the design and performance of the gondola pointing system for … · 2020. 1. 1. · (sufi) and the...
TRANSCRIPT
Journal of Astronomical Instrumentation
World Scientific Publishing Company
1
The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne
Stratospheric Solar Observatory
A. Lecinski†, G. Card‡, M. Knölker‡ and B. Hardy§ †High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, USA, [email protected]
‡High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, USA §Dynamics Analysis & Control, LLC, Longmont, CO 80503, USA, [email protected]
Received (to be inserted by publisher); Revised (to be inserted by publisher); Accepted (to be inserted by publisher);
At one meter, the Sunrise Balloon-Borne Stratospheric Solar Observatory is the largest solar telescope to leave the earth. Its aim
during its June 12 to June 17, 2013 flight was to study the magneto-convective processes of the sun at a resolution of better than
100 km. To obtain this goal, the gondola and telescope are required to point to an accuracy of better than 26 arc seconds for
extended periods of time. Pointing of the gondola and telescope was effected by the Sunrise Pointing System (PS). The PS
takes pointing error signals provided by a Lockheed Intermediate Sun Sensor (LISS) and passes the data through a cascade of up
to four digital biquadratic filters to calculate best voltages to move azimuthal and elevation motors. All filter settings can be
modified in flight to adapt to changing conditions. Using this design, the Sunrise Pointing System achieved the required goal,
pointing the gondola and telescope to better than 26 arc seconds for 60% of the flight and continuous time periods up to 99
minutes. In this paper we detail the design and performance of the PS during the 2013 flight.
Keywords: Balloon; pointing; attitude control.
1. Introduction
The second science flight of the Sunrise balloon-borne stratospheric solar observatory took place in June of 2013.
The stratospheric balloon flight began from ESRANGE (near Kiruna Sweden) and floated to northern Canada's
Boothia Peninsula. Floating above most of the earth's atmosphere at 36km, the Sunrise telescope is able to obtain
data in important, far ultra violet wavelengths (as low as 214 nm) and observe the sun without the detriment of
atmospheric seeing.
The Sunrise balloon-borne stratospheric solar observatory has been fully described by Barthol et al. (2011). The
Sunrise Pointing System (PS), developed by the High Altitude Observatory (HAO), has many functions: off-
pointing, flat fielding and engineering data collection. But the most critical function is to “coarsely” point the
gondola and telescope to a target on the sun within 26 arc seconds or better accuracy. Once the gondola and
telescope are stably pointing at better than ±26 arc seconds the Kiepenheuer Institute for Solar Physics (KIS)
Image Stabilization and Light Distribution (ISLiD) system and Correlating Wave-Front Sensor (CWS) can deliver
a stabilized image of a precision of 0.04 arc second (RMS) to the science instruments: the Sunrise Filter Imager
(SuFI) and the Imaging Magnetograph eXperiment (IMaX). Descriptions of ISLiD and SuFI are given in
Gandorfer et al. (2011). CWS is described by Berkefeld et al. (2011). Information for IMaX is given by
Martínez Pillet et al. (2011).
Pointing a 1920 kg balloon-borne telescope accurately and continuously to the sun is a non-trivial matter. In the
azimuthal direction there is nothing solid to push against. However, if one uses an appropriately sized flywheel*,
the reaction torque generated by its motor-driven acceleration (or deceleration) can be used to control the rotation
* The flywheel is also referred to as a reaction wheel.
2 A. Lecinski, et al.
of the gondola. Occasionally atmospheric disturbances require corrections that could force rotational velocities of
the flywheel beyond its capabilities. Thus the design of Sunrise uses both a flywheel (fine azimuth) and an
additional coarse azimuthal motor to minutely turn the entire gondola and allow the flywheel to slow down. In
the elevation direction pointing is much more straightforward, but care must be taken neither to induce nor
reinforce any pendulum motions of the gondola and telescope. The Sunrise elevation design uses an inclined
linear stage and a single motor to raise and lower the telescope. Fine detail pointing errors are measured by a
Lockheed Intermediate Sun Sensor (LISS). When PS first finds the sun, it employs less accurate sun sensors with
wide angle acquisition ranges: the ‘Precision Azimuth Sun Sensor’ (PASS), ‘Full Range Elevation Detector’
(FRED) and corner cells (azimuthal detectors). The LISS, PASS, FRED and corner cells are described in detail
below. A schematic of the design is shown in Figure 1.
