Orbiting approximately 400 km above the Earth, the International Space Station (ISS) is a unique research laboratory used to conduct ground-breaking science experiments in space. The ISS has eight Solar Array Wings (SAW), and each wing is 11.7 meters wide and 35.1 meters long. The SAWs are controlled individually to maximize power output, minimize stress to the ISS structure, and minimize interference with other ISS operations such as vehicle dockings and Extra-Vehicular Activities (EVA). The Solar Arrays are designed to operate at 160 Volts. These large, high power solar arrays are negatively grounded to the ISS and collect charged particles (predominately electrons) as they travel through the space plasma in the Earth's ionosphere. If not controlled, this collected charge causes floating potential variations which can result in arcing, causing injury to the crew during an EVA or damage to hardware [1]. The environmental catalysts for ISS floating potential variations include plasma density and temperature fluctuations and magnetic induction from the Earth's magnetic field. These alone are not enough to cause concern for ISS, but when they are coupled with the large positive potential on the solar arrays, floating potentials up to negative 95 Volts have been observed. Our goal is to differentiate the operationally induced fluctuations in floating potentials from the environmental causes. Differentiating will help to determine what charging can be controlled, and we can then design the proper operations controls for charge collection mitigation. Additionally, the knowledge of how high power solar arrays interact with the environment and what regulations or design techniques can be employed to minimize charging impacts can be applied to future programs

Minow, Joseph I.

Parker, Linda Neergaard

Willis, Emily M.

