Geometric satellite geodesy

The term geometric satellite geodesy is understood to mean those methods of satellite geodesy in which an earth satellite is used for purely geometric measurements, but its orbit (except for the precalculation of visibility) is irrelevant.

Methods

The term came up around 1960 when it became clear how much the emerging satellite geodesy would shape the future of geodesy . Its counter-term is dynamic satellite geodesy , while some special features of both groups of methods work together in the combined methods .

In the purely geometric method, the satellite only serves as a high target for direction or distance measurements from terrestrial satellite stations , while its orbital movement is theoretically disregarded. However, it is technically important because it makes observations by satellite cameras , LASER and other measuring instruments more difficult.

The most important measurement methods are:

• Optical direction measurement
  • visually (especially at the beginning and for difficult objects), see Moonwatch
  • photographic - see satellite photogrammetry
  • since around 1980 also optoelectronic , and since 1995 increasingly with CCD sensors
• Radio direction measurements
  • Bearing or radar
  • Interferometry (e.g. previous Minitrack monitoring system )
• Distance measurement
  • with LASER - see Satellite Laser Ranging (SLR)
  • with radar or other microwaves ( e.g. SECOR , PRARE )
• Height measurement over the sea or ice surfaces, see satellite altimetry
• partly also speed measurement (from distance measurements such as SST and according to the Doppler principle , the latter representing the transition to combined methods)
• further measurement methods from remote sensing (geometric measurements made by the satellite itself); in addition to visible light also in the infrared and radio range; see also PRARE , SAR radar.

Even some non-geometric measurement methods are geometrically available, for example in the field of remote sensing , from the satellite dynamics , the gradiometry and astronomy .

In order to calculate the visibility of an earth satellite in advance, you need to know your own location and a set of at least approximate orbit elements . The calculation is done by spherical trigonometry or by vector calculation . For optical direction observations, it is also necessary to consider whether and when the satellite enters the earth's shadow . A graphical solution to these tasks is also possible, for example with the so-called satellite slide (stereographic projection of the earth and the satellite orbit ).

For simultaneous methods (e.g. laser or satellite triangulation ), the simultaneous visibility of the planned two or more satellite stations must be checked, which is done using separate program modules.

historical development

While until about 1975 a clear distinction was made between geometric and physically dynamic satellite processes, it has been possible to solve very complex calculation models with tens of thousands of parameters for several decades. In addition to the orbital elements and their changes, these include the coefficients of the gravitational field and the earth models used , the exact coordinates of all observation stations and other parameters such as the slow movements of continental plates .

The first significant combination of geometric and dynamic models was the NNSS system of satellite navigation . With precise measurements of the Doppler effect on its 5–6 Doppler satellites , online accuracies of around 30 meters were possible from 1970, while large-scale surveying networks already achieved decimeter accuracies offline. Since the beginning of the 1990s, the combination methods have gained significantly in importance due to the development of GPS and GLONASS , so that today there is hardly any distinction between geometric and physical-dynamic satellite geodesy.

See also

• Cosmic geodesy , stellar triangulation
• Astronomical refraction , satellite refraction
• geodetic satellite , balloon satellite ,
• Ephemeris calculation , orbit determination , altimetry
• Pseudoranging , star coverage ,

literature

• Karl Ledersteger : The use of artificial satellites for geodesy. ÖZV, Baden 1961
• Kurt Arnold : Satellite Geodesy , Fachbuchverlag Berlin ~ 1965
• Rudolf Sigl , E. Groten: Dynamic Satellite Geodesy - An Overview. DGK Series A, Volume 49, Munich 1966
• Günter Seeber : Satellite Geodesy , ~ 1975 and 2000