[HN Gopher] Celestial Navigation for Drones
___________________________________________________________________
Celestial Navigation for Drones
Author : throw0101b
Score : 209 points
Date : 2025-01-20 12:02 UTC (1 days ago)
(HTM) web link (www.mdpi.com)
(TXT) w3m dump (www.mdpi.com)
| jcims wrote:
| I didn't see an explanation of what strapdown meant in this
| context, so I dug one up:
|
| "Traditional, stable-platform navigation systems commonly involve
| separate accelerators and fibers or laser-based gyroscopes, with
| all the components mechanically and rigidly mounted on a stable
| platform that is isolated from the moving vehicle. This leads to
| the drawbacks of large size, poor reliability, and high cost. In
| contrast, in strapdown navigation systems, the inertial sensors
| are fastened directly to the vehicle's body, which means the
| sensors rotate together with the vehicle. "
|
| https://www.mdpi.com/2504-446X/8/11/652
| plasticchris wrote:
| Or in short, the sensors are strapped down to the platform
| being measured - like your phone's sensors for example.
| boscillator wrote:
| Yes! It's in contrast to gimbaled systems. Putting the
| measuring instrument on a gimbal simplifies the math and
| often improves accuracy, but at the expense that you need
| this large moving object that needs more power.
| imglorp wrote:
| Gyros on gimbals have other drawbacks, such as drifting and
| gimbal lock.
| anothertroll456 wrote:
| Just gimbal lock. Drifting happens to all of them.
|
| But also the gimbal mechanisms, gimbal low response time,
| etc.
| UltraSane wrote:
| The ultimate example of this is the incredibly accurate and
| expensive and complicated floating Advanced Inertial
| Reference Sphere used on the Peacekeeper ICBM.
|
| https://en.wikipedia.org/wiki/Advanced_Inertial_Reference_S
| p...
| sandworm101 wrote:
| Except that a gimbaled system, if enclosed in a single
| box/pod, can also be described as a strapdown system. The
| term speaks more to separate modularity than how the system
| functions internally.
| kragen wrote:
| I wonder if you could dangle the star tracker below the drone
| on a long string, decoupling its attitude from the attitude of
| the drone. A kevlar or spectra string capable of supporting 100
| grams would be 20mm in diameter; 3 meters of it would weigh a
| milligram, which is significantly less than gimbals. A small
| weight a couple of meters below the star tracker on a carbon-
| fiber-composite rod would seem to be able to stabilize its
| attitude further except in yaw.
| adolph wrote:
| I wonder if GPS and the like will be used more for their clock
| features than for position. The emissions celestial bodies are
| perfect fiducial markers [0,1], but connecting them to position
| still requires accurate timekeeping [2], as the paper notes:
|
| _Provided the use of an accurate clock, the results presented in
| this paper will not degrade over time._
|
| 0. https://www.twz.com/17207/sr-71s-r2-d2-could-be-the-key-
| to-w...
|
| 1. https://timeandnavigation.si.edu/multimedia-
| asset/nortronics...
|
| 2. https://www.rmg.co.uk/stories/topics/harrisons-clocks-
| longit...
| KineticLensman wrote:
| They are perfect markers only as long as you can see them.
| Clouds and fog are your enemies here
| mpenet wrote:
| That. However that works just fine for ICBMs and the like...
|
| The future is more likely to be quantum accelerometers and
| quantum gyroscopes, as they have no "external dependency".
| cyberax wrote:
| More likely image-based navigation, where you just upload
| the entire imagery of the route and then use it to correct
| your inertial references.
|
| Encrypted positioning information from low-orbit satellites
| is another option.
| mpenet wrote:
| That doesn't work well in some conditions, it's not new
| either. Some cruise missiles that have TFR (Terrain-
| following radar) and actually do this already.
|
| It also is not really applicable when you are on a
| balistic course at *very* high altitude, course
| correction has to happen early in these case given the
| reentry speed/constraints.
| all2 wrote:
| I presume radio signal or certain frequencies of thermal
| would be viable for adverse weather conditions.
| the__alchemist wrote:
| The frequencies used in GPS: Yes. The frequencies used for
| celestial nav: No.
| rich_sasha wrote:
| I guess timekeeping is relatively easy? These systems would
| only operate independently for a few hours tops. I would
| imagine even a standard quartz movement would be accurate
| enough.
| GJim wrote:
| > I guess timekeeping is relatively easy...... would imagine
| even a standard quartz movement would be accurate enough.
|
| Good Lord! How wrong can you get!
|
| Very precise timing (often taken from GNSS for convenience)
| is needed for much of the modern word, from IP, cellular and
| DAB networks, to AC phase matching the electrical mains grid.
| Quartz clocks are nowhere near accurate enough for these
| purposes.
|
| This government report makes very sobering reading:
| https://www.gov.uk/government/publications/satellite-
| derived...
|
| TLDR: Our dependence on GNSS for timing almost dwarfs that
| for navigation. And we urgently need to consider using
| backups (be that local atomic clocks, or long wave time
| signals).
| withinboredom wrote:
| I mean, considering celestial navigation was a thing long
| before we had accurate clocks... I'd venture they aren't
| wrong at all. Or did you forget that people have been doing
| celestial navigation by hand for over two millennia?
| jjk166 wrote:
| Celestial navigation actually drove the development of
| accurate clocks
|
| https://timeandnavigation.si.edu/navigating-at-
| sea/longitude...
|
| Quartz clocks didn't overtake chronometers in terms of
| accuracy until the mid 20th century, and chronometers
| will still beat regular crystals like you'd find in cheap
| electronics.
