Atomic Clock Synchronisation using MSF

Accurate time using Atomic Clocks is available across Great Britain and parts of northern Europe using the MSF Atomic Clock time signal transmitted from Cumbria, UK; it provides the ability to synchronize the time on computers and other electrical equipment.

The UK MSF signal is operated by NPL – the National Physical Laboratory. MSF has high transmitter power (50,000 watts), a very efficient antenna and an extremely low frequency (60,000 Hz). For comparison, a typical AM radio station broadcasts at a frequency of 1,000,000 Hz. The combination of high power and low frequency gives the radio waves from MSF a lot of bounce, and this single station can therefore cover most of Britain and some of continental Europe.

The time codes are sent from MSF using one of the simplest systems possible, and at a very low data rate of one bit per second. The 60,000 Hz signal is always transmitted, but every second it is significantly reduced in power for a period of 0.2, 0.5 or 0.8 seconds: • 0.2 seconds of reduced power means a binary zero • 0.5 seconds of reduced power is a binary one. • 0.8 seconds of reduced power is a separator. The time code is sent in BCD (Binary Coded Decimal) and indicates minutes, hours, day of the year and year, along with information about daylight savings time and leap years.

The time is transmitted using 53 bits and 7 separators, and therefore takes 60 seconds to transmit. A clock or watch can contain an extremely small and relatively simple antenna and receiver to decode the information in the signal and set the clock’s time accurately. All that you have to do is set the time zone, and the atomic clock will display the correct time.

Dedicated time servers that are tuned to receive the MSF time signal are available. These devices connect o a computer network like any other server only these receive the timing signal and distribute it to other machines on the network using NTP (Network Time Protocol).

Correcting Network Time

Distributed networks rely completely on the correct time. Computers need timestamps to order events and when a collection of machines are working together it is imperative they run the same time.

Unfortunately modern PC’s are not designed to be perfect timekeepers. Their system clocks are simple electronic oscillators and are prone to drift. This is not normally a problem when the machines are working independently but when they are communicating across a network all sorts of problems can occur.

From emails arriving before they have been sent to entire system crashes, lack of synchronisation can causes untold problems across a network and it is for this reason that network time servers are used to ensure the entire network is synchronised together.

Network time servers come in two forms – The GPS time server and the radio referenced time server. GPS NTP servers use the time signal broadcast from GPS satellites. This is extremely accurate as it is generated by an atomic clock on board the GPS satellite. Radio referenced NTP servers use a long wave transmission broadcast by several national physics laboratories.

Both these methods are a good source of Coordinated Universal Time (UTC) the world’s global timescale. UTC is used by networks across the globe and synchronising to it allows computer networks to communicate confidently and partake of time sensitive transactions without error.

Some administrators use the Internet to receive a UTC time source. Whilst a dedicated network time server is not required to do this it does have security drawbacks in that a port is needed to be left open in the firewall for the computer to communicate with the NTP server, this can leave a system vulnerable and open to attack. Furthermore, Internet time sources are notoriously unreliable with many either too inaccurate or too far away to serve any useful purpose.

Why the Need for NTP

Network Time Protocol is an Internet protocol used to synchronize computer clocks to a stable and precise time reference. NTP was originally developed by Professor David L. Mills at the University of Delaware in 1985 and is an Internet standard protocol.

NTP was developed to solve the problem of multiple computers working together and having the different time. Whilst, time usually just advances, if programs are running on different computers time should advance even if you switch from one computer to another. However, if one system is ahead of the other, switching between these systems would cause time to jump forward and back.

As a consequence, networks may run their own time, but as soon as you connect to the Internet, effects become visible. Just Email messages arrive before they were sent, and are even replied to before they were mailed!

Whilst this sort of problem may seem innocuous when it comes to receiving email, however, in some environments a lack of synchronisation can have disastrous results this is why air traffic control was one of the first applications for NTP.

NTP uses a single time source and distributes it amongst all devices on a network it does this by using an algorithm that works out how much to adjust a system clock to ensure synchronisation.

NTP works on a hierarchical basis to ensure there are no network traffic and bandwidth problems. It uses a single time source, normally UTC (coordinated universal time) and receives time requests from the machines on the top of the hierarch which then pass the time on further down the chain.

