NASA/TM--2002-211715 IECEC-2002-20113

Comparison of ISS Power System

With Analytically Derived Data

for Shadowed Cases

Telemetry

H. James Fincannon

Glenn Research Center, Cleveland, Ohio

July 2002
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NASA/TM--2002-211715 IECEC-2002-20113

Comparison of ISS Power System

With Analytically Derived Data

for Shadowed Cases

Telemetry

H. James Fincannon

Glenn. Research Ce:nter, Cleveland, Ohio

Prepared for the

37fl_ In.tersociety Energy Conversion. Engin.eering Con:ference

sponsored by the Institute of Electrical and Electronics Engineers,

Electron Devices Society

Washington, DC, July 28-A ugust 2, 2002

National Aeronautics and

Sp ace Adm.inis tration

Glelm Research Center

July 2002
Acknowledgments

The author thanks Silvia Stewart from the NASA JSC Video Repository for help in acquiring ISS video and

NASA GRC% Thomas Kerslake for solar array current telemetry conversion formulae and expertise.

NASA Center for Aerospace Information

71121Standard Drive

Hanover, MD 211076

Available frorn

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.zrc.nasa.gov
COMPARISON OF ISS POWER SYSTEM TELEMETRY WITH ANALYTICALLY DERIVED

DATA FOR SHADOWED CASES

H. James Fincannon

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Phone: 216-433-5405

Fax: 216-433-2995

E-mail: Fincannon_

ABSTRACT

Accurate International Space Station (ISS) power

prediction requires the quantification of solar array

shadowing. Prior papers have discussed the NASA

Glenn Research Center (GRC) ISS power system tool

SPACE (System Power Analysis for Capability

Evaluation) and its integrated shadowing algorithms.

On-orbit telemetry has become available that permits

the correlation of theoretical shadowing predictions

with actual data. This paper documents the

comparison of a shadowing metric (total solar array

current) as derived from SPACE predictions and onorbit flight telemetry data for representative significant

shadowing cases. Images from flight video

recordings and the SPACE computer program

graphical output are used to illustrate the comparison.

The accuracy of the SPACE shadowing capability is

demonstrated for the cases examined.

INTRODUCTION

In December 2000, the first ISS U.S. photovoltaic

power system (designated as P6) was deployed.

Because of its size, some of the largest shadow

patterns ever observed on orbital solar arrays (-170

m^2) have occurred. Shadow patterns with significant

durations, varied distributions and shapes appeared

on the arrays. Significant shadowing events

happened during normal flight operation and transient

events such as STS (Space Transportation SystemSpace Shuttle) fly-around survey, flight mode

transition, vehicle docking/undocking, waste water

dumps, reboost, extravehicular activity (EVA) and

assembly operations free drift. As ISS modules are

added, power demands increase, especially during

assembly, and shadowing events increase in

frequency and magnitude in a complex fashion that

can only be predicted using computer programs.

Analysis tools have been developed to predict the

impact of shadowing on the ISS power system. Prior

papers have discussed the development of these

tools (SPACE and its integrated shadowing

algorithms) (Hojnicki, 1993) (Kerslake, 1993a)

(Fincannon, 1996) and analysis results using them

(Fincannon, 1995, 1999, 2002). SPACE models all

required hardware and integrates all necessary

analysis components to determine the time-varying

effect of load demand on the power system.

Because of SPACE's importance to ISS, measures

are taken to validate its various component parts. Its

shadowing algorithms and geometry models have

been validated manually for simple cases during the

ISS Verification Analysis Cycles. Successful

comparisons have also been made with shadowing

analyses from Russia's NPO Energia, Boeing and the

NASA JSC Mission Operations Directorate (each

using a different approach and/or implementation).

Given available P6 telemetry and video footage, it is

feasible to compare SPACE power system predictions

with the on-orbit data.

This comparison is of particular interest because

the shadowing algorithms and geometry data

necessitated key assumptions. These assumptions

were that 1) relatively low fidelity geometry models

were acceptable, 2) Sun subtense angle effects were

minimal, and 3) reflected energy from adjacent

hardware was minimal. Of these, the geometry model

was the main concern. SPACE geometry models are

a specially created lower fidelity derivation (<2000

polygons) of highly detailed ISS models (>10^6

polygons). Lower fidelity increases computational

speed for the integrated shadowing analysis and

reduces memory limitations. Variations in dimensions

(due to structural/thermal distortion, thermal blankets,

etc), missing hardware omitted in fidelity reduction,

unknown hardware modes (retraction or deployment)

are examples of areas of concern regarding model

accuracy. Quantifying the amount of these effects is

an important outcome of the comparison process.

