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GPS

ciechanow.ski

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Re: GPS

#41

Post by SamBam »

This is very nice. Very clear, and the 3D interactives are excellent at guiding the explanation.

One thing that I thought was a little confusing was right at the beginning, when we were estimating the position of the figurine and there was an area of uncertainty shown by the yellow circle.

It isn't clear how you're estimating position, and why we have an area of uncertainty. At first I figured it was going to explain it using triangulation (measurement of angles) but there's no reason triangulation wouldn't be exactly as accurate as the tape measure method on a 2D surface, so wouldn't explain the area of uncertainty.

The description merely says:

> Just by using these three reference points we can relate the figurine’s position in the environment to an approximate placement on the map as show with the yellow shape on the right.

I worry that having this ambiguity so early on might put some people off from reading the rest, because they figure they don't understand that and so won't understand the rest of it.

Re: GPS

#42

Post by beerandt »

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…

Geometric quality is easy to consider in terms of using trig and measured angles to solve for an (roughly) equilateral triangle vs a triangle with a very small measured internal angle.

Also, it's easier to understand variables vs uknowns of GPS if you consider that direct measurement is of velocity and/or acceleration, and position is the resultant derivative, after taking into account the probabilities of various solutions.

(Velocity and acceleration can be measured directly without making as many assumptions about various starting conditions.)

Re: GPS

#43

Post by mherdeg »

I'd be interested to see an illustration of how selective availability works -- if your goal is to ensure that a position cannot be determined accurately, how do you alter the signals produced by your satellites to correctly introduce only the desired amount of error?

Re: GPS

#45

Post by ragona »

I'm weirdly impressed by the "switch to metric/imperial" button that updates the article text. It's just so helpful.

Re: GPS

#46

Post by CaliforniaKarl »

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, so that it can correct for the extra time it takes for the signal to travel over the cable (for example, see https://www.meinbergglobal.com/download/docs/manuals/english... page 19).

Re: GPS

#47

Post by thehappypm »

SamBam wrote: post #41

This is very nice. Very clear, and the 3D interactives are excellent at guiding the explanation. One thing that I thought was a little confusing was right at the beginning, when we were estimating the position of the figurine and there was an area of uncertainty shown by the yellow circle. It isn't clear how you're estimating position, and why we have an area of uncertainty. At first I figured it was going to explain…

Literally left the article to come here to see if anyone addressed this yet. I can’t figure out what it means.

Re: GPS

#48

Post by copperx »

vladstudio wrote: post #37

If you have not yet seen the other articles by Bartosz, I am jealous :-) https://ciechanow.ski/archives/

Why are you jealous of someone not reading the other articles? Are they factually incorrect? I found them to be amazingly accurate.

Re: GPS

#49

Post by genewitch »

mherdeg wrote: post #43

I'd be interested to see an illustration of how selective availability works -- if your goal is to ensure that a position cannot be determined accurately, how do you alter the signals produced by your satellites to correctly introduce only the desired amount of error?

Civilian GPS does this two ways, clock skew and how many digits of resolution the timestamp is.

Re: GPS

#50

Post by _Microft »

copperx wrote: post #48
vladstudio wrote: post #37

If you have not yet seen the other articles by Bartosz, I am jealous :-) https://ciechanow.ski/archives/

Why are you jealous of someone not reading the other articles? Are they factually incorrect? I found them to be amazingly accurate.

The ones new to them can still experience the wonder of discovering these articles for the first time.
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