| littlestymaar wrote:
| > Celestial navigation actually drove the development of
| accurate clocks
|
| That's true, but that still doesn't change the fact that
| you don't need nanosecond precision for this purpose. At
| the equator, 1 second precision gives you roughly 500m
| accuracy, which is already much higher than what the
| celestial imagery allows here (4km in the paper).
|
| Clearly this method isn't limited by clock accuracy at
| all.
| jjk166 wrote:
| 1 second precision is a lot. A typical quartz resonator
| will drift by about .5 seconds per day at ambient
| conditions. In this paper they set the clock with GPS
| right before flight and they only fly for a few hours, so
| it's tolerable. But in a GPS denied environment where you
| can't set the clock right before flight, ie exactly where
| you are using this instead of gps, clock accuracy will
| become the dominant factor affecting your accuracy after
| a few days.
| littlestymaar wrote:
| > But in a GPS denied environment where you can't set the
| clock right before flight
|
| First of all I don't think the use-case involves the
| drones operators being deprived of GPS, but even if they
| were: you don't need GPS to get sub-second accurate time,
| any internet connection will do it thanks to NTP. Sure
| it's not as accurate as GPS, but it's still way more
| accurate than what you need for this to work. Heck, even
| sharing time through a phone call would work well enough.
| rich_sasha wrote:
| In the context of position keeping I think it's not too
| bad.
|
| If we focus on longitude, where timing I guess matters
| more, the equator moves at a speed of about 0.46 km/s. So I
| guess being out by 1 second translates to precisely 0.46km
| error. That's second order compared to the stated error of
| 4 km, and it will be smaller still away from the equator.
|
| I'm working off the assumption that such a drone can sync
| up to an accurate time source at launch, and then only
| needs maintain good timekeeping for its time in the air. I
| guess without the accurate initial time source, it gets
| bad. Being a minute out is suddenly 30km of latitude
| direction away.
| fisherjeff wrote:
| Plus I think most decent quartz oscillators have a drift
| measured in single-digit PPM (or less) so even 100ms
| error over a single sortie would be surprising.
| withinboredom wrote:
| > Our dependence on GNSS for timing almost dwarfs that for
| navigation.
|
| Galileo satellites also now sign the timestamp (IIRC) via a
| Merkle tree so you know it isn't spoofed.
| GJim wrote:
| Some clueless downvoting here!
| jenny91 wrote:
| Depends on what you're using time for. If you are doing
| advanced anti-jamming for comms for instance, you want
| extremely accurate timing (more accurate means you can
| frequency hop faster and do better anti-jamming).
| Animats wrote:
| The clock accuracy required for celestial navigation is on the
| order of seconds, not microseconds.
| areoform wrote:
| Perhaps I am too paranoid, but I've been told to avoid doing any
| DIY in this field of study.
|
| Apparently, or so I'm told, out of the many, many ways to end up
| on a list -- building a working celestial navigation system can
| lead to some very inconvenient outcomes. Second, only to ordering
| large quantities of certain chemicals online.
|
| Is this true?
|
| ------
|
| EDIT - from the paper, this is incorrect,
|
| > The introduction of GPS caused the interest in celestial
| navigation to wither due to its relative inaccuracy.
| Consequently, celestial navigation is primarily seen only in
| space-based systems, whose orientation must be known to high
| levels of precision. Nonetheless, celestial navigation was
| identified as a desirable alternative to GPS [2], primarily due
| its robustness against potential jamming. Critically, few GPS-
| denied alternatives exist that are capable of using passive
| sensors to estimate global position at night or over the ocean.
| For this reason, celestial navigation remains an important topic
| of research.
|
| The US and other militaries never stopped using these systems.
| They just stopped talking about them as much. Here's a literature
| search showing some of the slow & steady research on the topic,
|
| https://scholar.google.com/scholar?q=astro-inertial+navigati...
|
| Example systems that have been deployed in many (most? all???)
| American combat aircraft,
|
| https://theaviationist.com/2021/09/10/lets-have-another-look...
|
| https://www.gpsworld.com/honeywell-demonstrates-military-gra...
|
| https://ieeexplore.ieee.org/document/290940
|
| Alright. I'm ready to be on that list, Mr NSA agent.
| wolfram74 wrote:
| I've also been told learning too much about linux or the
| nuclear reactions in power plants or bombs puts you on a list.
| I just assume I'm on several.
| munchler wrote:
| Learning too much about Linux puts you on a list? That can't
| be a thing. Isn't Linux itself entirely a civilian project?
| wolfram74 wrote:
| The rationale mentioned was it was under the subheading of
| people interested in strong encryption, people who care
| about being unobservable might have something to hide.
| Maybe it's a good list? People who you might want to ramp
| up a new Bletchley Park? Probably not.
| Theodores wrote:
| Whatever you do, don't broadcast on the airwaves, as in
| pirate radio. That really does put you on the list.
|
| I don't believe they have the people to monitor those that
| know 'how to use grep' and put them on a list. It stands to
| no reason, government civil servants are rarely from the top
| drawer.
| pvg wrote:
| You're definitely on the list of people worried about being on
| lists now.
| areoform wrote:
| But that's the bestest list!
| RandomBacon wrote:
| The only people not on any lists, are boring people.
| y33t wrote:
| They're just kept on the list of all people not on a list.
| gessha wrote:
| Peaceful, not harmless.
