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Zulu Time or UTC

With the development of worldwide televsion and communications via satellite systems and the Global Positioning System (GPS), a standard time system used throughout the world is essential for international operators in government, military and commerce. Aviators and mariners have been using such a 'standard time' for many years. Other than international airlines, few civilian networks have used or have much knowledge of Universal Time Co-ordinated (UTC), its predecessor Greenwich Mean time (GMT) or in military parlance, "Zulu Time".

A standard international time system was created to solve many problems in the world caused by different countries, states, counties and towns. However, this did create difficulties in knowing what time was being referred to. UTC is merely local mean time at the Greenwich meridian, ie the 'prime meridian', zero longitude. As Zulu Time evolved, the word also acquired better navigation systems and more accurate and reliable timepieces.

However, before any of that could happen, somebody had to break the natural day into identifiable zones. For that, the world has to thank the forebears of today's Iraqis. While the rest of the world was still counting on fingers and doing base-ten arithmetic, the ancient Mesopotamians embraced the number sixty and used it to divide all sorts of things. By the time Mesopotamia became Iraq, the sexagesimal system was adopted - the twelve-month year (one-fifth of sixty), the twenty-four-hour day, the sixty-minute hour, the sixty-second minute and 360° in a circle definition of bearings.

People emerged from the Middle Ages with a more powerful yen to travel and took to the sea in large numbers. For a while, they just wandered around, not caring too much when they got home, but before long ships were carrying goods between the continents. As this import-export business grew more competitive, traders needed to know more than just which way was home. They had to know where they were at any given time and how to get where they were going by the shortest route.

European astronomers worked out a navigation system. In 1675. King Charles II built a major new observatory at Greenwich, England, where it started to produce star charts. With enough of those charts, a navigator could find his longitude and latitude any place in the world and keep to a precise course. All he needed was a sextant and a good clock.

What Was Missing

Although the Europeans had plenty of sextants, nobody had yet invented a really accurate timepiece. Ships still were getting lost at sea, sometimes permanently. By 1714, things were so bad that the British government offered a reward of 20000 pounds sterling to any one who could produce a state-of-the-art, reasonably accurate timepiece.

A mechanic named John Harrison came up with a chronometer that, after making a trip to Jamaica, had lost only five seconds. The British Government accepted it. Modern maritime navigation was off and running. With typical bureaucracy, Harrison did not collect his prize money for forty years.

However, following the development of the chronometer, world travellers faced another problem that technology couldn't solve by itself.

In the early 1830s, steam-driven trains, introduced in Britain and the United States, were moving people and goods around faster than ever before. The railroads opened up the country and provided the infrastructure and means to develop industry and 'connect' hundreds and thousands of towns and villages throughout the nations. As in many other countries in the world, the railways in Australia were the fabric of the nation and were responsible in opening up the country.

The development of the railway highlighted the problem of time - local, village and standardised. Communities in the world must keep time near to the local mean time to keep their lives in step with the sun. An 1841 timetable for Britain's Great Western Railroad told bewildered passengers the following: "London time is kept at all stations, which is about four minutes earlier than Reading time, five and one-half minutes before Steventon time, seven and one-half minutes before Cirencester time, and fourteen minutes before Bridgewater time."

As the railroads expanded, things got worse. By 1880, railroad companies around the world had laid more than 150,000 miles of track. Railroads in the United States alone were dealing with more than 100 separate time schemes. In 1883, Canadian and US railroad companies worked out a system to relieve the situation. One year later, the plan was adopted by an international conference.

The plan divided the Earth into twenty-four time zones, with the centre of each zone co-incident with each fifteen degrees of longitude, ie the meridian, (24 x 15 = 360 degrees). France wanted Paris to be the 'prime meridian' and the Americans promoted Washington. However, because Britain still ruled the waves, the time-organizers finally agreed to run the Prime Meridian through the old observatory at Greenwich.

Standard Time Gets Airborne

It was not until 1923 when US Army Douglas World Cruisers circled the earth that global aviation became of age. The value of the standard time system became apparent.

As aviation shrank the globe, accurate navigation became even more vital. Over land, pilots could follow highways and railroad tracks; by the mid-1920s, a system of lighted airways and radio beacons helped navigation, however, flying in bad weather, with no visual contact with the ground and over water was quite another story. Most pilots weren't trained to cope. Early in the 1930s, the US Army Air Corps created a unit at Bolling Field near Washington, D. C., to study the problem. It hired Harold Gatty, a navigator with expert knowledge on worldwide flight, as an advisor.

