control system, operating modes, and communications for pogolite

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control system, operating modes, and communications for pogolite

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 polarimeter

scheduled to be launched in the summer of 2011

from northern Sweden. The planned flight will have a circumpolar

route and will last around 20 days. Because the

payload will not be within the line of sight for most of

the flight, satellite-based Iridium modems must be used.

The low speed and reduced reliability of the connection

require careful design regarding the operations and communications.

The instrument has been made somewhat

autonomous in function and has its own redundant storage.

I describe the modes of operation, the communications,

control, and thermal regulation systems, and the

challenges encountered for a circumpolar flight.

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

1. INTRODUCTION

Figure 1. Photograph of the polarimeter inside its pressure

vessel, which is in turn inside the rotation frame.

The instrument is surrounded by polyethylene blocks for

passive shielding. The star trackers and the auroral monitor

unit are attached to the assembly. The frame of the

attitude control system can be seen in the background.

The Polarized Gamma-ray Observer (PoGOLite) [1, 2] is

a balloon-borne Compton-based polarimeter, with an energy

range of 25–80 keV. In the pathfinder instrument to

be flown in summer 2011, the detector system will employ

61 phoswich detector cells (PDC) and 30 side anticoincidence

shield (SAS) detectors made of BGO material

situated in an unbroken ring around the PDCs. The

full size PoGOLite instrument will contain 217 PDCs.

The polarimeter pressure vessel assembly is shown in

Figure 1.

The instrument is scheduled to be launched on its maiden

flight in summer 2011 from the Esrange Space Center in

northern Sweden. A circumpolar flight around the north

pole is planned, and is expected to last 17–25 days.

In addition to the polarimeter, a sophisticated pointing

system known as the attitude control system (ACS), employing

differential GPS, gyroscopes, magnetometers,

and two star trackers, is employed. This system has been

developed by DST CONTROL in Linköping, Sweden [3].

A flywheel is used to control pointing in the azimuthal

direction, and a motor is used to control the elevation.

To reduce systematic effects, the polarimeter is rotated

around its axis one full turn during each observation.

Figure 2. Photograph of the polarimeter attached to the

attitude control system.


Figure 4. Diagram of a PDC, approximately to scale.

Each of these units is about 1 m long. The slow scintillator

is depicted in blue, the fast scintillator is red, and the

bottom BGO is green.

2.2. Electronics

Figure 3. The PoGOLite detector array, showing the 61

PDCs in the center, surrounded by 30 SAS units. From

[4].

This negates the effect of different detector responses on

the polarization measurement. A photograph of the polarimeter

contained within the attitude control system is

shown in Figure 2.

The signal from each PMT in the detector array is attached

to one of eight channels on a flash analog to digital

converter (FADC) board. There are twelve FADC boards

in total. These boards provide control voltages for the

PMTs and store waveforms from the PMT signals. They

are also capable of some preanalysis, and issue triggers

and hit signals as well as veto signals according to a set

of predefined rules programmed into their FPGA chips.

The signals issued by the FADC boards are received and

processed by the digital input-output (DIO) board, which

in turn issues a signal to the FADC boards to store the current

waveforms in memory. The signals between the DIO

and FADC boards are mediated by another electronics

board which applies the required logic (AND/OR, etc.).

The FADCs contain a limited amount of volatile memory

and can each store 96 waveforms. To initiate data

acquisition and store the waveforms more permanently, a

SpaceWire to ethernet converter board is used, and this is

controlled by an onboard PC104 running Linux.

2. POGOLITE POLARIMETER HARDWARE

3. GOALS FOR THE MAIDEN FLIGHT

2.1. Detector array

3.1. Polarimeter

The PoGOLite detector array is designed with passive

shielding provided by a thick layer of polyethylene surrounding

the pressure vessels. The detector array is contained

within a pressure vessel, and consists of 61 PDCs

and 30 SAS units. The array is shown in Figure 3.

