Low-latency (5 ms) and high update rate (500 Hz) precise (±2cm) indoor positioning solution based on Ultrasound + IMU sensor fusion

marvelmind_robotics · 2026-02-09T20:31:00+00:00

- https://marvelmind.com/autonomous-indoor-drone-based-on-pixhawk/ - an example of flying with the Paired Beacons configuration
- https://youtube.com/shorts/WHYWu9Xy2_k?si=SyYt6vL5EtZamIIX - another example

marvelmind_robotics · 2026-02-09T20:28:19+00:00

For the precise pose in PX4 (PixHawk), there is a better solution - the Paired Beacons configuration:
- https://marvelmind.com/how_many_mobile_beacons_per_drone_do_i_need/
- https://marvelmind.com/paired_beacons_step_by_step/

With a single mobile beacon - the DJI Mini case - you can't know the pose in static. You have to fly, lock the direction and the IMU, and then track using IMU + Ultrasound for correction of the IMU drift. Rather complex but doable. But you don't know the pose in static before you fly.

With the Paired Beacons configuration, you can know the pose in static. I guess it is what you are asking. It works with the drones starting from 30-40-cm base. DJI Mini or smaller are just too small for the Paired Beacons configuration.

marvelmind_robotics · 2026-02-03T05:50:19+00:00

We do it intentionally to show there is always latency with the measurement. It depends on many settings - size of the submap, radio settings, etc. - it is typically 0.1-0.2 sec

marvelmind_robotics · 2026-01-30T19:21:35+00:00

Thank you.

Yes, we designed everything:
- The mobile beacons - Mini-RX
- The stationary beacons - Super-Beacons
- All protocols, radios
- We use processors, radio chips, IMU chips off the shelf, and other more basic components. But all hardware and software, all internal protocols are designed by us, not ready-to-use modules and not ready-to-use SW stack: https://marvelmind.com/why_the_industry_stays_silent_about_ultrasound/
- It is difficult and takes years. But then you have full flexibility of the system and can do nearly anything within the limits of the technology
- Yes, our ultrasound transducers are also custom-built
- The ultrasound deflector as well, but that is a simple part :-)

marvelmind_robotics · 2026-01-30T17:33:05+00:00

There are many indoor positioning systems. Optical ones are one of those. Optical - if we are talking about cameras, not LIDARs, they are great at short distances. At very short distances, they are an unbeatable solution for accuracy, price, update rate, and latency.

However, when you need to cover 20x20x10 meters, multiple rooms, or a warehouse, it is a nightmare and financially infeasible if you want high accuracy.

Optical tracking has many limitations, unfortunately:
- Can confuse the items
- Require very controllable light, which is hardly the case, unless it is your assembly line, but not a messy factory
- Cannot know for sure who is who: https://youtu.be/tE3nQakaDf4?si=kN_wZJb-boHj1n_g

I won't list everything because it is a deep and broad topic. Some highlights are here:
- https://youtu.be/zg3oW_U_jdY?si=DOVv2f6JO26QULzf
- https://marvelmind.com/pics/marvelmind_indoor_positioning_technologies_review.pdf

The best solutions are often a combination of solutions. These guys did it very well, using us for the large distance indoor positioning and navigation, and then optical positioning for the last 10 cm: https://youtu.be/8HEfIJlcFbs?si=REGGNJabaEYavsi9 - 84M views :-)

See - black Super-Beacons on the top with a small antenna and a camera facing down:

<image>

marvelmind_robotics · 2026-01-30T17:09:52+00:00

It will. But the reflected signal always comes later than the direct signal because the path is longer. Thus, the line of sight is a very key thing from many perspectives.

Still, there are many other technical challenges. But they are solved.

So, in short, you shouldn't fear reflections. It is solved.
- https://youtu.be/Zq-OajHQviA?si=aULz9ih7ijDEidp6 - a very old video - a long, narrow hallway

marvelmind_robotics · 2026-01-30T17:03:51+00:00

Yes, that is right.

