Solar Energy Materials & Solar Cells 75 (2003) 319–325

Analysis of generated power of ETS-VII duringsolar activity maximum period

Reiko Fujitaa,*, Mitsuru Imaizumib, Kazuhiro Aoyamab,Sumio Matsudab, Shinji Tokunagaa

aSpace Engineering Development Co., Ltd., ISSEI Third Building, 1-12-2, Takezono,

Tsukuba, Ibaraki 305-0032, JapanbNational Space Development Agency of Japan (NASDA), 2-1-1, Sengen, Tsukuba,

Ibaraki 305-8505, Japan

Abstract

The generated power degradation of a satellite in a low earth orbit during high solar activity

period has been compared with the power degradation of a satellite during low solar activity

period. A degradation prediction method is developed for this study. As a result, the effect of a

large solar flare on solar cell degradation is found to be negligible in a low earth orbit. This is

because the effects of shield thickness and inclination are thought to be greater than that of

degrees of solar activity. r 2002 Published by Elsevier Science B.V.

Keywords: Solar activity; Power; Low earth orbit; Degradation; Radiation; Fluence

1. Introduction

In space environment, several kinds of radiation exist. This radiation causesdegradation of solar cell, which leads to decrease in electric power from solar paddlesof a satellite. Therefore, it is important to analyze generated power and understand atendency of power degradation.The year 2000 was a maximum period of solar activity. In this period, large solar

flares occurred frequently and a large quantity of radiation was spouted out from thesun. This radiation may induce severe damage to solar cells of a satellite in an orbit.

*Corresponding author. Tel.: +81-298-52-1778; fax: +81-298-50-2017.

E-mail address: fujita.reiko@sed-c.co.jp (R. Fujita).

0927-0248/03/$ - see front matter r 2002 Published by Elsevier Science B.V.

PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 1 7 5 - 7

We have investigated power degradation of the solar paddles of the EngineeringTest Satellite-VII (ETS-VII), which has been in a low earth orbit. The level ofradiation exposure of the satellite was estimated from the amount of powerdegradation using a prediction method.The Japanese Earth Resources Satellite (JERS-1) is an earth observation satellite,

which was also in a low earth orbit during the solar activity minimum period. Thelevel of radiation exposure for JERS-1 was estimated by the same procedure as well,and the result was compared with that for ETS-VII.

2. Fundamental information

ETS-VII consists of two satellites, Chaser and Target, in order to perform dockingexperiment as one of its missions. Chaser has two solar cell paddles. Since thepaddles equip sun-tracking systems which control the paddles to face them towardthe sun, the solar incident angle is always perpendicular to the paddles. On the otherhand, Target has one paddle. Since it does not equip a sun-tracking system, thepaddle is fixed to the satellite. Therefore, the solar incident angle changes due to theposition of the satellite in the orbit. Chaser and Target exist in the same orbit, andthey utilize the same type of solar cells. As for JERS-1, it has one paddle which has asun-tracking system. It also utilizes the same type of solar cells to ETS-VII. Table 1shows comparison of fundamental information of ETS-VII (Chaser/Target) andJERS-1.

3. Analysis

3.1. Procedure

Fig. 1 shows the flow chart of the fluence analyzing method of radiation exposuredeveloped in this work.

Table 1

Fundamental information of ETS-VII (Chaser/Target) and JERS-1

Item Unit ETS-VII JERS-1

Launch day M/D/Y 11/27/97 02/11/92

Cell type (thickness) (mm) Si BSFR (200) Si BSFR (50)

Flight altitude km 550 570

Inclination deg 35 98

Surface shield g/cm2 0.026a 0.024

Backside shield g/cm2 0.1101b 0.0326

Paddle type — Rigid Semi-rigid

aSurface shield consists of coverglass and glue.bBackside shield consists of polyimide film, carbon fiber reinforced plastic, aluminum honeycomb core,

silver fluoroplastics and glue.

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Since ETS-VII was in a low earth orbit, changes in generated power dueto following factors were relatively large; (a) reflected sunlight from the earth,(b) solar insolation, and (c) temperature of solar cell. Therefore, at first, we examinedthe factors and the locations of the satellite both of which might affect electricpower output, in order to analyze under the same conditions. Next, we eliminatedthese factors from raw flight data of power, so as to standardize them. The powerdegradation tendency was obtained from the standardized data.On the other hand, we have developed a degradation prediction method which

estimates the level of radiation exposure based on ground irradiation test results. Thelevel of radiation exposure corresponding to the amount of power degradation wasestimated using this method. The same method was applied to estimate the level ofradiation exposure for JERS-1.

3.2. Power degradation

Fig. 2 shows trend of the actual generated power (raw flight data) and thecalibrated power (standardized by solar insolation and temperature) of Chaser andTarget. Some temperature sensors were attached on the backside of the paddles. Insuch case, the value of temperature was converted into the surface temperature byestimation from the consideration of the sensor position and temperature gradient.However, all the estimated cell temperatures were not likely to be accurate enough,thus the data with the estimated cell temperature between 65–751C for Chaser and58–691C for Target were selected for the following evaluation.