| biofox wrote:
| You will likely raise a flag somewhere if you publicise what
| you are doing, but I highly doubt there would be any issues if
| you're working on this in private as a hobby.
|
| As for chemicals, I can personally vouch that it is a terrible
| idea to order reagents (or even chemistry equipment) as an
| individual. I tried to teach myself organic synthesis in the
| summer before starting my doctoral studies, and ended up with
| MIB searching my house. Certainly on a list now :(
| the__alchemist wrote:
| LLC or nonprofit, with a business address. At least for bio
| reagants, they won't ship to you otherwise.
| 7thpower wrote:
| Please tell me there is a blog post or something documenting
| this experience. Sounds like a fun read.
| biofox wrote:
| Afraid not. I don't have much of an online presence, so
| didn't think to write anything.
| amarcheschi wrote:
| there's always a first time :)
| mavamaarten wrote:
| I remember watching a video about a dude who was building a
| mothership-launched glide drone that could land using
| camera vision. The idea was something like "the highest egg
| drop" or something like that. He was speaking with
| academics about his idea, who quickly told him to stop
| whatever he was doing because that would effectively be a
| forbidden military device. Guided artillery, basically.
|
| Sadly I don't remember who it was, it was a fun story. I
| thought it was maybe Mark Rober or Joe Barnard but I really
| can't find it anymore.
|
| Edit: found it! It was launched from a weather balloon, and
| it was both Mark and Joe. https://youtu.be/BYVZh5kqaFg
| Rallen89 wrote:
| Happened to codyslab, videos taken down now (but still on
| archive.org) of a uranium purification process and
| possibly nilered,no way to prove it but he had a 'making
| rocket fuels: part 1' that was never followed p on. Not
| totally sure though as people like BPS space on yt have
| some pretty in depth tutorials on solid rocket motors
| (does explicitly censor how to make the ignition
| component)
| cyberax wrote:
| Uhm, there are plenty of videos about making solid rocket
| fuel. Model rocketry doesn't need any special permits
| until you get to launching large rockets.
| Rallen89 wrote:
| I guess once you have the permits it doesnt matter. With
| the nilered vids, they were more of the more hypergolic
| variety
| trhway wrote:
| That is probably why Eric Schmidt (ex Google CEO)
| develops his AI combat drones in Estonia instead of US.
| myself248 wrote:
| Years ago, an acquaintance developed an autonomous flight
| controller for "real" helicopters. Cyclic-collective-tailrotor
| types. It would work on a full-size cargo helo just as well as
| an R/C model. He released it online, because why not? Drones
| are cool.
|
| Some very nice gentlemen showed up and explained that he
| couldn't do that. He didn't get in any actual trouble that I'm
| aware of, but they "asked" him to take down the published code,
| and definitely not fix any of the bugs it had.
|
| So, yeah, you're not wrong.
|
| There are nuances to the rules, involving things that're openly
| published online, but I don't understand it in the least. A
| hacker's guide to ITAR would be an interesting document indeed.
| notahacker wrote:
| > A hacker's guide to ITAR would be an interesting document
| indeed.
|
| I suspect producing something called "a hacker's guide to
| ITAR" really would get you put on a list...
| fullspectrumdev wrote:
| Knowing how stupid ITAR enforcement is, the guide would
| probably fall under ITAR :)
| hnuser123456 wrote:
| I'm sure there are thousands of datasets of the night sky, and
| a camera, gyrometer (to get camera angles), clock, and basic
| image recognition/pattern matching is all you'd need.
| notahacker wrote:
| yeah. Celestial navigation is a pretty standard thing to
| study if you're planning on taking up sailing or learning
| about satellite positioning. Celestial navigation with drones
| raises more _interesting_ possibilities, but I don 't think
| defence of key strategic assets against drones relies on the
| possibility it might be too difficult a problem to solve, and
| there are commercial solutions in the "drone navigation for
| GNSS denied environments" space. Don't even think the people
| that jailbreak consumer drones specifically to remove the
| geofences that prevent them flying near restricted areas get
| into trouble, at least not until someone spots them flying at
| the end of a runway or outside a military base.
| avs733 wrote:
| Can't find a source at the moment but cool side anecdote to
| this...working from memory
|
| Honeywell was largely the driving force behind developing
| terrain avoidance systems for commercial aircraft. Those
| initial systems worked based on comparing the terrain below to
| the flight profile of an aircraft using a radar altimeter.
|
| There was a CFIT (controlled flight into terrain) accident (I
| want to say AA in Peru?) where the mountains basically got to
| tall to fast to give the crew sufficient time to react because
| of that system. That caused Honeyweell to go back and look at
| ways to improve the system to be predictive rather than
| reactive - using a terrain database.
|
| Honeywell bought/came into posession of a russian world wide
| terrain altitude database to do the first generation of this. I
| can only imagine the US had the same thing, or more accurate,
| but this was far enough ago that US Government wasn't sharing.
| areoform wrote:
| You're right! I actually know about the system you're talking
| about! The US data was classified and Donald Bateman, the
| engineer behind this and bought the data post Soviet Union
| collapse.
|
| https://en.wikipedia.org/wiki/C._Donald_Bateman
|
| https://www.flightsafetyaustralia.com/2023/05/don-bateman-
| en...
| avs733 wrote:
| the amount of random 'stuff' like this that I've
| accummulated over the years could fill a book that is
| interesting only to me lol
|
| Thanks for the link!
