CONTROL SYSTEM, OPERATING MODES, AND COMMUNICATIONS FOR POGOLITE

Miranda Jackson (for the PoGOLite Collaboration)

KTH, Department of Physics and The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre,10691 Stockholm, Sweden. Email: miranda@particle.kth.se

ABSTRACT

PoGOLite is a balloon-borne high-energy X-ray po-larimeter scheduled to be launched in the summer of 2011from northern Sweden. The planned flight will have a cir-cumpolar route and will last around 20 days. Because thepayload will not be within the line of sight for most ofthe flight, satellite-based Iridium modems must be used.The low speed and reduced reliability of the connectionrequire careful design regarding the operations and com-munications. The instrument has been made somewhatautonomous in function and has its own redundant stor-age. I describe the modes of operation, the communi-cations, control, and thermal regulation systems, and thechallenges encountered for a circumpolar flight.

Key words: scientific balloons, X-rays, polarization.

1. INTRODUCTION

The Polarized Gamma-ray Observer (PoGOLite) [1, 2] isa balloon-borne Compton-based polarimeter, with an en-ergy range of 25–80 keV. In the pathfinder instrument tobe flown in summer 2011, the detector system will em-ploy 61 phoswich detector cells (PDC) and 30 side anti-coincidence shield (SAS) detectors made of BGO mate-rial situated in an unbroken ring around the PDCs. Thefull size PoGOLite instrument will contain 217 PDCs.The polarimeter pressure vessel assembly is shown inFigure 1.

The instrument is scheduled to be launched on its maidenflight in summer 2011 from the Esrange Space Center innorthern Sweden. A circumpolar flight around the northpole is planned, and is expected to last 17–25 days.

In addition to the polarimeter, a sophisticated pointingsystem known as the attitude control system (ACS), em-ploying differential GPS, gyroscopes, magnetometers,and two star trackers, is employed. This system has beendeveloped by DST CONTROL in Linkoping, Sweden [3].A flywheel is used to control pointing in the azimuthaldirection, and a motor is used to control the elevation.To reduce systematic effects, the polarimeter is rotatedaround its axis one full turn during each observation.

Figure 1. Photograph of the polarimeter inside its pres-sure vessel, which is in turn inside the rotation frame.The instrument is surrounded by polyethylene blocks forpassive shielding. The star trackers and the auroral mon-itor unit are attached to the assembly. The frame of theattitude control system can be seen in the background.

Figure 2. Photograph of the polarimeter attached to theattitude control system.

_________________________________________________ Proc. ‘20th ESA Symposium on European Rocket and Balloon Programmes and Related Research’ Hyère, France, 22–26 May 2011 (ESA SP-700, October 2011)

Figure 3. The PoGOLite detector array, showing the 61PDCs in the center, surrounded by 30 SAS units. From[4].

This negates the effect of different detector responses onthe polarization measurement. A photograph of the po-larimeter contained within the attitude control system isshown in Figure 2.

2. POGOLITE POLARIMETER HARDWARE

2.1. Detector array

The PoGOLite detector array is designed with passiveshielding provided by a thick layer of polyethylene sur-rounding the pressure vessels. The detector array is con-tained within a pressure vessel, and consists of 61 PDCsand 30 SAS units. The array is shown in Figure 3.

A diagram of a PDC unit is shown in Figure 4. Each PDCemploys a tubular hexagonal plastic scintillator unit at theentrance to the detector, a solid plastic scintillator for de-tecting Compton and photoabsorption events, and a BGOcrystal underneath. A specially designed low-noise pho-tomultiplier tube (PMT) is attached to each BGO crys-tal in the detector array to absorb all the light producedby the three scintillators. The time constant of the mid-dle plastic scintillator is much less than for the tubularor BGO scintillators, so interactions in the middle detec-tor can be immediately distinguished by the electronics,and other events, including those from the SAS, can beimmediately rejected. The resulting narrow field of view(∼ 5 square degrees) and low background make it idealfor observing astrophysical point sources.

Figure 4. Diagram of a PDC, approximately to scale.Each of these units is about 1 m long. The slow scintilla-tor is depicted in blue, the fast scintillator is red, and thebottom BGO is green.

