Archive for the ‘ACS’ Category

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.

Momentum Wheels

Momentum Wheels are used in a momentum biased control systems. Similar devices are used in zero momentum bias control systems and are called Reaction Wheels, they differ in use by centering their operational RPM range around zero and therefore do not provide significant gyroscopic stiffness in the spin axis.

Momentum Wheels function by spinning a wheel of a given mass at a targeted RPM to maintain gyroscopic stiffness in a given axis, and can induce movement in a secondary axis by varying the RPM. By orientating the location of the unit to provide stiffness in the Roll axis, variations in RPM will provide control over the Pitch axis. As a satellite progresses through its orbit, Yaw error will translate into Pitch error, therefore actively controlling Pitch errors effectively manages errors in Yaw, thereby achieving influences over all three axis through the course of the orbit.

With the orientation of the Roll axis held in a relatively fixed position by the momentum of the wheel any variations in the inclination of the orbit will appear as Roll errors. Periodic attitude adjustments are made to the momentum wheel spin axis to minimize Roll errors and momentum adjustments are made to maintain the wheel speed within the optimal RPM range.

Normal maneuvers are preformed to maintain the North/South (inclination) and East/West (eccentricity) drift of the satellite.

Reaction Wheel Assemblies

Reaction Wheel Assemblies are used on LEO and GEO satellites to control errors in Pitch, Roll and Yaw axises. The assembly consists of the electronics to monitor and control the wheel, a motor and the wheel its self. A Reaction wheel works by applying current to a motor that spins a wheel with a given mass and is based on the common principal that every action has an equal and opposite reaction. By applying current to increase or decrease the wheel speed control torques can be generated in the given axis. As the wheel is spun faster it gains stored momentum in the axis and therefore in systems that employ reaction wheels, periodic momentum management is required to prevent the wheel from exceeding its recommended speed range. Momentum unload procedures typically use opposing thruster firings to drive the wheel speed down or up and reduce the stored momentum.

In a zero momentum biased control system when the unload procedure is activated, it drives each axises momentum to zero momentum. In advanced control systems this function is enabled during the standard station keeping maneuvers and can conserve fuel.

When a fourth wheel is included for redundancy, the wheel’s orientation is set such that each wheel has an influence on 2 axis and the ACS or ADACS system uses the alignment of each wheel and axis to calculate the required torque and distribute them over the wheels to control errors in an axis. In this configuration a minimum of three wheels are required to operate the satellite and the fourth wheel can be turned on when needed to replace a failed or failing wheel. The system can also distribute the torques over all four wheels if desired, providing it has been designed to do so.

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.

Attitude Control System (ACS) 3-Axis GEO Satellites

Each satellite manufacturer has its own configuration so this is a basic description of the concept of the Attitude Control System on GEO Satellites.  The ACS hardware can be broken down to 2 basic categories, sensors and actuators.  Sensors collect measurements and information in reference to the spacecraft’s axises. This information is converted into error signals used to control each axis.  Sensors include but are not limited to, Earth Sensors, Sun Sensors and Gyros. Actuators allow control of movement in the axis. Typical hardware used as actuators includes but is not limited to,  Momentum Wheel Assemblies, Reaction Wheel Assemblies, Magnetic Torque  Rods and Reaction Control Thrusters. Antenna tracking systems and Solar Array systems have also been used as actuators, but are not part of the basic set of actuators normally used.  Although they are predominately used in 2-Axis GEO Satellites.

A third category, being the ACS software is normally referred to when talking about Attitude Determination and Control System (ADCS). The ACS software will collect the error signals from the sensors and convert them into control signals to be applied to the actuators to maintain control of each axis.

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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.