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====== Advanced radio-astronomical station ARAS01A ======

A station using a signal from antennae array that further processes it by digital algorithms.

===== Antennae array =====

==== Types of antennae ====

Previous experiments with [[en:bras|BRAS01A]] station showed, that the most advantageous antenna construction for small systems is a horizontally positioned loop antenna. The reasons are that it is less susceptible to receive interferences caused by electrostatic charge and it has slightly greater gain compared to inverted-V dipole. It’s disadvantage is worse polarisation selectivity.

==== Impedance matching and symetrisation ====

In case of using [[http://en.wikipedia.org/wiki/Loop_antenna|Loop antenna]] with a characteristic impedance of approximately 100 Ohm, the impedance matching can be realised by a 2:1 transformer, which transforms the antenna impedance to 50 Ohm.

==== Construction of antennae array ====

It should be possible to lay out the horizontal loop above the ground using four non-conductive columns. Power supply for the loop can be brought by means of one of the columns - the impedance transformer to 50 Ohm will be directly on the loop. The signal will be further transferred by a short coaxial cable to [[en:bp|BP01A]] filter, positioned on a column and then to a preamplifier, consisting of [[en:gb|GB01A]] module. From that point on, the SMA connector can be reduced to a better quality coaxial cable with a larger diameter, which can lead the signal to the receiver. There can be another selective filter and amplifier at the receiver.

===== Receiver and signal processing =====

===== Data processing software for station =====

The aims of software processing:

* Acquiring the raw record for later offline processing
  * Detection of interesting radioastronomical events
  * Data flow reduction - decreasing the amount of data saved for their further processing

Software implementation includes two tasks:
One is the realisation of DMA controller for FPGA that will enable data transfer from FPGA to computer’s memory and their further processing. 
Second is the realisation of PC’s driver, which would make it possible to reconstruct the former RF signal from data transferred to DMA for their further processing by parent applications (e.g. [[https://github.com/MLAB-project/radio-observer|Radio-Observer]]).

A general problem of this construction will be a need for massive reduction of data flow - this problem can however be resolved in a later application phase. Necessary computing power can be obtained either by changing the FPGA scheme that will preprocess the signal (e.g. filtration, decimation, demodulation) or by using PC’s GPU that can process parallel operations such as getting information about correlation of individual channels.

==== Existing software ====

There are two software packets used in online reduction of data from receivers, which could be used for data processing from multiple antennae. (There is more software for offline processing, but in such case, it is necessary to manipulate with enormously huge files).

* [[http://www.gnuradio.org/|GNUradio]] - open-source tool for experimental data processing from receivers. It is a part of gnuradio-companion - a GUI tool for ‘clicking’ a signal chain form ready-made blocks. It is suitable for algorithms prototyping. However, in order to run, it needs a quite powerful multi-core PC with a large memory. Intel Core i5 with 4GB RAM DDR3 is about the lower boundary of its functionality.  
    * [[https://github.com/MLAB-project/radio-observer|Radio-observer]], designed in a way, that a multi-receiver system can be easily integrated.

===== Observation types =====

==== Satellite observation system ====

For satellite observation a multi antenna array is useful. Antenna array brings opportunity to observe several satellites simultaneously by algorithmic steering of antenna array beam.

===== References =====

* [[http://pa0fri.home.xs4all.nl/Ant/Quad/quadeng.htm|Quad / Loop design]]
  * [[http://www.hardhack.org.au/book/export/html/18|Radio Astronomy]]
  * [[https://casper.berkeley.edu/wiki/Main_Page|CASPER project]]

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