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\chap Introduction 

\sec Current radioastronomy problems

From radioastronomer point of view its important radioastronomy has interest in primarily natural signals from surrounding universe. Radio astronomy do not have interest in terrestrial civilisation made signals. 

Radioastronomy has a big problem at now. It is because  many terrestrial transmitters are active at this moment. All terrestrial  transceivers made dense signal mixture which can cause troubles not only to radioastronomers. 

In consequence, there 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 contain special bands allocated to radioastronomy use. But for many reasons this bands are not clean enough for directly use in radioastronomy observations. As result we cannot work by same way as radioastronomers in the beginning of radioastronomy.  Many experiments namely, Cosmic microwave background detection and pulsar detection cannot be realised in its original form with acceptable results. 

Supporting evidence of such effect is RadioJOVE project. NASA engineers which come with  RadioJOVE project has great idea. 

RadioJOVE project brings opportunity for creating publicly available cheap radioastronomy receiver. But they used an old fashioned construction model which can work in desert, but it simply cannot work in modern civilisation as it is know in Europe.

Origin of its dysfunction is presence of strong radiofrequency interferences. This interferences are orders of magnitude stronger than Jupiter decametric emissions.

From practice about light pollution mitigation we also know that there are not much chance to improve this situation radically in radiofrequency spectrum. 

There are not other ways that searching for new methods for radioastronomy observations. New methods which allows us to work without completely clear radiofrequency bands and which allow us to see surrounding universe trough man made radiofrequency interference mixture.  One solution is use of already known natural radio frequency signals parameters. Natural signals usually have different signal properties from local interference. Natural object do not have a problem with transmitting in  bandwidth of tens megahertz in sub 100 MHz bands. This object are usually far away and the same signal could be received at almost half of Earth without any significant differences.  But it is also clear that signal parameters have drawbacks in reception power. The reception power of radioastronomy object is 1e9 smaller than signal power received from typical broadband radio transmitter.

From above mentioned facts about natural radio signals is clear one result. Modern requirements on radioastronomy receiver are complete different from requirements in history. Radioastronomy is not limited by access to electronic components today, but it is limited by presence of electronic everywhere.   

\sec Modern Radio astronomy receiver

In beginning of radioastronomy receivers were constructed as simple station with single antenna or multi antenna array with fixed phasing. This approach were used due to limits of previous electronics components and technology. Main challenges were noise number and sensitivity due to poor characteristic of active electronic components such transistors and vacuum tubes. 

Most of today operational radioastronomy equipments were constructed in this manner. They were constructed usually shortly after WWII or during The Cold War as parts of military technology.  

But today we have access to components with quality, repeatability and price is completely district from components accessible for previous generation of radioastronomers.  Then we could develop better radioastronomy equipment which will be powerful enough for make new astronomy discovery.\fnote{Most of astronomy related discoveries in 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

Today radioastronomy knows several observation types.

\begitems

* Spectral observations

* Intensity observations

* Velocity observations

\enditems

All of these observations 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/}