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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 issue. The problem arises from the fact, that there are too many terrestrial transmitters currently active. All of them are sources of a dense signal mixture which can cause trouble not only to radioastronomers.

As a consequence, there already exist efforts to control the radiofrequency spectrum. As result of these attempts to control the spectrum, the frequency allocation table has been created \cite[radio-astronomy-frequency]. The radio-frequency allocation table contains special bands allocated to radioastronomy use. However, for many reasons these bands are not clean enough to be used in radioastronomical observations directly. As a result, we cannot work in the same way as did the radioastronomers in the very beginnings of radioastronomy.  Many experiments, namely Cosmic microwave background detection or pulsar detection, cannot be realised nowadays in their original forms with satisfactory results.

Supporting evidence of such effect is RadioJOVE project. NASA \glos{NASA}{National Aeronautics and Space Administration} 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 unoccupied harsh environments like deserts, but on the other hand it simply do not meet the criteria that would make it possible to be used in modern civilisation, as we know it in Europe \cite[radio-jove].

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 allow 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 $1 \cdot 10^9$ smaller than the power of signal 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 electronics.

\sec Radio astronomy receiver

At the beginnings of radioastronomy, the receivers were constructed as simple stations with a single antenna or a multiple antennas arrays with fixed phasing. This approach was used because of the existing limits of electronic components and technologies. The main challenges of those times were problems 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 a similar manner. It was produced usually shortly after the WWII \glos{WWII}{Second World War} or during the Cold War as a part of the military technology.

Today, we have an access to components with much higher quality, repeatability and lower price as compared to the components accessible to previous generation of radioastronomers.  That is why we are able to develop a better radioastronomical equipment, powerful enough to make new astronomical discoveries possible.\fnote{Most of astronomy-related discoveries in the last fifty years came from the field of radioastronomy.}

We have the capacities necessary to develop a receiver which will have a wide bandwidth, a high third-order intercept point and preferably an option for phase and frequency locking to other receivers located at other radioastronomical sites on the Earth. Currently there exist several receivers with the above-mentioned parameters, for example USRP2, USRP B210 \cite[USRP-sdr] \glos{USRP}{Universal Software Radio Peripheral} or HackRF \cite[hackrf-sdr] which are commercially available. However all of them lack scalability and have higher prices unaffordable to our amateur radioastronomical network. Scalability and redundancy are the main requirements of the noise reduction algorithms and thus acted as a motivation for this diploma thesis.

New radio astronomical systems such as LOFAR\glos{LOFAR}{Low-Frequency Array}~\cite[lofar] are explicit examples of the scalability and redundancy approach. LOFAR has a completely different and novel structure developed to solve the problems of radioastronomy signal reception. It uses exclusively 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), the radiation intensity can be measured, the spectrum can be analysed for velocity measurement, etc.

LOFAR is an innovative radioastronomy system which uses a phased antenna array approach at an enormous scale. Thousands (around $2 \cdot 10^4$) of antennas are manufactured and deployed in the field. The centre of LOFAR system is situated in the Netherlands and peripheral antennas with the connection network extend to other European countries.

\midinsert

\clabel[lofar-antenna]{Lofar antenna configuration}

\picw=10cm \cinspic ./img/lofar_antenna.jpg

\caption/f One LOFAR LBA\glos{LBA}{Low Band Antenna} antenna element.

\endinsert

Due to the size of the whole system, the LOFAR project has to use a low cost hardware. A special construction techniques were employed to keep the overall project budget at acceptable level (including for example specially designed polystyrene supporting blocks for High Band Antenna (HBA\glos{HBA}{High Band Antenna})). Many of used components are manufactured on a massive scale for other than scientific use. LBA antennas' masts are made from a standard Polyvinyl chloride (PVC\glos{PVC}{Polyvinyl chloride}) plastic waste pipes \ref[lofar-antenna].  The entire project was designed by the Netherlands Institute for Radio Astronomy which produces many similarly sophisticated devices~\cite[astron-devices].

\sec Required receiver parameters

The novel approach of the receiver construction described above goes hand-in-hand with the new requirements on receiver parameters as well. No additional attempts to improve the signal-to-noise ratio on single antenna have been performed currently. There are however other parameters, that are requested nowadays.

\secc Sensitivity and noise number

Sensitivity and noise number are parameters which are tied together. However, the requirement for multi-antenna and multi-receiver arrays forces to keep the prices of receivers at the minimal value. This implies that the sensitivity and noise number have to be at least as good in the detection (signal $/$ noise $>$ 1 ) of an observed object, that the object would be detected by the majority of receivers connected to an observational network.

