Archive for the ‘Training Materal’ Category

Mercury The closest Planet to the Sun

Here is a video that I put together than will provide more information about the planet Mercury.

A good resource for more information is the NASA Planetary Page that is found at http://solarsystem.nasa.gov/index.cfm

How Comets get their tail.

Comets start out as a dark object traveling through deep cold space. This shows how they grow a tail as they enter the warmer regions of space. A comets tail can stretch for many thousands of miles.

If you would like to read along, here is the text!

Comets are believed to have a solid core, and accumulate additional dust and ices on its surface as they travel deep into the Oort Cloud. As the comets elliptical orbit brings it back closer to the Sun, it approaches the distance of the asteroid belt, outside the orbit of Mars, where its ices begin to turn to gas, releasing hydrogen, carbon, oxygen, nitrogen, and other substances in the form of vapors and dust particles. They are carried away from the comet by the Solar wind, forming a tail. On its return path it cools and the tail goes away until the next trip.

Our Solar System

This animation shows our Solar System with the planets and dwarf planets in order orbiting the Sun.

If you would like to read along, here is the text!

Here we see our Sun orbited by the planets. The planets and dwarf planets are shown in order from the closest to the Sun, they are Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake and Eris.

The Outer Solar System is home to the Gass Giants, Jupiter and Saturn, Ice Giants Uranus and Neptune, Comets, Centaurs, and the Dwarf Planets Pluto, Haumea, Makemake and Eris. Saturn, Uranus and Neptune are encircled by planetary rings of dust, ices and other small objects.

The 4 terrestrial planets, Mercury, Venus, Earth and Mars, the asteroid belt and the Dwarf planet Ceres make up the Inner Solar System.

The Solar System consists of the Sun and its planetary system of eight planets, dwarf planets, their moons, and other non-stellar objects. It formed 4.6 billion years ago from the gravitational collapse of a giant molecular cloud.

The Inner Solar System

This animation shows the inner Solar System with the planets orbiting the Sun.


If you would like to read along, here is the text!

The inner Solar System is the region comprising the 4 terrestrial planets, and the asteroid belt that includes the Dwarf planet Ceres. The asteroid belt resides between the Orbits of Mars and Jupiter, it is thought to be remnants from the Solar System’s formation that failed to form a planet because of the gravitational interference of Jupiter.

The four inner planets have dense, rocky compositions, which form their crusts and mantles, and metals such as iron and nickel, which form their cores. The closest, smallest, and fastest planet, is Mercury. Next is Venus the hottest, similar in structure, and size to Earth, the planet we live on, is the third, Then we have, Mars, a cold desert world. It is half the diameter of Earth.

Three of the four inner planets, Venus, Earth and Mars, have atmospheres substantial enough to generate weather. All orbit the Sun in a counter clock wise direction, have impact craters and tectonic surface features such as rift valleys and volcanoes.

The Inner Planets

Take a look at the first four planets in our Solar System. This animation shows each planet zoom in and rotate then zoom out. The textures of the planets are maps made by NASA and found on the WEB.

If you would like to read along, here is the text!
Mercury
Mercury Orbits, the Sun in 88 days, it’s the closest planet to the Sun, fastest, and the smallest planet in the Solar System, .055 the mass of Earth. Its only known geological features besides impact craters are lobed ridges, produced by a period of contraction. The almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.

Venus
Venus orbits, the Sun in 224.70 Earth days, it’s close in size to Earth, has a rocky mantle around an iron core, substantial atmosphere and evidence of internal geological activity. However, its atmosphere is ninety times as dense. The hottest planet, with surface temperatures over 400 °C, due to the greenhouse gases in the atmosphere.

Earth
Earth, the largest and densest of the inner planets, the only one to have current geological activity, and the only place where life is known to exist. Orbits the Sun in 365.26 days. Its liquid hydrosphere is unique among the terrestrial planets, and the only planet where plate tectonics has been observed. The atmosphere has 21% free oxygen.

Mars
Mars is half the size of Earth. Orbits the Sun in 686.98 Earth days. It’s atmosphere is .6% of that of Earth, made of Nitrogen, Argon, and mostly Carbon Dioxide. The surface has a vast number of volcanoes such as Olympus Mons, rift valleys such as Valleys Marineris . It’s red color comes from iron oxide, rust in its soil.

