---
title: "Q-CTRL navigates without GPS. Not without a gravity map"
locale: "en"
url: "https://irz.fr/en/articles/qctrl-no-gps-still-needs-gravity-map-en"
markdown_url: "https://irz.fr/en/articles/qctrl-no-gps-still-needs-gravity-map-en.md"
category: "tech"
tags: ["navigation", "quantum sensing", "gravimetry", "GNSS", "maritime"]
published_at: "2026-08-28T10:14:00.000Z"
author: "Hugo Marchal"
translation: "https://irz.fr/fr/articles/qctrl-sans-gps-pas-sans-carte-fr.md"
---

# Q-CTRL navigates without GPS. Not without a gravity map

Q-CTRL corrected inertial drift over an 83 km maritime run with GNSS excluded from the navigation chain. The quantum sensor does not output position: it measures gravity and matches it against a map prepared beforehand.

Switch off GPS and the ship still has one peculiar landmark: gravity. Not 'down', which is not terribly useful for finding Australia, but tiny changes in how strongly Earth pulls from one patch of ocean to the next. Q-CTRL used those changes as terrain.

Calling it quantum GPS would make the story simpler and the engineering wrong. The gravimeter never produces latitude or longitude. The ship's inertial system keeps doing the navigation; the gravity measurement gives it an occasional argument about where that navigation has wandered.[1](https://q-ctrl.com/blog/q-ctrl-achieves-worlds-first-gps-free-quantum-gravimetric-navigation-demonstration-in-maritime-field-trial)[2](https://arxiv.org/abs/2608.25563)

In the technical manuscript, that loop corrected navigation across an **83 km** maritime trajectory with GNSS excluded from the measurement chain and delivered bounded positioning at roughly **nautical-mile level**.[2](https://arxiv.org/abs/2608.25563)

> **This is not quantum GPS**
> - An IMU continuously estimates vessel motion, but its error accumulates with time.: 1
> - The gravimeter measures small local variations in Earth's gravity field.: 2
> - Software compares those variations with a gravity map prepared before the mission.: 3
> - The match provides a correction that reins in the inertial navigation solution.: 4
> The quantum sensor aids the INS. It replaces neither the INS nor the map.

## INS drifts

INS already works in the dark. Accelerometers and gyros do not need satellites, radio towers or a view of the sky. Their weakness is arithmetic: tiny measurement errors get integrated again and again.

An accelerometer bias first corrupts velocity, then position. A gyro that is slightly wrong about attitude lets a piece of gravity masquerade as motion. Leave that alone for long enough and the coordinates can look beautifully precise while describing the wrong patch of sea.[2](https://arxiv.org/abs/2608.25563)[4](https://research.birmingham.ac.uk/en/publications/preliminary-assessment-of-strap-down-operation-of-a-gravity-gradi/)

The escape hatch is that Earth is lumpy. Different rock and seafloor structures produce small gravity anomalies, and satellite altimetry has been used for years to turn those patterns into global marine-gravity grids.[6](https://topex.ucsd.edu/grav_outreach/)

Think terrain recognition, not a beacon. If a run of gravity highs and lows looks like one patch of the stored map, the filter gets a position clue it can use against inertial drift.

So Q-CTRL removes satellites from the **live navigation link**, not from the entire history of the data. The map used for matching can itself be satellite-derived.[2](https://arxiv.org/abs/2608.25563)[6](https://topex.ucsd.edu/grav_outreach/)

> Illustration: Q-CTRL diagram explaining Ironstone Opal gravity-map navigation. Q-CTRL's published diagram shows the core loop: local gravity measurements are matched to a map to correct inertial position. Credit: [Q-CTRL](https://q-ctrl.com/blog/q-ctrl-achieves-worlds-first-gps-free-quantum-gravimetric-navigation-demonstration-in-maritime-field-trial).