NASA/TM--2002- 211715 1
VIDEO OBSERVATIONS

P6 solar arrays are primarily videotaped from the

STS payload bay cameras. Video recordings

acquired at the GRC Engineering Support Room and

the NASA Johnson Space Center (JSC) Video

Repository were examined for shadow images. The

video covered a period from STS-97/ISS Flight 4A to

STS-105/ISS Flight 7A.1 (250 days and 6 assembly

flights). During this period, the primary shadowinducing hardware onto the P6 solar arrays were the

P6 thermal radiators, STS, Progress resupply module

(aft of Zvezda), Zvezda and Zarya solar arrays. Other

qualitative observations include 1) most shadowing

video was from moderate absolute solar beta angles

or below (<37 degrees), 2) shadowing occurred

primarily on one wing during an orbit and 3) large

shadows appeared dark and hard edged with no

apparent reflections of adjacent hardware.

CASE SELECTION METHOD

For the time period from P6 solar array

deployment until one year later, the ISS had traveled

through over 5500 orbits, with solar beta angles

ranging from 73 to -73 degrees, various flight

attitudes/methods, solar array pointing, and STS

docking locations. Since most orbits had some

shadowing and analyzing every orbit is time

prohibitive, criteria had to be developed to filter the

cases to those most illustrative in a comparison.

Video Imaqery

Although frequent shadowing occurs when there is

no video recording taking place, due to the difficulty in

understanding the shadow patterns with the limited

ISS telemetry, it was desired to coincide the telemetry

with actual video footage. Video is used to

qualitatively verify the shadow patterns and

understand geometry nuances. Quantitative

matching of the SPACE and video shadow patterns is

difficult and time-consuming because of camera

orientation, camera panning, camera zooming,

perspective and changing solar array angles.

Adequate Telemetry

Cases with video footage required that adequate

telemetry be available for the period. Most orbits

have telemetry dropouts (due to antenna blockage or

other communication problems) of various durations.

Complex maneuvers and gimbal activities with even

brief dropouts may lose enough information to make

the case impractical for comparisons.

Sufficient Shadow Current Drop

Because of the presence of uncertainty (e.g.,

albedo, solar array temperature) in the analysis,

cases with high shadowing-related current drops

(>25% of unshadowed) were required to overwhelm

these factors, thus depicting primarily shadowing

effects.

TELEMETRY PROCESSING

Acquiring telemetry data and correlating it with

predictions is complicated due to data drop outs, lack

of synchronized reporting of data elements, sensor

calibration, data conversion, and un-sensored data.

Solar array current telemetry is only available at the

solar array wing level, not the string level. Telemetry

was obtained for the shunted and unshunted solar

array string currents for each solar array wing (there

are two wings on P6, two blankets per wing).

Calibration formulae based on ground test data were

applied to the solar array current sensor telemetry to

improve their accuracy (they are typically too low by a

total current value of about 2% per wing).

Telemetry from the shunted solar array strings

were adjusted to represent unshunted strings. Solar

array string shunting is performed to regulate the

power distribution. Shunted current telemetry was

adjusted in two ways: 1) The shunted current

measured during string shadowing was reduced to

account for the difference in shunted and operating

string current dependencies. [For low shunted string

voltage drops (e.g., the result of shadowed cells in a

string not contributing voltage), shunt current is

largely insensitive to cell operating voltage due to the

cell current-voltage (IV) characteristic curve between

the short circuit and maximum power points.

Conversely, a high voltage, illuminated string current

(e.g., unshunted strings) is very sensitive to cell

operating voltage (as affected by shadowed cells in a

string not contributing voltage) since the cell IV curve

changes dramatically between the maximum power

and open circuit points.] 2) The remaining current

was reduced by a factor (0.9) representing shunted

string higher temperature operation, operating current

difference and voltage difference.

COMPARISON RESULTS

Descriptive data for each case is provided below.

