8 ms·

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, you

• jason0597 5y ago
  I don't quite understand how such questions help in the hiring process? It reminds me of the famous "why are pothole covers round" question, still not sure what it achieves.
  • goodpoint 5y ago
    Some are silly, some are effective ways to measure curiosity and ability to think through an abstract problem.
    • HeyLaughingBoy 5y ago
      Yes, but how many of them are relevant to determining if a particular candidate is a good fit for a programming position?
      • mhb 5y ago
        Engineers, not programmers.
        • dpark 5y ago
          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
          • mhb 5y ago
            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.
            • dpark 5y ago
              Let me rephrase my question. Are you nitpicking the difference between a different engineer and a programmer or are you interviewing a different sort of engineer?
              • mhb 5y ago
                The thread became about whether a good interview question for an engineer was how she thought GPS works. HeyLaughingBoy thinks that the qualities a question like that seeks to explore are irrelevant for a programming job. I think that it is a good question for any engineer. One explanation for why HeyLaughingBoy might have disagreed was that HeyLaughingBoy was making a distinction between a programmer and an engineer. I thought that highlighting the distinction between what HeyLaughingBoy wrote (programmer) and the parent (engineer) was a parsimonious way of expressing this.
      • mellavora 5y ago
        Well, obviously if the candidate is round then they fit. or was that the pothole cover? Sorry, I might have gotten confused.
      • goodpoint 5y ago
  • littlecranky67 5y ago
    Actually this is a very good question for engineers, as the (wrong) idea that a smartphone regularly sends data to a satellite shows fundamental lack of knowledge in some very basic physical understanding. The basic understanding should be: transmitting/sending = emitting energy (costly if a device runs on batteries), receiving/listening for a signal = almost for free nowadays. 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. Lots of room to show creative and analytical thinking here.
    • wonnage 5y ago
      I have a hard time imagining engineers in any other field (e.g a structural engineer) would be getting asked questions about GPS as a proxy for general engineering knowhow.
      • lazide 5y ago
        Do you have experience interviewing or seeing interview questions in those fields?
        • littlecranky67 5y ago
          Yes, you will be asked to showcase 2-3 side-projects such as houses, bridges and tunnels you built & designed in your backyard and discuss your design decisions on a whiteboard.
          • lazide 5y ago
            Not sure if serious?
          • rzzzt 5y ago
            Maybe a take-home exercise, like a small viaduct or a dome.
    • dpark 5y ago
• throwaway110535 5y ago
  Unless you're hiring them to work in that domain, I don't understand why you'd ask that question.
  • teraflop 5y ago
    Off the top of my head, I can think of a couple of things it tests for: * Ability to reflect on and critique their own ideas, using technical common-sense. If someone has never had a reason to research or think too hard about how GPS works, then it might be reasonable to initially assume that it sends a signal to a satellite. But hopefully, when prompted to think about it a bit more, they would realize: "Hey, if that's true, then somehow these satellites that were launched in the 80's and 90's were able to scale up their capacity and handle orders of magnitude more demand, now that everyone has a smartphone in their pocket. Maybe that's not how it works?" * General curiosity and interest in areas outside their field of specialty. This might not be strictly necessary to get a job done, but it probably has some correlation with other measures of technical aptitude that are hard to probe directly.
    • bobthechef 5y ago
    • alesua93 5y ago
      > General curiosity and interest in areas outside their field of specialty. This might not be strictly necessary to get a job done, but it probably has some correlation with other measures of technical aptitude that are hard to probe directly. I think this is a bit misguided, since the scope of things outside of a candidate's field of specialty is tremendously large. Picking a random piece of tech within that space and drilling the candidate about it seems rather unfair. A better way to handle this would be to actually ask the candidate about what else interests them outside of their specialty. But hey, maybe that doesn't stroke the interviewer's ego enough (:
      • Unklejoe 5y ago
        > random piece of tech IDK. GPS is probably one of the most prevalent technologies used today besides the Internet itself.
• daenz 5y ago
  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.