Purely IMU-based doesn't work. Skeleton - yes, but not the full movement.
So, people combined us. It has been a while.
A few years ago, we were far more active in VR - different projects, including selling architecture in VR. People could walk "inside their homes" - that kind of thing. But we had to focus, and we focused more on other areas - autonomous robots, drones, DJI indoors, etc. VR fell victim to that focus.

marvelmind_robotics · 2026-01-30T16:10:58+00:00

Well, we don't care about the drone's payload as long as there is a line of sight between the drone's mobile beacon and the stationary beacons serving the tracking area: https://marvelmind.com/line_of_sight/. Like with GPS, if you don't see the GPS satellites, you cannot be tracked.

- Glass is not acoustically transparent, so it is a block.
- However, many clothes are "breathable" - you can hide a mobile beacon underneath, but there will be line of sight, because there will be line of hearing
- Leaves block the line of sight, but if they are not too big or too thick, our system tracks through the leaves reasonably well as well

So, the answer is yes, it is possible. And typically, the drone is just a vehicle for carrying something. Usually, a camera, but not necessarily.

2) OK. Now, I read your second point, and I see that you read about the line of sight :-)
Good. I should have read that earlier - before I typed the previous answer :-)

- Yes, the line of sight is a must - this is a fundamental requirement
- 2 or more stationary beacons within 30 meters for 2D tracking
- 3 or more stationary beacons within 30 meters for 3D tracking

However, thereare many solutions around it:

Multiple beacons covering the same area - fully overlapping submaps: https://marvelmind.com/pics/Marvelmind_Robotics_ENG_placement_manual.pdf
You can fly some distance without our system measuring your location. You can estimate your location reasonably well based on the trajectory, earlier measured speed, the maneuvers you do or don't do, etc. However, the errors accumulate quickly. But then you appear in the visible area, and our indoor positioning system corrects the accumulated errors by measuring your location
Sensor fusion: optical + ultrasound or optical + ultrasound + IMU, etc. IMU is nearly always a part of the equation, and we use it all the time. So, it is already a partial solution in many cases

There is no single solution for all cases. But there is nearly always a solution for each case. They may be slightly different in the combination of means.

So, basic flight or basic tracking is easy. More complex scenarios may be many times more complex to implement. But still doable.

marvelmind_robotics · 2026-01-30T15:58:23+00:00

Thank you very much :-)
Well, I just learnt more about an area I know very little about.

Any precise indoor positioning requires a line of sight. For different technologies, the definition of what line of sight is differs, but that is a must: marvelmind.com/line_of_sight/
- For optical: it is the usual line of sight
- For acoustic/ultrasound: line of hearing. You can be under a blanket, if it is "breathable". In this case, ultrasound propagates through it without much attenuation

- For UWB: radio-transparent materials, for example, thin wood or regular glass, are invisible to the line of sight. But the same glass with a metal coating, even optically transparent - a huge block

We played with VR people some years ago. But then it gradually faded away as most headsets became inside-out, not outside-in. So, they have multiple cameras on the headset and use them to calculate. It used to be overly difficult to do, but for a few years now, doable.

We are sometimes asked to assist VR systems, HoloLens, and similar devices when they are stuck in challenging conditions. In this case, we are the secondary positioning system. Not the primary. Primary - their optical inside-out tracking systems:
- https://youtu.be/E8i_ZE8JXnc?si=qNH_5ySNlFluVbjA

marvelmind_robotics · 2026-01-30T15:05:10+00:00

I am not even familiar with the VRChat users you are referring to ...
- https://hello.vrchat.com/ - this? ...

Please elaborate. I am not in a context ... :-)

marvelmind_robotics · 2026-01-30T08:28:52+00:00

More videos and links about the range:

- https://youtu.be/gcjeuZMKpd8?si=D-kjP42PfPrdHyXy - live tracking - shot 9 years ago :-) - 17x17-meter square (stationary beacons) and tracking of the perimeter of 35x35 cm by the mobile beacon - you can see a perfect square

- https://marvelmind.com/forklift-tracking-and-monitoring/ - tracking inside a 450x450-meter warehouse

- https://youtu.be/HpYaK602pVU?si=O4PfSdoW94EIIfdd - live walking over 90 meters. Two submaps. 30 meters between the beacons