2.2. Electronics

The signal from each PMT in the detector array is at-tached to one of eight channels on a flash analog to digitalconverter (FADC) board. There are twelve FADC boardsin total. These boards provide control voltages for thePMTs and store waveforms from the PMT signals. Theyare also capable of some preanalysis, and issue triggersand hit signals as well as veto signals according to a setof predefined rules programmed into their FPGA chips.

The signals issued by the FADC boards are received andprocessed by the digital input-output (DIO) board, whichin turn issues a signal to the FADC boards to store the cur-rent waveforms in memory. The signals between the DIOand FADC boards are mediated by another electronicsboard which applies the required logic (AND/OR, etc.).The FADCs contain a limited amount of volatile mem-ory and can each store 96 waveforms. To initiate dataacquisition and store the waveforms more permanently, aSpaceWire to ethernet converter board is used, and this iscontrolled by an onboard PC104 running Linux.

3. GOALS FOR THE MAIDEN FLIGHT

3.1. Polarimeter

For the first flight, the intended primary targets are theCrab pulsar and nebula and Cygnus X-1, a high-massX-ray binary system, which are the brightest objects inthe PoGOLite energy range. A phase-resolved measure-ment of the polarization over the period of the Crab pul-sar, as well as a measurement of the steady-state polariza-tion from the Crab nebula, is desired. For this goal to beachieved, it is necessary for the timing resolution to be assmall as possible, ideally 1 µs or less. For Cygnus X-1, ameasurement of the polarization direction and degree andtheir evolution over time, if any, is sought.

3.2. Neutron detector

A neutron detector [5] is situated within the detector pres-sure vessel, enclosed by the polyethylene shield, and itwill measure the flux of <10 MeV neutrons, which areexpected to constitute a large portion of the total back-ground for the polarimeter.

Figure 5. Diagram of the overall control hierarchy ofPoGOLite.

3.3. Auroral monitor unit

An auroral monitor unit (AMU) [6] will be attached to theinstrument, and will measure the interaction of chargedparticles with the magnetic field of the earth. Becausesuch interactions may produce polarized X-rays, this rep-resents another source of background for the polarimeter[7], enhanced by the high latitude of the flight path. It isimportant to have an idea of the auroral contribution dur-ing the flight, though the measurement of the aurora is ascientific endeavour in itself.

3.4. Other goals

In addition to the scientific goals, there are many othergoals for the maiden flight. Two star trackers will beflown, one of which has been flown on balloons in thepast, (the “slow” star tracker), and the other of which isof a new and untested design (the “fast” star tracker). The“slow” and “fast” designations are from the original in-tentions for the instruments; the actual cameras, comput-ers, and software on the two trackers are virtually identi-cal. However, the new star tracker is smaller and lighter,has a wider field of view, and requires a smaller baffle toblock off-axis light. This flight will test the performanceof the new tracker design.

Many other hardware and software systems must betested during the flight. For example, while the polarime-ter detectors have been tested extensively in the con-trolled environment of an accelerator, this will be the firsttime that these detectors will be flown on a balloon, andit is necessary to test their performance and resilience. Inaddition, the control and pointing systems will be testedextensively. The lessons we learn from this flight will beused for future instruments and flights.

4. CONTROL STRUCTURE

The main control system for the instrument and point-ing system is known as the payload control unit (PCU).

Figure 6. Diagram of the PCU and polarimeter sys-tems. Blue lines denote ethernet connections, red denotesRS422 and RS485, green denotes SpaceWire connections,orange denotes LVDS, and black indicates other special-ized types of connections and power connections. The28V power connections and connections to the ACS arenot shown.

This system comprises computers and other electronics,and allows communication from the ground, performs au-tonomous functions, monitors the health of the systems,controls the pointing through the ACS, initiates data ac-quisition in the polarimeter, and stores polarimeter andhousekeeping data.

As shown in Figure 5, the PCU is connected to the ACS,the polarimeter, and one of the star trackers. The ACSin turn monitors the attitude of the gondola through dif-ferential GPS, gyroscopes, and a magnetometer, and con-trols various motors and monitors them by means of en-coders. Fine pointing is provided by a connection to asecond star tracker. The star tracker connected to the PCUserves as the backup for the one connected to the ACS.