NASA Technical Reports Server

Correlation of ISS Electric Potential Variations with Mission Operations
Emily M. Willis1, Joseph I. Minow1, L. Neergaard Parker2
1NASA, Marshall Space Flight Center, Huntsville, AL
2Jacobs , ESSSA Group, Huntsville, AL
Introduction
References
[1] S. Koontz, M. Valentine, T. Kepping, M. Edeen, W. Spetch, and P. Dalton, “Assessment and control of spacecraft charging risks on the International Space Station,” unpublished.
[2] K. H. Wright, Jr., C. M. Swenson, D. C. Thompson, A. Barjatya, S. L. Koontz, T. A. Schneider, J. A. Vaughn, J. I. Minow, P. D. Craven, V. N. Coffey, L. N. Parker, and T. H. Bui, “Charging of the International Space Station as observed by the Floating Potential Measurement Unit,” IEEE 
Trans. Plasma Science, Vol. 36, NO. 5, October 2008, p.2280-2293.
[3] Barjatya, A., C.M. Swenson, D.C. Thompson, and K.H. Wright, Jr., Invited article:  Data analysis of the Floating Potential Measurement Unit aboard the International Space Station, Rev. Sci. Instruments, 80, 041301,  2009.
[4] S. Simmons, L. McCullough, and K. Bolt, “Electrical Power System training manual,” unpublished.
[5] M. J. Mandell, V. A. Davis, B. Gardner, and G. Jongeward, “Electron Collection by International Space Station Solar Arrays,” unpublished.
Acknowledgment: The authors would like to thank the ISS Electrical Power Systems Team at Johnson Space Center for their help in collecting and understanding the ISS solar array data, especially Raymond Kaminski, Matthew Scudder,  and Aaron You.
Contact Information: emily.willis@nasa.gov
Floating Potential Measurement Unit
i. Normal eclipse exit: Characterized by a rise time less than ten
seconds and decay time greater than one minute. This is
typical of the ISS coming into sunlight. The arrays are pointed
into ram and string voltages increase quickly.
ii. Rapid eclipse exit: Characterized by a rise time less than ten
seconds and decay time less than one minute. These rapid
potential peaks correlate with lower plasma densities.
iii. Eclipse entry: Floating potential increases as the ISS moves
into darkness and arrays are unshunted.
iv. Auroral charging: With an inclination of 51.6 degrees, the ISS
occasionally travels through an auroral event and floating
potential increases occur from the precipitating auroral
electrons. These events are independent of solar array
operations.
v. Positive charging: Observed events have been less than one
second in duration. The largest event observed was 55 Volts.
The cause of these events is currently unknown.
vi. – ix) Additional uncharacterized peaks: These floating
potential fluctuations are possibly a combination of the others
caused by complex solar array operations. They occur during
the early part of insolation (sunlit portion of the orbit).
In order to determine charging
levels related to ISS operations, it
is important to understand how the
ISS solar arrays operate. This is a
basic block diagram of the
operations being investigated for
correlation with potential
fluctuations. There are eight Solar
Array Wings (SAW) on ISS, each
has 82 strings of solar cells that
are controlled individually to meet
the changing power needs of the
ISS.
During normal operations, the arrays are approaching ram at
eclipse exit and all strings are unshunted in order to supply
power to the loads and recharge the batteries. The potential on
the unshunted arrays quickly rises to 160 Volts. The exposed
portions of the semiconductor and interconnects collect
electrons increasing the negative floating potential of the ISS.
The cover glass on the arrays collects electrons developing a
potential barrier that slows the collection of electrons on the
conductive surfaces [5]. By orbital noon, the batteries are
charged which results in a decreased load. Many strings are
shunted due to this decrease in load. At the same time, the
arrays are no longer pointed in the ram direction, so the charge
collection, and therefore floating potential, is minimized.
During special operations, such as vehicle dockings, the
arrays are “parked”, meaning they do not track the sun but
stay in a designated position. This plot illustrates how this
“parked” configuration affects the floating potential. Instead
of the floating potential decreasing at orbital noon, it increases
because the arrays are pointed in ram and still receiving
enough sunlight to maintain a high positive voltage. When the
arrays are unparked and begin moving out of the ram
direction, the floating potential decreases accordingly.
This figure illustrates a similar effect as the previous one. In this
instance, one entire array was shunted due to an anomaly on ISS.
As the station entered insolation with the array shunted, the
positively oriented fluctuations occurred. Unshunting the array
resulted in a rapid charging peak with a large negative potential.
The array was again shunted, which had little effect on the
floating potential. These floating potential profiles are not well
understood, but occur regularly when an entire array is held in a
shunted state as the ISS enters insolation and then unshunted in
full sunlight. The low data rate (1/10 hertz) of ISS systems data
creates a challenge for observing any correlations for these fast
fluctuations.
The Floating Potential Measurement Unit (FPMU) is a collection of
four probes: the Plasma Impedance Probe (PIP), Wide-sweep
Langmuir Probe (WLP), Narrow-sweep Langmuir Probe (NLP), and
the Floating Potential Probe (FPP). Together, these instruments are
able to determine the ISS floating potential (FP) as well as the ion and
electron density (Ni, Ne) and electron temperature (Te) of the local
plasma environment [2,3]. The collected data is downlinked to the
FPMU Workstation at MSFC where it is processed and archived. The
Space Environments Team at MSFC analyzes the collected data with
respect to the ISS position along the orbital path, the timing of ISS
eclipse and sunlit phases, and the current space weather conditions to
determine the environmental sources causing significant changes in
floating potential. We focus on FPP and WLP observations for this
work. The FPP measures ISS frame potentials relative to the ambient
plasma environment at a rate of 128 Hz allowing detailed monitoring
of frame potential variations at high time resolution. Plasma density
measurements are obtained from the WLP Ni parameter at a rate of 1
Hz. Electron density is assumed to be equal to the ion density due to
quasi-neutrality. A description of the data analysis algorithms used to
obtain the potentials and plasma density parameters from the FPMU
instrument suite is described by Wright et al. 2008 [2].
Orbiting approximately 400 km above the Earth, the International Space Station (ISS) is a
unique research laboratory used to conduct ground-breaking science experiments in space.
The ISS has eight Solar Array Wings (SAW), and each wing is 11.7 meters wide and 35.1
meters long. The SAWs are controlled individually to maximize power output, minimize
stress to the ISS structure, and minimize interference with other ISS operations such as
vehicle dockings and Extra-Vehicular Activities (EVA). The Solar Arrays are designed to
operate at 160 Volts. These large, high power solar arrays are negatively grounded to the
ISS and collect charged particles (predominately electrons) as they travel through the
space plasma in the Earth’s ionosphere. If not controlled, this collected charge causes
floating potential variations which can result in arcing, causing injury to the crew during
an EVA or damage to hardware [1]. The environmental catalysts for ISS floating potential
variations include plasma density and temperature fluctuations and magnetic induction
from the Earth’s magnetic field. These alone are not enough to cause concern for ISS, but
when they are coupled with the large positive potential on the solar arrays, floating
potentials up to negative 95 Volts have been observed. Our goal is to differentiate the
operationally induced fluctuations in floating potentials from the environmental causes.
Differentiating will help to determine what charging can be controlled, and we can then
design the proper operations controls for charge collection mitigation. Additionally, the
knowledge of how high power solar arrays interact with the environment and what
regulations or design techniques can be employed to minimize charging impacts can be
applied to future programs.
The Charging Mosaic illustrates a number of floating potential
profiles currently being investigated for ISS. These are plots of
the ISS floating potential (Volts) versus time. Each plot is a
five minute time interval. They range from simple, well
understood potential fluctuations to complex potential
variations with currently unknown causes.
The arrays rotate via Beta Gimbal Assemblies (BGA) and Solar Alpha Rotary Joints
(SARJ) to maximize power production, minimize stress to the structure, and also move
for other specific operations such as vehicle dockings. During insolation, the Sequential
Shunt Unit (SSU) provides the automatic regulation of the arrays by turning strings of
cells on and off as needed to support loads. When a When a string is “shunted” all
generated current is sent back to the array, and the voltage on that string is zero. During
the eclipse portion of the orbit, the Battery Charge Discharge Units (BCDU) provide the
regulation of the power. The current flows into the Direct Current Switching Unit
(DCSU) and is sent to the ISS loads [4].
This figure illustrates the results of shunting operations during
insolation. In order to observe the effects of unshunting arrays
in sunlight, all eight arrays were commanded to a shunt state
until three minutes into insolation. While the arrays were
shunted, large positive floating potential fluctuations were
observed. As each array was unshunted, a large negative
fluctuation occurred.
https://ntrs.nasa.gov/search.jsp?R=20140011805 2019-08-31T19:20:24+00:00Z


