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GPS

ciechanow.ski

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Re: GPS

#261

Post by ninkendo »

askvictor wrote: post #222
ragona wrote: post #45

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

I wish this was standard in recipe webpages. It really makes a difference, and shows the author is thinking about their audience.

Adam Ragusea has a good video on why recipes aren't so easy to translate between imperial and metric: https://www.youtube.com/watch?v=TE8xg3d8dBg

It comes down to the fact that the ingredients we buy from the stores near us tend to come in nice round numbers in our local measuring system, and recipes tend to be tailored to that. For example, "1 cup of shredded cheese and 1lb sausage" may translate precisely to "236.9 mL shredded cheese and 0.45kg sausage", but your nearby store selling metric ingredients may have shredded cheese in a 300mL bag and sausage in 0.5kg packages. So you either measure very precisely and waste food (which makes the recipe a pain to get right), or try to use the local equivalent (and the recipe might not end up tasting the same as a result.) So there ends up being some "art" to doing a proper translation.

Re: GPS

#262

Post by mhb »

dpark wrote: post #139
mhb wrote: post #116

Earlier quoted context omitted.

Engineers, not programmers.

What kind of “engineers”? I’m not aware of most engineers going through this sort of interview. Mostly just software engineers who are indeed programmers even if they get uppity about the title

OK, I'll play. Some qualities that are valuable in an engineer are curiosity, creativity, and general knowledge. If your nomenclature has a programmer as a software engineer and a software engineer as an engineer, then these qualities are also valuable in programmers.

If you ask an interviewee how GPS works and he says something about the cell phone sending signals to a satellite, you would want to pursue that further, at least to make sure that he's asserting that with a sufficiently low confidence.

Re: GPS

#263

Post by jameshart »

marcellus23 wrote: post #259
YXNjaGVyZWdlbgo wrote: post #255

Earlier quoted context omitted.

You are assuming instead of knowing because you are talking about something you didn't read. LOL

No. I definitely did read that part of the article. I also went back and read the rest of the article this morning before I posted that comment, and he never goes on to explain that part. He just brushes past it and never clears it up.

It’s not really relevant to how GPS works? It’s just scene setting an intuition for, crudely, how you might estimate your position relative to landmarks.

If your take is then ‘well, he didn’t rigorously define how he derived the error bounds on crude position estimation, therefore all this stuff building into the level of precision GPS is capable of rests on a flimsy foundation of lies’, perhaps you are not the target audience for this kind of didactic presentation.

Re: GPS

#264

Post by WelcomeShorty »

skzv wrote: post #237
rvba wrote: post #236

Earlier quoted context omitted.

How many satelites are tracked by a typical android phone? 4? Or more? Can it track both GPS and other systems like glonass at the same time?

It depends on the GNSS chipset in the phone, but usually it'll be all the GNSS satellites visible in the sky, from all major constellations. The current Android platform supports GPS, SBAS, GLONASS, QZSS, BEIDOU, GALILEO, and IRNSS[0]. Each satellite adds another constraint, which helps, and with more satellites to choose from, you can drop the weakest or worst measurements. [0] https://developer.android.com/referenc…

Side note:

> The current Android platform supports GPS, SBAS, GLONASS, QZSS, BEIDOU, GALILEO, and IRNSS[0].

But it depends (and varies wildly) on the hardware of said Android. I buy 3 to 5 cheap (sub 100 euro/US$) Androids per year and it always surprises me which device supports which GNSS is supported and how accurate or not they are.

PS I use them to Wigle (https://wigle.net) mainly, and the WiFi reception is exactly as diverse as the GNSS.

Re: GPS

#265

Post by jpindar »

rvba wrote: post #236
skzv wrote: post #171

Amazing work. I work on the Android location team at Google, and I sent this article out to my team. GPS/GNSS is critical for accurate location/context, and there's still plenty of innovations happening in this field. One of our directors is Frank Van Diggelen - none other than the professor who taught the Stanford GPS MOOC referenced by the article :) I'm sure he is going to appreciate seeing the course called out t…

How many satelites are tracked by a typical android phone? 4? Or more? Can it track both GPS and other systems like glonass at the same time?

Mine's detecting 20 at the moment, and using 14 of those. This varies slightly from minute to minute. Many GPS apps display this data.

