Semi-related, X-37B is heading up next month, and unlike so many missions they're saying a little bit what this one is for. It'll attempt to do some laser communications demonstrations. And... it has a "quantum inertial sensor", as a would-be GPS alternative. https://www.airandspaceforces.com/x-37-eighth-mission-laser-...
The article says it uses magnetic and gravitational field sensing. I don't know how surprised I'd be to hear magnetic field sensing would work: I'd sort of assume there'd be some significant variance day-by-day week-by-week. But getting up there and seeing what you get is probably the way to find out?
If you happen to run three clusters of mag sensors (X,Y,Z on each wing tip and another group of three on a tail boom) then you are measuring a differential field against three points for each axis and sensing the induced ground change from a diurnally fluctuating field .. also getting different results depending on whether you are traveling North to South, East to West, or at some other angle.
Despite all that, maps are still produced of the leveled and normalized ground field (see geomagnetic maps).
Using the geomagnetic as an assist / backup to navigation reduces to something similar to the use of a DTM as an assist - not much use in flat areas, useful in areas with distinct features, easily confused if one valley looks much like another.
The daily flux is an issue, of course, but your software, hypothetically, would be looking at the preserved shape of waves on the ocean despite the fact that the tide is rising and falling all around.
Stating the obvious.
GPS gives you two things, absolute time and position... this Gravimeter, while interesting only allows you to deduce your position based on where you've been, and a somewhat accurate map of the world.
I think techniques like BPS might be more viable.
https://www.nist.gov/publications/time-transfer-performance-...
https://www.jeffgeerling.com/blog/2025/bps-gps-alternative-n...
The applications of BPS are entirely different.
BPS depends on terrestrial stations instead of satellites, like Loran, which was used in the past.
BPS is at least as easily jammed or disabled as GPS.
With the gravimeter discussed in the article, and with adequate maps, one can navigate autonomously, without depending on any external help.
Moreover, BPS appears to be intended to cover only continental USA. There are no plans to use radio frequencies that would allow global coverage, like Loran had in the past.
> BPS depends on terrestrial stations instead of satellites, like Loran, which was used in the past.
> BPS is at least as easily jammed or disabled as GPS.
So which is it? Because LORAN was much harder to jam than GPS. The jamming resistance of any positioning system is mostly based on the ability to overpower the legitimate signals arriving at the receiving antenna. The problem with satellite based positioning systems is:
1. Satellites have relatively little power available to them, which makes their broadcasts rather weak. Terrestial transmitters can be much more powerful.
2. Satellites are very far away, which causes the received power to be very attenuated. Terrestial transmitters can be much closer.
There are many valid reasons to choose GNSS over terrestial approaches, but jamming resistance is not one of them.
Source: I was a weapons engineering officer for the Dutch Navy for many years and jammability of our positioning systems was a constant concern, to the point of ships being equipped with LORAN-C receivers well into the 2000s as a backup for GPS.
In an EW contested environment, anything relying on external RF signaling is subject to denial or spoofing.
The war in Ukraine is teaching everyone a lot of lessons.
>Terrestial transmitters can be much closer.
Making them nice targets for the enemy
BPS would be substantially harder to jam, as the broadcast signal power would be local and in the many kW range (50+ kW transmitters, with multiple frequencies per city). GPS is one frequency (or a few if you use other countries' systems which has its own risks), with far lower signal strength.
Still in principle vulnerable to jamming, even if less so than the fairly weak signal from GPS. Though I suspect there will be an emphasis on using a combination of techniques, instead of relying on any single one.
https://www.naval-technology.com/news/uk-royal-navy-trials-a...
Related in submarines from 2024.
Are there any isotopic / gamma spectroscopy maps?
A gamma spectroscopy map would have many channels (a large number of independent monochromatic maps), the seas may look bland though.
a bit more context:
gamma radiation would be harder to block or deny
gamma radiation jamming could be more easily compensated for due to directionality of the line of gamma sight from jammer to receiver. its not even necessary to fully block the jammer (one could modulate different paths by rotating lead plates, or do other types of source separation)
Out of interest how well can you tell your location using the time, the position of an available known celestial object (sun, constellation, or moon) and a compass? Would that combination plus good instrumentation and computer provide a decent backup. I guess nighttime and cloudy poses an issue.
