Optical Communication Payload for an Experimental Microsatellite

Optical communication payload for an experimental microsatellite inlow-earth orbit

Javier Mendieta, Enrique Pacheco, Arturo Arvizu, Ramon Muraoka∗

Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE),Ensenada, Baja California, México, 22860

ABSTRACT

The SATEX I project, is a Mexican effort with the purpose of design, construct and operate an experimental microsatellite inlow-earth orbit, in a university and multi-institutional environment. The scientific mission is focused on electronictelecommunications research with a Ka band experiment and the optical payload; also, a CCD camera is included for remoteimage acquisition. The SATEX Optical Payload (SOP) is an experimental system aimed to perform BER and attenuationmeasurements. The SOP consists in a laser transmitter in the 830 nm and a quad-photo receiver in the 530 nm. Theexperiment is divided in two features: the downlink where the measurements will be performed and the uplink that will beused to perform the pointing of the experiment. The SOP has a control system to establish and keep a link by opto-electro-mechanical means, which tracks and acquires the optical beacon. This beacon is a non-modulated light source generated bythe earth station. When the satellite receives it, it is then used to acknowledge the location of the earth station; therefore, themodulated laser beam can be transmitted to Earth. The technical looks of design of all the subsystems and the prototypeobtained are presented and the link calculation is discussed.

Keywords: Free-space laser communications, lasercom, satellite communication, SATEX, microsatellite, opticalcommunication, downlink, uplink, tracking, pointing.

1. INTRODUCTION

The SATEX I project surged with the purpose to consolidate the development of space sciences and telecommunications inMexico. The development of space activities in Mexico is considered important because it generates state of the arttechnology, reaching towards other fields of science and industry with technical knowledge essential for technologydevelopment. It is also recognized as an opportunity to validate new technologies in telecommunications, and allowedachieving highly skilled human resources. Therefore, the objective of the SATEX I project is the design, construction andoperation of an experimental microsatellite in a university and multi-institutional environment. This allows the developmentof a scientific mission focused on electronic telecommunications research as well as the generation of a platform withevolution and adaptive capabilities to diverse requirements. The design of the satellite must combine simplicity, low cost andease of integration and reinforce the collaboration of different research groups.

The institutions taking part in this effort now are: Instituto de Ingeniería de la Universidad Nacional Autónoma de México(UNAM), Centro de Investigación en Matemáticas, Escuela Superior de Ingeniería Mecánica y Eléctrica del InstitutoPolitécnico Nacional (ESIME-IPN) through its Escuela de Aeronáutica, la Sección de Graduados, and the Centro deInvestigación y Desarrollo de Tecnología Digital (CITEDI), and CICESE. Each institution has developed a differentsubsystem of the satellite, whose design is based on the restrictions of the Ariane Structure for Auxiliary Payload (ASAP)rocket launcher. These restrictions are the maximum weight of 50Kg and in fact 47.5 Kg because the use of the structureadapter. The dimensions are 50x50x50 cm. The satellite is planned to be rotating in a low polar orbit of 780 km. Table 1shows a summary of the most important global data of the mission.

∗ email: [email protected], [email protected], [email protected], [email protected]. +52 6 175 0554 Fax.+52 6 175 0555http://www.cicese.mxUS mail address: CICESE Research CenterPO BOX 434944, San Diego, CA 92143-4944

Free-Space Laser Communication Technologies XIII, G. Stephen Mecherle, Editor,Proceedings of SPIE Vol. 4272 (2001) © 2001 SPIE · 0277-786X/01/$15.0016

The structure of the satellite has been designed in Escuela de Aeronáutica of ESIME-TICOMAN. The other working groupsdeveloped the remaining components. The satellite is planned to carry three payloads: an experiment of a Ka band radiotransmitter mainly for attenuation analysis; a CCD camera to obtain images and the SATEX Optical Payload (SOP). Theearth station for controlling the satellite is located in Ensenada, Baja California, in the north of Mexico.

