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\chap Introduction 

\sec Current radioastronomy problems

From a radioastronomer's point of view it is important that radioastronomy focuses its interest primarily on natural signals originating in the surrounding universe. It does not pay much attention to the man-made signals created by our civilisation. 

However, it is due to these artificial signals, that the current radioastronomy faces a disturbing problem. The problem arises from the fact, that there are so many terrestrial transmitters currently active and all of them are sources of a dense signal mixture which can cause trouble not only to radioastronomers. 

As a consequence, there already exist effort to control the radiofrequency spectrum. As result of attempts to control the radiofrequency spectrum, the frequency allocation table was created. \fnote{\url{http://www.ukaranet.org.uk/basics/frequency_allocation.htm}} Radio-frequency allocation table table contains special bands allocated to radioastronomy use. However, for many reasons these bands are not clean enough to be used directly in radioastronomy observations. As a result, we cannot work in the same way as had the radioastronomers in the very beginnings of radioastronomy.  Many experiments, namely Cosmic microwave background detection or pulsar detection, cannot be nowadays realised in their original forms with satisfactory results. 

Supporting evidence of such effect is RadioJOVE project. NASA engineers who originally created the  RadioJOVE project had a great idea. The RadioJOVE project brought an opportunity for creating a publicly available, cheap radioastronomy receiver. However, they used an old-fashioned construction design which, on one hand, can operate in harsh environments like deserts, but on the other it simply did not meet the criteria that would make it possible to be used in modern civilisation, as we know it in Europe.
The source of its dysfunction is a presence of strong radiofrequency interferences. These interferences are orders of magnitude stronger than Jupiter decametric emissions, whose detection was the main aim of the RadioJOVE project.
From what we have already seen in the light pollution mitigation pursuit, there is only a small chance to radically improve the situation in radiofrequency spectrum. 

The only way to overcome this problem is to search for new methods of radioastronomy observations. New methods which allows us to work without completely clear radiofrequency bands and which allow us to see the surrounding universe even despite the existence of man-made radiofrequency interference mixture.  One solution is to use already known natural radio frequency signals parameters. Natural signals usually have different signal properties than local interference. Natural objects do not have  problems with transmission in bandwidths of tens of megahertz in sub 100 MHz bands. These objects are usually far away and the same signal could be received at almost half of the Earth globe without any significant differences.  On the other hand, it is obvious that signals with such parameters have some drawbacks, namely in the reception power. The reception power of radioastronomical object is 1e9 smaller than signal power received from a typical broadband radio transmitter.

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

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. 

Most of the present-day operating radioastronomy equipment has been constructed in similar manner. It was produced usually shortly after the WWII or during The Cold War as a part of military technology.  

Today we have an access to components having quality, repeatability and price completely different from the components accessible by previous generation of radioastronomers.  That is why we can develop better radioastronomical equipment, powerful enough to make new astronomical discoveries possible.\fnote{Most of astronomy-related discoveries in the last fifty years came from radioastronomy.} 

We have the capacities necessary to develop a receiver which will have wide bandwidth, high Third-order intercept point and preferably an option for phase and frequency locking to other receivers located at another radioastronomical site at Earth. Currently there exist several receivers with the above-mentioned parameters, for example USRP2, USRP B210 or HackRF and all are commercially available. However all of them lack scalability and have high prices. It is exactly the scalability and redundancy that are the main requirements of noise reduction algorithms. 

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. 

\secc Sensitivity and noise number

Sensitivity and noise number are parameters that are tied together, but multi antenna and multi-receiver arrays force the price of receiver to be kept at minimal value. This implies that the sensitivity and noise number have to be at least so good in the detection (signal /noise > 1 ) of an observed object, that it would be detected on the majority of receivers connected to an observation network.  

\secc Dynamic range

Dynamic range represents a huge problem of current radioastronomical receivers. This parameter is enforced by everywhere present humans made EMI radiation on RF frequencies. The modern radio astronomy receiver must not be saturated by this high levels of signals but still needs to have enough sensitivity to see faint signals from natural sources. Dynamic range is limited either by the construction of analogue circuitry in receiver or by the digitalisation unit. 
The maximal theoretical dynamic range of ADC could be estimated from ADC bit depth using a following formula  \ref[dynamic-range]

\label[dynamic-range]
$$ D.R. (dB) = 20 * log(2^n) $$

The formula \ref[dynamic-range] gives values shown in table  below \ref[ADC-dynamic-range].

