Spacecraft Mission activities begin at the conception of the mission and end with the decommissioning phase. Mission activities involve incorporating various inputs and requirements during the design and developmental phase of the satellite, mission operations during launch followed by the commissioning phase, on-orbit phase management, and finally planning and executing the post-mission disposal.
Through mission planning, we translate the design into requirements for operating a spacecraft, finally leading to the realisation of the mission goals. Mission planning requires a thorough knowledge of spacecraft elements and interfaces with various entities. Spacecraft Operations involve all activities to be carried on a spacecraft to meet the mission objectives. It can be best described as a huge team activity involving mission operations teams, subsystem design teams, ground segment, and project teams. The operations procedures for nominal as well as contingency recovery are well laid out, documented, and reviewed.
The total ground segment for mission planning and operations mainly comprises elements related to ground station support for spacecraft mission operation and planning, TTC (Telemetry Tracking & Command) and payload data reception, data products generation, data interpretation, data exploitation, and data dissemination. Spacecraft operations need to be supported by a secure and reliable computer network and storage infrastructure, and reliable communication links connecting various ground stations and other facilities to the main control centre. In this chapter, we present an overview of spacecraft operations and mainly address the elements corresponding to the spacecraft control centre.
Space Mission Life Cycle
The life cycle of a space mission typically progresses through four phases:
- Concept exploration, the initial study phase, which results in a broad definition of the space
- mission and its components Detailed development, the formal design phase, which results in a detailed definition of the system components
- Production and deployment, the construction of the ground and flight hardware and software and launch
- Operations and support, the day-to-day operation of the space system, its maintenance and support
- End Of Life, phase in which the spacecraft needs to be removed from the operational Low Earth Orbit regime to avoid posing a possible collision risk to the other operational space assets as a part of the internationally accepted space debris mitigation practices
Various Mission Phases
Mission operations are organized and conducted in the following phases:
- Pre-launch simulation phase
- Launch and Early orbit Phase
- Initial phase
- Normal phase
- Decommissioning/End of Mission phase
Pre-Launch Phase on Spacecraft Mission
This phase involves the following activities
- Ensuring all hardware and software elements of the ground segment are in place as permission requirement
- Test and evaluation of all ground elements involved
- Training for the operations team to understand the spacecraft subsystem functioning and familiarize with the command sequence for various operations
- Prelaunch simulations to check the interface and compatibility of all the ground H/W and S/W elements involved, to exercise the initial sequence of operations etc.
Pre-launch simulation stages include autonomous checks at entity end and data flow checks for between individual stations. Further, checks are performed after integrating multiple network stations and finally with all supporting elements. Full Dress Rehearsals (FDR) are also performed with all entities to simulate the launch day scenario. Essentially, the following parameters are checked at the end of each simulation exercise:
- Conformance of support system and timeline of operations
- Adequacy of time allotted and time actually required for the conduct of each activity
- Adequacy of operations plan and procedures
- Proper functioning of all software elements
- Reliable and repeatable operations of all interfaces
- Possible areas of improvement and strive for the same
Launch and Early Orbit Phase (LEOP)
This phase includes
- Spacecraft power ON and initialization commanding operation
- Lift-off and Launch Phase monitoring
- Monitoring S/C Injection and solar panel deployment
- Monitoring S/C health, Test Command upload, and Ranging
- 3-axis earth/sun acquisition using thrusters
- Preliminary Orbit Determination immediately after injection
- Dissemination of orbital elements/ look angles to all tracking stations
- Critical evaluation of sub-systems by Designers
Initial Phase and Normal Phase
The initial phase starts with 3-Axes Acquisition till the start of the normal phase. The major activities performed here are:
Subsystem performance validation
- Payload commissioning
- Orbit acquisition and phasing
- Calibration exercises for Gyros and Payload systems
- Implementation of the final version of ground software for normal operations
- Tuning of alarm limits for health parameters
- Preparations for transition to Normal Phase.
This is the longest running phase, the following are the salient activities
- Real-time health monitoring of the spacecraft in all scheduled passes and analysis using the Playback data.
- Tele-commanding as per schedule
- Archival of all types of data
- Planning and execution of special operations, including Orbit Manoeuvre, Calibrations etc.
- Handling of Spacecraft anomalies and contingencies, this includes monitoring, reporting and corrective action plan execution
A typical day in the normal phase of mission operations for tracking and control involves receiving housekeeping telemetry from the satellite and analysing it for its health, performing daily routine command uplink to the satellite which includes the payload operations uplink. Special Operations like Orbit Manoeuvres (also called orbit maintenance manoeuvres) to keep the satellite within the ground track limits or maintain the phasing between satellites in a constellation are also carried out regularly. Close approach analysis to assess the collision risk of a given satellite due to other orbiting objects in its neighbourhood is carried out regularly.
An alert is issued whenever there is a likely conjunction between any of the operational satellites with any other object in space. This sometimes calls for urgent collision avoidance manoeuvre (CAM) on the on-orbit satellites. CAM can at times disturb the regular payload operations or the ground track limits. However, the design of the CAM itself in most cases is done to aid the ground track movement in a favourable direction. In case the CAM disturbs the orbit beyond its acceptable dispersion limits, a clean-up manoeuvre to nullify the effect of CAM is planned. This is usually carried out at the earliest possible opportunity after the conjunction time to not disturb the nominal mission operations.
This is the last phase of the mission life cycle.
- Post-mission disposal (removal of S/C from the useful orbital region at end of life by de- orbiting/re-orbiting)
- Electrical passivation by turning off all rotating mechanisms, battery drainage, powering off of AOCS system etc.
