Antenna Architecture of a Nano-Satellite forRadio Astronomy

Alexandru Budianu, Arjan Meijerink and Mark J. BentumTelecommunication Engineering Group

University of TwenteP.O. Box 217, 7500 AE, Enschede, the Netherlands

a.budianu@utwente.nl, a.meijerink@utwente.nl, m.j.bentum@utwente.nlDavid M. P. Smith and Albert-Jan Boonstra

Research and Development DepartmentASTRON, The Netherlands Institute for Radio Astronomy

Postbus 2, 7990 AA, Dwingeloo, the Netherlandssmith@astron.nl, boonstra@astron.nl

Abstract—Recent technological advancements have led to theemergence of a new miniaturized satellite platforms and thisopened up the path for a whole new range of applications.The Orbiting Low Frequency Antennas for Radio Astronomy(OLFAR) project is one of these applications, and aims to de-velop a low-frequency radio telescope for the 0.3 MHz to 30 MHzband. It uses a swarm of 50 or more identical nano-satellitesto synthesize a very large aperture to explore the ultra-longelectromagnetic (EM) waves in this frequency band. The swarmwill act similar to a wireless sensor network (WSN) that samplesthe cosmic noise, processes the data in a distributed manner, andthen sends the data to a base station (BS) for further processingand analysis. The satellites will have a very simple architecture(characteristic of a WSN node) that will focus on three mainfunctionalities: radio observation, data processing, and distri-bution (inter-satellite and downlink). However, the complexityof the application results in strict requirements for the designof the satellites components, especially for the antenna systems.Starting from OLFAR’s requirements and limitations, three sep-arate antenna systems for scientific observation, inter-satellitelink (ISL) and data downlink are designed and integrated into anano-satellite platform.

TABLE OF CONTENTS

1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 THE NANO-SATELLITE IMPLEMENTATION . . . . . . 2

3 OBSERVATION ANTENNAS . . . . . . . . . . . . . . . . . . . . . . . 3

4 INTER-SATELLITE LINK ANTENNA SYSTEM . . . . . 5

5 DOWNLINK ANTENNAS . . . . . . . . . . . . . . . . . . . . . . . . . . 6

6 FUTURE WORK AND CONCLUSIONS. . . . . . . . . . . . . 8

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

BIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1. INTRODUCTIONIn the past few decades radio astronomy related projectscaught a lot of interest from the science community, andattracted numerous resources as they opened a new path inunderstanding and broadening our knowledge of the Uni-verse. The radio observation of the cosmic backgroundradiation came as an aid for optical telescopes. It offered newinsights over already known celestial bodies and phenomena,

978-1-4799-1622-1/14/$31.00 c©2014 IEEE.1 IEEEAC Paper #2120, Version 2, Updated January 10, 2014.

and also revealed new ones. The potential of exploringnew scientific drivers enhanced the development of radiotelescopes all across the world. Both Earth- and space-basedinstruments were built or are currently under constructionto capture electromagnetic electromagnetic (EM) waves ofcosmic origin. At ground level very large dish telescopes(e.g. The Arecibo Observatory), and arrays (LOFAR [1],Square Kilometre Array [2]) were built or are currently underconstruction to observe EM waves with frequencies as low as30 MHz. Furthermore, spacecraft such as the Herschel SpaceObservatory and Planck were launched into space to imagethe Universe in far infrared and submillimeter wavebands,and, respectively, infrared and microwave bands.

One of the last unexplored frequency bands is the 0–30 MHzband. Observing the ultra-long-wavelength EM waves inthe previously mentioned frequency band is particularly in-teresting for revealing details about exo-planets and for abetter understanding of the evolution of the Universe [3].However, until now the required instrument proved to be toocomplex for our technological possibilities. Building a radiotelescope to explore the Universe in the very low frequencydomain needs to overcome a few limitations. First of all,an Earth-based instrument is not possible due to ionosphericproperties. The EM waves of interest are strongly affectedby scintillation, or cannot even penetrate the ionosphere(for frequencies below 15 MHz). Added to this, the largeamount of man-made radio interference make it impossibleto distinguish the ultra-long EM waves originating fromcosmic sources (at ground level) [4]. Secondly, it is verychallenging to build a space-based solution as well due to therequired size of the instrument. Ultra-long waves can onlybe observed with an aperture in the order of kilometers thatcannot be implemented in a monolithic fashion, and certainlynot integrated in a single spacecraft.

Recent developments in space industry have motivated engi-neers and scientists to reconsider the idea of a low-frequencyradio telescope in space. The evolution and ongoing minia-turization of technology have led to the emergence of newvery small spacecraft (nano-satellites) capable of performingsimple missions in outer space. With this, the concept of dis-tributed satellite systems became feasible, and more complextasks were thought of, such as weather monitoring [5] andradio astronomy [6].

