Archive for the ‘Training Materal’ Category

Nickel Hydrogen battery

Nickel hydrogen battery (NiH2 or Ni-H2) are used extensively on satellites. These batteries have replaced the use of NiCad batteries in almost all cases and continue to build a heritage based on high reliability. When properly operated and maintained they have provided reliable reserve power. Their design life exceeds the 15 years life expectancy required in on orbit operations of GEO satellites.

One of the most significant differences in these batteries is the power density to weight ratio allowing them to store more energy while reducing weight, These batteries are pressure vessels and the state of charge can be derived from the pressure and temperature of each cell. Another unique feature is that modules are available that contain 2 cells in one vessel. The design, testing and assembly of cells into a completed battery utilize similar process standards with minor changes. In addition to voltage, current and temperature, pressure monitoring sensors are included on the cells.

When monitoring cell voltages during eclipse or discharging, the voltage drop profile follows that similar to NiCad battery cells. The pressures will drop on a liner slope. Nearing the depletion of capacity the voltage will drop by as much as 0.1 to 0.2 volts over a few minutes resembling the initial rate at the start of discharge and below 1.0 volts dramatically drop off. These battery cells can be discharged safely below the 1.0 volt limitation placed on NiCad battery cells.

During charging, cell pressure and temperatures have to be closely monitored to ensure maximum charge and to prevent over pressure conditions that could lead to venting or bursting the pressure vessels. After discharging the batteries pressure and temperature are used in the determination of when the full state of charge is reached. Initially the battery will become endothermic and cool as it charges resulting in the battery heaters cycling on and off to maintain temperature. Once the battery state of charge nears 80% the battery will become exothermic and the temperature will start to rise. At this point the temperature rise rate should be monitored along with voltage and pressure. Monitoring the battery and cell voltages, the voltage will increase until they reach full charge and then slightly decrease before charging is complete. The pressure will rise and as it approaches full charge the ability to store energy at the high charge rate diminishes and the excess energy starts to be converted to heat account for the temperature increase and an increase in the rate pressure increases.

Prolonged short discharge and recharge cycles do not have a significant affect on the capacity of Nickel hydrogen batteries.

Battery recondition is not required and in most cases is preformed to obtain measurement and verification of battery aging. The open circuit stand is beneficial in minimizing the difference between the highest and lowest cell voltages, known as cell spreading and allows cells to reach a chemical balance and independent cell voltages to equalize.

Self discharge will occur in Nickel hydrogen batteries during storage periods due to the internal resistance of the battery. To overcome this affect a supplemental charge current at low level is applied to the battery known as a trickle charge.

Due to technological advancements in battery chemistry and design, driven by power storage density Nickel hydrogen batteries batteries are starting to be replaced by more efficient Lithium-ion batteries on GEO satellites.

Additional information about Nickel hydrogen batteries is available on Wikipedia

NiCad Batteries

Nickel-Cadmium batteries (NiCd or NiCad) have been used extensively on satellites. These batteries have a heritage based on high reliability that has been proven over time. When properly operated and maintained they have provided reliable reserve power well in excess of 15 years of on orbit operations on GEO satellites.

When monitoring cell voltages during eclipse or discharging operations, the voltage will drop by as much as 0.1 to 0.2 volts over the first few minutes then stabilize and decrease only 0.004 to 0.1 volts until it reaches less than 20 % of it’s capacity. Nearing the depletion of capacity the voltage will drop by as much as 0.1 to 0.2 volts over a few minutes resembling the initial rate at the start of discharge. It is critical to stop discharging when the voltage on any cell drops to 2/3 of the initial voltage. If the cell is rated at 1.5 volts the discharge termination voltage would be at 1.0 volt. To continue discharging below 1.0 volts could result cell reversal, cell failure or complete battery failure. During discharging of the battery the temperature will increase and stabilize.

After discharging the batteries the amount of energy removed must be calculated and 110 to 120% returned to reach the full state of charge. This is based on the internal resistance and inherent characteristics of the NiCad battery. Initially the battery will become endothermic and cool as it charges resulting in the battery heaters cycling to maintain temperature. Once the battery state of charge nears 80% the battery will become exothermic and the temperature will start to rise. Charging of NiCad batteries at higher temperatures can cause a chemical reaction that produces hydrogen or oxygen gasses in the battery. At this point the temperature rise rate should be monitored and if it exceeds a rate of 5 degrees per hour then charging should be terminated to minimize potential of gas buildup that could lead to cell failure or rupture. Monitoring the battery and cell voltages, the voltage will increase until they reach full charge and then slightly decrease before charging is complete.

Prolonged short discharge and recharge cycles can lead to a diminished capacity over time, this condition is known as memory discharge. In this condition the battery will discharge normally and then prematurely discharge rapidly to a secondary voltage level creating what appears as a step in the plotted voltage over time. By completing 2 full deep discharge and recharge cycles this condition can be minimized or eliminated. The process is called battery reconditioning it involves placing a large load on the battery and discharging it until the first cell reaches 1.0 volts (or 2/3 initial voltage) then reduce the load by one half. The voltage will slightly increase then decrease back to 1.0 volts again where the discharge is terminated. At this point the battery is left with no charge or load for a hour to allow the cells to stabilize. This open circuit stand is intended to allow cells to reach a chemical balance and independent cell voltages to equalize. This also minimizes the difference between the highest and lowest cell voltages, known as cell spreading. Charging current is applied and maintained until the battery reaches a full state of charge. This cycle is then repeated for a second time.

Self discharge will occur in NiCad batteries during storage periods due to the internal resistance of the battery. To overcome this affect a supplemental charge current at low level is applied to the battery known as a trickle charge.

Due to technological advancements in battery chemistry and design, driven by power storage density to weight ratio NiCad batteries have been replaced by more efficient Nickel hydrogen, and Lithium-ion batteries on GEO satellites.

Additional information about NiCad batteries is available on Wikipedia

Tilt in Earths axis

This demonstrates the tilt of 23.44 degrees in the Earths axis, it is referenced from the orbital plain, also called the Sun line. 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.

TCR Antenna pattern basics

Satellite control refers to Tracking Telemetry and Command (TT&C) as the operations interfaces to control the satellite throughout its mission life cycle. These links are maintained through the communications antennas system. For launch and contingency operations additional antennas are added to allow control links when the satellites Earth deck is not pointed directly at the Earth.

During launch and contingency operations it is essential to maintain command and telemetry links to the satellite. Until the satellite is placed into Geosynchronous orbit and the sensors are locked onto the earth it is spun at a target RPM to keep it stable through the orbit and the communications antennas are stowed and can not used.  Or in cases where Earth lock is lost the satellites communications antennas are not pointed at the Earth. For these conditions additional transmit and receive antennas are selected and positioned on the satellite to provide link coverage as close to 360 degrees around the satellite.

In this case horn type antennas with a 30 degree beam width are placed on the normally Earth facing deck and the opposing Aft deck. The  Earth facing horn antennas have a beam width that allows the operations team to stabilize the satellite and reestablish pointing control in the vast majority of cases before the satellite points away from the earth. For more saver contingencies Omni type antennas are selected for their toroidal radiation pattern and larger beam width +/- 35 degrees of their centerline to transmit and receive and are positioned for use as the offset increases to either side. The Aft antenna is selected for used when the satellite rotates  to an orientation with its back to the Earth. All of these antennas are measured at the 3dB or 6dB roll off point and will provide a diminishing signal level beyond the stated beam width. The RF engineering of these designs carefully take into account the link margins required to ensure complete 360 degree coverage.

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.

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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.