Geolocation and Time – An Evolution of the Millennial Pair

By Joc Triglav

A more extensive version of this article can be read in GeoInformatics issue 7 and 8 of 2009.

Abstract

This year’s 250-th anniversary of the invention of the famous watch H-4 that ultimately resolved the longitude problem and the 400-th anniversary of the first use of an astronomical telescope is also an opportunity to look at geodesy as a science of measuring the Earth’s shape as a function of time. The paper gives an insight in some basic developments and describes the historical development of geodesy by pointing out and demonstrating the relations between the Earth’s shape, geolocation and time measurements from the ancient times to the present time. Since the ancient times the flow of time was a natural measure of man’s movements in the physical space and of movements of the Sun, the Moon, the planets and all the other celestial bodies in the vastness of space. Providing the scientific means of how to reference the ‘where’ and ‘when’ through millennia, geodesy enables to put the ‘who’ or ‘what’ in the spatio-temporal space and to present the answers to ‘why’ and ‘how’ questions. Astronomy, geodesy, surveying and geography along with cartography, are the sciences that naturally combine the knowledge about space and time providing the natural and social sciences a framework for the development of the constantly growing knowledge of humankind. One of the crucial tasks of geodetic science in the modern era is to provide its unified spatial and temporal reference to geoinformation science and its wide area of application fields. Geodesy in the beginning of 21st century is defined essentially by the development of the Global Navigation Satellite System (GNSS), which enables the national geodetic organizations a gradual transition from their specific national terrestrial reference systems to the global terrestrial reference system. This way standardised information on geolocation is becoming a ubiquitous global utility, opening new windows of opportunities for geoinformation science and the humankind.

Keywords: Geolocation, Time, Geodesy, Reference Systems, GNSS.

1 Introduction
This year we remember 250-th anniversary of the John Harrison’s H-4, the famous watch that ultimately resolved the longitude problem, one of the toughest and most intriguing scientific problems of the larger part of human history. The watch, the perfect “Timekeeper for the Longitude” as the inventor called it, was finished in 1759 (NMM, 2009), (Sobel, Armstrong, 2005) and provided with an excellent timekeeping mechanism a purely mechanical solution to the longitude problem.

On the other hand, the most brilliant scientific minds from all over Europe were searching for centuries to provide an accurate and useful solution for measuring the longitude, pursuing their astronomic measurements and mapping the movements of the chosen known celestial bodies in the sky. The year 2009 has yet another important anniversary to remember. As a global celebration of astronomy and its contributions to society and culture and as a promotion of a greater appreciation of the inspirational aspects of astronomy that embody an invaluable shared resource for all nations this year is declared as The International Year of Astronomy 2009 (IYA 2009) and marks the 400th anniversary of the first use of an astronomical telescope by Galileo Galilei and the resulting journey of discovery for humanity (IYA, 2009).

Though the invention of the accurate watch overran astronomers’ scientific efforts and reached the ultimate longitude determinantion goal first 250 years ago, we can see from historic records and from today’s perspective that we need the best of both – time measurements and astronomic measurements – in order to define accurately a global foundation for measuring objects in space and time. In present times, the technology of accurate time measurements has moved from mechanical watches to cesium and rubidium atomic clocks, while in the technology of astronomic measurements the observations of natural celestial bodies retain their fundamental value in providing a unified reference of a modern global terrestrial reference system.
However, since the middle of the 20th century the natural celestial bodies are not the only objects of observation for astronomers and geodesists any more. With the Russian Sputnik I in the year 1957, we entered into the era of artificial satellites, which has evolved in a few decades to such a level, that the satellite techniques have gained an essential and actually indispensable role in global positioning, navigation and timing. The US Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS) system were established as Global Satellite Navigation Systems (GNSS), while the European Galileo and the Chinese BeiDou2-Compass systems are already in their initial operating phases. All these systems are actually based on the same concepts, i.e. on a constellation of Earth orbiting spacecraft emitting signals with precise orbital and time data. Suitable receiver equipment combines the signals from at least four spacecraft yielding the time and the three spatial coordinates.

In a certain way, the humanity is in a similar position nowadays as it was two and a half centuries ago, when H-4 was made. Then the long sought solution of the longitude problem was finally found and put into a mechanical pocket watch to serve the sailors and the public. Today, after a long and winding road filled with innumerable new technological inventions, we have reached a point of development, when we are able to put not only longitude, but also latitude, heighth and time solution all together in a small electronic device to serve positioning, navigation and timing professionals as well as the widest public across the globe. The article outlines in a few glimpses how the geodetic, surveying, positioning and navigation science have come this far and what steps can and should be done in order to allow the humanity to get the best use of knowing the combined geolocation and time data as accurately as possible. Due to the limited space of the paper, the presented contents are obviously selective and without a presumption or attempt of historical completeness.

2 A Short History of Relations between Geolocation and Time
Common sense tells us that spatial aspects of all existence are fundamental. Before an awareness of time, there is an awareness of relations in space. Space seems to be that aspect of existence to which most other things can be analogized or with which they can be equated. The concept of spatial relatedness is a quality without which it is difficult or impossible for the human mind to apprehend anything (Robinson, Petchenik, 1978), (Wilford, 2000). For this reason, a man in the earliest era of development developed a sense of relations between here and there and wanted to communicate these relations to the others. At the same time, a man wanted to acquire them from these others. The easiest way to do it was through maps. Maps constitute a common language used by men of different races and tongues to express the relationship of their society to a geographic environment (Harley and Woodward, 1987).

