SUNSAT - Launch and First Six Month's Orbital Performance

SSC99-1-4

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Garth W. Milne et al. 13th Annual AIAA/USU Conference on Small Satellites

SUNSAT - Launch and First Six Month's Orbital Performance.

Garth W. Milne, A. Schoonwinkel, J.J. du Plessis, S. Mostert, W.H. Steyn*, K vd Westhuizen,D.A. vd Merwe, H. Grobler, J.A. Koekemoer, N. Steenkamp.

Department of Electrical and Electronic Engineering, University of Stellenbosch, Private Bag X1,7602 Matieland, South Africa. Phone: +27 21 8084525, Fax: +27 21 8084981,

[email protected] http://sunsat.ee.sun.ac.za

* Presently at the University of Surrey

Abstract. South Africa’s first satellite, SUNSAT, has been operating in orbit since a NASA-sponsored launchon the USA Air Force P91-1 Argos Delta II mission on February 23, 1999. SUNSAT is a graduate student-developed satellite from the University of Stellenbosch in South Africa, and includes a NASA GPS receiver foroccultation research, laser reflectors, magnetometers, star cameras, Amateur Radio communications, and a 15-mresolution, 3456 pixel, 3-band, stereo-capable push broom imager. SUNSAT is controlled from the University,and most functions were operating in June 1999. Live PAL TV earth images are received well at Stellenboschon a 4.5 m diameter antenna, and good quality test images from the 15m imager have been obtained. As OS-CAR-35 (SO-35), SUNSATs Amateur Radio FM audio transponders provided strong signals on scheduledpasses over South Africa and the USA. http://sunsat.ee.sun.ac.za

1 - Introduction

On February 23 1999, a USA Air Force Delta IIlofted SUNSAT, South Africa’s first micro-satellite, into a precise 96.4 degree polar orbit.Cheers from staff, students and many otherswatching the live TV broadcast of the launch, her-alded South Africa’s entry into space.

The 64 kg satellite (Figure 1) has become OSCAR35 (SO-35) after starting South African and inter-national Amateur Radio activities, and has begunreturning 15-m resolution, 3456 pixel/line, 3-bandstereo imagery.

The successful commissioning of arguably themost complex student-developed satellite in space,has opened the door for South Africa and the Uni-versity of Stellenbosch to future space co-operation.

The SUNSAT programme has also achieved itsengineering educational goal by involved over 96graduate students, who have competed over 45graduate degrees. The programme has producednumerous spin-offs, including components in orbiton satellites of three countries.

This paper reviews the program’s goals and his-tory, and records and evaluates the orbital experi-ences from launch until 14 July 1999.

BackgroundIn 1991, the first four authors, who are in the Com-puter and Control Group in the Department ofElectrical and Electronic Engineering at the Uni-versity of Stellenbosch, decided that a micro-satellite project could satisfy the goals of:

• Adding a multi-disciplinary engineering re-search opportunity to the graduate portfolio.

• Stimulating significant international interactionthrough a challenging research initiative.

• Helping stimulate interest of the youth in sci-ence and technology through media exposureand schools programs.

Figure 1 SUNSAT in Handling Frame

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The initial goal was to launch a satellite in threeyears, but funding and launch delays stretchedSUNSAT’s development until launch in February1999.

Over 96 students were involved, and schools tech-nology stimulus activities spawned the MTN-SUNSTEP schools programme that has seen over25000 children assemble schools electronic kits(http://www.sunstep.sun.ac.za).

Educational aspects of the programme are coveredin previous papers.1, 2 The present paper concen-trates on the satellite engineering aspects of SUN-SAT.

Section 2 reviews SUNSAT’s goals, while sectionthree describes funding and schedule conse-quences. Sections four and five describe satellitedevelopment and environmental testing. Sectionssix through eight cover launch vehicle integration,the launch campaign, and early orbit operations.Sections seven and eight describe and evaluate theorbital status of SUNSAT in mid-July 1999. Sec-tions nine and ten give lessons learned, our interestin SUNSAT 2, and the conclusion.

2 - Goals of the SUNSAT satellite

The Master’s degree in the Department requires2400 hours of student effort. SUNSAT provided avehicle to stimulate this resource into creating asignificant engineering product.

A decision for in-house development of all elec-tronic systems and software except where facilityrequirements would be unreasonable, significantlyexpanded the system-wide understanding in ourgroup’s Electronic Systems Laboratory (ESL).

Industry sponsors also saw value in our studentsworking on systems that could possibly belaunched, thus developing engineers with increasedconcern for the quality and reliability of their work.

Implications of success or failureSuccess of SUNSAT was expected to have a sig-nificant effect on possible future satellite develop-ments in our country, so there was a strong moti-vation to succeed. This implied a satellite withmany functions, that would not become value-lessif a single failure occurred. To reduce risks andspace radiation environmental effects, a LEO satel-lite with basic gravity gradient stabilisation was alogical starting point.

A challenging main research payload was identi-fied, and options were reviewed for additionalfunctions. Amateur Radio’s track record and re-sources for stimulating interest in communication-based electronics led to a rewarding partnership. A

modular design approach had been selected, so anextra tray was easily added to accommodateNASA’s GPS when their launch offer arose. Ourown boom design could also be exploited to addwiring and a tip mass magnetometer, star camera,and laser reflectors when the symbiosis with theØrsted satellites data was appreciated.

