E. Hayun, A. Rokach, A. Fertman, G. Hajaj, H. Werner, Y. Cohen, A. Skorohod, A. Novikov, V.Shimankov, N. Sheinberg, S. Goldin
Faculty of Aerospace Engineering, Technion – Israel Institute of Technology
Dr. David Mishne, Dr. Fred Ortenberg
Asher Space Research Institute, Technion
The scientific objective of the student’s project mission LUNGRA (LUNar GRAvity) is the precise mapping of the lunar gravity field, based on satellite-to-satellite tracking. Theoretical and experimental studies have established that two or more co-operating satellites can provide more accurate gravity model. The educational goal of the LUNGRA student project was to demonstrate capability in the development of a very low mass, low power, low volume, and a low cost nanosatellite, while fulfilling an important and ambitious scientific mission.
The concept of two satellites formation flying is used in the LUNGRA student project. The daughter satellite LENS (Lunar Experimental Nano Satellite) will be externally attached to the mother spacecraft, and released upon arrival to the moon. The LENS satellite will be placed in the same orbit as the mother spacecraft, at a distance of 50 km. The measurements performed on-board the lunar orbit include the relative range-rate between the satellites, acquired by laser. The data will be down-linked to the Earth, through the mother spacecraft, and processed together with ground-based measurements of the positions of the satellites. The interface unit CLI with laser transmitter and receiver optical assembly, mounted on gimbals, is fixed on the mother spacecraft. The LENS satellite is equipped with a laser retroreflector, which allows the relative range-rate measurement.
A lunar circular polar orbit with altitude 100km was chosen. A study of the performance and operational analysis (stability, maneuvers, inter satellites distance, etc) were performed using by STK-6 simulations, using lunar gravity model with 70×70 coefficients. From range-rate laser measurements between two satellites, other measurements, such as attitude, absolute position and velocity, the coefficients of the gravity potential can be determined.
During the design process, the students performed several scientific and technical iterations from initial concept to preliminary design and development of both the LENS satellite and the CLI interface. The students gained valuable hands-on experience in the design and application of nanosatellite technologies, on- board laser and other modern space microtechnology. The team performed analysis of several alternatives for the LENS Attitude Control Systems (ACS), like gravity gradient, 3-axis, spin stabilization. An ACS based on 3-axis stabilization was chosen and developed. The LENS main operational subsystems (power, propulsion, telemetry, command and communication, on-board data handling) were also conceived and developed during the project. Preliminary architecture of the LENS, satellite detailed design, structure strength analyses were implemented. The estimated mass is less than 10 kg, with dimensions of 300X250X200 mm (Height X Width X Depth). The average power is about 6 Watts. Launch opportunities and maneuver control in orbit were also considered. Preliminary CLI design was carried out. The measurement technique, based on a laser transmitter/receiver mounted on the CLI, together with a retro reflector mounted on the LENS, is described in details. Manufacturing issues, product safety and cost analysis were briefly examined. The outcome of the design demonstrates that lunar experiment aimed to obtain precise gravity potential could be realized on the basis of nanosatellite and laser ranging technique. We appreciate the continual support of Prof. M. Guelman. Thanks for STK-6 assistance to M. Glikman-Raich.
Inner View of the LENS nanosatellite (without body mounted solar panels)
Results of LUNGRA orbital dynamic computations are presented in the article published in AGI’s December 2005 In-View newsletter, as an example of successful STK application www.agi.com/inview :