Maps have changed and developed through history as much as human mind and knowledge did.  The ways used to represent and organize these spatial relationships in a form of map changed and developed as well, from the simplest forms up to present sophisticated digital products. Astronomy, geodesy, surveying and geography along with cartography, are the sciences that enabled this development.

2.1 The Shape and the Size of Earth
Through the ancient times, several ideas and opinions regarding the shape of the Earth were prevailing among scholars, from a slab to a drum- or pillar-shaped world and through an idea of a circular form eventually to a spherical form. This spherical concept as a fundamental idea for later progress apparently had its beginnings among the ancient Greek Pythagorean philosophers in the 6th century BC and elaborated through the works of the philosopher Plato and his followers, including Aristotle (Thrower, 1999). By the time of Aristotle (384-322 BC), the spherical form of earth was generally accepted. The ancient Greeks also applied a system of dividing the circle into 360 equal parts, which they inherited from the Babylonian sexagesimal system (base-60) and their astronomers. Temple records from the city of Uruk in the late fourth millennium BC already include evidence of dividing a year into 12 months of 30 days each, i.e. in 360 days (Robson, 2004). Through observations of the Sun, Moon, stars and the visible planets, they noticed the circular track of the Sun's apparent annual path across the sky and knew that it took about 360 days to complete one year's circuit. Remember, in those times geocentric system was adopted. We have confirmed much later, that actually the Earth is revolving around the Sun and not vice-versa. In order to track each day's passage of the Sun's whole journey they therefore divided the circular path into 360 degrees. Egyptians made a step further around 1500 BC, dividing the day into 24 hours. Originally, the hours varied with the seasons, but Greek astronomers with their systematic approach made later the hours equal. About 300 to 100 BC, the Babylonians subdivided the hour into base-60 fractions: 60 minutes in an hour and 60 seconds in a minute. Based on actually the same roots of the Baylonian base-60 number system we have got a natural connection between measuring time, angles, and geographic coordinates which with slight modifications lives up to present times. The shape of Earth is thus logically inter-connecting the measurement of geolocation and time for thousands of years.

In addition, the first experimental defining of the size of Earth in the third century BC is closely connected with time by an ingenious measurement method of Eratosthenes (276-195 BC), the founder of geodesy (Torge, 2001). In his measurement, Eratosthenes used a known phenomenon at the well in Syene on the river Nile, present Aswan.
There the Sun shone directly into its deep waters at high noon on the longest day of the year, the summer solstice on June 21, while in Alexandria which lied approximately north of Syene, no such event had ever been seen. Instead, in Alexandria the Sun’s rays on the same day formed an angle with the direction of the plumb line. From the length of the shadow of a vertical staff produced in a hemispherical shell, Eratosthenes determined the angle as approximately 1/50 of a complete circle. This angle is equal to the central angle between these two places, as if measured from the centre of a spherical Earth. Eratosthenes applied the then estimated distance of 5000 stadia between Alexandria and Syene to provide the circumference of the Earth as 252000 stadia. Eratosthenes used the stade III = 158.73 m as the unit of length (Lelgemann, 2008). Calculating in metric units the measurements of Eratosthenes give a result of 40000km for the circumference of the Earth and consequently the result of 6365 km for the radius of the Earth ( = 252000/2π = 40100 stadia = 6365 km). This value is differing minimally from the radius 6371 km of the mean spherical Earth as derived with WGS 84 Ellipsoid, which represents the best global geodetic reference system for the Earth available at this time for practical applications of mapping, charting, geopositioning and navigation (NIMA, 2004). The principle that Eratosthenes has used is the measurement of the meridian arc in which he used the same specific moment in time on both ends of the arc to measure the central angle. The principle of meridian arc measurements was used with modifications in geodetic observations up to modern times.

2.2 Latitude, Longitude and Time
Being known at least three centuries BC, the lines of latitude and longitude were by A.D. 150 drawn also in the first world atlas by Ptolemy, the ancient scholar with many scientific interests who’s millennial fame and influence are mostly the result of his two books, one on astronomy and other on geography. In astronomy, he introduced among other things a simple but invaluable concept of subdividing an arc degree in 60 minutes (lat. partes minutae primae) and then further every minute in 60 seconds (lat. partes minutae secundae). Thus, we have minutes and seconds of time and minutes and seconds of arc degrees. In his book Geography Ptolemy defined geography as a “presentation in pictures of the whole known world together with the phenomena which are contained therein”, combining data on geolocation and time. He also defined the task of cartographer, which is “to survey the whole in just proportions” – that is, to draw maps to scale (Wilford, 2000). In his maps, Ptolemy used a grid system of latitude and longitude lines as a reference to geolocate the known items – such as lands, coasts, islands and towns – on a map. Longitude was expressed in fractions of hours while latitudes were designated by the number of hours in the longest day of the year (Brown, 1977). Once again, we come across relating a grid of geolocations on a map to the measures of time.

