141–150 of 296 posts
So who else spent five minutes playing with the flexible rope?
Earlier quoted context omitted.
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…
> 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…
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.
However, I do like the role GPS plays as a plot device in all kind of stories, where it's an active device, with GPS-enabled devices giving away position or there's no GPS in the wilderness, as mobile connections fail. So there are two versions of GPS, the popular plot device and the actual navigation device. (The more sinister version is that this has actually been planted as a cover-up for more realistic electronic…
The original study by Woodford and Nakamura (which laid the foundations for what eventually became NAVSTAR/GPS) has a really fascinating slide where they consider the tradeoffs of alternative configurations. What if GPS receivers had transmitters? What if the mathy computation was offloaded to a nearby ground station? What if every GPS receiver had an atomic clock? Or just a cheap quartz clock? How does that impact t…
You can still do this with GPS if you want. In the surveying community, it's not unusual to collect an extended period of raw GPS observations (e.g. 48 hours) and then submit them to NOAA's offline GPS computation service OPUS which will return a fix by email a while later. This can result in a more accurate fix but perhaps more importantly a more consistent fix, since OPUS will apply the exact same sophisticated solver used for other government geodetics like survey benchmarks. The tradeoff is that OPUS is slow enough that it tends to run on a queue.
In any case modern GPS involves surprisingly active receivers since smartphones commonly use AGPS over IP to accelerate receiving ephemera.
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…
Then you’d have a Stratum 0 source for a stratum 1 NTP server!
Considering GPS was originally created for military purposes, I find it fascinating that the largest holes are over the north and south poles. The northern polar region is where the US-launched missiles and bombers would travel. Does the lesser orbital coverage in that area not negatively affect GPS precision?
ICBMs don't use GPS. The almost universally use a inertial guidance system. For example: https://en.wikipedia.org/wiki/Advanced_Inertial_Reference_Sp... . Bombers use a similar inertial guidance system, but with updates from GPS and star trackers as applicable. The reduced precision from GPS doesn't matter too much, as the inertial guidance systems are pretty good now.
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'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 defect based on what a customer is reporting to me are very important skill sets to have.
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…
A lot of discrete GPS receivers have some nonvolatile storage where they "cache" fixes to reduce fix time. This has the amusing result that when you buy a GPS receiver and monitor its output immediately you usually find out the time and location where QA was performed, as the first fixes emitted without the quality flag.
How are these interactive visualizations made? As a senior machine learning engineer (with only rudimentary JS skills) it would be fantastically fun to make something like these.
https://ciechanow.ski/js/base.js https://ciechanow.ski/js/gps.js Looks like WebGL to me.
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…