Satellite engineering research goalThe research goal of SUNSAT is to demonstratethat a student-developed 50kg-class micro-satellitecan produce remote sensing imagery approachingthat of the SPOT-2 class of large remote sensingsatellites.

Choosing the satellite engineering research goal.Completing, launching and operating a first satel-lite would have met our training goals, but wasinsufficient for our research requirements. We thussought further challenges to make a research con-tribution to satellite engineering. The followingthought process led to SUNSAT’s satellite engi-neering research goal.

Imaging in L.E.O.The short visibility time in low earth orbits is agreat disadvantage for communications and opera-tions. The major attraction of this orbit for a singlesatellite is for imaging.

In 1991 the SPOT-2 satellite was producing thehighest resolution remote sensing data over SouthAfrica. SPOT-2 has a 20m multi-spectral resolu-tion, while similar-sized intelligence satellitesachieved meter-level resolution. There seemed tobe a great opportunity to increase the resolution ofimaging micro-satellites to well below the 100mlevels then common in microsatellites.

Lower cost and calibration need.We met numerous SPOT-2 users in South Africawho indicated a need and use for imagery that wasvisually similar to SPOT-2, but of lower cost. Usersoften did not utilise the accurate calibrations, andbelieved that lower cost data with less calibration

Figure 2 SUNSAT in-orbit configuration

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would permit work that was precluded by the highcost of SPOT data. We also learned that a stereocapability in the same pass, with SPOT’s green,red, and near IR bands would be an attractive com-bination of capabilities.

SensorsHigh-resolution imagery needs both small pixelspacing, and many pixels. CCD TV sensors werelimited to below 1000 pixels, but linear CCD sen-sors with up to 5000 pixels were available. Atwavelengths longer than visible light, (near-IR),photons penetrate deeply into Silicon sensors, andgenerate photo-electrons that can diffuse to adja-cent photo-sites. The MTF (Modulation TransferFunction ~ resolution) of sensors with fine pixelsthus drops significantly at longer (NIR) wave-lengths. To obtain good Chlorophyl-band imagery,we chose a TC104 3456 pixel linear CCD Siliconsensor with 10.7µm pixel spacing rather than asensor with a larger number of smaller pixels.

OpticsOptics development was beyond our skills goal, sowe teamed up with the Optical Engineering Groupof the CSIR in Pretoria. They had developed aseries of compact and light TeleMacro lenseswhich were a good starting point for a micro-satellite imager. They also proposed how to opti-cally combine three linear CCD’s into a single op-tical unit meeting the focus, alignment, and filter-ing needs for a pushbroom imager.

The real LEO!The LEO acronym is quite appropriate for SUN-SAT’s imager since a lion at the Pretoria zooplayed a role in the imager’s history. The CSIRhad taken a photograph of this lion using a 10cmdiameter, 1200mm focal length TeleMacro lens.The lion’s whiskers (estimated but not measured!as 1mm diameter) were in sharp focus. Simplescaling showed that 10m objects should be clearlyvisible from 700km. (1mm from 70m subtends thesame angle as a 10m object seen from 700km.)

Armed with MTF calculations, the 3456 elementlinear CCD’s performance, the CSIR’s optical ca-pability, and the lion’s personal endorsement, webecame confident of producing an imager forSUNSAT that would produce imagery that would

be visually similar to that from SPOT-2. A jointdevelopment with the Kitsat-3 project3 resulted incommon imagers which have been successful onboth satellites.

Trade-off and consequencesA trade-off study of light levels, MTF of optics,motion and CCD performance finally led to an op-tical system of 10cm diameter and 570mm focallength, using TC104 sensors. The pixel spacing of10.7µm subtends 18.7µrad (10.7µm/570mm), or15.01m from 800km. The swath is 3456 * 15.01 =51.9 km, and the field of view is 3.717 degrees. An8-bit image data handling system with independentelectronic gain control in each channel was de-signed, and a 64Mbyte RAM disk was added as anon-board image store.

While converging to a research goal of demon-strating 15m resolution, 52km swath imagery froma microsatellite, we had considered the implicationson other satellite subsystems. The real timedownlink capability of over 40 Mb/s could be metwith a 5W transmitter and a realistically sized 4.5mground station antenna. Demands for 1mradADCS accuracy when imaging could be met withfine visible-band horizon sensors, and reactionwheels that only needed to run when imaging.Coarse attitude control when not imaging couldstill be maintained with magnetorquing, which re-duced the consequences of reaction wheel failure.

3 - Program scheduling and completion

The concept design of SUNSAT as a sun-synchronous imaging micro-satellite was describedearlier.4 Funding has remained a major issuethroughout the program, and influenced many de-sign, contracting, and scheduling decisions. Thefirst consequence was that the planned auxiliarypayload launch on the Ariane Helios mission couldnot be funded and was missed, placing the wholeprogramme in jeopardy.

NASA-sponsored launchNASA saved the SUNSAT programme by offeringa launch on the US Air Force P91-1 Argos Delta IImission if SUNSAT could carry a JPL Turbo-Rogue GPS receiver for geodynamic research.Denmark’s Ørsted magnetic research satellite wasalready scheduled as a secondary payload, andSUNSAT could be carried in the opposite sectorinstead of a balance weight.