A diagram of a PDC unit is shown in Figure 4. Each PDC

employs a tubular hexagonal plastic scintillator unit at the

entrance to the detector, a solid plastic scintillator for detecting

Compton and photoabsorption events, and a BGO

crystal underneath. A specially designed low-noise photomultiplier

tube (PMT) is attached to each BGO crystal

in the detector array to absorb all the light produced

by the three scintillators. The time constant of the middle

plastic scintillator is much less than for the tubular

or BGO scintillators, so interactions in the middle detector

can be immediately distinguished by the electronics,

and other events, including those from the SAS, can be

immediately rejected. The resulting narrow field of view

(∼ 5 square degrees) and low background make it ideal

for observing astrophysical point sources.

For the first flight, the intended primary targets are the

Crab pulsar and nebula and Cygnus X-1, a high-mass

X-ray binary system, which are the brightest objects in

the PoGOLite energy range. A phase-resolved measurement

of the polarization over the period of the Crab pulsar,

as well as a measurement of the steady-state polarization

from the Crab nebula, is desired. For this goal to be

achieved, it is necessary for the timing resolution to be as

small as possible, ideally 1 µs or less. For Cygnus X-1, a

measurement of the polarization direction and degree and

their evolution over time, if any, is sought.

3.2. Neutron detector

A neutron detector [5] is situated within the detector pressure

vessel, enclosed by the polyethylene shield, and it

will measure the flux of


Figure 5. Diagram of the overall control hierarchy of

PoGOLite.

3.3. Auroral monitor unit

An auroral monitor unit (AMU) [6] will be attached to the

instrument, and will measure the interaction of charged

particles with the magnetic field of the earth. Because

such interactions may produce polarized X-rays, this represents

another source of background for the polarimeter

[7], enhanced by the high latitude of the flight path. It is

important to have an idea of the auroral contribution during

the flight, though the measurement of the aurora is a

scientific endeavour in itself.

3.4. Other goals

In addition to the scientific goals, there are many other

goals for the maiden flight. Two star trackers will be

flown, one of which has been flown on balloons in the

past, (the “slow” star tracker), and the other of which is

of a new and untested design (the “fast” star tracker). The

“slow” and “fast” designations are from the original intentions

for the instruments; the actual cameras, computers,

and software on the two trackers are virtually identical.

However, the new star tracker is smaller and lighter,

has a wider field of view, and requires a smaller baffle to

block off-axis light. This flight will test the performance

of the new tracker design.

Many other hardware and software systems must be

tested during the flight. For example, while the polarimeter

detectors have been tested extensively in the controlled

environment of an accelerator, this will be the first

time that these detectors will be flown on a balloon, and

it is necessary to test their performance and resilience. In

addition, the control and pointing systems will be tested

extensively. The lessons we learn from this flight will be

used for future instruments and flights.

4. CONTROL STRUCTURE

The main control system for the instrument and pointing

system is known as the payload control unit (PCU).

Figure 6. Diagram of the PCU and polarimeter systems.

Blue lines denote ethernet connections, red denotes

RS422 and RS485, green denotes SpaceWire connections,

orange denotes LVDS, and black indicates other specialized

types of connections and power connections. The

28V power connections and connections to the ACS are

not shown.

This system comprises computers and other electronics,

and allows communication from the ground, performs autonomous

functions, monitors the health of the systems,

controls the pointing through the ACS, initiates data acquisition

in the polarimeter, and stores polarimeter and

housekeeping data.

As shown in Figure 5, the PCU is connected to the ACS,

the polarimeter, and one of the star trackers. The ACS

in turn monitors the attitude of the gondola through differential

GPS, gyroscopes, and a magnetometer, and controls

various motors and monitors them by means of encoders.

Fine pointing is provided by a connection to a

second star tracker. The star tracker connected to the PCU

serves as the backup for the one connected to the ACS.

