GPS (2022)
ciechanow.ski
ciechanow.ski
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.
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I recommend the mechanical watch demonstration.
GPS - https://news.ycombinator.com/item?id=29981188 - Jan 2022 (285 comments)
Does anyone know of a diagram that shows this?
It seems as though the receiver signal is almost predicting the source data bit changes, which should not be the case.
I’ll see if I can find the paper it’s quite interesting.
Edit: I can't find the specific paper I'm thinking of. It's one that discusses how when other orbiting bodies want to use GPS they have to make a bunch of complex corrections that terrestrial users don't. But a search for Earth Centerered Inertial Frame will find you plenty of papers discussing the general concept.
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.
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.
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.
[1] https://gssc.esa.int/navipedia/index.php/Relativistic_Clock_...
First sentence of the introduction:
The Operational Control System (OCS) of the Global Positioning System (GPS) does not include the rigorous transformations between coordinate systems that Einstein's general theory of relativity would seem to require> 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.