Fig. 1 Schematic of the Sunrise Gondola.
The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne Stratospheric Solar Observatory 3
2. Design
2.1. Pointing System program overview
The PS computer, a Diamond systems PLT-N270XT-2G, runs the highly optimized PS program written in C++.
In addition to pointing, the PS program must also simultaneously perform other administrative tasks, and hence is
multi-threaded. Threads consist of:
Artificial Intelligence (AI) thread
o Computes running means and standard deviations of sun sensor data and motor voltages.
o Determines which sun sensors have valid data, which pointing plan to implement and which
servos to activate.
o Sets “use=1.0” or “don’t use=0.0” variables for the Pointing thread to use in its servo
calculations.
o Runs every 9 seconds.
Pointing thread
o Collects all sun sensor data.
o Collects all motor encoder data.
o Collects environmental data for PS components.
o Executes all pointing servo calculations.
o Sends output of servo calculations (voltages) to the elevation motor (El_), fine azimuth motor,
(Azf) and coarse azimuth motor (Azc).
o Saves collected data to memory.
o Runs at 150Hz.
Incoming command thread.
o Processes incoming commands from the Instrument Control Computer (ICU).
o Interrupt based.
CWS thread
o Processes incoming commands from CWS.
o Interrupt based.
CWS PS pointing lock thread
o Signals CWS when PS pointing is within ±6 arc seconds.
o Interrupt based.
Write data thread
o Gathers all collected data from memory and sends data to ICU. Data are retained on the gondola.
o Runs every 5 seconds.
House Keeping thread
o Gathers snippet of most recent data and sends to ICU. ICU transmits House Keeping to ground.
o Runs every second. May be commanded to run every 15 seconds.
Thumbnail thread
o If commanded, sends thumbnails of data to ICU. ICU transmits thumbnails to ground.
Thumbnails consist of trimmed down sun sensor data and motor velocities and voltages. They
contain sufficient information to determine how well the filters in the servos are behaving.
Thumbnail data were pivotal for fine tuning pointing servos and improving pointing accuracy.
The workhorses of the PS are the AI thread and the Pointing thread. They are described in detail below.
4 A. Lecinski, et al.
2.2. Sun sensors
The ensemble sun sensors used by the PS to determine orientation are:
low resolution coarse pointing, azimuth only, used during initial sun acquisition pointing:
o azimuth: four corner cells
The corner cells are mounted on the four corners of the gondola. They are wide range
photovoltaic cells with acceptance cones of 60° in elevation and 105° in azimuth. The PS corner
cells were calibrated such that the positon of the sun is known within a degree or so of accuracy.
intermediate pointing:
o azimuth: ‘Precision Azimuth Sun Sensor’ (PASS)
The PASS is mounted to the gondola frame, facing forward.
It is a shadow sensor type detector generating an azimuth (yaw) difference signal and Sun
Present intensity signal. It has a linear range of ±3° in azimuth and can capture the sun at any
elevation angle. It has an accuracy of a few to ~10 arc seconds.
o elevation: ‘Full Range Elevation Detector’ (FRED)
The FRED is mounted to the telescope frame. It is a shadow sensor detector with a linear range
of ±15° in elevation. For angles from ~15° to 60° (+/-) it gives a saturated signal which indicates
if the Sun is above or below the current telescope elevation. Within the linear range its accuracy
is a few arcminutes. The FRED has an azimuthal capture range of ±5°.
precise pointing:
o azimuth and elevation: Lockheed Intermediate Sun Sensor (LISS).