\midinsert \clabel[ADC-dynamic-range]{Dynamic range versus bit depth}
\ctable{cc}{
\hfil ADC Bits  & Dynamic range [dB] \cr
    8 &         48 \cr
    10 &        60 \cr
    12 &        72 \cr
    14 &        84 \cr
    16 &        96 \cr
    24  &       144 \cr
}
\caption/t Standard bit depths of ADC and its theoretical dynamic range. 
\endinsert

% dopsat cast o minimalnim dynamickem rozsahu ADC. 

If we look at actual spectrum occupancy in Europe (measured in power spectral density)  we see that signal dynamic range in spectra
\url{http://observatory.microsoftspectrum.com}
easily reaches more than 80 dB above natural noise levels. If we don't want to deal with receiver saturation or poor sensitivity we need a receiver and digitalization unit which has comparable dynamical range of with received signals. This imply use of least 14 bit ADC without any spare of range.   


\secc Bandwidth

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 

Only few digitalization systems dedicated for radioastronomy exists nowadays.  Currently existing systems use either custom design of the entire receiver or they are constructed from commercially available components. The attempts to use open-source solutions in the radioastronomical field are still rather scarce.

\secc Custom digitalization system

Custom designs usually use non-recurring engineering techniques for the development of specific solutions for observational projects. Thus the costs of these instruments are mostly very high as long as they are not produced in larger quantities. A typical example of an instrument developed and manufactured in one piece with enormous founding drawn is Arecibo ALFA survey multi beam feed Array. 
Another example illustrating the opposite of custom receiver and digitalization unit design is LOFAR system developed by Astron in Netherlands. \url{http://arxiv.org/abs/1305.3550}
 
LOFAR is an innovative radioastronomical system which uses the phased antenna array approach in extensive scale with thousands (around $2*10^4$) of antennas manufactured an deployed on the field. The centre of LOFAR system is situated in Netherlands and peripheral antennas and connection network are extended to other European countries. 

LOFAR project must use low cost hardware due to its large scale. Special construction techniques are used to keep the overall project budget at an acceptable level (take the specially designed polystyrene supporting blocks for HBA antennas as an example). Many of the components used are manufactured in mass scale for other than scientific purposes. LBA antennas masts are made of standard PVC plastic waste pipes and LOFAR uses low cost direct sampling receivers. The whole project has been designed by Netherlands Institute for Radio Astronomy, which produces many similarly sophisticated devices. \url{http://www.astron.nl/other/desp/competences_DesApp.htm}

\secc Modular digitalization systems
 
Due to the cost restrictions in science and astronomy instruments' development, a reuse of engineering work turns out to be very useful. There is only one modular digitalization and data processing system currently in existence - it is called CASPER and it is under development at Berkley university since around 2005. \url{https://casper.berkeley.edu/wiki/Main_Page} as CASPER's designers an engineers remarkably noticed a lack of such hardware in radioastronomy. Their ideas are summarised in the following paper \url{https://casper.berkeley.edu/papers/200509URSI.pdf}. Unfortunately they use proprietary connector standard and technology and develop modular system based purely on Tyco Z-DOK+ connectors family.  Z-DOK connectors are high quality differential pairs connectors, but price of these connectors (around 40 USD) is comparable with the cost of one ADC channel in the design described in our thesis. \url{http://www.digikey.com/product-detail/en/6367550-5/6367550-5-ND/2259130}. 

In contrast to professional astronomers which use proprietary digitalization units, amateur radioastronomers currently use multichannel sound cards \url{http://fringes.org/} or self designed digitalisation units. Devices constructed by amateurs are usually non-reproducible \url{http://wwwhome.cs.utwente.nl/~ptdeboer/ham/sdr/} . It is evident that current radioastronomy lacks a proper hardware which could be used by both communities - professionals and amateurs. Optimal solution for this situation would be an open-source hardware.