| hnthrow90348765 wrote:
| I'm told quantum navigation is the new hotness for being on
| lists these days
| Onavo wrote:
| These days celestial navigation is trivial. See my comment here
|
| https://news.ycombinator.com/item?id=42695079
| clarkmoody wrote:
| Plenty of homework assignments in graduate level aerospace
| engineering courses that are right up the alley of this paper.
| Star trackers as backup for GNSS would be of great interest to
| maritime vessels worried about spoofing. So there are plenty of
| non-military use cases for these algorithms.
| c_o_n_v_e_x wrote:
| Not NSA - you'd have someone from US Bureau of Industry and
| Security tracking you down (no pun) for most likely violating
| export controls if you were to openly share information on
| building the technology.
|
| Celestial tracking is a dual use technology (See 7A004 or
| 7A104) -
| https://www.bis.doc.gov/index.php/documents/regulations-docs...
| oasisaimlessly wrote:
| Openly publishing information in e.g. a book (or presumably a
| website) does not count as exporting. Releasing software is a
| bit more hazy, but has been defended[1].
|
| [1]: https://en.wikipedia.org/wiki/Pretty_Good_Privacy#Crimin
| al_i...
| maxglute wrote:
| Ctrl+F and 0 results for munitions or bombs. Seems like this is
| really about $25 controller gets drones to within 4km in GPS
| denied enviroments, after which a $50 infrared camera + DSMAC
| find targets to hit.
| the__alchemist wrote:
| I would assume the same. Operation in GNSS-denied environments
| is critical for military navigation systems. Comparatively, for
| civilian uses, it's an addon that provides low accuracy, and
| potentially high development or equipment cost (Maybe not for a
| cel nav camera, but for Ring Laser Gyro INSs etc)
|
| GNSS is very accurate, and receivers are cheap, but its reliant
| on satellite signals makes relying on it a liability in
| adversarial uses.
|
| Cel nav isn't self-contained in the way an INS is, because you
| need a clear LOS to the stars. But, it's useful on a clear
| night when your GPS is jammed.
| mapt wrote:
| Thanks for the summary.
|
| I suspect you could get this to FAR higher accuracy if you
| combined it with a recent upload of Starlink et al LEO
| constellation ephemera, an initial GPS fix at launch, and a
| planned flight path, because LEO constellations are bright
| foreground objects (high location-specific parallax differences
| against background stars) at apparent magnitude of about 5.0.
|
| This is simultaneously not reliant on perfect vertical attitude
| sensing coming off the autopilot IMU, you can do it purely
| photometrically.
|
| The limitation is that this is a dawn/dusk thing, in the middle
| of the night there isn't a ton of light reflected and in the
| day you're limited by scattered daylight.
|
| EDIT: Medium orbit satellites outside Earth's umbra but within
| view still provide some sort of visual fix. I wonder what the
| math is like for the GSO belt at midnight?
|
| EDIT2: Or the Moon.
| jcims wrote:
| That's a great idea. In the earlier days when they had about
| 2500 satellites in LEO I built a small visualizer from the
| fleet TLE data and it was remarkably simple with the skyfield
| library.
|
| If you're in the fringes of a GNSS denial area ADSB might be
| useful as well. Would need more hardware of course.
| BenjiWiebe wrote:
| Doesn't ADS-B get the location from GNSS?
| jcims wrote:
| Yes it does, but unless we're talking an entire system
| failue the GNSS denial does tend to have limits in range.
| I've picked up ADSB traffic from well over 200km with a
| simple ground antenna, so if you're in the fringes in
| could be a useful additional signal for similar reasons
| to the satellites.
|
| Just spitballing though really.
| maxglute wrote:
| IMO could synergize well for higher end celestia navigation -
| there are optics sensors for day time tracking, but daylight
| sensitivity is limitation, perhaps much less so when fixed to
| starlink. So maybe feasible $$$ hardware can make daylight
| celestial starlink navigation workable.
|
| Bringing component costs down seems like it would be much
| more useful for increasing capabilities / proliferating of
| lower end loitering munitions. You can already pack redundant
| navigation systems in more expensive platforms that gets them
| to area of operations. But being able to replace $20,000
| inertial navigation system with $200 board + IR camera makes
| a lot of somewhat cheap smart munitions much smarter, and
| mitigates a lot of expensive electronics warfare platforms.
|
| Starlink ubiquity does seem to open a lot of indirect
| strategic applications, i.e. research using starlink
| transmissions as bi/multistatic illumination source to detect
| stealth flyers.
| phoronixrly wrote:
| Don't get distracted
| https://news.ycombinator.com/item?id=42388354
| genewitch wrote:
| note: i used gpt to clean this up because i am ill and
| distracted by snowfall and cold. It muddied some of my
| points, but it removed a lot of PII and rambling. note over.
|
| I noticed some commenters questioning details in the article,
| like the Wi-Fi triangulation and the earthquake survivor
| detector. While it's fair to discuss technical aspects, I
| believe the focus should be on the broader implications
| rather than dismissing the story based on perceived
| inconsistencies.
|
| I haven't dealt with clearances or compartmentalization in
| years, but I know how serious these matters are. Disclosing
| specific names, dates, or events carries severe consequences
| --this isn't something covered by toothless NDAs. The
| penalties can include federal prison for treason. I've
| personally experienced the DoD investigating me just because
| I was listed as a reference. It's an intimidating process,
| and it makes sense why people who fear being doxxed rewrite
| their stories, swapping out modular details to obscure
| sensitive information.