The interaction with Boeing and NASA has beenone of the major learning experiences of the SUN-SAT programme. NASA was extremely supportive,but realistic and strict on SUNSAT’s launch inte-gration. This resulted in a smooth final integration

Figure 3 The SUNSAT LEO!

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process with Boeing’s Delta II rocket, and a perfectlaunch.

The 857 x 655 km elliptical polar orbit was dictatedby science requirements of the Oersted satellitewhich was the initial secondary payload. The orbitplane drifts an hour earlier every seventy days, andwas aligned with the sun in mid-July 1999.

The orbital drift makes long-term repeated imagingimpossible, and lowered the expected output ofSUNSAT to demonstrating what the satellite wouldbe able to achieve in a true sun-synchronous orbit.We retained the sun-synchronous design for thedrifting orbit, and intend to concentrate on commu-nication activities when the sun angle becomesunsuitable for imaging, but provides more averagesolar power.

Following a first technical integration meeting atBoeing in May 1994, a NASA visit in July 1994produced a series of technical milestones that weresuccessfully met. A launch MOU was signed inJuly 1996. The originally intended launch date ofSeptember 1996 slipped on numerous occasionsbecause of delays with the prime payload. Launcheventually occurred on 23 February 1999.

4 - Satellite final development

NASA took immediate control of the launch inte-gration process via a schedule of documents veri-fying SUNSAT’s design. The most urgent re-quirement was a correlated dynamic model of thespacecraft. The launcher interface design was fi-nalised, a finite element model was completed andmodal survey tests were performed at a dauntingschedule. Figure 4 shows one of the finite elementmodel printouts.

Tray structureSUNSAT is formed by a number of trays contain-ing the various subsystems. Figure 5 shows themechanical model of SUNSAT undergoing modalsurvey tests, and clearly shows the tray structure.

Bottom trayThe bottom tray is the deepest and most complextray since it has to carry components that need theview, a low position for C.G. reasons, or the mildtemperature and heat dissipation capability. It alsohas to incorporate the payload attach assembly(PAA). The bottom view in Figure 7 shows thelower mechanical components, including the largethreaded hole to accept the PAA.

The lower tray is 12cm high to contain the imager,which fits diagonally across it, and views the earthbelow via a 45-degree mirror. The imager me-chanics can be seen through the imager aperture inthe corner of the bottom tray in the Figure 7.

Imager mechanicsFigure 8 gives the top view of the base plate andthe imager mechanics at the beginning of lowertray assembly. The fused quartz optical system inFigure 9 was potted into mounting rings. It and theelectronic circuit boards were then mounted insidethe imager tube and subjected to comprehensiveoptical and environmental test before being handedover for integration with the lower tray.

Imager rotationFor stereo imaging along the ground track, the sat-ellite is flown with the imager normal to the veloc-ity vector. An approaching-image strip of up to700km length is scanned with the imager pointing24 degrees ahead of the sub-satellite point. The

Figure 4 FEM analysis

Figure 5 Modal survey tests

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imager is then rotated 24 degrees behind the sub-satellite point, to take a receding image of the samearea.

For imaging to the side of the ground track, thesatellite is flown with the imager pointing in thedirection of the velocity vector. Rotating theimager left or right allows scenes up to 350km

away from the ground track to be imaged.

The inner rotating tube in Figure 8 is needed tocorrect the orientation of the linear CCD’s whichdiffer by 90 degrees in the two cases. A potenti-ometer measures the coarse position of the imager,while fine wires passing through an optical inter-

ruptor’s beam give precise angular information at afew specific angles.

Lower tray wiringFigure 6 shows the lower tray after more items andthe wiring harness were added. SUNSAT’s S-bandPA and the UHF transmitter PA’s are mounted inthe lower tray for heat dissipation reasons. Theseall require cables, which add to those for reactionwheels, batteries, and solar panel strings to makethe harness a challenge.

Integrated lower trayThe complete lower tray seen in Figure 10 wasmore cluttered than desired. This resulted fromthermal and view-port needs of some components,and the need to place large mechanical componentsin the single ‘high bay’. Spot the four reactionwheels! In addition, the bottom plate represents50% of the antenna mounting area of the satellite,so had to carry its fair share of RF cables and con-nectors.

Standard traysThe remainder of the trays in SUNSAT were lessdifficult to package. Figure 11 shows the satelliteafter the power, telecommand, and UHF communi-cation trays had been added. The first two trays

Figure 6 Base plate with wiring harness.

Figure 7 Underside of base plate.

Figure 8 Base plate with imager.

Figure 9 Optics package inside imager.

Figure 10 Complete lower tray.

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contain large motherboards with two crossbarssupporting them via a number of screw attach-ments.

The RF trays used two crossbars to support a moth-erboard which carried the different RF subassem-blies, each in their own aluminium-screened boxes.

The construction of other trays followed the samecrossbar and large board concept with piggybackboard in the RAM tray and on the ADCS tray.

The S-band tray had to be at least 50mm high toaccommodate NASA-JPL Turbo-Rogue GPS re-ceiver. The non-Turbo-Rogue area was split with amezzanine floor, allowing two 25mm-high areas toaccommodate the L- and S-band modules and theschool experiment tray.

The top plateThe top plate of the satellite in its launch configu-ration is crowded, as can be seen in figures 12 - 15.The tip-mass occupies the center, and is attachedwith a single stainless steel bolt. The bolt passesthrough two pyrotechnic-activated guillotines thatcut it to release the tip mass.