## 83 kilometres

For the trial, all of this went onto a **29 m surface vessel**.[2](https://arxiv.org/abs/2608.25563) The gravimeter itself is already hybrid: atoms provide the long-term reference while a classical accelerometer helps bridge the ugly motion of a real boat. A separate navigation-grade IMU runs the inertial solution.[2](https://arxiv.org/abs/2608.25563)

The cabin was not turned into a floating metrology lab. No dedicated environmental stabilization, no special calibration regime. The researchers ran matching traversals first with a gimbal and then **strapdown**, with the instrument attached directly to the moving vessel.[2](https://arxiv.org/abs/2608.25563)

A boat is a rude place to measure gravity. It heaves, pitches, rolls and vibrates while the signal of interest is tiny. Strapdown means accepting that insult rather than mechanically hiding from most of it. Heave, roll, pitch and vibration are enormous compared with the small gravity anomalies the system is trying to recover. A gimbal physically removes some of that motion. Strapdown operation leaves much more of the separation job to sensing and software.

Q-CTRL reports comparable performance between the two configurations on the tested traversals.[2](https://arxiv.org/abs/2608.25563) That does not mean a quantum gravimeter can now be bolted anywhere on any ship. It means operation without a dedicated gyrostabilized platform worked under the conditions of this trial.

> Illustration: Maritime installation of Q-CTRL's gravity-aided navigation system. The demonstration is specifically about moving quantum gravimetry away from laboratory conditions and heavy stabilization. The manuscript describes an uncontrolled cabin installation. Credit: [Q-CTRL](https://q-ctrl.com/blog/q-ctrl-achieves-worlds-first-gps-free-quantum-gravimetric-navigation-demonstration-in-maritime-field-trial).

## Not the first ship

Q-CTRL's “world's first” only survives with a fairly precise boundary.[1](https://q-ctrl.com/blog/q-ctrl-achieves-worlds-first-gps-free-quantum-gravimetric-navigation-demonstration-in-maritime-field-trial)

The sea part is not new. A French team published shipborne atom-interferometry gravimetry in *Nature Communications* back in **2018**.[3](https://www.nature.com/articles/s41467-018-03040-2) The catch, and the reason that paper still matters here, is its **gyro-stabilized platform**. The hardware worked hard to keep the sensor pointed correctly while the ship misbehaved.[3](https://www.nature.com/articles/s41467-018-03040-2)

Birmingham then removed that comfort. Its quantum gravity-gradient sensor operated **strapdown** on a vessel, including at sea.[5](https://www.birmingham.ac.uk/news/2023/quantum-sensor-for-gravity-gradiometry-successfully-validated-at-sea) But the team's 2025 IEEE/ION paper was careful about what it had not done yet: this was sensor deployment on a moving platform, not a finished alternative-navigation chain. Integration and filtering still sat on the roadmap.[4](https://research.birmingham.ac.uk/en/publications/preliminary-assessment-of-strap-down-operation-of-a-gravity-gradi/)

The defensible novelty in Q-CTRL's manuscript is narrower and more useful. The authors claim the first published demonstration of **gravity map matching with a quantum gravimeter and a navigation chain fully independent of GNSS**, plus the first same-instrument comparison of gimbaled and strapdown mobile quantum gravimetry.[2](https://arxiv.org/abs/2608.25563)

One other brake on the superlatives: the Q-CTRL manuscript is currently an **arXiv preprint**. We can inspect the method and numbers; peer review has not happened yet.

## One mile

Q-CTRL highlights positioning accuracy of about **one nautical mile**, roughly 1.85 km, across the mission.[1](https://q-ctrl.com/blog/q-ctrl-achieves-worlds-first-gps-free-quantum-gravimetric-navigation-demonstration-in-maritime-field-trial) Beside a healthy phone GPS, 1.85 km sounds awful. But that is the wrong contest.

Nobody should choose this over healthy GNSS because one nautical mile sounds more futuristic. Its value appears after GNSS is gone: the gravity match gives the INS something that does not share the same accumulating error.[2](https://arxiv.org/abs/2608.25563)

The useful word in the paper is *bounded*. An unaided INS can keep printing a very precise-looking coordinate while moving steadily away from reality. Gravity map matching gives it another external constraint and keeps the error within an envelope.[2](https://arxiv.org/abs/2608.25563)

> **What the preprint demonstrates**
> - maritime trajectory used for the GNSS-free navigation demonstration: 83 km
> - length of the surface vessel: 29 m
> - reported positioning-accuracy level with gravity aiding: ≈ 1 NM
> - stationary test used to examine long-term sensor drift: 56 h
> These figures come from the Q-CTRL/arXiv preprint dated August 26, 2026.