The Equator as the zero-degree parallel of latitude was already known then and fixed from the laws of nature, i.e. from the observations of the apparent movements of the Sun and other celestial bodies. The astronomers knew from astronomic observations, that during the year the Sun is crossing the celestial equator twice a year on its way between the two extreme lines, where it turned around again on its yearly path. The celestial equator is an imaginary line, dividing the celestial sphere in half, and the Sun’s path intersects this equator on the beginning of spring and autumn, marking the vernal and autumnal equinox. Two imaginary extreme lines are known as tropics, positioned at the latitudes approximately 23.5 degrees north and south of the equator. On the other hand, the zero-degree longitude meridian line has no such natural phenomena to fix it down to the body of Earth. Ptolemy in his time has decided to put its origin at the western edge of the known world through the Fortunate (present Canary) islands in the Atlantic Ocean northwest of Africa. Through the centuries, the longitude line origin has been moved back and forth across the maps almost at the free will of later cartographers, until it finally settled in Greenwich, UK by an international political agreement. Namely, only in 1884 a conference in Washington of 25 world nations agreed that Greenwich would be the world's Prime Meridian of longitude, world time and time zones.

2.3 From the Spherical to the Ellipsoidal Earth
With latitude and longitude the principles of mapping the world were organized using a simple geometric proposition, that the intersection of two lines is a point (Brown, 1977), i.e. to geolocate a point on the Earth, we need to know the lines of its latitude and longitude. However simple this task may seem in principle at the first thought, its realisation in order to make an accurate map of any terrain presents an enormous effort and demands going into the field and actually measure and survey it. One of the crucial and fundamental goals in this process is to define those reference lines, to establish them physically in the field as a series of base lines from which later surveys can be made.

Since the Ptolemy’s times, during the dark middle ages up to the middle of the second millennium the question of the figure of the Earth has stood still. Then the arc measurements, based still on a spherical model of the Earth but benefiting from the advances in instrumentation technology and methodology, were pursued further in several European countries, mainly in France, Holland, Denmark, England and Italy. In that period of scientific renaissance new ideas from astronomy and physics have influenced the development of geodesy and fundamentally changed the view on the Earth and its position in space. Nicolaus Copernicus (1473-1543) formulated a scientifically based heliocentric cosmology that displaced the Earth from the center of the universe. Johannes Kepler (1571-1630) introduced the eponymous laws of planetary motion, while Galileo Galilei (1564-1642) opened a new era of astronomic observations with his improvements of the telescope and established the fundamental laws of falling bodies and of pendulum motion.

In the second half of 17th century, astronomic observations revealed the flattening of the poles of Jupiter and pendulum time measurements confirmed the effect of increase of gravity from the Earth equator towards the poles. Sir Isaac Newton (1643-1727) combined these observations and his theoretical work on gravitation and hydrostatics in his famous book Philosophiae Naturalis Principia Mathematica, published in 1687. In the book, Newton proposed as a model of the Earth a rotational ellipsoid, flattened at its poles by 1/230 because of the Earth’s rotation. Several geodetic arc measurements at various latitudes were performed in the next century to verify and possibly confirm Newton’s theory. The results of geodetic measurements financed by the French Academy of Sciences finally confirmed that the flattening of the Earth exists and is large enough to be measured.

Related to these arc measurements, there is a particularly significant year 1799 to remember. Namely, 210 years ago the French National Assembly specified, constructed and deposited the platinum metre bar, on 22 June 1799, in the Archives de la République in Paris, as the final standard of length defined as 1 / 10,000,000 of the meridional distance from the North Pole to the Equator. In order to establish the length of the meridian as the universally accepted foundation for the definition of the metre as the natural unit of length, the Bureau des Longitudes commissioned an expedition led by two astronomers and geodesists, Jean Baptiste Joseph Delambre and Pierre Méchain (Alder, 2004). Between the years 1792 and 1799, they measured the length of the meridian arc through Paris between Dunkerque and Barcelona as the basis for calculating the length of the half meridian, connecting the North Pole with the Equator.

With this new definition of the unit of length, France introduced the metric system. The creation of the decimal metric system at the time of the French Revolution and the subsequent deposition of two platinum standards in 1799 representing the metre and the kilogram was the first step in the development of the present International System of Units. Almost one century later, after agreeing upon a definition for the meter at the diplomatic Conference of the Metre and signing the Metre Convention in Paris in 1875, a more stable international prototype of platinum-iridium was realized and sanctioned in 1889 by the 1st General Conference on Weights and Measures. This original international prototype is still kept at the International Bureau of Weights and Measures (BIPM - Bureau International des Poids et Mesures) under conditions specified in 1889 (BIPM, 2006).

2.4 Transition to the Geoid and the Third Dimension in Geodesy
With the development of geodetic instrumentation and methodology in the early 19th century it was soon realized that in pursuing the measurements of high accuracy level the nature of the Earth is complicated more than an ellipsoidal model of the Earth can handle, therefore scientists attempted to define the figure of the Earth more precisely. In 1832, Carl Friedrich Gauss (1777-1855) strongly promoted the application of the metric system, together with the second defined in astronomy, as a coherent system of units for the physical sciences.
 