The tip mass comprises two halves and a bridgethat straddles the folded boom, which deploys us-ing its own stored energy. Screened signal leadspass through the interior of the boom to the starcamera and 3-axis magnetometer in the tip mass.

Figure 12 shows the fine roll and pitch sensors.These are only needed during imaging, so use visi-ble-band linear CCD sensors. Their black lensesare seen pointing 30 degrees downward. The fineyaw sensor used a similar CCD behind a split.

The phasing network for the VHF canted turnstileantenna is in the box at the lower right of Figure12, while the downward facing holes in the tipmass are to receive the laser retro-reflectors.

The tip mass is also an ideal place for a web-camera to view future satellites in orbit and moni-tor surface degradation. The small size and mass ofrecent matchbox or smaller cameras makes themvery useful for health and even attitude checks.

VHF AntennasSUNSAT has two VHF antenna systems. The sim-plest is a monopole at the center of the lower sur-face which passed through apertures in the PAAand PAF. The second system is a canted turnstile,radiating circular polarization below the satellite.Flexible 50cm Beryllium Copper ‘tape measures’were rolled up inside brackets on the tip mass dur-ing launch, and deployed once the tip mass wasreleased. Figure 13 shows these antennas, plus theshort UHF monopole on the far side of the topplate.

Boom deployment testFiring the guillotines while the boom’s elbowswere suspended with fine elastic in a high bay en-abled deployment to be tested. Figure 14 showsthe top plate with deployed VHF turnstile andboom. The boom is stiff enough to support its ownweight (but not the 4kg tip mass) under 1 g’, whicheases ground processing.

Figure 11 Lower four trays of SUNSAT.

Figure 12 Mechanical components of top plate.

Figure 13 Top plate during electrical wiring.

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The tip mass carried the eight NASA laser reflec-tors. These were mounted in 2.5 cm round housingsthat can be seen on the downward pointing edges ofthe tip mass. The baffle of the tip-mass star cameracan be seen aimed at the rear of the white GPS an-tenna above the right solar panel in Figure 15.Flanking the GPS antenna are the micro-meteoroidsensors from the Peninsula Technikon (Pentech) inSouth Africa and NASA in an experiment run byPentech. (www.EE.pentech.ac.za)

The closest corner in Figure 15 shows the topplate’s star camera, while the solar cells on thecoarse sun sensor can be seen above the top of theleft solar panel. The figure also shows the shortboom carrying the ADCS magnetometer.

Tray interconnections.Interconnection of the many telemetry and tele-command wires was a problem. The width of alltrays except the bottom tray was reduced to providea wiring cavity under a solar panel. This cavitycarries all ‘slow’ signals. A similar cavity on theopposite side was provided for data and address

bus interconnections between the OBC’s RAM,and ADCS. The separation of computer buses andsignals entering the analogue trays was imple-mented to reduced the level of RFI and EMI.

Figure 16 shows the satellite inverted in its han-dling frame, with the solar panel removed to showthe slow bus area. In later RFI correction measures,the wiring in the figure was totally covered by a0.3-mm Aluminum plate. It was regularly groundedto screen the VHF antennas from RF noise gener-ated inside the satellite.

RFIThe payload function on SUNSAT that is consid-ered to be the most important for stimulating tech-nical interest in young children is the VHF Parrot.Significant benefits accrue if the VHF receivers aresensitive enough to receive uplinked signals from1.5W hand-held radios.

Significant attention was given to receiver spuriousand image responses, and maintaining noise figuresnear 2 dB. However, RFI radiated from computersand in particular, from switching regulator pack-ages, was extremely difficult to control. To reducecosts, unshielded regulators were used in most lo-cations in SUNSAT. In the VHF tray, this provedinadequate, and the only way of controlling RFIwas to mount the regulator in its own Aluminumfiltered box that was attached to the sidewall of thetray.

The level of shielding needed is illustrated by asimple test done with an isolated 5V switchingregulator fed from a well-bypassed and chokedsupply. A 0.5m antenna mounted 50cm away fromthe regulator received a 650 kHz comb-spectrumwith -85 dBm spectral lines. This level is 44dBabove the -129 dBm noise floor of SUNSAT’sVHF receivers!

The switching regulator problem was very difficultto analyse and solve because the switching fre-quency changes slightly with supply voltage. Ten-

Figure 14 Top plate with boom deployed

Figure 15 Top plate before thermal blanketinstallation.

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millivolt changes in the supply, possibly caused byactivating other payloads, caused the spectral lines(± 223 harmonic) to drift through the receiver pass-band, making measurements inconsistent.

RFI tests and diagnosis proved to be very difficultas reported previously5. At one stage we were notsure if the RF filtering on leads to the top plate andthrough-seam leakage between trays would be ade-quate to reduce satellite emissions to a point whereour own receivers would not be de-sensitized bythe RFI.

The Faraday screens shown in Figure 17 were thusmade and attached around the satellite. With thesewe managed to prove that metallic screening of thebus cavities would be adequate to make our anten-nas noise-free.

5 - Environmental testing.

Thermal tests had to be done at affordable costs.We managed to obtain, and repaired a thrashingpump and diffuser vacuum pump and built the vac-uum chamber shown in Figure 18. Heater plateswere made to fit around SUNSAT as shown inFigure 19, and could fit into the vacuum tank.With these we could do room temperature and hotvacuum tests and bake-out.

