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

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. 

To je často velký problém, neboť na Zemi touto dobou existuje velké množství vysílačů, které pokrývají prakticky celé dostupné elektromagnetické spektrum a vytvářejí tak nepřebernou směs signálů, která se nejenom pro radioastronoma může stát nepřekonatelným problémem. Z tohoto důvodu byla již od počátků rádiového vysílání snaha udržet nad obsazením spektra určitou kontrolu a jedním z důsledků této snahy je například <a href="http://www.ukaranet.org.uk/basics/frequency_allocation.htm">tabulka přidělených kmitočtů pro radioastronomii</a>. Bohužel z mnoha důvodů nelze říci, že by tyto kmitočty byly dostatečně kvalitně čisté pro sériózní pozorování. Z toho vyplývá důležité zjištění, že v současné době nelze postupovat stejně jako v počátcích radioastronomie. A tedy i experimenty, vedoucí například k objevu reliktního záření nebo pulzarů, nelze v původní podobě zopakovat s uspokojivým výsledkem. 

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

There are not other ways that searching for new methods in radioastronomy obserevations. New methods which allows us to work without completely clear radiofrequency bands asd alow us to see surroundind universe trough man made radiofrequency interference micture.  One of sollutions is use of already known natural signals parameters. Natural signals usualy have different signal properties from local interference. Natural object do not have a problem with tansmission bandwith of tens megaherts in sub 100 MHz bands. This object also transmit the same signal for almost half of Earth without any difficulties.  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.

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.

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. 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.  

Řešením je pravděpodobně použít přijímač, který bude mít velkou šířku pásma, nejlépe řádu MHz, a vysokou vstupní odolnost. A pokud možno půjde sfázovat s nějakým dalším na jiném místě planety. Existuje několik zařízení, které tyto požadavky splňují. V naprosté většině jde o takzvané <a href="link wikipedie">SDR</a> přijímače, jako například USRP, USRP2, SDR-IQ a SSRP. Tato zařízení ale mají většinou zásadní nevýhodu, že jejich pořizovací cena je více jak 1000 USD a jsou velmi univerzální. Takže se moc nehodí na nějaké kontinuální pozorování, kde nebudou využity všechny jejich draze zaplacené vlastnosti. Poslední z uvedených <a href="link wikipedie">SSRP</a> je ale jednoduchá konstrukce 16bit AD převodníku připojeného k USB řadiči, který hrne všechna navzorkovaná data do PC. Je to velmi zajímavé zařízení s řádově nižší pořizovací cenou. Avšak v této podobě je omezené datovým tokem USB, které omezuje vzorkovací frekvenci na zhruba 30MSPS. Z čehož vyplývá, že v prvním Nyquistově pásmu není možné zpracovávat signály o frekvenci vyšší něž 15MHz. To je pro radioastronomické účely poměrně nízko. Řešením by byl přechod do vyšších Nyquistových zón, kde ale začne vznikat problém s vhodnou konstrukcí antialiasign filtru a omezením sample-hold obvodu na vstupu ADC.

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.} 

This new radio astronomy receivers have completely different approach to solve the problem of radioastronomy signal reception. They almost exclusive uses multi antenna arrays and mathematical algorithms for signal handling. Radio signal recorded by this metod can be used by many ways. Radio image can be computed (if sufficient cover of u/v plane is achieved), radiation intenzity can be measured, spectrum can be analysed for velocity measurement. etc. 

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. 

All of these observations ideally needs high frequency resolution and stability. Wide observation bandwidth in hundreds of MHz is usually desirable for easier discrimination of source types. 

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. 

This new approach of receiver construction has different requirement on receiver parameters. No signal to noise ratio on single antenna is improved. But other parameters are requested at now. 

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. 

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. 

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. 

From requirements mentioned above  

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. 

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

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/}