Height 780 KmOrbit PolarMass 50KgStabilization Boom and coils

PayloadKa ExperimentOptical Communications Experiment and,CCD Camera

Orbit period Aprox. 100 minutesMission Life One yearEstimated eclipse time 34 minutesSun exposure time 66 minutes in each orbitVacuum 10 -8 TorrsRadiation* 10 -3 rads

Table 1: Principal aspects for the SATEX I mission.*Approximate total of radiation dose in polar orbit for an estimated mission period of a year.

1.1. Orbital characteristicsThe definition of the space mission is in principle the one that determines the primary design characteristics andspecifications for whatever subsystems needed onboard the satellite. Since the Earth revolves over its polar axis during thistime, the satellite never passes two consecutive times over the same place. For that reason it will be visible from the earthstation only at certain times, when its orbit allows the satellite to be useful. SATEX I allow a useful visibility time of onlybetween 4 to 10 minutes, depending upon its trajectory over the earth station. This means that we only have that time tocontact the satellite, to upload command and control data, download telemetry and experimental results, and prepare it for thenext pass. The distance between the earth station and the satellite varies between a maximum of 1,362 km at the horizon to aminimum of 780 km above the earth station, as shown in Figure 1. This implies a longitude of arch (traveled distance) of2,370 km to an instantaneous tangential velocity of almost 27,000 km/ h (7.5 km/ s)1.

Figure 1: Orbital characteristics for the SATEX I mission.

Proc. SPIE Vol. 4272 17

2. SOP EXPERIMENT LINK CALCULATIONS

The SATEX Optical Payload (SOP) is an experimental system aimed to perform BER and attenuation measurements. Thisexperiment is based on the proposal presented by the Jet Propulsion Laboratory (JPL)2. It is divided in two features: thedownlink where the measurements will be performed and the uplink, which will be used to perform the pointing of theexperiment. In the figure 2 is showing the general diagram of the SOP experiment.

Figure 2: General diagram of the SOP experiment.

Proc. SPIE Vol. 427218

The SOP has a control system to establish and keep a link by opto-electro-mechanical means, which tracks and acquires theoptical beacon. This beacon is a non-modulated light source generated by the earth station. When the satellite receives it, it isthen used to acknowledge the location of the earth station. Therefore, the modulated laser beam can be transmitted to Earth.The goal is to obtain the minimum coupling transference for the motion of the optical antenna of the satellite, high sensitivityand a considerable dynamic range for the earth detector. Once the link is locked with the satellite beacon, data is transmittedwhile the alignment is kept; with the support of the orbital data transmitted to the earth station by the TT&C link and isemployed to position the telescope in the earth station, so as to optimize the acquisition process3.

The communication process is based on a sequence of three steps: acquisition, pointing and tracking.Acquisition. The system has to acquire the laser beacon signal from its sight fieldPointing. The satellite system aligns its mirror systems with the line-of-sight of the earth receiver, which has to center thelaser beam in the detection area.Tracking. In this stage, the system is kept aligned to start sending information. The transmitter antenna is adjusted to keep thebeam centered with the detector. A dedicated micro controller that converts the data of the positioning system and thedetector in suitable signals controls the transmitter antenna. The micro controller interacts with the on-board main computerof the satellite.

The laser beacon of the earth station emits a wavelength of 532 nm while the wavelength of the transmitter in the SATEX I is830 nm. Both beams share the optical path between the dichroic splitters of the satellite and the earth station.

The transmit/receive telescope in the SOP is a single lens. The small lens allows a large enough beam that the problem ofpointing is simplified. The laser transmitter is a semiconductor laser element with a single-lobe far field pattern. The trackingdetector is a quadrant avalanche photodiode (QAPD), the output is used for the motors that control two mirrors to keep thelaser beacon signal centered on the detector. Two one-axis steering mirrors were selected because the need of a larger angularrange of motion that offered a one two-axis mirror. It allows covering the total field of view of the detector and catching thebeacon faster. A collimating lens is positioned to form the required beam width that will be transmitted. The anamorphicprisms are included to circularize the elliptical beam that is generated by the laser diode. The beam splitter is highlytransmissive at the beacon wavelength and highly reflective at the lasing one4.