Cold thermal vacuum tests required additional fa-cilities and complications, so we reviewed their

necessity. We knew that the internal temperature ofSUNSAT would not become very low, and we pro-vided no heaters except to heat the imager optics inan attempt to dislodge any condensed outgassingproducts from the satellite.

We reasoned that high temperature tests needed tobe done in vacuum to prove that losing the thermaltransfer properties of air would not cause localoverheating. We could not see the need to do coldtests in vacuum if we had no heaters to test, so de-cided to do all cold testing at atmospheric pressure.

Vibration testsFull level, 3-axis random and sine vibration testshad been completed on sub-assemblies and thecomplete engineering model of SUNSAT. Thesehad verified the packaging concept and identifiedisolated component mounting problems, which

Figure 16 Slow bus wiring area.

Figure 17 Faraday screen during RFI diagnosis.

Figure 18 Vacuum chamber.

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were corrected. The flight model experienced nofailures during its vibration test series at theHouwteq test facility. (Figure 20)

6 - Launch vehicle integration

Launch vehicle physical integration was done byBoeing, initially by design drawings and then con-

firmed by a fit check of a volumetric model to thesecond stage flight hardware. The location on thesecond stage is shown later in Figure 23.

Fit and shock testsThe flight PAF and flight PAA were integrated inSouth Africa during a visit by NASA and Boeing.A separation shock test was also completed to as-sess clamp-band firing shock loads. Figure 21

shows SUNSAT suspended with PAF attached andbeing readied for clamp band firing.

Figure 22 shows the separated PAF after the clampband firing, and with SUNSAT’s VHF monopoleand UHF canted turnstile antennas in their orbitconfiguration. Shock levels were monitored andfound to be within normal ranges, and no damageto optical components occurred.

7 - Launch campaign

Our mission nearly set the record for the launchcampaign length. Our four-man launch team ar-

Figure 19 Heater jacket around SUNSAT

Figure 20 Flight model vibration tests.

Figure 21 Preparations for clamp band firing.

Figure 22 Hey guys -- it worked! No glass!

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rived at Vandenberg on 23 November 1998, andproceeded with satellite checkout. The NASAResident Office and its contractors, and the wholeBoeing team, have become our reference standardfor willingness and quality of support. Ørsted andSUNSAT were mated with the launch vehicle onDecember 8 and 9 (Figure 23), and left on moni-tored trickle charge over Christmas, with launchscheduled for 12 January 1999.

Figure 23 shows SUNSAT installed on the launchvehicle, with Ørsted on the opposite sector. TheArgos satellite is attached on top of the white sup-port ring, but is not visible because of the inter-vening floor of the mission support tower.

LaunchA number of Danish and South African visitors,including a cabinet minister attended, making for abusy public affairs programme. The 30-th SpaceWing at Vandenberg made all visitors extremelywelcome, and did a great job handling VIP’s androckets alike!

WaitingWeather and two technical hitches caused tenlaunch scrubs, with launch eventually occurring on23 February 1999. By that time, even the keenestacademics had returned to South Africa and under-graduate classes, and only a single SUNSAT engi-neer was left monitoring trickle charging. The restof the team watched the launch attempts via the TVlink, and each time experienced the anticipation of

launch as LOX vented and then puffed from justunder the SUNSAT insignia on the launch vehicle(Figure 25).

Go goThe simplicity of the go, go, go on the countdownnet on 23 February was almost an anti-climax afterthe dramas of earlier occasions. The auditorium atour Department was full of students and the die-hard launch watchers. The auditorium erupted asthe Delta ignited and lifted smoothly into the nightsky. We then watched in awe as the solids sepa-rated, and the range cameras tracked the Delta upuntil second stage shut down. We waited, doubtingvelocity vectors, Kepler and Newton, until theHartbeesthoek tracking station in South Africa re-ceived confirmation of Argos’ separation, and thenuntil the words ‘secondary separation confirmed’were heard from Vandenberg. Then it was up tous!

8 - Initial commissioning

Immediately after separation, our launch teammember hosted at W6AB, the Vandenberg SatelliteAmateur Radio Club, uplinked commands to SUN-SAT to activate its power relays and telemetry

transmitter, but no downlink was heard. Half anorbit later, over Stellenbosch -- still no telemetry,and yet again for the next pass. Joy of joy’s, te-lemetry was received during the night pass, andindicated that battery voltage and temperature werefine. Life started again!

Being our first satellite, we were extremely carefulof any action that could cause us to lose the satel-lite through command lock-out by on-board trans-mitters. SUNSAT has time-outs on all transmittersto prevent lock-outs but we weren’t prepared torisk the satellite on their unproven post-launch per-formance.

Figure 23 SUNSAT and Ørsted in place!

Figure 24 A beautiful sight.

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The delayed telemetry had stalled our commis-sioning sequence, and allowed the satellite tem-perature to rise. It had also made us nervous andscared of losing telemetry or tele-command. Greatjoy occurred when we booted OBC1 for the firsttime and it transmitted it’s ‘I’m alive’ audio signal.