|
| Regarding the Wi-Fi triangulation: this is well within the
| realm of possibility. Many years ago, I purchased a Hydra SDR
| radio with inexpensive RTL-SDR chips. With four matched
| antennas arranged in a line or an X, connected to a Raspberry
| Pi 4, I could triangulate signals and visualize the results
| on a map. The hardware wasn't advanced, but it worked. Even
| in 2012, there were rumors about using Wi-Fi signals to see
| through walls. Whether or not the article is perfectly
| accurate, the point is to consider the ethical and societal
| consequences of such technologies, not to nitpick technical
| details.
|
| As for the earthquake survivor detector, the underlying
| principle is related. Identifying survivors using leaked
| signals like Bluetooth or cellular emissions isn't
| fundamentally different from using Wi-Fi for similar
| purposes. The scenarios may involve different actors--
| military versus contractors--but the capabilities are
| converging.
|
| I've worked at a defense contractor that manufactured
| components for Boeing and McDonnell Douglas jets. While I
| avoided involvement with military projects, I know how
| extensive and layered the contractor ecosystem is. Comments
| suggesting "there are only a few" don't align with my
| experience.
|
| On a personal note, I've always struggled with the ethical
| implications of the work I've done. This has made my career
| difficult. I don't judge others who take these roles--someone
| else will do the work if they don't--but my own scruples have
| been a constant challenge. For example, I once worked on a
| project at a large entertainment company based on an idea I
| had years earlier. The demanded i eventually sit in the
| office and handle tier 3 phone calls. I had a minor breakdown
| in the stairwell; i didn't even let my children consume their
| content, but i was too jazzed to work on the thing that i
| pitched to apple 7 years earlier. That was over a decade ago,
| but i'm still annoyed at myself.
|
| I believe stories like this should be taken seriously.
| Dismissing them based on perceived inconsistencies seems like
| rationalization, to me.
|
| thanks for the link!
| cyberax wrote:
| Ukrainian and Russian drones already do that. They use simple
| visual navigation for terminal guidance:
| https://www.rockingrobots.com/ukraine-drones-able-to-navigat...
|
| At this point, it's pretty clear that this type of
| functionality is out of the bag. Any significant actor can
| easily replicate this with minimal effort, given the advances
| in AI.
| alexpotato wrote:
| Fun fact:
|
| The SR-71 and U2 planes had automated celestial navigation
| systems b/c GPS wasn't around when they came out.
|
| There a story in the book about Lockheed Martin's Skunk Works
| where they mention turning on the system while one of the planes
| was in the hangar and it locked on to a hole in the roof (sun was
| shining through the hole and system thought it was a start).
| perihelions wrote:
| And, it's a bit older than that: the SR-71's derived from ICBM
| targeting systems,
|
| https://en.wikipedia.org/wiki/Missile_guidance#Astro-inertia...
| ( _" the latter of which was adapted for the SR-71..."_)
|
| (Actually the very first one, in that history, was an
| intercontinental _cruise missile_ --a jet weapon that slightly
| predated (~1958) rockets powerful enough to cross oceans.
| ICBM's came a bit later. I'm pretty sure the first generation
| were pure-analog circuits, but I forgot where I read about
| that).
| alexpotato wrote:
| Reminds me of the "the distance between the rails of a
| railway are due to the width of Roman horse drawn carts"
| story.
| littlestymaar wrote:
| Isn't that one a hoax though?
| all2 wrote:
| There are a few standards for rail-line widths. I know
| the US is on one standard (I think the narrow width lines
| died out almost 100 years ago at this point). I know that
| Europe has two, or maybe more.
|
| https://en.wikipedia.org/wiki/Standard-gauge_railway <<
| This makes for fun reading if you're interested in that
| sort of thing.
|
| Relevant passage A popular legend that
| has circulated since at least 1937[8] traces the origin
| of the 1,435 mm (4 ft 8+1/2 in) gauge even further back
| than the coalfields of northern England, pointing to the
| evidence of rutted roads marked by chariot wheels dating
| from the Roman Empire.[a][9] Snopes categorised this
| legend as "false", but commented that it "is perhaps more
| fairly labeled as 'Partly true, but for trivial and
| unremarkable reasons.'"[10] The historical tendency to
| place the wheels of horse-drawn vehicles around 5 ft
| (1,524 mm) apart probably derives from the width needed
| to fit a carthorse in between the shafts.[10] Research,
| however, has been undertaken to support the hypothesis
| that "the origin of the standard gauge of the railway
| might result from an interval of wheel ruts of
| prehistoric ancient carriages".[11]
| cdot2 wrote:
| I dislike the incorrect usage of "prehistoric". The
| Roman-era is not prehistoric.
| umeshunni wrote:
| in theory, the carts could predate Roman history.
| Neywiny wrote:
| My preferred one for EE folks is that reportedly the first
| Arduino boards (now 20 years old?) had a mistake in their
| eCAD where the second pair of headers was 0.05 instead of
| 0.1" apart. But it was too late by the time they caught it.
| And now, 20 years later, even high end microcontroller
| boards ship with that same gap to be compatible.
| progbits wrote:
| Small correction, one pair is 0.2" apart (so skipping one
| 0.1" pitch space), but the other is 0.16".
| sandworm101 wrote:
| Lookup why torpedo's are almost universally 21" in
| diameter. The short version: because that was how big they
| were last time. There is no reason beyond 21" being usrd
| once upon a time and nobody wanting to break from it and
| have the old torpedos not work in the new boats.
| magicalhippo wrote:
| H I Sutton, a naval defense analyst, made a nice video on
| this topic.
|
| [1]: https://www.youtube.com/watch?v=cuS0yhwSPMc
| persolb wrote:
| I've actually used this fact in a related way, for
| wayfinding.