For the downlink calculations we consider the space to ground link, which can be represented by the figure 3. Thesecalculations will determine the allowed beam divergence and the sizes of the collector and detector diameters. We supposethe satellite has already acquired the beacon laser beam and its motions are completely compensated successfully by a controlof the transmitting beam.

The transmitter is characterized by the following parameters: Pt, Dt, lasing wavelength λ, the beam quality BQ, the pointingefficiency (Tp), the transmission of the optical components (Ttx). The receiver parameters are: Dr, the receiver focal number(f/#), the optical bandwith of the filter (OptBW), the receiver optical components (Trx) and filter transmission (Tfil). Theoperating characteristics of the space channel depend primarily on the properties of the medium involved, in our case: freespace. This would characterize propagation paths in or outside the Earth’s atmosphere. The principal effect is the propagationlosses defined as:

24

1

RLp π

= (1)

were R is the distance which the power is transmitting over.

Absorption and scattering phenomena in space channel involve both amplitude and spatial effects. Amplitude effects causetime variations in the electromagnetic field (power loss, power fluctuations, and frequency filtering). Spatial effects appear asvariations in the beam direction or as distortion effects across the beam front. The corresponding transmission loss factor ortransmissivity can be expressed as:

Rt

teL α−= (2)

where SCat ααα += is a per unit length loss coefficient.


Absorption coefficients aα and scattering coefficients SCα are readily available as a function of the wavelength and

altitude, essentially derived from years of empirical studies. Particulate scattering in the optical range, produces weak andstrong scattering effects. The first one corresponds to a beam movement while essentially retaining the structure of the beamfront. In this case, the beam is refocused by the medium, which causes:• The direction of propagation moving.• The orientation of the plane wavefront changing• Beam spreading.This phenomenon generally happens with a quiet channel and when the beam width is narrower than the cross section of theimpurities.

Figure 3: Schematic down link, were: TFOV is the Transmit Field of View, RFOV is the Received FOV, Dt is the transmitaperture diameter, Pt is the transmit power and Dr is the diameter of the cassegrain’s reception telescope.

The strong scattering occurs when the medium particles are particularly dense (smoke, fog, rain) and the beam diameter ismuch larger that they’re cross sections. In this case, each particle is an independent scatterer, producing multiple scatteringand refocusing of different part of the field and the beam is spatially distorted.

After a extensive calculation were we had take in account the variations and dependence between the divergence of thetransmitting beam, the size of the collective area, and the receiver diameter in function of the emitting power. We obtain theresults for the link calculation resumed in the table 2. As we can see, the expected performance is better in the night duemainly by the infrared noiseless environment.


PARAMETERDayHi BW

DayLo BW

NightHi BW

NightLo BW

LASERWavelength λ m 8.30E-07 8.30E-07 8.30E-07 8.30E-07Power Pt W 0.1 0.1 0.1 0.1SYSTEMRange Km 1000 1000 1000 1000OPTICSTrans. Beam Quality BQ wI/D 5 5 5 5Trans Diameter Dt m 0.0025 0.0025 0.0025 0.0025Rec Diameter Dr m 0.3 0.3 0.3 0.3Rec Obsc. Diameter Dobs m 0.05 0.05 0.05 0.05Tx. Optical Efficiency Ttx 0.78 0.78 0.78 0.78Rec. Optical; Eff w/oband. Filter

