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Some common problems in geodesy and astrometry after establishing ICRF
Victor V. Popadyov, Centre de geodesie et cartographie (CNIIGAiK) Svetlana A. Tolchel'nikova, GAO Pulkovo


Distribution of duties between astrometry and geodesy
· Connection between the 2 sciences was established by the plumb-line near Earth surface · Astrometry: Earth's axis in space (known, need only more precise determination) · IERS (formerly International Latitude Service): determination of polar motion (,)(,)
­ Pole was common for terrestrial and celestial frames difficulties in division of Earth's axis motion about CF and TS


Distribution of duties between astrometry and geodesy
· Geodesy
­ The main directions:
· Normals to ellipsoid (don't cross in his centre) · Plumb-lines (don't cross in centre of masses)

­ The main tasks:
· Establishing of terrestrial system connected with geocentre (geometrical method) · Earth gravitational field (physycal method)


Former astrogeodetic connection
Astrogeodetic and gravimetric measurements allowed to establish ellipsoid for (,)(B,L) (semimajor axe ae and flatness 1/ of ellipsoid need for precise satellites orbit modeling)
· "normal field" in geodesy ­ ellipsoidal, but in celestial mechanics ­ spherical

­ Astrogravimetric leveling allowed to transfer (for reduction to ellipsoid)


Reference ellipsoid mean/main Earth ellipsoid


Revolution in astrometry
· "Nothing will remain the same as it was before" [Walter and Sovers, Astrometry of fundamental catalogues. The evolution from optical to radio reference frames]


Revolution in astronomy
The arguments
­ Increased of accuracy of radioastrometrical measurements ­ Declared absence of proper motions ­ The low value of atmospheric refraction, especially with VLBI

Problems
­ New system is not connected with ecliptic and equator (vernal point) ­ Unsolved question with radioobservations of Soon and Solar system bodies

Paradox
­ more easily to connect between (quasi)inertial reference system with TRF, that with one celestial [Robertson 1981]


Problem of origin
· Transitions from barycentre ICRS to the Earth center is difficult because nobody can observe from the centers of Earth and Sun · For this transition we need 4 models (E-M orb. motion, M. rot., E. rot., pole motion). Transition from triad connected with barycenter to other triad performs with relativistic effects. · This corrections follow from impossibility of making measurements from origins
­ For rotating motion studies is required a triad X,Y,Z and point, center of masses (unobservable, but computed by relativistic hypothesis, periodically updated and complicated) ­ Problem: quantity of parameters to determine grow up, but quantity of independent equations is too small ­ Position of centre of CF is indeterminate, accuracy of arc measurements will 0,000 001" (!)
· star observations: place origin in any point of Earth · quasars observations: place origin in any point of terrestrial orbit


Establishing of radiosystem
· Question of coincidence of optical- and radiosources
­ RA = 01m 32s ­ DEC = ­1532

· Question on the radiorefraction See Lipovka A., Lipovka N. On the transition to the radio system coordinates ICRF


Geodesy problems
· Old problems (astronomy is no more needed):
­ Determination of LAT and LONG (time) (but is desirable periodically control) ­ Projection of geod. measures on ref.-ellipsoid for further adjustment

· Rests 2+1 problem:
­ Astroorientation systems for photosurvey ­ Astronomical azimuth (for ballistics) ? ­ Rotations of plumb-line (, ) - can be solved by only geod. and grav. meas., but more complicated... Radiosources are unobservable from points of geodetic net.


New reality


Conclusion
· In postrevolutionary situation the original empirical relation between the three branches of the same science is disturbed
­ Astronomy ­ Geodesy ­ Gravimetry

· Pulkovo! · It is necessary to save the any empirical connection between them