Lunar Orbit of the Earth

This demonstrates the orbit of the Moon around the Earth and is a good companion to the earlier post Tilt in Earths axis. NASA defines the period of the Moon’s orbit to be one complete orbit in 27.3215 days that translates to 27 days, 7 hours, and 43 minutes. The cycles or phases of the Moon are due to it’s orbit around the Earth and the angle that the light from the Sun illuminates the Moon. As demonstrated in this animation as the Moon progresses through it’s orbit it will cycle from full illumination (the Full Moon) to total darkness (called new Moon) and back to full illumination as viewed on the earth. The tilt in the Moons axis is 1.54 degrees and the orbit has a 5.14 degree inclination to the Sun line, when added together the resulting tilt in axis is a total of 6.68 degrees.

This animation shows the spin of the Earth and movement of the Moon relative to the Moons orbit period. You can also see how the the phase of the Moon is based on it’s location in the orbit.

Solar Array Deployment

At launch the satellite solar panels are folded against the body of the satellite (stowed configuration) to minimize space and allow the satellite to fit into the faring of the launch vehicle. Once the satellite has been placed into Geosynchronous orbit the ACS system is activated to point the proper axis of the satellite at the earth. This process is called ACS initialization and Earth capture activation. During this period the satellite is being powered by the batteries. Once the Earth sensors are activated and the ACS system is locked onto the Earth, the solar panel release mechanization can be actuated to allow the arrays to unfold and lock into a deployed position. With the arrays deployed the solar array drive system is activated and commanded to track the Sun. The following animation shows the release of the North panel, then the South panel, followed by the positioning of the panels to point directly at the sun.

This would be the worst case condition where the arrays are pointed 180 degrees away from the Sun, the arrays are typically deployed at a time of day that allows them to be pointed at the Sun so they only require minor pointing changes to peak their power output. With a 180 degree pointing offset as shown the arrays would be commanded to move in opposite directions (one clockwise and the other counter clockwise) at the same time. From the ACS control standpoint the torques on the body of the satellite would relatively cancel and minimize the error correction required by the ACS system. After the the arrays are peaked on the Sun the Solar array drive system is commanded to normal tracking mode to maintain this pointing through a full 360 degrees rotation over the course of the day. As soon as there is adequate power being generated from the arrays, the power load will transition from battery to array power and battery charging can be started.

Solar Array Drive

Solar Array Drive (SAD) refers to the system that positions the solar panels on the satellite to efficiently convert s the sun light into electrical energy. This system is comprised of the Solar Array Drive Assembly (SADA) and the Solar Array Drive Electronics (SADE).

The SADA includes the stepper motor, drive gear assembly, Barring And Power Transfer Assembly (BAPTA) and the mounting hardware required to mount the solar panel to the body of the satellite. The solar panel is mounted to the SADA and allowed to move by means of the barrings in the BAPTA. It also includes a slip ring assembly used to transfer the power generated by the solar panel to the EPS bus. The drive gear assembly is installed to smooth the step movement of the motor and provide a steady constant travel through 360 degrees of rotation. Stepper motors are known for their ability to provide high torque and controlled movement, this makes them ideal for this application. Rather than mounting two motors in the assembly, to increase the reliability these stepper motors are designed with redundant coils, this effectively reduces weight and provides improved redundancy.

Each SADA is controlled by independent Solar Array Drive Electronics (SADE) unit that provides the interface between it and the EPS subsystem or flight computer. It is comprised of all of the circuitry required to generate and shape the pulses needed to drive the stepper motors. Telemetry monitoring sensors collect data such as current, temperature, position, speed, rotational direction of travel, step counts, and status indicating movement or stopped for the SADA.

In operation the rotation of the solar panel is started when the SADE is enabled and step pulses are received, movement stops between steps or when it has been disabled. Control of the direction of travel, can be commanded to rotate in either forward or reverse direction to allow for pointing position optimization. The SADE is normally configured to output a consistent train of pulses at the required intervals to maintain movement through the 360 degree rotation called Normal Mode. With the panel rotating less than 15 degrees each hour it is easily commanded at less than 5 times a second allowing for rest intervals between movement. Speed and torque can be adjusted by increasing or decreasing the rate of pulses per second (pps) sent to the motor. If the motor is normally driven at 5 pps then driving it at 25 pps will increase the speed by a factor of 5. This system is typically design for the solar panel to complete a full 360 degree rotation in less than 15 minutes at the highest pps command rate. When commanding the movement of the SADA, commands are processed by the SADE, stored and then pulses are sent at a 5 pps or 25 pps rate until the complete commanded step count is reached. At completion the SADE must be commanded back to the Normal Mode or transitions back automatically.

Satellite Solar Array positioning

As a satellite travels along the orbital path the Solar Arrays must remain pointed at the Sun to produce power. This is accomplished in a number of methods based on the satellite design. Satellites with fixed arrays must point their surfaces at the Sun or spin around the axis that allows them to rotate into the Sun light. With movable solar arrays they either actively track the Sun or are driven at a constant rate to maintain their pointing.