Attitude is specified in terms of (Yaw, Pitch, Roll) or

(Y,P,R) in degrees. Only XvvZnadir flight mode cases

were examined. Here, the ISS +X axis is in the

velocity vector, the +Z axis in the nadir direction, the

+Y axis on the right/starboard side (with observer's

feet at nadir and facing in +X direction) and -Y axis

on the 'port' side. The solar array wing designated

'2B' is on the starboard side and '4B' on the port side.

Note that Yaw is about Z, Pitch about Y, and Roll

about X.

To bound the variable, several orbit-constant

albedo (abbreviated as 'ALB' in the figures) values

were assumed. Nominally, the average albedo of

27% was used. In addition, a 'high' albedo of 40%

and a 'low' albedo of 15% were used when warranted.

Albedo contributes significantly to power production

but its actual value (which changes through the orbit)

is not known for the following cases. Historical

averages are normally used but seasonal/

geographic/weather variations and albedo shadowing

NASA/TM--2002- 211715 2
give it a broad uncertainty. Short term albedo varies

from 0.05 to 0.60.

Each figure shows the solar array current based

on telemetry and the SPACE prediction, selected

representative video frames and SPACE graphical

output (Sun's eye view cartoon and projected shadow

pattern of wing of interest). Synchronization of video

and SPACE output is difficult due to rapid changing

shadow patterns. However, in general, these showed

a good relative comparison for each case.

To aid in understanding the amount of shadowing

relative to the total amount of available solar array

current, a quantity called Shadowing_Impact was

calculated. This value was the percent reduction

caused by shadowing of the combined solar array

amp-hours for both wings. To quantify the difference

between the telemetry and prediction current values,

a quantity called Shadowing_Difference was

calculated. To determine this value, the solar array

currents for prediction and telemetry were summed

(averaged and time-weighted) for only the time period

when the solar array is shadowed (to eliminate other

variables). The percent difference between prediction

and telemetry provided the Shadowing_Difference.

Note that this value was determined only for the solar

array that was most shadowed. The resulting value

represents the net amp-hour difference between the

actual value and the SPACE prediction, a more

important measure than instantaneous current

differences (with regards to energy balance).

ISS Fli,qht 4A (121812000)

The P6 solar arrays were added in this flight

(Figure 1). Insolation began at 18:57 GMT with a

solar beta angle of 28.4 degrees and altitude of 374

km. Both solar arrays were locked. Insolation began

with a pitch of YPR (3,147,-4) which peaked at YPR

(0,160,-3) but drifted back down to YPR (0,8,3) by the

end of insolation. This attitude change was due to a

free drift mode during a radiator loads validation test.

At orbit dawn, the backside of one solar array wing

was shadowed by the STS and forward (+X) ISS

radiator. The remainder of the orbit was

unshadowed. This case had 62% shadowing of one

wing (-170 m^2). Because the front of the solar array

was facing toward Earth early in the orbit, it was more

sensitive to albedo variation. Examination of satellite

images and ground tracks showed that heavy cloud

cover during the early part of the orbit justified an

albedo higher than the average. This case had

Shadowing_Impact of 3%. This case had a

Shadowing_Difference of 20% below telemetry for

average albedo and 2% below telemetry for high

albedo.

ISS Fli,qht 6A (4128100)

During remote manipulator assembly operations

activity, significant shadowing occurred early in the

orbit (Figure 2) while the remainder was unshadowed.

Insolation began at 15:15 GMT with a solar beta

angle of-27.5 degrees, altitude of 392.7 km and flight

attitude of YPR (0,23,0). Solar arrays were fully

tracking. Shadowing by the P6 radiator and STS-100

occurred on previous and subsequent orbits.

Reasonable variation of albedo had minimal effect on

the results so only the average case is presented.

This case had a Shadowing_Impact of a 2%. This

case had a Shadowing_Difference of 4% above

telemetry.

ISS Fli,qht 7A.1 (8114101)

Prolonged significant shadowing occurred during a

maneuver into reboost (by the STS-105) attitude

(Figure 3). The solar arrays were fully sun-tracking

except for Zvezda's arrays which were locked.

