Thoroughly enjoyable read, and explained a complex system excellently. Well done.
GPS
151–160 of 296 posts
Re: GPS
#152One 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).
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.
Re: GPS
#153If 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.
Modern cell phones use A-GPS (Assisted GPS): https://en.wikipedia.org/wiki/Assisted_GNSS I also remember back in the early 2000s we were using a GPS-quipped PDA as a turn-by-turn navigation device, and for the first ten minutes the device simply asked us to wait.
At that time the signal was intentionally degraded in a process called Selective Availability (https://www.gps.gov/systems/gps/modernization/sa/). I didn't have any experience with that.
Re: GPS
#154Earlier quoted context omitted.
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…
Ok, but feeding the candidate half of the information is pretty different from coming up with the rough design with “no idea” how it works. Yes, if you basically tell them how it should work, then they can probably figure out how to calculate the time difference and do some trig.
I mean if the candidate would insist that the smartphone sends a signal to the satellite, you can go on and ask "what kind of data?" -"Nothing, just a PING to which the satellite answers with a PONG that contains its position". This is not at all how GPS works, but this system would also work (in a sense you could determine your position on earth based on the RTT + transmitted satellite coordinate). It is not that the candidate should come up with a perfectly correct solution (actually this could be bad because he could just blindly repeat what he/she read without understanding it) - its whether he/she can come up with a solution for the problem at all. In the interview you could still go into discussion about that approach then (what about undirected energy dissipation for contacting a thousands-of-km away satellite, and problems with time-multiplexing transmissions now on the satellite that has to answer individual requests vs broadcast etc.).
Re: GPS
#155This blog turns around high quality explorations for things so quickly. It is astonishing.
Re: GPS
#156Earlier quoted context omitted.
Do you have experience interviewing or seeing interview questions in those fields?
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.
Not sure if serious?
Re: GPS
#157Earlier quoted context omitted.
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.
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…
Please don’t fall into the strawman argument of “if people criticize the idea it’s because they’re offended”.
Re: GPS
#158Earlier quoted context omitted.
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).
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.
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.
Re: GPS
#159Earlier quoted context omitted.
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'll firmly disagree. I find logic questions, and how they solve a problem vastly more important than other aspects of the interview. I'm often looking for someone I can give a problem to and they return with a potential solution. That involves thinking through problems, how they work, how they can't work, etc. Asking someone how they think GPS works, or asking them a question about how they'd try to track down a def…
The overarching skillset that really smart people use everyday to explain how x works or investigate a customer issue is...google
Re: GPS
#160This 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…
I had the same question. I moved on in the article, and its great, but this still puzzled me. If the author cleared this up, this article would be an absolute masterclass.