TOPEX/Poseidon Electrical Power System - Performance

This paper shows that the power system performance (batteries, solar array, power regulator) on-board the TOPEX/Poseidon satellite has met or exceeded pre-launch predictions, and has successfully managed the performance of NiCd batteries which had shown anomalous performance on other missions such as UARS and GRO. The battery performance is addressed through the following parameters: end-of-discharge voltage, peak charge current, charge to discharge ratio, and voltage differential. The solar array performance discussion includes voltage, current and power. There is also a discussion of the power regulator efficiency and the satellite load power history. INTRODUCTION TOPEX/Poseidon (TOPEX), jointly conducted by NASA and the French Centre National d’Etudes Spatiales (CNES), was launched successfully on August 10, 1992. The 3.7 year successful operation to date of the TOPEX satellite and power subsystem has enabled the science community to make enormous gains in measuring and understanding global ocean phenomena such as circulation, tides, and their effects on climate. TOPEX is powered by the Modular Power Subsystem (MPS) containing 3 NASA Standard 50 Ah capacity Nickel Cadmium (NiCd) batteries manufactured by McDonnell Douglas and a deployable, sun tracking, rigid, single-wing, rectangular silicon solar array with overall dimensions of approximately 26 by 11 feet. Batteries of identical design and similar manufacturing history on board other NASA satellites experienced battery anomalies early in life. These satellites include the Extreme Ultraviolet Explorer (EUVE), the Upper Atmosphere Research Satellite (UARS), and the Gamma Ray Observatory (GRO). These satellites exhibited large divergence of the half-battery voltage within the first 4-7 months after launch. This deteriorating condition normally would be exhibited near the battery end-oflife, which was designed to be on the order of 3-5 years in a Low Earth Orbit regime. In order to avoid similar battery problems on TOPEX, an Investigation Team and the Battery Management Team were formed prior to launch to make operational recommendations for the TOPEX batteries. The recommendations included using novel battery management techniques to maintain the battery health throughout the primary mission. These techniques have been referred to as TLC or Tender-Loving-Care. The TLC techniques included: limit the battery peak charge current to less than 20 A, control the recharge ratio to 1.05±.03, and use the low current sensor for recharge ratio calculations when charge current is less than 3 A. These successful operational techniques for the TOPEX batteries have been continued during the extended mission in anticipation of battery life well beyond the primary mission. The TOPEX satellite has an orbit with a period of 112.5 minutes and eclipse duration varying from 0 to 35 minutes. The occultation periods alternate between approximately 42 days and 84 days with periods of full sun of about 10 to 20 days in between. This behavior repeats throughout the duration of the mission. The primary source of power for TOPEX is a rigid single-wing solar array. The solar array provides power to the satellite loads and charges batteries while in orbit daytime. The array tracks the sun and is offset from normal to the sun line by (currently) 50.5° to reduce the charge current on the batteries. The solar array temperature varies between -80° to +40°C and the differential temperature between the front and back of the array varies from 0 to 10°C. The load requirements on the TOPEX power system range from 700 to 1050 W. The variation is caused by heater cycling and switching between the NASA altimeter and the CNES altimeter. Battery charging on TOPEX is carried out by the Standard Power Regulator Unit (SPRU) using 2 different modes: peak power tracking mode and voltage limit mode. Peak power tracking is utilized at the beginning of each satellite orbit until the selected charge voltage/temperature (V/T) level is reached. At this point, the SPRU enters voltage limit mode, also known as taper charge mode. The V/T controller in the SPRU senses the battery voltage and limits the voltage to the preset limit defined by the V/T level. The V/T levels are used effectively in controlling battery recharge. TOPEX operates using V/T levels 2,3 & 4 from the NASA standard Ni-Cd battery manual. V/T level 2 is used during periods of full sun. V/T level 3 is used when the earth occultation periods are < 28 minutes and V/T level 4 is used during earth occultation periods between 28 and 35 minutes. The battery temperature varies between 5-8° C and is controlled using heat pipes and heaters. BATTERY GROUND TESTS Four months prior to launch, a mission simulation ground test using TOPEX cells from the flight lot was initiated as standard operational procedure in support of flight projects. The objective of this test was to predict mission performance and to provide quantitative data to aid in the management of the spacecraft batteries. Because the mission simulation test was initiated prior to the launch, the operational changes for the Tender-Loving-Care environment were not yet implemented and not incorporated in the test. In addition, another ground test was started 2 months after launch to assess the effects of charging at a lower temperature. The cell temperature was 10° C for the mission simulation test and 0° C for the temperature effect test. In both tests, a pack of 5 flight cells was charged at 25 amperes to V/T level 5. The discharge rate was 10 amperes to a maximum 12% depth of discharge (D.O.D.). The orbit duration was 112 minutes with a discharge duration ranging from 23.5 minutes to 35.5 minutes maximum. The discharge duration simulates the TOPEX/POSEIDON satellite eclipse duration. SOLAR ARRAY PERFORMANCE The solar array power is a function of the solar intensity and the solar array offset angle. The solar intensity increases from aphelion (July 1) to perihelion (January 3) and then decreases back to aphelion. By monitoring the solar array degradation and solar intensity cycles, the array was positioned to maintain the peak charge current within the recommended operational range (15-20 A). The following table lists solar array offset angle changes since launch.