The LISS is mounted to the telescope frame. It consists of 5 photodiodes beneath a square
aperture window. They are arranged such that the central diode provides a Sun Present signal,
the right/left diodes providing an azimuthal (yaw) difference signal, and the top bottom diodes an
elevation (pitch) difference signal. When the Sun Present signal is fully saturated, zero readings
from LISS yaw and LISS pitch indicate the sun is centered. The capture range of the LISS is ±3°
in azimuth and elevation. Its linear range is ±15 arcminutes. Within the linear range the
accuracy of the LISS is 1 to 2 arc seconds.
The LISS is mounted on a two axis motorized tip-/tilt stage with precise sub-arc second
resolution encoders. By moving the LISS stage, the telescope can be pointed in any direction up
to a distance of 4 degrees from sun center. This allows the telescope to observe features located
anywhere on the sun and even beyond the solar limb. Co-alignment between the LISS and the
telescope is accomplished by moving the LISS stage as CWS looks for the solar limb. Once the
location of the north, south, east and west limbs have been found, orientation and spatial scale is
computed, stored in memory and saved to the PS disk.
The data from these sun sensors is acquired at 150Hz as part of the Pointing thread, described in more detail
below.
2.3. AI thread
To acquire and point to the sun, the PS AI thread employs three pointing stages: low, intermediate and precise.
The thread initially determines the sun’s location in azimuth. Low resolution pointing mode then slowly rotates
the gondola towards the sun. Once the thread has determined that the gondola is stably pointing to the sun within
a few degrees in azimuth, intermediate azimuthal pointing mode begins. Simultaneously, light should now be
available on the elevation sensors, so the thread may now activate the intermediate elevation pointing mode.
When the AI thread determines that azimuthal or elevation pointing is stable to within a few arcminutes, it
The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne Stratospheric Solar Observatory 5
activates the precise (arc second) pointing mode for that axis. The AI thread treats each axis independently. That
is, the precise elevation pointing mode may be activated while the intermediate azimuthal pointing mode may still
be engaged. Additionally, steps can be skipped so that the precise pointing mode can start immediately as long
as certain criteria are met.
The AI thread autonomously determines which pointing plan to activate. As mentioned above, the AI thread
computes running means and running standard deviations of all the sun sensor data. If the means and standard
deviations simultaneously meet certain criteria, a pointing plan is activated. A pointing plan sets “use=1.0” or
“don’t use=0.0” for sun sensors and servos. For example, for the intermediate azimuthal pointing mode, the “use”
settings are:
UseCC = 0.0 (corner cells)
UsePassAz = 1.0 (PASS Difference)
UseLissAz = 0.0 (LISS yaw)
UseAzcTrack = 0.0 (low resolution azimuthal pointing servo, see below)
UseAzfToAzc = 1.0 (medium and precise azimuthal pointing servo, see below)
UseAzfTrack = 1.0 (medium and precise azimuthal pointing servo, see below)
These “use” values are a key strategy to avoid any variation in the execution time of the highly optimized
Pointing thread, described below.
The AI thread also compensates for misaligned sensors. For example, it was not possible for the PASS to be
perfectly aligned with LISS yaw. Thus when the gondola was in intermediate azimuthal pointing mode, the AI
thread had to nudge the gondola, check if the statistics for LISS yaw improved, and if so, it would keep nudging
the gondola until LISS yaw statistics met the established criteria. If the statistics did not improve, the AI thread
would nudge the gondola in the reverse direction. The slight nudging is accomplished by adding small offsets to
the PASS Difference signal. Once a good offset was found, it was applied to the PASS Difference signal,
retained in memory and saved to a file on the PS disk. The PS also applied this strategy between the alignments
of the corner cells to PASS, corner cells to LISS yaw, and FRED to LISS pitch. Thus if any misalignment
between sun sensors existed or changed at launch or occurred during the flight, it was automatically handled by
the AI thread.
The offsets could be set via commands from ground. And similarly, threshold settings for statistics and pointing
offsets could be modified in flight via commands sent from ground. Auto-pointing could be commanded to be
enabled, or disabled and commanded to go into manual mode.
