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GPS (2022)

ciechanow.ski

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Re: GPS (2022)

#21

Post by cornflake23 »

OldGuyInTheClub wrote: post #18
sillysaurusx wrote: post #12

Earlier quoted context omitted.

If you do run across the paper, please post it. It sounds quite interesting.

"Relativity in the Global Positioning System" by Neil Ashby of the University of Colorado is excellent. Chapter 6 discusses relativistic corrections for satellites using GPS. Living Rev. Relativity, 6, (2003), 1 https://link.springer.com/content/pdf/10.12942/lrr-2003-1.pd... Ashby knows whereof he speaks. He is credited with properly accounting for general relativistic effects in what is now the GPS constellation. ht…

This is a fantastic reference! Thank you!

Quote from the paper: “Relativistic principles and effects which must be considered include the constancy of the speed of light, the equivalence principle, the Sagnac effect, time dilation, gravitational frequency shifts, and relativity of synchronization.“

So many factors to unpack yet the solution is right there.

Re: GPS (2022)

#22

Post by londons_explore »

By the way...

Gold codes (this article calls them chipping codes) can be used to solve a lot of engineering problems where you need to synchronise two things. Eg. "I want to transmit an infrared signal and receive it somewhere else, but the signal isn't bright enough to detect! - no worries, use a 1023 bit gold code, and suddenly you get a 30x effective brightness increase, and perfect time sync, with no hardware changes!"

However... beware that you should never transmit the codes once per millisecond. Thats the GPS rate, and GPS is very easy to accidentally interfere with if you use their gold codes. Instead transmit at some other data rate (ie. once per 1.2 milliseconds), or generate your own gold code set, and you won't disrupt GPS.

Re: GPS (2022)

#23

Post by OldGuyInTheClub »

cornflake23 wrote: post #21
OldGuyInTheClub wrote: post #18

Earlier quoted context omitted.

"Relativity in the Global Positioning System" by Neil Ashby of the University of Colorado is excellent. Chapter 6 discusses relativistic corrections for satellites using GPS. Living Rev. Relativity, 6, (2003), 1 https://link.springer.com/content/pdf/10.12942/lrr-2003-1.pd... Ashby knows whereof he speaks. He is credited with properly accounting for general relativistic effects in what is now the GPS constellation. ht…

This is a fantastic reference! Thank you! Quote from the paper: “Relativistic principles and effects which must be considered include the constancy of the speed of light, the equivalence principle, the Sagnac effect, time dilation, gravitational frequency shifts, and relativity of synchronization.“ So many factors to unpack yet the solution is right there.

You're very welcome. I revisit it periodically and understand a little more each time.

Re: GPS (2022)

#24

Post by endorphine »

Off-topic: I was looking at the first visualization (the globe with the satellites) and thought to myself: "how cool is that the author also drew the stars in the galaxy behind the globe?". Only after 3 seconds I realized this was just dust on my screen.

Re: GPS (2022)

#26

Post by IndigoIncognito »

endorphine wrote: post #24

Off-topic: I was looking at the first visualization (the globe with the satellites) and thought to myself: "how cool is that the author also drew the stars in the galaxy behind the globe?". Only after 3 seconds I realized this was just dust on my screen.

Haha

Re: GPS (2022)

#27

Post by nameequalsmain »

IndigoIncognito wrote: post #25

Correct me if I am wrong, but was this not a duplicate of a post (top of all time on HN)? Aren't duplicates not allowed

> Are reposts ok?

> If a story has not had significant attention in the last year or so, a small number of reposts is ok. Otherwise we bury reposts as duplicates.

https://news.ycombinator.com/newsfaq.html

Re: GPS (2022)

#28

Post by martyvis »

endorphine wrote: post #24

Off-topic: I was looking at the first visualization (the globe with the satellites) and thought to myself: "how cool is that the author also drew the stars in the galaxy behind the globe?". Only after 3 seconds I realized this was just dust on my screen.

I went back and noticed the same. Most be the genuine blackness of AMOLED screens.

Re: GPS (2022)

#29

Post by defrost »

cornflake23 wrote: post #21
OldGuyInTheClub wrote: post #18

Earlier quoted context omitted.

"Relativity in the Global Positioning System" by Neil Ashby of the University of Colorado is excellent. Chapter 6 discusses relativistic corrections for satellites using GPS. Living Rev. Relativity, 6, (2003), 1 https://link.springer.com/content/pdf/10.12942/lrr-2003-1.pd... Ashby knows whereof he speaks. He is credited with properly accounting for general relativistic effects in what is now the GPS constellation. ht…

This is a fantastic reference! Thank you! Quote from the paper: “Relativistic principles and effects which must be considered include the constancy of the speed of light, the equivalence principle, the Sagnac effect, time dilation, gravitational frequency shifts, and relativity of synchronization.“ So many factors to unpack yet the solution is right there.

Interestingly while all those points factor in it would be possible to have a working GPS system without "understanding" the error or having any grasp of relativity.

To expand on that, consider if the satellites went up (with ideas of basic triangulation from beacons orbiting) and then the drifting error for a supposedly fixed ground position was noticed what could be done?

The 'unknown cause' error function over time twixt fixed position and uncorrected GPS calculation can be fed into a Kalman filter to derive weights that eliminate the error.

Typically what happens in many instrumentation applications is models are created to derive functions to emit answers, errors are noticed, people think hard to add extra terms to account for errors and eventually either all errors are accounted for or some residual 'wobble' remains which can be smoothed away by an adaptive error model.

To this day in high precision GPS applications post processing runs are used to improve accuracy that account for relativity factors, atmospheric twinkle factors, (other factors I'd have to look up), and still there's a bit or error left over that can be sweep away (for a time) with a Kalman filter.

Re: GPS (2022)

#30

Post by fanf2 »

sgtnoodle wrote: post #20
themerone wrote: post #15

Earlier quoted context omitted.

GLONASS also depends on leap seconds, which is why Russia is opposed to eliminating them by 2035.

Leap seconds are completely arbitrary by committee, aren't they? I'm curious how there could be a technical dependency.

GLONASS transmissions have fields for earth orientation parameters that are too small to allow for unbounded DUT1: it has a built-in assumption that its system time is close to earth rotation angle.
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