GPS • Hacker News Board

Re: GPS

#252

Post by kubanczyk » Wed, Jan 19, 2022, 11:18 AM UTC

mellavora wrote: post #101

gsibble wrote: post #75

One of my favorite engineering tech interview questions is asking an engineer how they think GPS works. You won't believe how many people start with: "Well, your cell phone sends a signal to the satellite......." Amazing that even engineers don't understand that GPS receivers don't talk to the satellites.

You've received some flack for this, but I'm not sure the context. If most of the questions in this round are of this nature, then maybe it is a "why are manhole covers round" thing. But as a one-off, and if asked in a forgiving way to explore how they think, then I'd consider this question valid. Maybe "Have you ever thought about how GPS works?" if "yes", then let them explain, if "no", then make it easy for them t…

I don't know about you, but this part of the interview is for me just another annoying obstacle on the way to get the job.

The "Have you ever thought about how GPS works?" question gets a scripted "No, but " answer. It's fully automatic, because there are now two possibilities:

- I have thought about GPS before. I will now fake my creative process. No hard feelings, but you are just another obstacle for me to get the job, that's it.

- I have never thought about GPS before. We will now try to design "together" a super-complicated system having so many boundaries and hidden constraints, while I will be also panicking on the inside the whole time. No wait, that would be dumb. I know, I will just refuse to discuss it further. Next question.

(Also, TIL that "flack" in addition to its primary meaning is apparently an acceptable spelling of "flak" too.)

Re: GPS

#253

Post by marcellus23 » Wed, Jan 19, 2022, 11:49 AM UTC

throwaway64643 wrote: post #192

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…

Was taught in college that if we get stuck, just move on forwards, things ahead can clear that up. And it works in this case for me. That part really doesn't matter much. At the end of the day, our brain is not a linear programming interpreter.

I stopped reading there because frankly, if there’s such a flaw in the article that early, I was worried about the accuracy of the rest of it.

The concept is just not explained at all. I spent too long making sure I didn’t miss some sentence somewhere explaining what was happening in that diagram. It felt like the article was wasting my time, and that maybe the author himself didn’t really understand what was happening.

Re: GPS

#254

Post by michaelt » Wed, Jan 19, 2022, 11:52 AM UTC

myself248 wrote: post #198

floatrock wrote: post #110

Earlier quoted context omitted.

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…

It's been said that week number rollover occupies a "sweet spot of awfulness" where it happens infrequently enough that it doesn't get much testing, but often enough to impact equipment deployed in the real world. The designers of GPS either should've made it use like 64 weeks so WNRO would happen constantly and we'd have to get good at handling it, or 32768 weeks so we could ignore it for the entire life of the syst…

> 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 rollovers and GPS receivers will increment their rollover counters. In 99% of cases there's no way to decrease them, and now your GPS receiver is convinced it's year 2100.

Re: GPS

#255

Post by YXNjaGVyZWdlbgo » Wed, Jan 19, 2022, 12:16 PM UTC

marcellus23 wrote: post #253

throwaway64643 wrote: post #192

Earlier quoted context omitted.

Was taught in college that if we get stuck, just move on forwards, things ahead can clear that up. And it works in this case for me. That part really doesn't matter much. At the end of the day, our brain is not a linear programming interpreter.

I stopped reading there because frankly, if there’s such a flaw in the article that early, I was worried about the accuracy of the rest of it. The concept is just not explained at all. I spent too long making sure I didn’t miss some sentence somewhere explaining what was happening in that diagram. It felt like the article was wasting my time, and that maybe the author himself didn’t really understand what was happeni…

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

Re: GPS

#256

Post by grkvlt » Wed, Jan 19, 2022, 12:30 PM UTC

ezconnect wrote: post #194

gfd wrote: post #166

Dumb question, but how does this deal with security? Can't anyone broadcast valid but malicious data on 1575.42 MHz? (e.g. to crash planes/missiles etc) EDIT: found wiki from some quick googling https://en.wikipedia.org/wiki/Spoofing_attack#Global_navigat...

It's illegal in most countries to jam or transmit on that frequency.

it's already illegal to cause a plane to crash, regardless of the mechanism used, and the legality of the mechanism generally has no bearing on its effectiveness...!

but, gps (or other gndss) spoofing or jamming is effective, and i think that commercially available jammers simply broadcast reasonably broad spectrum noise around that frequency, which can overwhelm nearby recievers; although the article describes how noise rejection is performed, it has its limits. spoofing is more difficult, but still possible, including simple re-broadcast attacks using data received at another location, however i believe these things are non-trivial for military receivers due to countermeasures like beam steering?

Re: GPS

#257

Post by zb » Wed, Jan 19, 2022, 12:35 PM UTC

skzv wrote: post #238

zb wrote: post #225

Earlier quoted context omitted.

Fun fact: when I was looking into patenting this concept in c. 2007, the prior art search revealed that it had already been patented by someone in Japan in c. 1997. Amazing to think that ideas people had back when Selective Availability was on for the foreseeable future are just now becoming available to consumers. Congrats on getting it done!

Cool! Were you involved in GPS/localization R&D back then?

Yes, I was working for a company making standalone car navigation devices (remember those?), amongst other GPS-related things - I also worked on a lot of OEM GPS receivers. There was even a guy (not me!) who had written the firmware for a GPS receiver basically from scratch, so I got a look at everything from the very low level up to the navigation UI.

At this point the iPhone hadn’t come out yet, but building data was just starting to show up the maps we used for navigation.

Re: GPS

#258

Post by grkvlt » Wed, Jan 19, 2022, 12:41 PM UTC

jandrese wrote: post #36

taubek wrote: post #28

If all of the teaching materials would be so good... I've encountered first GPS devices back 1997.i remember when my coulegues were explaining them to me. At that time you wouldn't get precise measurements right away. You had to wait for correction factors or something like that. The GPS signal was scrambled at that time.

In order to determine where you are you need to know where all of the satellites are. For a standalone receiver this involves downloading a almanac of the satellites from the signal, but GPS receivers have small antennas and the satellites don't blast out at tremendous power so the available bandwidth is very low. This means the effective bitrate of a GPS signal is only 50 bits per second so it takes twelve and a hal…

surely you need to know where you are not, to know where you are. if the difference between where you are not and where you were, or vice versa, is correct, then you are being targeted by the missile.

Re: GPS

#259

Post by marcellus23 » Wed, Jan 19, 2022, 1:02 PM UTC

YXNjaGVyZWdlbgo wrote: post #255

marcellus23 wrote: post #253

Earlier quoted context omitted.

I stopped reading there because frankly, if there’s such a flaw in the article that early, I was worried about the accuracy of the rest of it. The concept is just not explained at all. I spent too long making sure I didn’t miss some sentence somewhere explaining what was happening in that diagram. It felt like the article was wasting my time, and that maybe the author himself didn’t really understand what was happeni…

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.

Re: GPS

#260

Post by elxr » Wed, Jan 19, 2022, 1:15 PM UTC

masswerk wrote: post #27

jakub_g wrote: post #3

Auto-upvoted based on domain name. See all submissions from Bartosz: https://news.ycombinator.com/from?site=ciechanow.ski

Yes, great educational content and great interactive animations! (Also, a constant reminder to learn WebGL. ;-) )

This article just pushed WebGL to the top of my to-learn list!

Those graphics were lovely, and makes me realize how underutilized this stuff is in the modern web.