Lecture 5 - Modern Reference Systems Flashcards
What is a reference system?
A reference system is the complete conceptual definition of how a coordinate system is formed.
Introduced in order to model observations as a function of unknown parameters of interest
How can reference systems be defined?
Defines the origin, the orientation of fundamental planes or axes of the system.
Includes the underlying fundamental mathematical and physical models.
Theoretical concept accompanied by practical realization (reference frames)
What are 3 types of reference coordinate systems? Distinguish between them.
Terrestrial coordinate systems: earth-fixed – spin and revolve with Earth
Celestial coordinate system: do not spin, but may revolve around the sun with the same velocity as the Earth does.
Orbital coordinate systems: do not spin with the Earth, but revolve with it
What is an inertial reference frame?
Space-fixed frame (celestial reference frame) represents an approximation to inertial reference system. Describes time and space homogeneously, isotopically and in a time-independent manner. All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not accelerating (in the sense of proper acceleration that would be detected by an accelerometer)
What is ICRS?
International Celestial Reference System
Fixed with respect to distant matter of universe on grounds that these sources are so far away that their expected proper motions are negligibly small.
What is ICRF?
International Celestial Reference Frame
Dynamical: the celestial bodies move in such a way that equations of motion have no kinematic acceleration. Dynamical reference frames are defined by the dynamics of the solar system. These reference frames have time-dependent orientation
Kinematic: there exists in the universe a class of objects which have no global systematic motions and therefore are not rotating in the mean.
Only stars with well determined proper motions are used for the ICRS realization
A new set of standard algorithms were adopted to transfer data into observable quantities. These include precession-nutation model, new definition of the celestial pole, and new reference point for measuring the rotation angle of the Earth around its instantaneous axis
What are ITRS and its definition?
International Terrestrial Reference System.
Fixing terrestrial system wrt static locations.
Similar to CRS, need to define a terrestrial reference system that is kinematical in character.
Astronomical vision: study translational and rotational motion of the Earth in the inertial space.
Earth science: build physical models of the earth system and its various components.
What is the Earth’s wobble effect?
An annual oscillation forced by the seasonal displacement of air and water amasses
What is ITRF?
International Terrestrial Reference Frame.
What are other effects when it comes to observe the stars?
Gravitational bending, gravitational lensing, lense-thirring (dragging of reference frame)
What is dynamical ICRS?
Assuming a certain number of celestial bodies, their motion must be represented in the frame of classical mechanics by a solution of a system of differential equations written in a fixed triad.
The ideal CRS is based on the form that equations of motion take in a non-rotating, non-accelerating frame of reference which is imposed on the actual motion of celestial bodies.
What is kinematical ICRS?
Assuming that the visible universe does not rotate, so that very distant objects in the universe do not have a group motion that could be interpreted as a rotation
The ideal CRS defined geometrically is essentially based on the kinematic behaviour of very distant objects assumed to be random
What is IERS?
International Earth Rotation and Reference Systems Services.
