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.
_________________________________________________
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 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.
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.
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.
POGOLITE POLARIMETER HARDWARE
3.
2.1.
Electronics
Detector array
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.
3.1.
GOALS FOR THE MAIDEN FLIGHT
Polarimeter
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 <10 MeV neutrons, which are
expected to constitute a large portion of the total background for the polarimeter.
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
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.
5.1.4.
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.
“Initialize”
“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.
“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.
5.1.2.
“Acquisition”
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.
“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.
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].
6.
6.1.
COMMUNICATIONS AND CONTROL
E-Link
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
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
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.
7.
7.1.
THERMAL REGULATION
Need for a cooling system
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.
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.
8.1.
ADDITIONAL CHALLENGES
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.
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.
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
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.
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[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
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