# Measuring short distances using radio waves

#### Kevil

Joined Jun 28, 2020
161
What would you advise for measuring distance to within 1 cm using radio waves. This is a DIY project for a lawnmower that would be controlled in space using a couple of radio beacons to mark out the corners of the workspace. My idea is that each radio beacon would transmit a pre-programmed ID. What parts, affordable for an amateur, would you recommend?

#### MrChips

Joined Oct 2, 2009
29,200
I would use sonar or LiDAR.

#### BobTPH

Joined Jun 5, 2013
7,518
My idea is that each radio beacon would transmit a pre-programmed ID
How does that help determine the distance?

#### Kevil

Joined Jun 28, 2020
161
This should help keep the lawnmower within the area marked by the radio beacons. There would be no need to teach a lawnmower where it can move by driving around the perimeter of an area if a GPS system was used. I'll use the collision avoidance system against obstacles, too.

#### Papabravo

Joined Feb 24, 2006
20,378
Considering the speed of light is 0.98357 feet/nanosecond, your goal of measuring distance to 1 cm resolution seems ambitious at best, seeing as how there are approximately 30+ cm/ft

#### Kevil

Joined Jun 28, 2020
161
I know I needs time measurement in picoseconds. 1 cm is 33.36 ps.

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#### MrChips

Joined Oct 2, 2009
29,200
I do have plans to build a robotic lawn mower. At the moment I am still working on the mechanical parts.
Here are the plans I have for navigation.

1) I would use a drone and take an aerial photograph of the lawn area.
2) I would map the coordinates of the lawn area into the system.
4) I would have two or three ultrasonic beacons strategically located to calibrate the position.

#### Papabravo

Joined Feb 24, 2006
20,378
I know I needs time measurement in picoseconds. 1 cm is 33,36 ps.
Yeah. Good luck with that project.

#### ZCochran98

Joined Jul 24, 2018
273
Look up FMCW (frequency-modulated continuous-wave) radar. Use something in the 915-930 MHz band (because that's open - in the US anyway). You won't need to measure in picoseconds when you can measure in phase differences. Precision may vary....

#### Papabravo

Joined Feb 24, 2006
20,378
Look up FMCW (frequency-modulated continuous-wave) radar. Use something in the 915-930 MHz band (because that's open - in the US anyway). You won't need to measure in picoseconds when you can measure in phase differences. Precision may vary....
Designing and building circuits for that frequency range may be challenging for a hobbyist.

#### ZCochran98

Joined Jul 24, 2018
273
Designing and building circuits for that frequency range may be challenging for a hobbyist.
Oh, absolutely. Especially if you want to measure more than a few centimeters away in a noisy environment. But a couple cheap SDRs ("cheap") can function for this, depending on budget. Otherwise...that frequency range is a bit out of range for most for FMCW.

#### Kevil

Joined Jun 28, 2020
161
I found that MAX35101EHJ+T can measure "Time-to-Digital Accuracy Down to 20ps" for a very affordable price. Peridically I would send a request with the radio beacon ID and measure the response time.

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#### Papabravo

Joined Feb 24, 2006
20,378
I found that MAX35101EHJ+T can measure "Time-to-Digital Accuracy Down to 20ps" for a very affordable price. Peridically I would send a request with the radio beacon ID and measure the response time.
How exactly do you plan to use this part for your application? I don't suppose you could whip up a quick schematic for our edification and perusal. Could you do that for us?

#### Kevil

Joined Jun 28, 2020
161
I would send the signal with ID (number) of required transceiver through the 433 MHz transmitter to the transceiver and measure the response time. My device and transceiver will use the same 433 MHz transceiver. They will all receive my signal, but only the transceiver with the same ID will respond.

#### Papabravo

Joined Feb 24, 2006
20,378
I would send the signal with ID (number) of required transceiver through the 433 MHz transmitter to the transceiver and measure the response time. My device and transceiver will use the same 433 MHz transceiver. They will all receive my signal, but only the transceiver with the same ID will respond.
I ask for an explanation of how will use the MAX35101 and a schematic. Do I get either one of those? No. You go off on another tangent. In this tangent I have no earthly idea how that will help you with your problem.

#### Kevil

Joined Jun 28, 2020
161
I don't know now. You can have a look at Typical Application Circuit in the TDS. I also noticed the TI TDC7200 Time-to-Digital Converter.

I need to consult it with Analog and TI.

#### Ya’akov

Joined Jan 27, 2019
8,153
I don't know now. You can have a look at Typical Application Circuit in the TDS. I also noticed the TI TDC7200 Time-to-Digital Converter.

I need to consult it with Analog and TI.
You are not including the latency of that cheap ASK module in your thinking. What you are trying to do is not really practical, there are other ways to do it that are.

#### Papabravo

Joined Feb 24, 2006
20,378
I don't know now. You can have a look at Typical Application Circuit in the TDS. I also noticed the TI TDC7200 Time-to-Digital Converter.

I need to consult it with Analog and TI.
It seems a bit disingenuous to offer up a proposed "chip solution" with no idea how you are going to apply it. IMHO you would do well to consider the already mentioned alternative solutions.

#### MrAl

Joined Jun 17, 2014
10,588
It seems a bit disingenuous to offer up a proposed "chip solution" with no idea how you are going to apply it. IMHO you would do well to consider the already mentioned alternative solutions.
A long tape measure and a motor and rotory encoder