2.4. Pointing thread
The voltages seen by the above sun sensors are converted to digital units with a Diamond Systems Analog to
Digital (A/D) Converter / Counter Timer board, model DSC_DMM32DXAT. The total intensity and intensity
differences from these sun sensors are sampled and processed at a constant rate, fast enough to achieve the desired
pointing consistency and accuracy. The lowest rate to achieve the desired pointing accuracy was calculated to be
~100Hz. Faster rates yield improved performance. Code optimization allowed the PS Pointing thread to run
quickly, so the Diamond Counter/Timer board was set to run at 150Hz.
Each interrupt of the 150Hz rate calls the highly efficient C++ Pointing thread which captures the current sample
of all of the data provided by the A/D board:
6 A. Lecinski, et al.
Corner cell (cc) intensities: ccRearRight, ccRearLeft, ccFrontRight, ccFrontLeft
FRED difference signal
PASS sum (PASS sun present)
PASS difference signal
LISS yaw difference signal
LISS pitch difference signal
LISS sun present
Amplifier temperatures for: Azc, Azf and El_
Motor temperatures for: Azc, Azf and El_
Upper gondola accelerometer data in X, Y and Z directions
Lower gondola accelerometer data in X, Y and Z directions
Telescope accelerometer data in X, Y and Z directions
PS computer environmental data: temperature, pressure, humidity
and also queries the Azc, Azf and El_ encoders for current positions.
The Pointing thread distributes the sun sensor data and flywheel velocity through the appropriate servo suites. A
servo suite consists of up to four sequential user selectable digital biquadratic filters (low pass, high pass, lead,
lag, integrator, notch, peak or one-to-one). The output from a servo suite calculation is a voltage that is sent to
the appropriate gondola/telescope pointing motor:
coarse azimuth motor (Azc)
fine azimuth (flywheel) motor (Azf)
elevation motor (El_)
The Pointing thread is highly optimized and coded to insure that it consumes identical clock times and identical
CPU cycles every time it is trigged for execution. No ‘if’ or ‘case’ statements are used. Instead, all calculations
for all light sensors and all servos are computed in each sample. The outputs of the servos are multiplied by the
“use=1.0” or “don’t use=0.0” values provided by the AI thread, and then they are added all together and sent to
the appropriate motors. Since the Pointing thread just does sampling and numerical calculations, and never has
to choose what light sensor to use or what servos to use, the thread always runs in the same amount of time. The
CPU cycles used by the Pointing thread easily fit within 150Hz.
2.5. Pointing servos
The servo suites are:
AzcTrack coarse azimuth pointing using Corner Cell intensities for input.
This servo is only used during initial sun acquisition pointing.
The output from AzcTrack suite is a voltage to the Azc motor which rotates the entire
gondola.
AzfTrack intermediate azimuth pointing when using the PASS difference signal as input,
or precise azimuth pointing when using the LISS yaw difference signal as input. The output from the AzfTrack suite is a voltage to the Azf motor which accelerates or
decelerates the flywheel. The acceleration/deceleration of the flywheel rotates the
gondola by small, precise amounts. A desired Azf velocity of 10 RPM ensures that the
Azf motor is nearly always in motion.
AzfToAzc spins off excessive Azf (flywheel) velocity to Azc.
The input is the difference between the desired Azf velocity and the actual Azf velocity.
The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne Stratospheric Solar Observatory 7
The output from the AzfToAzc suite is a voltage to the Azc motor which will very
slightly rotate the entire gondola. This very slight rotation allows the flywheel to slow.
El_Track intermediate elevation pointing when using the FRED difference signal as input,
or precise elevation pointing when using the LISS pitch difference signal as input.
The results from the El_Track suite are passed to the El_Local servo suite.
El_Local acts to reduce the effects of bearing friction and changes the output of El_Track from a
motor current (the effects of which will change with temperature) to a calibrated velocity.
The results from the El_Local suite are passed to the El_Frict function.
El_Frict Friction is reduced further using this Friction Compensation function.
