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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 attention to the man-made signals created by our civilisation. 

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

As a consequence, there already exists attempts to control 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 and pulsar detection, cannot be nowdays 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 desert, but on the other it simply did not meet the criteria allowing its use in modern civilisation, as it is know 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 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 a 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 transmitting 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 beginning 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. 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 operational radioastronomy equipments were constructed in similar manner. They were 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 it new astronomical discoveries possible.\fnote{Most of astronomy-related discoveries in the last fifty years came from radioastronomy.} 

We could develop a receiver which will have wide bandwidth, high Third-order intercept point and ideally has an option for phase and frequency locking to other receiver on another radioastronomy site of planet.  Several receivers which have such parameters currently exists USRP2, USRP B210 or HackRF and are commercially available. But all of them lacks scalability and have high prices. However scalability and redundancy is main requirement which is requested by noise reduction algorithms. 

New radio astronomy systems such LOFAR are explicit examples of scalability and redundancy approach. LOFAR has completely different and new structure to solve problems of radioastronomy signal reception. LOFAR exclusively uses multi antenna arrays and mathematical algorithms for signal handling. Radio signals recorded  by LOFAR can be used by many ways. Radio image 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 theme ideally needs 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

New approach of receiver construction described above has new requirements on receiver parameters. No additional attempts for signal to noise ratio on single antenna are performed. But other parameters are requested at now. 

\secc Sensitivity and noise number

These parameters are are tied together, but multi antenna and multi receiver arrays requires to keep price of receiver at minimal values. This implicates that sensitivity and noise number must be least as good to detect (signal /noise > 1 ) observed object on majority of receivers connected to observation network.  

\secc Dynamic range

Dynamic range is huge problem of current radioastronomy receivers.  This parameter is enforced by anywhere 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 should be limited by construction of analogue circuitry in receiver or by digitalisation unit. 
Maximal theoretical dynamic range of ADC could be estimated from ADC bit depth according to formula  \ref[dynamic-range]

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

Formula \ref[dynamic-range] gives values shown in table  \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. 

\secc Bandwidth

Historically bandwidth parameter of radioastronomy receiver was in kilohertz range. Small bandwidth was acceptable because observations were processed directly by listening or by paper chart intensity recorder. Chart recorder integrate energy of signal over defined small bandwidth which was suitable for detection of intensity variance in microwave background. No wideband transmitters  exist in this era (except of TV transmitters) and eventually tuning to other neighbour silent frequency was easy. Parallel observations from several places was unnecessary because conditions were nearly same at all locations. 

% dopsat 


The system requires proper handling of huge amount of data. 

Professional astronomers uses uses proprietary digitalisation units \url{http://arxiv.org/abs/1305.3550} or by multichannel sound cadrd on amateur levels \url{http://fringes.org/}