Satellite observation

The observation of artificial earth satellites is mainly used to precisely determine their orbits around the earth. This orbit determination is the prerequisite for using the satellites in fields of knowledge such as earth observation (remote sensing), geography and biology , satellite navigation and satellite geodesy , material science and many others. The observation is mainly carried out by terrestrial satellite stations , but in very different ways.

Measurements on board satellites, which are used for remote sensing of the earth's surface or for astronomical measurements of celestial bodies, are sometimes referred to as satellite observation .

Observation methods based on principle

The measurements can be made by geometric or physical methods:

through angle measurement with telescopes, with cameras or through interferometry
by measuring distances with laser beams or radio waves
by measuring speeds , mostly using the Doppler effect
by analyzing orbit disturbances , which are calculated from the above methods
by measuring gravity gradients and accelerations in the satellite itself.

Methods according to observed wavelength

The observation methods are often classified according to the wavelength of the measured signal.

Observation in the range of optical wavelengths

It mainly takes place in visible light , which also corresponds to the order of technical development. Only isolated measurements are made in the near UV or infrared , for which optical satellite telescopes are also suitable.

Optical direction or angle measurement with powerful and quickly swiveling measuring telescopes. These are - similar to a theodolite - equipped with partial circles for the direction and the elevation angle . Between about 1955 and 1965, a. developed the Kinetheodolite and telescopes such as the Moonwatch telescope . The time measurement, which is necessary to at least 0.01 seconds, is carried out either through automatic registration or manually with precise stopwatches and time signal transmitters . The instruments are aimed at pre-calculated positions in the sky and then follow the satellite track in front of the stars. Methods are being developed that will also enable daytime observations .

Visually by observing the star background , which requires a telescope with a large field of view . All you need is a simple pair of binoculars , supplemented by an electronic stopwatch and a suitable star atlas .

Photographically through a special satellite camera . As a ballistic camera , it is permanently set up during the measurement. As an astrograph , it is tracked to the stars (see equatorial mount ), or to the satellite itself on a three-axis mount. a. developed the Baker-Nunn and the Hewitt satellite camera, in whose control computer the current orbit elements are to be entered.
Distance measurement using active laser telescopes . They send a highly focused, monochromatic beam to the satellite, which reflects it. The received photons trigger the measurement of a time interval counter that was started when the laser beam was emitted. This transit time measurement basically determines the current distance to the satellite (i.e. after subtracting some corrective variables) .
- The reflected laser beam is also occasionally photographed in front of the starry sky.

Observation by radio waves

Here, too, the measurement methods are diverse. The oldest is the radio direction finding , with which the preliminary orbits of the first earth satellites were determined - as soon as possible after the satellite launch . More precise procedures are:

Direction measurement using radio wave interferometry , for example in the US Minitrack network (now closed)
Direction measurement by means of rapidly rotating radio telescopes . These have antennas in the form of metallic parabolic mirrors, which gives them a useful directional characteristic . The measurement accuracy is, however, much lower than with the optical methods (see above). In addition, radio telescopes are only suitable for measuring higher (slower) missiles, but they are of great importance for determining the orbit of space probes to other planets or to comets .

Occasionally, cylindrical mirrors are also used, which are similar to the radar antennas used in air traffic control . The radio telescopes of both types cannot be larger than about 50 meters because they have to be able to move quickly due to the high angular speed of the satellites (up to about 2 ° per second). In general, in addition to the direction, you can also measure the exact distance to the spacecraft:

Distance measurement using the transit time of the radio signal. As a two-way measurement, twice the transit time (there and back) of the signal answered by a transponder is determined. With the one-way method, however, the time system on the ground and in the satellite must be exactly synchronized .

Pseudoranging , as used in GPS and the other GNSS systems. Codes and time stamps are stamped on the radio signals , which enables high-precision runtime measurement with synchronized clocks in the satellite and on the ground station. The small residual error will ...

Velocity measurement using the Doppler effect , see e.g. B. the former transit NNSS system of the USA or the modern, worldwide DORIS network of France.

Distance difference through integration of the Doppler shift - see also hyperbolic method

Observation with other methods

These include above all

the satellite Alimetrie , a kind of radar altimetry to sea ,

the methods of satellite gradiometry , in which irregularities of the earth's gravity field at the level of the satellite orbit are determined and analyzed,

the satellite-based acceleration measurement with accelerometers - based on a similar principle as the inertial navigation of aircraft, and

the Satellite-to-Satellite Tracking .

Technical literature and web links

Kurt Arnold : Methods of Satellite Geodesy , Chap. 5 observation methods . Akademie-Verlag, Berlin 1970
Megan Donahue , Jeffrey Bennett et al .: Astronomy. The cosmic perspective , chapter S1 (measurement method) and chap. 6 (telescopes). Ed. Harald Lesch , 5th, updated edition 2010. Pearson Studium Verlag, Munich, ISBN 978-3-8273-7360-1 .
Günter Seeber : Satellite Geodesy: Foundation, Methods, and Application. De Gruyter, Berlin 1993, ISBN 3-11-012753-9 ; 2nd edition 2003, ISBN 3-11-017549-5 .
Meyers Handbuch Weltall, guide through the world of astronomy . 1994 (7th, revised edition), ISBN 3-411-07757-3 .
W. Steiner, M. Schagerl: The orbits of artificial earth satellites . In space flight mechanics: dynamics and control , Springer-Verlag 2004, pp. 145-162
S. Kern, D. Flocco et al .: Satellite observation and numerical modeling of coastal polynes in the Antarctic (on the 2nd topic: remote sensing), Meteorologentagung DACH 1st volume, COSIS pdf abstract 2007