This is not a digital biquadratic filter but rather a function which compensates for stiction
in the motor/drive as well as gravitational effects. It is detailed below.
The output from this function is a voltage applied to the El_ motor which then drives the
telescope lever arm along the inclined elevation stage. Movement of the lever arm raises
or lowers the telescope.
With the exception of the El_Frict function, the above servo suites are
all cascaded digital biquadratic filters. Settings for all of the servos and
functions can be modified in-flight via commands sent through the ICU.
Modification of the settings allows the PS to adapt to nuances of
evolving flight conditions. Thumbnails containing 6 seconds of 150Hz
sun sensor data, motor voltages and velocities provided detailed servo
performance information. Analysis on the ground by servo engineers
determined where improvements were needed in the servo settings.
Improved settings could quickly be sent up to the PS and activated. The
settings in Table 1 produced a continuous 99 minute period during
which pointing was within ±26 arc seconds of the target.
Selection of the optimal suite gains and filter frequencies ensure that a
servo remains stable and resonances are not excited in the mechanical
structure of the gondola. Vibrations of the telescope are undesirable as
they negatively affect the CWS performance. High amplitude vibrations
prevent the CWS from achieving a closed loop control lock of the image
stabilization system. And low amplitude vibrations can create residual
image smear.
The deleterious effect of too large a suite gain is shown in data from the
commissioning phase of the mission. During the commissioning phase,
servo settings similar to those of the 2009 flight were initially tried and
subsequently adjusted. A trial setting applied on 2013 June 12 at
10:08:24 used fairly high servo suite gain for AzfTrack (-400.). As seen
in Figure 2, this resulted in a very high noise level, particularly at 10Hz.
A downloaded thumbnail revealed the problem to the PS engineers, who
determined a smaller AzfTrack servo suite gain was required. At
10:08:38 an AzfTrack servo suite gain of -150.0 was uploaded to the PS,
and as seen in Figure 3, the smaller gain resulted in greatly improved
pointing and much less noise at 10Hz.
Table 1. Servo settings for the 2013 Sunrise flight.
Settings shown produced a 99 minute period where
pointing was continuously within ±26 arc seconds.
AzfTrack settings: suite gain -150.00
Order Type Settings
2nd Low pass Frequency=5. Quality=1.
1st Lead Zero=0.1 Pole=20.
1st Integrator Frequency=0.11
AzfToAzc settings: suite gain -900.00
Order Type Settings
1st Lead Zero=0.1 Pole=1.0
1st Integrator Frequency=0.005
El_Track settings: suite gain -0.100
Order Type Settings
2nd Low pass Frequency=5. Quality=1.
1st Integrator Frequency=0.125
2nd Notch Frequency=0.43 Quality=4.7
2nd Notch Frequency=5. Quality=5.
El_Local settings: suite gain 2.0
Order Type Settings
1st Integrator Frequency=100.
1st Lead Zero=5.0 Pole=20.
El_Frict settings: El_FrictSlope 10.0
El_FrictThresPlus 1.2
El_FrictThreshMinus -1.2
8 A. Lecinski, et al.
Fig. 2 Plot showing deleterious effects of too large servo suite gain for AzfTrack: a 10Hz gondola resonance has been excited.
Fig. 3 Reduction of the AzfTrack servo suite gain by a factor of ~2.7 greatly improves pointing and decreases noise, notably the
10Hz gondola resonance.
The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne Stratospheric Solar Observatory 9
Unlike the above servos, the El_Frict friction compensation function is not a biquadratic filter, but rather a simple
function. For most motors, a non-linear friction region exists where it is more difficult to initiate the rotation of
the axle. In this area, more force, hence higher voltage must be used. If one does not compensate for this
behavior, any small voltage sent within the friction region will not move the motor the desired amount, and the
next servo loop will see the increased pointing error and send ever larger voltages to the motor. This typically
causes oscillating overshoot/undershoot (‘limit cycle’) and very poor pointing performance.