Re: GPS

#266

Post by jameshart »

eminence32 wrote: post #6

There's a lot of really great info in here. One random things I learned from this: > As that angle increases, the signal from a satellite travels more sideways and its larger portion gets affected by the atmosphere. To account for this, GPS receivers ignore ranges measured from satellites at very low elevation angles. ... atmospheric effects are primary source of GPS inaccuracies. (I know GPS has inaccuracies, but I…

There’s an interesting chicken-and-egg problem there. You don’t know what angle the signal is coming from (unless you have some kind of sophisticated multi receiver setup) - so first you need to estimate your position, then figure out whether the satellite is low in the sky, then you can determine whether to trust the timing of the signal from that satellite.

Re: GPS

#267

Post by rasz »

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.

Its not all that bad, for example googling 'GPS FPGA receiver' will give you equally excellent http://www.aholme.co.uk/GPS/Main.htm

Re: GPS

#268

Post by jameshart »

littlecranky67 wrote: post #142
dpark wrote: post #137

Earlier quoted context omitted.

> Actually this is a very good question for engineers What kind of engineers are you interviewing? > I also like the question because even if you have no idea how GPS actually works, you could quickly come up with the rough idea yourself given that you have a bunch of satellites that can emit arbitrary signals. I find this really doubtful. Someone who doesn’t already understand how GPS works is unlikely to derive it…

If you make some simplifications and assumptions such as a GPS receiver has a highly-precise & synchronized clock (it doesnt), you can conceive GPS basically as a system where each satellite simply broadcasts its position & own current timestamp and then determining your position of the earth surface becomes basic triangulation calculating the time-in-flight of the received signal from multiple satellites. Of course…

You’re still going to wind up assuming a lot of baseline common knowledge that, while it might be stuff you find interesting and have come across in pursuit of your personal brand of geekery, are likely not actually necessary for the job.

Just consider that there are people - perfectly intelligent, capable of learning - who don’t actually know what ‘GPS’ stands for. They maybe don’t know that the GPS system is separate from their phone’s cell connection. They may not even know that satellites are involved. They may have not taken enough of an interest in space technology to have internalized how satellite orbits work, to have intuitions for how orbital speeds and altitudes are correlated. Or they may have picked up a common misconception at some point in the past about how cellphones work, thinking cellphones are always talking to satellites. So they might have made some reasonable intuitions about how GPS works that - because they haven’t been exposed to the true answer - cause them to make some erroneous assumptions that seem like dumb mistakes a poor engineer would make.

Not having had the opportunity to come across those things is not the same thing as not being able to incorporate that knowledge into your worldview when you encounter it.

Re: GPS

#269

Post by Bedon292 »

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…

Think about it as if you are standing somewhere in relation to the monuments and are trying to figure out where you are on the map. You don't have precise measurements of distances or angles, you only have the estimated distance / angle you are from each of them that you get by looking at them. And without precise measurements there is uncertainty in where you are located.

You probably know exactly where you are on the map if you are within a meter of a monument. As you move farther away from the monuments your estimation of the distance from each one becomes less precise. At least when I am estimating a distance things end up rough really quickly. At 10m I might be off by 1m. By 50m I am off by 10m, and so on. Now translate that into an exact position on the map. It not possible, there is always some level of uncertainty.

I didn't realize it at first, but all of the examples are interactive. You can move the figure around, I found that pretty helpful and fun as well. In the very first example: place the figure somewhere, and then try and point to where it is on the map. I found myself circling areas naturally, even though the scale is relatively small. Especially when viewing it from an angle. The second example is quite exaggerated as far as the circles go but it is representative of the idea.

Re: GPS

#270

Post by marcellus23 »

jameshart wrote: post #263
marcellus23 wrote: post #259

Earlier quoted context omitted.

No. I definitely did read that part of the article. I also went back and read the rest of the article this morning before I posted that comment, and he never goes on to explain that part. He just brushes past it and never clears it up.

It’s not really relevant to how GPS works? It’s just scene setting an intuition for, crudely, how you might estimate your position relative to landmarks. If your take is then ‘well, he didn’t rigorously define how he derived the error bounds on crude position estimation, therefore all this stuff building into the level of precision GPS is capable of rests on a flimsy foundation of lies’, perhaps you are not the targe…

> well, he didn’t rigorously define how he derived the error bounds on crude position estimation

or more correctly, he used an example and a diagram without every explaining how we should think about it. I think running into a nonsensical, unexplained diagram is a good heuristic for whether or not an article is worth reading — even if, in this case, it turns out the rest of the article is well-written.

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