# Inertial navigation: is a Schuler pendulum still involved? **URL:** \ **Category:** Factual Questions\ **Created:** [May 12, 2026, 4:34pm UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022 "2026-05-12T16:34:40Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Timz](https://avatars.discourse-cdn.com/v4/letter/t/6a8cbe/32.png) [@Timz](https://boards.straightdope.com/u/Timz)\ **Post date:** [May 12, 2026, 4:34pm UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/1 "2026-05-12T16:34:40Z")
The front page of the NY Times for 18 April 1957 has an article on inertial navigation. The previous day, Charles Stark Draper gave a talk describing the trip he made in 1953, Boston (Hanscom) to LA in a B-29 using inertial navigation. The article says the mechanism “simulates the Schuler pendulum”, the latter being a pendulum with length equal to Earth’s radius. Never heard of that. Has the INS that’s been common in aircraft for decades ever done that? (The article doesn’t say whether the mechanism controlled the plane itself, or just told the pilot where he was.) ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 12, 2026, 4:44pm UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/2 "2026-05-12T16:44:39Z")
> [@Timz](#): > > a pendulum with length equal to Earth’s radius Even in 1957, this was not a physical pendulum. ETA even today, AFAIK mechanical gyrocompasses have not completely disappeared from ships and 'planes, although I am not sure to what extent, so do not quote me on that. Of course, today you can get fiber-optic gyroscopes and laser velocity meters, but also various MEMS devices. ---
**Author:** ![Llama\_Llogophile](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@Llama\_Llogophile](https://boards.straightdope.com/u/Llama_Llogophile)\ **Post date:** [May 12, 2026, 4:45pm UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/3 "2026-05-12T16:45:46Z")
Inertial navigation systems have largely been supplanted by GPS. The bizjet I fly has an Inertial Reference System (actually several, for redundancy), but these are laser devices, not a big box of spinning gyroscopes of years prior. Our IRS does a number of jobs, but navigation is a secondary function. It’s the primary source of attitude information, being much more sensitive than the old mechanical units. I would assume our laser IRS units do something akin to [Schuler Tuning](https://en.wikipedia.org/wiki/Schuler_tuning), but that’s beyond my pay grade as a mere pilot. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [May 12, 2026, 4:47pm UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/4 "2026-05-12T16:47:19Z")
I never heard of this before, but there’s a Wikipedia page on “Schuler tuning” > **[Schuler tuning](https://en.wikipedia.org/wiki/Schuler_tuning)** > > Schuler tuning is a design principle for inertial navigation systems that accounts for the curvature of the Earth. An inertial navigation system, used in submarines, ships, aircraft, and other vehicles to keep track of position, determines directions with respect to three axes pointing "north", "east", and "down". To detect the vehicle's orientation, the system contains an "inertial platform" mounted on gimbals, with gyroscopes that detect motion connected to a servo system to keep it pointing ... > A pendulum the length of the Earth’s radius is impractical, so Schuler tuning doesn’t use physical pendulums. Instead, the electronic control system of the [inertial navigation system](https://en.wikipedia.org/wiki/Inertial_navigation_system "Inertial navigation system") is modified to make the platform behave as if it were attached to a pendulum. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [May 12, 2026, 6:36pm UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/5 "2026-05-12T18:36:57Z")
> [@CalMeacham](#): > > A pendulum the length of the Earth’s radius is impractical, I’m so tempted to add “[citation needed]” to that sentence. ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [May 12, 2026, 9:31pm UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/6 "2026-05-12T21:31:13Z")
IIRC, many modern inertial guidance systems use a coil of fiber optic for each axis; laser interferometry measures the difference in path as the device is turned by counting the wavelengths. If for example, you turn the device clockwise, the laser light going clockwise has to go a longer distance to get back to the source than the counterclockwise, and the difference can be counted from wave interference. (I.e. one wave pulse started when the source was at 12 o’clock, now it’s at 1 o’clock, so 30° further. While counterclockwise went 30° shorter. Obviously, we’re talking about microscopic differences, not 30 degrees, but the concept is there - and best of all - no moving parts. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 12, 2026, 9:58pm UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/7 "2026-05-12T21:58:17Z")