In the Distributed Aperture Array for Radio Astronomy InSpace (DARIS) project [7] a study about the possibility of do-ing low-frequency observations with multiple small satellites

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was conducted. A scenario with eight slave spacecraft anda central spacecraft was proposed, in which the nodes willdo the sensing part while the mothership will have additionalprocessing and communication tasks. One of the conclusionsof the study was that technology has reached a level wherethis type of scenario is realistic and can be implemented withcommercial off-the-shelf (COTS) components. However,having a central spacecraft increases the risk of failure of thesystem.

In a similar manner the Orbiting Low Frequency Antennas forRadio Astronomy (OLFAR) project aims to develop a radiotelescope in space for the 0–30 MHz domain by employing acompletely distributed system. A swarm of 50 or more nano-satellites orbiting faraway from terrestrial radio-frequencyinterference (RFI) will be used to sense and sample thecosmic noise, process the information by means of distributedcorrelation, and send the processed data to a base station(BS) on Earth (see Figure 1). Thus, each member of theswarm will have to fulfill three main tasks—radio observa-tion, data distribution and processing, and downlinking. Fromthe hardware point of view separate subsystems will haveto be designed and integrated on a miniaturized spacecraftplatform. Multiple antenna systems will have to be hostedby the nano-satellites to be able to support the data flow into,within, and out of the swarm.

Figure 1. OLFAR swarm on a lunar orbit.

In this paper we summarize previous work on the inter-satellite link (ISL) and radio observation antenna systems,and extend it with the design considerations of the downlinkantennas. We propose a simple radio architecture for thesatellites and discuss a cubesat implementation of the OLFARswarm in Section 2. In Sections 3, 4 and 5 we follow throughwith the design of the three separate antenna systems for thethree data-related tasks. Conclusions are drawn in Section 6.

2. THE NANO-SATELLITE IMPLEMENTATIONSeveral aspects confer OLFAR an unique approach and makethe project distinguish itself from previous studies. Unlikethe DARIS project, the system proposed by OLFAR will becompletely distributed. The satellite swarm will compriseof a large number (50 or more) of identical spacecraft. Al-though the assigned functionalities may differ from memberto member (e.g. communication master [8]), no differencewill be made (from the hardware perspective). This increasesthe robustness and reliability [9] of the swarm, and will alsoenhance the processing of the radio astronomy data.

In Figure 1 a possible deployment location for the OLFARswarm is exemplified and the different tasks are illustrated. Inthis case, the advantages and the constraints of a lunar orbitresult in a temporal separation of the tasks. One orbital periodcan be divided into three (optionally four) stages.

1. Observation: in the radio-silent region [10], satellites willonly sample the cosmic background radiation.2. Data distribution and processing: once sampled, thedata is shared among all the members of the swarm, andprocessed by means of distributed correlation.3. Downlink: while facing Earth, satellites will send theprocessed data to a BS on Earth.4. The fourth stage is optional. If it is necessary this stagecan be used to finalize the distribution and processing task orit can be an idling stage when the only task is the solar powerconversion.

Another key feature of OLFAR is the engineering mindset.The project is aimed at developing an instrument and all thenecessary hardware rather than only analyzing its feasibil-ity. A new satellite platform was recently developed—thecubesat platform—and was proven to be eligible for spaceapplications. The many successful launches [11] showedthat the platform is suitable for more complex missions thantechnology demonstrations. Therefore, OLFAR is planningto use three-unit (3U) cubesats, similar to Delfi-C3 [12], tofulfill its science task. The satellites will have a very simpleand robust architecture as shown in Figure 2. The threemain subsystems will exchange information and inter-operatewhile having support from other secondary subsystems (electric power system (EPS), propulsion, attitude control).

Figure 2. Functional diagram of a small satellite.

The simple radio architecture has to be backed up by suitablehardware (antennas) custom built to fit the cubesat platform.Since the main tasks are fundamentally different, separateantenna systems will be placed on the 3U cubesat, and usedfor capturing low-frequency cosmic waves, distributing databetween the members of the swarm and sending the result ofthe processing to a BS on Earth.

Accommodating multiple antenna systems on a simple nano-satellite platform is very challenging because of the limited

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volume, area and mass. The cubesat standard [13] limitsthe mass of a 3U cubesat to 4, 000 grams and specifiesa parallelepipedic shape of 30 cm × 10 cm × 10 cm, thuslimiting the size of possible patch or wire antennas. Dueto launch restrictions, such as the size of the standardizedcubesat orbital deployer, long wire antennas must be housedinside the satellite during launch, and only be deployed oncethe satellites have reached orbit. Furthermore, deploymentmechanisms should be kept simple or even avoided as muchas possible to preserve the robustness and reliability of thesystem.

Taking these limitations into account in the next sections weproceed to the design of three antenna systems, one for eachof the main functionalities of the OLFAR satellites.

3. OBSERVATION ANTENNASChallenges and requirements

To operate over the multi-octave band of 0.3 MHz to 30 MHzit is necessary to have an observation antenna with dimen-sions in the order of meters to hundreds of meters. Thiscannot be realized in the form of patch antennas or apertureantennas mounted on the exterior of the satellite, due to thesatellite dimensions. Reflector antennas are excluded dueto the complex deployment mechanism required for such aconfiguration.

