[thesh4d0w]

I don't understand this article. If the GPS signals are jammed, what purpose does it serve to have an atomic clock on board your plane? You still need accurate signals with time data to measure against.

Am I missing something?

[touisteur]

You can get a very accurate timestamp from GNSS. What lots of people do then is slave a PLL based on a local oscillator, to be able to get time between two GNSS captations. Or to be able to extrapolate when they have no GNSS signal.

Now suppose someone is spoofing your GNSS signals, it's pretty hard to replace a constellation with another one whilst maintaining time consistency for you. One way to detect spoofing is comparing what a local clock is saying to whatever the GNSS is giving. A local, unfudgeable, stable, accurate clock is a good reference for this.

[DannyBee]

The article is seriously confused. What you are talking about is easy - chip scale atomic clocks are easy to get. I can have one shipped to me today. Hell, i have one on a time card in my basement.

Assume you want it even super accurate.

Great, 3k for an SA65 https://www.microchip.com/en-us/products/clock-and-timing/co...

Holdover would be fine for even a very long flight.

Hell, even a good rubidium oscillator doing 1PPS will stay within 200 nanoseconds over 12 hours.

If you are trying to do navigation while jammed, none of these help you.

You still need good reckoning, which is the hard part

We done solved the clock problem enough already :)

[touisteur]

I was under the impression (and from experience too) that the very stable oscillators were finicky and sensitive to temperature swings and in general costly to use properly in "hard" environments.

I'm happy to learn this is not the case for every good oscillator. TIL.

[thesh4d0w]

Ahhhhh, that makes sense. Treating this as security mechanism rather than an anti-jamming one.

[simne]

As I read from book about gyroscopes, most sensitive achieve so fine accuracy, they detect daily Earth rotation and even yearly Earth rotation.

But when they speaking about near zero temperatures, looks like they talking about something like Rydberg atoms - extremely sensitive matter, which could be considered as nuclear scale gyroscopes, or quantum gyroscopes, or read more about quantum accelerometer.

And current inertial navigation could be used to calculate relative coordinates like automobile odometer, but from integrating accelerations. But classic accelerometer is just not fine enough, and at this place appear quantum accelerometer and quantum gyroscope.

And I agree, article is terrible. I don't know why they use so abstract language, when could just say, navy already tested quantum navigation.

[simne]

To be more concrete, space rockets nearly all fly with inertial navigation, but they are extreme case, because most use only inertial navigation just few minutes (so all those classic gyros/accelerometers integrated errors are small enough to successful enter stable orbit, and then using some sort of radio or optical fine measurements and making corrections with fine engines).

Planes flights are much more lengthy than rockets - I think, typical ~40 minutes or more (most long I hear 20 hours), so INS could integrate huge mistake.

[foldr]

I believe submarines navigate long distances using INS. I don’t know how accurate it is, or how often they have to make corrections using other data. But ballistic missile submarines can’t really use active sonar or surface with any frequency, so I’m not sure what other method they’d use.

[simne]

I talked with captain of submarine. He said, in real life navigation was not reliable, so they have to go to surface and make adjustments with some classic navigation - radio beacons and star navigation.

And civilian education now close to forgot star navigation, but navy still train people to navigate with stars and learn Morse code.

https://news.ycombinator.com/item?id=43319923

[simne]

> ballistic missile submarines can’t really use active sonar or surface with any frequency

Detect semi-surfaced submarine at night is really hard, if don't have intelligence data that it will surface on some non-random position.

From experience of Ukrainian war, my country have success with eliminating surface military ships, because have constantly monitoring their moves with satellites, but I cannot remember any case when semi-surfaced submarine was hit.

[foldr]

Are they not easy to detect on radar? Even during WWII, radar got good enough to detect submarine periscopes. It's hard to imagine that a partially surfaced submarine wouldn't have a significant radar return. That doesn't mean that they're easy to detect at long ranges, but I would have thought that partially surfacing or raising a periscope would be a significant risk to a submarine if the enemy knew its rough location.

At a guess, Ukraine probably can't deploy naval assets with powerful radar close enough to where Russian subs are operating. But an adversary with a more powerful navy might be able to.

[simne]

> if the enemy knew its rough location

In these words you hit bull eye.

During WWII, submarines was just very special type of boat. You could check wikipedia about German u-boats - exist about TEN subtypes, from which only latest types have really significant underwater range, but all others was extremely limited in underwater activity.

But, surface ships of that time was even more limited, many could not achieve even half of surface speed of u-boat, so become easy prey.

But if you will try to find some artificial object on sea surface, that is really hard question. Just because sea is huge, so you need to check extremely large space in short time.

Radars are better to spot artificial object on sea surface than visual, just because radar easier to automate. But nothing more. Radar is also have problem of square distance, very similar to visual. So, as it is hard to spot partially surfaced submarine visually, it also hard to spot such sub with radar, because much less part will be on surface, so radar will have much less signal to detect.

