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Comment by t0mas88 | Hacker News Reader

You don't need GPS to figure out the correction for this. Inertial navigation systems in aircraft (which use very stabilised platforms with a lot of math involved) worked before GPS was available.

It helps to have a rough indication of the current latitude on startup, but you can also figure it out from the gyro outputs. Just takes longer.

With modern sensors (solid state laser gyroscopes) it has all become a lot smaller so if you really want to you can do this in a camera. It's just probably going to be too expensive for what it brings, because 6+ stops of stabilisation is a lot already.

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> You don't need GPS to figure out the correction for this.

Perhaps not, but a lot of cameras already have it for geotagging purposes (EXIF), so why not use it:

* https://en.wikipedia.org/wiki/List_of_cameras_which_provide_...

* https://www.digitalcameraworld.com/buying-guides/best-camera...

Because location doesn't give orientation, which you need to know to get acceleration to know how to correct.

Simply using the accelerometers and gyro directly does give the info you want. GPS is useless.

Aerospace grade laser gyroscopes are incredibly expensive (and bulky), and even then, they still have massive drift after several hours. If you don't have GPS to relocalize precisely at least every day, there is no way you can know the location of the camera on earth for more than a day, even with state of the art aerospace stuff

Realistically GPS is the answer, but it’s notable that you could also use a simple light sensor combined with accurate clocks to get your position on earth:

https://en.m.wikipedia.org/wiki/Light_level_geolocator

> Recording light levels over time Wonder how much time is needed to determine location.

You just need to determine the time of sunrise and sunset relative to a known location and you get a rough idea of latitude and longitude.

> they still have massive drift after several hours. If you don't have GPS to relocalize precisely at least every day

I think you may be confusing two concepts: Measurement of true north and latitude via gyro (what the GP is talking about) and inertial navigation systems (which, yes, do drift).

You can measure those two things with just a single-axis gyro and no external references using a technique called "gyro-compassing". In fact, most internal navigation systems use gyro-compassing to directly measure true north and latitude to align the system on initial startup.

But the location of the camera doesn't matter. You only need to figure out very roughly at which latitude you are to know by how much to compensate for earth rotation. And you can do that with the sensors that you're already using to do the stabilisation. That was my point... no need for GPS.

> worked before GPS was available.

Worked makes it seem like you throw a switch and it just gives you position data. Those units take anywhere from 6 to 10 minutes to align, if you move the platform, it will error out and you must restart the alignment. The current systems take their initial fix from GPS, but the initial systems, the operator had to manually know and then key that information into the unit.

"Worked" with extreme care operated by a qualified professional.

I mean, surely if you are doing something that requires this level of precision, you could just ask the user to input its current known location? I doubt that even if the user misdialed by ten or twenty meters the difference in compensation would matter (or even if the camera was actually moving around).

> Inertial navigation systems in aircraft (which use very stabilised platforms with a lot of math involved) worked before GPS was available.

Inertial measurement units for aircrafts and submarines cost as much as a house in California. Good luck putting those in a phone.

The IMUs that existed on aircraft before the invention of GPS have been superseded by the ones which actually are in your phone, in much the same way and for much the same reason that a $20 Casio F-91w keeps better time than a fancy Rolex that costs more than a house in California: electronics are cheaper and better than mechanical systems.

We have, naturally, also made better IMUs for places where it matters, ones which won't fit in your phone.

The question is therefore not suited to "aircraft grade, yes or no?", it's "how expensive is the cheapest IMU that's good enough for the specific need?" which in this case itself depends on how many stops is desired.

Actually that F91W does not keep very good time.

There pretty excellent if you keep them on a shelf bit if you run around outside in the hot and cold (you know, like people use a watch) they'll deviate quickly. Because they don't have a temperature controlled (or even compensated) oscillator. A real TXCO (basically putting the crystal inside a temp calibrated oven) is not feasible on a watch battery but compensation sure would be.

I picked it not because it's good, but to illustrate the cheapest digital is still better than any analog mechanism that money can buy.

Ah ok,I didn't realize analog watches were that bad.

> A real TXCO (basically putting the crystal inside a temp calibrated oven) is not feasible on a watch battery but compensation sure would be.

Nitpick: You're thinking about OCXO for crystals inside an oven, TCXOs are temperature compensated crystal oscillators

Well, that, and there's no such thing as a "solid state laser gyro". I believe the GP is confusing MEMS solid-state gyros and laser-ring gyros (which can use a solid state laser, but AFAIK aren't ever called "solid state laser gyro").

MEMS gyros have too much bias drift (both on a unit basis due to fab processes and on a temperature basis) to be practically useful here. You can measure the earth's rotation with a MEMS gyro, but you're really at the limit.

Laser ring gyros are referred to as solid state laser gyros.

Heh, while I get what you're saying... despite being somewhat pedantic... there are in fact MEMS FOGs now too :). https://www.anellophotonics.com/technology

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