«Univ.-Prof. Dr.-Ing. Prof. h.c. Günter Seeber anlässlich seines 65. Geburtstages und der Verabschiedung in den Ruhestand. Wissenschaftliche ...»
Univ.-Prof. Dr.-Ing. Prof. h.c. Günter Seeber anlässlich seines 65. Geburtstages
und der Verabschiedung in den Ruhestand. Wissenschaftliche Arbeiten der Fachrichtung
Geodäsie und Geoinformatik der Universität Hannover Nr. 258: 81-99, 2006
Status of Geodetic Astronomy at the Beginning of the 21st Century
Christian Hirt1 and Beat Bürki2
Institut für Erdmessung, University of Hannover, Germany
Geodesy and Geodynamics Lab, ETH Zurich, Switzerland
At the beginning of the 21st century, a significant technological change took place in geodetic astronomy. The use of digital imaging sensors strongly improved the degree of automation, efficiency and accuracy of methods for the observation of the direction of the plumb line and its vertical deflection. This paper outlines the transition of astrogeodetic techniques and applications from the analogue to the digital era and addresses instrumental developments and recently completed projects. Particular attention is given to Digital Zenith Camera Systems representing astrogeodetic state-of-the-art instrumentation. Moreover, accuracy issues, present application examples for highly-precise astrogeodetic gravity field determinations and some future applications are described.
1 Introduction – Review of Astrogeodetic Tasks and Methods The most important objective of geodetic astronomy is the determination of astronomical longitude Λ and latitude Φ at selected points on the Earth’s surface by measuring directions to celestial bodies, primarily stars. Astronomical longitude Λ and latitude Φ represent the orientation of the local gravity vector g in space and hence the direction of the local plumb
line. The local gravity vector g is completed by the gravity acceleration g (Torge 2001):
cos Φ cos Λ g = – g cos Φ sin Λ . (1) sin Φ A second objective is to determine astronomical azimuths A of terrestrial points by combining direction measurements to terrestrial and celestial targets.
Geodetic astronomy is the only discipline which provides methods for the direct observation of the direction of the plumb line. Until the middle of the last century, exclusively astrogeodetic methods allowed the absolute determination of longitude and latitude related to the global terrestrial coordinate system. Essential early applications were navigation at sea, positioning (e.g. on expeditions), determination of Earth dimensions (historical arc measurements), orientation of geodetic networks or reference ellipsoids, determination of geoid profiles using the method of astronomical leveling and the observation of Earth orientation parameters.
With the evolution of satellite-based positioning techniques, particularly the U.S. Global Positioning System GPS, an essential part of traditional astrogeodetic tasks was taken over (details are given e.g. by Seeber 2003 and Beutler 2006 in this volume). Satellite-based positioning and navigation – available at all times, everywhere and independent of weather conditions – established itself as flexible and easily applicable universal geodetic technique.
Christian Hirt, Beat Bürki Optical astrogeodetic methods were applied for determining Earth orientation parameters (pole coordinates and time) from 1900-1990. Up to the 1950’s, observations using (optical) zenith telescopes of the International Latitude Service (ILS) and Bureau International de l’Heure (BIH) were visually performed. Subsequently, stationary Photographic Zenith Tubes (PZT) automated the optical observation procedure. The PZTs operated within the framework of the International Polar Motion Service (IPMS). Later modern space geodetic techniques like Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Lunar Laser Ranging (LLR) and GPS were combined for monitoring Earth rotation by the International Earth Rotation and Reference Systems Service (IERS). The space geodetic techniques replaced the optical astrometric methods by the end of the 1980’s.
While disadvantageously for pure positioning purposes, the direction of the plumb line and its vertical deflection provides information on the structure of Earth’s gravity field – as seen from a physical point of view. For several decades, astrogeodetic observations were primary gravity field observables and used for astrogeodetic geoid determinations (e.g. Helmert 1913 in the Harz mountains, Northern Germany; Heitz 1968 in Germany; Bomford in Europe (1971) – just to mention some of the classical works). Observations were performed visually; therefore requiring not only dedicated instruments (DKM3A, T4, Astrolabes) but also well-trained and skilled observers. Major improvements of these observation techniques could be achieved since the 1970’s when transportable photographic zenith cameras (named TZK1,2,3, see Figure 1-3) were successfully designed and constructed at the University of Hannover (cf.
