1 / 27

# 3.1. Coordinate-systems and time. Seeber 2.1. - PowerPoint PPT Presentation

3.1. Coordinate-systems and time. Seeber 2.1. Z. NON INERTIAL SYSTEM. Mean-rotationaxis 1900. Gravity-centre. Y- Rotates with the Earth. CTS : Conventional Terrestrial System. Greenwich. X. CIS.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about '3.1. Coordinate-systems and time. Seeber 2.1.' - rasha

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

Z

NON INERTIAL SYSTEM

Mean-rotationaxis

1900.

Gravity-centre

Y- Rotates with

the Earth

CTS:

Conventional Terrestrial System

Greenwich

X

• Zero-meridian for Bureau Internationale de l’ Heure (BHI) determined so that star-catalogues agree in the mean with observations from astronomical observatories.

• The connection to an Inertial System is determined using knowledge of the Z-axís (Polar motion), rotational velocity and the movement of the Earth Center.

• We obtain an Quasi-Inertial system, CIS.

• More correct to use the Sun or the centre of our galaxe !

Kap. 3 POLAR MOTION

• Approximatively circular

• Period 430 days (Chandler period)

• Main reason: Axis of Inertia does not co-inside with axis of rotation.

• Rigid Earth: 305 days: Euler-period.

Ch. 3 POLBEVÆGELSEN

• .

Kap. 3 POLAR MOVEMENT

• Coordinates for the Polen and Rotational velocity

• IERS (http://www.iers.org)

• International Earth Rotation and Reference System service (IAG + IAU)

• http://aiuws.unibe.ch/code/erp_pp.gif

• Metods:

LLR (Laser ranging to the Moon)

SLR (Satellite Laser ranging)

GPS, DORIS

• Polbevægelse, 1994-1997, Fuld linie : middel pol bevægelse, 1900-1996

Kap. 3. International Terrestrial Reference System (ITRS)

• Defined, realised and controlled by IERS ITRS Center. http://www.iers.org/iers/products/itrs/

• Geocentric, mass-centre from total Earth inclusive oceans and atmosphere.

• IERS Reference Pole (IRP) and Reference Meridian (IRM) konsist with BIH directions within +/- 0.005".

• Time-wise change of the orientations secured through 0-rotation-condition taking into account horizontal tectonic movements for the whole Earth.

• ITRS realised from estimate of coordinates for set of station with observations of VLBI, LLR, GPS, SLR, and DORIS. See: ftp://lareg.ensg.ign.fr/pub/itrf/old/itrf92.ssc

• Paris, 1 July 2003 Bulletin C 26

• INFORMATION ON UTC - TAI

• NO positive leap second will be introduced at the end of December 2003.

• The difference between UTC and the International Atomic Time TAI is :

• from 1999 January 1, 0h UTC, until further notice : UTC-TAI = -32 s

• Leap seconds can be introduced in UTC at the end of the months of December or June, depending on the evolution of UT1-TAI. Bulletin C is mailed every six months, either to announce a time step in UTC, or to confirm that there will be no time step at the next possible date.

• http://www.iers.org/iers/products/eop/leap_second.html

• Precession

• Nutation

• Rotation+

• Polar movement

Sun+Moon

• Example: t-t0=0.01 (2001-01-01)

• .

• Movement takes place in Ecliptica

• Suppose θ=0, xp=yp =1” (30 m)

• .

2 May 1994:

x”=0.1843”=0.000000893,

y”=0.3309”=0.0000014651

(x,y,z)=(3513648.63m,778953.56m,5248202.81m)

Compute changes to coordinates.

• 1 cm at Equator is 2*10-5 s in rotation

• 1 cm in satellite movement is 10-6 s

• 1 cm in distance measurement is 3*10-11 s

• We must measure better than these quantities.

• Not absolute, but time-differences.

• Siderial time: Hour-angle of vernal equinox in relationship to the observing instrument

• LAST: Local apparent siderial time: true hour angle

• GAST: LAST for Greenwich

• LMST: Local hour angle of mean equinox

• GMST: LMST for Greenwich

• GMST-GAST=Δψcosε

• LMST-GMST=LAST-GAST=Λ

xp

• UT= 12 hours + Greenwich hourangle for the mean sun. Follows siderial time.

• 1 mean siderial day = 1 mean solar day -3m55.909s.

• UT0B is time at observation point B, must be referred to conventional pole

• UT1= UT0B + ΔΛP

• ET: Ephemeis time (1952) to make equatins of motion OK.

• TDB= Barycentric time – refers to the Sun

• TDT=Terrestrial time

• From general relativity: clock at the earth moving around the sun varies 0.0016 s due to change in potential of sun (Earth does not move with constant velocity).

• TDB=ET on 1984-01-01

• GPS time = UTC 1980-01-05

• Determined from Clocks in GPS satellites

• GPS time – UTC = n * s-C0,

• With GPS we count cycles. Expect the fequency to be constant.

• Precision quarts crystal: temperature dependent, aging

• Rubidium: good stability, long term

• Cesium: stable both on short term and long term – transportable, commercially available.

• Hydrogen masers: 10-15 stability in periods of 102 to 105 s.

• Pulsars: period e.g. 1.6 ms.