• Mayur Pawar

Do you know: How satellite systems work together to position it into the orbit?

We live in a world where Artificial Satellites are the coherent component of our lives even if it doesn't a direct part of our lives. Our most of the wireless communication done by artificial communication satellites in orbit around the Earth. There are other types of an artificial satellite, like weather satellites, navigation satellites, reconnaissance satellites, astronomy satellites and many other kinds. Let's get to the topic to understand satellite systems.

Satellite control system (SCS) is a core, essential subsystem that provides to the satellite capabilities to control its orbit and attitude with a certain performance that is required for satellite mission and proper functioning of satellite payload operation. However, the first mandatory task for SCS is assuring satellite safe functionality; providing sufficient electric power, thermal and communication conditions to be able for nominal functioning during a specified lifetime at different sun lightening conditions (including potential eclipse periods), protecting against life-critical failures proving to satellite safe attitude in Safe Hold Mode (SHM). Without SCS or satellite guidance, navigation and control (GNC) system, any Earth-orbiting satellite could be considered just as artificial space body, demonstrating the launcher capability for the satellite launch. As soon as a satellite is assigned to perform a certain space mission, it has to have SCS and a kind of special device (s)-payload (s), performing scientific, commercial or military tasks that are dedicated to this mission.

A large diversity of satellites serving for different missions is in space now. A widespread point of view is that all of them are transportation platforms delivering and carrying in orbit dedicated to the planned space mission payload system, as a VIP passenger. For example, it could be the postman for the postal horse carriage for many years ago. Namely, the satellite with its control system (SCS) provides to the payload all conditions required for the mission performance (orbit, attitude, power, pressure, temperature, radiation protection and communication with ground mission control centre (MCC)). That is why from the mission integration point of view, the SCS can be seen as the space segment integration bases that set their development and operation process in corresponding order. In turn, SCS as a satellite subsystem also can be reviled and established in satellite onboard equipment architecture, combining the group of subsystems that are dedicated to orbit and attitude determination and control tasks. It could be done rather from the System Engineering than from the commercial practice point of view and would significantly streamline satellite development order and the degree of responsibility of all the developers.

Today, for many satellites, GNC system onboard equipment subsystems, performing related functions Global Positioning System (GPS)—onboard satellite orbit and time determination, Propulsion system—orbit/attitude control system, Attitude Determination and Control System (ADCS)—satellite attitude determination and control Integration of these subsystems can be named as attitude and orbit determination and control system or Spacetronic system. Typically, AODCS includes components Onboard computer system (OBCS) or dedicated to AODCS electronic cards (plates) in Central Satellite Computer System (e.g., command and data handling computer (C&DH)), Sensors, Actuators Basic AODCS architecture is presented in Figure below. OBCS, onboard computer system; TLM, telemetry data and commands; PL- payload; PS- propulsion system; RW- inertia reaction wheels; MTR- magnetic torque rods; GPS- satellite navigation Global Positioning System; MAG- 3-axis magnetometer; SS- 2-axis Sun sensor; HS- horizontal plane sensor; ST- star tracker; RS- angular rate sensor; EP- electric power; TR- temperature regulation; VP- vacuumed protection; RP- radiation protection.

Depending on the required reliability and lifetime, each component can be a single or redundant unit. Unlike aeroplanes, the satellite is an inhabitant space vehicle that is operated from the ground. The operation is usually performed via a bidirectional telemetry radio link (TLM) in S-band (2.0–2.2 GHz). Payload data downlink radio link (unidirectional) is usually performed via X-band (7.25–7.75 GHz;). For both links, usually, the same data protocol standards are applied.

Two subsystems can be allocated in AODCS architecture, namely, orbit determination and control subsystem (ODCS) and attitude determination and control subsystem (ADCS). Practically both subsystems are dynamically uncoupled; however, orbital control requires the satellite to have a certain attitude (as well as orbital knowledge itself), and attitude control requires orbit knowledge also. Hence, orbit (its knowledge) is essentially continuously required on satellite board where it is propagated by special orbit propagator (OP). Due to orbital perturbations (residual atmospheric drag, gravity and magnetic disturbances and solar pressure), satellite orbit changes over time and OP accumulates errors; its accuracy is degraded.

Before the application of satellite onboard GPS receivers, the satellite position and velocity were periodically determined on ground by the ground tracking radio stations (GS, dish antenna), and calculated on-ground orbital parameters were periodically uploaded to satellite OBCS to correct OP, to provide available accuracy. Now with GPS satellite, the orbit can be calculated onboard autonomously, and OP can propagate data only during relatively short GPS outage periods. For some applications, orbital data uploaded from the ground still can be used, at least, for fusion with GPS-based OP. For newly developed satellites with GPS, orbit maneuvers (correction, deorbiting, collision avoidance, special formation flying and orbit servicing missions) can be executed autonomously onboard at the planned time or from ground operators using orbital knowledge and TLM commands to activate satellite orbit control thrusters.