Trec 0.78 0.78 0.78 0.78

Peak trans of optical band.Fi.,filter

Tfilt 0.75 0.75 0.75 0.75

Optical filter Bandwidth Um 0.01 0.01 0.01 0.01Trans. Pointing Eff Tp 0.933 0.933 0.933 0.933Receiver Optics f/# 5 5 5 5Rec. Obsc. Efficiency Tobs 0.972 0.972 0.972 0.972Rec. Focal length f m 1.5 1.5 1.5 1.5Trans. Beamwidth, FWHH TFOV Rad 1.660E-03 1.660E-03 1.660E-03 1.660E-03 FW1/e^2 Rad 2.82E-03 2.82E-03 2.82E-03 2.82E-03Detector IFOV RFOV Rad 6.67E-04 6.67E-04 6.67E-04 6.67E-04Rec. Beamwidth, FWHH Rad 2.77E-06 2.77E-06 2.77E-06 2.77E-06Trans. Raleigh Range Zrtx Km 5.91E-03 5.91E-03 5.91E-03 5.91E-03Rec. Raleigh Range Zrrx Km 8.52E+01 8.52E+01 8.52E+01 8.52E+01ATMOSPHEREOne way transmission 0.375 0.375 0.375 0.375DETECTOR EG&GType C30954E C30954E C30954E C30954EElement Size um 1000 1000 1000 1000Quantum Efficiency η 0.85 0.85 0.85 0.85Gain M 120 120 120 120Noise factor equation keff Keff 0.02 0.02 0.02 0.02Gain Noise Factor F(M) 4.35 4.35 4.35 4.35Responsivity (unity gain) Ro A/W 0.569 0.569 0.569 0.569Responsivity Total R A/W 68.27 68.27 68.27 68.27Dark Noise A/Hz0.5 1.00Ê-12 1.00Ê-12 1.00Ê-12 1.00Ê-12Shot Current Noise ld A 3.96E-11 8.86E-12 3.96E-11 8.86E-12Sky Radiance UW/cm^2/sr/um 300 300 0 0Sky Radiance W/m^2/sr/um 3 3 0 0Sky backgrd Rec. power pb W 4.33E-10 4.33E-10 0 0Sky backgrd DC current A 2.96E-09 2.96E-09 0 0Backgrd shot noise current lb A 9.65E-10 2.16E-10 0 0

Table 2: Optical downlink parameters and calculations.


PARAMETERDayHi BW

DayLo BW

NightHi BW

NightLo BW

RECEIVERReceiver power (average) Pr W 2.52E-10 2.52E-10 2.52E-10 2.52E-10Visibility Eff Tvisi 0.6 0.6 0.6 0.6Signal photocurrent, unitgain

Lph A 8.6E-11 8.6E-11 8.6E-11 8.6E-11

Matched receiver BW BW Hz 1000 50 1000 50Noise equivalentBandwidth

BWn Hz 1.57E+03 7.85E+01 1.57E+03 7.85E+01

Preamp Noise A/Hz^1/2 5.00E-12 5.00E-12 5.00E-12 5.00E-12Preamp Noise current La A 1.98E-10 4.43E-11 1.98E-10 4.43E-11noise current w/obackground

Ldn A 2.02E-10 4.52E-11 2.02E-10 4.52E-11

Total noise current Ln A 9.86E-10 2.2E-10 2.02E-10 4.52E-11NEP at detector w/obackgrd

W 4.93E-11 1.10E-12 4.93E-12 1.10E-12

NEP at detector NEP W 2.41E-11 5.38E-12 4.93E-12 1.10E-12convert is to isn^2; divideby in^2

3.24E+07 3.24E+07 7.71E+08 7.71E+08

H, increase factor of NEPdue to signal

1.001392 1.001396 1.032633 1.032633

SNR (elec power) SNR 1.10E+02 2.2E+03 2.4E+03 4.9E+04Id dB 20.38 33.39 33.88 46.89

RANGE CALCULATIONRequired SNR SNR dB 20 20 20 20Required received power Pru W 2.41E-10 5.38E-11 4.93E-11 1.1E-11Given by pru=NEP*SQRT(SNR) ,NEP calculated w/o the noise generated by the signal currentRange R Km 1023 2164 2260 4779System Margin (Pr/Pru) SM 1.05 4.68 5.11 22.84

Table 2: Optical downlink parameters and calculations (continue).

