121–130 of 130 posts
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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.
Gyros on gimbals have other drawbacks, such as drifting and gimbal lock.
But also the gimbal mechanisms, gimbal low response time, etc.
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).
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...
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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 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 coalf…
I dislike the incorrect usage of "prehistoric". The Roman-era is not prehistoric.
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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.
> 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.
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 t…
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.
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> 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.
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 affe…
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.
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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.
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 :)
Definitely! I wasn't expecting to see a mention of BICEP while reading HN from Pole, particularly not on something as arcane as its star camera.
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).