The only available option is wire antennas mounted on theexterior of the satellite, connected as either monopoles ordipoles. Furthermore, these antennas are required to havegood sensitivity in as large a field of view (FoV) as possible,in order to sample the full 4π steradian range. Also, threeorthogonal antennas are required, in order to extract theamplitude, phase and orientation of the incoming EM waves.

Design and implementation

Dipoles much shorter than a wavelength have favorable an-tenna pattern shapes with 3 dB beam widths of 90◦. How-ever, they are inefficient. Increasing the length to the orderof a wavelength improves the efficiency, while maintainingdecent antenna pattern shapes with 3 dB beam widths of 75◦.Above one wavelength, the antenna patterns start to lose theirshape through becoming dominated by side lobes, therebyconstraining the dipole lengths to one wavelength.

This restricts the dipoles to 10 meters, this being a fullwavelength at the highest frequency in the operational bandof 30 MHz [14]. However, this is the theoretical limit foran infinitesimally thin dipole, while for a physical antenna,depending on the wire radius, side lobes will start to appearat frequencies below 30 MHz. Therefore, the dipoles arechosen to be 9.6 meters to shift the theoretical full wavelengthresonance to 31 MHz and, hence, shift side lobes outside ofthe operational band.

For optimal performance, the corresponding monopole pairsfor each dipole should be deployed opposite to each other,sharing two planes, with the minimal feed gap size possibleseparating them. Also, the three dipole pairs should beorthogonal to each other, sharing the same geometric center.However, due to mechanical constraints this is not possible.The cramped confines within the satellite prohibit an electricconnection for one of the dipole pairs.

Instead, this pair must be electronically coupled, resulting in

a pair of pseudo-antipodal monopoles mounted on a finiteground plane. Also, due to the deployment mechanism, eachdipole pair has a different geometric center. In additionto this, due to the varying port impedance over the wideoperational band, the astronomical antennas are used as activeantennas, where an active antenna does not remove energyfrom the field being measured, but instead acts as an electricfield probe.

The resulting astronomical antenna is composed of six wireantennas deployed in three orthogonal directions [15], asdepicted in Figure 3. Antennas are internally housed duringlaunch. To minimize the volume taken up by the payloadwithin the satellite, the antennas are deployed in groups ofthree at either end of the satellite. Antipodal pairs C–D andE–F are electrically coupled and pseudo antipodal pair A–Bis electronically coupled to form dipole pairs.

Figure 3. Nano-satellite exterior. The solid back and graylines are used to sketch the nano-satellite, and the solar pan-els, respectively, the dashed lines represent the wire antennas,and the arrows are used for dimensioning.

The exterior of the satellites is also used to mount thetwin deployed solar panel arrays [16], the surface-mountedpatch antennas for inter-satellite communication and Earthdownlink, the integrated propulsion system, and the surface-mounted astronomical source sensors.

The monopoles are lengths of copper wire enclosed by radio-frequency (RF)-transparent structures with a triangular rol-lable and collapsible (TRAC) [17] cross section (as depictedin Figure 4) to keep the antennas rigid. The diameter ofthe copper wire is 0.24 mm, which is twice the skin depthof copper at the lowest frequency of the operational bandof 300 kHz. This diameter ensures that the antenna lossresistance is at a minimum while keeping the antennas as thinand as light as possible.

Simulations

On each satellite, the observation antenna system consists ofthree orthogonal dipoles designed to work within the con-straints of a nano-satellite. Due to mechanical constraints, thedipoles are not optimally integrated into the nano-satellitesfrom an antenna point of view. Therefore, the effects ofthe finite non-uniform ground plane, the non-symmetricalantenna deployment, and the non-infinitesimal dipole gap onthe antenna properties need to be investigated.

Using WIPL-D [18], a model of the exterior of a satellite,

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w

w

t

Figure 4. Antenna cross section.

with solar panels and astronomical antennas, is simulated toderive multiple properties of the astronomical antenna. Insimulating the passive port impedances, the feed lines can bedesigned and the resonance frequencies of the monopoles canbe determined. In exciting the monopole under test, its radi-ation efficiency can be simulated. Also, the electromagneticport parameters of each monopole are simulated to derive theantenna patterns.

The resonance frequencies of the monopoles are determinedby simulating the reactance of the monopoles. From thesimulations, the monopoles have three resonance frequenciesof interest, as depicted in Figure 5. The first, around 16 MHz,is equal to the half-wave resonance. The second, around29 MHz, and the third, around 33 MHz, are at either sideof the full-wave resonance. The offsets in the resonances canbe attributed to the finite and non uniform ground plane.

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Figure 5. Reactance as a function of frequency.

The radiation efficiency of the monopoles is determined bysubtracting the resistance of the ideal monopoles from theirrealized versions, thereby removing the conduction loss re-sistance and the dielectric loss resistance from the radiationresistance. From the simulations, the monopoles have lowradiation efficiencies in the low end of the operational band,with nearly perfect radiation efficiencies above the half-waveresonance, as depicted in Figure 6.