The handicap of short passes and marginal com-munications was sorely felt in the first few days.The extra accesses from W6AB proved invaluablein understanding what was happening on boardSUNSAT. OBC1 started collecting whole orbittelemetry (WOD) reliably, and our insight in-creased to a point where we could re-consider de-tumbling and boom deployment.

Initial ADCS activationSUNSAT’s 3-axis magnetometer from HermanusMagnetic Observatory of the CSIR performedflawlessly and showed that SUNSAT was coningwith its lower plate exposed to the Sun, and with itstemperature rising. Our attitude control systemwas started and slowed the Z-spin using the torquecoils. Figure 29 shows WOD magnetometer data asthe Z-spin was controlled. The large square wavesignal that starts at 20% of the time axis is magne-torquer current, which clearly reduces the fre-quency of the spin shown by the sinusoidal x-axismagnetic field.

We experienced some resets on OBC1 which thenreset the ADCS computer (this has now beenchanged by software upload). It was thus impossi-ble for the ADCS to enter the planned boom de-ployment process. Meanwhile, we had an increas-ing satellite temperature because of the attitude-stabilizing spin, and no gravity moment to breakthis pattern

Boom deploymentWe reviewed our options, and concluded that thelowest risk strategy would be to deploy the boom,even though SUNSAT’s was pitching at one degree

Figure 25 LOX venting below SUNSAT em-blem

Figure 26 Go!

Figure 27 Orbit trace until secondary separa-tion

Figure 28 Solids separate

Figure 29 X-axis magnetometer during de-spinning

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per second. We translated the rotational energy of0.3 mJ, into the energy of one gram at three centi-meters height, or a 6 µF capacitor charged to 10Volts. We decided this could not damage anything,particularly since our boom deployment tests hadshown the boom to straighten out in less than 5seconds, and decided to deploy the boom.

The Z-axis magnetometer readings during boomdeployment that are given in Figure 30 show howthe angular rate normal to the Z-axis dropped by alarge factor as the boom deployed. This particulartelemetry trace caused great relief since it con-firmed that our own-developed boom had de-ployed. Our communications team wanted deploy-ment to occur to release the VHF turnstile antennawhich immediately improved uplink telecommandperformance.

The first action after boom deployment was to acti-vate the libration damper controller on the ADCS.The satellite managed to establish gravity-gradientlock in the correct orientation within a few orbits.

With SUNSAT earth-pointing with a slow yaw spinfor temperature equalization and a stable and satis-factory temperature, our thermal concerns wereover. The main battery voltage was in limits, thebattery charge controller was cycling correctly, andthe state of charge indicator was giving credibleinformation. It was time to celebrate a successfulinitial commissioning phase.

First software updatePrior to shipping SUNSAT, we knew we were run-ning behind schedule on real-time software, andhad to choose between completing a reliable kerneland AX25 communications software and drivers, ortesting the applications to run on this. The kernelgot preference, and proved able to boot OBC1 fromsimulated intermittent RF links, even if all flashmemories had been erased, and then re-program theflash memories.

This effort, plus code for other controllers absorbedmore software effort than was expected or avail-

able, so we missed a bug in our Diary commandsystem which triggered crashes in OBC1. Thesewere first blamed on SEU’s, but the correlationwith diary execution was noted, and then repro-duced on the ground. Our first software uploadcorrected this simple bug and made other smallimprovements which left the software on OBC1 ina good condition.

9 - Orbital status

SUNSAT is a complex satellite with a number ofredundancies. Apparent failure of the high-speed S-band modem will significantly reduce the imagingthroughput of the satellite, but hasn’t prevented usachieving our main research goal.

The high-speed modem failure has been located tothe QPSK S-band exciter, which no longer drivesthe S-band PA (power amplifier). The same PAcarries the live PAL TV signal, and on three earlyoccasions transmitted an un-modulated carrier tothe OTB ground station in South Africa. Nomodulation was possible at that stage since theRAM tray had not been commissioned, and ourown S-band reception system was not operating.There is thus still a remote possibility that the ex-citer may be coaxed back into operation by somesequencing combination.

Failure of an interface to two of the four reactionwheels will also complicate ADCS operation. Wehave wheels operating in yaw and pitch, which isadequate for fine pointing control. This work wasstill in process in July 1999.

The pitch reaction wheel has also proved to be in-valuable when inverting the satellite during a singlepass, following occasional inverted gravity lockconditions that have followed computer crashes.

General performanceSUNSAT is functioning well as an Amateur com-munications satellite. The bus is in good conditionwith typically 20 degree-C temperature, and thepower, charge-control, and monitoring systemworking well. Telecommand is reliable, and wholeorbit data gathering works well. OBC1 has noknown faults but crashes about once per week, pos-sibly as a result of SEU’s. After improving theground station performance, we have reliablecommunications from horizon to horizon.

SO-35SUNSAT has been allocated the Amateur designa-tion of SO-35 (SUNSAT OSCAR 35) and has beensupporting Amateur radio FM repeater operationsin South Africa and the USA since mid-June andJuly respectively. The 436.291 MHz uplink istransponded down onto 145.825 MHz, giving astrong signal (-101 dBm) even on a hand-held re-

Figure 30 Z-axis magnetometer duringboom deployment

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ceiver. Initial USA Amateur QSO’s were recordedand made available on the www.amsatnet.com website, and other links are on the SUNSAT web sitehttp://sunsat.ee.sun.ac.za.