Correlation of ISS Electric Potential Variations with Mission Operations

Spacecraft charging on the International Space Station (ISS) is caused by a complex combination of the low Earth orbit plasma environment, space weather events, operations of the high voltage solar arrays, and changes in the ISS configuration and orbit parameters. Measurements of the ionospheric electron density and temperature along the ISS orbit and variations in the ISS electric potential are obtained from the Floating Potential Measurement Unit (FPMU) suite of four plasma instruments (two Langmuir probes, a Floating Potential Probe, and a Plasma Impedance Probe) on the ISS. These instruments provide a unique capability for monitoring the response of the ISS electric potential to variations in the space environment, changes in vehicle configuration, and operational solar array power manipulation. In particular, rapid variations in ISS potential during solar array operations on time scales of tens of milliseconds can be monitored due to the 128 Hz sample rate of the Floating Potential Probe providing an interesting insight into high voltage solar array interaction with the space plasma environment. Comparing the FPMU data with the ISS operations timeline and solar array data provides a means for correlating some of the more complex and interesting ISS electric potential variations with mission operations. In addition, recent extensions and improvements to the ISS data downlink capabilities have allowed more operating time for the FPMU than ever before. The FPMU was operated for over 200 days in 2013 resulting in the largest data set ever recorded in a single year for the ISS. In this paper we provide examples of a number of the more interesting ISS charging events observed during the 2013 operations including examples of rapid charging events due to solar array power operations, auroral charging events, and other charging behavior related to ISS mission operations

(Abstract No# 149) 
Correlation of ISS Electric Potential Variations with 
Mission Operations 
 
