Why didn't GPS crash? All the Numbers - Numberphile 1 day ago   12:20

More on the GPS roll-over.

"In the Future the Modernized GPS Navigation (CNAV and MNAV) message has a 13-bit week number, which for all practical purposes solves this ambiguity"

See me turn on my TomTom:

If you want to see more hilarious tales about roll-over errors then, well, READ MY BOOK.

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- Sorry: I used “overflow” to refer to when the value bursts into the adjacent memory as opposed to just rolling back to zero. But an overflow can simply mean it has rolled over.
- I add the extra week bits to the wrong side of the number! Unbelievable. I even point to the other side of the screen in the video. Somehow I totally missed this until it was too late. I’m surprised loads of people are not pointing it out. So far only Kyle K has noticed. Good on you, Kyle.
- Let me know if you spot anything else!

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Comments 767 Comments

"it'll be fine"
The word "epoch" rhymes with "epic"
main takeaway here is: if you stare at screens all day you miss all of life's dogs
A message to the GPS maintainers: Please use only storage capacities that are powers of two. Would it really hurt that much to go to 16 instead of 13?
Ethanagor S-D
between 6:15 and 6:24 is the only important part of the video because that's when the dog is on screen
Garry Jones
Very interesting post.
I really like all this stuff.
Interestingly or not older computers think that time began on Jan. 4, 1980.
When an old computer loses its settings and returns to its default state, that is the only date it remembers in its BIOS software. (Pre flash upgrades). This can often be recreated by removing the cmos CR 2032 battery in the mother board. In fact even in new Windows systems you'll often find files with a creation date of 4th Jan 1980. I wonder if this is connected to the GPS date?
In the 70's I worked on mainframe computers. Storage space on disk drivers was money so files were given a File Retention Date (FRD) in the format YY/DDD. So 79/043 was the 43rd day of 1979 (when the programmer knew it would not be necessary to require the file after that date). After the FRD the physical space on the disk was released and could be overwritten. System files and other permanent files were given an FRD of 99/365. (The 31st December 1979).
In the summer of 1980 I asked an IBM software engineer what would happen on 1st January 2000. At first his answer was "We'll cross that bridge when we come to it". He gave it some further thought and eventually we asked the question on via shoebox modem to the IBM Knowledge base in the States. A rollover was not possible because computers would be overwriting themselves and deleting their own system and bootstrap files. It led to the first articles of the year 2000 problem and I got a (small) degree of notoriety.
Great vid, love the isle of wight
Frank Yanish
Most cell phones don't use GPS. They geolocate using cell towers.
If you reverse the battery you will kill the etrek GPS. ask me how i know lol
martijn van weele
Speak for yourself. I'm not sorry.
TIL Why GPS didn't crash ... and Matt Parker's wife loves him very much. ;)
L Dewey
Brilliant, funny and enjoyable video! (At least you didn't drop your phone into a toilet... like I once did!) Keep these videos coming!
"The military are convinced we'll be fiiiine... in 150 years, it'll be somebody else's problem, WE'll be fine"
Why those old devices picks 4 satellites in 15 minute while a modern smartphone picks ~10 satellites in just a few seconds?
My GPS unit is now useless, it doesn't know when it is between 1999 and 2099 :\
Jintaro Kensei
Jesus man, lower the damn sfx.
Hi Matt, why is my computer clock 3-4 minutes behind my other electronics?
Henry Giles
Виталий Матин
2\2147483647 = ... -1= PRIME
Mo Poppins
Nice fingernails, Matt. Do you get manis?
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All the Numbers - Numberphile Why didn't GPS crash? 1 day ago   14:27

Matt Parker talks about numbers - as he often does. His book "Humble Pi" is at: http://bit.ly/Humble_Pi
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Yes we went for the sometimes English spelling constructable... Constructible is more common and probably better!

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