Title: Distance Measurement Shows Double the Actual Value on nRF52840

[Usha]

I am using an nRF52840 and  nrf52832 to measure distance, but the reported value is almost twice the actual distance. For example, when the actual distance is 1 meter, the reading is around 2 meters.

What could be causing this issue? Is there a setting or calibration step I need to adjust in the Nordic SDK?

I am currently working on this project and would appreciate a quick response.

[Simonr]

Hi

So, while what Andrew is suggesting is correct, you should be able to get far better measurements than almost a 1 meter margin of error. I assume you're using the Distance Measurement sample in the nRF Connect SDK for testing here. And the main issue when seeing large error margins here is that you need to do zero distance calibration, which is explained in the documentation of the sample here: https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/bluetooth/nrf_dm/README.html#zero-distance_calibration 

Additionally, right above it you can see a section on high precision calculations that is a config you can enable to increase the precision of your distance measurement.

Lastly, as somewhat of a side note since the following is not supported on the nRF 52 series. On the nRF54L15, we have support for the new BLE distance measurement solution from the Bluetooth spec. called channel sounding. We have some experimental samples for that available as well here https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/bluetooth/channel_sounding_ras_initiator/README.html, but it's quite a bit to dig into, so I'd suggest starting out with the technical overview of the technology available here: https://www.bluetooth.com/channel-sounding-tech-overview/ 

Best regards,

Simon

[snoopy20]

Still pretty incredible that it can do that resolution.

Then I started thinking how it would be done without a much higher clock and I thought if the receiver was able to echo the signal with a known latency then the required clock would be half and only one precision clock would be needed. Even so that's still high but if there was multiple echoing each time would half the clock requirement at the expense of doubling the time needed to do the measurement.

[Usha]

I am measuring the distance between two devices placed 60 cm apart, but the reported distance fluctuates between 0.88 m and 1.43 m. What could be causing this variation, and how can I accurately calibrate the offset?

[Simonr]

Hi

It's a bit strange that it fluctuates by this much. Are you testing on two nRF52 DKs or custom boards? And can you share some details on what environment you're testing in? Are the devices in field of view of each other, and is there a lot of other activity on the 2.4GHz band in the space you're testing here? 

The zero distance calibration can also be done at 0cm, and the offset would be whatever the distance you see there that you will have to implement manually in your application so that the ranging result always adds/subtracts the offset before printing them to the terminal.

Best regards,

Simon

[Usha]

"I am measuring in an office environment, with both antennas facing opposite to each other."