181–190 of 296 posts
It's so depressing that – at some point in the future – I'm going to want a really clear and precise explanation of how GPS works and the likelihood that an internet search directs me to this excellent, clean, and ad-free blog is essentially nil.
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Then you’d have a Stratum 0 source for a stratum 1 NTP server!
Which is what Google did to have a quality time-source for synchronized time for their global database: https://www.wired.com/2012/11/google-spanner-time/
Except they decided to "smear" leap seconds [0], instead of handling them appropriately.
For that reason, if you "require correct timestamps for legal purposes" [1], you may want to make sure that you aren't using Google's NTP servers.
(N.B.: Amazon (AWS) too, FWIW.)
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It's so depressing that – at some point in the future – I'm going to want a really clear and precise explanation of how GPS works and the likelihood that an internet search directs me to this excellent, clean, and ad-free blog is essentially nil.
I searched for an excellent, clear and precise explanation of GPS that is clean and ad-free, and landed on your comment.
I know not by what tools Web 3.0 content will be found, but Web 4.0 will be searched using hn comment breadcrumbs.There's a lot of really great info in here. One random things I learned from this: > As that angle increases, the signal from a satellite travels more sideways and its larger portion gets affected by the atmosphere. To account for this, GPS receivers ignore ranges measured from satellites at very low elevation angles. ... atmospheric effects are primary source of GPS inaccuracies. (I know GPS has inaccuracies, but I…
Something not mentioned: the "new" L5 signal at 1176Mhz, combined with the existing L1 signal at 1575Mhz, allows the receiver to estimate the atmospheric effects and reduce the uncertainty, allowing for a much better position fix. Think centimeters instead of meters. One more thing I've wondered: the system depends on the sattelites knowing and broadcasting their exact position, but how do you determine this position…
It's so depressing that – at some point in the future – I'm going to want a really clear and precise explanation of how GPS works and the likelihood that an internet search directs me to this excellent, clean, and ad-free blog is essentially nil.
I searched for an excellent, clear and precise explanation of GPS that is clean and ad-free, and landed on your comment.
Maybe internet searches can be improved by changing the search from "how gps works" to "excellent, clear, and precise explanation of gps"?
From the toy model it seems like if in addition to distance you could also get your angle to the landmark then you could get your position from a single landmark. Is that something which is practical? Knowing which angle a radio signal came from? It seems like if you had several receivers you could use the timings of when they receive a message to determine the approximate direction of the satellite in the sky. Given…
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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.
Again, that's not necessarily how that's intended. The observation on its own is just an observation, it doesn't indicate anything about how interview performance was evaluated. For example when I interview, I have a question related to chess, which requires a basic understanding of how the pieces move. I was amazed that even engineers don't understand the basic movement rules of chess. However as far as the intervie…
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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.
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.
The satellites only know their own position to a certain precision, and there are only so many bits to express it in the data packet. More bits wouldn't make sense because the measurements aren't that good in the first place.
So what you get "live" is naturally limited by both of those things. Single-frequency unassisted solutions are usually good to a few meters, dual-frequency to a meter or so.
But ground stations can determine, after observing the satellites for a long time, where they _were_ to a much higher accuracy. It's a complicated process involving a whole network of ground stations, whose own positions are precisely surveyed, etc.
The product of that network is known as "precise ephemeris", and it's available in an "ultra-rapid" (3-9 hours later), "rapid" (24 hours later), and "final" (13 days later) version. With these data, the initial observation can be post-processed to get very good solutions. Down into the millimeters.
The RTKLIB manual has a lot more detail if you're curious.
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…
It's a bit more complex than this, the entire GPS fix is 4D since position depends on time and vice versa. The time reported by a GPS receiver, once fix is attained, is not just the time from one of the satellites but the time resulting from the 4D fix in space and time. This eliminates (to within a certain precision) the latency. A lot of discrete GPS receivers have some nonvolatile storage where they "cache" fixes…