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Figure 6. Radiation efficiency as a function of frequency.

From the simulated results of Figure 6, it is evident that amonopole as long as possible is desired, as this would resultin a higher radiation efficiency, and in turn higher gain, inthe lower half of the operational band. However, from thesimulated results of Figure 5, it is evident that this wouldshift the antenna pattern at longer wavelengths within theoperational band, with the desire to not exceed a wavelengthto avoid side lobe domination in the antenna pattern.

The antenna pattern above the third resonance of 33 MHzis simulated, as depicted in Figure 7. The antenna patternfor each monopole consists of multiple lobes of various gain,as depicted in Figure 7a. The desired antenna pattern of asingle beam cannot be recovered by forming dipole pairs, asdepicted in Figure 7b. However, the desired single beam isstill present at frequencies around 35 MHz, thereby limitingthe dipole lengths to about one wavelength.

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Figure 7. Antenna Pattern at 45 MHz.

The antenna pattern around the third resonance of 33 MHz issimulated, as depicted in Figure 8. The antenna pattern foreach monopole consists of multiple lobes of various gain, asdepicted in Figure 8a. The desired antenna pattern of a singlebeam is recovered for each pair by forming dipole pairs, asdepicted in Figure 8b. Therefore, forming dipole pairs is

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required to maintain the desired antenna pattern. However,the beam is narrower than the desired 3 dB width of 90◦.

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Figure 8. Antenna Pattern at 33 MHz.

The antenna pattern around the second resonance of 29 MHzis simulated, but there are no obvious aberrations in theantenna pattern of any of the monopoles. Also, based onthe simulations with the twin-deployed solar panel arraysadded to the model, the presence of these solar panels has noobvious effect on the antenna patterns until around the thirdresonance of 33 MHz, which is outside of the operationalband, and therefore of no concern.

The antenna pattern around the first resonance at 16 MHzis simulated, as depicted in Figure 9. The antenna patternfor each monopole consists of four lobes of various gain,as depicted in Figure 9a. While the desired antenna patternof a single beam is recovered by forming antipodal dipolepairs C–D and E–F, the antenna pattern for pseudo-antipodaldipole pair A–B is undesired, as depicted in Figure 9b. Thisis attributed to the 30 cm gap between these two monopoles.

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Figure 9. Antenna Pattern at 16 MHz.

The problem band around the half wave resonance can behandled in a number of ways. Either the half wave resonancecan be shifted to be outside of the operational band by reduc-ing the monopole lengths to be at most 2.4 meters. However,this would result in a decrease in the radiation efficiency.

Alternatively, the problem band can be filtered out by using anotch filter, thereby marginally reducing the operational band.Or, specialized calibration techniques can be derived for thispart of the band, thereby reducing the sensitivity.

4. INTER-SATELLITE LINK ANTENNA SYSTEMChallenges and requirements

In the OLFAR swarm the ISLs will have to support high-speed data transmission (exceeding 6 Mbit/s/satellite) overconsiderable distances [19]. The satellites will be randomlyspread in a 100-km diameter cloud and will form baselinesof different length (10 to 100 km) and orientation (betweenthem). Starting from these assumptions a link budget anal-ysis was conducted in [20]. It resulted in an antenna gainrequirement of 5 dBi. Since the ISL must work for any linkdirection, the antenna systems should be able to provide thegain for any direction of transmission or reception. Addedto this, the ISL antennas should cover the whole 4π-sr (solidangle) range.

Design and implementation

The cubesat implementation is an important factor in thedesign of the ISL antenna system since it limits the use of verycomplex deployment mechanisms, and the available surfacearea. Therefore, the best scenario would be to use smallconformal antennas and to keep the number of antennas beas low as possible.

Radiating patches make an excellent candidate for cubesats.They provide reasonable gain figures (up to 9 dBi), and canbe easily integrated into planar structures. Added to this, theycan be highly efficient (around 80%), are easy to match, andrequire no deployment.

In [21] an ISL antenna system for a 3U cubesat is proposed.It uses a configuration of six patches, one on each facet ofthe satellite, as shown in Figure 10. For any direction oftransmission or reception, at most three antennas will be used(X , Y and Z), as illustrated in Figure 11a.

Figure 10. Proposed antenna configuration for a 3U cubesat.

(a) (b)

Figure 11. Proposed (a) global coordinate system and (b)individual antenna coordinate system.

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In order to meet the gain requirement and ensure that thesystem functions properly, a beamforming approach is usedto control the antennas. Maximum directivity is achievedfor any link direction by assigning different weights to thecontributing antennas (X , Y and Z). The complex weightfor each individual antenna i (where i can be X , Y or Z) iscalculated as:

wi =√ci e

jβi , (1)

where

ci =Di(θi, φi)∑

j

Dj(θj , φj)(2)

is the power coefficient for optimal distribution of power,

βi = k di sin θi. (3)

is the necessary phase correction for in-phase adding of theE-fields, (θ, φ) is the link direction as referred to the globalcoordinate system (as shown in Figure 11a), (θi, φi) the linkdirection referred to its local coordinate system (as shownin Figure 11b), Di(θi, φi) is the normalized directivity ex-pressed in the local coordinate system, di is the distance fromthe antenna system coordinates’ origin to the other satellite,and k is the wave number.

