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Are accurate clocks really the limiting factor in cheap Inertial Navigation?

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rs20:
Hi,

In this article, in which scientists have apparently developed some sort of portable atomic clock, they claim that with this clock, the need for GPS satellites is rendered obsolete:


--- Quote ---Dr. Alessia Pasquazi from the EPic Lab in the School of Mathematical and Physical Sciences at the University of Sussex explains the breakthrough: "With a portable atomic clock, an ambulance, for example, will be able to still access their mapping whilst in a tunnel, and a commuter will be able to plan their route whilst on the underground or without mobile phone signal in the countryside. Portable atomic clocks would work on an extremely accurate form of geo-mapping, enabling access to your location and planned route without the need for satellite signal.

--- End quote ---

Now of course you need more than just the time to figure out one's location; on the surface these claims make no sense. I sent an email to the author for clarification, and got this response:


--- Quote ---Long story short, precision clocks are needed for what is called 'inertial navigation', where you map your position with time using accelerometers.  ships have used for a long time, and a  version of this navigation is already on our phones. it is how they get the direction when you start the navigation.
because you calculate the path by measuring time and acceleration/angular moment, if you do not have a good clock you make a lot of errors, and this is why portable atomic clocks are needed. exciting  research by our colleagues on quantum  gyroscopes should make this thing even better, but the first bottleneck at this point is to make a portable high accurate clock.

--- End quote ---

So in short, the claim is that we could have dead reckoning, non-satellite backed INS systems in our cars (maybe even mobile phones), except for one hurdle: modern (quartz?) clocks being too inaccurate. My intuition/understand was the MEMS 6-axis sensors, while amazing feats of engineering, aren't all that accurate and certainly aren't hampered by the inaccuracies presented by a quartz clock.

Is my understanding out of whack? Or are the claims in the link completely unreasonable?

PA0PBZ:
Even when the time is absolute correct then still the smallest error from the accelerometer will keep multiplying, so I don't see how this is going to work. If you use it in a car and keep correcting based on where the roads are.. .maybe.

Berni:
Does not make sense to me too.

Sure time is a factor, but we can already make very precise TCXOs that are easily orders of magnitude more accurate than needed. But accelerometers and gyroscopes are way too noisy and drifty to make this work. Acceleration integrated gives you velocity and velocity integrated gives you position. Any slight offset error in the accelerometer causes the velocity integral to drift away in time, and since this feeds into another integrator it means it will drift faster and faster until it just flies off towards infinity. Its an incredibly unstable system.

We do make use of intertial navigation tho. Intercontinental missiles for one have a dead reckoning system that allow the missile to keep on course even if all outside navigation signals are jammed. But they use very fancy accurate accelerometers and special light interferomentry gyroscopes to get it accurate enough to not drift off in the few minutes it takes the missile to hit the target.

CatalinaWOW:
In every navigation application I have encountered clock errors are several orders of magnitude below the other error sources.  By my rough estimate even the incredibly exotic inertial navigation systems used in submarines have at least an order of magnitude between the clocks and other error sources, though I don't really have any definitive information on the components of those systems.

While he didn't admit it to you, the professor may have one real application up his sleeve which he is hiding for proprietary reasons.  A really good clock can be used to estimate altitude.  Which combined with a known starting position, good geodata and a crude inertial system could give you excellent results.  Unclear how much better they would be than a similar system with just the known start, good geodata and a crude inertial system.  My GPS units do a pretty good job of estimating position in tunnels based on these types of inference. 

But my own prejudice, based on many encounters with research professors, is that they are often incredibly naive outside their specific expertise and that he had never actually dealt with real world errors in navigation systems.  My favorite example of this naivety came in a grad class in the early 1980's.  The prof was actually a noted expert in his field and as well as I could judge the reputation was well earned.  But my respect for his overall knowledge came when he announced in class that there was a new invention out, something called a microprocessor that might someday allow data operations in small fielded units.  Many of us in the class were already employed in industry and had fielded such devices years ago.  The prof was at least three product cycles behind the curve.

StillTrying:

--- Quote from: Berni on March 25, 2019, 02:06:37 pm ---But accelerometers and gyroscopes are way too noisy and drifty to make this work.
...
very fancy accurate accelerometers and special light interferomentry gyroscopes

--- End quote ---

I'd agree that some other very accurate other stuff is needed as well as the accurate time. Perhaps they haven't figured them out yet so they thought it best not to mention them.

"which range from partnerships with the UK aerospace industry, which could come to fruition within five years,"

Sounds like they're after another 5 years of funding!

"We will be starting work on these projects on 1st December 2014 (i.e. next Monday) and the initial funding is for five years."
https://telescoper.wordpress.com/2014/11/26/quantum-technologies-at-sussex

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