\label[dynamic-range-theory] \secc Dynamic range

The dynamic range represents a huge problem of current radioastronomical receivers. It arises from the  ubiquitous human activity creating an electromagnetic inference (EMI\glos{EMI}{Electromagnetic interference}) radiation in the radio frequency (RF\glos{RF}{Radio frequency}) band. The modern radioastronomical receivers must not be saturated by these high levels of signals but still need to have sufficient sensitivity to see faint signals from sources being observed. The dynamic range is limited either by the construction of analog circuitry in the receiver or by the digitization unit.

The maximal theoretical dynamic range of an analog-to-digital converter (ADC)\glos{ADC}{analog-to-digital converter} could be estimated from ADC bit depth as

\label[dynamic-range]

$$

dynamic range \; [dB] = 20 \cdot \log(2^n) \eqmark

$$.

The maximum theoretical dynamic range for standard bit depths of ADC is shown in the 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

If we look at the actual spectrum occupancy in Europe (measured in the power spectral density), we see, that the signal dynamic range in spectra easily reaches more than 80 dB above the natural noise levels~\cite[spectrum-observatory]. If we do not want to deal with the receiver's saturation or the poor sensitivity, we need a receiver and digitization unit which has a dynamic range comparable to the received signals. This implies the use of at least 14 bit ADC which would not, however, leave any part of the range spare. The 16 bit range seems more optimal as it offers some spare range for the strongest RF signals. In addition, the two byte sample range has a good efficiency in the use of standard data types, that mostly have a length in the powers of 2. That is why we have selected the 16 bit digital range as the optimal one for our design.

\secc Bandwidth

Historically, the bandwidth parameter in radioastronomical receivers used to be within the kilohertz range. Such a narrow bandwidth was acceptable because observations were processed directly by listening or by a paper chart intensity recorder. The chart recorder integrated the energy of a signal over a defined narrow bandwidth which was suitable for detecting the intensity variance of a microwave background. No wide-band transmitters existed in that era (except for TV\glos{TV}{Television} 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 the same at all locations. 

Currently we are not able to use any unused part of the radio-frequency spectrum for radioastronomical observations. 

%%

\sec State of the art in receivers' digitization units

Only a few digitization systems dedicated to radioastronomy currently exist. Today's systems use either a custom design of the whole receiver or they are constructed from commercially available components. Open-source principle attempts are very rare in the radioastronomy field.

\secc Custom digitization system

Custom designs usually use non-recurring engineering for the development of a specific solution for an observational project. Consequently, such instruments are very costly if they are not reproduced multiple times. A typical example of the instrument developed and manufactured in a single piece with enormous funding requirements was the Arecibo ALFA\glos{ALFA}{Arecibo L-Band Feed Array} \cite[alfa].

Another example, this time a custom-designed receiver and digitization unit design but duplicated many times is LOFAR system developed by Astron in the Netherlands~\cite[lofar]. Since thousands of antennas are used the project must use a low cost hardware due to system scale. The solution chosen in this project is based on low cost direct sampling receivers, combining RF part and digitizer in a single block.

\secc Modular digitization systems

Due to cost restrictions in science and astronomy instruments development, a reuse of engineering work is preferable. There is one good example of a modular digitization and data processing system - a system Collaboration for Astronomy Signal Processing and Electronics Research (CASPER\glos{CASPER}{Collaboration for Astronomy Signal Processing and Electronics Research}), that has been in development at the University of Berkeley~\cite[casper-project] since around 2005. CASPER designers and engineers noticed a remarkable lack of such hardware in radioastronomical science. Their ideas are summarised in the following paper~\cite[casper-paper]. Unfortunately, they use a proprietary connector standard and technology. They have developed a modular system based purely on Tyco Z-DOK+ connectors family. CASPER data processing board with Z-DOK connectors is shown in the Figure~\ref[casper-roach]. Z-DOK connectors have a relatively high pricing (around 40 USD)~\cite[Z-DOK-connectors], but represent high quality differential pairs connectors. However, the price of these connectors is comparable with the price of one ADC channel in the design described in this diploma thesis.

\midinsert

\clabel[casper-roach]{CASPER's ROACH data processing board}

\picw=10cm \cinspic ./img/Roach2_rev0_2xcx4mezz.jpg

\caption/f CASPER project ROACH-2 \glos{ROACH}{ Reconfigurable Open Architecture Computing Hardware (ROACH) board} data processing board. White Z-DOK connectors for daughter ADC Boards can be easily seen in the front of the board.

\endinsert

On the contrary to the professional astronomers, who use proprietary digitization units, amateur radioastronomers have been using multichannel sound cards~\cite[amateur-fringes] or self designed digitalisation units. Devices constructed by amateurs are usually non reproducible \cite[amateur-sdr]. It is evident that the current radioastronomy lacks a proper hardware which could be used by both communities, professionals and amateurs. The optimal solution in such situation should be an open-source hardware, a concept that we further develop in this diploma project.