Archive for the ‘Animation’ Category

Earth Acqusition basics

In preparation for initial Earth Acquisition or as part of contingency operations to recover Earth pointing the approach is the same.  The earth sensor is configured for wide scan operations increasing its field of view.

The operations team monitors the sensor output, once earth presents is detected the sensor will provide a measurement of the pointing error that can be used to determine the rate of spin and adjust it to an acceptable range if needed.  The error will decrease as the earth approaches the center of the scan range. The ACS system is configured for Acquisition mode and then enable earth capture as the error becomes zero.  Based on the spin rate the there may be an oscillation around zero pointing as the subsystem controls the errors based on its momentum and the torque authority of the control system employed (seen in this demonstration).  The transition back to the normal operational range on the earth sensor is made either automatically or by command when the errors have settled and it is safe to do so.

Earth Sensor Error basics

The Attitude Control System ACS uses sensors to determine errors in the pointing of the satellite. The primary sensor is an Earth sensor, it has the ability to measure errors in both roll and pitch. This shows the use of 2 detectors offset to scan north and south of the equator.

By measuring the length of time the detectors see the earth and comparing the results, the difference is converted into the roll error. To determine the pitch error a measurement is taken from when the detector scenes the starts of earth presents to a center of sensor reference and compares it to that measured from the center of sensor reference to the end of earth presents, the difference is converted into the pitch error. This animation shows how the scans change as the satellite moves in roll, pitch and yaw.

With the sensor pointed at the center of the earth the resulting north and south scans will be the same.  As the sensor is moved down from center the south scan will decrease and the north scan will increase. Conversely as the sensor is moved up from center the north scan will decrease and the south scan will increase.   As the sensor moves in pitch you can see how the measurements change from the starts of earth presents to a center of sensor reference and from the center of sensor reference to the end of earth presents.

In this animation I show the Earth to make it easier to depict the interaction between movement and scan changes.  The satellite movement is exaggerated due to the sensitivity of the sensor, pointing requirements are on the order of +/- 0.05 degrees and that would be difficult to detect.

There is no significant change with yaw movement. Yaw measurements require the use of data collected from Sun sensors.  As the satellite moves along the orbit, yaw will gradually translate into roll over a 6 hour period and back on the next 6 hour period. As the yaw translates to roll the ACS system will measure and manage these errors.

Orbit Eccentricity basics

Eccentricity in an orbit causes the satellite to appear to drift east and west over the course of the day. As eccentricity increases the orbit changes from circular to elliptical path. When eccentricity is zero the orbit is circular without the appearance of any drift.  The gravitational  affects of the Earth and moon on the satellite are the primary influences that result in this gradual increase in eccentricity and drift.

To control the drift, maneuvers are carefully planed and executed to fire thrusters and reduce the eccentricity returning the orbit to its circular path.  One standard approach is to plan these maneuvers in two parts separated by 12 hours where one is an East correction and the other is a West correction.  These maneuvers are referred to as Delta-V (where V is a velocity change), or East/West depending on the preferred terminology.  They are designed to maintain the satellite in a specific orbital location, plus or minus an acceptable or defined margin called the orbital box.  A typical box is +/- 0.25 to +/- 0.5 degrees this restriction can be tighter based on the the owners requirements.  This is not to be confused with attitude pointing requirements that are much tighter and on the order of +/- 0.05 degrees or less.  To conserve fuel single maneuvers can be planed to allow the satellite to drift to the edge of the box, then execute the maneuver, reversing the drift at a rate that will slow and naturally reverse again before reaching the opposing side of the box.

In addition Start and Stop Drift maneuvers utilize the same principals, typically they are longer in duration, and are preformed to move a satellite from one orbital slot to a new one.  Drift maneuvers are normally used after launch to position the satellite in it’s orbital slot or at the end of the life as part of decommissioning.

NEC Inferred Earth Sensor Basics

One of the more popular Earth sensors in use today is produced by NEC. These sensors are mounted on the earth facing deck of the satellite and are used to measure Roll and Pitch errors. This earth sensor uses inferred detectors to sense the Earth and measure the duration of its presence in the field of view. Two inferred detectors are mounted in a fixed location in the earth sensor and an oscillating mirror is used to reflect light in the inferred spectrum into the detector.

Detector locations are offset to produce north and south scans that are compared to calculate pointing errors in the roll axis , used for control of north/south pointing.  East/west error in the pitch axis is calculated by comparing the measure start of the scan to a center reference and center reference to the end of scan to produce east/west pointing errors.

While operating in the Normal mode on this sensor the oscillating mirror travels plus and minus approximately 15 degrees of its center point. For course measurements the mirror travels can be commanded to Wide scan mode widening the scan range to plus and minus 30 degrees, used during contingency operations and during initial acquisition of the earth.

Earth sensor Roll and Pitch error signals are acted on by the ACS subsystem to maintain pointing accuracy.

Orbit Inclination Basics

Inclination is the angular difference between the orbit and the equatorial planes.  Inclination Maneuvers adjust the orbital plane by aligning it with the equatorial plane.  As satellites orbit the Earth, the Moon and Sun have noticeable affects on its orbit that cause the inclination angle to increase over time.

North/South station keeping maneuvers are designed to control inclination and are scheduled on a regular basis. Thrusters located on the North face of the satellite are used for this purpose along with a set of thrusters that will be used to control the attitude disturbances generated by the thruster firings. During preparation, thruster sets are selected, thruster firing durations are calculated and the resulting period is centered on the ascending node of the orbit to reduce the inclination angle and maintain the orbit.

SPM Orbit alignments

During the launch phase the satellite is placed into an elliptical orbit. To maintain a stable orientation the satellite is spun to add gyroscopic stiffness to the axis aligned with the orbit plain. Prior to a orbit raising maneuver or a solid fuel motor firing to circularize the orbit it is critical to align the spin axis with the targeted orbit plan. This is done by performing a Spin Precession Maneuver. Based on the satellites design, this can be accomplished by using thruster firings or momentum wheel torques.

This shows a simplified depiction of  the satellite’s movement to reinforce the concept.

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.


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.

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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.