Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

GPS

ciechanow.ski

Post reply on HN
Print view View original

31–40 of 296 posts

Re: GPS

#31

Post by beerandt »

masswerk wrote: post #25

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…

There are way more (real-world) versions than two, with quite a few arrangements that can cancel out some of the systematic error, both with and without inputs requiring a data connection, and with some augmented solutions passively broadcast as "one-way" data.

The funny thing is that the James Bond/ Tomorrow Never Dies plot device has turned out to be the most realistic, but doesn't actually require the theft of some encoding device, with various record and delay replay attacks.

Other underused plot device: lots is made out of over reliance on GPS and what would happen in the event of an attack on the system. But most ignores the fact that the US NAVSTAR GPS constellation is multipurpose, and is also a (confirmed) primary component of worldwide nuke detection, with some (afaik unconfirmed) claims of a missile launch detection capability, as well.

Re: GPS

#32

Post by brandmeyer »

mnw21cam wrote: post #29
eminence32 wrote: post #6

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…

Not only that. You can use a ground-based fixed station to listen to GPS signals and work out how much they have been affected by the atmosphere. This then gets fed back into the weather prediction models used by many weather forecasting services.

Even better, use a receiver in low-earth orbit. See also: COSMIC, GeoOptics, Spire, and PlanetiQ.

Re: GPS

#33

Post by jvanderbot »

brandmeyer wrote: post #30
jvanderbot wrote: post #18

> Naturally, large range uncertainty increases the ambiguity of position, but the relative position of the satellites also matters. If they aren’t well spread, the exactness of calculated location also suffers. (see the excellent example in OP) Fun tidbit, the resulting error is known for the system in closed form as Geometric Dilution of Precision, and is a 3x3 (edit: or 4+x4+ if you are estimating bias or quantitie…

The natural expression of the DOP matrix is 4x4, since the receiver is computing a solution in 4D space-time. Its pretty common for the dominant eigenvector to be along the time-vertical axis for a terrestrial receiver.

Great point, thanks. I edited.

Re: GPS

#36

Post by jandrese »

taubek wrote: post #28

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.

In order to determine where you are you need to know where all of the satellites are. For a standalone receiver this involves downloading a almanac of the satellites from the signal, but GPS receivers have small antennas and the satellites don't blast out at tremendous power so the available bandwidth is very low. This means the effective bitrate of a GPS signal is only 50 bits per second so it takes twelve and a half minutes to transmit the entire list.

Cell phones get around this by downloading the almanac from the internet. Standalone receivers also keep the almanac in nonvolatile storage, but the almanacs eventually go stale if you leave the receiver off for too long.

Re: GPS

#38

Post by modeless »

taubek wrote: post #28

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 article explains the delay. The satellites transmit their ephemeris data and other important data very slowly, 50 bits per second, so you have to listen to the signals for a long time to get all of it. Not explained in the article is that modern GPS receivers in phones download this data separately from the internet, so they can calculate positions instantly without waiting for the data to finish transmitting.

Re: GPS

#39

Post by jandrese »

zeeb wrote: post #15

GPS applies the theory of relativity directly to your everyday life... pretty cool!

GPS is great because it has to take into account both special and general relativity. Very few consumer goods can make this claim.

Re: GPS

#40

Post by whiteboardr »

hwers wrote: post #11

This person deserves to get filthy rich off of their patreon.

Not just filthy rich i guess.

Bartosz is a one of a kind explainer of things - no matter what topic he touches, he always manages to outright nail the communication of the core concepts in a manner almost anyone can understand.

Let alone the interactive visualizations.

There should be some sort of Nobel Prize for people that contribute to humanity’s education - and methods therof.

Kudos!

Post reply on HN