|
| Old school Open-CV was able to see tracks well from an
| onboard monocular camera, but calibration and scale was
| annoying. Track width is accurate enough that I was able to
| use it to input a bunch of head-end video to map the
| tracks.
|
| It was mostly just a modified edge detect where the tracks
| approximately would be. Once finding the tracks, you could
| automatically calculate the camera's height, lateral
| location, and angle.
| andylynch wrote:
| I understand these still do incorporate celestial navigation.
|
| Since GPS is quite likely going to be unavailable at the time
| of use.
| perihelions wrote:
| - _" pretty sure the first generation were pure-analog
| circuits"_
|
| This Wikipedia entry isn't what I had in mind, but it
| describes an interesting analog mechanism,
|
| - _" For guidance systems based solely on star tracking, some
| sort of recording mechanism, typically a magnetic tape, was
| pre-recorded with a signal that represented the angle of the
| star over the period of a day. At launch, the tape was
| forwarded to the appropriate time.[2] During the flight, the
| signal on the tape was used to roughly position a telescope
| so it would point at the expected position of the star. At
| the telescope's focus was a photocell and some sort of
| signal-generator, typically a spinning disk known as a
| chopper. The chopper causes the image of the star to
| repeatedly appear and disappear on the photocell, producing a
| signal that was then smoothed to produce an alternating
| current output. The phase of that signal was compared to the
| one on the tape to produce a guidance signal.[2]"_
|
| https://en.wikipedia.org/wiki/Star_tracker
| throw0101c wrote:
| The sensor was sensitive enough that it could detect stars
| during daylight:
|
| * https://theaviationgeekclub.com/the-sr-71-blackbird-astro-
| na...
|
| * https://www.twz.com/17207/sr-71s-r2-d2-could-be-the-key-
| to-w...
|
| * https://en.wikipedia.org/wiki/Missile_guidance#Astro-
| inertia...
| gunian wrote:
| That's insanely cool what kind of cameras / telescope are
| strong enough to do that? My guess is it was primarily
| hardware and not software bacuse of compute limits
|
| Did the planes have to fly above clouds?
| sho_hn wrote:
| Check out the CuriousMarc video series I linked under the
| OP, which gets into the sensor used and the encoding
| scheme.
| gunian wrote:
| it is kind of crazy and just a testament to people's
| creativity the plane basically flies using an ipdated
| version of whag the medieval ships used for navigation
| totally mind blown
| joshvm wrote:
| It would work on the ground, I believe the pilots
| (normally) had to get a fix before takeoff. You do need to
| see the sky without cloud cover, but spy satellites were
| less of a concern back then so less risk of being overflown
| during a daylight setup. The cameras are basically visible
| telescopes with very narrow fields of view and good
| baffling. Only a few stars are bright enough that you can
| sight off them, but it can be done. The device does a scan,
| so it's only accepting a small area on the sky and the
| initial fix can be sped up because you know where/when the
| aircraft is taking off. A lot of tricks to minimize the
| need for "plate solving", like knowing which direction the
| aircraft is pointing within some tolerance.
|
| Info here: https://www.sr-71.org/blackbird/manual/4/4-3.php
|
| It wasn't exactly a simple instrument to use, and it relied
| on a ton of planned course information. You could also do a
| cold midair start after a power outage, but preflight would
| be much more preferable!
|
| Some modern microwave telescopes like BICEP3 have an
| additional optical telescope for star pointing that are
| daylight-usable, but in summer you need to use a big baffle
| tube. The images are taken with a high sensitivity CCD
| camera and you can pick out brighter target stars
| surprisingly well in the images.
| mpetroff wrote:
| BICEP3 actually uses a >20 year old CCD camera with
| analog video output (BICEP Array uses newer cameras, with
| more modern sensors). Daytime star pointings are possible
| by using a low-pass filter to block visible light and
| take advantage of the sensitivity of CCD / CMOS sensors
| to the near infrared, where the daytime sky is more
| transparent, combined with baffling.
| gunian wrote:
| how hard would this be to set up for a total hardware
| noob? and how good or useful would the data be?
|
| i know gaia data for instance is available for free but
| if one used just a homemade telescope could any useful
| celestial data be acquired?
| mpetroff wrote:
| With images taken at night, you can run the images
| through Astrometry.net, which is a blind astrometric
| solver and will provide you with RA / Dec for most
| images, as long as you have at least a dozen or two stars
| visible. The code compares asterisms formed by multiple
| stars to index files built from Gaia or other similar
| data. This is the technique that's used more frequently
| for microwave telescopes located where there's a normal
| diurnal cycle, e.g., CLASS. The smaller the field of the
| view, the higher the precision, but it also works fine
| with a camera with a zoom lens.
|
| BICEP, however, is located at the South Pole on a moving
| ice sheet, requiring frequent updates to its pointing
| model, and has six months of continuous daylight, so
| daytime star pointing observations are required. This
| requires a different technique. Instead of looking at
| asterisms with multiple stars, the optical pointing
| telescope is pointed at a single star using an initial
| pointing model, the telescope pointing is adjusted until
| the star is centered, and the offset is recorded. This
| measurement process is repeated for the few dozen
| brightest stars, which acquires the data needed for
| refining the pointing model.
| joshvm wrote:
| It depends what you mean by useful. On its own, all
| you're doing is taking pictures of the sky and figuring
| out where the camera was pointing (and its field of
| view). Where it's useful is calibrating the pointing
| direction of other systems. It's fun to try the software
| at home (there is a public web interface), you just need
| a camera that can take long enough exposures to see stars
| without too much noise.
