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GPS

ciechanow.ski

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Re: GPS

#101

Post by mellavora »

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 to start reasoning about the system and then see how they might design it.

Seems to me as fair as asking "have you ever thought about how a double-linked list works?" or "a basic way to ensure database consistency when the DB is replicated at two different physical machines"?

It's not like GPS is something esoteric that they are unlikely to have interacted with, odds are they used it to get to the interview location.

Re: GPS

#104

Post by daenz »

Mindless2112 wrote: post #95
throwaway110535 wrote: post #77

Earlier quoted context omitted.

Unless you're hiring them to work in that domain, I don't understand why you'd ask that question.

You either find out the candidate knows how GPS works or you find out the candidate's willingness to say "I don't know". It's valuable to know how someone will handle a question they don't know the answer to -- whether they recognize their own ignorance and whether they are willing to admit to it.

There are plenty of ways to get to the bottom of what a candidate knows without resorting to unreasonable questions. What you're really testing for is a candidate's patience for being toyed with.

Re: GPS

#105

Post by mellavora »

HeyLaughingBoy wrote: post #94
goodpoint wrote: post #85

Earlier quoted context omitted.

Some are silly, some are effective ways to measure curiosity and ability to think through an abstract problem.

Yes, but how many of them are relevant to determining if a particular candidate is a good fit for a programming position?

Well, obviously if the candidate is round then they fit.

or was that the pothole cover? Sorry, I might have gotten confused.

Re: GPS

#106

Post by littlecranky67 »

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.

Bonus/Followup question: Explain why GPS is not working very well to determine height.

Once you understand the signals are timecoded, this boils basically down on your ability to picture the intersection of multiple sphere in 3D space and it will become obvious (amongst other, very technical reasons that should not be an interview question).

Re: GPS

#107

Post by Unklejoe »

daenz wrote: post #81
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.

This is going to come across as harsh, but if that is your favorite interviewing question, and you are not hiring GPS engineers, then it is a ridiculous question to ask and serves no purpose but to "haze" candidates.

I think it depends on the job. If this was for an embedded/firmware engineering position, I'd expect a level of EE knowledge that would at least make the concept of everyone's phone sending data to a satellite be a non-starter.

I think it's more relevant than asking an embedded engineer which sorting algorithm is the fastest.

Re: GPS

#108

Post by daenz »

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 understand why you think linked lists and db replication, which I think are both are very appropriate questions for a software engineer (less relevant to FE engs), are as relevant as how GPS works.

Re: GPS

#109

Post by skilled »

What even is this explanation!

Holy crap. I wish I knew more people to share it with.

Re: GPS

#110

Post by floatrock »

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 after the completion of week 1024. Honda’s head unit supplier did not code their head units to account for the rollover and, on January 1, 2022, reverted to a date and time 1024 weeks in the past [1024/52 = 19.7, so 20 years in the past or 2002]

So despite all the almost-magic level of engineering that has gone into the GPS system that has stayed consistent for 40-some-odd years, a classic integer overflow has ruined it all because some subcomponent test engineer didn't think to check the inputs against the expected lifetime duration of the car's equipment.

Another fun issue with these is DST databases. The satellites will tell you the time, but it's up to you know how your location translates into a DST zone. And if you have long-running offline equipment (say, a car), and the DST dates change, well, your smarts are only as smart as the update procedure.

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