References

Cho, Y.J. and Cho, B.H., ''A novel battery charge-discharge of the regulated peak power tracking systems,'' in Proceedings of the 34th Intersociety Energy Conversion Engineering Conference, SAE, 1999, Paper No. 01-2445. 2. Veerachary, M., Senjyu, T., and Uezato, K., ''Voltage-based maximum power point tracking control of PV system,'' IEEE Transactions on Aerospace and Electronics Systems, 38(1), 262-267, 2002. 3. Gietl, E.B., Gholdston, E.W., Manners, B.A., and Delventhal R.A., ''Electrical...

Summary of Major Component Ratings

Solar array power output rating 5839.2 watts Battery Wh energy storage rating 6803.5 Wh Battery Ah rating with 80 cells 68.90 Ah PCU charge converter rating 2489.3 watts PCU discharge converter rating 3045.5 watts FIGURE 7.8 Power flow and energy balance analysis example for a 3000-W LEO satellite. electronics and distribution harness. Various power electronic converters are used to change the voltage as needed at various points in the power system. Their efficiencies are accounted for in the...

Fully Regulated

In a fully regulated direct energy transfer bus, commonly known as the regulated bus, the bus voltage is controlled within a few percent during the entire orbit period. The typical bus voltage variation is 2 to 5 of the nominal voltage. The architecture of this bus is shown in Figure 4.8. It has the following components. PEMS Power and Energy Management Software. d(2) and d(4) bypass diodes, double and quad FIGURE 4.8 Fully regulated direct energy transfer architecture. PEMS Power and Energy...

Charge cutback mode

When the battery is approaching full charge, the charge rate is cut back to control the battery temperature. This mode includes trickle charge rate when the battery reaches a 100 state of charge. The charge cut-back mode is also activated in case the solar power exceeds the load requirement, but not enough to supply the required charge current to the battery. In such a case, the battery charge power is cut back to the level that will maintain the bus voltage within the allowable limits.

Bus Voltage Trades

For communications satellites, selecting one of the standard busses offered by prime contractors may be the most economical option. The bus voltages are standardized for various ranges of the load power. However, the voltage selection may be open for designing a special science or defense spacecraft. Even in such cases, the bus voltage is sometimes set by the heritage and the preferred database. Otherwise, a trade study is needed to select the optimum voltage for the mission on hand. Developing...

Bus Voltage Control

The regulated bus in normal operation maintains the bus voltage between the specified upper limit and the lower limit. The mode control scheme for GPS IIR type 28-V regulated MEO bus with about 1000 W load, may be set up as follows Shunt mode regulates the bus voltage by using a shunt dissipator. In this mode, the batteries are fully charged and the solar array power generation exceeds the spacecraft's needs, so the excess current from the solar array is shunted. This mode is established when...

Peak Power Tracking

The solar array generates more power at higher voltage at the beginning of life and when cold coming out of an eclipse. The maximum power can be extracted only if the bus voltage is varied with years in service and with temperature. However, the load must be supplied power at the same voltage, generally lower than the maximum power producing voltage of the solar array. A suitable switching regulator between the solar array and the load, as shown in Figure 4.11, remedies this disparity between...

The International Space Station 160 to 120V

The International Space Station is truly the largest and most complex space structure ever built, with 16 international partners. The complete assembly shown in Figure 4.15 weighs one million pounds and the total interior space of its six laboratories equals two Boeing 747 aircraft. It is taller than a 30-story building and wider than the length of a football field. The attitude is controlled within 1 stabilized at a rate less than 0.1 s. It is in 335 to 500-km low Earth, 51.6 inclined orbit...

Damage to the Power Electronics

The charged particles can interact with sensitive regions of semiconductor devices and degrade the electrical performance and even trigger catastrophic failures. The photons, protons, and electrons cause gradual degradation in performance with increasing radiation dose. The space environment also contains high-energy protons, neutrons, and heavy ions that can cause single-event displacement damage by introducing high-energy transients in the sensitive region of the power MOSFET. The effect can...

Solar Array Trades

A mass reduction in the solar array has multiple benefits at the satellite level, such as reduced drag, inertia, and propellant mass. Such win-win trades must be thoroughly investigated at an early design stage. The solar array area is linearly proportional to the required power generation. At the same time, it also has significant influence on the propulsion and attitude control systems. A large array increases the drag and moment on the satellite, more so in LEO satellites and in low Earth...