Another very important and influential aspect of the ISL is thechoice of the individual antenna. The performance of the linkbetween the satellites strongly depends on the characteristicsof the antennas. Properties such as gain, bandwidth, radiationpattern and polarization are key elements for a good imple-mentation. A circularly polarized patch with a resonancefrequency of 2.45 GHz was initially selected so that it fitsthe cubesat’s facets, and meets the 5 dBi gain and the 90◦half-power beamwidth requisites. Nonetheless, the limitedbandwidth and the unsatisfying axial ratio might need animproved solution.

Simulations and measurements

The ISL antenna system will consist of six circularly polar-ized patches (placed one on each facet of the 3U cubesat), andan antenna system controller that will maximize the directiv-ity in the desired direction of transmission or reception. Ananalytical model was developed to evaluate the performanceof the system. Figure 12 consists of the resulted radiationpattern for an arbitrarily selected link direction (θd, φd) of(58.1◦, 38.3◦) and the radiation pattern of an antenna se-lection scheme. It can be observed that the beamformingprovides a gain increase for the proposed link direction.

The peak gain of the considered antenna system was calcu-lated also for other link directions, and the results are plottedin Figure 13(a). The graph proves that our solution meets the5-dBi requirement for any direction of the ISL.

Furthermore, an evaluation platform with COTS componentswas built to test the antenna system. It consisted of atransmitting antenna and a two antennas for reception, placedon a cubesat platform, on the perpendicular ’faces’. Theresult of the measurements is displayed in Figure 13(b). Thisresult differs substantially from the one of the ideal scenariosimulated before (see Figure 13(a)) due to multiple causes,such as the imperfections of the testing environment and ofthe setup, and the inaccurate model for the real antennas.However, the measurement result fundamentally supports theproposed antenna system as it shows that a beamformingapproach provides a better directivity than using individualantennas.

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Figure 12. Antenna system radiation pattern for (θd, φd) =(58.1◦, 38.3◦), both using antenna selection and conformalbeamforming.

5. DOWNLINK ANTENNASChallenges and requirements

A different approach was used when designing the antennasfor the communication link between the swarm and the BS.It has been shown that, if we consider a lunar orbit, placing asingle patch antenna on the cubesat’s outer surface will notsuffice for establishing a reliable downlink [22]. Yet, theswarm diversity can be used to improve the signal-to-noise(SNR) of the link by as much as 10 dB.

The pointing of the downlink antennas will not matter asmuch as ISL antennas, and, as a result, they can be placedon a single facet of the spacecraft. A good solution for thisis to place the antennas on the backside of the solar panels.After considering the design of proposed by Klein in [16],the available space for downlink antennas resulted to be twoareas of 41.4 cm × 34 cm (one on each of the two panels),and a thickness of 0.7 mm with an FR-4 available substrate.

With these being said, the driver of the design was to build ahigh gain (more than 10 dBi) antenna system that would helpthe system downlink data towards Earth while loosening therequirements for the EPS and for the ground station.

Design and implementation

The objective of the design process of the downlink antennasystem was to optimize the RF properties of the system (di-rectivity, side-lobe level, beamwidth) while efficiently usingthe available resources (materials, area, thickness).

Also in this case the cubesat platform is a very importantlimiting factor in the design process. The low power availableto establish a link with a ground BS imposes the need ofa high-gain system. Furthermore, the use of a standardizeddeployer bounds not only the available space but also thetype of structure to be used. Since the solar panels have tofit into the ISISPOD [23] alongside the cubesat, they requireto be modular foldable structures. The solution proposed byKlein [16] specifies that each solar panel is made out of fivepieces, each 34 cm × 8.2 cm. As a result, a single antennaper panel implementation would not use the area efficiently,since it could occupy only one fifth of the available surface.

A more suitable implementation is to build an array of simpleplanar antennas and feed them through integrated microstriplines. Aperture-fed patches make an excellent candidate andcan be implemented using the FR-4 substrate. We opted, thus,

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Figure 13. Antenna system gain for different link directions (for θ = θd = 90◦).

for a patch antenna array with a resonant frequency fdown

of 2.4 GHz. The central frequency was chosen so that thewidth of antennas fits the 8.2-cm width of the subpanels andcreates a spacing between the elements of the array that islarger than a quarter of the wavelength and smaller than halfof the wavelength.