To compensate, a friction compensation function can be implemented. Friction compensation functions assume
many shapes depending on what details are included in the friction model. For Sunrise, a simple Coulomb
friction model was used and was designed to match the specifics of the Sunrise elevation motor and elevation
stage geometry. The Sunrise friction compensation function simply boosts the voltage within a small region
around the origin with a multiplicative factor, El_FrictSlope. Outside of the region offsets are added. The
equations follow:
𝑖𝑓 ( 𝑉𝑖𝑛 ≥ 𝑇ℎ𝑟𝑒𝑠ℎ𝑝𝑙𝑢𝑠
𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒) 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 + 𝑇ℎ𝑟𝑒𝑠ℎ𝑝𝑙𝑢𝑠 −
𝑇ℎ𝑟𝑒𝑠ℎ𝑝𝑙𝑢𝑠
𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒
𝑒𝑙𝑠𝑒 𝑖𝑓 ( 𝑉𝑖𝑛 ≤𝑇ℎ𝑟𝑒𝑠ℎ𝑚𝑖𝑛𝑢𝑠
𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒) 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 + 𝑇ℎ𝑟𝑒𝑠ℎ𝑚𝑖𝑛𝑢𝑠 −
𝑇ℎ𝑟𝑒𝑠ℎ𝑚𝑖𝑛𝑢𝑠
𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒 (1)
𝑒𝑙𝑠𝑒 𝑉𝑜𝑢𝑡 = 𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒 × 𝑉𝑖𝑛 .
Figure 4 shows the output voltage as a function of the input voltage for the cases in Eq. (1).
To accommodate evolving conditions,
telescope loading, temperature changes or
other issues affecting the friction
characteristics of the motor, El_FrictSlope,
Threshminus and Threshplus could be
individually modified. During the flight only
symmetric adjustments of Threshminus and
Threshplus were needed to maintain good
motor performance and good pointing.
These adjustments were likely necessary due
to the non-linearity of the elevation stage
design as well as diurnal temperature
variations.
As Figures 5 and 6 indicate, the use of the
El_Frict friction compensation function was
critical to maintaining good pointing. In
Figure 5, where no friction compensation
was in use, pointing was erratic, elevation
motor voltage was quite high, and elevation
motor velocities varied widely and rapidly.
In Figure 6 where friction compensation was
implemented, pointing was greatly improved,
motor voltages were smaller and velocities
excursions were greatly diminished.
Fig. 4 El_Frict function overcomes stiction within the elevation motor. Within the
static friction region the voltage is boosted to compensate for the additional force
needed to overcome static friction. El_Frict assumes a simple Coulomb friction model,
following Eq (1). In the above example, El_FrictSlope=10, ThreshMinus=-1.0 and
ThreshPlus=+1.0 .
10 A. Lecinski, et al.
Fig. 5. Friction compensation not in use. Note large pointing errors, voltages and velocity excursions.
Fig 6. Friction compensation in use. Pointing is greatly improved. Voltages and volocities no longer over shoot.
The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne Stratospheric Solar Observatory 11
When the friction compensation function is not used, the extra push caused by the servo overshoot can induce
pendulum motions. As seen in Figure 5, the large excursions with a ~8 second periodicity appear to be due to an
overshoot/undershoot induced pendulum motion of the telescope. The smaller excursions with a ~2 second
periodicity exist in both Figures. These are likely caused by the short period oscillation of the gondola on its
suspension system (flight train).
3. Results
During the 2013 flight of ~120 hours, the
gondola/telescope was pointing within ±26
arc seconds for more than 50% of the flight.
If one adds all time periods during which
the gondola/telescope was pointing within
±26 arc seconds for at least 1 minute
continuously, the total sum is over 72 hours,
or 60% of the flight.
Long periods of continuous good pointing
are needed for scientific goals, e.g. long
exposures, calibrations and movies. Thus
continuous, long duration pointing periods
of accuracy better than ±26 arc second are
highly desirable. Table 2 shows the total
sum of time over the entire flight that the gondola/telescope was pointed continuously within ±26 arc seconds of
the target for time periods of 30 seconds, 1 minute, 2 minutes, 5 minutes or 10 minutes.