PV Cell Trades

The fundamental design requirement is to generate the required electrical power at the end of life. The degradation of the solar array power output under charged particles is a trade within the power system design that often does not cross the system boundary. The PV cell selection depends on the mission environment and the design life. Some cells, such as low substrate resistivity cells, have high BOL power, but degrade fast, while others do the opposite. The orbit environment and the mission...

Equivalent Electrical Circuit

The steady state equivalent electrical circuit of the cell is shown in Figure 8.5, which represents the complex physics of the PV cell. The cell acts as a FIGURE 8.5 Equivalent electrical circuit of PV cell. FIGURE 8.5 Equivalent electrical circuit of PV cell. constant current source shunted by a perfect diode. The circuit parameters are as follows. The series resistance Rs represents the internal resistance to the current flow. It is primarily due to the resistivity of the material. The shunt...

Solar Array Degradation

The solar array is the most affected component in the power system under space radiation. The degradations are listed in Table 5.1 in order of their importance. The array performance degrades with the increasing level of fluence, but not in a linear relationship. The damage is more in the initial years than in the latter years. The impinging particles produce defects in the crystalline structure of the PV cells. The resulting defects degrade the voltage and current outputs of the cell....

Design Process Phases

In today's competitive environment it is not enough to ensure that the system and the equipment are designed to be capable of meeting high standards of performance and reliability. A consideration of manufacturing processes and expected tolerances is part of good and complete design. A thorough and correct design the first time is of paramount importance for maintaining the program schedule and cost. The following Rule of Ten can be a strong motivator to the design engineer Not doing something...

Electrostatic Discharge in the Solar Array

The arcing in a solar array due to electrostatic discharge (ESD) is known to have caused power loss in both LEO in GEO satellites. Small ESD discharges do not have enough energy to cause damage. However, the risk is increased in a high-voltage array, particularly in the low Earth orbits. Bus voltages greater than 120 V inherently increase such a risk because of current leakage and arc discharges. The mechanism of damage is as follows. An anamolously large charge collection from the space...

Self Derived Requirements

As mentioned earlier, the electrical power system requirements are seldom found in the customer specifications for the spacecraft under procurement. They are derived from the spacecraft level requirements and in-house trade studies. The EPS self-derived requirements are based on various analyses performed on the power system under design. The final requirements generally come from the operational orbit analyses, but other orbits must also be analyzed to ascertain that the proposed requirement...

Worst Case Error Margin

Various uncertainties in input parameters may introduce probable errors in the design estimates. Such errors are managed by providing appropriate margins. If no guidance is available from the management, a conservative engineer may stack all probable errors to derive the total error that is the sum of all individual errors. The probability of occurrence of such a total error is incredibly slim, of course. In situation where life safety in not Table 7.6 Typical design analyses required for the...

Power Regulator Unit

The power regulator unit (PRU) provides an interface between the solar array bus and the battery. The battery voltage varies widely with the cell voltage varying from 1.0 V when fully discharged to 1.55 V when fully charged. The discharge converter in the PRU boosts the battery voltage to the bus voltage during an eclipse and the charge converter bucks the array voltage to the battery voltage during sunlight. For GEO missions, the charge converter rating is much smaller than the discharge...

Predicting Damage

Radiation damage to spacecraft components can be predicted fairly accurately now. For example, the damage found on the solar array of MIR, when examined in 2000, was as predicted for the effects of radiation, micrometeoroids, thermal absorptivity, and emissivity, for example. The MIR solar array did not have internal bypass diodes, only string isolation diodes. PV cells were silicon with 14 efficiency and 76 fill factor. No arcing or soldering damage or open circuits were found. Some cells were...

Micrometeoroid and Debris Impacts

The solar array gradually becomes damaged by micrometeoroids punching small holes or shattering large surface areas of the cell. In the case of small holes of a couple of millimeters diameter, the impacted cell would still work without losing the whole string, but with a degraded performance. In the case of a shatter, the cell fails open, losing the whole string. In the past a few spacecraft have incurred such damage under micrometeoroid showers. The damage is indicated by a sudden decrease in...

Solar Array

The solar array is made of numerous PV cells stacked in series-parallel connections to obtain the desired voltage and current from the assembly. It converts the incident photon energy into d.c. voltage, which drives current through the external load circuit. The solar array works more like a constant current source over its normal operating range. Its terminal voltage versus current characteristic, referred to as the I-V curve, is shown in Figure 4.1. The end of life (EOL) curve must meet the...