In Figure 14, a sketch of the patch implementation is pre-sented. The following notations have been used:

• Wpanel is the width of a subpanel;• Lpanel is the length of a subpanel;• Wpatch is the width of the patches;• Lpatch is the length of the patches;• hsubstrate is the thickness of the substrate;• hground is the thickness of the ground plane;• hpatch is the thickness of the patches;• darray is the array spacing, and represents the distancebetween two adjacent radiating edges;• PEC refers to a perfect electrical conductor, and was thematerial chosen only for simulations.

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Figure 14. Sketch of the patch antennas implementation.

In [24] the parameters of the individual patch antenna arecalculated. The values are summarized in the Table 1.

Table 1. Downlink antenna parameters.

Parameter Value [Unit]Wpanel 8.2 cmLpanel 34 cmWpatch 3.68 cmLpatch 2.88 cmdarray 4.62 cmhpatch 0.01 mmhground 0.01 mmhsubstrate 0.68 mmεr 4.4fdown 2.4 GHz

Having calculated the width and length of each patch antenna,it resulted that each subpanel can accommodate an array offour patches. Therefore, on the back of each of the two solarpanels a five-by-four rectangular array of patches was placedas shown in Figure 15.

Figure 15. Proposed solar array (left) and patch array (right)integrated in the deployable panels of a 3U-cubesat.

The feeding of the antennas plays an important role in theperformances of the array. Since the two solar panels areseparated by the cubesat structure and the sustaining rods, thedistance between them is roughly a wavelength, and, thus, inour model we considered the two five-by-four arrays as twoseparate systems. For each of the panels, we distributed thepower to the antennas using a binomial scheme [25].

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Simulations

To evaluate the performance of the proposed downlink ar-ray we built an analytical model using CST and conductedfarfield simulations. The model made use of available ma-terial characterizations for the FR-4 substrate. The metal-lic parts were modeled as perfect electric conductor (PEC)as their influence is not significant. Figure 16 shows theradiation pattern of the five-by-four array of patches whenconsidering uniform and binomial power distribution in thefeed network. It can be observed that in the binomial casethe performance in terms of beamwidth and side-lobe level isbetter, at the cost of a lower directivity.

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Figure 16. Farfield radiation pattern of a single panel array.

The selection of the power weighting scheme will also de-pend on other subsystems of the cubesat. For example,a stabilized cubesat with precise pointing capabilities willmake use the higher directivity scheme while an unstabilizedspacecraft would be better off with a scheme that has a largerbeamwidth and radiates less power outside the main beam.

In addition, we simulated a setup consisting of two identicalpanels that were placed 56.4 cm apart (center-to-center). Thevalue of the spacing is the result of adding the sizes of thepanels, the size of the cubesat and the size of the supportingrods (yokes) for the solar arrays. Binomial power distributionwas assigned to both arrays. The resulted farfield radiationpattern is plotted in Figure 17.

It can be seen that the scheme exhibits a 3 dB increase. Thisis due to the fact that the spacing between the radiating edgesof the two arrays is roughly one wavelength. However, ifspacing between the panels will increase or the wavelengthwill decrease, this will result in a high-level of the side-lobes. The 3 dB increase in directivity can still be attained

−15

−5

15

35 dBi

30°

210°

60°

240°

90°

270°

120°

300°

150°

330°

180° 0°

Dual panelSingle panel

Figure 17. Farfield radiation pattern of the dual-arraysystem.

for any desired direction of transmission with the adequatephase shifting between the two arrays. A similar algorithm asin the ISL scenario can be employed.

6. FUTURE WORK AND CONCLUSIONSCurrent work focuses on the RF aspects of a low-frequencyradio telescope implemented with a swarm of nano-satellites.Although based on a well-known theoretical consideration,fulfilling the science task will be very challenging from thehardware point of view. Morever, the novel cubesat imple-mentation takes all the challenges to a new level.

We proposed a simple functional architecture for a nano-satellite to be used for low-frequency radio astronomy, andto that the design considerations of the multiple requiredantenna systems were attached. Three separate systems wereimplemented to fulfill each of the main sub-tasks: radioobservation, ISL, and downlinking processed data to the BS.The astronomical antennas were designed according to theneed to detect signals from all directions, thus, the systemuses of three orthogonal very long dipoles with finite groundplanes. Simulations were performed to analyze the effect ofthis non ideal configuration.

Due to the finite ground plane the monopoles do not have an-tenna patterns that match the theoretical values for monopoleson infinite ground planes. However, in forming dipoles ofantipodal pairs, the expected patterns are recovered for twoof the three pairs over the full operational band. Due to thenon symmetrical antenna deployment and the extended dipole

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gap, the antenna pattern of the third pair is not recovered fora small range around the half wave resonance.

A conformal antenna system using circular polarized patcheswas designed to establish and maintain a communication linkbetween cubesats regardless of their relative orientation. Abeamforming approach was necessary to provide the requiredgain for any link direction. The simulation and measurementsconfirmed the plausibility of the system. Further work on theindividual antenna needs to be done to meet the polarizationand bandwidth requisites.