Insolation began at 17:04 GMT with a solar beta

angle of 14.6 degrees and altitude of 394.9 km. The

attitude started at YPR (0,23.8,0), then transitioned in

pitch to YPR (2,-122,3). This resulted in the ISS

being upside-down and backwards from its normal

flight attitude. This case had the highest relative

shadowing with a Shadowing_Impact of 6%. This

case had a Shadowing_Difference of 6% above

telemetry. The main deviation occurred at the point

when the Zvezda solar array shadow passes between

the P6 blankets. Since the telemetry was not

continuous, this transient current increase was

missed. Note that removal of the one point reduced

the differences to 3% and 5% respectively.

Reasonable variation of albedo had minimal effect on

the results so only the average case was presented.

ISS Fli,qht 5A (2116101)

After the STS undocks, the video/photographic

inspection of ISS called fly-around starts at orbit dawn

and continues until orbit dusk. Fly-around shadowing

is important because the distance of the shadowing

object enhances Sun subtense angle effects.

Implementation of fly-around depends upon STS

docking location. The variables for fly-around include

the STS displacement (X,Y and Z) from ISS, the rate

and starting time. Fly-around is assumed to occur

with the STS passing in a circle around the ISS in the

orbit plane (X-Z plane) from ISS-forward to ISS-zenith

then to ISS-aft. The orientation of the STS is held

such that the STS payload bay is always pointing

toward the ISS. Because of variable STS passage

rates (4-8 degrees/minute), it was assumed the rate

at shadow occurrence happened throughout flyaround with the start time adjusted to reflect this fact.

Insolation began at 14:24 GMT (Figure 4) with a

solar beta angle of -2.5 degrees, altitude of 382 km

and flight attitude of YPR (10,-10,0). The gimbals

started in a locked position for undocking and then

transitioned to sun tracking prior to the start of

shadowing. For this stage, the STS was docked in

the ISS nadir location the tail of the STS facing in the

ISS velocity vector. Upon undocking, the STS moved

away along the ISS +Z axis towards Earth. At 120-

150 meters (130 meters at shadow time), the STS

loitered until orbit dawn. Then, the STS "backed-up"

around ISS. The STS was offset from the ISS

coordinate center by 6 meters in the -Y direction.

This case had a Shadowing_Impact of 2%. This case

NASA/TM--2002- 211715 3
had a Shadowing_Difference of 4% above telemetry

for low albedo and 8% above telemetry for average

albedo.

ISS Fli,clht 7A.1 (8120101)

Insolation began at 14:51 GMT (Figure 5) with a

solar beta of -12.6 degrees and altitude of 396 km.

The flight attitude transitioned from YPR (3 ,22, 2) to

YPR (-5,7,0). The STS-105 was docked to the Lab

forward PMA2 location, with its tail in the nadir

direction and bottom in the velocity vector direction.

The STS departed by moving away along the positive

ISS X axis. Fly-around proceeded at dawn with the

STS nose-first. This case had 62% shadowing of one

wing (-170 m^2). This case had a

Shadowing_Impact of 2%. This case had a

Shadowing_Difference of 2% below telemetry for low

albedo and 1% above telemetry for average albedo.

CONCLUSION

This paper compared a subset of the available

ISS telemetry data with SPACE predictions. Caveats

of this analysis were that temperature effects could

not be properly assessed due to lack of telemetry and

albedo data could not be definitively quantified.

Although shadowing effects caused by all ISS

hardware under all orbital conditions could not be

considered, for the cases that were examined, the

SPACE shadowing algorithms and geometry models

predicted shadowing effects at acceptable difference

levels from telemetry (within 6% of solar array wing

amp-hour capability during a shadowing event).

Comparisons of back and front side solar array

shadowing of solar illumination were presented. Sun

subtense angles effects, even with distant shadowing

objects, were not discernable. The effect of reflections

on shadow patterns was not apparent using video

imagery or telemetry. The remaining comparison

amp-hour differences are likely due to a combination

of geometric model fidelity (primarily) and Sun

subtense angle effects.

REFERENCES

Fincannon, J., 1995, "Analysis of Shadowing

Effects on Spacecraft Power Systems," NASA TM106994, Cleveland, OH.

Fincannon, J., et al., 1996, "Load-Following Power

Timeline Analyses for the International Space

Station," NASA TM-107263, Cleveland, OH.

