Archive for the ‘Training Materal’ Category

Satellite Orbit Basics

Satellites in an equatorial orbit follow an orbital path that is synchronized with the Earth’s rotation. This is accomplished by adjusting the velocity of the satellite to complete one orbit in the same time it takes for a complete rotation of the Earth. This results in the satellite  appearing to be a stationary location in the sky.

During the spring and fall seasons in witch the equinoxes occur the satellite will pass behind the Earth during a portion of it’s orbit resulting in an eclipse as seen by the satellite.  The duration of the eclipse periods range from a few minutes to as long as 70 minutes on the actual day of equinox.

For more information please see my post titled Earth’s Orbit of the Sun or Eclipse.

Eclipse

One of the most active periods during the operations of a satellite in Geosynchronous Earth Orbit is during eclipse seasons. These period are centered around the vernal and autumnal equinoxes. On equinox the satellite will pass through the longest period of the eclipse season having a duration of approximately 70 minutes.

Due to refraction of the light passing through the Earth’s atmosphere the sunlight gradually fades in intensity from full sunlight to total darkness over a period of approximately 2 minuets proceeding and following the eclipse. This is call the penumbra and is depicted in gray. The area of total darkness is called the umbra and is 70 minutes in length.

The satellite operations team will prepare each satellite for eclipse before entry into the penumbra, monitor it through the umbra and  either verify or configure the charging system to recharge the batteries after the completion of the eclipse.

Before the scheduled eclipse the charging system is commanded to charge the batteries to 100% state of charge (SOC) this is shown by the increase of the green bar on the indicator. This bar turns yellow as the solar array power decreases. When the solar arrays can no longer support the power requirements of the satellite the load transitions to the batteries and this is indicated by the change to red on the status bar. During the eclipse the SOC of the batteries will decrease as the stored power is removed. By design the batteries are selected for their capacity and the ability to support the total power requirement with no less than a 25 percent margin at worst case. The SOC decreases to 25 % on exit. As the satellites exits into the penumbra the load is transitioned back to solar array power as it becomes available, the bar turns yellow again. In the absences of sunlight the solar arrays will dramatically cool to ruffly -200 degrees and will be more efficient on entry into the sun. This is shown in a slight bounce in the status indicator on exit.  When back in full sunlight the charging system is enabled and the batteries are recharged at the high charge rate.

The demo only shows the return to a 50% state of charge at the end. With an eclipse of 70 minutes, it typically requires high charge of the batteries for approximately 8 to 10 hours. This varies based on the battery type, the power load of the satellite, the initial charge state of the battery and a number of other variables.

A higher resolution AVI of this demo can be obtained through Turbosquid.

Earth’s Orbit of the Sun

This is a simple AVI that demonstrates the Earth’s orbit of the Sun. It starts at the left of the screen in Winter solstice with the Earth axis tilted away from the Sun and progresses through the full orbit and cycle of seasons. This shows that the Earth’s axis remains tilted at a 23.45 degree angle throughout it’s orbit. Based on Newton’s Law’s, an object in motion will remain in motion, the effects of friction, resistance and gravitational effects of other celestial bodies in space are minimized due the Earth’s mass and velocity.  As we all know the Earth spins 360 degrees in a day and this gives it gyroscopic stiffness in this axis orientation throughout it’s orbit helping it to resist these outside influences. The common misconception is that the Earth tilts back and forth throughout the year, where in fact it’s precession is caused by a function of it’s position in the orbit.

The intention was to provide a means to stimulate discussion in a training environment and to point out significant attributes of the Earth’s orbit from a general perspective. Any constructive comments are welcome and reasonable suggestions can be incorporated in future updates.

This has also been rendered as an AVI that runs 12 seconds start to finish in letterbox format at a resolution of 736×398. The file stops one frame from the starting point so it is set to loop smoothly.  To obtain a copy, follow the link to Turbosquid.

Movement around the Z-Axis is Yaw

In 3-Axis satellite control movement around the Z-Axis is called Yaw. Errors in this Axis will shift the pointing of the communications transmit pattern of the satellite in what appears to be a clockwise/counterclockwise movement on the earth. Through the normal progression of an orbit Yaw will translate to Roll in a quarter of the orbit.  Both errors can be managed by active control and typically Roll control is chosen to be controlled if only one of the two Axis is to be controlled. The diagram and animation show a visual representation of this movement.

Movement around the Y-Axis is Pitch

In 3-Axis satellite control movement around the Y-Axis is called Pitch.  Errors in this Axis will shift the pointing of the communications transmit pattern of the satellite in what appears to be an east/west movement on the earth. The diagram and animation show a visual representation of this movement.

Movement around the X-Axis is Roll

In 3-Axis satellite control movement around the X-Axis is called Roll.  Errors in this Axis will shift the pointing of the communications transmit pattern of the satellite in what appears to be a north/south movement on the earth.  Through the normal progression of an orbit Roll will translate to Yaw in a quarter of the orbit.  Both errors can be managed by active control and typically Roll control is chosen to be controlled if only one of the two Axis is to be controlled.  The diagram and animation show a visual representation of this movement.

Reason for Seasons

The Earth’s pole is offset by 23.45 degrees, as it orbits the Sun it appears to precess above and below the equator.Earth's Seasons

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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.