 
Abstract— Spacecraft charging on the International Space 
Station (ISS) is caused by a complex combination of the low 
Earth orbit plasma environment, space weather events, 
operations of the high voltage solar arrays, and changes in the 
ISS configuration and orbit parameters.  Measurements of the 
ionospheric electron density and temperature along the ISS orbit 
and variations in the ISS electric potential are obtained from the 
Floating Potential Measurement Unit (FPMU) suite of four 
plasma instruments (two Langmuir probes, a Floating Potential 
Probe, and a Plasma Impedance Probe) on the ISS.   These 
instruments provide a unique capability for monitoring the 
response of the ISS electric potential to variations in the space 
environment, changes in vehicle configuration, and operational 
solar array power manipulation.  In particular, rapid variations 
in ISS potential during solar array operations on time scales of 
tens of milliseconds can be monitored due to the 128 Hz sample 
rate of the Floating Potential Probe providing an interesting 
insight into high voltage solar array interaction with the space 
plasma environment.  Comparing the FPMU data with the ISS 
operations timeline and solar array data provides a means for 
correlating some of the more complex and interesting ISS electric 
potential variations with mission operations.  In addition, recent 
extensions and improvements to the ISS data downlink 
capabilities have allowed more operating time for the FPMU 
than ever before. The FPMU was operated for over 200 days in 
2013 resulting in the largest data set ever recorded in a single 
year for the ISS.  In this paper we provide examples of a number 
of the more interesting ISS charging events observed during the 
2013 operations including examples of rapid charging events due 
to solar array power operations, auroral charging events, and 
other charging behavior related to ISS mission operations. 
Keywords—ISS; FPMU; plasma; charging; potential; array  
I. INTRODUCTION                  
Orbiting approximately 400 km above the Earth, the 
International Space Station (ISS) is a unique research 
laboratory used to conduct ground-breaking science 
experiments in space.   The ISS has eight Solar Array Wings 
(SAW), and each wing is 11.7 meters wide and 35.1 meters 
long. The SAWs are controlled individually to maximize 
power output, minimize stress to the ISS structure, and 
minimize interference with other ISS operations such as 
vehicle dockings and Extra-Vehicular Activities (EVA). The 
Solar Arrays are designed to operate at 160 Volts. These large, 
high power solar arrays are negatively grounded to the ISS and  
collect charged particles (predominately electrons) as they 
travel through the space plasma in the Earth’s ionosphere. If 
not controlled, this collected charge causes floating potential 
variations which can result in arcing, causing injury to the crew 
during an EVA or damage to hardware [1].  The environmental 
catalysts for ISS floating potential variations include plasma 
density and temperature fluctuations and magnetic induction 
from the Earth’s magnetic field. These alone are not enough to 
cause concern for ISS, but when they are coupled with the 
large positive potential on the solar arrays, floating potentials 
up to negative 95 Volts have been observed. Our goal is to 
differentiate the operationally induced fluctuations in floating 
potentials from the environmental causes. Differentiating will 
help to determine what charging can be controlled, and we can 
then design the proper operations controls for charge collection 
mitigation. Additionally, the knowledge of how high power 
solar arrays interact with the environment and what regulations 
or design techniques can be employed to minimize charging 
impacts can be applied  to future programs. 
II. FLOATING POTENTIAL MEASUREMENT UNIT 
The Floating Potential Measurement Unit (FPMU), shown 
in Fig. 1, is a collection of four probes: the Plasma Impedance 
Probe (PIP), Wide-sweep Langmuir Probe (WLP), Narrow-
sweep Langmuir Probe (NLP), and the Floating Potential 
Probe (FPP).  Together, these instruments are able to 
determine the ISS floating potential (FP) as well as the ion and 
electron density (Ni, Ne) and electron temperature (Te) of the 
local plasma environment [2,3]. The collected data is 
downlinked to the FPMU Workstation at MSFC where it is 
processed and archived.  The Space Environments Team at 
MSFC analyzes the collected data with respect to the ISS 
position along the orbital path, the timing of ISS eclipse and 
sunlit phases, and the current space weather conditions to 
determine the environmental sources causing significant 
changes in floating potential. 
We focus on FPP and WLP observations for this work.   
The FPP measures ISS frame potentials relative to the ambient 
plasma environment at a rate of 128 Hz allowing detailed 
monitoring of frame potential variations at high time 
resolution.  Plasma density measurements are obtained from 
the WLP Ni parameter at a rate of 1 Hz.  Electron density is 
assumed to be equal to the ion density due to quasi-neutrality. 
A description of the data analysis algorithms used to obtain 
the potentials and plasma density parameters from the FPMU 
instrument suite is described by Wright et al. 2008 [2]. 
 