Shown in the fourth column of Table 2 are the statistics from the 2009 flight of Sunrise (Sunrise I). Comparison
shows Sunrise II pointing statistics are a factor of two better than Sunrise I. During Sunrise I, thumbnails were
not available, so servo engineers on the ground had very limited information to fine tune the servos. Having the
benefit of thumbnails and the ability to upload informed servo settings were pivotal in the improved performance
of the 2013 flight.
Shown in Figure 7 is the histogram of the durations of all good pointing periods in the 2013 flight. There are
several time periods greater than an hour where the gondola/telescope was continually pointing within ±26 arc
seconds of the target. Notably, a 99 minute period of continuous good pointing was achieved.
Scientific objectives required re-pointing of the gondola/telescope for geometric scaling, flat fielding and
targeting of interesting solar features. To save time, re-pointing was done quickly and these sudden offsets
disrupted the continuity of the good pointing. Without these perturbations, the PS could have attained longer
periods of continuous good pointing. To test this hypothesis, the statistics for long periods of good pointing were
rerun by setting all LISS errors to zero for 30 seconds after any re-pointing command was received. The re-
computed histogram is shown in Figure 8.
Table 2. Total duration of flight with the gondola/telescope pointing within ±26 arc
seconds of target. The sum only includes continuous times of a minimum duration.
Minimum
continuous time
period with
gondola/telescope
pointing within
±26 arc seconds.
Sum of time in
2013 (Sunrise II)
flight with
gondola/telescope
pointing within
±26 arc seconds.
(hours)
Percent of entire
2013 (Sunrise II)
flight with
gondola/telescope
pointing within
±26 arc seconds.
Percent of entire
2009 (Sunrise I)
flight with
gondola/telescope
pointing within
±26 arc seconds.
30 seconds 80.9 67% 48%
1 minute 72.5 60% 40%
2 minutes 62.5 52% 30%
5 minutes 47.0 39% 18%
10 minutes 33.3 28% 12%
12 A. Lecinski, et al.
As can be seen from Figure 8, adjusting for commanded re-pointing has combined several of the shorter periods
together. Remarkably, there are now two periods of good pointing that exceed 135 minutes. In comparison with
the 2009 flight, as reported by Barthol et al. (2011), the longest period with telescope pointing within ±46 arcsec
of the target was 45 minutes.
Termination of many good pointing
periods resulted from elevation
pointing failure, an example of which
is shown in Figure 9. Since these
failures were sporadic events,
thumbnails were not available for
engineers to diagnose the problem.
Post flight analysis revealed the failure
was due to the very high 100Hz
frequency setting for the integrator
filter in the El_Local servo suite.
Reducing the frequency by a few
percent should eliminate the failure
and still maintain pointing
performance.
Fig. 7 Histogram of the length of periods of continuous pointing
within ±26 arc seconds of the target during the 2013 flight of Sunrise.
Fig. 8 Recomputed histogram of the length of periods of continuous
pointing within ±26 arc seconds of the target during the 2013 flight of
Sunrise after compensating for commanded re-pointing.
Fig. 9 Termination of a good pointing period due to elevation pointing failure.
The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne Stratospheric Solar Observatory 13
Occasionally, azimuthal pointing failure occurred as well. The AzfTrack servo did not include a friction
compensation function. Thus, under circumstances where the velocity of the flywheel approached zero,
overshoot/undershoot of the Azf motor voltage occurred. An example is given in Figure 10. Around 14:35:32, a
typical atmospheric disturbance has perturbed the pointing, causing the flywheel to slow. Around 14:35:46 the
velocity required by the servos was positive, but very close to zero. However, friction caused the flywheel to stall
and the pointing to further degrade. Larger LISS yaw error signals caused the servos to demand more voltage,
beginning the overshoot/undershoot pattern. Although not obvious from the Figure, the Azc motor also exhibits
stiction. The large Azc velocity excursions at 14:35:52 and 14:36:08 reveal the friction effects on the motor’s
performance.