Most of the needed tools were at hand. The Germans had developed an aerial sextant. Accurate drift meters had appeared soon afterward, along with slide rules for solving course and distance problems. The nautical almanacs had been adopted to aerial almanacs, (which many navies adopted over the years). The aviators had watches as accurate as the marine chronometers. The main problem was to teach flyers to use the new technology.

The Air Corps set up a navigation training program for pilots and later expanded it into a full five-month course for navigators. At the heart of the new air training was a technique based on the triangle of velocities, the basis of navigation. Together with an analogue, or vector, of the aircraft's course and airspeed and a vector representing the wind speed and direction, the navigator can determine the heading, ground speed and the position of the aircraft. The position is deduced rather than positively identified; it is known as a dead reckoning (DR) position. Some historians suggest it was called "deduced reckoning," then first word was abbreviated to "ded," and the phonetic pronunciation "dead reckoning " became the accepted term.

Dead reckoning involves taking a series of visual positions (pinpoints) using maps, radio position lines and celestial fixes, from a series of celestial observations, using star charts based on Greenwich Mean Time.

Western Air Forces adopted GMT for navigation and for timing worldwide operations. World War II navigators lived by it, as did their successors. The pre-mission "time hacks" took on the solemnity of religious rituals and GMT bound the faithful together like some secret password.

Air Forces navigators (and some pilots) set their 'nav watches' on GMT and did the conversion to standard time readily - it separated the knowledgeable from the 'rest'. Some aircrew wore two watches for the different times, one on local and the other always on GMT. The duo analogue and digital watch was a godsend.

Since the World War II, technology has taken much of the drudgery out of navigation, and in many aircraft, black boxes have replaced human navigators altogether. The GPS is the modern 'celestial' system where the aircraft position is derived from satellites and displayed continuously to the crews. In strike aircraft, the derived position updates the weapon delivery parameters for the changing tactical situation.

Time systems themselves have gone through a series of changes. Until well into this century, Greenwich Mean Day began at noon because astronomers didn't want to change dates during their overnight vigils. That practice confused other people, but it was not until the 1920s that the schedule was revised. The stargazers still kept their system but renamed it Greenwich Mean Astronomical Time. The rest of the world went on a midnight-to-midnight routine. Officially it is "Universal Time Co-ordinated (UTC)," but many people continue to call it GMT.

Zero, Zebra, Zulu

Illustration by Bob Stevens

The military has its own term. Since Greenwich is the site of the "zero meridian," the military called GMT "Zero Time" or simply "Z-Time." In the phonetic alphabet used by the aviation world, "Z" became "Zebra." When the alphabet was changed, the call sign became "Zulu."

Zulu Time no longer emanates from Charles II's old observatory. In 1958, the Greenwich astronomers moved to the English coast to escape London's fog and city lights. They still correct their observations to show the time at the Greenwich meridian, but the old building there has been converted to a museum.

Nor are the 1884 time zones still the neat parallel meridians first defined. They vary around international borders and state boundaries and are redrawn every now and then. Some countries change the time zones to suit national requirements; in others, the time zones are based on half-hour differences from GMT. One country has shifted from one side of the international date line, to the other, so that they are the first to welcome in the new year (and the millennium in 2000).

Some scientists and chronologists would like to dump the old, sexagesimal system of keeping time and come out with metric clocks. That could result in 20 hour days with 100 minutes per hour.

Digital clocks made a great inroad with many aviators because navigation is 'done on numbers'. Many 'fast jet' aircrew used digital watches as analogue watches were hard to read accurately in low light level cockpits (eg, night and red instrument lighting) and turbulent conditions. Also, analogue time requires conversion to numbers for use in navigation calculations. However, digital watches will not take over completely, because one can not determine position from the sun in survival situations nor could one determine "bogies at ten o'clock" or "check your six". Nowdays, modern systems provide most of the answers for air navigation and watches are only required as a backup and, when back in the crewroom, to remind you when it is coffee time or when to go to the mess.

We're probably in no immediate danger of going base number 10 or all-digital for time keeping. Minor variations are creeping in; time used to be expressed in hours and minutes, but in the latest navigation systems, time is defined as a decimal in the base 10 system, eg 4hrs 35 mins is now 4.55h. Latitude is also being expressed in the same format, eg S35.6 instead of 3536S. The second was the smallest division of the solar day. Now atomic clocks in GPS, the modern time standard source, use the natural resonance of caesium atoms to divide time even more finely, cutting it into millionths and billionths of a second.

These times are base 10 systems, where one millionth of a second is one micro second or 1 x 10-6 second and one nano second, or 1 x 10-9 second.

Original article in AIR FORCE Magazine, by Bob Challender. Illustration by Bob Stevens.

Lance Halvorson
ACT Division