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. "
Or in short, the sensors are strapped down to the platform being measured - like your phone’s sensors for example.
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.
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_Sp...
Gyros on gimbals have other drawbacks, such as drifting and gimbal lock.
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.
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).
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).
- "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]"
Reminds me of the "the distance between the rails of a railway are due to the width of Roman horse drawn carts" story.
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.
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.
Small correction, one pair is 0.2" apart (so skipping one 0.1" pitch space), but the other is 0.16".
Isn't that one a hoax though?
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]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.
I understand these still do incorporate celestial navigation.
Since GPS is quite likely going to be unavailable at the time of use.
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...
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?
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.
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.
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?
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.
Check out the CuriousMarc video series I linked under the OP, which gets into the sensor used and the encoding scheme.
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
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 :)
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.
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.
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.
They have some of these on ships already.
400 Bucks Sensors is a touch rough.
This would only work at night, right?
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...
They are perfect markers only as long as you can see them. Clouds and fog are your enemies here
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”.
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.
I presume radio signal or certain frequencies of thermal would be viable for adverse weather conditions.
The frequencies used in GPS: Yes. The frequencies used for celestial nav: No.
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.
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).
> 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).
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.
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.
> 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.
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?
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.
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.
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 :(
LLC or nonprofit, with a business address. At least for bio reagants, they won't ship to you otherwise.
Please tell me there is a blog post or something documenting this experience. Sounds like a fun read.
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
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)
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.
That is probably why Eric Schmidt (ex Google CEO) develops his AI combat drones in Estonia instead of US.
Afraid not. I don't have much of an online presence, so didn't think to write anything.
there's always a first time :)
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...
You're definitely on the list of people worried about being on lists now.
The only people not on any lists, are boring people.
They're just kept on the list of all people not on a list.
Peaceful, not harmless.
But that's the bestest list!
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.
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.
Learning too much about Linux puts you on a list? That can't be a thing. Isn't Linux itself entirely a civilian project?
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.
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.
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.
> 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...
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.
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.
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.
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...
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!
These days celestial navigation is trivial. See my comment here
I'm told quantum navigation is the new hotness for being on lists these days
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.
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.
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.
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.
Doesn't ADS-B get the location from GNSS?
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.
Don't get distracted https://news.ycombinator.com/item?id=42388354
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.