The constituents of and the connections within and fromthe PCU are shown in Figure 6. Three PC104s are used,two within the PCU enclosure, and one in an indepen-dent enclosure. See §8.3 for a thorough description ofthe functions of these computers. The purpose of the sec-ond enclosure is to provide extra redundancy should oneof the enclosures be damaged during landing. The twoPC104s inside the PCU are each connected to an Irid-ium modem, for communication while the payload is notwithin the line of sight of the launch facility (see §6). Asmall real-time computer provided by DST CONTROL,known as a module PC board (MPB), is controlled andmonitored by the PC104, and is connected to the MPB inthe ACS through a high-speed RS485 bus. An interfaceutility board (IUB), also connected to the MPB throughthe high-speed bus, is used to control power switches andto monitor temperatures and currents, etc. A second IUBperforms similar functions in the PVA. Additional elec-tronics boards provide switches and interfaces to the sen-sors in both the PCU and the PVA, and the board in thePVA also provides logic for the polarimeter electronics.The IUBs are also used to pass the PPS signal from theGPS into the polarimeter electronics. A DC/DC converterin the PCU provides 5V and 12V to the polarimeter.

5. MODES OF OPERATION

Because the function of the instrument will be largely au-tonomous during the flight, it is convenient to use nu-meric modes which define procedures and functions ofthe instrument and pointing system. The modes are di-vided into those for the instrument and those for the atti-tude control system. Some of the modes on the two sys-tems are linked.

5.1. Instrument modes

The following modes are the instrument modes, whichdefine the operations of the polarimeter, including the de-tectors, electronics, and related hardware:

5.1.1. “Power save” or “Do nothing”

The purpose of this mode is to preserve battery powerwhen the instrument does not have to perform any tasks.The instrument arrives in this mode when it is first pow-ered and after a power failure. In this mode, all FADCsand PMTs are unpowered, and when the instrument isswitched into this mode, the control voltages of the PMTsare ramped down and then the FADCs and PMTs areswitched off. Other non-essential equipment such as thecooling system are also powered down.

5.1.2. “Initialize”

When the acquisition mode is changed, the “initialize”mode determines which units are required and powersthem on. The FADCs required for the target acquisitionmode (usually all of them) are checked for proper func-tionality and the PMTs are ramped up to the appropri-ate predefined levels. Parameters such as trigger thresh-olds are read from the appropriate detector mode file andare set for each channel on each FADC. The ACS is ini-tialized as needed as part of this mode, and instructed topoint at a particular predefined target. The instrument isrolled to one of the end points (180◦ or−180◦) to prepareit for a 360◦ rotation through 0◦.

5.1.3. “Ready”

After the initialization in the above “initialize” mode, theinstrument arrives in ready mode. This mode is specific tothe particular acquisition mode that has been requested,and changing the acquisition mode will cause the instru-ment to go back into ”initialize” mode to prepare for ac-quisition with the new parameters.

5.1.4. “Acquisition”

The acquisition mode is reached by a simple transitionfrom “ready” mode. When this mode is set, the instru-ment will already be pointed at the desired target and dataacquisition will begin. At the same moment, the instru-ment starts rolling axially at a speed which will take it360◦ in the acquisition time (usually 15 minutes). Theinstrument must be rolled during observation to removethe systematic effects from varying responses of the de-tectors. Once the acquisition finishes, the instrument re-turns to “initialize” mode in order that the FADCs canbe rechecked and any parameters can be modified for thenext data run.

5.2. Pointing system modes

Like the polarimeter, the ACS also requires the use ofmodes. The following is a summary of the modes usedby the pointing system:

5.2.1. “Startup”

When the ACS is powered on, it is placed in this mode,indicating that none of the motors or encoders has beeninitialized. Before the ACS is capable of useful functions,it must be initialized in the next mode.

5.2.2. “Initialize”

This mode operates each motor in turn from one ex-treme point to the other, checking and calibrating the en-coders. The flywheel assembly, the elevation motor, andthe rolling apparatus are initialized in this way. Variousother systems, such as the GPS and magnetometer, areinitialized and tested.

5.2.3. “Stow” and “Power save”

The instrument is returned to vertical position and thelocking magnets are applied. This is used for low powersituations where the instrument will be powered down inorder that the solar panels can recharge the batteries, orwhen pointing is otherwise not required.

