Subsystems

Subsystems

• Structure
• Separation System
• Communications Architecture
• Power
• Command & Data Handling Software
• GPS / Attitude Determination System
• Microdischarge Plasma Thruster (MDPT)

Structure

The constraints on size, weight, and rigidity are some of the most significant challenges associated with designing a nanosatellite. For this reason, a hexagonal iso-grid type structure composed of 6061 T-6 aluminum was chosen for the FASTRAC satellites. The mass of the two nanosatellite structures consume nearly a third of the total mass budget, weighing in at approximately 5 kg each, including fasteners. A single structure is composed of two base plates, six side plates, and six vertical square tubes and square tube inserts. The completed structures measure 20.84 cm in height and 47.50 cm in width.

The next figure shows the orientation of the two nanosatellites in their stacked, launch configuration. Once in orbit, this entire canister will be jettisoned from the launch vehicle before the satellites are exposed to space. The launch vehicle for the mission has not yet been defined. However, the FASTRAC payload is being designed for flight on the Shuttle with the intent that flight on any other launch vehicle as a secondary payload will have less stringent requirements.

Separation System

The separation system, design by Planetary Systems Corporation (PSC), will be used to eject the FASTRAC pair from the ICU canister as well as control the separation of FASTRAC-A and FASTRAC-B. Shown in Figure 4, the PSC LightBand Separation System is composed of two spring loaded rings and a motorized release mechanism.

Communications Architecture

The communications architecture is based on a system which is currently being flown on PCSAT2 onboard the International Space Station (ISS). The FASTRAC implementation consists of two transmitters and one receiver. The primary satellite uplink is conducted on the amateur 2-meter band at 1200 baud. The satellite's downlink is conducted on the amateur 70-cm band at 9600 baud. The satellite's transmitted signal is used for two primary purposes; it acts as the data downlink to the earth, as well as the umbilical data link between the FASTRAC satellites. Therefore, each satellite is also required to have a 70-cm receiver, which can also act as a secondary uplink receiver if necessary. The figure below shows a schematic of the FASTRAC satellite communication system. A single Kantronics TNC is used to decode and encode the 2-meter and 70-cm signals.

Power

The constraints for the power system have primarily been defined by stringent requirements set forth by the Shuttle. AFRL has procured and tested a set of NiCd D-cell batteries for use in space, and will supply them for the FASTRAC flight. There is also a requirement that the batteries be completed discharged at launch. Therefore, the power system will only become active after the ICU canister is ejected from the launch vehicle and the FASTRAC satellite pair has been jettisoned from the ICU. After the batteries sustain a sufficient charge, through solar charging, nominal operations will begin. A set of Spectrolab single junction GaAs/Ge CIC 19% efficient solar cells, will be used to charge the batteries. Because the bottom satellite will be connected to the ICU as well as the top satellite, it will have less space for solar cell placement and thus less total energy generation. The current design allows for 18 strings on top satellite, and 16 strings on bottom satellite. On average, the design currently collects more solar power than is required for most modes of operation. The figure below shows a schematic of the current power system design including inhibits and monitoring circuitry.

Command & Data Handling Software

The command and data handling system is based on an architecture design by Santa Clara University for their Emerald satellite mission. This system utilizes a distributed architecture based on 8-bit Atmel (AVR) microprocessors. Intersystem communication is conducted over a 2-wire I2C bus which is built into the AVR microprocessors.

GPS / Attitude Determination System

The GPS position and attitude determination system was designed and built by student researchers at The University of Texas's GPS Research Lab. The system utilizes GPS code measurements, as well as antenna signal-to-noise ratio (SNR) and 3-axis magnetometer measurements to provide estimates of position, velocity, and attitude. Each satellite will have redundant GPS receivers and dual cross-strapped antennas. A GPS simulator, shown in the figure below, will be used to test the on-orbit receiver algorithms and dual antenna switching circuit.

Microdischarge Plasma Thruster (MDPT)

The microdischarge plasma thruster was designed and built by researchers at The University of Texas's Propulsion Research Lab. The thruster channels and superheats an inert gas through a microchannel nozzle producing a microNewton level of thrust. The operation of the thruster will be automated by the spacecraft OS using the attitude measurements provided by the GPS attitude determination system. The thruster will be fired in such a way as to alter the lifetime of the spacecraft. In order to magnify the effects of the MDPT, one satellite's thruster will attempt to increase its orbit lifetime while the other is attempting to decrease its orbit lifetime. Initial investigation has shown that the thrust produced by the system will be comparable to the atmospheric drag at 300 km altitude. Since the satellites have no attitude control during the majority of the mission, multiple thruster nozzles will be utilized to increase the probability of proper thruster alignment.