The constituents of and the connections within and from

the PCU are shown in Figure 6. Three PC104s are used,

two within the PCU enclosure, and one in an independent

enclosure. See §8.3 for a thorough description of

the functions of these computers. The purpose of the second

enclosure is to provide extra redundancy should one

of the enclosures be damaged during landing. The two

PC104s inside the PCU are each connected to an Iridium

modem, for communication while the payload is not

within the line of sight of the launch facility (see §6). A

small real-time computer provided by DST CONTROL,

known as a module PC board (MPB), is controlled and

monitored by the PC104, and is connected to the MPB in

the ACS through a high-speed RS485 bus. An interface

utility board (IUB), also connected to the MPB through

the high-speed bus, is used to control power switches and

to monitor temperatures and currents, etc. A second IUB

performs similar functions in the PVA. Additional electronics

boards provide switches and interfaces to the sensors

in both the PCU and the PVA, and the board in the

PVA also provides logic for the polarimeter electronics.

The IUBs are also used to pass the PPS signal from the

GPS into the polarimeter electronics. A DC/DC converter

in the PCU provides 5V and 12V to the polarimeter.


5. MODES OF OPERATION

5.1.4. “Acquisition”

Because the function of the instrument will be largely autonomous

during the flight, it is convenient to use numeric

modes which define procedures and functions of

the instrument and pointing system. The modes are divided

into those for the instrument and those for the attitude

control system. Some of the modes on the two systems

are linked.

5.1. Instrument modes

The following modes are the instrument modes, which

define the operations of the polarimeter, including the detectors,

electronics, and related hardware:

5.1.1. “Power save” or “Do nothing”

The purpose of this mode is to preserve battery power

when the instrument does not have to perform any tasks.

The instrument arrives in this mode when it is first powered

and after a power failure. In this mode, all FADCs

and PMTs are unpowered, and when the instrument is

switched into this mode, the control voltages of the PMTs

are ramped down and then the FADCs and PMTs are

switched off. Other non-essential equipment such as the

cooling system are also powered down.

5.1.2. “Initialize”

When the acquisition mode is changed, the “initialize”

mode determines which units are required and powers

them on. The FADCs required for the target acquisition

mode (usually all of them) are checked for proper functionality

and the PMTs are ramped up to the appropriate

predefined levels. Parameters such as trigger thresholds

are read from the appropriate detector mode file and

are set for each channel on each FADC. The ACS is initialized

as needed as part of this mode, and instructed to

point at a particular predefined target. The instrument is

rolled to one of the end points (180 ◦ or −180 ◦ ) to prepare

it for a 360 ◦ rotation through 0 ◦ .

5.1.3. “Ready”

After the initialization in the above “initialize” mode, the

instrument arrives in ready mode. This mode is specific to

the particular acquisition mode that has been requested,

and changing the acquisition mode will cause the instrument

to go back into ”initialize” mode to prepare for acquisition

with the new parameters.

The acquisition mode is reached by a simple transition

from “ready” mode. When this mode is set, the instrument

will already be pointed at the desired target and data

acquisition will begin. At the same moment, the instrument

starts rolling axially at a speed which will take it

360 ◦ in the acquisition time (usually 15 minutes). The

instrument must be rolled during observation to remove

the systematic effects from varying responses of the detectors.

Once the acquisition finishes, the instrument returns

to “initialize” mode in order that the FADCs can

be rechecked and any parameters can be modified for the

next data run.

5.2. Pointing system modes

Like the polarimeter, the ACS also requires the use of

modes. The following is a summary of the modes used

by 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 been

initialized. 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 extreme

point to the other, checking and calibrating the encoders.

The flywheel assembly, the elevation motor, and

the rolling apparatus are initialized in this way. Various

other systems, such as the GPS and magnetometer, are

initialized and tested.

