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/dokumenty/skolni/diplomka/introduction.tex
16,7 → 16,7
From the above mentioned facts concerning the natural radio signals we can conclude that modern requirements imposed on a radioastronomy receiver are completely different from the requirements existing back in the history. Radioastronomy is no longer limited by an access to electronic components, today it is rather limited by the everywhere presence of electronic.
\sec Modern Radio astronomy receiver
\sec Radio astronomy receiver
In the beginnings of radioastronomy, the receivers were constructed as simple stations with single antenna or multi antenna array with fixed phasing. This approach was used because of the existing limits of electronic components and technologies. The main challenges of those times were the problem of noise number and low sensitivity, both present due to the poor characteristics of active electronic components such as transistors and vacuum tubes.
28,18 → 28,6
New radio astronomy systems such LOFAR are explicit examples of the scalability and redundancy approach. LOFAR has completely different and novel structure developed to solve the problems of radioastronomy signal reception. It exclusively uses multi antenna arrays and mathematical algorithms for signal handling. Radio signals recorded by LOFAR can be used in multiple ways: radio images can be computed (if sufficient cover of u/v plane is achieved), radiation intensity can be measured, spectrum can be analysed for velocity measurement, etc.
\secc Observation types
Current radioastronomy knows several types of observations.
\begitems
* Spectral observations
* Intensity observations
* Velocity observations
\enditems
All of them prefer high frequency resolution and stability. Wide observation bandwidth in hundreds of MHz is usually desirable for easier differentiation of source types.
\sec Required receiver parameters
The novel approach of receiver construction described above goes hand-in-hand with new requirements on receiver parameters as well. Currently no additional attempts to improve the signal-to-noise ratio on single antenna are performed. There are however other parameters requested nowadays.
83,7 → 71,7
Historically, the parameter of bandwidth in radioastronomical receiver used to be within the kilohertz range. Small bandwidth was acceptable because observations were processed directly by listening or by paper chart intensity recorder. Chart recorder integrated energy of signal over defined small bandwidth which was suitable for detecting the intensity variance of microwave background. No wide-band transmitters existed in that era (except for TV transmitters) and tuning to other neighbouring frequency was easy as they were mostly vacant. Parallel observations from several places were unnecessary as well because the electromagnetic conditions were nearly same at all locations.
\sec Current status of receivers digitalization units
\sec State of the art receivers digitalization units
Only few digitalization systems dedicated for radioastronomy currently exists. Currently existing systems uses either custom design of whole receiver or they are constructed from commercially available components. Open-source principle attempts are very rare in radioastronomy field.