> [@Llama\_Llogophile](#): > > Inertial navigation systems have largely been supplanted by GPS. Not really. You wouldn’t use one to figure out your location after a trans-continental flight, but almost all navigation systems do still use inertial navigation. The “GPS” on a phone is actually a compilation of data from many different sources, one of which is the inertial sensors. And the inertial sensors are the biggest source of information for information on short distance or time scales. (if you’re curious, other sources of “location services” data include which WiFi or cell tower signals are visible and their strengths, and extrapolation from maps). ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [May 13, 2026, 12:11am UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/8 "2026-05-13T00:11:28Z")
Yes, Schuler-tuning is still a part of internal navigation design, and will be as long as the Earth has the same surface gravity and radius. See here for a 2025 article on the subject [https://www.sciencedirect.com/science/article/abs/pii/S0263224124017470](https://www.sciencedirect.com/science/article/abs/pii/S0263224124017470) ---
**Author:** ![Llama\_Llogophile](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@Llama\_Llogophile](https://boards.straightdope.com/u/Llama_Llogophile)\ **Post date:** [May 13, 2026, 12:31am UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/9 "2026-05-13T00:31:25Z")
> [@Chronos](#): > > Not really. You wouldn’t use one to figure out your location after a trans-continental flight, but almost all navigation systems do still use inertial navigation. The “GPS” on a phone is actually a compilation of data from many different sources, one of which is the inertial sensors. And the inertial sensors are the biggest source of information for information on short distance or time scales. OK, but I was referring to the old mechanical inertial systems - the “boxes of gyroscopes” I mentioned. GPS has certainly replaced those. Though I take your point about what some aircraft manufacturers call “blended navigation”. Multiple inputs are considered and they come from more sources than one might imagine. My jet takes into account GPS, IRS, VOR and DME to compute position. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [May 14, 2026, 1:37am UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/10 "2026-05-14T01:37:57Z")
These guys ([Inertial Navigation Systems and Units - Safran - Navigation & Timing](https://safran-navigation-timing.com/solution/inertial-navigation-systems/)) are still selling inertial systems for things like underwater operations, and GPS-denied environments, using gyros (made of vibrating crystals) ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [May 14, 2026, 1:52am UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/11 "2026-05-14T01:52:46Z")
A Foucault Pendulum “rotates” every 24 hours (that is, it does not rotate, but the earth rotates under it). But also, _the centre of the swing describes a small circle_ around the “true” midpoint. That 24 hour rotation _and the small circle_ are important if you are an Inertial Navigation System. The 24 hour rotation would have you going around in a 24 hour circle, and the small circle would have you moving sideways and fore/aft in a small circle around your “true” course. Unfortunately, with an INS, the “small circle” translates to a fore/aft east/west acceleration, so your actual course correction could be much larger than the error term. If your Foucault Pendulum was mounted on a platform orbiting the earth at orbital speed, you wouldn’t see that east/west for/aft uncertainty in your position. (It is conventional to have Foucault Pendulums orbiting the earth on a 24 hour orbit, which is much slower that orbital speed for sea level, and requires continuous support to prevent the system falling into the centre of the earth.) The Schuler correction is the correction for the small circle. (If you are building a Foucault Pendulum, you use a collar beneath the pivot point.) As you will remember, orbital speed depends on altitude, so the Schuler correction depends on altitude. I don’t know if the altitude correction is important in practice. The theory of Schuler pendulums and Foucault Pendulums is under-graduate level. I only have a High-School level understanding: everything I wrote may be wrong. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 15, 2026, 12:18am UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/12 "2026-05-15T00:18:43Z")
> [@Melbourne](#): > > A Foucault Pendulum “rotates” every 24 hours (that is, it does not rotate, but the earth rotates under it). Only if it’s mounted at a pole. Anywhere else on the Earth will have a longer period. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [May 15, 2026, 12:24am UTC](https://boards.straightdope.com/t/inertial-navigation-is-a-schuler-pendulum-still-involved/1030022/13 "2026-05-15T00:24:43Z")
Up to an infinite period (i.e. no rotation at all) at the equator.