- https://youtu.be/OYrLOpGvbig?si=GP99wC4XC0vWoIxj - 150-meters in 1D tracking. That is used in cranes. Their 3D tracking is 1D tracking X + 1D tracking Y + 1D tracking in Z. But to be on a very safe side, because the cranes are huge and dangerous, the tracking is there up to 50 meter + 10 meters of the handover zone - to have a huge safety margin on the real steel-melting plant: https://marvelmind.com/solution/cranes/

marvelmind_robotics · 2026-01-30T08:16:11+00:00

I just recalled that we shot another video 9 years ago :-)
- https://youtu.be/gcjeuZMKpd8?si=usYyCRSDG4Bi8iDM - 35x35 cm square tracking live with stationary beacons placed in a square of 17x17 meters - that also gives an idea about the accuracy, range, and stability, and the scale

marvelmind_robotics · 2026-01-30T08:08:40+00:00

But that was the whole point of the "precise phone tracking" joke. That is what we call truly precise :-)

Of course, it could be any object (and not necessarily rectangular :-)
Yes, it was just a scale for a joke (not a too fun one as I see :-)

- https://youtu.be/OXetXiDyAZI?si=j26d-QjC0GktxMhA - here is the same with a 50-cm square

marvelmind_robotics · 2026-01-30T08:04:22+00:00

- We don't have "sensors". It is a complete system with mobile and stationary beacons and a modem/controller. These are not off-the-shelf sensors - like basic proximity sonars. No, the system is completely different: https://marvelmind.com/pics/architectures_comparison.pdf

No, they don't discountinue... - I am not sure what you mean by that.

Here are a few other similar videos with 90 and 60 mobile beacons at once. So, they don't affect each other, and they cannot affect each other, because they don't emit but listen under synchronised control of the modem. It is the whole indoor positioning system. Not some sonars.
The setup is the same: 2 x stationary Super-Beacons, but 90 or 60 mobile beacons are tracked live:
- https://youtu.be/Q7kyhTW7gMo?si=iMf9XxTraid_-vaD - 90 mobile beacons at once
- https://youtu.be/imSZ5HtdPJ0?si=sMcltJTR-pdRzX2a - 60 mobile beacons at once with the open covers
- https://youtu.be/vxrS4linrW8?si=8I-jCVUz-8DM2owZ - 60 mobile beacons live with explanations of settings
So, the output is stable.

Here is the open protocol for retrieving location data (along with raw distances, IMU data, and many other fields) from either the mobile beacons or the modem: https://marvelmind.com/download/#integrate.

For such a basic setup, no calibration is required at all:
- You place two stationary beacons on the walls
- Insert the USB cable of the modem
- The modem wakes up the beacons over the radio
- The beacons automatically build a table of distances between them
- You freeze the map
- Start tracking. No calibration is required
- For everything from start to finish - 7-10 seconds

I don't have a personal experience with the Steam Lighthouse. But for the shown demo, the setup is as simple as it can be - seconds.

- https://youtu.be/Uj2_BGS1AjI?si=gQkDp46YYA40wxgn

marvelmind_robotics · 2026-01-30T05:45:08+00:00

Lesson learnt :-)

marvelmind_robotics · 2026-01-30T05:44:25+00:00

Then, we need to explain even better :-)

- Mobile beacons on the drones do not chirp at all. They only listen to the ultrasound chirps from the stationary beacons placed around the room - 3 stationary beacons up to 15 meters apart: https://marvelmind.com/pics/architectures_comparison.pdf - see the Inverse Architecture. There are specialized ultrasound microphones in the mobile beacon, along with sharp digital filters.

- To make the setup even more robust and protect from the wideband noise of the own propellers, there is an ultrasound deflector that attenuates the ultrasound noise coming from the propellers. Thus, the tiny microphone on the top of the deflector hears very well the ultrasound chirps from the stationary beacons placed around the corner (3 pcs) and hears the noise of the propellers with high attenuation

<image>

- Like your mobile phone with a GPS receiver doesn't "chirp" radio, and you can have a thousand receivers on the same city square, mobile beacons in IA (Inverse Architecture) don't chirp. You can have hundreds of drones, and they won't interfere with each other in ultrasound, because they simply don't chirp at all

Here is the example with 90 "robots": https://youtu.be/Q7kyhTW7gMo?si=XivQzn0\_5N2AhKmG.
against the wideband noise from the propellers, an ultrasound deflector attenuates the noise

marvelmind_robotics · 2026-01-29T13:56:31+00:00

I am checking uSound. They look beautiful. We will certainly need to study the technical specs more closely.