Gessler and Pilowski 1972, Gessler 1975, Seeber 1978, Wissel 1982, Torge and Seeber 1985, Wildermann 1988 and Bürki 1989). Similar developments took place in Italy (Birardi 1976) and Austria (Chesi 1984). Thanks to a fully automated registration of exposure epochs and level readings, this new instrumental type overcame the problem of personally influenced results (personal equation effects) to a large extent. Moreover, compared to classical visual techniques, the zenith cameras allowed a significantly accelerated and simplified observation procedure. Such automated instruments were extensively applied in European and American countries (Switzerland, Austria, Germany, Denmark, Italy, Spain, Portugal, Greece, France, Norway, Canada, Brazil, Venezuela) for local and regional geoid determination and geophysical applications up to the nineties of the 20th century.
Figure 1-3: Transportable Zenith Cameras TZK1 (1976), TZK2 (2000) (both cameras operated by the Institut für Erdmessung, University of Hannover) and the twin model of TZK2, called TZK3 (1986), operated by the Geodesy and Geodynamics Laboratory of ETH Zurich.
Compared with current geodetic standards, the main drawback of the (analogue) photographic technique was the acquisition of star coordinates which was performed manually or semiautomated using a comparator (Figure 4, see also Wissel and Seeber 1979). The processing of a single station usually lasted 3-5 hours thus requiring man-power and keeping the costs of astrogeodetic measurements at a relatively high level. In addition, due to the increasing number of available gravity observations and satellite data, gravimetric computation techniques advanced with the result that more gravity field models based on gravimetric techniques became available (e.g. Denker 1988, Geiger 1990). As a consequence, the significance of astrogeodetic methods for the determination of the Earth gravity field decreased considerably in the 1990’s.
2 Digital Era of Geodetic Astronomy, Today’s Instrumentation
The invention of extremely light-sensitive digital imaging sensors (CCD) which are able to replace the analogue photographic techniques has revolutionized observational astronomy in general and astrometry in particular (e.g. Smith 1976, Buil 1991, Janesick 2001). Also the field of geodetic astronomy did not remain unaffected by these developments. CCD-sensors used for digital imaging of stars basically provide the image data directly after observation which is indispensable for online-processing. This way the conceptual disadvantage of photographic observations – the chemical development of the photo and the need for manual evaluation by means of a comparator – could be completely overcome so that the efficiency of astrogeodetic methods increases significantly.
First developments at the beginning of the digital era were published in the field of direction measurements to artificial satellites more than one decade ago (Schildknecht 1994, Ploner 1996). In consequence of this new technique, concepts for the automated observation of the plumb line were presented by research groups in Munich (cf. Eissfeller und Hein 1994, Schödlbauer 2000) and Vienna (Gerstbach 1996, Bretterbauer 1997). First instrumental approaches were published by e.g. Fosu (1999), however, without progressing to fieldcapability yet. This requires robust implementation of hardware and electronic components, the development of suited processing software and proper calibration procedures.
At the Institut für Erdmessung (IfE), University of Hannover and the Geodesy and Geodynamics Laboratory (GGL), ETH Zurich, intensive research in astrogeodetic use of CCDtechnology has started at the beginning of the 21st century. The existing photographic zenith Christian Hirt, Beat Bürki cameras TZK2 and TZK3 were re-designed and equipped with CCD imaging sensors to Digital Zenith Camera Systems (DZCS). The digital development, which was initiated by Prof. Günter Seeber at the Institut für Erdmessung, is called TZK2-D (Transportable Zenitkamera 2 – Digitalsystem), whereas the DZCS implemented at ETH Zurich is named Digital Astronomical Deflection Measuring System (DIADEM). Both systems are shown in Figure 5.
