Mission operation (EO-LEO mission)
Stage I – communication
The operation of a satellite in orbit begins with the moment of deploy – the ejection of satellites from the rocket, which gain the status of separate, independent objects belonging to companies and institutions from all over the world. At the time of the satellite’s deployment, a so-called power switch is turned on, which allows the satellite to go from unpowered mode to stand-by mode, the satellite begins to charge using solar panels. Once the batteries powering the entire system obtain a sufficient level of charge, the stage of communication with the satellite begins. Determining the location of the satellite at this stage is not practically possible, so the satellite transmits a signal that attempts to be picked up by a ground station. The orbit in which the satellite is located is not yet determined, but it is possible to determine approximately when to expect the next communication windows after receiving the first signals (beacons). Once the so-called two-line orbital elements (TLEs) have been determined by the satellite’s external detection and localization systems, orbital simulations are made so that communication sessions can be carefully planned, with commands sent to the satellite in the first phases to test launch other subsystems and collect logs to assess whether the subsystems are working properly.
Stage II – positioning / detumbling
Most satellites are equipped with an Attitude Determination&Control System (ADCS). The advancement of such systems can be at different levels. Generally, in micro and lighter satellites, these systems are not critical. However, if the purpose of the mission is EO imaging, then such a system must allow relatively high control over the satellite’s orientation.
◼️ Determination – that is, establishing what position and orientation the satellite is in. This is done using measurements from sensors such as gyroscopes, accelerometers, magnetometers, sun and star sensors.
◼️ Control – that is, controlling this position and orientation in coupling with the above measurements. Control over the orientation (that is, de facto rotation) of the satellite is most often realized by systems of so-called magnetorquers (special electromagnet) and reaction wheels. With their help, torque is generated and allows the satellite to aim at the target. In the case of larger satellites, small thrusters are also used, which, in addition to orientation, can also affect the change of the orbit profile.
Our satellite has just entered the phase of first attempts to gain control of its orientation. The power subsystem is running at full steam, regular communication sessions are being recorded successfully, while telemetry from the on-board computer indicates stable operation of the satellite.
Detumbling, as this phase is expertly called, is the phase where the angular velocity imparted by the moment generated at the so-called deploy (release of the satellite from the rocket) is lowered. The initial angular velocity usually does not exceed a few degrees per second. Detumbling was realized with magnetorquers, which will allow stabilization of the position and control of the satellite to a certain extent. In order to achieve much greater control over orientation and greater accuracy of targeting, next steps will be attempted using reaction wheels.
Stage III – imaging
To begin with – what conditions should be met to perform imaging in EO-LEO missions:
◼️ we know where the satellite is located and can determine its position in orbit with high accuracy and simulate where it will be in the near future
◼️ we have measurement and control of the satellite’s orientation, to properly point at the target
◼️ based on reference data, we know at what sensor settings the image acquisition is to take place
◼️ on the basis of weather data, we can estimate the risk whether during imaging there is a chance of high cloud cover, which in the case of VIS-NIR imaging can obscure the image of the target
Imaging operation begins with the system user himself, who defines what place on earth he would like to image, under what lighting conditions it is to be imaged, and other aspects, such as what elements of the scene are most important. With such input, the Mission Operations team simulates the orbit propagation, based on which it can determine when the satellite will be in the given conditions. Once we have gained knowledge of the most temporal imaging windows, then we can move on to the next steps. By this we mean preparing a simulation of the sensor’s operation in the given lighting conditions and determining the imaging angles and associated coordinates.
Once the commands and scripts for the on-board computer are properly prepared, they are sent via the ground station to the platform. Thanks to them, the satellite will be able to enter the imaging mode and properly prepare for the target imaging operation. Excluding operations related to the designation of imaging windows, the whole operation takes no more than 1 day.