Lernkarten

Roland Schenkel
Karten 101 Karten
Lernende 12 Lernende
Sprache English
Stufe Universität
Erstellt / Aktualisiert 17.01.2012 / 23.01.2021
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0 Exakte Antworten 89 Text Antworten 12 Multiple Choice Antworten
Fenster schliessen

Which parameter can, which cannot be determined by VLBI?

Radioi Sources (ICRF)

Nutation

Polar Motion

UT1

Length of Day

Fenster schliessen

Which parameter can, which cannot be determined by VLBI?

High-Frequency Earth Rotation Parameters

Ocean tide amplitudes

Coordinates and Velocities (ITRF)

Geocenter

Coefficients of Gravity Field

Fenster schliessen

Which parameter can, which cannot be determined by VLBI?

Orbit Parameters

Orbit Determination for Low Earth Orbiter

Ionosphere

Troposphere

Clocks (time transfer)

Fenster schliessen

Which parameter can, which cannot be determined by GNSS?

Radio Sources (ICRF)

Nutation

Polar Motion

UT1

Length of Day

Fenster schliessen

Which parameter can, which cannot be determined by GNSS?

High Frequency Earth Rotation Parameters

Ocean tide amplitudes

Coordinates and Velocities (ITRF)

Geocenter

Coefficients of Gravity Field

Fenster schliessen

Which parameter can, which cannot be determined by GNSS?

Orbit Parameters

Orbit Determination of Low Earth Orbiter

Ionosphere

Troposphere

Clocks (time transfer)

Fenster schliessen

Which parameter can, which cannot be determined by SLR?

Radio Sources (ICRF)

Nutation

Polar Motion

UT1

Length of Day

Fenster schliessen

Which parameter can, which cannot be determined by SLR?

High Frequency Earth Rotation Parameters

Ocean tide Amplitudes

Coordinates and Velocities (ITRF)

Geocenter

Coefficients of Gravity Field

Fenster schliessen

Which parameter can, which cannot be determined by SLR?

Orbit Parameters

Orbit Determination of Low Earth Orbits

Ionosphere

Troposphere

Clocks (time transfer)

Fenster schliessen

Which parameter can, which cannot be determined by LLR?

Radio Soruces (ICRF)

Nutation

Polar Motion

UT1

Length of Day

Fenster schliessen

Which parameter can, which cannot be determined by LLR?

High Frequency Earth Rotation Parameters

Ocean tide Amplitudes

Coordinates and Velocities (ITRF)

Goecenter

Coefficients of Gravity Field

Fenster schliessen

Which parameter can, which cannot be determined by LLR?

Orbit Parameters

Orbit Determination of Low Earth Orbiters

Ionosphere

Troposphere

Clocks (time transfer)

Fenster schliessen

How can the high angular resolution of VLBI be explained?

angular resolution = lambda / D, Here D is the diameter of the telescope. Because the same signal is measured in more than one telescope it’s possible to take the distance between these telescopes as D an not only the real diameter.

Fenster schliessen

Which instruments/sensors are required for a VLBI station?

large telescope, receiver, high-precision frequency standard, recording device

Fenster schliessen

Which are the most important error sources for the VLBI of today?

structure of the radio sources (quasars don’t appear as point sources), antenna structure (deformation, thermal expansion), troposphere (water vapor)

Fenster schliessen

Which relativitic effects have to be considered in VLBI analyses (withhout formulas)?

a)“Clock error” (see formula 2.21): The stronger the gravity field is, the more slowly the clocks are running and moving clocks are running more slowly than clocks at rest // b) Signal goes across a Gravity Field (eg. Earth, Sun) (see formula 2.26): a signal is slowed down when propagating through a gravitational field (only relevant when the signal passes very closely to a gravity body)

Fenster schliessen

Principle of the correlation of the two signals from the two telescopes

crosscorelation: shift one data series to the other until the best match is found.

Signal to Noise Ratio very bad => long observation (up to 10’)

Fenster schliessen

What do you see? (Fig. 2.3)

Map of known and used (ICRF) Quasars

Fenster schliessen

Wie funktioniert VLBI?

Minddestens zwei Radioteleskope messen die Signale, welche von eine Quasar ausgesendet werden. Dabei werden die Daten mit einem Zeitstempel versehen um sie danach korrelieren zu können. So können danach die Laufzeitunterschiede vom Quasar zum einen oder anderen Teleskop ermittelt werden.

Daraus können Positionen abgeleitet werden.

Fenster schliessen

How big are Quasars?

Quasars are cores of galaxies. Seen from Earth they have often a complex structure and an extension of up to 60''.

Fenster schliessen

Where in de Observation Equation of VLBI do you find the coordinates of the Quasars?

In the unit vector (e) poiting to the Quasars (barrycentric coordinate system)

Fenster schliessen

What is a barycentric intertial system?

Static coordinate sysstem with origin in the barycenter of the solar-system (or Earth-Moon-System)

Fenster schliessen

Transformation from barycentric to earth fixed system? Parameters?

Lorentz-Transformation, Speed is the only parameter

Fenster schliessen

What is that strange PNUXY?

Transformation from Earth-fixed to space-fixed, P: Precession, N: Nutation, U: Rotation matrix with Greenwich sidereal time as argument, X,Y: Polar motion

Fenster schliessen

Formular 2.21, discuss, what speed is v? For GPS?

U = newtonian gravitationl potential at the location of the clock // va = velocity of the atomic clock // c = speed of light // delta_tau_geo = purely geometrical (or newtonian) delay difference between the arrival times of a wavefront at both telescopes. v ist die Geschwindigkeit der Atomuhr verglichen mit dem Massenzentrum des Systems. ca. 4km/s (Speed of a GPS-Satellite with respect to the Earth) ca. 29km/s (Speed of Earth with respect to the sun)

Fenster schliessen

Charactericits of the GPS satellite constellation?

a: 26'600km, Periode: 11h58, I: 55°, 6 Ebenen mit 60° spacing, 24 Satelliten, CDMA (PRN-Codes)

Fenster schliessen

Structure of the GPS signals?

L1: f1 = 1575 MHz, _ = 19cm, C/A-Code, P-Code // L2: f2 = 1227 MHz, _ = 24cm, P-Code

Fenster schliessen

C/A-Code?

(clear access / coarse acquisition), 1023bits (chips), repeats after 1ms,

chip length (1 bit) = 293m, only modulated on L1, each GPS-Satellite sends own code-sequence.

Fenster schliessen

P-Code?

(protected / precise), repeats after 266.4 days (=2.35*10^14 chips), chip length = 29.3m, transmitted on L1 and L2

Fenster schliessen

Nav-Message of GPS Signal?

on L1 and L2, contains broadcast ephemerides (pseud-keplerian elements), satellite clock corrections (polynomial of 2nd degree), almanach data (approximate orbit info for long-term predictions), information about ionosphere, health status of the satellites