The UHF transmitters can also be heard on hand-helds, but are weaker, as expected. We have alsosuccessfully tested a VHF uplink to SUNSAT froma 1.5W VHF hand-held transmitter and obtained aclear UHF return from the satellite when at 18 de-grees elevation. The implication of the good VHFuplink and downlink performance is that the VHFParrot system should work well when its softwareis installed.

Ground station downloadsOur ground station has a 4.5-m diameter trackingdish that we have started using for the UHF down-link. The additional gain relative to a set of Yagiantennas means that we get constant –98 dBm sig-nals from SUNSAT for virtually any elevation, andhave been able to download over 710kB at 9600Baud in a single pass.

PayloadsSUNSAT’s PAL TV camera is working well, anddownlinks S-band images of South Africa that arereceived with no speckles on the 4.5m S-band dishat Stellenbosch. Figure 31 shows an image grabbedfrom this video stream. Note the tape antennas onSUNSAT protruding into the picture.

The NASA/JPL Turbo-Rogue GPS receiver isworking and software updates are being prepared tosupport full instrument operation.

The push-broom imager is working well, and inearly July 1999 was storing 3456 pixel by 3500

Figure 31 PAL TV frame of the Western Cape

Figure 32 1000 x 350 pixels from a SUNSATimage of 3456 pixel width

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pixel images on the 64 Mbyte RAM tray. TheRAM is used simultaneously as a file system and tostore raw image data.

With the high speed modem malfunction, the greatstream of real-time image data expected fromSUNSAT has not occurred, so we have had todownload images at 9600 Baud over a number ofpasses. The process will be speeded by uploadingJPEG image compression software.

Sample high-resolution images.This section presents some image samples gatheredfrom SUNSAT in the first two weeks of imagercommissioning. SUNSAT’s image width of 3456pixels rapidly fills disk space, and cannot be ade-quately communicated without photographic qual-ity reproduction. Small sections of magnified im-

ages are presented to aid assessment.

Figure 32 is a 1000 x 350-pixel excerpt from animage of mountains near Riviersonderend in SouthAfrica.

Figure 33 was taken over the Middle East andshows varying fields and a round irrigation system.The irrigator is expanded further in Figure 34

The full color versions of these images are in theelectronic version of this paper and on the web sitehttp://sunsat.ee.sun.ac.za, They show the near-IRband in red, clearly highlighting the growingvegetation in the sector from 5’o clock to 11’oclock, and the worse development at the 9’o clockposition.

10 - Preliminary evaluation of SUNSAT

After four and a half months orbital experience of anew satellite design and a still-evolving groundstation, it is too early to give a full evaluation ofSUNSAT. Performance is improving on a weeklybasis.

The commissioning pace is slow because of SUN-SAT’s complexity; initial ground station problems,short LEO access times, and the small operatingteam that has other normal university responsibili-ties.

SUNSAT OSCAR 35 is a great success with RadioAmateurs, as evidenced by e-mail and on-air reac-tions of American and South African Radio Ama-teurs. VHF up-link tests using VHF hand-heldtransceivers show that the Parrot should work wellwhen its software is activated. At present, power isneeded for commissioning support.

The Middle East and Riviersonderend images showthat SUNSAT is able to take 15m resolution im-ages. The quality of the S-band TV link indicatesthat the S-band link margins for the data downlinkwould be adequate for regular image data transmis-sion.

By mid-July, the accurate ADCS performance ofSUNSAT was not what was desired, but we expectto bring this to full performance in due course.

11 - Lessons learned

Good things.We feel good about a number of features on SUN-SAT that are worth sharing.

1. Survive-all RF boot capability for OBC’s.2. Good performance of the AX25 software.3. Good performance of the RAM disk and FTP.4. Ability to upload new software into FLASH.

Figure 33 147 x 172 pixel segment of image

Figure 34 48 x 46 pixel segment of irrigator.

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5. Our boom that deployed well.6. Audio boot messages on the OBC’s.7. Using high-power VHF and UHF transmitters.8. Multiple antennas avoid single point failures.9. Excellent images produced by our jointly de-

veloped imager on KITSAT. (Identical toSUNSAT’s imager.)

10. Good sample images from SUNSAT’s imager.11. Sub-sampling software for quick-look images.12. A simple reaction wheel in pitch plane for in-

verting upside-down gravity locked satellites.13. State-of charge integrator in power system.14. Deploying the boom early with no problems.15. Time-outs on all transmitters.16. Periodic time-ins on de-activated receivers to

provide backup comms in case of need.17. 1200-Baud telemetry is a good wake-up call.18. Redundant audio buses enabling audio signals

to be ‘patched’ around the satellite.19. Redundancy in, and by means of the VHF and

UHF systems.20. Apparently successful RFI measures resulting

in good receiver sensitivity on VHF and UHF.21. Positive feedback from Amateurs using SO-35.22. Live TV from SUNSAT, and noise-free.23. Solithane’d commercial TV camera works.24. Befriending folks at NASA, Boeing, and

USAF.25. Sharing much with the Ørsted launch team.26. Our students performed like stars!