These errors did not occur frequently since the Azf motor was nearly always moving. But the easy addition of
simple friction compensation functions to the output of the AzfTrack and AzfToAzc servos could completely
eliminate this failure mode.
Fig. 10 Pointing failure due to stiction effects of the Azf flywheel. Stiction effects in the Azc motor are also evident.
14 A. Lecinski, et al.
4. Conclusions
The Sunrise II Pointing System was able to maintain stable pointing within the required ±26 arc seconds of the
target for a substantial portion of the 2013 flight. With this success, ISLiD and CWS were able to perform well,
and the science instruments, IMaX, and SUFI were able to obtain high quality data. Using Sunrise II
observations, Riethmüller et al. (2013) and Danilovic et al. (2014) have presented the first high resolution images
of quiet and active regions of the Sun in the Mg II k 2796 Å line. An upcoming special issue of The
Astrophysical Journal (Supplement Series) will be devoted to Sunrise II and describe the many exciting results
stemming from its observations.
Were a third flight of Sunrise to occur, easily made improvements to the PS, e.g. El_Local filter adjustments and
friction compensation functions for the fine (AzfTrack) and coarse azimuth (AzfToAzc) servos, would provide
even better pointing and longer duration times of continuous good pointing.
Acknowledgments
This work was performed under NASA grant number NNX13AE95G.
The authors thank the talented staff of NCAR’s Earth Observing Laboratory, Design and Fabrication Services.
Their contributions to the pre-flight and post-flight efforts are greatly appreciated. Additionally the authors wish
to thank Peter G. Nelson and especially Clemens Halbgewachs who worked tirelessly in determining optimal
servo settings. The authors are very grateful to Piyush Agrawal, Justus Brosche, Rebecca Centeno, Courtney
Peck, and Jack Fox. Their wonderful help was invaluable in during PS set up and flight operations. The Sunrise
II flight would not have been a success without the expertise of the dedicated staff of CSBF, ESRANGE, MPS,
KIS, and IMaX. We thank them.
The National Center for Atmospheric Research is sponsored by the National Science Foundation.
References
Barthol, P., Gandorfer, A., Solanki, S.K., Schüssler, M., Chares, B., Curdt, W., Deutsch, W., Feller, A., Germerott, D., Grauf,
B., Heerlein, K., Hirzberger, J., Kolleck, M., Meller, R., Müller, R., Reithmüller, T.L., Tomasch, G., Knölker, M., Lites,
B.W., Card, G., Elmore, D., Fox, J., Lecinski, A., Nelson, P., Summers, R., Watt, A., Martínez Pillet, V., Bonet, J.A.,
Schmidt, W., Berkefeld, T., Title, A.M., Domingo, V., Gasent Blesa, J.L., del Toro Iniesta, J.C., López Jiménez, A.,
Álvarez-Herrero, A., Sabau-Graziati, L., Widani, C., Haberler, P., Härtel, K., Kampf, D., Levin, T., Pérez Grande, I.,
Sanz-Andrés, A., Schmidt, E. : [2010], “The Sunrise Mission”, Solar Phys, 268, 1, doi:10.1007/s11207-010-9662-9.
Berkefeld, T., Schmidt, W., Soltau, D. et al. [2011] Solar Phys, 268, 103, doi:10.1007/s11207-010-9676-3.
Danilovic, S., Hirzberger, J., Riethmüller, T. L., et al. [2014], ApJ, 784, 20, doi:10.1088/0004-637X/784/1/20.
Gandorfer, A., Grauf, B., Barthol, P. et al. [2011] Solar Phys, 268, 35, doi:10.1007/s11207-010-9636-y.
Martínez Pillet, V., del Toro Iniesta, J.C., Álvarez-Herrero, A. et al. [2011] Solar Phys 268, 57, doi:10.1007/s11207-010-
9644-y.
Riethmüller, T. L., Solanki, S. K., Hirzberger, J., et al. [2013], ApJL, 776, L13, doi:10.1088/2041-8205/776/1/L13.