Most networks that utilise NTP will use a dedicated network time server to receive their UTC time signal. These can receive the time from the GPS network or radio transmissions broadcast by national physics laboratories. These dedicated NTP time servers are ideal as they receive time direct from an atomic clock source they are also secure as they are situated externally and therefore do not require interruptions in the network firewall.

New Waterproof GPS Mushroom Antenna

Galleon Systems’ new mushroom GPS antenna provide increased reliability in receiving GPS timing signals for NTP time servers.
The new Exactime 300 GPS Timing and Synchronization Receiver boasts waterproof protection, anti-UV, anti-acidity and anti-alkalinity properties to ensure reliable and continual communication with the GPS network.

The attractive white mushroom is smaller than conventional GPS antennas and sits just 77.5mm or 3.05-inch in height and is easily fitted and installed thanks to the inclusion of a full installation guide and CD manual.

Whilst an ideal unit for a GPS NTP time server this industry standard antenna is also ideal for all GPS receiving needs including: Marine Navigation, Control Vehicle Tracking and NTP synchronisation
The main features of the Exactime 300 mushroom antenna are:

• Built-in patch antenna • 12 parallel tracking channels • Fast TTFF (Time to first fix) and low power consumption • On-board, rechargeable battery sustained Real-Time Clock and control • parameters memory for fast satellite acquisition during power-up • Interference filter to major VHF channels of marine radar • WAAS compliant with EGNOS support • Perfect Static Drift for both of speed and course •  Magnetic Declination compensation • Is protected against reverse polarity voltage • Support RS-232 or RS-422 interface, Support 1 PPS output.

Utilising UTC

To receive and distribute and authenticated UTC time source there are currently two types of NTP server, the GPS NTP server and the radio referenced NTP server. While both these systems distribute UTC in identical ways the way they receive the timing information differs.

A GPS NTP time server is an ideal time and frequency source because it can provide highly accurate time anywhere in the world using relatively cheap components.  Each GPS satellite transmits in two frequencies L2 for the military use and L1 for use by civilians transmitted at 1575 MHz, Low-cost GPS antennas and receivers are now widely available.

The radio signal transmitted by the satellite can pass through windows but can be blocked by buildings so the ideal location for a GPS antenna is on a rooftop with a good view of the sky. The more satellites it can receive from the better the signal. However, roof-mounted antennas can be prone to lighting strikes or other voltage surges so a suppressor is highly recommend being installed inline on the GPS cable.

The cable between the GPS antenna and receiver is also critical. The maximum distance that a cable can run is normally only 20-30 metres but a high quality coax cable combined with a GPS amplifier placed in-line to boost the gain of the antenna can allow in excess of 100 metre cable runs. This can provide difficulties in installation in larger buildings if the server is too far from the antenna.

An alternative solution is to use a radio referenced NTP time server. These rely on a number of national time and frequency radio transmissions that that broadcast UTC time. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom’s national time reference, there are also similar systems in the USA (WWVB) and in France, Germany and Japan.

A radio based NTP server usually consists of a rack-mountable time server, and an antenna, consisting of a ferrite bar inside a plastic enclosure, which receives the radio time and frequency broadcast. It should always be mounted horizontally at a right angle toward the transmission for optimum signal strength. Data is sent in pulses, 60 a second. These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

2008 Will be a second longer Leap Second to be added to UTC

New Year’s celebrations will have to wait another second this year as the International Earth Rotation and Reference Systems Service (IERS) have decided to 2008 is to have Leap Second added.

IERS announced in Paris in July that a positive Leap Second was to be added to 2008, the first since Dec. 31, 2005. Leap Seconds were introduced to compensate for the unpredictability of the Earth’s rotation and to keep UTC (Coordinated Universal Time) with GMT (Greenwich Meantime).

The new extra second will be added on the last day of this year at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time — 6:59:59 pm Eastern Standard Time. 33 Leap Seconds have been added since 1972

NTP server systems controlling time synchronisation on computer networks are all governed by UTC (Coordinated Universal Time). When an additional second is added at the end of the year UTC will automatically be altered as the additional second. #

Whether a NTP server receives a time signal fro transmissions such as MSF, WWVB or DCF or from the GPS network the signal will automatically carry the Leap Second announcement.

