The Communication and Spectrum MonitoringSystem of Smog-1 PocketQube Class Satellite

Levente Dudas, dr. Andras GschwindtDept. of Broadband Infocommunications and Electromagnetic Theory

Budapest University of Technology and Economics, Budapest, Hungary-1111Email: dudas@mht.bme.hu, gschwindt@mht.bme.hu

Abstract—As one possible continuation of development ofMasat-1, the first Hungarian satellite, in 2015, a group of student,from Faculty of Electrical Engineering and Informatics andMechanical Engineering, decided to realize a PocketQube-classsatellite, called Smog-1. Smog-1 is a 5 cm cubesat as PocketQubewith less than 180 g mass. The aim of Smog-1 is to measurethe radio frequency (RF) smog caused by human beings on theDVB-T band (Digital Video Broadcasting-Terrestrial).

Index Terms—Smog-1, PocketQube, Spectrum Monitoring,Satellite Communication

I. INTRODUCTION

The planned trajectory of Smog-1 satellite will be a polarcircular Low Earth Orbit (LEO) with 600 km apogee andperigee (with Dnyepr rocket from Russia). Because of thehigh distance from the Sun (150 million km), the solar powerdensity on a LEO is 1360 W/m2 in space. On Earth surface,the solar power density is only 1000 W/m2, because of theatmosphere (mainly by the ozone layer). In case of 5 cmsatellite, the incoming power is 2.5 W . The efficiency ofthree-layers solar cells of Smog-1 is 28 %, so the DC (DirectCurrent) power is 0.7 W . The time period of LEO of Smog-1 is 100 min: 60 min in light, and 40 min in dark. So, theaveraged input DC is 0.4 W , with at least 100 mW fluctuationcaused by the almost random movement of Smog-1 duringorbiting: 300 − 500 mW.

All on-board systems of Smog-1 will be cold-redundantand single-point-failure tolerant. On-board subsystems of thesatellite are: EPS (Electrical Power System), OBC (On-BoardComputer), COM (Communication System), SP (SPectrummonitoring system) as a payload.

II. THE PAYLOAD OF THE SATELLITE

The payload of Smog-1 will be a spectrum monitoringsystem (SP) [6]. The spectrum monitor is a single-chip ra-dio transceiver (in receiver mode) from Silicon LaboratoriesSI4464. This receiver is working from 119 to 960 MHz with1 − 850 kHz bandwidth. The monitored frequency band is430 − 860 MHz DVB-T band, where a lot of high-powerTV transmitters are working on producing interference anddisturbances on LEO as electromagnetic smog (the outgoingRF power to space is lost power). The block scheme of thesingle-chip transceiver is in Fig. 1.

The receiver part of this transceiver is a conventional super-heterodyne receiver with digital IF (Intermediate Frequency)

Fig. 1. Single-chip radio transceiver as a spectrum monitor

unit. This chip contains a wide-range fractional-PLL (PhaseLocked Loop) as a local oscillator, a wide-band LNA (LowNoise Amplifier) and mixer, PGA (Programmable Gain Am-plifier) as IF amplifier with low pass filter, I-Q ADCs (Ana-logue to Digital Converters) and digital OOK-FSK MODEM(MOdulator and DEModulator).

In case of spectrum monitor, the task is to tune the carrierfrequency of the receiver from 430 to 860 MHz and readthe RSSI (Received Signal Strength Indicator) register of thereceiver chip.

The measured amplitude transfer function of IF filter with800 kHz bandwidth is in Fig. 2. The effect of local oscillatoris highly visible near the center of the curve. The mirrorselectivity of the receiver is better than 45 dB. This curveis measured by setting the carrier frequency of the receiver to434 MHz and tuning the RF CW (Continuous Wave) generatorwith a fix power level and reading the RSSI value.

The used bandwidth equals to 800 kHz (the maximalbandwidth of the receiver chip is 850 kHz). This bandwidthis ten times less than the bandwidth of the DVB-T channels,so 10 dB correction is necessary between the RSSI value andthe real received power level. The measured RSSI curve isshown in Fig. 3. From −100 to −10 dBm input power rangethe curve is linear in dBm.

III. PRELIMINARY IN-SITU SPECTRUM MONITORMEASUREMENTS

A simplified spectrum monitoring system of Smog-1 satel-lite was flown by HAB (High Altitude Balloon) to measurethe DVB-T signal levels versus altitude. The experimentalmeasurement system is in Fig. 4 [6].

978-1-5090-2214-4/16/$31.00 ©2016 IEEE

Fig. 2. Transfer function of IF filter of the spectrum monitor if RBW(Resolution BandWidth) is 800 kHz

Fig. 3. Measured RSSI curve of the spectrum monitor versus input RF power

Fig. 4. The payload of high altitude balloon measurements

This measurement system contains a single power supplyunit with linear voltage regulator (for heating the payload: −60degrees environmental temperature), the spectrum monitorreceiver with differential antenna input, a radio transceiver tobe able to communicate with the ground station, a C8051F326type 8 bits micro-controller and GPS module in flight-modefor positioning the balloon.

