A Small Satellite Design for Deep Space Network Testing and Training
Dennis McWilliams, et. all
May 8, 1993
Mankind's quest for knowledge of space has led scientists to loft expensive, complex space probes that gather a multitude a data. Current spacecraft can gather more data than their communication system can transmit during limited transmit times. In order to increase efficiency as well as the amount of data a satellite can retrieve, faster data transmission methods must be found. The fastest data transmission rates now occur over X-band. However, a debate by scientists currently focuses on whether NASA should switch to another radio frequency, like Ka-Band, or if they should move on to a more advanced scheme such as laser communication. Mockingbird Designs hopes to aid the debate by studying Ka transmissions. Although the Ka-Band could offer higher data transmission rates, the effect of the atmosphere on the signal is unknown. Therefore Mockingbird designed and plans to produce a small satellite, named BATSAT, that will transmit a test signal over both Ka and X bands to study atmospheric signal attenuations.
The BATSAT satellite incorporates small satellite design principles to provide a simple, low-cost design ideal for university production and testing. The satellite is an 0.64 m diameter space-frame truss structure built of 6061-T6 aluminum truss members. The skin panels will be built of a honeycomb sandwich material, which will be mounted the outer truss structure using locking bolts. In lieu of an axial strut to support launch loads, an inner strut configuration will provide stiffness. The inner strut design also provides more space inside the spacecraft for component placement. The components will be placed in an insulated disk that will be isolated from the structure using nylon cords. Using a NASTRAN structural model, Mockingbird found that the spacecraft has a large structural design margin while maintaining a low mass. This could be very beneficial if one wished to increase the component mass by adding scientific instruments.
The communication system includes Ka and X-Band transmitters that are used to carry the test signal. Mockingbird also plans to include an S band transmitter that will be used to help train DSN personnel. By purposely turning certain antennae off, DSN trainees could determine the attitude motion of the satellite via blinking techniques. The spacecraft will transmit the 1 mW test signal over four omni-directional antennae provided for each band. The increase in signal interference due to these antennae can be compensated for using filter techniques available at DSN, but most importantly these effects are distinguishable from atmospheric effects. Commands will be up-linked via an X-Band transponder and a step logic circuit will change the spacecraft configuration for different subsystems. Also, to insure that the satellite can always be in a known state, a reset command was designed to return the satellite to an initial state.
Thermal control is often a problem for small satellites. To compensate, Mockingbird Designs cold biased the BATSAT satellite. The outer surface will be covered with solar arrays, and all remaining surfaces will be painted white. This prevents the satellite from exceeding hot temperature limits. To prevent certain satellite components from getting to cold, small thermal strip heaters with a thermostat will be used to heat up components. Since all components will be placed in a circular disk, components can also be warmed via thermal blankets. Also, the circular disk can be radiation hardened to increase satellite reliability.
When the satellite is transmitting over both X and Ka-Bands with heaters on, it will need 8.6 W of power. This maximum power requirement will be fulfilled using silicon solar arrays and nickel-cadmium batteries. The solar arrays will be surface mounted to the honeycomb skin structure to offer omni-directional solar flux coverage. A peak power tracker will be used to smooth out the power from the arrays. Both the solar arrays and the batteries are designed for a 10 year lifetime. Four batteries will be used: one for the first five years, one for the second five years, and two redundant batteries.
The BATSAT satellite will be lofted into an 1163 km, 70¡ orbit by the Pegasus launch system. This orbit fulfills the DSN dish slew rate requirement of .4 deg/s while also keeping the satellite out of the heaviest regions of the Van Allen radiation belts. Currently, only the Goldstone, California DSN dish can receive in Ka-Band, but the Canberra, Australia and the Madrid, Spain dishes will soon be upgraded. Each of the three DSN stations capable of receiving Ka-Band (Goldstone, Canberra, and Madrid) will have an average of 85 minutes of view-time per day. A seven day repeating groundtrack will allow for easy planning and view time estimates. The Pegasus launch system by Orbital Sciences Corporation will provide the launch services for BATSAT. Pegasus offers the lowest priced commercial launch option while providing many performance options that decrease satellite complexity. While the Pegasus system is relatively cheap (around $9 million), Mockingbird Designs is researching cheaper options such as shared launch and military surplus.
The current BATSAT design offers an excellent spaced-based platform for testing the merits of Ka-Band transmissions. The satellite fulfills the mission goals while keeping complexity and costs down. However, the most important aspect of the BATSAT satellite is its receptiveness to university design. By using students to design, fabricate, and test the satellite structure, costs can be kept to a bare minimum. Many of the electronic components have been built by other university groups and can be adapted to the BATSAT design. Even though this satellite was designed specifically for the DSN/JPL testing project, the small satellite can easily be modified to carry scientific instruments. Mockingbird hopes that other missions will wish to use the BATSAT design for their small satellite applications.
CSR/TSGC Team Web