GPS • Hacker News Board

Re: GPS

#291

Post by gsich » Thu, Jan 20, 2022, 1:30 PM UTC

floatrock wrote: post #110

CaliforniaKarl wrote: post #46

One of the things I love about GPS is: Since you know your exact position, you can pick a good GPS satellite (one of the satellite's you're using to calculate your position), look at the timestamp from that satellite, and use it as a highly-accurate time source! Purpose-built GPS time servers (like those from Meinberg) give you an option to enter the length of the coax cable connecting the receiver and the antenna, s…

My car does this. Unfortunately, it's a Honda/Acura, and there's a downstream bug in the way the receiver sends the info to the clock display that this year, almost all older Hondas/Acura's are reporting the wrong time: https://didhondafixtheclocks.com/ > Honda’s head unit receives a GPS signal for date and time including a number representing a week, coded in binary. These digits count from 0-1024 and rollover to 0…

Your car does it somewhat different then those Meinbergs. Yes you can get datetime from GPS, but what you really want is a clock signal that triggers 1 PPS.

Re: GPS

#292

Post by myself248 » Thu, Jan 20, 2022, 2:49 PM UTC

myself248 wrote: post #276

michaelt wrote: post #254

Earlier quoted context omitted.

> It's been said that week number rollover occupies a "sweet spot of awfulness" It's even worse than that: The traditional way of handling week number rollover is a rollover count in nonvolatile memory, incremented every time a rollover is seen (for a standalone receiver there's no other source for how many rollovers there have been) So a griefer with a software-defined radio can radio out repeated week number rollov…

Perhaps traditional in devices without a UI. My car's stereo (which is where the GPS/nav functions also live) has an epoch dropdown in the settings menu. I presume you're referencing https://users.ece.cmu.edu/~dbrumley/pdf/Nighswander%2520et%2...

Ooh it just hit me. Maybe that rollover counter itself can be rolled over, so just transmit 253 more rollover events....

Re: GPS

#293

Post by jvanderbot » Thu, Jan 20, 2022, 4:59 PM UTC

simsla wrote: post #240

jvanderbot wrote: post #18

> Naturally, large range uncertainty increases the ambiguity of position, but the relative position of the satellites also matters. If they aren’t well spread, the exactness of calculated location also suffers. (see the excellent example in OP) Fun tidbit, the resulting error is known for the system in closed form as Geometric Dilution of Precision, and is a 3x3 (edit: or 4+x4+ if you are estimating bias or quantitie…

Do you know a good source to learn about the FIM? (Postgraduate level stats/maths, mostly applied, tiny bit pure.)

For these kinds of problems, literature has multiple derivations of FIM for the purposes of tracking & estimation (and path planning for sensing -- my former specialty). Shameless plug: https://josh.vanderhook.info/media/pdf/thesis.pdf chapter 3.

Most of that was distilled from literature or basic math (and probably contains errors -- thanks grad school).

Bishop https://www.sciencedirect.com/science/article/pii/S000510980... was always a good reference for me,

as was

B. Grocholsky, “Information-theoretic control of multiple sensor platforms,” Ph.D. dissertation, University of Sydney. School of Aerospace, Mechanical and Mecha- tronic Engineering, 2006

And here's a tutorial that might help:

https://www.sciencedirect.com/science/article/abs/pii/S00222...

Re: GPS

#294

Post by jvanderbot » Thu, Jan 20, 2022, 5:04 PM UTC

uoaei wrote: post #285

jvanderbot wrote: post #18

> Naturally, large range uncertainty increases the ambiguity of position, but the relative position of the satellites also matters. If they aren’t well spread, the exactness of calculated location also suffers. (see the excellent example in OP) Fun tidbit, the resulting error is known for the system in closed form as Geometric Dilution of Precision, and is a 3x3 (edit: or 4+x4+ if you are estimating bias or quantitie…

Why is it called "Geometric Dilution of Precision" and not "covariance matrix"? In what ways is the former not the latter?

GDOP is sometimes taken to mean the largest eigenvalue or trace of the covariance matrix. It's a metric for the badness of the estimate.

Often, GDOP is broken into components HDOP, VDOP, etc for the values corresponding to some earth-fixed coordinate frame. That starts to look more like statistics about the covariance matrix.

Here's a derivation: https://en.wikipedia.org/wiki/Dilution_of_precision_(navigat...

Here, it ends up being (usually) the sqrt(trace(covariance))

Re: GPS

#295

Post by hdjrudni » Fri, Jan 21, 2022, 8:59 AM UTC

coryfklein wrote: post #176

It's so depressing that – at some point in the future – I'm going to want a really clear and precise explanation of how GPS works and the likelihood that an internet search directs me to this excellent, clean, and ad-free blog is essentially nil.

There's an article similar to this but about Sound.... I wonder if I'll ever find it again.

Re: GPS

#296

Post by kortilla » Sat, Jan 22, 2022, 7:15 PM UTC

kubanczyk wrote: post #251

FabHK wrote: post #161

Earlier quoted context omitted.

Related: When I fly, I like to cache a map of the region I fly over on my phone, particularly the airport region at the destination. Then, you can hold your phone to the window for a few minutes and get a GPS fix, even if in airplane mode, because as you say GPS is purely passive. Then you can follow along nicely where you are, at the resolution you want. (Sure, many airlines have moving maps, but they're not as good…

Tsk, tsk. Keep bragging and soon enough we all hear: All GPS devices MUST be switched off for the duration of the flight. Because terrorism. /s

Terrorism isn’t the reason you can’t use cell networks during a flight. Early on it was ostensibly interference concerns but those are not a real problem (if they were they wouldn’t let you carry cell phones onto a plane).

At this point it’s an explicit request from the cell phone companies to not have a plane with 200 people ripping through their cell networks with association attempts that are most likely going to fail and then immediately become stale.