NIST spins atomic gyroscope to allow navigation without GPS The United States' National Institute of Standards and Technology (NIST) thinks it can use a cloud of atoms as a gyroscope. The point is navigation: the agency reckons the little gyro is part of work on ultra-precise navigation for applications like space and submarines. It measures rotation by analysing patterns of “ …
By my reckoning, 8 million Rb atoms weighs ~1.1 x 10^-15 grams... assuming that Rb has a relative atomic mass of ~85 g.
Of course, it's been nearly two decades since I aborted my attempt at a B.A. in Chemistry in favor of Psychology.... so take it with a planet of salt.
Frickin' lasers....
Our destroyer was equipped with laser ring gyros feeding into the inertial navigation system. Pretty sizeable themselves. GPS was a brought into the mix on an experimental basis (I was beta testing a commercial navigation set.) These days with cruise missiles all over the damn place, it helps to know where you're launching from, which is why we had it.
Also submarines, huge things, you could easily load one on one of those. Worth remembering Subs still use a laser gyro inertial nav system as they can only get a GPS signal at periscope depth. As I understand it, having only ever looked for them, they get a fix and then once submerged navigate using an expanding circle of errors based on the known drift rate of the system. i.e. the longer they're down the wider a berth they have to give obstacles on the grounds they're less sure of where they are.
Still better than Bing maps.
'I'm impressed subs can pick up a GPS signal underwater at all. Though, I admit, I've never calculated the attenuation for the frequencies involved.'
When I said at periscope depth I really meant they had to stick something out of the water while they were there to receive the signal, poor phrasing on my part. I doubt you could get a decent signal without having the antenna so close to the surface wave motion would reveal it.
Or the 1970's ELF of "Project Sanguine" https://en.wikipedia.org/wiki/Project_Sanguine
transmitted from the bedrock antenna of the state of Wisconsin (with Texas as a back-up) or its replacement in the 1970's or point to point comm via blue-green laser from some other platform nearby.
Loran is gone by eLoran is back. Submarines could get a Loran fix via a variety of different antennass while still submerged ("whip it with a pig tail",or via Buoyant Cable Antenna).
Current kit to do the same job, better than the old ship/submarine INS, is already as small as a postage stamp.
""While falling under gravity" implies that it's a one-shot measurement."
No it doesn't, it implies that it would be a pulsed measurement rather than a strictly continuous one, but that's not at all the same thing.
"And the kit to do this is huge."
No it isn't. 300 cm^3, plus a bit more for some supporting structure. This is a tabletop experiment, not some building-sized dedicated construction.
"This is not a portable device, even in theory."
Yes it is. In it's current form it would only be portable by a relatively large vehicle, but if you actually read the paper you'll notice that the entire point of this research is to develop smaller, more portable systems. The first paragraph of the paper:
"Over the last few decades, light-pulse atom interferometers (LPAIs) have proven their outstanding sensitivity and stability for inertial measurements.1 These sensors are now making their way out of the laboratory into field demonstrations. Transportable LPAIs have realized accelerometry in a zero-g plane flight,2 precision gravimetry,3–5 gravity gradiometry,6 and measurements of the Earth's rotation.7 However, applications such as inertial navigation will need systems that are smaller, lighter, and require lower power than the mobile systems demonstrated so far. This will require new techniques optimized for use in a compact volume."
In summary, every single word of your whole post is utterly wrong, and this could easily have been avoided by simply taking a few seconds to actually read the thing under discussion instead of inventing some nonsensical fantasy inside your own head.
"And the kit to do this is huge."
The epilogue to a 1930s book on clock mechanisms noted that the recent use of a quartz crystal provided the best accuracy to date. It stated that owing to the required equipment's complexity and size - the technique would always be confined to just a few laboratories. Just over forty years later came the Sinclair "Black Watch" for under £20.
One of the most ingenious older techniques was the friction-free transfer from a master clock for distribution to slave clocks. The pendulum was designed to cast a graduated shadow on a photocell - producing a sine wave for the electrical distribution of the signal.
But if it's measured by 'falling under gravity' how will the system compensate for the fact that gravity is not a nice constant value? (9.7639 m/s2 being the lowest in Peru, while the highest is at the surface of the Arctic Ocean, at 9.8337 m/s2...google in this case being a friend) thus causing variation over time (hence the earlier stated 'drift').
I would also assume that 'falling under gravity' simply means they have some form of system to recycle the atoms back to the top and thus keeping them in an internal cycle rather than only having a very short window of measurement.
Though that is alot of conjecture on my part.
"I would also assume that 'falling under gravity' simply means they have some form of system to recycle the atoms back to the top and thus keeping them in an internal cycle rather than only having a very short window of measurement."
Or... they could just turn the device upside down and start over again..(like an hour-glass). Gravity is relative, after all..
"But if it's measured by 'falling under gravity' how will the system compensate for the fact that gravity is not a nice constant value?"
It's not measured by falling under gravity, it's just measured whilst falling under gravity.
It's the interference pattern between all of the atoms that is used for measurement and all of those atoms will be influenced to the same degree regardless of the local gravity gradient.
