# Lernkarten

Roland Schenkel
Karten 101 Karten 12 Lernende English Universität 17.01.2012 / 23.01.2021 Kein Urheberrechtsschutz (CC0)
0 Exakte Antworten 89 Text Antworten 12 Multiple Choice Antworten

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Difference in the influence of the troposphere on VLBI, GPS, SLR?

For radio waves in the frequency range from 100 MHz to about 15 GHz (as in case of VLBI and GPS) the refractive index n is independent of the frequency (--> identical delay for all frequencies) // For optical wavelengths (SLR/LLR with a frequency of around 5*1014Hz ) the tropospheric refraction is dispersive

Order of magnitude of the tropospheric delay for VLBI, GPS, SLR?

The total delay (dry and wet part) amounts to about 2.3 m in the zenith direction (~8 ns in time units) and about 25 m at an elevation of 5°.

The wet part is maximal 40 cm.

What is a mapping function? Expression for the simplest mapping function

Often the TEC value in zenith direction (VTEC = Vertical Total Electron Content) is modelled and not directly the TEC in a given direction. With the mapping function mion(z) and the VTEC EV as input parameter it is possible to compute the TEC E for an arbitrary angle z.

Difference between the mapping function for the ionosphere (single layer) and for the troposphere

(z … zenith angle) ionosphere: mion(z)=1/cos(z') and sin(z')=R/(R+H)*sin(z) // troposphere: mtrp(z)=1/cos(z)

How are troposphere parameters estimated? What are the difficulties?

GPS (estimation) and Meteo-Measurements // big local and temporal differences

Formula 4.58. What is that? P, T, e?

Saastamoinen-Modell, Modellierung der tropospährischen Refraktion. Erster Term in runder Klammer: dry Part. Zweiter Term in runder Klammer: wet Part. Dritter Term in runder Klammer: Erdkrümmung. P: Luftdruck, T: Temperatur, e: Wasserdampfdruck , berechnbar aus relativer Luftfeuchtigkeit

Formula 4.58, Was von den dreien (P,T,e) hat den grössten Einfluss?

Temperatur (3-27 mm / 1°C), Druck (2mm / mbar), relative Luftfeuchte (0.6 - 4 mm / 1%)

Formula 4.58, Weshalb 1/cos(z)? Wie heisst dieser Teil? Warum tan^2(z)?

Einfachste Mapping-Function <= Mapping Function // Erdkrümmung

Wie gross ist der Einfluss der Troposphäre bei GPS, VLBI, Altimetrie?

GPS: (non-dispersive) 2.3m (Zenith) (wetpart: 5-40cm), 25m (5° elev) // VLBI: (non-dispersive) dito // Altimetrie: (non-dispersive) dito // SLR/LLR: (dispersive) 2.45 (Zenith) (wetpart: 0.1-0.6cm), 7m (20° elev)

Wie gross ist der Einfluss der Ionosphäre bei GPS, VLBI, Altimetrie?

GPS: (dispersive) 1-15m (Zenith), 150m (low elev) // VLBI: (dispersive) S-Band (2GHz), wie GPS, X-Band (8GHz) 4-60cm // Altimetrie: (dispersive) 2-20cm // SLR/LLR: (non-dispersive) ionospheric refraction doesn’t play any role!4

Rotation der Erde: Ist das etwas konstantes?

Nein. Die Erdrotation wird durch die Gezeitenreibung verlangsamt. Die Erde hat das gleiche schon mit dem Mond “gemacht”

Weshalb besteht ein Zusammenhang zwischen Verlangsamung der Erdrotation und Entfernung des Mondes?

Es gibt drei Erhaltungssätze in der Physik, hier: Drehimpuls muss erhalten bleiben

Was für Effekte bewirkt der Mond sonst noch? (abgesehen von der Gezeitenreibung)

Feste Erdgezeiten, Schwankungen in der Erdrotationsachse, davon abgeleitet auch Polgezeiten, er wollte noch auf generelle Relativität bei Mondnahen VLBI-Messungen hinaus.

Wie kann man die Erdrotation und ihre Änderung messen?

GNSS, LLR, VLBI

Wo in der ausgleichungsrechnung werden die parameter für die

troposphärische refraktion eingesetzt? Was kommt raus, etc?

Page 101, In the Observation Equation

=> The Partial derivative of the Obs. Equ. gives the Mapping Function.

Which parameter can, which cannot be determined by VLBI?

Nutation

Polar Motion

UT1

Length of Day

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

Which parameter can, which cannot be determined by VLBI?

Orbit Parameters

Orbit Determination for Low Earth Orbiter

Ionosphere

Troposphere

Clocks (time transfer)

Which parameter can, which cannot be determined by GNSS?

Nutation

Polar Motion

UT1

Length of Day

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

Which parameter can, which cannot be determined by GNSS?

Orbit Parameters

Orbit Determination of Low Earth Orbiter

Ionosphere

Troposphere

Clocks (time transfer)

Which parameter can, which cannot be determined by SLR?

Nutation

Polar Motion

UT1

Length of Day

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

Which parameter can, which cannot be determined by SLR?

Orbit Parameters

Orbit Determination of Low Earth Orbits

Ionosphere

Troposphere

Clocks (time transfer)

Which parameter can, which cannot be determined by LLR?

Nutation

Polar Motion

UT1

Length of Day

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

Which parameter can, which cannot be determined by LLR?

Orbit Parameters

Orbit Determination of Low Earth Orbiters

Ionosphere

Troposphere

Clocks (time transfer)

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.

Which instruments/sensors are required for a VLBI station?

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

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)