Emily M. Willis, Joseph I. Minow, Linda Neergaard Parker 
 
E. M. Willis and J. I. Minow are with the Marshall Space Flight Center, 
Huntsville, 35812,  USA (e-mail: emily.willis@nasa.gov) 
L. Neergaard Parker is with Jacobs Technology, ESSSA Group, 
Huntsville, 35812,  USA 
https://ntrs.nasa.gov/search.jsp?R=20140011804 2019-08-31T19:20:13+00:00Z
(Abstract No# 149) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
III. ISS FLOATING POTENTIAL PROFILES 
Fig. 2, “The Charging Mosaic”, illustrates a number of  
floating potential profiles currently being investigated for ISS. 
These are plots of the ISS floating potential (Volts) versus 
time. Each plot is a 5 minute time interval. They range from 
simple, well understood potential fluctuations to complex 
potential variations with currently unknown causes.  
i) Normal eclipse exit: Characterized by a rise time less 
than 10 seconds and decay time greater than 1 minute. 
This is typical of the ISS coming into sunlight. The 
arrays are pointed into ram and string voltages 
increase quickly. 
ii) Rapid eclipse exit: Characterized by a rise time less 
than 10 seconds and decay time less than than 1 
minute. These rapid potential peaks correlate with 
lower plasma densities.  
iii) Eclipse entry: Floating potential increases as the ISS 
moves into darkness and arrays are unshunted.   
iv) Auroral charging: With an inclination of 51.6 degrees, 
the ISS occasionally travels through an auroral event 
and floating potential increases occur from the 
precipitating auroral electrons. These events are 
independent of solar array operations.  
 
 
v) Positive charging: Observed events have been less 
than one second in duration. The largest event 
observed was 55 Volts. The cause of these events is 
currently unknown. 
vi) – ix)  Additional uncharacterized peaks: These 
floating potential fluctuations are possibly a 
combination of the others caused by complex solar 
array operations. They occur during the early part of 
insolation (sunlit portion of the orbit). 
 
 
 
 
 
 
 