5.2.4. ‘Exercise”

As the balloon rises through the atmosphere after launch,it will likely encounter layers of very cold temperatures,and it may be delayed in such a layer for an unpredictableamount of time. It is important that the motors are kept

moving so that the lubricant is not given a chance to be-come adhesive or to solidify. The purpose of the “Exer-cise” mode is to keep the motors moving back and forthat a slow speed.

5.2.5. “Pointing” and “Tracking”

These modes are used for fine pointing of the instrumentfor data acquisition. The GPS, magnetometers, and gy-roscope are used to point the instrument, and one of thestar trackers is used for fine control. With this system itis possible to keep the instrument pointed within 0◦.1 ofthe target, which is well within the requirements, giventhe field of view [8].

6. COMMUNICATIONS AND CONTROL

6.1. E-Link

While within the line of sight of the launch facility, up to500 km away, it will be possible to communicate with theinstrument on a high-speed connection known as E-link[9]. Thus, it will be possible to control and monitor theinstrument in real-time, and to download entire datasetsat a speed of 1–3 Mbit/s, for analysis on the ground. Therange of this connection may be extended with the useof the transmitting station at the Andøya rocket range inNorway.

6.2. Iridium

Because the balloon will not be within the line of sight ofthe Esrange facility for most of the flight, a satellite-basedIridium Router Unrestricted Digital Information Connec-tivity Solution (RUDICS) [10] will be used for commu-nications when E-Link is not possible. For this reason, acontinuous connection cannot be maintained and many ofthe instrument operations must be autonomous. In addi-tion, the bulk of the scientific data will not be downloadedto the ground until the end of the flight. A significantamount of preprocessing of the data will be performedonboard and the results will be sent to the ground, to en-sure that the instrument works as expected and producesscientifically valid results.

6.3. Autonomous function and ground control

Because the instrument and pointing system are requiredto work autonomously, a predefined set of modes and tar-gets will be established before the flight. A variety ofpredictable errors and failures have been accounted forand will be automatically corrected. For example, thecooling system will automatically be activated when the

components in the polarimeter reach a certain temper-ature. Polarimeter data will be automatically preanal-ysed, and small summary files as well as housekeepingdata files concerning the data acquisition and instrumenthealth will be available for transfer to the ground.

The Iridium system will allow for checking every fewminutes, so the instrument can be monitored at least onceevery acquisition run. When the available files are down-loaded and examined, it will be clear whether any adjust-ments are needed to the equipment or instructions. Be-cause the functions of the instrument are stored in thepayload, there is no need to provide instructions unlesssomething must be changed, and then it involves only up-loading a single text file with the new instructions.

7. THERMAL REGULATION

7.1. Need for a cooling system

The PMTs produce heat, which can raise the tempera-ture of the detector system. The energy deposition from alow energy photon is small, and therefore the dark cur-rent must be kept to a minimum. Since the dark cur-rent increases with temperature, it is necessary to keepthe PMTs at a preferably constant and uniform low tem-perature.

If they are not cooled, the FADCs, which produce a totalof over 100 W of heat, will overheat within a few minuteswhen enclosed in the pressure vessel. The FADCs mustbe kept well below 60◦C to maintain optimal function.

7.2. Cooling system constituents and functions

The instrument is cooled with the use of radiatorsmounted on the outside of the gondola. Paratherm LR[11] heat transfer fluid is pumped from the radiators andthrough the polarimeter pressure vessels. To remove heatfrom the vicinity of the PMTs, a cooling plate is installedthrough which the cold fluid flows. The fluid also flowsthrough plates which hold the FADCs in place, and fansare used near the FADCs to circulate cool air throughoutthe area.

The radiators are mounted at an angle so that the sun,which will be at a low elevation throughout the flight, willnever shine directly onto them. Nevertheless, the targetsand radiator placement must be chosen carefully beforethe flight to reduce the chance that the cooling systemwill be heated by the sun. A safeguard is in place thatwill stop the pump if the fluid entering the system fromthe radiators is too hot. In this event, the polarimeter elec-tronics will be without cooling and must be shut downimmediately.