The **70×** number in the paper is easy to misuse. It comes from a 56-hour stationary test where atomic referencing reduced long-term sensor drift versus the classical channel.[2](https://arxiv.org/abs/2608.25563) It does **not** mean the boat suddenly knew its position 70 times better.

## Two modes

Then the researchers changed jobs without changing the instrument. For **gravity surveying**, GNSS came back. This time it supplied reference position while the team judged the quality of the gravity measurements themselves.[2](https://arxiv.org/abs/2608.25563)

Trials reached conditions up to **Sea State 4**. The authors report mGal-level agreement with gravimetric maps and sub-mGal repeatability and stability.[2](https://arxiv.org/abs/2608.25563) They also report resolving anomalies down to an along-track scale of roughly **300 m**, much finer than the effective spatial wavelength of the satellite map used for navigation.[2](https://arxiv.org/abs/2608.25563)

The same box is doing two different experiments:

- in navigation mode, the map already exists and measured gravity corrects the INS;
- in survey mode, GNSS provides reference position while the instrument measures gravity finely enough to build or check better maps.[2](https://arxiv.org/abs/2608.25563)

That dual use hints at systems that could eventually improve the data later missions depend on. The preprint does not yet demonstrate an autonomous fleet continuously maintaining a global map in service.

## The map stays

So “GPS-free” survives the fact check, with an asterisk. The correction does not arrive over radio during the run. An onboard gravity map cannot be jammed in the way a GNSS receiver can lose or accept a bad satellite signal.[1](https://q-ctrl.com/blog/q-ctrl-achieves-worlds-first-gps-free-quantum-gravimetric-navigation-demonstration-in-maritime-field-trial)

Maps have their own failure modes. Resolution may be poor. Two stretches may look too similar. A geologically bland area gives the matcher less to grab. And none of this excuses a bad INS, bad motion estimate or bad filter.[2](https://arxiv.org/abs/2608.25563)[4](https://research.birmingham.ac.uk/en/publications/preliminary-assessment-of-strap-down-operation-of-a-gravity-gradi/)

That is the useful catch. A navigation system can remove live external infrastructure while still depending on information prepared before departure.

> Illustration: Q-CTRL Ironstone Opal quantum gravimeter. Ironstone Opal measures gravity. Position only appears once that measurement is fused with an INS and a reference map. Credit: [Q-CTRL](https://q-ctrl.com/blog/q-ctrl-achieves-worlds-first-gps-free-quantum-gravimetric-navigation-demonstration-in-maritime-field-trial).

The gravimeter is not an exotic GPS replacement. Its job is smaller and more believable: add **a physically different reference** to an inertial system that would otherwise keep drifting.

Q-CTRL removed GNSS from the navigation run. It did not remove prior knowledge.

The map stayed onboard, quietly doing the part that makes a gravity reading useful for position.

## References

1. [Q-CTRL — GPS-Free Quantum Gravimetric Navigation Demonstration, August 27, 2026](https://q-ctrl.com/blog/q-ctrl-achieves-worlds-first-gps-free-quantum-gravimetric-navigation-demonstration-in-maritime-field-trial)
2. [Everitt et al. — GNSS-free quantum gravity-aided navigation and fine-scale marine surveying with a strapdown quantum gravimeter, arXiv:2608.25563, August 26, 2026](https://arxiv.org/abs/2608.25563)
3. [Bidel et al. — Absolute marine gravimetry with matter-wave interferometry, Nature Communications, 2018](https://www.nature.com/articles/s41467-018-03040-2)
4. [Seedat et al. — Preliminary Assessment of Strap-down Operation of a Gravity Gradient Quantum Sensor in Maritime Contexts, IEEE/ION PLANS, 2025](https://research.birmingham.ac.uk/en/publications/preliminary-assessment-of-strap-down-operation-of-a-gravity-gradi/)
5. [University of Birmingham — Quantum sensor for gravity successfully validated at sea, 2023](https://www.birmingham.ac.uk/news/2023/quantum-sensor-for-gravity-gradiometry-successfully-validated-at-sea)
6. [Scripps Institution of Oceanography — Marine Gravity from Satellite Altimetry](https://topex.ucsd.edu/grav_outreach/)