 
Fig. 1.  The Floating Potential Measurement Unit (FPMU) deployed on the ISS measures the ISS floating potential and the density 
and temperature of the local plasma environment. It was developed by Space Dynamics Laboratory in collaboration with Utah State 
University under contract to the NASA/Johnson Space Center
(Abstract No# 149) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
IV. SOLAR ARRAY OPERATIONS 
In order to determine  charging levels related to ISS 
operations, it is important to understand how the ISS solar 
arrays operate. Fig. 3, Solar Array Block Diagram, shows a 
basic block diagram of the operations being investigated for 
correlation with potential fluctuations.  
There are eight Solar Array Wings (SAW) on ISS, each has 
82 strings of solar cells that are controlled individually to meet 
the changing power needs of the ISS. The arrays rotate via 
Beta Gimbal Assemblies (BGA) and Solar Alpha Rotary Joints 
(SARJ) to maximize power production, minimize stress to the 
structure, and also move for other specific operations such as 
vehicle dockings.  
During insolation, the Sequential Shunt Unit (SSU) 
provides the automatic regulation of the arrays by turning 
strings of cells on and off as needed to support loads. When a 
string is “shunted” all generated current is sent back to the 
array, and the voltage on that string is zero. During the eclipse 
portion of the orbit, the Battery Charge Discharge Units 
(BCDU) provide the regulation of the power. The current flows 
into the Direct Current Switching Unit (DCSU) and is sent to 
the ISS loads [4]. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 2, The Charging Mosaic, illustrates the range of  ISS floating potential profile currently being 
investigated. The y-axis shows the floating potential in Volts and the x-axis for each plot is a 5 minute 
time interval 
Fig. 3, Solar Array Block Diagram, shows the main components involved 
in causing floating potential variations. There are eight Solar Array Wings 
(SAW) on ISS. The arrays rotate via Beta Gimbal Assemblies (BGA) and Solar 
Alpha Rotary Joints (SARJ). During insolation, the Sequential Shunt Unit 
(SSU) provides the automatic regulation of the arrays. During the eclipse 
portion of the orbit, the Battery Charge Discharge Units (BCDU) provide the 
regulation of the power. The current flows into the Direct Current Switching 
Unit (DCSU) and is sent to the ISS loads. 
(Abstract No# 149) 
V. FLOATING POTENTIAL CORRELATIONS 
The following sections demonstrate how the ISS floating 
potential varies according to five  parameters:  
1) number of unshunted solar array strings (% of the total 
number of strings) as calculated from ISS data. 
2) projection of the solar array area in the ram direction 
(CA) calculated from ISS SARJ, BGA, and quaternion 
data. This value is plotted as a percentage of the active 
array area projected in the ram direction. At 100%, all 
eight arrays are pointed in ram. Negative 100% 
indicates all eight arrays are pointed in wake. Zero 
percent indicates all arrays are “edge-on”, meaning the 
edge of the arrays are pointing in the ram direction. 
3) plasma ion density (Ni) from the FPMU WLP 
4) geographic latitude from Satellite Tool Kit (STK). The 
geographic latitude is an indication of the expected 
contribution to floating potential fluctuations from 
magnetic induction. Higher latitudes result in increased 
floating potential, with some correction required for the 
difference between magnetic and geographic latitude. 
5) sunlight intensity from STK. 
 
Each Figure has 4 panels. The first is a plot of the ISS 
floating potenial as measured by the FPMU FPP. The 
following three panels display the percentage of unshunted 
strings, CA, plasma density (Ni) and geographic latitude. The 
background color of all panels is an indication of the sunlight 
intensity where gray represents eclipse (no sunlight on the 
arrays) and white represents insolation. The x-axis on all plots 
is Greenwich Mean Time.  
As the arrays enter insolation, the potential on the 
unshunted arrays quickly rises to 160 Volts. As the potential 
across a string of solar cells increases, the exposed portions of 
the semiconductor and interconnects collect electrons, which 
increases the negative floating potential of the ISS. The 
coverglass on the solar arrays collects electrons and begins 
developing a potential barrier which slows the collection of 
electrons on the conductive surfaces of the arrays [5]. As the 
potential barrier increases, electron collection decreases. The 
floating potential continues to fluctuate through the orbit as 
the number of unshunted strings, position of the arrays, and 
magnetic induction vary.  
 