8. ADDITIONAL CHALLENGES

8.1. Power use

In the circumpolar gondola design, the solar panels aremounted in a skirt around the bottom of the gondola, andthus the sun will shine on at least one of them at anypointing angle. Because the flight will be above the arcticcircle during the summer, the sun will be in the sky at alltimes. For the long duration flight, the batteries must becontinuously charged so that there is enough power avail-able for the instrument to function. It is possible that theinstrument will occasionally need to be shut down for afew hours in order to replenish the batteries. The possi-bility to do this is provided by the “power save” modesin the various systems, and will allow the system to workfor the entire flight.

8.2. Timing

The timing challenges are not specific to the circumpolarflight, but since the balloon will be traveling a long dis-tance, it is more of a challenge to store the coordinatesat a given time in a precise way. As mentioned in § 3.1,it is important that the timing resolution be as small aspossible, particularly for measurements of pulsars. Pho-ton arrival times from pulsars must be shifted to an iner-tial frame of reference, such as the solar system barycen-ter. For this calculation to be performed accurately, theGPS coordinates, including altitude, must be measuredto within a few metres.

When the FADCs store waveforms, they base the storedtimestamp on a clock within the FADCs themselves.Thus, the stored times have no concrete relation to the ac-tual time. To match the times stored with the waveformsto actual times, a pulse per second (PPS) from the GPSsystem is used. PPS events are stored as empty wave-forms in the FADCs, and the timestamps saved with thesePPS events can be used to calibrate the times of the truepolarimeter events.

8.3. Data storage

Because the entirety of the polarimeter data will not betransferred to the ground during the flight, it is importantthat the data be stored as redundantly as possible. ThreePC104s with industrial specifications are employed, onefor the instrument control, one for preprocessing, and thethird as an additional safeguard of the data. All of thesecomputers will be able to function as controllers and pre-processors, should one be rendered inoperative. Eachof the three PC104s has a RAID array of 4 solid statedisks (SSD) and an additional SSD attached directly tothe motherboard. This will allow six complete copies ofthe data to be stored onboard, and even if the computers

are all destroyed in the parachute deployment and land-ing, it is likely that at least a few of the SSDs will surviveand the data will be retrievable.

9. CONCLUSIONS

A circumpolar flight will allow us to do much more sci-ence than in a shorter flight, but there is much more po-tential for minor failures and errors. Thus, it is impera-tive that all systems on PoGOLite be designed in a waywhich will allow autonomous operation with as manysafeguards and redundancies as possible.

The lessons learned from the maiden flight will be ap-plied to future instruments and flights. We will knowwhich equipment performed well and which has a ten-dency to fail under the harsh conditions at 40 km abovethe surface of the earth. We will also be able to constructa better plan for autonomous control and failure recov-ery, once it is more clear which types of failures are mostlikely.

PoGOLite has a great potential to change the face of highenergy astrophysics as we know it. Measurements ofthe polarization represent an entirely new dimension ofknowledge for objects such as pulsars and black holes.

REFERENCES

[1] Kamae, T., et al. Astroparticle Physics 30 (2008) 72.[2] Pearce, M., these proceedings.[3] Stromberg, J.-E., these proceedings.[4] Kiss, M. (2011). Pre-Flight Development of the

PoGOLite Pathfinder, KTH Doctoral thesis, Stock-holm, Sweden, 113.

[5] Takahashi, H., et al. A Thermal-Neutron Detectorwith a Phoswich System of LiCaAlF6 and BGO Crys-tal Scintillators onboard PoGOLite, 2010 IEEE NSSMIC Conference record, in press.

[6] Jokiaho, O., et al. ESA-SP-671, ESAPAC Proceed-ings, Bad Reichenhall, Germany,195-200, 2009.

[7] Larsson, S., et al., ESA-SP-647, ESAPAC Proceed-ings, Visby, Sweden, 513-516, 2007.

[8] Marini Bettolo, C. (2010). Performance studies andstar tracking for PoGOLite, KTH Doctoral thesis,Stockholm, Sweden, 139.

[9] Jonsson, L.-O. ESA-SP-671, ESAPAC Proceedings,Bad Reichenhall, Germany, 215-218, 2009.

[10] http://www.Iridium.com/products/RUDICS.aspx(accessed 26 May 2011).

[11] http://www.paratherm.com/Paratherm-LR/LR-heating-cooling-fluid.asp (accessed 26 May 2011).