Celestial Navigation for Drones

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

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).

Here's the camera used.[1]

It's not exotic. It's a 1936 × 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-...

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.

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...

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.

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.

4 km seems kind of coarse. Could you combine it with knowledge of satellite imagery or something to increase precision?

400 Bucks Sensors is a touch rough.

This would only work at night, right?

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?

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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.

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.