INTERNATIONAL SPACE SCIENCE INSTITUTE No 10, June 2003 Published by the Association Pro ISSI Satellite Navigation Systems for Earth and Space Sciences Editorial It was in 1915 when the German Global satellite navigation systems Impressum scientist Alfred Wegener pub- are based on a constellation of lished the “The Origin of Con- Earth orbiting spacecraft emitting tinents and Oceans” outlining the signals with precise orbital and concept of continental drift for time data. Suitable receiver equip- SPATIUM the first time. According to this ment combines the signals from at Published bythe theory, all the continents on Earth least four spacecraft yielding the Association Pro ISSI had formed from one single mass time and the three space coordi- twice a year about 300 million years ago,called nates. The US Global Positioning Pangaea. Later on, Pangaea had System (GPS) for instance consists split, and its pieces had been mov- of 24 spacecraft orbiting the Earth INTERNATIONAL SPACE ing away from each other ever in six different planes. The Russ- SCIENCE since. Where such continental ian Glonass system as well as the INSTITUTE plates collide, mountain ranges planned European Galileo system Association Pro ISSI may be pushed up, like for exam- are based on the same concepts. Hallerstrasse 6,CH-3012 Bern ple the Alps that are the collision Phone +41 (0)31 631 48 96 product of the African and the Of course, the GPS originally was Fax +41 (0)31 631 48 97 Eurasian plate. not conceived as a scientific tool but much more as a sophisticated President Initially, Wegener’s theory was military infrastructure for the ful- Prof.Heinrich Leutwyler, strongly contested not least be- filment of navigational tasks. It University of Bern cause he did not provide convinc- was only after its commissioning Publisher ing explanations for the mechan- that its scientific value became ap- Dr.Hansjörg Schlaepfer, ics allowing for the movement of parent. The continental drift ve- legenda schläpfer wort & bild, the continents through the oceans locities measurement is just one of Winkel (as it was seen at that time). Today a number of fascinating scientific Layout we know that below the continen- applications of global navigation Marcel Künzi,marketing · kom- tal plates there is a hot liquid zone, systems to which the present issue munikation, CH-8483 Kollbrunn the asthenosphere, on which the of Spatium is devoted. We are Printing continents f loat. The plates are greatly indebted to Prof. Gerhard Druckerei Peter + Co dpc separated by ridges, where fresh Beutler, Astronomical Institute, CH-8037 Zurich magma spills out from the as- University of Berne for his kind thenosphere providing the thrust permission to publish herewith a to the continental plates moving revised version of his exciting them apart thereby. lecture of November 12, 2002 for the members of our association. In the meantime such different sci- entific branches as geology, palae- ontology, botany and zoology have provided ample proof in support Hansjörg Schlaepfer of Wegener’s theory. Even space Zürich, June 2003 technology has contributed by the direct measurement of the conti- nental drift.As an example,the drift Front Cover: The Galileo naviga- velocity between America and Eu- tion satellite in an artist’s view rope has been determined to be in (credit: European Space Agency the order of 2.5 centimetres/year. ESA) SPATIUM 10 2 Satellite Navigation Systems for Earth and Space Sciences *) Gerhard Beutler, Astronomical Institute, University of Bern Navigation and Figure 1 also shows that relative rived either from the Sun [local local and absolute positioning was solar time] or from the stars [side- Science in the performed with the same instru- rial time]) between the unknown ments, the so-called cross-staffs, in site and Greenwich. The problem Past and Today Apian’s days. Global positioning resided in the realisation of Green- meant the determination of the wich time at the observing site in observer’s geographical latitude and the pre-telecommunication era. The introduction to Peter Apian’s longitude (relative to an arbitrarily One astronomical solution to this Geographia from 1533 in Figure 1 selected site – first Paris, then problem, illustrated in Figure 1, nicely illustrates that positioning in Greenwich was used for this pur- consisted of measuring the so- the “good old times” in essence pose). The latitude of an observ- called lunar distances (angles be- meant measuring angles –the scale ing site was easily established by tween bright stars and the Moon). was eventually introduced by one determining the elevation (at the With increasing accuracy of the known distance between two sites observer’s location) of the Earth’s (prediction of the) lunar orbit the (as indicated by the symbolic rotation axis, approximately repre- angular distances between the measurement rod in the centre of sented by the polar star. In princi- Moon and the stars could be accu- the wood-cut). ple, longitude