Difficult things.We found the following to be difficult

1. Stopping RFI from de-sensitizing VHF receiv-ers due to stray coupling and via antennas.

2. Meeting software milestones with adequatetesting.

3. Unwanted power consumption or latch-upfrom driving unpowered logic.

4. S-band QPSK modulators and demodulators,particularly without QPSK test equipment.

5. Incidental phase modulation on the QPSKsynthesizer and modulator.

6. Maintaining continuity of knowledge with astudent-manned project.

7. Funding.

Regrets.We are sometimes sorry that:

1. The bottom tray was so tightly packed.2. Harnesses were so complex.3. The batteries are difficult to remove for condi-

tioning.4. We didn’t do RFI tests on the VHF tray on its

own.5. We didn’t build a sensor stimulation simulator

facility to be able to do full ADCS tests beforelaunch.

6. We didn’t have time to operate SUNSAT re-motely for a month before shipping.

7. We didn’t account for magnetic torques fromsolar panel currents in design. (Are OK.)

8. We didn’t include a simple high speed FSKmodulator.

Next time we will:1. Add a ‘spy’ TV camera on the tip mass.2. Add an IR earth-sensor3. Appreciate that VHF receivers imply high RF

cleanliness, and use S-band uplinks if possible,with UHF as backup.

4. Do RFI tests with prototype hardware, satellitereceivers and antennas.

5. Plan on 2-month EMC testing and problemsolution with the Engineering Model.

6. Make an extra set of flight-identical computersfor software testing only.

7. Split OBC processors and I/O multiplexers bymodularizing or by serial bus systems.

8. Put micro-controllers on most UART’s to re-duce interrupt load on main processors.

9. Specify brownout performance of all modules.10. Plan to allow all uplink receivers to be pow-

ered down to save power, but automaticallycycle on for 5% of the time.

11. Get software finished earlier.12. Incorporate JPEG in imaging system design.13. Don’t make it all yourself.14. Arrange additional telemetry and ground con-

trol stations for early orbit operations.15. Ensure all power consuming items driven by

computers time-out if not regularly triggered.16. Ensure all processors signal the main OBC that

they are alive so they can be reset and logged.17. Develop a standard timeout circuit and apply.

12 - SUNSAT 2

After the positive experiences with SUNSAT, ourgroup is exploring options for an upgrade to alarger format. Research work is also in progress onimproved imagers, ADCS and data handling sys-tems.

In the last five years, the University of Stellenboschhas demonstrated its ability to participate success-fully in international missions. These include car-rying piggyback instruments for NASA, supplyinginstruments for other satellites such as Safir-2, andconsultation to various organisations around theworld.

The SUNSAT program looks forward to maximis-ing the return from SUNSAT, and to participatingin future satellite missions with international part-ners.

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13 - Conclusion

SUNSAT is a successful start to South Africa’smicro-satellite activities and has brought manybenefits. It has succeeded in training graduate stu-dents, stimulating international interaction betweenour Department and other significant space entities,and stimulating interest in science and technologyat schools --- the three original goals.

We are supporting the NASA payload and interna-tional Amateur Radio services, and are able todemonstrate our highest goal of returning 15-mresolution images from SUNSAT.

SUNSAT has been a wonderful journey. We haveshared common goals and loads with hobbyists andprofessionals from many parts of the world. Wehave experienced the satisfaction of achieving withthem, what to a new nation in space, are wondrousthings. NASA, Boeing, and Vandenberg havebecome words that mean wonderful friends, part-ners, and shared experiences. Thanks, 73’s andHOO - AAAH

Acknowledgements:SUNSAT has been completed through friends andcolleagues in many organisations. The followingorganisations have supplied funds or facilities:

Altech-Alcatel, ECS, First National Bank, FRD,Grintek, Houwteq, Irdeto, Malaysian Airlines,MTN, Orbicom, Plessey, Reumech, SARL, SAAMSAT, Siemens, Somchem, Telkom, Universityof Stellenbosch, Vodacom.

Colleagues at the university have built capabilitiesthat were used on SUNSAT. Students and supportorganisations have done great work.

The first four authors have had the joy of leading ahistoric project. To Marisa, Gil, Larry, John L,John D, Bill, Sybrand, Tjaart, Christo, and Hans, --you are the tops!

References:1. Schoonwinkel A and Milne GW. Managing a

Microsatellite Development Programme withinthe University Environment. Technovation,Vol. 17, No. 1, January 1997, pp 1-9.

2. Schoonwinkel A and Milne GW. UniversityResearch and Development of a Microsatellite.R&D Management, Vol. 27, No. 1, January1997, pp 37-44.

3. KITSAT-3 web site:http://satrec.kaist.ac.kr/english/SaTReC.html

4. Milne GW, Bakkes PJ, Schoonwinkel A, duPlessis JJ, Enslin JHR, Steyn WH, Mostert S,Palmer KD and Weber DM. SUNSAT, Stel-lenbosch University and SA-AMSAT’s RemoteSensing and Packet Communications Micro-satellite. Proceedings of the Conference on

Small Satellites #7, Utah, USA, September1993.

5. Milne GW, Jansen E, Roux JJ, Koekemoer J,Kotze PPA. EMC and RFI problems and solu-tions on the SUNSAT micro-satellite. Pro-ceedings of the South African Symposium onCommunications and Signal Processing.COMSIG ’98 ISBN 0-7803-5054-5

6. SUNSAT web site: http://sunsat.ee.sun.ac.za

Emails:[email protected],[email protected],[email protected],[email protected]

SUNSAT - Launch and First Six Month's Orbital Performance

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