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Comment by DamnInteresting | Hacker News Reader

Strange that they don't use the age-old term for this sort of position-tracking: dead reckoning.

https://en.wikipedia.org/wiki/Dead_reckoning

They use dead reckoning.

Dead reckoning requires speed information. They have invented an application specific way to reduce cumulative errors in speed from IMU unit.

Accelerometers and gyroscopes can't differentiate between constant linear speed and staying still (both have zero acceleration) and errors in speed accumulate constantly. Airplances and ships have ways to measure speed without GPS, even if currents or wind prevent accurate ground speed measurements. Small inexpensive devices like this don't have that.

This application uses foot movement to calibrate speed in every step. The same can be done with laser, ultrasound etc. but doing it with IMU sensors only is a neat trick.

The GP was saying that they don't use the term itself, though.

Dead reckoning is like pure IMU integration. In this work they detect the instant of zero velocity and do something like a Kalman update with a velocity measurement

No. Dead reckoning is using relative measurements. Your algorithm is irrelevant.

Yes, but those "relative measurements" are IMU measurements, which are accelerations and angular rates. The only way to get position and attitude from that is to integrate. All that is to say, the parent comment was dead (sorry) on.

You’re being needlessly narrow minded. Any improvement on that integration that doesn’t use an absolute source of position is still dead reckoning. Do you think ancient sailors used IMUs?

He's not totally wrong here. The "IMU" you're using consists of a fix, clock, compass, and airspeed indicator (in my case). You're doing the same basic stuff that inertial guidance would do but slower and with less accuracy.

And going back to the great-great-great grandparent comment, I've heard from instructors and other pilots that the 'dead' bit of the word came originally from a shortening of 'deduced'. Something like 'deduced' -> 'ded' -> 'dead' or so. I'm not totally sure of the accuracy of that etymology but it sounds plausible to me.

Wikipedia disagrees:

The term "dead reckoning" was not originally used to abbreviate "deduced reckoning," nor is it a misspelling of the term "ded reckoning."

https://en.wikipedia.org/wiki/Dead_reckoning

I agree that without absolute measurement, it’s still just dead reckoning. However, the article is dedicated to describing a novel source of absolute measurement.

Do you... know what these words mean? Absolute positioning has to be calculated from an absolute frame, like GPS. This is relative positioning, corrected by detecting periods when there should be no motion.

Am I an old man now? Where is my cane? Get off my lawn!

Maybe, but I (selfishly) believe old dogs can learn new tricks. Leaving off the first sentence of your comment would convey the same meaning while keeping any well-earned crotchetiness hidden from view.

True but drift is inherent in dead reckoning due to cumulative errors and evidently the trick this guy used eliminates the drifting or limits it (the prototype was accurate to 5 meters).

Well, Wikipedia defines dead reckoning as:

> the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time and course.

It still seems applicable, even if the hardware has advanced sufficiently that errors are minimized.

Dead reckoning is exactly as described in that pull quote, but the problem is that when you integrate, e.g., accelerometer readings to get velocity, you also integrate its bias, which varies over time, and in the case of MEMS sensors will cause very significant errors.

The solution described in this article significantly improves on that by using the pressure data on foot strike as an observation to estimate the IMU's biases and correct the IMU-only position and attitude.

EDITED TO ADD: In many inertial navigation systems - presumably including this one - dead reckoning will still be used as a fallback in the case where corrections fail (i.e., no GPS or insufficient pressure data)

I think the position error will still be unbounded over time with this approach. What it does give you is sharply bounded velocity error, which in turn makes position error grow much more slowly.

It’s basically dead reckoning as you’d see in a car (where you always know your speed pretty accurately) versus in a plane (where speed is an output of the dead reckoning process as well as an input).

The cumulative errors that this device reduces are inherent to the measurement, drift in dead reckoning is just the sum of the errors in measurement and the assumptions thereof. This device simply provides a more accurate dead reckoning with cheap IMU sensors.

Actually the term is apparently Zero Velocity Update (ZUPT somehow).

https://www.omicsonline.org/open-access/understanding-the-pe...

In general, inertial navigation has one significant difference over traditional dead reckoning: it is unaffected by currents. This is effectively irrelevant when traveling on land, of course, but then again, I don't recall seeing 'dead reckoning' being used to describe navigation on land.

That's because it isn't simple dead reckoning. Dead reckoning is just a part of inertial navigation.

It's the other way around: Inertial navigation is just a very accurate implementation of dead reckoning.

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