3. SOP EXPERIMENT DESCRIPTION

The on-board transmitter consists of the laser transmitter subsystem and the beacon signal receiver. Figure 4 shows a blockdiagram of the system. The total weight of the SOP are 4.88 Kg. As see, the SOP is divided in different subsystems, this arethe mainly parts of the experiment in the satellite5. These modules are:

• Computer module R3.• Power module R3.1• Motors module R4• Laser module T1• Avalanche photo diode module R2

3.1. Computer Module R3The on-board computer is based on a 16 bits RISC micro controller to support the whole experiment. The system providesthe interfaces to the remaining elements of the SOP and coordinates its actions. In short, the SOP Computer (SOPC) has thefollowing functions.


A minimum monitor to perform:• Self-text• Configuration for the peripheral devices of modules R3 and R4.• Communication protocol with the main computer or the redundant computer for receiving internal or external

commands and programs, as well as the transmission of status report of the board and telemetry• Decoding and execution of commands• Switching to new programs• Telemetry of module T1.• Real-time clock

Use of open and closed loops to control mirrors, for this purpose:• It calculates the open loop path• It measures the optical detected power from module R2• It controls the positioning of the step motors• It performs optical tracking of the closed loop

Figure 4: Diagram of the on-board system


3.2. Power Module R3.1This module consists of multifunction boards to reinforce and perform the optocoupling of the lines involved in thecomputer, as well as the decoupling and conditioning of the power supply lines. The power is taken from the unregulatedpower supply bus of SATEX I, since the SOP has its own DC-DC converters in order to supply the different voltagesrequired in the system.

3.3. Motor Module R4Two micro-steps motors with their respective drivers comprise the motor module. The two motors move the mirrors thatallow performing the tracking and compensation of the satellite motion. Some problems remain to determine the bestsupporting structure for the launching. Some proposals are under evaluation in order to guarantee the right protection andoperation of the mirrors. It’s is important mentioned that we have use the same motors for coarse and fine pointing, and bysoftware we are changing the operational mode to have the ability of make different pointing steps.

3.4. Laser Module T1The photo transmitter system of the spacecraft uses a high optical power laser diode (200 mW). However when laser diodesare used, such factors as temperatures changes, laser diode wear out and extra perturbations related with the laser diodenature operation, have to be taken into account, because they can produce unwanted changes in the optical output power.Therefore, a temperature controller, an optical power controller and protection circuitry of the laser diode power supply havebeen implemented and tested. A collimator lens has been employed to create the beam width required to be transmitted. Theanaphormic prism converts the elliptic beam generated by the laser diode in a circular beam6.

The beam divider is optimized to work at the wavelength of the beacon in the earth station and it is highly reflective at thewavelength of the laser ray in the satellite. Figure 5 shows a photograph of the SOP prototype, where the different modulesare identified for easy location.

Figure 5: Photograph of SOP.


3.5. Avalanche Photodiode APD Module R2The system shares the optical part of the transmitter system, such as the dichroic beam divider and the mirrors. It also has acollimated lens, optical filter and photoreception board with low noise electronic amplification. The photo reception boardcontains a four-quadrant avalanche photodiode and current to voltage converters in each of them with additional electronicsto condition the signals at the input of the analog to digital converter of the SOPC. SATEX-I will operate with the laserbeacon of the earth station as a pointing reference (see Figure 2 and 4). Once it is aligned with the beacon of the earth stationwith a small angle in the light of sight of SATEX I, the four-quadrant APD provides information about the relative positionof the beacon. The four outputs of the photo detector are amplified using low noise preamplifiers, then are digitized fromthese signals in order to obtain the command signals for each of the mirrors of an axis, which are adjusted to align the SOPwith the optical earth station. For convenience purposes, the digitalization process is limited to the analog signal bandwidth,to avoid the generation of a phenomenon known as aliasing7.

Figure 6: Detail of module T1.

4. OPTICAL EARTH STATION

A functional diagram of the optical earth station is shown in figure 7. It is comprised of a laser beacon transmitter subsystemand the modulated signal receiver. The laser beacon transmits at a wavelength of 532 nm and the satellite transmitter at 830nm. The beam divider is optimized to operate at the wavelength of the earth station beacon and it is highly reflective to thewavelength of the satellite laser ray.

