Celestial Navigation for Drones
39 comments
·January 20, 2025alexpotato
sho_hn
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 :)
perihelions
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
Reminds me of the "the distance between the rails of a railway are due to the width of Roman horse drawn carts" story.
throw0101c
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
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
Check out the CuriousMarc video series I linked under the OP, which gets into the sensor used and the encoding scheme.
jcims
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. "
plasticchris
Or in short, the sensors are strapped down to the platform being measured - like your phone’s sensors for example.
boscillator
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
Gyros on gimbals have other drawbacks, such as drifting and gimbal lock.
adolph
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
They are perfect markers only as long as you can see them. Clouds and fog are your enemies here
mpenet
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”.
rich_sasha
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.
areoform
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.
biofox
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
LLC or nonprofit, with a business address. At least for bio reagants, they won't ship to you otherwise.
pvg
You're definitely on the list of people worried about being on lists now.
RandomBacon
The only people not on any lists, are boring people.
y33t
They're just kept on the list of all people not on a list.
areoform
But that's the bestest list!
hnuser123456
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
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.
wolfram74
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.
myself248
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
> 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...
hnthrow90348765
I'm told quantum navigation is the new hotness for being on lists these days
avs733
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
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
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!
null
maxglute
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.
mapt
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
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.
the__alchemist
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.
null
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).