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I bet you heard about GPS, shortened for Global positioning system.
It is owned and operated by the US government, but it is freely available to everyone in the world – and, boy, is it widely used?
GPS is a fantastic feat of science and engineering whose implementation is not simple, but it is explained quite simply.
A number of satellites in orbit (31 are active at the moment) are continuously broadcasting their position in space and at the present time.
Radio receivers on Earth listen to these broadcasts and as long as they can "hear" signals from three different satellites at the same time and have their own way of measuring time, they can solve a system of mathematical equations. calculate their own position.
The calculations are based on the fact that the time required for the signal to get from the satellite to the receiver determines its distance, and with three distances, you can lock your position in only three dimensions.
The time elapsed between the satellite and the receiver accurately indicates the distance because the radio waves are moving at a constant speed. distance = speed × time.
Radio waves, collectively referred to as electromagnetic radiation (EMR), move to what is commonly known as the speed of light, because light is only a special type of radio wave in the appropriate frequency range. to trigger the detectors of the human retina. This speed is noted c, as in the famous equation E = mc2, and is defined in the GPS standard as 299,792,458 meters per second.
In a fascinating way, GPS position calculations must take into account Einstein's theories of relativity.
Satellites move very fast in relation to a terrestrial receiver, giving the impression that their clocks seem slow.
So we see satellites taking 7 million seconds of delay each day.
On the other hand, the fact that we are much closer to the center of Earth's gravitational field than GPS satellites means that our clocks seem slow.
Thus, satellites see us retreat 45 million seconds per day.
This means that they actually gain 45 microseconds because of our gravity, while also losing us 7 microseconds due to their speed.
These daily drifts of 38 microseconds (45 – 7) must be taken into account in GPS calculations.
Four heads are better than three
GPS receivers actually lock on four (or more) satellites simultaneously, instead of three, in order to solve equations that calculate both their position. and the current time, with amazing precision.
This fourth satellite signal means that GPS receivers do not need their own atomic clocks. So they can be very small, and because they just need to listen, never to emit, they do not consume a lot of energy.
Indeed, modern GPS receivers are so small and energy-efficient that they can be grouped into a single 5mm x 5mm chip, so most modern phones can use GPS, as well as speedos from bike, smart watches, drones, etc. other consumer devices.
In fact, given their price, GPS receivers are fantastic reference clocks, even if the receiver is attached to a building and you do not care to measure its position.
Absolute time relative to relative time
If all you need to know is how many seconds have elapsed since midnight on the previous Sunday morning, for example, because you can track the date yourself, you never have to worry about numbers greater than 604,800, which is the number of seconds. in a week (60 × 60 × 24 × 7).
But that would mean that every GPS receiver would need at least a basic clock, though accurate, after a half-week that would work even if the receiver itself was turned off.
The GPS signal alone would contain only enough information to decode the time relative to the current week.
So, GPS includes a Week Number (WN) field that gives an absolute time reference, representing the number of weeks since the midnight hour that started on Sunday, January 06, 1980 (1980-01-06T00: 00 : 00Z).
Thanks to the WN, you can theoretically indicate the time absolutely: WN = -5 would start on the second day of December 1979, for example, while WN = +4 is the first week of February 1980.
Your GPS receiver can therefore be autonomous, requiring only GPS satellites as an external data source and requiring no writable computer memory (RAM) able to keep the date when the device is turned off.
The tyranny of distance
GPS relies on precise electronic devices, including atomic clocks, which are launched into space and then used in space.
By convention, space begins only 100 km from the surface of our planet; GPS satellites are about 20 000 km, almost double the diameter of the Earth.
Because space is a hostile environment for computers, their performance is measured more in terms of durability than speed. It is useless to have a multi-gigahertz processor and a multi-megabit network link if they quickly end up running at zero speed.
In addition, GPS was invented and built in the 1970s and 1980s, while even terrestrial modems were able to send data at 1,200 bits per second.
As a result, GPS downlinks send data to billions of GPS receivers around the world at only 50 bits per second.
So, every gesture counts and nothing can be wasted.
There is no "protection of this variable until the next 64-bit limit" or "storage of this unique character in a 32-bit DWORD file" in the GPS protocol.
As a result, the GPS standard had to make compromises in storage, especially that the WN field was assigned only 10 bits. It can therefore represent numbers between 0 and 1023. It is then returned to 0 and the count starts. .
1024 weeks is a little under 20 years old, and since GPS time – as such things are called strangely in technical circles – started in 1980, the GPS had its own Y2K-like moment in 1999.
In simple terms, the "terrestrial time" GPS that immediately follows 1999-08-21T23: 59: 59Z is not, as one might expect, 1999-08-22T00: 00: 00Z.
Day zero revisited
During the turnaround, the hour advanced quite naturally, from one minute to midnight, then to midnight, but the date came back to zero, January 6, 1980, when the GPS started.
Of course, you can program around this, to a certain extent, as some have done for Y2K, assuming, for example, that years 00 to 49 correspond to AD2000 to AD2049, while the years 50 and following cover the 1950s to AD1999.
But for this type of compromise to work, you must be sure that you never have to represent AD1949 because you can not.
Wherever you redirect your day to zero in the year 2000, you are still struggling with a time that can not last more than 100 years.
Likewise, you are stuck with up to 1024 weeks in GPS. (The most recent version of the GPS will extend this duration to 8196 weeks, more than 150 years, but there is still a strict limit as to the duration of the time.)
A tip that you can use in GPS receivers that can not receive data from anywhere except satellites, and that do not have non-volatile RAM (memory that can survive a power failure), is to process the release date of the product. in compensation at the time, so you get 19.7 years from the WN beach from your own starting point.
Since you can not run your firmware code before compiling it, you can burn the build date reliably into your firmware image and use it as a vintage extension.
As long as you receive a firmware update to all your users over the next 19 years, you will be able to reset and rerun your own adjusted time again and again, and you will never make mistakes when converting data. Gross GPS. in absolute terrestrial timestamps.
Already seen again
Guess what?
If you advance another 1024 weeks, or 19.7 years, from the moment of tipping of the 1999 GPS, you find yourself facing the stroke of midnight that divides …
… Sunday's tomorrow!
It is Saturday, April 06, 2019 which will turn into Sunday, April 07, 2019.
What to do?
Should you panic?
Will your {bike computer, your car, your mobile phone, your drone, will it insert the name of the device here} go on Sunday morning?
The answer is "very unlikely".
Unless you have a very old GPS device that can not get firmware updates or a newer device but have never updated, you should be fine.
Time can not go back. Therefore, any properly compiled GPS device running with firmware compiled after 1999 already knows that the date can not go back to 1999, and can detect and automatically adjust to the failover.
Networked computers that synchronize their clocks from external sources are also unlikely to run in circles.
First, most modern computers (with the notable exception of the Raspberry Pi series of computers, which still restart in 1970) have sufficiently accurate backup clocks to detect inaccurate external time sources and ignore them. .
Secondly, most modern computers preserve the accuracy of their clocks by using a protocol called Network Time Protocol (NTP) that does not depend on a single time source.
So it is unlikely that you would wake up and find Limp Bizkit on the radio, the Spice Girls on TV and the Apple title at $ 1.50 (as splendid as at least one of those results would be).
Yet you could as well check for updates to satnav devices or GPS compatible devices right now, In case…
… and you could as well make sure you have your flow capacitor with you Saturday night.
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