A. ISS Floating Potential during Normal Operations  
Fig. 4, Floating Potential during Normal Operations,  
illustrates the potential profile for normal ISS operations. 
During normal operations, the arrays are approaching ram at 
eclipse exit, all strings are unshunted in order to supply power 
to the loads and recharge the batteries. By orbital noon, the 
batteries are charged which results in a decreased load. Many 
strings are shunted due to this decrease in load. At the same 
time, the arrays are no longer pointed in the ram direction, so 
the charge collection, and therefore floating potential, is 
minimized. In this instance, the magnetic induction and ram 
collection contributions are in synchronization. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
B. ISS Floating Potential with Parked Arrays  
During special operations, such as vehicle dockings, the 
arrays are “parked”, meaning they do not track the sun but are 
fixed  in a designated position. Fig. 5, Floating Potential 
Increase Correlated with Parked Solar Arrays, illustrates how 
this “parked” configuration affects the floating potential. 
Instead of the floating potential decreasing at orbital noon, it 
increases because the arrays are pointed in ram and still 
receiving enough sunlight to maintain a high positive voltage. 
When the arrays are unparked and begin moving out of the 
ram direction, the floating potential decreases accordingly.  
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 4,  Floating  Potential during Normal Operations, illustrates a 
typical charging profile. There is an initial increase in floating 
potential when sunlight  is incident on the arrays, then it 
decreases through the rest of the orbit as strings are shunted and 
arrays rotate. 
(Abstract No# 149) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
C. ISS Floating Potential with Arrays Shunted and 
Unshunted during Insolation  
Fig. 6, Floating Potential with All Arrays Shunted During 
Insolation, illustrates the results of shunting operations during 
insolation. In order to observe the effects of unshunting arrays 
in sunlight, all eight arrays were commanded to a shunt state 
until three minutes into insolation. While the arrays were 
shunted, large positive floating potential fluctuations were 
observed. As each array was unshunted, a large negative 
fluctuation occurred.  
Fig. 7, Floating Potential with One Array Shunted during 
Insolation, illustrates a similar effect as Fig 6. In this instance, 
one entire array was shunted due to an anomaly on ISS. As the 
station entered insolation with the array shunted, the positively 
oriented fluctuations occurred. Unshunting the array resulted in 
a rapid charging peak with a large negative potential. The array 
was again shunted, which had little effect on the floating 
potential.  
These floating potential profiles are not well understood, 
but occur regularly when an entire array is held in a shunted 
state as the ISS enters insolation and then unshunted in full 
sunlight. The low data rate (1/10 hertz) of ISS systems data 
creates a challenge for observing any correlations for these fast 
fluctuations.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 7, Floating Potential with One Array Shunted during Insolation, 
illustrates the effects on floating potential of shunting and unshunting 
one entire array during insolation 
Fig. 5, Floating Potential Increase Correlated with Parked Solar Arrays, 
illustrates an increase in floating potential midday due to arrays being 
parked in the ram direction. 
Fig. 6, Floating  Potential with All Arrays Shunted During Insolation, 
illustrates the effects on floating potential of shunting and unshunting 
all eight arrays during insolation 
(Abstract No# 149) 
SUMMARY 
 While the environmental causes of ISS floating potential 
variations are relatively well characterized, there remain some 
unknowns regarding the impact of ISS operations. 
Characterizing the effects of ISS operations is a difficult task 
due to the complexity of the array design and operations and 
the limited array data available. This is especially true of the 
fast fluctuations measured by the FPMU FPP. It is important to 
attempt to characterize the effects of ISS operations so that we 
can design proper operations controls for charge collection 
mitigation and incorporate lessons learned into future vehicle 
designs.  
ACKNOWLEDGMENT 
 
The authors would like to thank the ISS Electrical Power 
Systems Team at Johnson Space Center for their help in 
collecting and understanding the ISS solar array data, 
especially Raymond Kaminski, Matthew Scudder,  and Aaron 
You. 
 
REFERENCES 
 
[1] S. Koontz, M. Valentine, T. Kepping, M. Edeen, W. Spetch, and P. 
Dalton, “Assessment and control of spacecraft charging risks on the 
International Space Station,” unpublished. 
[2] K. H. Wright, Jr., C. M. Swenson, D. C. Thompson, A. Barjatya, S. L. 
Koontz, T. A. Schneider, J. A. Vaughn, J. I. Minow, P. D. Craven, V. N. 
Coffey, L. N. Parker, and T. H. Bui, “Charging of the International 
Space Station as observed by the Floating Potential Measurement Unit,” 
IEEE Trans. Plasma Science, Vol. 36, NO. 5, October 2008, p.2280-
2293. 
[3] Barjatya, A., C.M. Swenson, D.C. Thompson, and K.H. Wright, Jr., 
Invited article:  Data analysis of the Floating Potential Measurement 
Unit aboard the International Space Station, Rev. Sci. Instruments, 80, 
041301,  2009. 
[4] S. Simmons, L. McCullough, and K. Bolt, “Electrical Power System 
training manual,” unpublished. 
[5]  M. J. Mandell, V. A. Davis, B. Gardner, and G. Jongeward, “Electron 
collection by International Space Station solar arrays,” unpublished.  
 


Correlation of ISS Electric Potential Variations with Mission Operations

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