5.2.3. “Stow” and “Power save”

The instrument is returned to vertical position and the

locking magnets are applied. This is used for low power

situations where the instrument will be powered down in

order that the solar panels can recharge the batteries, or

when 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 unpredictable

amount of time. It is important that the motors are kept


moving so that the lubricant is not given a chance to become

adhesive or to solidify. The purpose of the “Exercise”

mode is to keep the motors moving back and forth

at a slow speed.

5.2.5. “Pointing” and “Tracking”

These modes are used for fine pointing of the instrument

for data acquisition. The GPS, magnetometers, and gyroscope

are used to point the instrument, and one of the

star trackers is used for fine control. With this system it

is possible to keep the instrument pointed within 0 ◦ .1 of

the target, which is well within the requirements, given

the field of view [8].

components in the polarimeter reach a certain temperature.

Polarimeter data will be automatically preanalysed,

and small summary files as well as housekeeping

data files concerning the data acquisition and instrument

health will be available for transfer to the ground.

The Iridium system will allow for checking every few

minutes, so the instrument can be monitored at least once

every acquisition run. When the available files are downloaded

and examined, it will be clear whether any adjustments

are needed to the equipment or instructions. Because

the functions of the instrument are stored in the

payload, there is no need to provide instructions unless

something must be changed, and then it involves only uploading

a single text file with the new instructions.

6. COMMUNICATIONS AND CONTROL

7. THERMAL REGULATION

6.1. E-Link

7.1. Need for a cooling system

While within the line of sight of the launch facility, up to

500 km away, it will be possible to communicate with the

instrument on a high-speed connection known as E-link

[9]. Thus, it will be possible to control and monitor the

instrument in real-time, and to download entire datasets

at a speed of 1–3 Mbit/s, for analysis on the ground. The

range of this connection may be extended with the use

of the transmitting station at the Andøya rocket range in

Norway.

6.2. Iridium

The PMTs produce heat, which can raise the temperature

of the detector system. The energy deposition from a

low energy photon is small, and therefore the dark current

must be kept to a minimum. Since the dark current

increases with temperature, it is necessary to keep

the PMTs at a preferably constant and uniform low temperature.

If they are not cooled, the FADCs, which produce a total

of over 100 W of heat, will overheat within a few minutes

when enclosed in the pressure vessel. The FADCs must

be kept well below 60 ◦ C to maintain optimal function.

Because the balloon will not be within the line of sight of

the Esrange facility for most of the flight, a satellite-based

Iridium Router Unrestricted Digital Information Connectivity

Solution (RUDICS) [10] will be used for communications

when E-Link is not possible. For this reason, a

continuous connection cannot be maintained and many of

the instrument operations must be autonomous. In addition,

the bulk of the scientific data will not be downloaded

to the ground until the end of the flight. A significant

amount of preprocessing of the data will be performed

onboard and the results will be sent to the ground, to ensure

that the instrument works as expected and produces

scientifically valid results.

6.3. Autonomous function and ground control

Because the instrument and pointing system are required

to work autonomously, a predefined set of modes and targets

will be established before the flight. A variety of

predictable errors and failures have been accounted for

and will be automatically corrected. For example, the

cooling system will automatically be activated when the

7.2. Cooling system constituents and functions

The instrument is cooled with the use of radiators

mounted on the outside of the gondola. Paratherm LR

[11] heat transfer fluid is pumped from the radiators and

through the polarimeter pressure vessels. To remove heat

from the vicinity of the PMTs, a cooling plate is installed

through which the cold fluid flows. The fluid also flows

through plates which hold the FADCs in place, and fans

are used near the FADCs to circulate cool air throughout

the area.

The radiators are mounted at an angle so that the sun,

which will be at a low elevation throughout the flight, will

never shine directly onto them. Nevertheless, the targets

and radiator placement must be chosen carefully before

the flight to reduce the chance that the cooling system

will be heated by the sun. A safeguard is in place that

will stop the pump if the fluid entering the system from

the radiators is too hot. In this event, the polarimeter electronics

will be without cooling and must be shut down

immediately.