  • Unklejoe 5y ago
    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.
    • deleted 5y ago
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  • Youden 5y ago
    I think you're unfairly assuming something about how the question was used. If the intent was to find out if people __knew__ how GPS worked when they didn't need it for the position, sure. However I see two other possibilities: - How does a candidate react when they don't know something? Do they try to bullshit their way through? Do they admit it and ask what you'd like them to do? - Can a candidate, __with guidance__, come to a basic understanding of how GPS works and come up with a reasonable (note: not necessarily correct) suggestion for how to implement it?
    • daenz 5y ago
      I agree with the meta interview concept, but I don't think that applies here because of the OP's last sentence: >Amazing that even engineers don't understand that GPS receivers don't talk to the satellites. They're amazed at the lack of knowledge, not that a candidate wasn't able to talk it out, or that they bs-ed through it. That tells me the question is asked in bad faith.
      • Youden 5y ago
• u385639 5y ago
  Not sure why you're getting flack it's a great question and you can learn a lot about your candidate as you work through it.
  • mwfunk 5y ago
    I assume the responses are from people who didn't know that and are now offended that they would've flubbed what the interviewer considered common knowledge in their field. I could see someone thinking it's not a relevant question for certain types of positions, but anyone getting defensive about it is a little bit of a red flag. Anyone doing development relying on location services or navigation systems should have a general baseline intuition for how those things work, even if it's only the broadest strokes. If someone feels angry about not knowing this, well congratulations, you now know this and no longer have anything to get defensive about.
    • hk__2 5y ago
      Please don’t fall into the strawman argument of “if people criticize the idea it’s because they’re offended”.
• HeyLaughingBoy 5y ago
  Why do you think it's a good question?
• mellavora 5y ago
  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.
  • daenz 5y ago
    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.
    • avianlyric 5y ago
      Depends on what you’re evaluating the candidate on. If you’re evaluating the candidates ability to reason through a problem, and communicate the reasoning, the. GPS is arguably a better question for a software engineer as they’re unlikely to have studied it. Thus you can watch them work through a problem they haven’t thought about before, but which should probably have the basic tools to solve. There should be no expectation of them coming up with the “correct” answer. But they should be able come with an answer and explain it clearly, warts, holes and all. Using something real like GPS also ensures that the candidate understands why the problem is a useful problem to solve, and what the objective of the solution is I.e. a system that lets you locate yourself on earth.
    • lazide 5y ago
      Only reason I can think of is it is a real world example of distributed coherency/information problem and if you squint hard could give insights into things like paxos (and why it does what it does), why certain types of keying/sharding/distributed computing are done the way they are done. But directly talking about those topics is probably handy.
• littlecranky67 5y ago
  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).
  • areyousure 5y ago
    Can you say a little more about this? This is a topic I am interested in, and I was hoping that the OP went into it, but it didn't seem like it did.
    • littlecranky67 5y ago
      The linked article does neatly, but they use 2D projection (so it's circles not spheres). Very simplified you use the broadcasted position + timestamp of multiple GPS satellites to find your location on the earth. So for a single satellite, you can (using time-in-flight of the signal) estimate the distance to that satellite at a point in time when you received its data. Once you know the distance D to that single satellite, you will know that your position (within margin of error) is anywhere on a sphere with the satellite as its center and radius D (of course this is a bit silly as you know you are definitely not farther away from earth surface as the satellite but rather somewhere 6000-6500km away from Earth center). So when using multiple satellites, you get a set of spheres and (simply viewed) would intersect those spheres - you will be somewhere in this intersection. Now if you can safely assume that your are always on the surface of the earth, you would project this (3d volume) intersection to 2d onto the earth surface - this projection is rather "small" hence less error. But if you would project this intersection using any other 2dimensions, the resulting area would be "wider" in terms of height/depth relative to the surface. You have to picture those spheres given that the satellites are roughly at same orbital height relative to earth center and you always receive signals within your "field of view" on the nightsky - and of course assume the earth is round.
      • areyousure 5y ago
• deleted 5y ago
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