Space Segment refers to the satellite itself and the various elements that constitute the satellite. These can be broadly classified as Satellite Main frame and Payload systems. The Satellite Main frame consists of different subsystems like Power, Attitude and Orbit Control System (AOCS), Reaction Control System (RCS), TTC, Thermal and Sensors. Payload and Data Handling System is responsible for Payload data acquisition and dissemination to the required users to meet the objectives of the mission.
Main Frame Systems
Any electronics/electrical/electro-mechanical system needs electric power for its functioning and it is the lifeline for the spacecraft. In addition to supporting the spacecraft bus, it should also support the payloads. It should be capable of generating sufficient power till the end of life to support the full mission with sufficient margins. Sufficient storage capacity to be configured to meet the spacecraft bus load and the full payload operation during eclipse. The functional elements of the power sub- systems include Solar panels to generate power, batteries for storage and circuits for power conditioning.
Attitude and Orbit Control Subsystem
This sub-system provides the desired orientation and stability against various disturbance torques acting on the S/C. It acquires attitude data from sensors, estimates the errors, computes correction torques required and generates the control signals to energise the actuators. It also does the function of implementing and controlling the commanded change in velocity for orbit control and is coupled to other subsystems on board, especially actuators and Sensors
Sensors are instruments used for measuring the attitude errors with respect to a referenced object, for measuring the temperatures, for measuring the fuel tank pressures etc. Various types of sensors are configured in the spacecraft according to application, accuracies and attitude reference sources which include absolute and inertial Sensors like Sun sensors, Earth sensors, Star sensors, Inertial sensors etc. Temperature sensors used include thermistors, thermocouples Platinum Resistance Thermometers etc.
Telemetry Tracking and Command Subsystem
This sub-system does the acquisition, multiplexing and transmission of health data via downlink carrier with a defined data rate. It receives and decodes the commands sent from the ground to execute routine tasks, payload operations and any contingency recovery actions. It is equipped with a transponder to carry out ranging operations necessary for the ground orbit termination process
The functional elements include transmitters, receivers, coder/decoder, modulation/demodulation, encryption/decryption packages etc.
Propulsion/Reaction Control Subsystem
This system is used to provide thrust generated by momentum exchange between the exhaust and the vehicle as in a rocket to change translational velocity used for orbit control and also impart torques for attitude control/ reorientation and desaturation of reaction wheels. This system is configured either as a monopropellant system or bi-propellant system having functional elements like propellant tanks, pressurizing gas, flow control valves, latch valves, monitoring sensors, plumb lines, heaters, thrusters etc.
Thermal Control Subsystem
This subsystem ensures the required thermal environment for all the spacecraft bus elements and the payloads taking into account the individual power dissipation of equipment, heat absorption from the Sun, heat absorption from Earth (IR and albedo), heat lost to space through radiation etc. This is implemented using active elements (heaters, thermostats, Peltier coolers, heat pipes) and passive elements (Coatings & paintings, multi-layer insulation (MLI Blankets), Optical Solar Reflector (DSR), Grease, Space radiators, Cold plates, Thermal isolators)
Payload and data handling Subsystem
This subsystem has the function of collecting the instrument data, formatting the data along with auxiliary information, coding the data, storing the data on-board, and playing back the data to the ground. The functional elements include multiplexers, data formatters, solid state recorders etc.
Spacecraft Control Centre Elements
The ground segment consists of all ground-based facilities required to operate the spacecraft. It mainly comprises a network of ground stations, spacecraft control centre (SCC), payload data collection centre, and computer and communication network interfacing them. The spacecraft control centre is the hub of all activities related to the conduct of mission operations through suitable interfaces with various supporting elements, namely spacecraft operations software and hardware, telecommand and telemetry systems at various ground station locations, network control centre, communication links, computer systems, flight dynamics system, scheduling system, and space situational awareness control centre.
Spacecraft Health Monitoring and Command Operation.
The spacecraft health monitoring and control operations at the satellite control centre are carried out to ensure the normalcy of the health of the spacecraft and enable payload services to the users till the end of mission life. This is done by health monitoring on a round-the-clock 24×7 basis in real- time and offline modes, uplinking the commands for payload programming to operate payloads, carrying out attitude and orbit maintenance and recovering spacecraft in case of spacecraft contingencies.
There are several hardware and software packages with interfaces designed and implemented to carry out health monitoring and commanding activities.
The hardware is specifically suited for real-time data processing, dissemination and displaying. Various ground software are implemented to process the raw spacecraft data, display, analyze, archive, disseminate etc. Ground automation is implemented for all the tasks like health monitoring
in real-time as well as offline and generating alerts, preparing and verifying the command sequences and transmitting the commands in auto mode once the prerequisites are met, collecting the various types of data downloaded and archiving in the storage etc. AI/ML engines can also be utilised to determine the health of the spacecraft. The entire spacecraft data is trained using various models and different sub-systems are modelled, the trends and anomalies are captured by the models. Typical examples of anomalous behaviours are shown below.
Operational constraints that need to be considered for planning operations include
- The visibility duration of the supported ground stations
- Maximum command upload rate
- Data download rate and on-board storage: limits the amount of data that can be recorded on- board and played back/downloaded over a finite time
- Multi-satellite support- constrains dedicated support for a given satellite
- Eclipse – drainage of battery and power related constraints for payload operation duration, high-torque operations etc.
- Attitude maintenance in case of Sun-moon intrusion in sensor Field Of View
- South-Atlantic-Anomaly for high-voltage payload operation
- On-board failures imposing additional constraints
Such constraints add complexities to both planning and execution of mission operations.