Lastly, a downlink array was developed and accommodatedby the solar panels. The residual space available on thedeployable structures was used to create a high-directivitysystem. This can be exploited to make requirements for othersubsystems of the cubesat (e.g. EPS) less harsh. As stated inSection 5, the performance of this system strongly dependson the downlinking strategy or the pointing capabilities of thesatellites.

Until now the antenna system was individually designed.Future work will have as a mindset the final implementationof the OLFAR cubesat. The integration of the systems intothe platform and compatibility analysis still have to be done.

ACKNOWLEDGMENTSThis research is supported by the Dutch Technology Founda-tion STW, which is part of the Netherlands Organization forScientific Research (NWO), project #10556 (OLFAR).

The authors would like to acknowledge Teodoro J. WillinkCastro, and Jan Idserda of ASTRON for their contributionand support.

REFERENCES[1] A. W. Gunst and M. J. Bentum, “The lofar phased array

telescope system,” in Phased Array Systems and Tech-nology (ARRAY), 2010 IEEE International Symposiumon, Waltham, MA, October 2010, pp. 632–639.

[2] J. Bregman, System Design and Wide-field ImagingAspects of Synthesis Arrays with Phased Array Stations:To the Next Generation of SKA System Designers. Uni-versity Library Groningen, 2012. [Online]. Available:http://books.google.nl/books?id=b2GklAEACAAJ

[3] S. Jester and H. Falcke, “Science with a lunar low-frequency array: From the dark ages of the universe tonearby exoplanets,” New Astronomy Reviews, vol. 53,no. 1, pp. 1–26, 2009.

[4] M. J. Bentum and A.-J. Boonstra, “Low frequency as-tronomy — the challenge in a crowded rfi environment,”in General Assembly and Scientific Symposium, 2011XXXth URSI, Istanbul, Turkey, August 2011, pp. 1–4.

[5] E. K. A. Gill, P. Sundaramoorthy, J. Bouwmeester,B. Zandbergen, and R. Reinhard, “Formation flyingwithin a constellation of nano-satellites: The {QB50}mission,” Acta Astronautica, vol. 82, no. 1, pp. 110 –117, 2013.

[6] M. J. Bentum, C. J. M. Verhoeven, and A.-J. Boon-stra, “Olfar, orbiting low frequency antennas for radioastronomy,” in ProRISC 2009, Annual Workshop onCircuits, Systems and Signal Processing, Utrecht, the

Netherlands, November 2009.

[7] N. Saks, A.-J. Boonstra, R. T. Rajan, M. J. Bentum,F. Beilen, and K. van ’t Klooster, “DARIS, a fleet ofpassive formation flying small satellites for low fre-quency radio astronomy,” in The 4S Symposium (SmallSatellites Systems & Services Symposium), (ESA andCNES conference), Madeira, Portugal, May–June 2010.

[8] A. Budianu, R. T. Rajan, S. Engelen, A. Meijerink,C. J. M. Verhoeven, and M. J. Bentum, “Olfar: adaptivetopology for satellite swarms,” in 62nd InternationalAstronautical Congress, Cape Town, Republic of SouthAfrica, October 2011, pp. 1–9.

[9] S. Engelen, E. K. A. Gill, and C. J. M. Verhoeven, “Onthe reliability, availability and throughput of satelliteswarms,” IEEE Transactions on Aerospace and Elec-tronic Systems, 2013, (Accepted, July 2013).

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[11] R. Nugent, R. Munakata, A. Chin, R. Coelho, andJ. Puig-Suari, “The cubesat: The picosatellite standardfor research and education,” Aerospace Engineering,vol. 805, pp. 756–5087, 2008.

[12] E. D. van Breukelen, A. R. Bonnema, W. J. Ubbels, andR. J. Hamann, “Delfi-c3: Delft university of technolo-gys nanosatellite,” in Proceedings of the 4S Symposium:Small Satellites, Systems and Services, Chia Laguna,Italy, September 2006.

[13] “Cubesat design specification,” August2010, revision 12. [Online]. Available:http://www.cubesat.org/images/developers/cds rev12

[14] D. M. P. Smith, M. J. Arts, A.-J. Boonstra, and S. J.Wijnholds, “Characterisation of astronomical antennafor space based low frequency radio telescope,” inAerospace Conference, 2013 IEEE, Big Sky, MT, March2013, pp. 1–9.

[15] K. A. Quillien, D. M. P. Smith, S. Engelen, E. K. A. Gill,M. J. Arts, and A. Boonstra, “Astronomical antennafor a space-based low frequency radio telescope,” inAIAA Annual Conference on Small Satellites, Logan,UT, August 2013.

[16] J. M. Klein, A. Budianu, M. J. Bentum, S. Engelen, andC. J. M. Verhoeven, “Design of an electric power systemwith incorporation of a phased array antenna for olfar,”in 64th International Astronautical Congress, Beijing,China, September 2013, pp. 1–6.

[17] F. A. Roybal, J. A. Banik, and T. W. Murphey, “Devel-opment of an elastically deployable boom for tensionedplanar structures,” in 48th AIAA Structures, StructuralDynamics, and Materials Conference, vol. 1838, Hon-olulu, HI, April 2007.