Gauss was the first to make absolute measurements of the Earth’s magnetic field in terms of a decimal system based on the three mechanical units millimetre, gram, and second for, respectively, the quantities length, mass, and time (BIPM, 2006). Gauss developed his theory of surfaces (Moritz, Hofmann-Wellenhof, 1993) and introduced the geoid as the “mathematical figure of the Earth”, defined as a level surface of the gravity field. The geoid deviates from a well-fitting ellipsoid by less than 100 meters (Hofmann-Wellenhof, Moritz, 2006). The efforts of geodesists in the 19th and in the early 20th century have concentrated on the measurements of large triangulation chains in order to provide the parameters of ellipsoids, fitting best to the geoid in the areas of measurements. Based on such geodetic measurements, which have often demanded enormous human and scientific efforts of geodesists and their crews (Keay, 2000), several referential ellipsoids were introduced to support the triangulation of the national geodetic surveys, which provide control points for national geodetic reference systems, mapping, positioning, etc. up to present time.

Within these national geodetic surveys, the geodesists observed and evaluated horizontal and vertical control networks separately, because heights were calculated regarding to the mean sea level as a surface close to the geoid, defined by long-term observations at a tide gauge.

Further development of the geodetic methodology demanded a common mathematical model for calculations of horizontal and vertical networks. After the first ideas on such three-dimensional concept of geodesy in the second half of the 19th century, the concept revived after the Second World War, especially with the invention of the electromagnetic distance measurements in the 1950’s and with the first artificial satellite Sputnik I in 1957. Geodetic observations to orbiting satellites started providing data for determination of control point geolocation in three-dimensional system (Fischer, 2005) and led to the development of satellite geodesy.

Also in the second half of the 20th century, a new essential space geodetic method and technique of Very Long Baseline Interferometry (VLBI) was developed for measuring a large selection of quasars, which has lead to the present definition of the celestial reference frame as a realization of a highly accurate and stable inertial reference system. Space geodesy developed its techniques in the last decades of 20th century through several characteristic periods based on measurement method or technique. The periods overlap and begin with the optical period, followed by Doppler, Satellite and Lunar Laser Ranging (SLR, LLR), VLBI, Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), altimetry, SAR, InSAR, gravity and present GNSS period (Beutler, 2004).

In the 1980’s, the USA started to establish the NAVSTAR Global Positioning System (GPS) and since the 1990’s this system became fundamental in the geodetic measurement techniques worldwide. This way, using GPS and other satellite techniques, the geometry of the Earth can be determined to a great level independently of the gravitational field and global reference system is established (NIMA, 2004). This development allows geodesy to concentrate on solving practical problems of transformation of the existent horizontal and vertical networks of control points to the global reference system and of establishing high quality relations between the geoid and the global reference ellipsoid.

2.5 Fourth Dimension in Geodesy
The development of geodetic methodology has made its first steps into four-dimensional geodesy at the end of 19th and in the beginning of 20th century with the detection of earth polar motion and the observations of the earth tides as well as with measuring crustal motions and deformations due to earthquakes and postglacial rebound. Then in the second half of the previous century, space geodetic methods like laser ranging and VLBI techniques have developed and are being applied to support the scientific field of geodynamics (Fischetti, 1982), which includes the study of the interior structure and composition of the earth, its crustal motions and deformations, the rotational dynamics, and the terrestrial potential fields. Thus, the concept of the figure of the Earth has definitely widened from three-dimensional rigidity to four-dimensional time-dependence (Fischer, 1975), (Fischer, 2005).
Technology developments of the last decades especially in the field of laser technology, signal processing, atomic clocks, time transfer, IT developments, etc., were supporting the development of space geodetic techniques and the remarkable progress in their accuracy. This development brings us to the definition of geodesy as a science of measurement and presentation of earth's surface and its external gravitational field, including its temporal changes. Modern geodesy is based on three pillars (Torge, 2001), (Torge, 1996), (Rummel et al., 2002):
• Geometrical shape of the Earth as a function of time,
• Orientation of Earth in space as a function of time and
• Earth gravitational field as a function of time.

The four-dimensional aspect of geodesy allows geodesists to employ improved modelling of their observations in evaluation and presentation of the slow long-term changes of the Earth and its gravity field (Lambeck, 1988). This way a firm scientific foundation is set for other natural and social sciences, which can use it as a skeleton for the development of their geospatial-temporal fields of work in order to provide access to a common digital model of temporally geolocated information on the Earth. The already achieved high accuracy and the anticipated further progress open new fields for research and multidisciplinary applications in the 21st century.

3 Geodesy and its Scientific Field in the Beginning of 21st Century
Geodesy was the science and profession, which widely opened an insight into the secrets of mathematics, geometry and trigonometry to geodesists and navigators in the beginning of 17th century with the manuals written in English language. Until then these secrets were wrapped in Latin and Greek writings, which were accessible to only a very narrow group of scholars (Linklater, 2002). The primary task of geodesists in the past periods of development of geodesy and society was at the basic level of measuring the size and shape of the Earth and its gravitational field. Using geodetic application technologies the geodesists were able to measure, monitor, supervise and register the data on geolocation of objects in the agreed reference systems of countries, regions, continents or the world, with the most accurate mutual mathematical relations established.

