Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

We built a new GPS receiver engine

blog.coresemi.io

Post reply on HN
Print view View original

81–90 of 122 posts

Re: We built a new GPS receiver engine

#81

Post by durkie »

joshvm wrote: post #79
durkie wrote: post #61

Mostly off-topic here, but it seems like the best place to get an answer: several years ago in my lab at Georgia Tech, a technician I worked with said he had a friend who invented a device for doing celestial navigation, but that it 1) worked in the daytime somehow and 2) used digital technology so that it was more precise than a sextant / traditional celestial navigation. The technician was prone to a bit of exagger…

This is the best reference I've found so far on the SR-71 system (the NAS-14V2): https://airandspace.si.edu/webimages/collections/full/NAS-14... That's some kind of flight/operational manual for the astroinertial navigation system (ANS), pretty detailed. Quite amazing that it's been declassified - you can see the link is from the Smithsonian Air and Space Museum. It also discusses details of how missions are loaded a…

Very cool! Can you expand on the idea that it's just a single IR photodiode? This would get rid of visible atmospheric light, but wouldn't there still be residual IR light from the sun (whatever isn't absorbed by the atmosphere) interfering with collecting the light from stars?

Re: We built a new GPS receiver engine

#82

Post by ohazi »

bjt2n3904 wrote: post #80
Arjuna wrote: post #68

Earlier quoted context omitted.

The speed limit in the US is 1000mph, else it’s a munition. Please note: my apologies in advance for a comment that may be perceived as pedantic, but I just wanted to further illuminate this statement. The U.S. regulation, which technically falls under the Export Administration Regulations (Navigation and Avionics), does not exclusively regulate GPS navigation in munitions applications; rather, it serves to regulate…

According to Google, the F22 can do 1500 mph. Looks like civillian GPS is still good as long as you keep it under Mach 1.8 at sea level ;) ICBMs on the other hand, reportedly go ten times the speed of an F22.

Neither of which would be using a civilian GPS receiver?

Re: We built a new GPS receiver engine

#83

Post by labcomputer »

durkie wrote: post #61

Mostly off-topic here, but it seems like the best place to get an answer: several years ago in my lab at Georgia Tech, a technician I worked with said he had a friend who invented a device for doing celestial navigation, but that it 1) worked in the daytime somehow and 2) used digital technology so that it was more precise than a sextant / traditional celestial navigation. The technician was prone to a bit of exagger…

It's real. There are some neat patents from the mid-40's through the mid-60's from the usual crowd of aviation companies.

This is probably the best one: https://patents.google.com/patent/US3165632?oq=%22star+track...

The core technologies are IR filter (to increase contrast) + an optical modulator + a synchronous detector (lock-in amplifier).

There are a bunch of variations (mostly, I think, to avoid patent infringement). The basic idea is to have a sensor offset from the optical axis of a telescope (and revolving around the OA--or, more commonly, to rotate the OA about the sensor). The telescope is focused at infinity.

Then modulate the entire field in various clever ways (to avoid the gradient of the sky brightness from affecting the measurement), and detect the total field brightness with a PMT (nowdays you'd use a CMOS sensor or photodiode).

They typically include a servo to point the telescope OA at the star (conveniently, this also tells you how the star is oriented relative to the body axis of whatever the tracker is mounted on).

Re: We built a new GPS receiver engine

#84

Post by oyvind »

detaro wrote: post #37
Accujack wrote: post #30

Earlier quoted context omitted.

RTK gps is better and quite affordable now. Look it up, you can use two receivers costing maybe $15 each for it.

Do you happen to know a good introduction to how one would use it? I've looked at tools around it a few times, but always didn't find the point where it made "click" on how to actually do anything with them.

Take a look at rtklib.com for software and instructions on https://wiki.openstreetmap.org/wiki/RTKLIB. There are quite a few u-blox receivers that work, also receivers from other brands. If you want to try dual frequency for improved accuracy, u-blox launched an affordable dual frequency chip in 2019.

Re: We built a new GPS receiver engine

#85

Post by JoshuaDavid »

colechristensen wrote: post #77
durkie wrote: post #61

Mostly off-topic here, but it seems like the best place to get an answer: several years ago in my lab at Georgia Tech, a technician I worked with said he had a friend who invented a device for doing celestial navigation, but that it 1) worked in the daytime somehow and 2) used digital technology so that it was more precise than a sextant / traditional celestial navigation. The technician was prone to a bit of exagger…

He might have been exaggerating the performance of his friend's "invention", but you can track the sun during the day, stars at night, and possibly other celestial bodies during the day with sensitive enough instruments. If you can calculate the angles between local gravity, the height of the sun, and you know what time it is, there is a lot of location information there, especially if you track changes over time and…

Don't forget that the surface of the sun is not perfectly uniform: if you have a recent picture of the surface of the sun and sensitive enough instruments, you can effectively treat it as a "cluster of known celestial objects" (e.g. sunspots, solar flares) half a degree apart from each other.

