General question about RF accuracy

Hi there!

I have a question regarding the general accuracy in wireless distance measuring using RF.

I want to measure the distance between two objects in real time. It must be wireless and without laser, since there might be a few objects like a tree In between.
The measurement should be on the millimeter accurate.

Someone told me that decawave is the company that comes closest to that. But still from what I read, it can only get a cm accuracy.

Is there any other technology/way to measure the distance that accurate?

Thanks a lot in advance!
Best, Nate

You could potentially look at mm wave radar systems. TI have a few demo kits that give you an out of the box example.
But getting mm accurate is tricky, it’s hard to do that even with lasers.
And as soon as there is anything in the path forget about that level of accuracy.

Thanks Andy for the swift reply! Is there any chance there will be an improvement of accuracy in the near future (5-10 years) ?
Or is it just physical constraints that can’t get changed?

I’m sure things will improve a bit but probably not that much. Radio goes down to wavelengths of a mm or so, getting a radio system to give sub-wavelength levels of accuracy is tricky. That said GPS can give 1-2 cm accuracies on wavelengths of 20 cm so it is possible but it isn’t easy.

So with a clear line of sight radio could potentially get to the accuracy you need. But if you have a clear line of sight why not use a laser.

As soon as you put anything in the way you will lose accuracy, the signal will either not pass through it sufficiently to get a measurement which means you won’t get a reading or you’ll measure a reflection and think it’s good. Or the signal will pass through the object but will travel at a lower speed for that time. And since ranging systems assume the signal is traveling at the speed of light that speed change will introduce an error in the measurement.

Really understandable explanation, appreciate it!
Sorry for this naive question: But do you see any other possibility that such accurate wireless distance measurements could be theoretically achieved?
I just came across motion measuring/tracking, which seems to be not super accurate either…

Assuming you can walk between the two points you could do it with an inertial system. Motion tracking systems lose accuracy exponentially with time but the rate at which this happens depends a huge amount on the quality of the sensors. Cheap cell phone type parts will drop off to the point where accuracies are in the cm within a second or two. Spend a few thousand dollars on the sensors and you can keep the error under a few cm for 20 seconds or more. Spend a stupid amount of money (and get the appropriate import licenses / clearances) and a military grade inertial system would do what you want.

Alternatively you could use an optical system placed somewhere that can see both of the points you wish to measure between. Or place the optical system at the two points and measure locations that are visible from both points. The more locations you can do this from the more accurate the results as the errors will average out. Generally the angular measurement on a total station is more accurate than the range finder so if you can get enough measurements to solve it by triangulation that will normally be more accurate.

Or if both points have a good view of the sky then long time period GNSS RTK or PPP measurements at both points may be an option and should get the errors down to the level you need.

And ultimately the question should always be asked as to why you need that level of accuracy. A number of times I’ve asked that question and the answer has been along the lines of: Well my tape measure is marked in mm and I need the same level of accuracy. Since they were adding multiple measurements up and not allowing for stretch / thermal expansion in the tape measure their actual accuracy with the tape measure was at least 10 times worse than they thought it was. But the perception was that it was mm accurate and so any replacement for it also had to be that accurate.

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