For centuries, the role of geodesy was in production of plans and maps, therefore the majority of public still understands this as its main goal. In present time, the above mentioned (see 2.5) three pillars are equally important in spite of the fact that a large part of information provided by geodesy is still mainly in the domain of geolocating. Geolocating as a subject of this paper belongs to the first above mentioned pillar of geodesy; it is that task of geodesy, which is best understood by most of the people. Geolocating is absolute determination of coordinates on land, at sea or in space, in relation to terrestrial reference system. With a group of points, an entire space can be discretely described; therefore the term of geolocation of an individual point is used. The problem of geolocating is defining point coordinates, if known coordinates of measured extra-terrestrial objects like stars or satellites exist, and if quantities, connecting terrestrial point with these objects are measured.

Geolocation is defined in a reference system, which is only an agreement, a definition of mathematical, physical and geodetic rules and constants that define how to write a certain geolocation in the form of algebraic numbers, i.e. with coordinates in a reference system. In order to make a reference system usable in practice, a realization (materialization) of the reference system is unavoidable. For a materialized reference system – with physical or virtual objects having defined geolocation with coordinates of a chosen reference system – the conception of the geoinformation infrastructure is also used. A reference system from a practical point of view is all the space, defined with geolocation coordinates.

Ensuring a further improvement of geolocating quality and a simplification of the geolocating procedures to the users will be in the focus of development of geodetic science and profession, aiming at the expanding geolocation usage in various fields. This basis has to be accessible on a global level in a simple way equally to all relevant scientific disciplines and to the entire global community in its widest spectre of daily life applications, where geolocation and timing are relevant. The bases for the three-dimensional temporally dependent geolocating on a global, regional or national level are the geodetic reference systems and frames. For this reason, the basic scientific geodetic research will be aimed at the definition and realization of the global and regional reference systems and frames, as well as at the development of the analysis and methods of processing geodetic observations. Various techniques of terrestrial, airborne and satellite measurements will continue to be an object of research regarding their accuracy and reliability, their advantages and weaknesses as well as their development potentials. A large part of the research in the field of geodesy has to be devoted right with the daily life applications in mind and with a strong emphasis to the significance of geolocation and timing for them.

3.1 Base Quantities and Units
Due to the importance of a set of well defined and easily accessible units universally agreed for the multitude of measurements that support today’s complex society, units should be chosen so that they are readily available to all, are constant throughout time and space, and are easy to realize with high accuracy. The International System of Units (SI) defines seven base quantities – length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity – and their corresponding base units. Among them are length, mass and time, which are used as base quantities also in geodesy. The definitions of the base units of the SI were adopted in a context that takes no account of relativistic effects. These units are known as proper units (BIPM, 2006).
Although all the base quantities are by convention regarded as independent, their respective base units are in a number of instances interdependent. For example, the definition of the metre incorporates the second. 

3.2 Reference Systems
For the purpose of this paper, two groups of reference systems are essential: celestial reference system for locating celestial positions in space and terrestrial reference system for geolocating points on Earth.

The basis for realization of these reference systems is the movement of Earth and satellites. Earth itself has two periodic movements, that are important in this context – it is moving around the Sun in the ecliptic plane and it is rotating around its polar axis.
Equatorial plane is perpendicular to the Earth’s axis of rotation. The intersections of the both planes with the imaginary celestial sphere define the ecliptic and celestial equator. The vernal equinox, where the Sun transits from the southern to the northern hemisphere, is the intersection of the equator and the ecliptic. It also defines the direction of the X-axis of the celestial reference system. The axis of rotation is the Z-axis and Y-axis lies perpendicular to both in the equatorial plane. Terrestrial reference systems are tied to the Earth and so they rotate and move around the Sun along with the Earth. Celestial reference systems do not move around the Sun, but they can rotate with the same angular speed as the Earth does. Among the celestial system is also a group of reference systems of satellite orbits, which do not rotate with the Earth, but they move around the Sun along with the Earth (Vani?ek and Krakiwsky, 1986).

Celestial reference systems are in use in different forms for millennia. In modern times, the most precise realization of the celestial reference system is based on extra-galactical radio sources. Until the advent of highly precise space geodesy techniques in the 1960s and 1970s there was no need to take into account the deviations from Newtonian physics in the solar system. Since then the theory of relativity had to become considered in defining celestial reference systems. In 1991, the International Astronomical Union (IAU) adopted the conceptual definitions within the framework of General Relativity for a system of space-time coordinates of the Barycentric Celestial Reference System (BCRS) with its origin at the solar system barycenter and of the Geocentric Celestial Reference System (GCRS) with its origin at the geocenter. The IAU 24th General Assembly defined both systems of space-time coordinates with the resolution in the year 2000 (IAU, 2000), (Soffel et al., 2003).