Van Allen Belts

Van Allen Belt Distance From Earth

The Earth's magnetic field covers the magnetosphere, a doughnut shaped region of space influenced by the Earth's magnetic field. It acts on the electrons and protons coming from the space. The Van Allen radiation belts are parts of the magnetosphere that contain a large number of particles. The magnetosphere normally shields the Earth from these particles. However, when a disturbance on the sun radiates an abundance of particles, some reach the Earth's atmosphere near the magnetic poles and...

And PV Characteristics

The electrical characteristic of PV cell in sunlight in the useful working range is represented by the I-V curve in the first quadrant only, as shown in Figure 8.7(a). Two most important parameters widely used for describing the cell electrical performance are the open circuit voltage, Voc, and the short circuit current, Isc. The short circuit current is measured by shorting the output terminals, and measuring the terminal current under full (a) l-V characteristic (b) P-V characteristic FIGURE...

Total Radiation Fluence

The energy levels of various sources of the natural radiation in the Earth's orbit are as follows Trapped electrons Trapped protons Solar flare protons Solar flare alphas Galactic cosmic rays > 1 GeV 0.1 to 7 MeV 0.1 to 100 MeV 1 to 200 MeV 1 to 300 MeV FIGURE 2.9 Trapped proton environment in 852-km, 99 -inclined circular orbit. (Source NASA GSFC.) FIGURE 2.9 Trapped proton environment in 852-km, 99 -inclined circular orbit. (Source NASA GSFC.) The total fluence is calculated for a given...

Small Satellite

The following power system features are typical in small satellites A solar array covering a wide range of solar flux and temperature with wide swings in the I-V characteristics. SA power 1.1 to 2.0kW 30 to 42V 8 SA power 1.1 to 2.0kW 30 to 42V 8 C Discrete commands c Serial data bus Power distribution (300 to 2200W 14 TO 23V) S-band conv. Payload 1 & 2 D & C prim. & red. SA W1 Solar Array Wing 1 SA W2 Solar Array Wing 2 SADA Solar Array Drive Assembly FIGURE 4.19 Sun-regulated low...

Fully Regulated Versus Sun Regulated

Power system design must give equal importance to conditioning and distributing power from both the solar array during sunlight and from the battery during eclipse. This poses a difficult design problem since both power sources have fundamentally different characteristics. The solar array is inherently a constant current source whose output voltage is limited by the forward junction voltage of the PV cell. On the other hand, the battery is inherently a constant voltage source of low internal...

Load Power Profile

Once the satellite level trades are settled, and the EPS architecture has been selected, the next step in the design process is to compile a detailed load power profile of each component to be powered. It is also called the power budget or the power allocation in an early phase of the design. The input power for the on and off durations of each item of equipment 'peak power when on, quiescent power when off, and the thermal dissipation are compiled. The component dissipation may vary...

Contents

Acronyms and Abbreviations xiii Systems of Units and Conversion Factors xx 1.2.1 Communications and data handling 4 1.2.2 Attitude and orbit control system 4 1.2.3 Tracking, telemetry, and command system 5 1.2.4 Electrical power system 5 1.2.5 Thermal control system 5 1.2.6 Structure and mechanisms system 6 1.3 Earth orbit classification 6 1.3.1 Geostationary orbit 8 1.3.2 Geosynchronous orbit 9 1.3.3 Highly elliptical orbit 11 1.3.5 Sunsynchronous orbit 12 1.5 Satellite stabilization methods...

Micro Satellite

Very small satellites with load power requirements in watts can use simple and lean architecture shown in Figure 4.20. The solar array, the battery, and the loads are permanently connected in parallel. The battery feeds to the loads automatically during an eclipse, and recharges itself during sunlight by imposing its drained voltage on the bus. Once fully charged, the battery voltage is relatively constant, and the battery works as the buffer. Any excess current is absorbed by the shunt...

Solar Array Sizing

The load power profile developed above is then used to determine the solar array output power requirement. For example, for a communications satellite having a 2500-W communication payload power, the solar array power generation requirement is shown in Table 7.5 in two seasons and during an eclipse. The autumnal equinox day, having the longest eclipse, in this case determines the solar array output requirement of 3940 W, which includes a 5 power margin and a 5 reliability margin. Allowance must...

System Voltage Options

Early spacecraft with loads of a few hundred watts used 28 V d.c., primarily based on the product specifications readily available for the aircraft power system at the time. Since then, the power levels have increased significantly. As power is the product of voltage and current, high power requires a highvoltage bus in order to keep the current at a reasonable level. Otherwise, the excessive power loss in switching devices and the I2R loss in conductors reduce the system efficiency...