Fig. 1. Flow of the fluence analyzing method of radiation exposure developed in this work.

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These values should include only degradation due to radiation exposure. The slopeof the fitted line (negative value) confirms that there is significant decrease in thepower due to the degradation of the solar cells. The quantity of the annual powerdecrease and its rate for Chaser and Target are summarized in Table 2.

3.3. Fluence of radiation exposure

The level of radiation exposure (fluence) was calculated from the degradationshown in Fig. 2. The fluence is expressed as a value equivalent to 1MeV electronswith consideration of shielding effect by such as cover glass.

Fig. 2. (a, b) Trend of raw flight data and standardized data of generated power of ETS-VII.

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The estimated fluence at the end of the mission term is 4.1� 1012 (e/cm2) forChaser and 3.9� 1012 (e/cm2) for Target. The fluence for Chaser is slightly largerthan that for Target. Fluence of the two satellites for 1.5 years after launch was alsocalculated. The result was 2.3� 1012 (e/cm2) for Chaser for ETS-VII and1.2� 1013 (e/cm2) for JERS-1. Therefore, the fluence for ETS-VII is less than thatfor JERS-1 despite that the mission term of ETS-VII was in solar activity maximumwhile that of JERS-1 was solar activity minimum. Fig. 3 shows the fluence obtainedfrom decrease in the generated power values shown in Fig. 2.

4. Results and discussion

4.1. Comparison between Chaser and Target

With regard to the degradation rate, the rate of Chaser is larger than that ofTarget. As a result, the fluence for Chaser is slightly larger than that for Target, but

Fig. 3. Fluence of radiation exposure of ETS-VII (Chaser/Target) and JERS-1. X-axis is in UT (days after

launch).

Table 2

Annual degradation and its rate

Annual degradation (W) Degradation rate (%) Distribution (%)

Chaser 15.3 0.5 3.8

Target 2.8 0.3 4.8

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Chaser and Target showed the same order of magnitude of fluence. Cover glassthickness and the backside shield materials are the same for Chaser and Target, andthe orbits of these two satellites are also the same. Therefore, the differences in therate and the fluence are supposed to be due to the fact that the paddles of Chaserhave sun-tracking systems, while the paddle of Target does not. However,distribution of the data for Target is larger than that for Chaser. Thus, it is difficultto distinguish whether this is due to the sun-tracking or not.

4.2. Comparison between Chaser of ETS-VII and JERS-1

As for the fluence for Chaser of ETS-VII and for JERS-1, the fluence for Chaser isless than that for JERS-1, despite the difference of solar activity. There are twopossible reasons as follows.(1) Difference of inclination angle: Radiation environment becomes severe as

inclination angle approaches 901. JERS-1 is the solar synchronous orbital satellitewhose inclination angle is 981. On the other hand, ETS-VII has an inclination angleof 351.(2) Difference in paddle structure (difference in shield thickness): With regard

to the surface shield thickness (cover glass thickness), ETS-VII and JERS-1are almost the same. However, backside shield thickness is different. ETS-VIIhas rigid type paddles, while JERS-1 has a semi rigid type paddle. Rigid typepaddle has an aluminum base, while semi rigid type has not. Their backsideshield areal densities are 0.0326 (g/cm2) for JERS-1 and 0.110 (g/cm2) forETS-VII. Therefore, the shield of ETS-VII is 3.8 times thicker than that ofJERS-1.(3) Difference in cell thickness: The cell thickness of ETS-VII (200 mm) is thicker

than that of JERS-1 (50 mm). Generally, thinner cell is more radiation tolerant thanthicker cell. However, the degradation of the cells of ETS-VII is less than that ofJERS-1. This should be due to that the effects of (1) and (2) is greater than thethickness effect.

4.3. Correlation to solar activity

It was expected that the degradation of ETS-VII is greater than that of JERS-1,because the solar activity of the mission term of ETS-VII is more active than that ofJERS-1. However, the result was contrary to expectation. Because of the reasonsdescribed in Section 4.2, the effect of shield thickness and inclination was consideredto be greater than that of degrees of solar activity in the case in a low earth orbit.Distinct the power degradation due to the degradation of solar cells cannot beobserved, even on the day when a huge flare occurred (July 14th, 2000). Therefore, ina low earth orbit, the effect of a solar flare on solar cells of a satellite was consideredto be negligible.

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5. Summary

Decrease in generated power due to degradation of solar cells of ETS-VII wasanalyzed by means of developed method for this study. The effect of a large solarflare was found to be negligible in a low earth orbit. This is because the effects ofshield thickness and inclination angle are thought to be greater than that of degreesof solar activity.

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