8. ADDITIONAL CHALLENGES

8.1. Power use

In the circumpolar gondola design, the solar panels are

mounted in a skirt around the bottom of the gondola, and

thus the sun will shine on at least one of them at any

pointing angle. Because the flight will be above the arctic

circle during the summer, the sun will be in the sky at all

times. For the long duration flight, the batteries must be

continuously charged so that there is enough power available

for the instrument to function. It is possible that the

instrument will occasionally need to be shut down for a

few hours in order to replenish the batteries. The possibility

to do this is provided by the “power save” modes

in the various systems, and will allow the system to work

for the entire flight.

8.2. Timing

The timing challenges are not specific to the circumpolar

flight, but since the balloon will be traveling a long distance,

it is more of a challenge to store the coordinates

at a given time in a precise way. As mentioned in § 3.1,

it is important that the timing resolution be as small as

possible, particularly for measurements of pulsars. Photon

arrival times from pulsars must be shifted to an inertial

frame of reference, such as the solar system barycenter.

For this calculation to be performed accurately, the

GPS coordinates, including altitude, must be measured

to within a few metres.

When the FADCs store waveforms, they base the stored

timestamp on a clock within the FADCs themselves.

Thus, the stored times have no concrete relation to the actual

time. To match the times stored with the waveforms

to actual times, a pulse per second (PPS) from the GPS

system is used. PPS events are stored as empty waveforms

in the FADCs, and the timestamps saved with these

PPS events can be used to calibrate the times of the true

polarimeter events.

8.3. Data storage

Because the entirety of the polarimeter data will not be

transferred to the ground during the flight, it is important

that the data be stored as redundantly as possible. Three

PC104s with industrial specifications are employed, one

for the instrument control, one for preprocessing, and the

third as an additional safeguard of the data. All of these

computers will be able to function as controllers and preprocessors,

should one be rendered inoperative. Each

of the three PC104s has a RAID array of 4 solid state

disks (SSD) and an additional SSD attached directly to

the motherboard. This will allow six complete copies of

the data to be stored onboard, and even if the computers

are all destroyed in the parachute deployment and landing,

it is likely that at least a few of the SSDs will survive

and the data will be retrievable.

9. CONCLUSIONS

A circumpolar flight will allow us to do much more science

than in a shorter flight, but there is much more potential

for minor failures and errors. Thus, it is imperative

that all systems on PoGOLite be designed in a way

which will allow autonomous operation with as many

safeguards and redundancies as possible.

The lessons learned from the maiden flight will be applied

to future instruments and flights. We will know

which equipment performed well and which has a tendency

to fail under the harsh conditions at 40 km above

the surface of the earth. We will also be able to construct

a better plan for autonomous control and failure recovery,

once it is more clear which types of failures are most

likely.

PoGOLite has a great potential to change the face of high

energy astrophysics as we know it. Measurements of

the polarization represent an entirely new dimension of

knowledge 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] Strömberg, J.-E., these proceedings.

[4] Kiss, M. (2011). Pre-Flight Development of the

PoGOLite Pathfinder, KTH Doctoral thesis, Stockholm,

Sweden, 113.

[5] Takahashi, H., et al. A Thermal-Neutron Detector

with a Phoswich System of LiCaAlF6 and BGO Crystal

Scintillators onboard PoGOLite, 2010 IEEE NSS

MIC Conference record, in press.

[6] Jokiaho, O., et al. ESA-SP-671, ESAPAC Proceedings,

Bad Reichenhall, Germany,195-200, 2009.

[7] Larsson, S., et al., ESA-SP-647, ESAPAC Proceedings,

Visby, Sweden, 513-516, 2007.

[8] Marini Bettolo, C. (2010). Performance studies and

star tracking for PoGOLite, KTH Doctoral thesis,

Stockholm, Sweden, 139.

[9] Jönsson, 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/LRheating-cooling-fluid.asp

(accessed 26 May 2011).