Since 1997, the International Celestial Reference System (ICRS) is adopted as the idealized barycentric coordinate system to which celestial positions are referred. The axes of the ICRS are defined by the adopted positions of a specific set of extragalactic objects, which are assumed to have no measurable proper motions. It is kinematically non-rotating with respect to the ensemble of distant extragalactic objects.
ICRS is in practise realized by the International Celestial Reference Frame (ICRF) (Figure 13), which is a set of extragalactic objects whose adopted positions and uncertainties realize the ICRS axes and give the uncertainties of the axes (IERS, 2006). ICRF is also the name of the radio catalog whose 212 defining sources are currently the most accurate realization of the ICRS (NFA, 2007).

For solving practical problems of geosciences, we need reference systems, which are tied with the body of the Earth, i.e. terrestrial reference systems of the highest achievable quality. The International Union of Geodesy and Geophysics (IUGG) adopted in 1991 at the Vienna IUGG General Assembly the Conventional Terrestrial Reference System (CTRS) for analysis of data from different measurement techniques or for combination of solutions of individual techniques, e.g. the already mentioned VLBI, SLR, LLR, GPS and DORIS.

The Perugia IUGG General Assembly in 2007 endorsed the definitions of a Geocentric Terrestrial Reference System (GTRS) and of the International Terrestrial Reference System (ITRS) as the specific GTRS. GTRS is a system of geocentric space-time coordinates within the framework of General Relativity, co-rotating with the Earth, and related to the GCRS by a spatial rotation, which takes into account the Earth orientation parameters. It replaces the previously defined Conventional Terrestrial Reference System (NFA, 2007). In addition, IUGG adopted the ITRS as the preferred GTRS for scientific and technical applications and urged other communities, such as the geospatial information and navigation communities, to do the same (IUGG, 2007).

Realizations of the ITRS system are the responsibility of the International Earth Rotation and Reference Systems Service (IERS). The ITRS is the recommended system to express positions on the Earth and is realized in the form of International Terrestrial Reference Frames (ITRF) by a set of instantaneous coordinates (and velocities) of reference points - mainly space geodetic stations and related markers - distributed on the topographic surface of the Earth. Currently the ITRF provides a model for estimating, to high accuracy, the instantaneous positions of these points.
A system that has to be mentioned here is the World Geodetic System 1984 (WGS84), which is used in the GPS measurements. The latest realization of the WGS 84 Reference Frame implemented since January 2002 is designated as WGS 84 (G1150) and compared to ITRF2000 shows a RMS difference of one centimeter per component, which is significantly smaller than previous solutions. Precise geodetic applications must account for temporal effects, such as plate tectonic motion and tidal effects (NIMA, 2004). Further improvements and future realizations of the WGS 84 Reference Frame are anticipated.

Very important for Europe is the European Terrestrial Reference System 1989 (ETRS89), which is the European reference system, used in practice as the EUREF reference frame, based on the actual European terrestrial reference frames ETRF-YY. The ETRF frame for a selected year YY is a part of a wider ITRF-YY, comprised of the European reference points of the ITRF frame. ETRS89 is used in Europe as a horizontal reference system, while the height reference system is the European Vertical Reference System (EVRS). Systems ETRS89 and EVRS together form the European Spatial Reference System (ESRS), which is a stable system of homogeneous properties for all geodetic, geophysical, geodynamical and other purposes.

The rules of terrestrial reference systems are transferred by numerical-graphical representation from the real into the virtual world. Using the advanced development of computer graphic technologies and digital mobile telecommunications it is possible to present the data and information about the real world or from the real world in the virtual world of three-dimensional digital graphics varying with time. Combining of geolocated data and information of airborne or satellite image remote sensing, digital elevation model, GNSS, LiDAR, InSAR and other sensors, it is possible to represent such a virtual world as an approximation of the real world. The quality of this approximation depends mostly from the quality and precision of the used sensor data and the quality of algorithms, i.e. geodetic methodology for their combined use. It is characteristical that all these measurements are based on emitting and/or receiving various characteristic electromagnetic signals and determining either their time of travel, signal intensity or spectral characteristics. One of the basic goals of such a virtual world representation is a Digital Earth model capable of showing the timelined geolocated knowledge of our planet as it is at present, as it was in the past and as it is modelled to be in the future (Gore, 1998), (Crampton, 2008), (Grossner, 2008).

3.3 Time Systems and Scales
Throughout history the concept of time has been refined. New discoveries and technological development allow new understanding and gradually leads to the introduction of a new time scale. A very precise definition of the unit of time and the rules of time measurement are indispensable for science and technology. This is especially true for geodesy where most of the measurement methods apply electromagnetic waves and their signal travel time in order to calculate geolocations and a uniform time scale is needed for modelling artificial satellites' motion.  Measurement of time is based on a specific periodic natural phenomenon, which defines a time system with a description of the phenomenon, its origin and its advancement interval rate. Time systems are divided in two basic different groups, based either on the SI system second or on the rotation of the Earth. For the purpose of this paper only a very short informative description of both types of systems follows.