Semicrystalline and Polycrystalline

Instead of drawing single crystals using seeds, the molten silicon is poured into a rectangular crucible. This process under a controlled cooling rate forms partial and or multiple crystals. The rectangular ingot thus produced eliminates the squaring-up process and the associated waste of materials. The remaining fabrication process follows the steps of the single crystal silicon cell. This is a relatively fast and inexpensive process for manufacturing thick crystalline cells. The conversion...

Mass Erosion under Atomic Oxygen

Atomic oxygen (AO) is a significant environmental constituent for spacecraft in a low Earth orbit. It is formed by photo-dissociation of diatomic oxygen by ultraviolet radiation from the sun, more in LEO, where recombination or the formation of ozone is improbable. It reacts with many commonly used spacecraft materials resulting in degradation and mass erosion of exposed spacecraft surfaces. The AO can oxidize and damage surfaces, especially in the ram direction with the spacecraft ram velocity...

Sun Regulated

If the design objective is to minimize complexity, an obvious approach is to distribute power from both sources the solar array and the battery directly to the load. Although such a direct energy transfer bus is sometimes known as unregulated bus, the bus voltage is regulated by shunt control during sunlight, and is unregulated only during an eclipse. For this reason, such a bus is also known as the partially regulated bus, sunlight regulated bus, or sun-regulated bus. Typical sun-regulated bus...

Photovoltaic Cell

Photovoltaic Cell Physics

A photovoltaic cell converts sunlight into direct current electricity. The PV effect was discovered in 1839 by the French physicist Antoine Henri Becquerel. It remained in the laboratory until 1954, when Bell Laboratories produced the first silicon solar cell. It powered the first Russian satellite in 1958, followed by applications in the U.S. space programs. Since then, it has been an important source of power for satellites because of its high power output per unit mass. Having developed...

Pyro Power Trades

In the case where a fully regulated direct energy transfer bus has been selected, another top level trade is whether to power the EED pyro circuits directly from the regulated bus or from the battery, bypassing the battery discharge converter. This trade is depicted in Figure 7.5. It is not for cost or mass reduction, but for the EMI and reliability improvements as described hereafter. Both the primary and backup converters for each battery are normally active during operational orbit. During...

Satellite Overview

A satellite consists of various systems designed to meet the mission specific requirements. All but the simplest satellites require a common set of systems shown by the solid lines in Figure 1.1. Complex satellites require additional systems shown by the dotted lines. The systems are classified into two groups, the payload and the bus. The payload consists of the communications equipment in commercial satellites or science instruments in research satellites. The bus consists of all remaining...

Space Flight Constants

Acceleration due to Earth gravity Earth equatorial radius (mean) Geosynchronous orbit altitude Obliquity of ecliptic at Epoch 2000 Precession of the equinox Earth-sun distance (AU) Mean lunar distance Sidereal year a'b'c'd Solar year Solar flare maxima year Solar flux radiation Earth annual mean albedo Earth's long wavelength radiatione'f'g Earth's effective black body temperature Permittivity of free space, o Permittivity of free space, so Velocity of light in free space Vo 9.806 7 m s2...

Bus

A standard fully regulated DET bus operating at 100 V d.c. is offered by Boeing Satellite Systems under the trade name BSS-702 Its power system architecture is shown in Figure 4.17.4 The BSS-702 bus is a dual voltage bus, primary 100 V d.c. for high power equipment and secondary 30 V d.c. for low power component. It uses a NiH2 battery and silicon triple junction solar cells. A xenon electric ion-propulsion system is used for N-S station keeping, which is powered from the 100-V bus. After each...

Beta Angle

The beta angle, ft, is defined as the angle between the Earth-Sun line and the orbit plane when the spacecraft is closets to the sun orbit noon . It varies seasonally between 0 and i io , where i orbit inclination, and io angle between equator and ecliptic plane, which is 23.45 . The value of ft 90 results in the greatest solar flux on the satellite body, and ft 0 gives zero flux. In most satellites with the array always pointed to the sun by the solar array drive motor, the ft angle has an...

Solar Flux

The energy received from the sun in space varies with the distance squared. The Earth's orbit around the sun is approximately circular with a slight eccentricity of 0.01672. The distance, therefore, varies within 0.01672 times the average distance between the sun and the Earth, which is 149.6 million km, defined as one astronomical unit AU of distance. Thus, the solar flux varies over 1 0.01672 2 or 1 0.034 of the yearly average. Within these small variations, the Earth is closest to the sun...