|
| One of the more "useful" backyard astronomy tasks that is
| achievable for a dedicated amateur is variable star
| observation (eg AAVSO), because many stars don't need
| huge telescopes to observe and it's very expensive for a
| big observatory to stare at a single patch of sky for
| weeks. Nowadays we have instruments like LSST which is
| basically designed for this sort of surveying, but public
| data are still useful. And you do need to know exactly
| where you're pointing, so either you do this manually by
| pointing at a bunch of target stars, or you can use a
| guide scope that solves the field for you.
| joshvm wrote:
| I would add it also uses an ancient analog TV for manual
| sighting in combination with the GUI for semi-auto
| centroiding. I always thought that was funny to see, but
| it seems to work well enough. Also, inserting that baffle
| is somewhat terrifying because it slots into a hole next
| to the main vacuum window and if you dropped it on the
| membrane, bad things would happen. Always fun to bump
| into Polies here :)
| sho_hn wrote:
| The excellent CuriousMarc YouTube channel just started a new
| video series refurbishing a B-52 astrotracker, going over all
| of this in some detail:
|
| https://www.youtube.com/watch?v=GkEjLqu-JH0&list=PL-_93BVApb...
|
| Recommended.
|
| It also immediately occured to me how much easier this should
| be on a copter, since you don't need a gimbal'd platform :)
| fooblaster wrote:
| why would you think this has stopped? All military aircraft and
| missiles need to operate in gps denied environments and near
| universally have dead reckoning or celestial navigation still.
| Sylamore wrote:
| I recently watched this channels videos on B-52 Astro tracking
| navigation system repairs:
| https://www.youtube.com/watch?v=nkvN74wuT8w&list=PL-_93BVApb...
|
| He's got a bunch of other vintage electronics stuff that's from
| the early space program as well, interesting stuff to see the
| insides of that gear.
| UltraSane wrote:
| I read that the US military wants a modernized version of
| celestial navigation to reduce dependence on GPS. With modern
| light amplification technology it might be able to work during
| the day.
| pclmulqdq wrote:
| They have some of these on ships already.
| Havoc wrote:
| 400 Bucks Sensors is a touch rough.
|
| This would only work at night, right?
| littlestymaar wrote:
| The full title is:
|
| > An Algorithm for Affordable Vision-Based _GNSS-Denied_
| Strapdown Celestial Navigation
|
| Emphasis mine.
|
| In what kind of context do you expect drones to operate in an
| area where GNSS is disabled by electronic warfare devices? Do
| you really think that a $400 cost is of any issue for military
| use?
| Havoc wrote:
| > Do you really think that a $400 cost is of any issue for
| military use?
|
| If your name is Ukraine then yeah. Effectively halves the
| number of drones you can build
| littlestymaar wrote:
| > Effectively halves the number of drones you can build
|
| You're confusing the price tag of an FPV drone (for which
| this tech has no use, 4km precision is roughly the range of
| such drone, so even without a positioning device you'd get
| such a precision...) with the one of a long-range drone
| which is hundreds of magnitudes larger, even for
| Ukrainians.
| fullspectrumdev wrote:
| FPV used in combat these days go a lot further than 4km,
| you can punch out to 25km+ with a good repeater setup
| _easily_ , 10-15km with a good mast setup, and strikes
| using FPV quads out to 45km have been documented (but
| these are rare).
|
| You just need to plan your battery selection and consider
| the electronic warfare environment to go the distance.
|
| There's also the optical fiber drones which come in spool
| lengths up to 20km...
| littlestymaar wrote:
| You're picking nits here. Again, you barely even need any
| kind of positioning system in an FPV, let alone a
| positioning system with 4km accuracy.
|
| Such a system only make sense for use in long-ranged
| drones.
| _joel wrote:
| Sorry, this is wrong. They'd be based on long range UAV's -
| not short range FPVs. Ukraine have been building about half
| a dozen types of different long range drones and are
| producing a good number of those every day now (some with
| jets making them analogous to cruise missiles). They're
| much more expensive than a cheap FPV with an RPG warhead
| strapped to it.
| rkagerer wrote:
| 4 km seems kind of coarse. Could you combine it with knowledge of
| satellite imagery or something to increase precision?
| cdot2 wrote:
| The drone on mars uses visual navigation based on known imagery
| of the terrain. I suspect that would be easier on this planet.
| kragen wrote:
| Not at sea.
| anovikov wrote:
| An idea: use satellites for navigation. No, not the satellite
| signals, but the satellites themselves. Use NORAD orbital
| elements data for satellites to deduce land coordinates using
| time and pixel coordinates of satellites observed. Low orbit
| satellites will be only observable for two hours or so after
| sunset and before sunrise, but there are enough medium Earth
| orbit satellites that are still bright enough for a small camera
| and are visible whole night.
| nuccy wrote:
| If you see satellites then likely you see even more stars.
| Unlike satellites the stars barelly move (actually they do, see
| "proper motion" [1]) relatively to each other, so a catalogue
| of stars (two coordinates values and two proper motion values)
| along with the time of observation is sufficient to be used
| over decades, unlike NORAD orbit elements requiring regular
| updates. With stars you need just one image at a known time to
| find your location, with satellites it is much much more
| complicated: you need to know where the sun is, you need few
| images of a satellite or even a video (likely on top of image
| of stars anyway) to distinguish it from the stars and to solve
| the trajectory.