Notice of Leap Second from the International Earth Rotation and Reference Systems Service (IERS)

SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE ET DES SYSTEMES DE REFERENCE

SERVICE DE LA ROTATION TERRESTRE
OBSERVATOIRE DE PARIS
61, Av. de l’Observatoire 75014 PARIS (France)
Tel.      : 33 (0) 1 40 51 22 26
FAX       : 33 (0) 1 40 51 22 91
e-mail    : services.iers@obspm.fr

http://hpiers.obspm.fr/eop-pc

Paris, 4 July 2008

Bulletin C 36

To authorities responsible for the measurement and distribution of time

UTC TIME STEP
on the 1st of January 2009

A positive leap second will be introduced at the end of December 2008.
The sequence of dates of the UTC second markers will be:

2008 December 31,     23h 59m 59s
2008 December 31,     23h 59m 60s
2009 January   1,      0h  0m  0s

The difference between UTC and the International Atomic Time TAI is:

from 2006 January 1, 0h UTC, to 2009 January 1  0h UTC  : UTC-TAI = – 33s
from 2009 January 1, 0h UTC, until further notice       : UTC-TAI = – 34s

Leap seconds can be introduced in UTC at the end of the months of December

Atomic Clocks The Future of Time

Methods of keeping track of time have altered throughout history with ever increasing accuracy has being the catalyst for change.

Most methods of timekeeping have traditionally been based on the movement of the Earth around the Sun. For millennia, a day has been divided into 24 equal parts that have become known as hours. Basing our timescales on the rotation of the Earth has been adequate for most of our historical needs, however as technology advances, the need for an ever increasingly accurate timescale has been evident.

The problem with the traditional methods became apparent when the first truly accurate timepieces – the atomic clock was developed in the 1950’s. Because these timepieces  was based on the frequency of atoms and were accurate to within a second every million years it was soon discovered that our day, that we had always presumed as being precisely 24 hours, altered from day to day.

The affects of the Moon’s gravity on our oceans causes the Earth to slow and speed up during its rotation – some days are longer than 24 hours whilst others are shorter. Whilst this minute differences in the length of a day have made little difference to our daily lives it this inaccuracy has implications for many of our modern technologies such as satellite communication and global positioning.

A timescale has been developed to deal with the inaccuracies in the Earth’s spin – Coordinated Universal Time (UTC). It is based on the traditional 24-hour Earth rotation known as Greenwich Meantime (GMT) but accounts for the inaccuracies in the earth’s spin by having so-called ‘Leap Seconds’ added (or subtracted).

As UTC is based on the time told by atomic clocks it is incredibly accurate and therefore has been adopted as the World’s civilian timescale and is used by business and commerce all over the globe.

Most computer networks can be synchronised to UTC by using a dedicated NTP time server.

Atomic Clocks and the NTP Server Using Quantum Mechanics to Tell the Time

Telling the time is not as straight forward as most people think. In fact the very question, ‘what is the time?’ is a question that even modern science can fail to answer. Time, according to Einstein, is relative; it’s passing changes for different observers, affected by such things as speed and gravity.

Even when we all live on the same planet and experience the passing of time in a similar way, telling the time can be increasingly difficult. Our original method of using the Earth’s rotation has since been discovered to be inaccurate as the Moon’s gravity causes some days to be longer than 24 hours and a few to be shorter. In fact when the early dinosaurs were roaming the Earth a day was only 22 hours long!

Whilst mechanical and electronic clocks have provided us with some degree accuracy, our modern technologies have required far more accurate time measurements. GPS, Internet trading and air traffic control are just three industries were split second timing is incredibly important.

So how do we keep track of time? Using the Earth’s rotation has proven unreliable whilst electrical oscillators (quartz clocks) and mechanical clocks are only accurate to a second or two per day. Unfortunately for many of our technologies a second inaccuracy can be far too long. In satellite navigation, light can travel 300,000 km in just over a second, making the average sat-nav unit useless if there was one second of inaccuracy.