Also the moon is sometimes visible during the day, and since the topography of the moon stays constant over time that trick will work with the moon (with craters etc) even without a recent picture.

Re: We built a new GPS receiver engine

#86

Post by miahi »

implements wrote: post #57
miahi wrote: post #11

Earlier quoted context omitted.

There are two limits: 60000ft and 1000 knots. Some manufacturers use "AND" between them, others use "OR"[1]. So you can have faster drones if you manage to stay below 60000 ft. [1] https://ukhas.org.uk/guides:gps_modules

Some manufacturers have imposed lower limits eg some older Garmin handhelds won’t display info at speeds over 90 mph, though they still maintained a fix. I believe it was to stop people using them in light aircraft.

That's probably because Garmin also makes avionics and flight systems, so they just segmented the market.

Re: We built a new GPS receiver engine

#87

Post by kayson »

Very cool but a far cry from an actually usable open source hardware receiver. Implementing the RF front end is not at all trivial and highly dependent on the manufacturing process, which is probably why they didn't release one. There's also the issue of fabs all requiring NDA's for their PDK's (process development kit), and transistor-level netlists would probably leak some of that information. It would be great to see an open PDK at something like a 65nm or 40nm node which would really allow for open source hardware design.

Re: We built a new GPS receiver engine

#88

Post by labcomputer »

durkie wrote: post #81
joshvm wrote: post #79

Earlier quoted context omitted.

This is the best reference I've found so far on the SR-71 system (the NAS-14V2): https://airandspace.si.edu/webimages/collections/full/NAS-14... That's some kind of flight/operational manual for the astroinertial navigation system (ANS), pretty detailed. Quite amazing that it's been declassified - you can see the link is from the Smithsonian Air and Space Museum. It also discusses details of how missions are loaded a…

Very cool! Can you expand on the idea that it's just a single IR photodiode? This would get rid of visible atmospheric light, but wouldn't there still be residual IR light from the sun (whatever isn't absorbed by the atmosphere) interfering with collecting the light from stars?

Typically the FOV of the telescope is too narrow to see the sun directly, so you only have to worry about scattered light from the atmosphere (hence the IR-pass filter).

Referencing the link from my other comment, the nutator and optical modulator referenced in the patent combine to remove the residual sky background and apply FM modulation to the star aligned with the telescope OA.

By measuring the phase and depth of the FM modulation, you get the angle and distance of offset between the star and OA.

Re: We built a new GPS receiver engine

#89

Post by wiml »

sho wrote: post #72
durkie wrote: post #61

Mostly off-topic here, but it seems like the best place to get an answer: several years ago in my lab at Georgia Tech, a technician I worked with said he had a friend who invented a device for doing celestial navigation, but that it 1) worked in the daytime somehow and 2) used digital technology so that it was more precise than a sextant / traditional celestial navigation. The technician was prone to a bit of exagger…

They used astro-navigation as far back as the SR71, the "guidance group" system nicknamed R2D2. Cool video of a pilot talking about it here: https://youtu.be/CeBu6mRDaro?t=3180

Not just exotic aircraft! Celestial navigation was used by all kinds of long range aircraft, military and civilian, until the 1960s or so. Early jetliners (including the first models of the 747, for example) had sextant ports in the top of the fuselage; older aircraft might have "astrodomes". There's a continuous line of use from age-of-sail seafarers to jet-age aircraft. I think the military trained with sextants for air navigation until the 1990s, as a backup method for inertial and satellite.

The main difference being that star sights were taken manually, with an instrument and an eyeball, instead of automatically like the SR71.

Of course satellites did and still do use star trackers. The Apollo astronauts took star sights too, presumably for orientation rather than position.

Re: We built a new GPS receiver engine

#90

Post by azinman2 »

It seems like they did this for SpaceChain, which I had never heard of before. I read their information, taking about combining blockchain-technology and satellites, and I'm still very confused. Other than buzzwords, what is this actual business and why is the blockchain necessary here? dApps in Space? What? Why?
Post reply on HN