|
| 1. https://en.m.wikipedia.org/wiki/Proper_motion
| anovikov wrote:
| How do you find your location from one image of stars? It is
| possible if you have a precise vertical but you don't have a
| precise vertical on a moving UAV. That is, you need an
| inertial system on top that will provide you with a vertical.
|
| With satellite images, you don't need anything apart from
| time. And no, you don't need to "make a video to see
| satellites move", you start with your approximate location,
| make an image and find satellites within a circle where each
| of them might be, starting with the slowest moving - furthest
| away from you - ones (they provide poorest precision of
| coordinates because parallax is small, but you need to start
| with something, but their search circle will also be
| smaller), locating those, you get better coordinates of yours
| and the search circle for each satellite becomes smaller,
| then you can find faster moving satellites too to get precise
| coordinates of yourself.
| nuccy wrote:
| You are right: to find a location from a star image you
| need a true horizon, but unless UAV is pulling some Gs even
| a basic accelerometer would give you the horizon, accuracy
| of that estimation will limit the accuracy of your
| location.
|
| Regarding satellites: so "starting with the slowest moving"
| requires a series of images, doesn't it? Then how do you
| know "your approximate location"? From stars? In theory I
| understand what you say but practically it would be much
| more complicated and the obtained accuracy would not be
| better than with the stars, since in either case you also
| need a horizon to know your location.
| anovikov wrote:
| No you just "start looking" on a single image.
|
| Know your approximate location: by dead reckoning. You
| will need coordinate fixes once every few minutes anyway
| and you know your direction precisely enough from the
| same stars, error only comes from wind direction not
| being precisely known. So we are speaking of correcting
| for at most tens of kilometers of error. 10km at a
| typical distance of 1000km to a low orbit sat is <1
| degree and only about 10 arcmin to a typical medium earth
| orbit satellite.
|
| Astrometry allows for locating objects down to about 0.2
| pixel reliably and to 0.1 pixels in optimal conditions,
| so a typical wide-angle camera that might have about 40
| arcsecond pixels will easily give 8 arcsecond precision,
| for a satellite 4000km away (about 2000km orbit at 30
| degrees elevation), that's 170 meters of location error,
| which is more than good enough for navigation (final
| targeting is done by optical pattern recognition on the
| ground anyway).
|
| >since in either case you also need a horizon to know
| your location.
|
| No you don't. Benefit of using satellites is that the
| source of coordinate data is the parallax of satellites
| vs stars. It works without having a vertical/horizon.
|
| Simply put, we calculate that in a predicted location the
| satellite will be at a certain pixel distance from a few
| of the closest stars on the photo. And it will be a few
| pixels off that predicted point. Distance and direction
| of that error allows for calculation of discrepancy of
| predicted vs real location (and repeating this process on
| several satellites visible on same photo, allows to
| decrease the error by removing outliers - which might be
| noise/space rays on images or errors in star catalogs or
| orbital elements data, or satellites changing their
| orbits - and averaging the results).
| m2fkxy wrote:
| yeah, and then you need to get refreshed orbital elements for
| those satellites. not good if you are in an airtight
| environment.
|
| celestial ephemerides don't change nearly as much.
| anovikov wrote:
| No problem, in a few hours orbits of satellites don't change
| much, a day or two days' old ephemeris are ok. Especially not
| those on medium earth orbits which are the ones to be used
| (geostationary and other high orbital ones are too dim + too
| far away to provide precise coordinates; low orbit ones are
| not visible most of the night)
| Animats wrote:
| Here's the camera used.[1]
|
| It's not exotic. It's a 1936 x 1216 Sony sensor with a C-mount
| lens. That's below current phone camera resolution. It's
| monochrome, which makes sense in this application.
|
| They have bigger collecting optics than a phone, and you get
| better sensitivity without the color filters.
|
| I'm not clear on how they get their "down" reference. It's clear
| how they get heading; that's easy if you can see the stars. But
| you need an accurate horizon or vertical to get latitude and
| longitude. One degree of error in the vertical is maybe 100 km of
| error in position. How good are drone AHRS systems today in
| attitude? They have a correction system that works if you fly in
| a circle, but that just corrects for constant misalignment
| between camera and down reference.
|
| [1]
| https://www.alliedvision.com/fileadmin/pdf/en/Alvium_1800_U-...
| kragen wrote:
| One degree of error in the vertical for the drone's control
| system, if it's hovering by blowing air downward at 5 meters
| per second, would be a ground speed of 87 mm/s (sin(1deg)x5m/s)
| in whichever direction the tilt is. Also without any correction
| in the propeller speed it would result in a loss of altitude
| averaging 0.76mm/s (2.7 m/hour, (1 - cos(1deg) 5m/s). But that
| could also be caused by something like a mild downdraft, while
| the horizontal drift could be caused by an imperceptibly weak
| breeze.
|
| So I don't really know how this is normally done. If you can
| set the drone on the ground for a few minutes, you should be
| able to get a very good reference up-vector, but I don't know
| how long the MEMS gyros can preserve that up-vector without
| GNSS once it takes off.
|
| At sea you can probably look at the horizon with a camera
| unless it's foggy.
| leoc wrote:
| A 2021 PopMech article about the US military's revival of
| interest in celestial navigation:
| https://www.popularmechanics.com/military/research/a36078957...
| It mentions a handheld system designed for special forces units
| to use, but I assume that that would incorporate something like a
| camera gyro stabiliser, presumably making the calculations easier
| than when relying on "strapdown" sensors.
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