"'dead reckoning' navigation, which measures change of position without reference to landmarks or start" Not true. There may be a very few circumstances where you only need to know where you are relative to your start but normally Dead Reckoning is used relative to a known starting point. Mariners take a known starting point on their maps and calculate from that. I'm struggling to think of a use without a known starting point... Here's one: you are in the dessert you do not know where you are and you want to get out. You could use dead reckoning to ensure you don't go round in circles and head off in the direction you think a town or road might be.
you are in the dessert you do not know where you are and you want to get out. You could use dead reckoning to ensure you don't go round in circles and head off in the direction you think a town or road might be.
Dessert - wot, like custard?
I'd want to get out too if I landed in it - not sure about towns -as for the road, yellow brick I presume...
The article wasn't well written in that respect, but you will always have a starting reference. Anywhere you have differential GPS available you can correct any accumulated errors in your dead reckoning. If it was accurate enough - lets say a nanometer of accumulated error per second under normal use - you wouldn't need to return to "known" location for months. A sub travel underwater for six months with no access to GPS would have an accumulated error of only 3cm. I have no idea what the accumulated error of this dead reckoning would be, that may be a bit optimistic...
Be nice if they could make it smaller/cheaper, as it would be very useful for self-driving cars.
That's 3 cm error from gyro drift. Plus X Nautical Miles error from drift as in 'something floating submersed in an ocean' that is also moving, and not consistently. Doesn't hurt to get a fix on your position every now and then.
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"Cold Atom" gyros are almost here. Old NADC in Warminster, PA was developing quantum spin gyros in the early 1980's but I am not sure what became of those gyros. I remember the now old (new at the time) Electro-statically Supported Gyro Navigators (ESGN) on SSBN's and the old SINS and warm standby SINS and Dual SINS on SSN's. Besides GPS (and NAVSAT prior to that), U. S. submarines also had other alternative external navaids to provide position fixes to their INS (to compensate or ameliorate the effect of gyro drift-rate). In the open literature: Kerr, T. H., “Modeling and Evaluating an Empirical INS Difference Monitoring Procedure Used to Sequence SSBN Navaid Fixes,” Proceedings of the Annual Meeting of the Institute of Navigation, U.S. Naval Academy, Annapolis, Md., 9-11 June 1981 (reprinted in Navigation: Journal of the Institute of Navigation, Vol. 28, No. 4, pp. 263-285, Winter 1981-82). Also see: Clautice, W.G., "Submarine Navigation," Journal of the Institute of Navigation, Vol. 25, No. 1, pp. 34-39, Spring, 1978. (Commander Clautice was head of SP-24 at that time.) The historical INSes mentioned above were extremely accurate (most accurate in the world) but were big and expensive. This aforementioned predated ring laser gyros and fiber-optic gyros and MEMS gyros. All gyros incur inherent drift rate of their gyros and external position fixes are used to ameliorate or compensate for the drift rate that degrades the gyros accuracy with time, MEMS have very low accuracy and so needed GPS fixes at a fairly high rate to help them. Also see: http://www.tekassociates.biz/AbstractMiniatureAutonomousSystems3.pdf I learned later that strapdown mechanized INS can be made to appear as any other standard INS mechanization. In space, one can also use star sensors, horizon sensors, and sun sensors, gravimeters, and magnetometers to obtain external fixes for the on-board INS to compensate for its inherent drift rate that would otherwise degrade its accuracy as time elapses unless fixes were used. .
The Saturn V rocket flew with an ST-124M inertial navigation unit providing acceleration and roll, pitch and yaw information to the flight control system. It was about 60cm in diameter with beryllium gimbals and frame and tungsten gyro rotors and weighed a chunk along with its associated nitrogen gas supply and electronic control box. The MEMS accelerometer in a cheap phone today would do better.
As far as accumulating errors in position over time, subs can cheat by using topographic maps of the sea bottom and comparing them with sonar images instead of coming up close to the surface to get a GPS lock. There's a reason for all those hydrographic survey vessels many navies employ, after all. It's only really useful though in a few rare circumstances and it requires them to use their active sonars, not something the Silent Service likes doing unless they're shooting at someone.
"subs can cheat by using topographic maps of the sea bottom and comparing them with sonar images instead of coming up close to the surface to get a GPS lock"
Or they could just rush up to the giant panoramic windows and look at the topography then if they are unsure they could detach the flying submarine and take a really good look.
Bathymetric mapp-matching is what was historically used with the characteristics that you mentioned as being abhorrent to The Silent Service for the reasons that you conveyed and for its duration (another drawback). However, there are modern day variants to the same idea that are less noisy:
http://www.nauticalcharts.noaa.gov/hsd/lidar.html
and there are other geological characteristics that can be mapped and exploited as an alternative that are less noisy and more passive (less observable to enemy surveillance) such as a classified technique known in the 1960's as "Project Linear Chair"..
The duration of the mission dictates the accuracy requirements of an on-board INS. Missiles have a much shorter mission time so cruder (less accurate and less expensive) gyros suffice. Nuclear submarines have a mission time of 3 months so more accurate INS is required. They know where they start from dockside but require external position fixes as well to compensate for the degradation incurred from gyro drift-rate. The spinning beryllium spheres of two degree[of freedom ESGN with two input axes could be used as an accelerator as well.
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