In the history all time definitions were based on the rotation of the Earth, because it provided a natural measure of time. However, the rotation of the Earth as a basis of time is variable with long and short periodic variations and containing irregularities. Therefore the rotation of the Earth is continuously monitored, in the last decades primarily by VLBI measurements. To establish relations between Earth-based and inertial space based systems the adjacent time systems had to be introduced – the sidereal and solar (universal) time. In everyday life solar time is used for practical reasons. The second as the unit of time is considered to be the fraction 1/86 400 (1 day =24 x 60 x 60 seconds) of the mean solar day, which is based on the diurnal rotation of the Earth and defined by the interval between two apparent transits of the sun through the meridian. Mean solar time is termed Universal Time (UT), if referred to the Greenwich mean astronomical meridian.  UT now almost always refers to the specific time scale UT1, which is by the IAU definition (IAU, 2000) strictly proportional to the Earth Rotation Angle (ERA) around the moving rotation axis.

The SI second is independent of Earth rotation and provides a constant and precise measure of time and came into use for time definitions and measurements in the second half of the 20th  century, following the introduction of the first atomic clocks in the 1950s. In 1972 based on SI second International Atomic Time TAI (Temps Atomique International) was officially introduced by BIPM as a uniform commonly used time scale of high accuracy for practical applications. TAI is realized by a large number of atomic clocks and has always been a statistical combination of the atomic time TA(k) data provided by a large number of operating  atomic clocks available from the participating labs around the globe. TAI as a uniform time scale does not keep in step with the irregular rotation of the Earth.

For practical purposes in the worldwide system of civil time, another uniform scale has been defined (UTC - Coordinated Universal Time), which is a hybrid time scale using the SI second on the geoid as its fundamental unit and differing from TAI by an integer number of seconds. This means that TAI and UTC have the same unit, the SI second. To avoid the uniform scale diverging indefinitely from that of the Earth's rotation, a positive leap second is introduced every few years in UTC whenever necessary by international agreement, so that it is kept tightly synchronised within 0.9 secons of UT1. The choice of the dates and the announcement of the leap seconds is under the responsibility of the IERS (BIPM, 2009b).

Also, relations of UTC and TAI time with GPS and GLONASS time are reported monthly by BIPM (BIPM, 2009a). The C0 values provide a realization of GPS time as provided by the Paris Observatory and C1 values provide a realization of GLONASS time as provided by the Astrogeodynamical Observatory Borowiec.

3.4 GNSS – Global Navigation Satellite Systems
In the last two decades, intensive preparations and procedures are taking place in the national geodetic organizations of the developed countries for a gradual transition from their national reference systems to the global terrestrial reference system. This development was possible with satellite geodesy and its constant improvements. Simultaneous measurement of the coherent microwave signals (Figure 16) emitted by several satellites and recorded by receivers on the Earth’s surface quickly evolved into the most used and best known space geodetic technique, providing instantaneous positioning and revolutionizing geodesy, surveying, navigation and timing.
Geodesy in the beginning of 21st century is thus defined essentially by the development of the Global Navigation Satellite System (GNSS). We are witnessing the present renewal of the American Global Positioning System (GPS), accelerated employment in the last few years of the Russian Global Navigation Satellite System (GLONASS) and the successful introduction of the European system for satellite navigation GALILEO. Additionally, China is joining this club with the development of its own Compass-BeiDou2 satellite navigation system, while Japan and India are developing their augmented satellite systems MSAS and GAGAN, functioning as the regional equivalents of the European EGNOS system. The current high pace development allows the realization and usage of terrestrial reference system through GNSS implementation, which has a great impact on a wide range of other sciences and on global society as a whole.

Scientific mission of geodesy in the beginning of 21st century is its contribution to the establishment (realization) of GNSS as the means for realization of a unified terrestrial reference system for geolocating data and information, taking into account the time component. Societal mission of modern geodesy is to establish conditions to achieve the widest possible use of global navigation satellite systems in the geospatial components of all areas of our daily lives, so that the global terrestrial coordinate system becomes a unified universal basis for geolocating. Geodesy will achieve both missions by establishing conditions for usage of GNSS in combination with other geodetic application technologies, geoinformation tools and digital mobile communication technologies.

GNSS as a system is not intended only for geodetic use. On the contrary, geodesists are developing and establishing it in cooperation with other geosciences and technologically advanced industries with a much broader goal, i.e. to provide data about static and dynamical geolocation and to ensure a wide geoinformation basis to the widest field of users in the private, business and public sector.

Of special interest are the interdisciplinary aspects of geodesy and its data in order to exploit the advantages of using the advanced geodetic knowledge in the field of geolocating to support scientific, environmental, economical and social activities of humankind. Rapid development of electronics, computers and space technologies in the last half of the century provided geodesy new efficient tools with a great impact to the meaning, precision, reliability, amount and renewal cycle of spatial data. Global geodetic community seized the opportunity to establish a global terrestrial reference system. To provide quality, precise and reliable use of the rapidly growing amount of global spatial data from various sources, geodesy has to use a dynamic and interdisciplinary approach now to empower it as a basis of precise spatial geoinformation infrastructure (Kouba et al., 2000).

3.5 Combined Geodetic Methodology
For determination, monitoring and registration of geolocation changes the geodetic methodology is applied. Individual fields of geolocation applications are characterized by different levels of change dynamics, which demand from geodesy an adapted quality approach for every field in order to define geolocation as a function of time.

