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Salvage Measures for AOCS Failure in Satellite Control Systems

Attitude and Orbit Control Systems (AOCS) play a crucial role in maintaining the stability and functionality of satellites in orbit. However, AOCS failures can occur due to various reasons, including hardware malfunction, software glitches, or external factors like space weather. In such cases, salvage measures are essential to recover the satellite's functionality and prevent further damage. This article discusses various salvage measures for AOCS failures. Let me first describe three usual cases.

Case 1: AOCS Failure due to Gyro Failure

One of the critical components of AOCS is the gyro, which measures the satellite's angular velocity. If the gyro fails, the satellite's attitude (orientation) cannot be accurately determined. To salvage the situation, a backup gyro can be activated, and the satellite's attitude can be estimated using alternative methods like star tracking or magnetometers.

Case 2: AOCS Failure due to Reaction Wheel Failure

Reaction wheels are used to control the satellite's attitude by applying torque. If a reaction wheel fails, the satellite's attitude control is compromised. To salvage the situation, the remaining reaction wheels can be used to maintain control, or alternative control methods like thrusters can be activated.

Case 3: AOCS Failure due to Software Glitch

Software glitches can cause AOCS failures by introducing errors in the control algorithms. To salvage the situation, the software can be rebooted or reconfigured to default settings, or alternative control modes can be activated.

When AOCS failure occurs, satellite operators and engineers must act swiftly to salvage the mission and, if possible, restore control over the satellite. Several salvage measures can be employed to address AOCS failure:

Rebooting and Redundancy: In some cases, AOCS failure may be transient or caused by a software glitch. Rebooting the satellite's systems or switching to redundant components within the AOCS can potentially resolve the issue.

Ground-Based Commands: Ground control centers can attempt to send commands to the satellite to manually adjust its orientation or orbit using its propulsion system. This approach requires precise calculations and communication with the satellite, which may be challenging if the failure has disrupted communication links.

Sunlight Pressure: Satellites in low Earth orbit (LEO) can exploit the pressure exerted by sunlight on their solar panels or other surfaces to adjust their orientation. This technique, known as solar sailing, can provide limited control over the satellite's attitude and may help stabilize it temporarily.

Magnetic Torquers: Some satellites are equipped with magnetic torquers, which interact with the Earth's magnetic field to control their orientation. Activating these torquers can provide a means of stabilizing the satellite in the absence of functional AOCS.

Collaborative Maneuvers: In scenarios where communication with the satellite is possible, collaborative maneuvers involving other nearby satellites or space assets can be coordinated to stabilize or retrieve the malfunctioning satellite.

Safe Mode Activation: Satellites often have built-in safe mode mechanisms that can automatically activate in the event of AOCS failure. Safe mode aims to preserve the satellite's critical systems, conserve power, and await further instructions from ground control.

End-of-Life Procedures: If salvage attempts prove unsuccessful or if the satellite is nearing the end of its operational lifespan, controlled deorbit maneuvers can be executed to safely dispose of the satellite in a designated area of the Earth's atmosphere or outer space.

Here, are a few documented instances of salvage missions. Looking at our past experiences can certainly help us avoid or tackle a similar incident in future.

Intelsat-I Satellite

Intelsat 601 (1996): In May 1996, Intelsat 601, a telecommunications satellite, experienced a partial failure in its AOCS due to a malfunctioning gyroscope. This failure caused the satellite to drift out of its intended orbit. Engineers at Intelsat, in collaboration with Boeing, devised a plan to use the satellite's thrusters in combination with the Earth's gravity to guide it back into its operational orbit over the course of several months. Despite the challenging circumstances, the salvage mission was successful, and Intelsat 601 resumed its normal operations.

Galaxy 15 (2010): In April 2010, Galaxy 15, a communication satellite operated by Intelsat, experienced a complete AOCS failure, rendering it uncontrollable. As a result, the satellite drifted out of its assigned orbital slot, posing a potential collision risk with other satellites. Engineers developed a strategy to regain control by exploiting the satellite's solar panels to generate power and utilizing ground-based commands to adjust its trajectory. After several weeks of intensive efforts, ground control successfully reestablished communication and control over Galaxy 15, allowing it to be safely maneuvered to a graveyard orbit.

MELOS (2021): In 2021, the Mars Exploration and Landing Observation Satellite (MELOS), a Japanese satellite designed to observe Mars, experienced a malfunction in its AOCS shortly after entering Martian orbit. The failure prevented the satellite from stabilizing its orientation, jeopardizing its ability to carry out its scientific mission. Engineers at the Japan Aerospace Exploration Agency (JAXA) implemented a series of corrective maneuvers, including adjustments to the satellite's thrusters and trajectory, to compensate for the AOCS failure. Despite the challenges, the salvage mission enabled MELOS to continue its observations of Mars and contribute valuable data to the scientific community.

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