Is that a real question? As in you don't actually know?
That's how ships navigated for centuries. The instruments are simple by modern standards. You don't need a computer, although it's nice to have. The fixes aren't (normally) as precise as you get from GPS, but they're much better than you'll ever get from gravimetry. It can take time to get one, though.
Celestial navigation was a huge driver for the development of timekeeping and supporting technologies. They still teach it in "ship school". It was big news when the US Naval Academy stopped requiring it, and I think they've now backed off and brought it back because of the GPS jamming. It can be and has been automated, although the resulting instrument is a lot bigger, touchier, and more expensive than a GPS receiver.
If you need a lot of accuracy, you normally use more than one object and don't trust the compass. Individual stars are fair game and have a lot of advantages over extended objects.
But, yeah, clouds. I assume clouds are the reason people are building these big complicated expensive devices. It also gets harder to do it, at least manually, if the ship is rolling around a lot.
"I must go down to the seas again, to the lonely sea and the sky And all I ask is a tall ship and a star to steer her by."
With all due deference to Harrison's clocks, Sobel's Longitude, Eco's The Island of the Day Before, and other such prose or poetry of maritime navigation, I do prefer to mutter the lyrics to XTC's All You Pretty Girls (Quiet or witty girls by the quayside).
Nighttime makes it easier, I suppose (if no clouds).
Whay do you do once you measured the gravitational fluctuations so precisely? Is there some kind of map of fluctuations you compare to? Arent these almost the same on adjacent points? Or is it a function of local terrain topography (mountains, lakes etc?)
Indeed.
>The system deduces the strength of Earth’s gravity in every given point of the journey from the motion of the atoms illuminated by laser beams inside a vacuum chamber and compares that data with gravity maps compiled from satellite measurements.
Is that like TERCOM but using strength of gravity rather than terrain height?
Yup, I was just about to comment the same. I find it shocking that there's enough information/variation in gravitational field that you can match on.
The variation is very small, and that is why it can be exploited only with an extraordinarily sensitive gravimeter, like the one discussed in the article.
Very Star Trek technobabble vibes, except for real!
Precisely.
What kind of raw signals can the device generate? Can it measure gravity in various directions? Or is the output just a single scalar?
It's a rack sized multitude of sensors on a boat on the ocean.
It's 'locked' to the tidal surface (meaning an internal tide model and a mean global ocean surface model are onboarded) and perturbed by wave action, the heaving pitch and yaw presaging a decent chunder (it's being trialed on an Australian Navy ship)
Likely single channel gravity signal and three axis inertial measures .. as outlined in this abstract:
Software Ruggedized Atom Interferometry for Strapdown Mobile Quantum Inertial Sensing
Here, we report world-first demonstrations of a dual quantum gravimeter operating in a relevant maritime environment, with an unattended uptime of >144 hours, ...
In addition, we present a compact 3-axis quantum inertial sensor capable of high repetition rate operation in extremely harsh environments.
> “You can’t spoof gravity without literally moving a mountain,” said Biercuk.
What about weather conditions? Clouds, heavy rain, hurricanes, etc.
Relevant if you're trying to do extremely fine gravity measurements, but for navigation (and with the performance of these sensors), it's not going register much, the natural contours of earth's gravity over longer distances are large compared to these effects.
Though the more interesting part is that with such sensitivity you would likely be able to infer nearby masses moving around - shipsnor stealth planes for example.
Difficult to do over any significant range: gravity drops off with 1/r^2, gravity gradient like what they seem to be using here (easier to measure while in motion) drops off with 1/r^3. The variations in the earth are due to fairly large density variations in massive sections of the crust, and a ship and especially an aircraft is absolutely miniscule in comparison, even if the density contrast is higher.
Not sure about weather conditions. What about magma flows?
Moon too (but that is predictable)