Geodetic methodology is defined with the necessary quality of individual demands of the application. For example, the range of necessary position precision is 0.1 mm in deformation monitoring of built environment or a few centimetres in registering land property borders to several metres or more in navigation and location based services (LBS). On the other hand, geolocation of objects or states as a function of time changes with different rates in individual relevant application fields. For example, in monitoring geodynamic phenomena the geolocation change rate is measured in mm/year, geolocation change rates in built environments can reach cm/month, while in LBS the geolocation change rate can reach several tens of metres/second.

To the entire variety of relevant application fields geodesy has to provide appropriate geodetic space-time framework that is fulfilling the requirements of the standard quality model for spatial data regarding the overall quality elements of purpose, usage and source and quantitative quality elements of completeness, logical consistency, positional accuracy, temporal accuracy and thematic accuracy. In order to succeed in this enormous task, one of the most important issues for geodesy is a widespread adoption of International Standards inside the geodetic/surveying profession and bussines as well as in the relations with the wide user community of the relevant application fields (Greenway, 2005), (Greenway, 2006). It is important to understand, that the adoption of geodata standards is closely connected with the software standards (Delphi Group, 2003).

Location-aware technologies (LAT), including the GNSS and radiolocation methods, backed up by telecommunication systems of mobile networks, enable measurements of the basic entities and their relations in spatial and temporal resolutions, which were almost unimaginable some decades ago. The procedure of geolocation change registration in space requires a synthesis of usage of GNSS in combination with other geodetic application technologies. Among them are technologies like INS - Inertial Navigation Systems, terrestrial geodesy, aerial and satellite photogrammetry, remote sensing imaging techniques like LiDAR - Light Detection And Ranging, InSAR - Interferometric Synthetic Aperture Radar. Lately, new technologies like Wi-Fi – Wireless LAN Positioning, UWB – Ultra Wide Band Indoor Positioning, RFID – Radio-Frequency Identification and other technologies are used in combination with geodetic application technologies. All these and other technologies supply data to a variety of geoinformation management, analysis and presentation systems, using digital and mobile communication techniques, like GSM - Global System for Mobile communication, GPRS - General Packet Radio Service, UMTS - Universal Mobile Telecommunications System, TCP/IP - Transmission Control Protocol/Internet Protocol, etc.. A concept of sensor web, measuring dynamic geospatial, spectral, and temporal characteristics using a new intelligent data collection system paradigm, is already evolving (Habib, Talabac, 2004), especially in remote sensing applications supported by underlying communications fabric facilitating the exchange of sensor measurement data and results.

4 Conclusion
Geolocation and timing are basic information. Acquiring, maintaining, servicing and representing spatial data on geolocation as a function of time and with the different levels of quality, is the working field of geodetic science and profession with a vast area and variety of possibilities for modernization and improvement. Every scientific effort to increase systematic development in this field is an important contribution to science. It will lead to a more optimal usage of the data, provided by geodetic science, profession and service in all those segments of society, where geolocation and timing information is relevant. With the analysis of geolocation significance as a function of time in describing the real and virtual world, geodesy and the geodetic methods of geolocating have to provide the bases for the the entire range of individual geospatial and timing application fields. Geodesy is constantly advancing the methods and techniques of geolocating with new scientific knowledge and technological achievements. Using them, geodesy is upgrading the existent and introducing new algorithms, which are increasingly sophisticated on the inside, so that on the outside they can be more and more simple and reliable to the users, while their results are of required quality and instantly accessible using the means of the most modern information communication technologies. In a very simplified manner, it could be said that geodesy intentionally has to complicate its »life« on the inside with the principle goal to ease it to the greatest possible extent to the rest of the world and thus provide the best possible service to the global community.

Today, at the beginning of 21st century, geodesy is the science, which knows how to incorporate its achievements into most advanced technologies in order to allow their widest availability and usage. This way geodesy enters into various daily life fields of work and living of an individual and of the society as a whole. Geodesy makes these steps modestly, almost imperceptible to the wider public, but on the other side with growing reliability and efficiency.

The relevant geolocation and timing application fields are among the key segments of a functioning society, economy and individual. Therefore, it is necessary that they are lead safely, reliably, efficiently, with accuracy and great care for the environment. Safety, security and protection of people, environment and property have a constantly rising significance in the modern world. For this reason, the applicability of the GNSS and geodetic application technologies in these fields has to be constantly developing. In this context there is an especially large and to a great extent still unexploited potential of multi-sensor applications, like in the application fields of transport, engineering and construction, car navigation, personal navigation, navigation inside the buildings’ spaces, logical and geometrical topology in the building information space, etc. To exploit this potential better for a mutual benefit, the international geodetic community needs to perform a systematic analysis of the requirements of individual relevant application fields with regard to applying GNSS and geodetic application technologies as well as the analysis of advantages of their usage in individual relevant application fields for the society, economy and the individual. The international geodetic community can reach this goal only in tight cooperation with all the interested parties. The space and time for this cooperation is now!

Joc Triglav ( jtriglav@geoinformatics.com)  is a professional surveyor and editor of  GeoInformatics. In the last 20 years, he published more than 300 articles in various professional and technical magazines, mostly in the fields of geoinformatics and geodesy. Geoinformation science and applications determine the entire author’s professional life, while geodesy, cartography and geography fuel his enthusiasm and imagination since the early childhood.

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