Finding offset using AM

Hello Erik,

I don't think 4ft at 1MHz is going to cut the mustard. You need a much higher frequency and directional antennas. Think GHz.

Make sure that the source of 1MHz carrier is the same for both transmitters, otherwise you will have a beat note problem between the two.

Then your receiver would just compare the amplitudes of the two modulation frequencies.

...Jim Thompson

Hello Jim,

That could be a challenge with metal cars. You'd really need plastic vehicles. From recent years I only know one but it was discontinued when communism was discontinued in former East Germany:

In the UK we had the Reliant Robin ("plastic pig") 3 wheeler but that ceased production some short while ago, as I understand it the French have introduced a plastic car which I am told is to replace the plastic pig in the UK market and can be driven by people with only a motorcycle license - the law must have been changed because it used to specify no more than 3 wheels and 425kg unladen.

"Plastic pig"? That's so appropriately French ;-)

...Jim Thompson

Why not just use sound, or ultrasound? Who needs the RF.

...Jim Thompson

Naw, just use a CLASS IIA or lower laser. Search eyesafe erbium laser or eyesafe diode laser if you want to be expensive Keep driver exposure to 330 microwatts per cm^2 or less which is OSHA Standard or look up data for max power vs pulse rate for CDRH in US or whatever handles laser safety in Europe/Asia for class I exposure. All agencies around the world use pretty much the same exposure chart, and you should be able to do this at energy levels far below the eye safety limit.

place a IR led strobe on the lead car. Also on the lead car place a diode laser with a galvanometer scanner. General Scanning model G734 or G124 low cost open loop scanner for example, similar parts using feedback are made by Cambridge technologies, Galvoline, GSI-Lumonics etc, and cloned fairly well by the Chinese. On the chase car mount three detectors. one on center, one on left side of bumper, one on right side. IR strobe is different wavelength then laser, use dichroic filters to pass each wavelength, ie 1100 nm led, 750 or 808 nm laser, Strobe signals start of scan, send a ramp waveform to the galvo scanner, scan the beam outwards towards the other car. Measure time till center of laser beam passes over each detector using microcontroller. Expand the laser beam in its vertical axis using a cylinder lens to compensate for differences in road and bumper height.

overly complex, perhaps, but much easier then a RF solution. Since the laser beam will be 1-2 mm in diameter , accuracy is good. With work you'll have both offset and distance from car to car. Based on Omni and Consolan navigation techniques, its easy to compute using say 16 bit micro.

Steve Roberts own opinions only.

!!!

A modulation scheme measuring amplitudes is going to be a fairly complex tuning and adjustment job, maybe even nightmare. A laser-based scheme is even worse. An ultrasound scheme will be sensitive to everything in the area.

Just build a 2Mhz transmitter, divide a 1Mhz signal from it, put one out one side and the other out the other, and measure the phase (i.e., time between crossover) between the two signals. You use a single receiving antenna, a preselector filter for each frequency, and divide the 2Mhz copy by two. Put each into one port of a synchronous detector. (It's not strictly necessary to divide the 2Mhz signal, with some logic).

Simple, easy to build.

Stable, resistant to weather, dirt, dust, etc.

Simple one-time tuning to get the transmitted phases matched.

Easy calculation of angle based on easy-to-measure timing.

Why all the exotica?

John Perry

didnt the Luftwaffe have something similar when they started bombing london? ISTR the poms chief scientific advisor was a kiwi, who figured out what they were doing and organised to fool, rather than jam, the system - resulting in a great deal of countryside being bombed.

Cheers Terry

Hmm. Discriminating at most 3ns from 1000ns will make things somewhat touchy. Better to go with (Joerg's?) higher frequency, though Ghz would make things tougher, given phase ambiguities.

A 10Mhz system (3ns out of 100) would still be easy to build; a 30Mhz (3ns out of 33) wouldn't be hard. Discriminating would get easier up to 50'ish Mhz. Then you start getting into serious nonlinearity issues (the crest of the sine wave), unless you do it with fast digital logic. Then the limit would be 150Mhz. What kind of accuracy would you want at angles far from center? Is this for indication or for measurement?

jp

Jim Thompson a écrit :

You must mean "plastic frog" - albeit, i miss the joke wit "plastic".... peter

I'm thrilled that so many ideas have been put forward! I suppose I should have mentioned that this design will be mounted on a small UAV, so weight is definitely a consideration at my end--I can't just pick up a box and throw it in the cargo hold!

I've modified my design somewhat from my original posting, it's as followed:

The transmitting craft has two transmitters operating at two different FM-band frequencies (say 88MHz and 100MHz). The wing of the aircraft has a directional, rearward-facing antenna on one wingtip transmitting one signal, with the other antenna on the other wingtip broadcasting the other signal.

The receiving aircraft has two forward-facing antennas in a mirror configuration. When the two signals are of equal strength, the aircraft knows it's lined up.

I have two main concerns of this design, however. For example, how will the transmitting craft know to which side it's drifting? Also, I realize I will need to take great care in designing a (light, small) Yagi antenna of some 1/2^n size to the wavelengths I'm operating on.

Any thoughts?

If this is intended for cars on public streets, no system that relies on resources on a different vehicle will do. Within a month, you would see law suits of the kind "My car hit that tree because your's told it so.." Remember that it's dangerous to sell hot coffee and Audi lost a case where the customer mistook the gas for the brake.

ILS is something different. It is calibrated officially in regular intervals and it's OK by default.

And, don't forget that your system must be immune to reflections. Imagine a bridge or tunnel made from steel and the leading car aproaches one side or the other. That could easily swap right and left as your receiver sees it.

I think that centering a leading car would be an easy add-on to the

regards, Gerhard

There is no courteous way to tell you that you are hopelessly out of your league on this one. Stick to the aerospace aspect of the project, whatever that may be, and leave the radionavigation to an expert. The ILS may be old but it is highly sophisticated in concept and depth of antenna propagation application. The so-called "localizer" transmitter, which broadcasts the amplitude modulated VHF tones for determination of error off runway center in the horizontal plane, is a complex antenna array with resultant beam pattern such that the "depth of modulation" of the VHF tone is a precise function of angle off boresight, with 20% on center and varying by +/-7.55% at the specified fine angle error limits, with the two tones modulation depth varying in opposite sense so as to maintain a constant total depth of modulation over angle off boresight.

I realized from the outset that I'm hopelessly out of my league, but that isn't really going to stop me from finding a solution to my problem. Luckily for me, I work right above a group working in high-energy EM radiation, so I have a depth of knowledge to tap. Again, this is just a project for which I'm searching for a rough solution (a valid first step in any endeavor)--It's not something I'm expecting to sell to DARPA in two months or something like that.

OMG! - they've taken to the air!!!

I believe this might work, but you may want to go up a bit in frequency to keep the antennas smaller than the craft. You only need a single frequency and antenna on the leader. On the follower you have two slightly diverging yagis and two receivers. Detect and take the difference of the receiver amplitudes. When it's zero you're lined up, otherwise the polarity will tell you which way to steer. Don't follow too closely, the near field will mess things up. Something like 20 wavelengths or more should do it. The amplitude might be used to keep a constant distance from the leader. Complication: In an aircraft you also have to consider the up-down dimension.

This was the basis for the German Junkers JU-88 night fighter radar, the pilot centered a spot on a CRT until he came into visual range. The spot was actually a circle, the nearer to the target, the smaller it got. Ever seen a photo of them with that cumbersome hardware in the nose?

Come to think of it, a modulated LED or laser and cross-eyed optical detectors might work too.

- YD.