[18] B. M. Kolundzija and A. R. Djordjevic, WIPL-D:Electromagnetic Modelingof Composite Metallic andDielectric Structures. Software and User’s Manual.ArtechHouse, 2000.

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[20] A. Budianu, T. J. Willink-Castro, A. Meijerink, andM. J. Bentum, “Communication schemes for olfar’sinter-satellite links,” in 63rd International AstronauticalCongress, Naples, Italy, October 2012, pp. 1–5.

[21] A. Budianu, T. J. W. Castro, A. Meijerink, andM. J. Bentum, “Inter-satellite links for cubesats,” inAerospace Conference, 2013 IEEE, Big Sky, MT, March2013, pp. 1–10.

[22] A. Budianu, A. Meijerink, and M. J. Bentum, “Swarm toearth communication in olfar,” in 64th International As-tronautical Congress, Beijing, China, September 2013,pp. 1–6.

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[25] C. A. Balanis, Antenna Theory: Analysis and Design.Wiley-Interscience, 2005.

BIOGRAPHY[

Alexandru Budianu (S’12) receivedhis engineering degree in the field ofTelecommunications, from the TechnicalUniversity of Iasi, Romania in 2009 .He is currently a Ph.D. candidate withinthe University of Twente, working in theOLFAR project. His research focuses ondeveloping the communication layer forswarms of nano-satellites. His interestslie mostly in the inter-satellite links and

satellite-to-Earth communication for CubeSats.

.

David Smith (M’12) received his BEng,MScEng and PhD in Electronic Engi-neering from Stellenbosch University inthe period 2001 until 2010. From 2010until 2012, he held a post doctoral po-sition at Stellenbosch University work-ing on a LNA for APERTIF, a SKAPathfinder Project. Currently, he holds apost doctoral position at ASTRON work-ing on the OLFAR Project.

Arjan Meijerink (S’00–M’06–SM’11)received the M.Sc. and Ph.D. degrees inElectrical Engineering (both with hon-ors) from the University of Twente, En-schede, the Netherlands, in 2001 and2005, respectively. In 2000 he wasa Trainee at Ericsson Business MobileNetworks in Enschede. From 2001 hehas been working at the University ofTwente, initially as a Research Assistant,

and from 2005 as a Postdoctoral Researcher. Since 2007 hehas been an Assistant Professor in short range radio. In 2009he was a Visiting Lecturer at the Queen’s University, Belfast,U.K., and in 2010 he was a Visiting Scholar at the Universityof Southern California, Los Angeles, U.S.A. Dr. Meijerink

has published more than 50 papers in international journals,conferences and symposia. He is a member of IEEE, URSI.and the Dutch Electronics and Radio Society NERG, andhas reviewed for various journals, conferences and symposia.Currently he is Junior Secretary of the IEEE Benelux Section,and he is a member of the Executive Committee of the IEEEBenelux Joint Chapter on Communications and VehicularTechnology.

Albert-Jan Boonstra (M’09) receivedhis MSc in Applied Physics from Gronin-gen University in 1987 and his PhD fromDelft University of Technology in 2005.From 1987 until 1991, he worked at theLaboratory for Space Research Gronin-gen. Currently, he heads ASTRON’sR&D Department. His research is signalprocessing, specifically in RFI mitiga-tion for radio astronomy.

Mark J. Bentum (S’92–M’95–SM’09)was born in Smilde, the Netherlands, in1967. He received the M.Sc. degreein electrical engineering (with honors)from the University of Twente, Enschede,the Netherlands, in August 1991. InDecember 1995 he received the Ph.D.degree for his thesis “Interactive Visu-alization of Volume Data”, also from theUniversity of Twente. From December

1995 to June 1996 he was a Research Assistant at the Uni-versity of Twente in the field of signal processing for mobiletelecommunications and medical data processing. In June1996 he joined the Netherlands Foundation for Researchin Astronomy (ASTRON). He was in various positions atASTRON. In 2005 he was involved in the eSMA project inHawaii to correlate the Dutch JCMT mm-telescope with theSubmillimeter Array (SMA) of Harvard University. From2005 to 2008 he was responsible for the construction of thefirst software radio telescope in the world, LOFAR (LowFrequency Array). In 2008 he became an Associate Pro-fessor in the Telecommunication Engineering Group at theUniversity of Twente. He is now involved with researchand education in mobile radio communications. His currentresearch interests are short-range radio communications,novel receiver technologies (for instance in the field of radioastronomy), channel modeling, interference mitigation andsensor networks. Dr. Bentum is a Senior Member of theIEEE, Secretary of the Dutch URSI committee, initiator andchair of the IEEE Benelux AES/GRSS Chapter, board memberof the Dutch Electronics and Radio Society NERG, memberof the Dutch Royal Institute of Engineers KIVI NIRIA andthe Dutch Pattern Recognition Society, and has acted as areviewer for various conferences and journals.

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