an ADC, followed by a DSP, the ns resolution implies a sample rate of

1 GHz, in some sense.

I have seen programmable delay circuits which were adjustable in small steps (picoseconds) but they incorporated clever analog stuff along with digital clocks.

In the application you are talking about, you would need to have some kind of analog detection (time to voltage circuit, perhaps) which would then be sampled.

Well, the transmitter doesn't have to be high bandwidth. You just need a high bandwidth (fast) switch between it and the antenna. The antenna does need to have high bandwidth.

The receive chain, including the antenna, I think, does need to be wideband. That opens you up to all kinds of noise, which is problematic. I'm not saying its impossible, I'm just saying that it isn't trivial or easy.

Another problem with 2GHz is that it is difficult to get a narrow beam antenna that can fit unobtrusively into the car's styling.

[snip]

You could use a dual frequency scheme where you send out a pulse at f1, during which the PLL can lock, then abruptly change the frequency to f2. As soon as the transponder PLL detects the step in frequency, it can turn on its transmit gate.

The transmitter would use the frequency step as the synchronizing time. I don't know that this would work it is just a thought.

--Mac

Modulate the microwave pulse frequency. There are modulation schemes (a quick ramp up in frequency, or chirp for example) that will allow the detection of both range and speed from the reflected signal. The DSP might get a little more expensive than what is needed for Doppler alone.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
If you can't beat them, arrange to have them beaten.
                                -- George Carlin

I talked to one of my co-workers today, and he said that as a very rough order of magnitude estimate, the receive bandwidth needs to be about 1/T, where T is the pulse duration. So if you want a 10 ns pulse, you will need on the order of 100 MHz of receive bandwidth.

The situation is somewhat analogous to sending a digital pulse through a bandwidth-constrained channel (filter). Depending on the nature of the filter, it may ring or just ramp up slowly.

If the bandwidth is too narrow, you may not see the pulse at all.

There are other practical problems to overcome.Some of the other practical problems with this system are that unless the beam width is kept narrow, strong returns from objects on the side of the road will swamp the receiver.

Anyway, it is fun to think about it.

--Mac

'swhat I suggested earlier. You don't need DSP though, WWII radar fused bombs used this technique, and they did NOT have dsp! :)

Makes sense to me.

Sure, filters have non-zero response time. Again, makes sense. So with a 10ns pulse one should be able to measure down to 5' without too much trouble. One ns resolution shouldn't be all that difficult (gate delays on the order of 10-20pS aren't all that big of a deal).

Look for the first return. That's the object that has the highest probability of hitting you the soonest, thus the most "interesting". ;-).

Sure. There are other issues, such as "you aren't the only one on the road", but that's all a simple matter of engineering.

--
  Keith

What you are describing is a linear FM homodyne radar. I believe this is how the actual radars now available on some cars work. For a large variety of reasons, I agree with you that this is the best way to do it.

Because you can transmit almost continually, you can get by with transmit power in the milliwatt range.

--Mac

AIUI, the reflection from a single sweep can't give you Doppler. You would like to keep the single sweep duration short enough so that the object doesn't move much during the sweep.

Doppler would manifest itself over the course of several chirps as a gradual phase shift in the IF.

If your chirp repetition rate is fast enough, you could still get very frequent (100's of Hz) Doppler updates.

--Mac

--Mac

Chirp dosen't do doppler, it measures the distance. So if you want rate of change of distance, you'll have to compare distances over time.

Correct. There are some more complex modulation schemes where both Doppler (target velocity) and distance can be derived. That's what got me thinking about the complex DSP. But distance is what the OP wanted anyway.

The chirp modulation ramp is selected to give a difference frequency that is much higher than the Doppler shift created by target motion for any reasonable distance measurement precision.

I didn't see your original post, Dave. It was in a different branch of the thread. But great minds think alike. :)

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
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