It could mean a "negative" leap second.

Even time did not escape 2020 unscathed.

The 28 fastest days on record (since 1960) all occurred in 2020, with Earth completing its revolutions around its axis milliseconds quicker than average. That's not particularly alarming — the planet's rotation varies slightly all the time, driven by variations in atmospheric pressure, winds, ocean currents and the movement of the core. But it is inconvenient for international timekeepers, who use ultra-accurate atomic clocks to meter out the Coordinated Universal Time (UTC) by which everyone sets their clocks. When astronomical time, set by the time it takes the Earth to make one full rotation, deviates from UTC by more than 0.4 seconds, UTC gets an adjustment.

Currently, GPS satellites use atomic clocks as part of navigation systems, but there are limits to the approach. They rely on atoms confined by a gas cell to serve as a meter for the clock, but the long-term stability of gas-cell clocks can suffer when the atoms collide with the "walls" of the cell, causing drift. GPS systems get signals from earthbound atomic clocks to correct the instability. But that gets trickier the deeper you go into space.

Trapped-ion atomic clocks, in which charged atoms are prevented from colliding with walls by an electromagnetic cage, have been around since the early 2000s and are more accurate than earlier approaches to atomic time-keeping, which have been around since the 1950s. NASA's Deep Space Atomic Clock loses one second every 10 million years, as proven in controlled tests on Earth.

The table below lists the time servers used by the NIST Internet Time Service (ITS). The table lists each server's name, IP address, and location, organized geographically within the US from North to South and then from East to West. Please note that while we make every effort to ensure that the names of the servers are correct, we control the names of only the nist.gov servers. If you have difficulty using the name of a system, you can access a server using the IP address directly.

Q: As we all know, atomic clocks are being used to measure time and the GPS system. But I was wondering based on what was the first atomic clock calibrated and how accurate this calibration was based on our standards nowadays?

A: More specifically, caesium atomic clocks realize the second (see this Q&A for the meaning of realization) or, said another way, they are a primary frequency standard. Generally, when a new primary standard is being developed—for whatever quantity, not only time—and has not yet become, by international agreement, a primary standard, it should be calibrated against the primary standards of the time.

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According to a study last year commissioned by the National Institute of Standards and Technology, GPS has about $1 billion a day in economic impact in the US. Its reach is, simply, mind-blowing.

"Gauging the overall value of GPS is nearly impossible," writes Greg Milner in Pinpoint, a 2016 book about how the space-based system came to be and the effect it's having on the world. "It has become difficult to untangle the worth of GPS from the worth of everything." //

"The timing aspect of this is probably more widely used than the where-are-you aspect," says Goward. //

The origins of GPS stretch back to secret work by the Department of Defense in the 1970s, in a quest for precision targeting. As Milner recounts it, GPS chief architect Brad Parkinson summed up that goal in the phrase "Drop five bombs in the same hole." //

In 1983, after a Korean Air Lines passenger jet strayed into Soviet airspace and got shot down, killing 269 people, President Ronald Reagan declassified GPS to give civilian aircraft access to the navigation signals. Almost a decade later, GPS famously earned its stripes as a military resource during Operation Desert Storm, when it helped guide US and allied forces across desert expanses to a swift victory over Iraq during the Gulf War. //

Though the funding to keep things running goes through the Pentagon -- the Space Force GPS program had a 2020 fiscal year budget of $1.71 billion -- there's civilian oversight as well. The Defense Department and the Transportation Department co-chair the US government's National Executive Committee for Space-Based Positioning, Navigation and Timing, which coordinates GPS-related matters across federal agencies and includes representatives from Boeing, Garmin, Google, Ohio State and Stanford.

Note the keywords in that committee name: positioning, navigation and timing, or PNT. Where you are, where you're going, and when the signals hit a receiver. It's a term that's inescapable when you're talking with folks who live and breathe GPS.

The most frequent sounds heard on WWV and WWVH are the seconds pulses. These pulses are heard every second except on the 29th and 59th seconds of each minute. The first pulse of each hour is an 800 ms pulse of 1500 Hz. The first pulse of each minute is an 800 ms pulse of 1000 Hz at WWV and 1200 Hz at WWVH. The remaining seconds pulses are short audio bursts (5 ms pulses of 1000 Hz at WWV and 1200 Hz at WWVH) that sound like the ticking of a clock. //

Both WWV and WWVH broadcast standard frequency audio tones that alternate during most minutes of the hour. Most minutes feature a 500 or 600 Hz audio tone. However, a 440 Hz tone is broadcast once per hour, and some minutes do not include any audio tones at all.

We need to sync all the servers across many data centers with sub-millisecond precision. For that we tested chrony, a modern NTP server implementation with interesting features. During testing, we found that chrony is significantly more accurate and scalable than the previously used service, ntpd, which made it an easy decision for us to replace ntpd in our infrastructure. Chrony also forms the foundation of our Facebook public NTP service, available from time.facebook.com. In this post, we will share our work to improve accuracy from 10 milliseconds to 100 microseconds and how we verified these results in our timing laboratory. //

In comparing ntpd with chrony, our measurements indicate that chrony is far more precise, which is why we’ve migrated our infra to chrony and launched a public NTP service. We’ve found that the effort to migrate is worth the immediate improvement in precision from tens of milliseconds to hundreds of microseconds.

Using hardware timestamps can further improve precision by two orders of magnitude. Despite its improvements, NTP has its own limitations, so evaluating PTP can take your precision to the next level.

Atomic clocks are used around the world to precisely tell time. Each "tick" of the clock depends on atomic vibrations and their effects on surrounding electromagnetic fields. Standard atomic clocks in use today, based on the atom cesium, tell time by "counting" radio frequencies. These clocks can measure time to a precision of one second per every hundreds of millions of years. Newer atomic clocks that measure optical frequencies of light are even more precise, and may eventually replace the radio-based ones.

Now, researchers at Caltech and the Jet Propulsion Laboratory (JPL), which is managed by Caltech for NASA, have come up with a new design for an optical atomic clock that holds promise to be the most accurate and precise yet (accuracy refers to the ability of the clock to correctly pin down the time, and precision refers to its ability to tell time in fine detail). Nicknamed the "tweezer clock," it employs technology in which so-called laser tweezers are used to manipulate individual atoms.

NTP Pool Servers

Time.is - exact time for any time zone
Time.is displays exact, official atomic clock time for any time zone (more than 7 million locations) in 49 languages.