Periscope size is nearly undetectable on surface, if it used carefully, just outside detection range of radar.

So, to conclude, Ukraine problem is, we cannot detect partially surfaced submarines on open sea, but they could fire missiles. Fortunately, Russians have very few submarines on Black sea, and after they was hit at harbors, their usage become very limited.

[crote]

A big issue during WWII is that the submarines were trying to find and approach the ships in order to sink them - and the ships in turn were looking out for the submarines. The submarine is forced to be close to ships equipped with radar.

Ballistic missile submarines are a completely different story. They aren't chasing anyone. Their entire goal is to be unpredictable and stay hidden, so if there's anyone with a radar around they are just going to keep quiet and move somewhere else.

Finding a sub prowling a shipping route is quite doable. Finding a sub in the middle of the Pacific Ocean? Not a chance.

[maratc]

> Are they not easy to detect on radar? Even during WWII, radar got good enough to detect submarine periscopes.

They are, if you can get your radar on top of the periscope, e.g. mounted on a plane that flies above the sea.

[p_l]

INS essentially was expensive and AFAIK once GPS became available started to drop off in use outside of military. And with GPS availability coinciding with switching to more modern integrated Flight Management System/Computer, a lot of planes simply don't have INS installed.

[simne]

Your words are near to truth. Before GPS from nearly 1950s used LORAN navigation system, with similar to GPS principles, but used long waves and have relatively low precision - about kilometer at best.

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

Before LORAN, used radio beacon navigation and star navigation (from Newton time), and good human navigator could achieve about 50km precision.

You could easy see signs of star navigation on good preserved old planes - they all have some sort of fully glass dome, or blister, to provide good near semi-sphere view. And sure, all those before-GPS era planes have separate navigator job position, sometimes shared with mechanic.

https://www.reddit.com/r/WWIIplanes/comments/59xfkz/pby_wais...

You could ask, how planes could fly with 50km precision? Answer is easy - at all plane routes built ground structures easy seen from air and last mile navigation become essentially visual flight, nothing more, nothing less.

On some places ground navigation structures preserved now, for examples:

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

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

[p_l]

LORAN was mostly long range over sea, on the ground we had NDB, DME, VOR, etc all ultimately linking into "airways" for higher altitude operations where earth might be not visible due to cloud cover for example

[scrlk]

> a lot of planes simply don't have INS installed

Perhaps in general aviation, but I can't think of any modern commercial airliner without an INS via the air data inertial reference unit.

[p_l]

Except ADIRU isn't INS.

The INS unit is separate and often has its own set of gyros and has to be connected as separate input to FMS or other navigational computers, same as connecting GPSes or other radio nav components.

For example the current model of popular Universal Avionics UNS1 series of NCU (navigational computer part of FMS) come with built-in augmented GPS receiversz but do not mention INS functionality at all even in extended models. Don't have access to manuals at the moment, but I'd expect to see INS as optional to connect over one of the external connectors on the MCU, as it was on the older models without integrated GPS

[scrlk]

In that case, I stand corrected.

I had assumed that the ADIRU’s inertial reference data from its gyroscopes and accelerometers would feed into the FMS to provide INS capabilities in case GNSS was unavailable.

[simne]

Many small planes don't have INS in typical meaning, but their pilot is INS computer, calculated approximate nav from air data (air speed + weather data + compass or radio compass).

[magnetometer]

> most sensitive achieve so fine accuracy, they detect daily Earth rotation and even yearly Earth rotation

Daily rotation is 360°/23.934h, so 0,25°/min, which is acutally quite a lot if you want to use a device to track your orientation.

[simne]

Unfortunately, these numbers considered state of art for modern classic gyroscopes.

Better are quantum navigation systems, using quantum matter as sensor, but they was too bulky to be used on planes, only last years appear more compact systems, sized like common home fridge.

[RandomBacon]

> daily Earth rotation and even yearly Earth rotation.

Minor FYI: the earth rotates daily, but it revolves around the sun yearly.

    revolve /rĭ-vŏlv′/
    intransitive verb
    To orbit a central point.
    "The planets revolve around the sun."

[HPsquared]

I took it to mean "able to measure a rotation rate of 1 turn per year".

[lupusreal]

> they detect daily Earth rotation

This is the principle gyrocompasses work on; when left running for a while they align themselves with true north; the axis of Earths rotation.

[BenjiWiebe]

I didn't read the article, but: a GPS receiver must calculate/find both it's time and position to get a fix. So maybe by having the time already available really accurately it makes the job of finding position easier?

[gorbypark]

From my (very basic) understanding of GPS you need at minimum four satellites to calculate the time. If you had a local atomic clock in sync with the GPS satellites, you'd only need three satellites to get a position fix. It would (probably, maybe?) also speed up the time to first fix / time to a precise position fix.