Fincannon, J., et al., 1999, "Effects of Solar Array

Shadowing on the Power Capability of the Interim

Control Module," NASA TM-209378, Cleveland, OH.

Fincannon, J., 2002, "Incident Energy Focused

Design and Validation for the Floating Potential

Probe," NASA TM-211349, Cleveland, OH.

Hojnicki, J.S., and Kerslake, T.W., 1993, "Space

Station Freedom Electrical Performance Model,"

NASA TM-106395, Cleveland, OH.

Kerslake, T.W., and Hojnicki, J.S., 1993a, "System

Performance Predictions For Space Station

Freedom's Electrical Power System," NASA TM106396, Cleveland, OH.

33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 9_:

49 min. _ 50 min.

FIGURE 1 : ISS FLIGHT 4AFREE DRIFT- SOLAR ARRAY CURRENT, SPACE GRAPHICS AND VIDEO FRAMES

NASA/TM--2002- 211715 4
220

2OO

_- 180

<.5.16o

E 140

_D

120

loo

80

'< 6O

-_ 4o

O

CO 2O

0

::,:: 2B-Telemetry

z_ 2B-SPACE, alb=27%

÷ 4B-Telemetry

4B-SPACE, alb=27%

33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93

Orbit ]1me (Minutes)

35 min. 38 min. 41 min.

FIGURE 2: ISS FLIGHT 6AASSEMBLY- SOLAR ARRAY CURRENT, SPACE GRAPH ICS AND VIDEO FRAMES

240

220

_- 200

_- 180

160

140

5 12o

_, loo

8O

6O

5

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0_

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+ 4B-Telemetry ....

Z_ 4B-SPACE, alb=27% .....

_- J J J J I I I _

33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93

Orbit ]1me (Minutes)

o

83 min. 84 min.

FIGURE 3: ISS FLIGHT 7A.1 REBOOST- SOLAR ARRAYCURRENT, SPACE GRAPHICS AND VIDEO FRAMES

NASA/TM--2002- 211715 5
240

220

200

E 180

160

*_ 140

120 100

80

6o

co 4O

2O

:i.....

33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93

Orbit ]]me (Minutes)

57 min. i_ _ _ "" :z_59min" """_:".... _

FIGURE 4: ISS FLIGHT 5A FLY-AROUND- SOLAR ARRAY CURRENT, SPACE GRAPHICS AND VIDEO FRAMES

220

18o " I

_ 160 _"!_r _ 1 140 :il 2B-Telemetry

120 _. /

 1oo o

80 _._ '::" z_ 2B-SPACE, alb=27%

_: 60 _ _:_!: + 4B-Telemetry

40 _ _ 4B-SPACE, alb=I 5%

20 I _ _,_ + 4B-SPACE, alb=27%

33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93

Orbit ]]me (Minutes)

74 min. _ ]=LJ 75min .......... _ 76 rnin.

FIGURE 5: ISS FLIGHT 7A.1 FLY-AROUND- SOLAR ARRAY CURRENT, SPACE GRAPHICS AND VIDEO FRAMES

NASA/TM--2002- 211715 6
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Comparison of ISS Power System Telemetry With Analytically Derived Data

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H. James Fincannon

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Prepared for the 37th lntersociety Energy Conversion Engineering Conference sponsored by the Institute of Electrical

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It. James Fincannon, organization code 6920, _ 16-43._-5405.

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13. ABSTRACT (Maximum 200 words)

Accurate International Space Station (ISS) power prediction requires the quantification of solar array shadowing. Prior

papers have discussed the NASA Glenn Research Center (GRC) ISS power system tool SPACE (System Power Analysis

for Capability Evaluation) and its integrated shadowing algorithms. On-orbit telemetry has become available that permits

the co_Telation of theoretical shadowing predictions with actual data. This paper documents the comparison of a shadowing metric (total solar array current) as derived from SPACE predictions aid on-orbit flight telemetry data for representative significmlt shadowing cases, hnages from flight video recordings and the SPACE computer progrmn graphical output

are used to illustrate the comparison. The accuracy of the SPACE shadowing capability is demonstrated for the cases

examined.

14. SUBJECT TERMS

International Space Station; Power generation; Shadowing; Perfbrmance modeling;

Solar arrays

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