Geoscience Reference
In-Depth Information
3. Because of its direct relation with the alteration of the Earth's rotation
ʸ
g, the
of the radio source cannot be calculated independently from
the observed quantities of the delay and the delay rate. As a result, VLBI alone
cannot determine the origin of the right ascension of the radio source reference
system and other technologies are required to do so.
4. The observed quantity of the delay rate does not contain the effect of the baseline
component
right ascension
ʱ
Z, which cannot be calculated just by the observation of the delay
rate. Besides, adding the data of delay rate to that of delay will not reduce the
level of radio sources, which are to be observed to solve all the unknown
parameters. In data processing and parameter solution, the delay rate is adopted
only as a supplementary observation while the observed quantity of the delay is
decisive.
ʔ
The VLBI System
The VLBI system, as shown in Fig.
2.14
, contains antennae, receivers, local
oscillators, samplers, recording devices, related processors, and other units. The
following is a brief introduction of the process of data collection of VLBI.
1. First, two antennae in the system receive the radio signal emitted by the observed
radio source on the focus points of the antennae's paraboloid. Then, the feed
source transfers the collected electromagnetic wave into high-frequency current
and sends it to the receiver. The accuracy of VLBI observation (time delay, time
delay rate) with regard to celestial and geodetic observation is directly propor-
tional to the signal-to-noise ratio (SNR) of the system, and the SNR is directly
proportional to the antenna aperture. Due to the weak signals from extragalactic
radio sources,
the VLBI antenna aperture is often over 20 m to obtain
enough SNR.
2. The receiver's main function is to amplify this signal into a radio-frequency
signal by using a high frequency amplifier and then convert it to an intermediate-
frequency signal with a certain bandwidth through a mixer. In mixing, the mixer
requires a local oscillating (LO) signal, which is provided by the station's local
oscillator.
3. The intermediate-frequency (IF) signal from the receiver reaches the data
recording terminal device, which nowadays employs an MK3 system or
upgraded MK4 and MK5 systems. The MK3 recording system mainly contains
two IF distributors, 14 video converters, a format cell data collection system, a
magnetic tape recorder, and a computer that takes control of the data collection
system and the recorder. The IF signal from the receiver is sent to the IF
distributors and then to these 14 video converters, which convert the signals in
different IF frequency ranges into video signals (also named Base Band) at 0-
2 MHz that can be recorded by the magnetic tape recorder. The video signal
output by the video converters is sent to the format cell. The main function of the
format cell is to digitalize the 0-2 MHz video signals through one-bit sampling;
then a format encoder supplies the precise receiving time for each datum and
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