De-Orbiting Control and Re-Entry of a CubeSat

Andrea Corte-Real, University of Porto

Abstract

This paper presents the approach for the de-orbiting and re-entry of GAMASAT, a partnership between University of Porto and TEKEVER to launch a CubeSat in the scope of the QB50 mission. This CubeSat will deploy a re-entry capsule that carries and will further have communications, navigation, attitude determination and atmospheric study subsystems. The approach for the re-orbiting control of the 3U GAMASAT is to periodically actuate on the drag based on a continuously running landing point forecast algorithm and then release the capsule shortly before re-entry. The control will be made according to the navigation data obtained from GPS and targets a short list of re-entry points that ensures safe landing. The Attitude Determination and Control System (ADCS) will maintain the CubeSat aligned with the velocity vector within an error envelope of 5o with the science payload facing the apparent wind. Once it reaches below 250Km and until the 120km, specific reaction wheels will change the satellite exposed area in a few seconds and adjust the ballistic coefficient with a factor of 3. Actuation will occur in cycles of 30 minutes, each cycle initiating with a new GPS reading. One of the larger faces will be exposed to the apparent wind during a Ton time interval and then the satellite will be returned back to the normal attitude. The magnitude of Ton will be computed at the beginning each cycle, by the algorithm that uses the information collected from the updated TLE and predicts the landing spot. A simulation routine computes a landing location for given Ton and given density variations of the atmosphere. This algorithm computes assuming that the following Ton intervals have a average value of 25% of the cycles and that the capsule is released at 120km. This algorithm results from simplifications of more elaborate procedures and corrective parameters, yet giving a sufficiently accurate forecast. The optimal Ton is chosen as the minimizer of a cost function that includes a blend of average landing (stochastic solution) and also a worst-case scenario for the possible drag variations. The re-entry spot needs to guarantees the safety and accessibility to allow for its recovery. For an object’s landing to be considered safe on the ground, its energy is limited to 15 J. A capsule with 0.1 Kg has a estimated kinetic energy around 14 J, which is to close to the limit. Since, for this project the capsule will about 0.15 Kg, the algorithm will assume a re-entry with splashdown on water. Therefore it is planned to land within the Exclusive economic zone (EEZ) of Portugal exclusive of the Atlantic. The re-entry will be performed by the capsule that contains a SDR radio transmitter, another GPS receiver, batteries, sensors and a passive dumping actuator. The landing will occur on the sea and for it recovery the communication will be performed through UHF ARGOS messages for location determination. The capsule will have the general shape similar to the successful Apollo 11 with a 9.5 cm diameter and height about 6.5 cm. It will weigh about 0.15 kg, most of it due the ceramic material that will protect the interior with little ablation. A cork composite ablative material will cover it.

 
Aug 10th, 10:15 AM

De-Orbiting Control and Re-Entry of a CubeSat

This paper presents the approach for the de-orbiting and re-entry of GAMASAT, a partnership between University of Porto and TEKEVER to launch a CubeSat in the scope of the QB50 mission. This CubeSat will deploy a re-entry capsule that carries and will further have communications, navigation, attitude determination and atmospheric study subsystems. The approach for the re-orbiting control of the 3U GAMASAT is to periodically actuate on the drag based on a continuously running landing point forecast algorithm and then release the capsule shortly before re-entry. The control will be made according to the navigation data obtained from GPS and targets a short list of re-entry points that ensures safe landing. The Attitude Determination and Control System (ADCS) will maintain the CubeSat aligned with the velocity vector within an error envelope of 5o with the science payload facing the apparent wind. Once it reaches below 250Km and until the 120km, specific reaction wheels will change the satellite exposed area in a few seconds and adjust the ballistic coefficient with a factor of 3. Actuation will occur in cycles of 30 minutes, each cycle initiating with a new GPS reading. One of the larger faces will be exposed to the apparent wind during a Ton time interval and then the satellite will be returned back to the normal attitude. The magnitude of Ton will be computed at the beginning each cycle, by the algorithm that uses the information collected from the updated TLE and predicts the landing spot. A simulation routine computes a landing location for given Ton and given density variations of the atmosphere. This algorithm computes assuming that the following Ton intervals have a average value of 25% of the cycles and that the capsule is released at 120km. This algorithm results from simplifications of more elaborate procedures and corrective parameters, yet giving a sufficiently accurate forecast. The optimal Ton is chosen as the minimizer of a cost function that includes a blend of average landing (stochastic solution) and also a worst-case scenario for the possible drag variations. The re-entry spot needs to guarantees the safety and accessibility to allow for its recovery. For an object’s landing to be considered safe on the ground, its energy is limited to 15 J. A capsule with 0.1 Kg has a estimated kinetic energy around 14 J, which is to close to the limit. Since, for this project the capsule will about 0.15 Kg, the algorithm will assume a re-entry with splashdown on water. Therefore it is planned to land within the Exclusive economic zone (EEZ) of Portugal exclusive of the Atlantic. The re-entry will be performed by the capsule that contains a SDR radio transmitter, another GPS receiver, batteries, sensors and a passive dumping actuator. The landing will occur on the sea and for it recovery the communication will be performed through UHF ARGOS messages for location determination. The capsule will have the general shape similar to the successful Apollo 11 with a 9.5 cm diameter and height about 6.5 cm. It will weigh about 0.15 kg, most of it due the ceramic material that will protect the interior with little ablation. A cork composite ablative material will cover it.