The transmitter/receiver telescope of the earth station is based on a simple, relatively reduced lens, in order to simplify thetracking system. The Optical Earth Station, (OES) consists of:• Laser beacon which transmits at 532 nm wavelength.• The collector of the satellite transmitted signal, which is a Cassegrain, type telescope.• The filter to reduce the received signal background noise and to improve the signal to noise ratio• The electronics allows the control of the guidance feedback of the telescope and the BER measurement.


The divergence allowed by the beam and the collector and detector diameters was determined by the downlink calculations.The uplink transmission system is comprised of a laser emitter a collimating system of the transmission side. A green colorlaser was chosen as a transmission source because of the quality of these lasers. The main problem was the background noise,because the radiation emitted by the sun reaches its maximum at the wavelength of the green laser.

TransmissionAnalyzer

ManchesterDecoder

ThresholdDetector

PLL

AmplifierandBPF

Beam Splitter

Optic Filter

532 nmBeacon

APD

Clock Signal

830 nm

BER

Figure 7: Schematic diagram of the Optical Earth Station

5. CONCLUSIONS

With the experiment described in this work, we are trying to do an optical link analysis in a space application. Currently,operational tests with the system completely integrated have been performed and allowed to fine tune the interaction amongthe different modules. The features regarding the program, operation and interaction with the platform have been concluded.Since the engineering prototype is finished, the flying prototype will be integrated making some components substitutions tohave a more reliable system, we intend to use military qualified parts. The project was delayed due to budget problems,fortunately we can continue now and actually, we are scheduling a launch window for the end of this year. The primary goalis to obtain the knowledge to participate in the development of optical communications technology that permit us to preparedhuman resources in these field.

It is important to comment that we have received supporting from Hughes Aerospace to determine the feasibility of theexperiment. This was possible under the contract for two communications satellites that Hughes manufactured for theMexican government.

AKNOWLEDGEMENTSWe wish to acknowledge the Ministry of Communications and Transport (Secretaría de Comunicaciones y Transporte SCT)by means of the COFETEL for their support to this project. Also thanks to the CONACYT for the support; as well as oursponsor, the Instituto Politécnico Nacional IPN.


REFERENCES

1. E. Pacheco, et al. "Telemetry and Command System for Space Applications” Instrumentation & Development, Journal ofthe Mexican Society of Instrumentation, vol. 4, nr.4, 2000. 46-50.

2 Hamid Hemmati and James R. Lesh. "Laser Transmitter Aims at Laser Beacon" NASA Tech Briefs. November 1993.

3. Fco. Javier Mendieta, et al. "Carga Util Optica y Subsistemas de Comunicación para el Satélite Experimental SATEX I".Third Euro-Latin American Space Days, Proceedings of an International Conference held in México City, 10-14 November1997. (ESA SP-412, May 1998).

4. Fco. Javier Mendieta, Sylviane Gaonach. "Conception and Experimentation of an Optical Free Space Link for theSATEX". Informe Técnico. Comunicaciones Académicas, Serie Electrónica y Telecomunicaciones, CICESE. 49 pp. 1994.

5. Héctor Mejía, et al. "Subsistema de Control e Interfaces de la Carga Util Optica de Comunicaciones del MicrosatéliteSATEX 1". SOMI XII Congreso de Instrumentación. Sociedad Mexicana de Instrumentación. S.L.P, S.L.P. 1997. P.P. 259-263.

6. Ramon Muraoka Espiritu, Oscar Villalvazo Castellanos. "Foto-Receptor para Apuntamiento Láser con Aplicación enComunicaciones Opticas en el Espacio Libre". Tesis de Licenciatura. Escuela de Ingeniería Unidad Ensenada, UABC, 1998.

7. H. Mejía, H. Gómez, F. J. Mendieta "Control de Apuntamiento y seguimiento espacial. Carga Útil Óptica". Diseño de lacomputadora e interfaces. Informe Técnico, CIETT9513 CICESE, 1996.


Optical Communication Payload for an Experimental Microsatellite

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