Autonomous satellite navigation system (sensor) is the inertial navigation system (INS/inertial measurement unit (IMU)). It can be used for the determination of satellite position, velocity, orientation and angular rate simultaneously. INS is based on measuring with linear accelerometers and angular rate sensors (“gyros”) the two vectors: satellite linear active acceleration a and angular rate ω. After integration, the system provides satellite position, velocity, attitude and angular rate. It is also assumed in INS theory that the vector of Earth gravity acceleration g is not measured by the system accelerometers, but it is computed from referenced mathematical Earth gravity field model. Essential INS disadvantage is that its errors grow with time. That is why it has to be periodically corrected by such navigation aids as a pair of VMD used for the direct attitude determination.

GPS receiver is a radio range measuring device that measures the distance from the desired satellite to navigation satellite constellation (NAVSTAR, USA; GLONASS, Russia; and GALILEO, Europe) and computes its position and velocity. GPS measures the distance R (R = (Rx^2 + Ry^2 + Rz^2 )) of the vector from the desired satellite to the navigation satellite, and this system is invariant of the system orientation (satellite attitude). The distance between the desired satellite and navigation satellite is measured by measuring the time delay Δt between the time t^s of the radio pulse transmitted by navigation satellite and the reception time t^r of its reception by GPS receiver installed on the desired satellite Δt = t^r - t^s. Measuring the distance allows to determine the desired satellite relative position (relatively to navigation satellite), and using known navigation satellite position that is continuously received by the receiver for every tracking satellite in the navigation message (NM) converts it in absolute position.

The satellite propulsion system is usually designed for satellite orbital and/or angular control. In the first case, PS is commanded from the ground OC by TLM commands in some cases when satellite orbit has to be changed (orbit correction, deorbiting, collision avoidance), in the second controlled automatically from onboard AODCS. It consists of such typical elements as orbital and attitude thrusters (number and installation scheme depending on certain application), propulsion tank with associated pipes, valves, regulators, and electronics. General principles of PS act independently of the type (ion thrusters (0.01–0.1 N), liquid propellant and solid motor (100–10,000 N), cold gas (1-3 N)).

Independently of system architecture; it is separate dedicated to AODCS computer, or a special AODCS card within central satellite C&DH computer, it is the integration element of AODCS. AODCS system may consist of the computer (computer card) itself (OBC) and auxiliary intercommunication electronic units (electronic cards) AEU carrying DC/DC electric power conversion and I/O (analogue and digital) interface and commutation functions. OBC can be divided into two parts: the hardware (HW, power convertor, processor, input/output [I/O] convertors, non-volatile and volatile memory) and the software (SW, operation system [OS] and vital or functional software [VS/FS]).

What makes the satellite OBC essentially different for the aeroplane OBC is that its SW can be uploaded and updated from the ground and during operation and scheduled maintenance. OS OBC includes generic computer programs: the program of I/O interface, time schedule (dispatcher), embedded test, timer and standard mathematic functions. Satellite SW often is considered as satellite SW subsystem that is verified during development (with mathematical high-fidelity Matlab/Simulink simulators and semi-natural processor-in-the-loop (PIL) simulators). SW subsystem should be tested to meet SW requirements. The flight version of the SW subsystem is supported with operation real-time satellite simulators (RSS) located in the operation centre. It should be mentioned that only final AODCS (OBC (HW + SW), sensors, and actuators) functional test that should be performed in the Space Qualification Laboratory during satellite Space Qualification and Acceptance campaign can really minimize the risk of launching a not ready satellite and prevent against AODCS refinishing in orbit during commissioning and operation. VS can be separated into two parts, ODCS SW and ADCS SW. For both parts, I/O interface with sensors and actuators is determined in special interface control document(s) (ICD), describing the type, certain connectors, and electrical parameters of the exchanging data. These data before using them for functional tasks are pre-processed in OBC with special algorithms.

Satellite data processing unit works like:

Convert data into required physical parameters and units, taking into account certain sensor input-output scale function.

• Transform data in certain device frame and compensate device misalignment, bias and scale function errors if it is possible, monitor device state, establishing “on/off,” “work/control,” “data bad/good” flags.

• Transfer to C&DH TLM data about sensor/actuator state and their data.

• Perform some other auxiliary functions if they are required.

If orbit maneuver is required, then it can be commanded by AODCS SW autonomously, or special control commands TLM (uploaded command tables) are sending to satellite AODCS, and in predetermined time they are executed activating at the scheduled time for the calculated period Δt the orbital thrusters that provide for the required orbital correction/manoeuvre pulse( FΔt). This is how satellite knows its position and work along with the subsystems to change position if needed.

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