determination was rately predicted and tabulated in Figure 1 simple, as well: One merely had to astronomical and nautical al- Geographia by Peter Apian,dated 1533. determine the time difference (de- manacs with Greenwich local th *) Pro ISSI lecture, Bern,12 November 2002 SPATIUM 10 3 time as argument. Lunar distances were used for centuries for precise positioning. For navigation on sea the method became eventually ob- solete with the development of marine chronometers, which were capable of transporting accurately Greenwich time in vessels over time spans of weeks. Figure 2 shows the first chronometer developed by the ingenious British watch- maker J.Harrison (1693–1776). The principles of precise global po- sitioning and precise navigation remained in essence the same from Apian’s times till well into the second half of the 20th century. The development of the accuracy was dramatic: The cross-staff was replaced by increasingly more so- phisticated optical telescopes. More precise star catalogues (fun- damental catalogues) were pro- duced and the art of predicting the motion of planets was developed in analytical celestial mechanics.A long list of eminent astronomers, mathematicians, and physicists, from L.Euler (1707–1783), P.S. de Laplace (1749–1827), to S. New- comb (1835–1909) were steadily improving the ephemerides.High- ly precise pendulum clocks and marine chronometers allowed it eventually to time-tag the obser- vations in the millisecond accura- cyrange. The relationship between science on one hand and precise position- ing and navigation on the other hand were truly remarkable: The discipline of fundamental astronomy emerged from this interaction be- tween theory and application. In Figure 2 Harrison I, first marine Chronometer (credit: D. Sobel und W.J.H. Andrewes: fundamental astronomy one de- Längengrad, Berlin Velag 1999). fines and realises the global terres- SPATIUM 10 4 trial and the celestial reference sys- tems including the transformation The Earth’s rotation axis in inertial space between the systems. The terres- pole of ecliptic instantaneous trial system was realised by the pole of rotation geographical coordinates of a net- work of astronomical observato- ries. Until quite recently the celes- tial reference system was realised through fundamental catalogues of stars. Celestial mechanics, so to speak the fine art of accurately de- scribing the motion of celestial 23.5º bodies in the celestial reference systems, is also part of fundamen- tal astronomy. Earth’s axis The establishment of the transfor- mation between the two systems implies the monitoring of Earth rotation in inertial space. Figure 3 illustrates that the rotation axis of the Earth moves in inertial Sun space. Earth’s orbit It is well known that the rotation Earth’s orbital plane axis approximatelymoves on a ° straight cone inclined by 23.5 Figure 3 w. r.t. the pole of the ecliptic, an ef- Precession and Nutation. fect known as precession, which was already discovered in the Greek era (and usually attributed to the great Greek astronomer Year Discoverer Effect Hipparchos). This motion is not fully regular but shows short-peri- 300 B.C Hipparchos Precession in longitude (50.4”/y) od variations, which is why the as- tronomers make the distinction 1728 A.D. J. Bradley Nutation (18.6 years period, amplitudes between precession and nutation. of 17.2” and 9.2” in ecliptical longitude A study of ancient solar eclipses and obliquity, respectively) revealed eventually that the length 1765 A.D. L. Euler Prediction of polar motion of day was slowly (by about 2 msec (with a period 300 days) per century) growing. The Earth 1798 A.D. P.S. Laplace Deceleration of Earth rotation axis also moves on the Earth’s sur- (length of day) face, an effect known as polar mo- 1891 A.D. S.C. Chandler Polar motion, Chandler period of 430 days tion.This and other discoveries re- and Annual Period lated to Earth rotation made in the era of optical astronomy are sum- Table 1 marised in Table 1. Discoveries related to Earth rotation in the optical era of fundamental astronomy SPATIUM 10 5 The Global Posi- vantages over optical navigation intersecting three spheres with were considerable: the observa- known centres and radii. (One tioning System tions were weather-independent may remember from school that in and available in digital form from general two solutions emerge (GPS) the outset. The system worked re- from the intersection process, but markably well. Using the observa- one easily recognises that one solu- tions of one pass of one satellite tion – lying far above the Earth’s over a receiver (duration about surface –may be ruled out).As one Sputnik-I, launched on October 4 5–10 minutes), the (two-dimen- wishes to use rather cheap oscilla- of the International Geophysical sional) position of the observer tors in the receivers,one cannot as- Year 1957, did broadcast a (rela- could be established with an accu- sume that the receiver clock error tively) stable radio-frequency f.