The results already obtained in the development phase have been encouraging as they clearly indicated the potential of automated astrogeodetic methods for local gravity field studies (Hirt 2001, Hirt and Seeber 2002, Hirt and Bürki 2002). In 2003, hardware and software details of both DZCSs were completed and full field-capability was achieved for the first time. The performance of this new generation of astrogeodetic instruments for the determination of vertical deflections could be successfully demonstrated during a first common field campaign in the frame of the “European Combined Geodetic Network (ECGN)” which took place in Switzerland in autumn 2003 (Müller et al. 2004, Brockmann et al. 2004, Hirt 2004). In the sequel, both systems have been extensively used for local and regional gravity field determinations in Switzerland, Greece, Portugal, Northern Germany, Bavaria and the Netherlands at more than 450 new field stations (for details see section 5).
Figure 5: Digital Zenith Camera Systems TZK2-D (IfE) and DIADEM (GGL), depicted on the left side, during parallel observations at the Geostation Zimmerwald in Switzerland, autumn 2003.
Today, in the digital era of geodetic astronomy, the determination of vertical deflections using the digital zenith camera systems DIADEM and TZK2-D requires just “one mouse click”.
This is due to the completely automated observation process – including leveling, exposure, data acquisition and data transfer – and processing. The observation of a single vertical deflection takes about half a minute. Under normal conditions about 50 single observations are carried out at field stations requiring a total of 20 min observation time. Depending on the accessibility of the station, station spacing and duration of darkness, about 5 to 20 vertical deflection stations can be observed during one night. If compared to the analogue era, the accuracy of astrogeodetic observations remarkably improved from about 0.4-0.5” to 0.05-0.1” Status of Geodetic Astronomy at the Beginning of the 21st Century (cf. section 4). In comparison with modern satellite positioning, the new astrogeodetic observation technique has achieved a similar degree of automation, ease and real-time capability as such complying with today’s geodetic standards. For technical details on the Swiss observation system DIADEM, the reader is referred to Bürki et al. (2004) and Müller et al. (2004). Details on the realization of the system TZK2-D are given in the PhD thesis of Hirt (2004), which includes a comprehensive description of instrumental calibration procedures and algorithms used for automated data processing.
System ICARUS for on-line deflection and azimuth measurements
At the GGL, ETH Zurich, a theodolite measurement system called ICARUS has been developed which allows an almost fully automated determination of astronomical azimuths A and the direction of the plumb line (Φ,Λ). The equipment consists of a motorized total station TCA 1800 by Leica Geosystems, a small GPS receiver for timing and geodetic positioning, a field computer, and the online-processing software ICARUS/AZIMUTH. Once the orientation has been realized by pointing to a star (in most cases to α Ursae Minoris/Polaris) the system moves its telescope automatically towards the terrestrial target, and to the chosen star, respectively. The observer’s only task is to center the telescope towards the star, and once to the target. The related accuracy of ICARUS azimuth observations and vertical deflection measurements is about 0.5”. Recent projects focus on the use of CCD sensors instead of the human eye in order to minimize the impact of the personal equation and to enhance the efficiency.
3 Observation Principle, Status of Astrogeodetic Observables and Related Techniques The simple astrogeodetic observation principle remained – in essence – unchanged throughout the decades: The direction of the plumb line (Φ,Λ) is obtained by means of direction measurements to celestial objects, primarily stars, whose equatorial coordinates right ascension α and declination δ are given in the International Celestial Reference System ICRS.
The astronomical coordinates longitude Λ and latitude Φ define the spatial direction of the plumb line with respect to the International Terrestrial Reference System ITRS (see Figure 6).
ITRS and ICRS are linked by Greenwich Sidereal Time GAST being a measure for Earth’s rotation angle. Astrogeodetic methods use the equivalence of astronomical coordinates (plumb
line Φ,Λ) and equatorial coordinates (zenith direction α,δ) for a star exactly located in zenith:
Φ=δ Λ = α – GAST. (2) Since a corresponding star is usually not exactly transiting through zenith, the star field surrounding the zenith is imaged on a CCD and used for interpolation. Vertical deflections (ξ, η) are easily obtained by calculating the difference between the local plumb line and the
camera’s geodetic coordinates (ϕ,λ) to be determined with GPS (see Figure 7):
ξ=Φ–ϕ η = (Λ – λ) cos ϕ. (3) A closer look onto the observation principle and processing chain leads to an extended observation equation which schematically shows the quantities needed for data processing.