How does your smartphone always keep the exact time?

One of the actions we do most often with our cell phone is checking the time. But How does your smartphone always keep the exact time? The answer must be sought in a plurality of technologies which, combined together, allow our electronic devices to always be very precise in indicating the correct time. To get more specific, smartphones adjust the time based on atomic clocks present in the laboratories of NTP server and in satellites.

The “atomic” precision of the Clock app

To put it simply, smartphones regularly synchronize with servers that provide the exact time based on the so-called UTC (Coordinated Universal Time) or Universal Coordinated Time, which is the time standard used globally. In practice, when a device connects to the Internet servers the time is synchronized and transmitted via the NTP protocol (Network Time Protocol), which is the one used within a packet switching network (just like the Internet).

But how do servers that transmit the exact time via the NTP protocol know the exact time? Thanks to atomic clocks, which are sometimes hosted in the laboratories where the servers in question are located. This is the case ofINRiM (National Institute of Metrological Research), located in Italy, which on its official website states:

This time signal is derived from the creation of the Italian time scale called UTC (IT) and used by the Bureau international des poids et mesures (BIPM) to create the international time scale UTC (Universal Time Coordinated).

NTP servers can receive the exact time to then transmit to devices connected to the Internet also thanks to the atomic clocks hosted in satellites in orbit around the earth, the so-called GNSS (Global Navigation Satellite System), in Italian “global navigation satellite system”. The satellites communicate the exact time of the atomic clocks to the receivers on the ground, including those integrated into our smartphones, which are thus able to update the time even when they do not have the possibility of being connected to the Internet.

Let's do one more zoom-in in the topic by going deeper how atomic clocks worksuch as those present in the United States satellite positioning and navigation system, that is GPS (Global Positioning System), or like those present in the satellites of the European project Galileo.

Atomic Clock |  Geopop
Hydrogen maser clock. Credits: ESA.

As their name suggests, atomic clocks exploit the oscillations of a particular atom as a “metronome” (unlike quartz clocks where a quartz plate oscillates to mark time), thus making the measurement the most accurate and precise as possible.

Since, as the special theory of relativity and of general theory of relativity by Albert Einstein, time in space flows differently than time on earth, corrections are even made to ensure that the clocks in orbiting satellites are synchronized with the clocks on the ground. In this way the time transmitted by satellites is precise and reliable.

But why do systems like GPS and Galileo need to integrate such precise clocks? Because otherwise they could not work correctly, given that four or more satellites are usually involved in calculating the exact position of a receiver (such as the one integrated into our smartphones). This because the distances are calculated by measuring the travel time that the signal takes to travel the distance between the satellite itself and us. This explains, therefore, why atomic clocks are used in satellite positioning and navigation systems, useful for determining the exact position on earth as well as allowing our smartphones (and computers) to always keep the exact time.

When Android smartphones were 15 seconds ahead of the iPhone

We close with a little curiosity. Did you know that, at least until a few years ago, Android devices were 15 seconds ahead of iPhone? Interviewed by The Vergethe American astrophysicist and science communicator Neil DeGrasse Tyson (known for his hilarious character), had reported:

The iPhone has accurate time unlike Android-based phones… Most Android devices get their time from GPS satellites. The timing system for GPS satellites was defined until 1982. And since 1982, 15 leap seconds have been added to civil time. And those seconds aren't included in Android's timekeeping because they get their time directly from GPS, while the iPhone compensates by re-entering those 15 seconds. As a result, the iPhone has the correct time unlike most Android phones which are exactly 15 seconds faster.

We would like to point out that the interview we have just referred to dates back to 2012. In the meantime, the developers of the Android operating system seem to have corrected their aim, at least from what we have had the opportunity to verify with the devices in our possession (but we do not exclude that the problem may still affect some smartphones with an obsolete version of the “green robot”).

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Sources ESA Business Insider The Verge (YouTube Video) National Institute of Metrological Research Professional PC