The solution to finding an accurate method of measuring time has been to examine the very small – quantum mechanics. Quantum mechanics is the study of the atom and its properties and how they interact. It was discovered that electrons, the tiny particles that orbit atoms changed the path that they orbit and released a precise amount of energy when they do so.

In the case of the caesium atom this occurs nearly nine billion times a second and this number never alters and so can be used as an ultra reliable method of keeping track of time. Caesium atoms are use din atomic clocks and in fact the second is now defined as just over 9 billion cycles of radiation of the caesium atom.

Atomic clocks
are the foundation for many of our technologies. The entire global economy relies on them with the time relayed by NTP time servers on computer networks or beamed down by GPS satellites; ensuring the entire world keeps the same, accurate and stable time.

An official global timescale, Coordinated Universal Time (UTC) has been developed thanks to atomic clocks allowing the whole world to run the same time to within a few thousandths of a second from each other.

How a GPS Time Server Works

A GPS time server is really a communication device. Its purpose is to receive a timing signal and then distribute it amongst all devices on a network. Time server s are often called different things from network time server, GPS time server, radio time server and NTP server.

Most time servers use the protocol NTP (Network Time Protocol). NTP is one of the Internet’s oldest protocols and is used by the majority of machines that use a time server. NTP is often installed, in a basic form, in most operating systems.

A GPS time server, as the names suggests, receives a timing signal from the GPS network. GPS satellites are really nothing more than orbiting clocks. Onboard each GPS satellite is an atomic clock. The ultra-precise time from this clock is what is transmitted from the satellite (along with the satellite’s position).

A satellite navigation system works by receiving the time signal from three or more satellites and by working out the position of the satellites and how long the signals took to arrive, it can triangulate a position.

A GPS time server needs even less information and only one satellite is required in order to receive a timing reference. A GPS time server’s antenna will receive a timing signal from one of the 33 orbiting satellites via line of sight, so the best place to fix the antenna is the roof.

Most dedicated GPS NTP time servers require a good 48 hours to locate and get a steady fix on a satellite but once they have it is rare for communication to be lost.

The time relayed by GPS satellites is known as GPS time and although it differs to the official global timescale UTC (Coordinated Universal Time) as they are both based on atomic time (TAI) GPS time is easily converted by NTP.

A GPS time server is often referred to as a stratum 1 NTP device, a stratum 2 device is a machine that receives the time from the GPS time server. Stratum 2 and stratum 3 devices can also be used as a time servers and in this way a single GPS time server can operate as a timing source for an unlimited amount of computers and devices as long as the hierarchy of NTP is followed.

Keeping Time with the Rest of the World

A time server is a common office tool but what is it for?

We are all used to having a different time from the rest of the world. When America is waking up, Honk Kong is going to bed which is why the world is divided into time zones. Even in the same time-zone there can still be differences. In mainland Europe for instance most countries are an hour ahead of the UK because of Britain’s seasonal clock changing.

However, when it comes to global communication, having different times all over the world can cause problem particularly if you have to conduct time sensitive transactions such as buying or selling shares.

For this purpose it was clear by the early 1970’s that a global timescale was required. It was introduced on 1 January 1972 and was called UTC – Coordinated Universal Time. UTC is kept by atomic clock but is based on Greenwich Meantime (GMT – often called UT1) which is itself a timescale based on the rotation of the Earth. Unfortunately the Earth varies in its spin so UTC accounts for this by adding a second once or twice a year (Leap Second).

Whilst controversial to many, leap seconds are needed by astronomers and other institutions to prevent the day from drifting otherwise it would be impossible to work out the position of the stars in the night sky.

UTC is now used all over the world. Not only is it the official global timescale but is used by hundreds of thousands of computer networks all over the world.

Computer networks use a network time server to synchronise all devices on a network to UTC. Most time servers use the protocol NTP (Network Time Protocol) to distribute time.

NTP time servers receive the time from atomic clocks by either long-wave radio transmissions from national physics laboratories or from the GPS network (Global Positioning System). GPS satellites all carry an onboard atomic clock that beams the time back to Earth. Whilst this time signal is not strictly speaking UTC (it is known as GPS time) because of the accuracy of the transmission it is easily converted to UTC by a GPS NTP server.