LionSat (Local Ionospheric Measurements Satellite) is a new student project at Penn State.  This mission is a part of the University Nanosat Program in which students from diverse technical backgrounds design and build a flight-worthy spacecraft that will measure properties of the ionosphere.  If the satellite is successfully built, it may be launched into space aboard a shuttle or other space vehicle.

The primary objectives for LionSat are to:

●  Map the ram and wake plasma structure surrounding a small satellite

●  Collect data on ionospheric plasma in a variety of  locations in low Earth orbit

●  Test, on orbit, a miniature RF ion thruster

     The first task is to determine the controls and power of Control Unit-A and build a complete working prototype.  Power Control Unit-A is a power subsystem which charges the satellite's auxiliary power source.  Current taken from the solar array is used to charge batteries.  The charging rate is determined using voltage and temperature measurements from the batteries.  There is an 80% chance that the NiCad batteries will be used, therefore the design will be completed according to the specification for this battery type using 10 cells in series to produce 12 volts.

      LionSat, once launched, will not be able to be maintained.  The design of the power control units had to be such that they could stand up to the harsh space environment.  All of the parts chosen for the design have space rated parts available that have similar electrical characteristics and are radiation tolerant.  As a safety requirement, all power systems must be completely discharged upon launch.  Therefore, the design must be able to function when starting from a state of zero charge. 

     This figure is a block diagram of Power Control Unit-A.  This control unit serves as a battery charger and monitor.  The primary power for the satellite comes from a solar array affixed to the exterior.  When the Earth is between the sun and the satellite, a series of NiCad batteries are used to power the satellite's systems. 

     Power Control Unit-A uses a 555 timer to pulse width modulate the voltage sent to the batteries.  The duty cycle of the oscillation is determined by current, voltage, and temperature readings taken from the battery cells.  An analog computer based on a series of op amps determines the best charging scheme for the batteries based on the temperature and state of charge of the cells.

     In the above schematic, the 555 timer is the lm555 by National Semiconductor.  There is also a JAN spec 555 timer available from National Semiconductor which is space rated.  The op amp is the Analog Devices op400 which consumes an extremely low amount of power.  Maxwell Technologies produces a radiation hardened op400 which is identical to the Analog Devices prototype version.

     This figure is a block diagram of Power Control Unit-B.  This control unit switches the power supplied to the low duty cycle loads aboard the satellite.  This is done for power conservation reasons.  This unit also uses DC/DC converters to supply the proper voltages to the various loads.  The primary power bus on the satellite is 12 VDC which is accomplished using a string of ten 1.2 volt NiCad battery cells.

     This diagram shows the power coming into the control unit from the 12 volt bus.  DC/DC converters are connected to the bus to create supply lines of varying voltages.  These supply voltage lines then run to power MOSFETs which act as power switches.  Controlling the MOSFETs is an 8-bit serial to parallel shift register.  The shift register accepts a serial data line and clock signal from LionSat's main CPU.  The control scheme set by the main CPU is implemented by manipulating the serial data line and clock to switch the power of the MOSFETs.

     The shift register used for prototyping is a Texas Instruments cd74act164 serial in/parallel out 8-Bit shift register.  The flight version of this chip is made by Intersil and is the model hcs164ms radiation hardened shift register..  The pin configuration and electrical performance is identical.  Also shown are power MOSFETs.  The prototype versions are the International Rectifier irfb23n20d, which are 200V high power n-channel MOSFETs.  For the flight version, the International Rectifier irhn7250 are radiation hardened and almost identical in electrical characteristics.

                                                Above is a more complex schematic of Power Control Unit-B

     Penn State LionSat Website