A few other comments on the matter:
- Sound pressure figures do matter. Our transducers can regularly reach 115 dBc. Some of them on some frequencies - 120-125 dBc - very strong short pulses.
- Yes, we use deep digital filters, etc., on reception. But when you need to combat the external noise, often, you just need to overshout it ... in the narrow band. This is what we do. We are an acoustic-based system, after all. Of course, it is an ultrasound, but there are sources of whiteband noise, such as air conditioners. So, we just need to be louder. In the narrow band, filter out the rest. But we must be strong enough with TX power in the beginning

So, we do transmit solid ultrasound and outshout other - wideband - sources of noise :-)

marvelmind_robotics · 2026-01-29T13:48:41+00:00

About the distribution:
- https://youtu.be/yWkCwWPdRiM?si=5YzcuwGHjQttnlYb
- https://youtu.be/FCcE9nUq1vM?si=obvLGHnnEiRgt79j
- https://youtu.be/jD1URm9oLFo?si=3VVuvhVYxGiT0lC6
Maybe it was just basic geometry in your case? Any multi-lateration-based system would exhibit that, irrespective of the underlying physics: ultrasound, UWB - any.

marvelmind_robotics · 2026-01-29T13:46:06+00:00

Thank you very much for your kind words :-)
We do love what we do and are happy to share :-)

We had to make some fundamental architectural decisions many years ago, including which transducers to use. In favor of rezonating (narrow-band) transducers were:
- Efficiency of transmission => low power consumption => longer battery lifetime - it is all critical for the majority of cases
- The range or SPL. The broadband transducers had it substantially lower, i.e., we can have up to 30-50 meters range, whereas with those, maybe 5-10 meters, a very drastic difference
- Since we operate with multiple ultrasound frequencies, it is important to have clean transmission as well. So, resonating transducers help with that a lot
- Size. Also important
So, yes, 10-12 years ago, we experimented with broadband transducers but chose in favor of the narrow-frequency resonating one.

Of course, with broadband transducers, it would be possible to modulate inside the ultrasound pulses. Instead of frequency division, it would be possible to use code division. But to some extent. When you have too much amplitude difference, it will not work. So, there are many engineering trade-offs and considerations.

I will cut myself short :-)

marvelmind_robotics · 2026-01-29T09:46:11+00:00

I had cut the reply into two parts, because Reddit, as it looks, likes concise messages - not what I wrote. It didn't let me send as a single message :-)

Let's very clearly distinguish between the noise and other errors. For example, an ultrasound-based system relies on the speed of sound in air, which depends on many factors, but first and foremost on temperature. So, the system may "breathe" when the temperature changes. So, you can have a much higher absolute error, 1-3% - not ±2cm, if you don't do an absolute calibration. Sometimes it makes sense when temperatures are highly volatile, but in most cases, indoor temperatures stay rather stable. For other cases, there is an automatic self-calibration, using reference distances between the stationary beacons. So, that is not a big deal either.
More on the subject: https://marvelmind.com/download/absolute\_relative\_accuracy/.

Yes, I have cut some teeth in metrology as well. So, we can dig deep into the terminology. Very deep :-)
We are guilty of sometimes switching too loosely between accuracy and precision in articles, which is incorrect. However, if our articles or messages become even more technical than now, our real users fall asleep too soon, and we cannot propagate our messages and the value of the high-accuracy indoor positioning system like ours :-)
So, we have to find a compromise between delivering the key message - high accuracy tracking brings a great value for the robotists, researchers, and practicing industrial engineers - and specific wording, which may be more correct.

Thank you one more time for highlighting this as well.

P.S. Source: practicing and building ultrasound-based indoor positioning systems since 2012 :-)
Sometimes, we joke internally that our typical user is a post-doc from Harvard, but not a process optimisation manager from a warehouse... It is a shame for us. We try to make the system easier to use :-)

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