The antenna of the spectrum monitor is a 2 x 180 mmdipole, used in wide frequency range: 430 − 860 MHz.

The impedance curve of this dipole antenna is in Fig. 5.The real part of the input impedance of dipole in DVB-

T frequency range is between 100 and 2000 Ω . There isno matching network in such wide frequency range, but thedifferential input impedance of the receiver is 450 Ω parallelwith 1.2 pF. So, the measurement dipole is connected directlyto the input of receiver and the task is to calibrate themeasurement system in our anechoic-chamber. The calibrationmeasurement arrangement was: transmit CW RF signal withknown power level using log-periodic wideband antenna in

Fig. 5. The input impedance of measuring dipole

front of the spectrum monitor, and read the RSSI value of thereceiver while tuning the generator and the receiver carrierfrequency. The result of this measurement is a calibrationvector related to the incoming RF signal to the measurementantenna and the recorded RSSI value.

After calibration, there were 4 in-situ measurements flownby HAB. The recorded power spectrum of DVB-T band versusaltitude is in Fig. 6.

Fig. 6. DVB-T power spectrum versus altitude flown by HAB

In accordance with the HAB measurement results, a hugeamount of RF power is radiated to the space and this power islost power (it is heating the space and causing RF disturbancesin case of LEO satellite communication).

IV. THE COMMUNICATION SYSTEM OF SMOG-1SATELLITE

The communication system of Smog-1 is based on a RF-micro-controller from Silabs SI1062. It contains a SI4460digital radio module and C8051F930 micro-controller in asingle QFN (Quad Flat No-Lead) package.

The block scheme of the communication and spectrummonitoring system of Smog-1 is in Fig. 7.

All of on-board systems of Smog-1 is cold-redundant anddesigned as single-point failure tolerant system [3], [4]. So,two different UHF (Ultra High Frequency) telemetry trans-mitters and two different UHF telecommand receivers areconnected to the communicational antenna across SWR-bridge

Fig. 7. Block scheme of COM and SP of Smog-1

(Standing Wave Ratio) to be able to monitor the forwarded andreflected RF power of the on-board transceiver.

On transmitter side, the output of the 4460 radio is con-nected to a power amplifier FET (Field Effect Transistor). Theoutput signal is low pass filtered for the right harmonic sup-pression and connected to RF switch realized by PIN (PositiveIntrinsic Negative) diodes. The bias-point of the power FET isdigitally controlled with DAC (Digital to Analogue Converter)to get the optimal efficiency and consumption versus powersupply voltage level.

On receiver side, the communicational antenna is connectedto a LNA with RF BPF (Band Pass Filter) and then to thereceiver input.

The communication system contains the spectrum monitorreceivers. The spectrum monitor receivers are connected dif-ferentially to the measurement dipole antenna using also PINdiodes.The on-board systems of Smog-1 have own local intelligence,so the micro-controller of COM and SP is connected tothe OBC with digital UART bus (Universal AsynchronousReceiver / Transmitter).

The internationally coordinated operation frequency of thecommunication system of Smog-1 is 437.345 MHz. Thelicensed bandwidth are 12.5 kHz for uplink and 20 kHz fordownlink.

The modulation of Smog-1 telemetry and telecommandsystem is OOK (On-Off Keying) and 2-GMSK (GaussianMinimal Shift Keying as Frequency Shift Keying). Slow basictelemetry data connection can be realized with OOK (Morse-code) and 1200 bit/s 2-GMSK modulation, with less than 2400Hz bandwidth, which is a conventional radio-amateur SSBradio bandwidth (Single Side Band) - radio-amateurs all overthe world will be able to receive Smog-1 signal.

The data of spectrum monitoring system can be downloadedwith 13333 bit/s (20 kHz BW) data rate GMSK after groundstation telecommand. GMSK modulation is necessary for theoptimal spectral efficiency. The BER (Bit Error Rate) curve

of (G)MSK signal is in Fig. 8 [1].

Fig. 8. Minimal Shift Keying Bit Error Rate curve versus Eb/N0 in dB

The required uncoded BER value is 10−3, which means 8dB Eb/N0.

Because of the limited input DC power, the transmit powerof Smog-1 is 100 mW (20 dBm) with 30 % efficiency.

According to 600 km apogee/perigee of the orbit, themaximal distance between the satellite and the ground stationis 3000 km (communication in zero degree elevation angle -horizon). On the 437.345 MHz, the free-space loss is (1).

a0 = 20lg(4πd/λ) = 155dB (1)

Where d = 3000 km, and λ is the wavelength (0.69 m).The on-board communication antenna will be a quasi-

omnidirectional antenna. The simulated 3D radiation patternis in Fig. 9.