Spin stabilized or 2 axis satellites typically have fixed solar panels and positioning is achieved by the orientation of the spin axis of the satellite. Attitude or Spin Precession Maneuvers are periodically preformed to maintain the spin axis alignment.

Solar arrays on a 3 axis stabilized satellite track the Sun by the implementation of positioning mechanisms that either step the arrays or actively position them. To simplify the tracking system a geared drive system is driven by stepper motors that are selected to provide the required torque and move the array smoothly. The number of steps are calculated using the angular step size of the stepper motor, the gear ratio, and the angular change needed. This is used to generate the required number of pulses per second. Pulses are generated in the solar array drive electronics then applied to the motor to allow them to complete one complete 360 degree revolution a day. To increase accuracy the sidereal day measurement of approximately 23:56:04 hours is used. These systems can provide accurate positioning with minimal operational requirements. Over time periodic adjustments are made to minimize any tracking errors and optimize positioning and output power.

Active positioning systems also employ the use of stepper motors and the output of the solar array is sampled and converted to pulses that are applied to the stepper motor to maintain the peak output power. One important characteristic of this system is that it can not be used during eclipse periods, during these periods the system normally reverts to a stepped mode and the active control is resumed when the satellite returns to full sunlight. When these systems are fully optimized it significantly reduces operational intervention to maintain peak power. Failures in active tracking systems can lead to a rapid loss of power and must have antiquate procedures or automated sequences to prevent the arrays from being driven off the Sun.

Lithium-ion batteries

Lithium-ion batteries (Li-ion) are increasingly being used on satellites as a replaced for Ni-Cad and Ni-Hyd batteries. They continue to build a heritage based on high reliability. When properly operated and maintained they provide reliable reserve power. Their design life meets the 15 years life expectancy required in on orbit operations of GEO satellites.

The most significant differences in these batteries are related to their operation. The deterioration of the state of charge capacity is attributed to the chemical breakdown of the Lithium component in the battery over time. To slow this process, during periods when the batteries are not being discharged and charged (storage) between eclipse seasons, the batteries are maintained at a lower temperature and at a reduced state of charge. Typically at 50 % state of charge and the temperature is reduced by 10 degrees C, this varies based on the manufactures recommendations. Automated command sequences are stored in the flight computer and triggered by battery telemetry monitors. The flight computer maintains the battery state of charge at the recommended level and during emergencies will turn off units in a predefined sequence to reduce the power load and extend the time on batteries.

Eclipse operations have also been automated requiring additional preparations. Initially the batteries are warmed up by changing the heater set-points to bringing them up to the normal operation temperature. The battery discharge for each eclipse is calculated and then the batteries are charged to a higher state of charge to account for the expected discharge. At the start of eclipse season the periods are only a few minutes, at the center or longest eclipse is 70 minutes then gradually decrease back to a few minutes in duration. Plotting the battery charge and the battery discharge over the eclipse season shows curves that resemble a football and is loosely referred to as the football curve. These charging values are entered into a table in the flight computer and the charging profile is enabled allowing charging to be completed autonomously. Implementation of a day counter allows the flight computer to progress through the charging schedule.

Over charging of Lithium-ion batteries will lead them to catastrophic failure. The battery is comprised of modules vs cells on other types of batteries. Independent charge/discharge circuits are included in each module and to protect the battery, bypass relays are installed to isolate week or failed modules from the circuit. If a module is bypassed the module is completely discharged forcing it to permanently fail. Once a module has been bypassed it can no longer be used. Each module operates at 4 to 4.5 volts one design for a 36 volt battery has 9 modules. With all modules functioning, each module is charged to 4.0 volts and with one module in bypass the remaining 8 modules are charged to 4.5 volts to compensate.

Prolonged short discharge and recharge cycles do not have a significant affect on the capacity of these batteries (noted at this time).

Manual battery recondition has been replaced by automated sequences with ground commanded table values.

Due to technological advancements in battery chemistry and design, Nickel hydrogen batteries batteries are starting to be replaced by more efficient Lithium-ion batteries as they prove their reliability.

Additional information about Lithium-ion batteries is available on Wikipedia

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INFORMATION

Shining light on satellites and how they operate. Drawing from over 30 years of knowledge and experience in all phases of the life of a satellite from concept, to operations, and through end of life. You will find short topics intended to give you an understanding of how they work, the general concepts, and principals used along with information on ground systems. There is also a section dedicated to topics that can be used as basic concept training along with links to animations and 3D models I have created.