Fig. 9. 3D radiation pattern of COM antenna of Smog-1

In practice, the measured fluctuation of the radiated RFpower is less than 10 dB, caused by the almost randommovement of the satellite.

The radiated power from Smog-1 is 20 dBm, and theautomated and remote controlled ground stations have at least16 dBi gain antenna system with azimuth and elevation rotator.So, the received power level is: 20dBm− 155dB+ 16dBi =−119dBm.

The thermal noise power level is (2).

Pn = 10lg(kTB) = −128dBm (2)

Where k is the Boltzmann-constant, B is the maximalbandwidth of the telemetry signal (20 kHz), T is the equivalentnoise temperature (500 K) of the receiver system includingantenna. The calculated SNR (signal to Noise Ratio) is:−119dBm − (−128dBm) = 9dB, means better than 10−3

non-coded BER in worst case, so the communication link isclosed.

V. THE REALIZED COM AND SP OF SMOG-1

The prototype of the communication and spectrum moni-toring system is in Fig. 10.

Fig. 10. The realized prototype of COM + SP of Smog-1

The laboratory measurement results of COM & SP are inTable I.

TABLE ICOM & SP MEASUREMENT RESULTS FROM 3.3V

Mode DC Current DC PowerTX 100mA 330mWRX 41mA 136mWSP 32mA 106mW

In TX mode (transmit), the output RF power is 20 dBmwhile the DC power is 330 mW, so, the total efficiency of TXis 30 %. The lost 230 mW power heats the Li-Ion battery tobe able to operate in dark side of the orbit.

In worst case scenario, the ratio of TX and RX (reception)mode will be 50 − 50 % (half duplex). So, the averagedconsumed power of COM & SP is 233 mW. It is less thanthe minimal input DC power from EPS 300 mW, so there willbe enough energy for the normal-mode operation of Smog-1.

VI. CONTROLLING THE SATELLITE FROM THE EARTH

The other side of the communication system of Smog-1is the satellite control station [5]. There are two automatedand remote controlled ground stations in Budapest (primary,JN97ML) and Erd (secondary, JN97KJ), in Hungary. Theseground stations are able not only to receive but control thesatellite (Masat-1 in past, Smog-1 in future).

The realized antenna system of the primary ground station(GND) of Smog-1 is in Fig. 11 [2].

Fig. 11. The antenna of primary ground control station of Smog-1

GND in Budapest (BUTE) has a parabolic reflector typeaperture antenna with a circular back-fire helix primary radi-ator on 437 MHz. The antenna gain is 21 dBi for linear (24dBi circular) polarized RF signal (from Smog-1), the width ofthe main lobe of the antenna is: 10 deg. (-3 dB), 18 deg. (-10 dB) and 22 deg. (between null-points).

GNDs are remote controlled via Internet and automatized.The control computer of GND is Raspberry PI, it can controlthe azimuth-elevation antenna rotator, calculate the route oftracked satellite and the Doppler-shift of RF signal, controlthe radio transceiver including PA and LNA. The output RFpower of primary GND is 300 W RF + 21 dBi antenna gain,secondary GND is 120 W RF + 16 dBi antenna gain.

VII. CONCLUSION

In accordance with the measurement results, the communi-cation and spectrum monitoring system of Smog-1 are workingproperly not only in laboratory, but in-situ environment. Therealized GNDs of Smog-1 are able to control and receive notonly the Smog-1 satellite automatically and remote controlled.

ACKNOWLEDGEMENT

The development of Smog-1 satellite and upgrading ofprimary ground station of Smog-1 are sponsored by NationalMedia and Infocommunications Authority, Hungary.

REFERENCES

[1] RODDY, D., Satellite Communications, Fourth Edition, McGraw HillHigher Education, 2006

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

[3] Levente DUDAS, Lajos VARGA, Rudolf SELLER, The CommunicationSubsystem of Masat-1, the First Hungarian Satellite, Signal ProcessingSymposium, Jachranka, Poland, 2009

[4] Levente DUDAS, Lajos VARGA, Masat-1 COM, Antenna Systems &Sensors for Information Society Technologies COST Action IC0603,Dubrovnik, 2010

[5] Levente DUDAS, Levente PAPAY, Rudolf SELLER, Automated andRemote Controlled Ground Station of Masat-1, the First HungarianSatellite, International Conference Radioelektronika, Bratislava, Slovakia,2014

[6] Levente Dudas, Laszlo Szucs, Andras Gschwindt, The Spectrum Mon-itoring System of Smog-1 Satellite, 14th Conference on MicrowaveTechniques, CO- MITE 2015, Pardubice, pp. 143-146, ISBN:978-1-4799-8121-2