Without the electronic brain of the system, our satellite would merely be an expensive piece of space junk. The microcontrollers and internal processing units are responsible for all control and command of the satellite. Due to the limited amount of actual contact time with the satellite, all communications, diagnostics, power management, and data collection have to be fully automated and dependable.

Considering our very strict space and size limitations, we need a processing unit that is versatile, yet small. To this end, we took advantage of the recent improvements in circuit miniaturization. For the email server we are using a miniature 386 processor with 256 Mbytes of flash memory and a DOS 3.0 operating system. We purchased this board, which is merely 2" x 4", from Jkmicro, a company which supplies miniature computer parts for many different applications

Fig. 7—386 computer on a board.

The 386 computer uses 3.17 Watt hours per day in order to operate. So, in order to limit power consumption we programmed two 16C774 Motorola microchips to handle the majority of satellite control and maintenance. The first microchip, Brain, handles data collection, component deployment, internal communication, and memory storage. The second microchip, Pinky, handles all diagnostics, power management functions and Ham radio communication. This microchip will be able to disconnect different satellite subsystems from the main power source based on power availability and need.

On Board Communication System

Our on board communication system consists of two parts. The first and most heavily used is an Inter-Integrated Chip, or IIC, bus. This bus functions much the same way as the main bus on a personal computer and allows data transmission between the different microcontrollers, the real time clock, the secondary switching microchip, and the four memory chips. It can be controlled by either Brain or Pinky and can pass data to or from any of the other devices on the bus to those two chips.

The second major form of onboard communication is a serial link between the 386 and Brain. This link will be used to pass data that we need to store for later transmission to the base groundstation. Because the 386 can access 256 MBytes of data and the off chip memory can only store 256 KBytes this link will allow us to store significantly more information than we could if we used the off chip memory only.

The microchips will have Ham communication with groundstations at a rate of 120 bytes per second. This interface, which is directed through Pinky, will be used to transmit control and command codes as well as to download diagnostic data from the satellite. All major functions, including deployment, data collection rate, and power control, will be ground modifiable at any time when the satellite is over the main groundstation. This system will also be used to transmit a beacon during the early stages of the satellite's mission in order to facilitate location of the satellite before it's path is completely mapped.

As mentioned above, the 386 processor will handle the e-mail server. When the satellite passes over a ground station a series of events occurs. First, Brain will detect the initiation signal from the 900 MHz spread, sent from the ground station, and send a signal to Pinky to turn on the 386. This signal will be transmitted at 115 Kbaud, or roughly 11 Kbytes, per second. Once the processor is running, it sends a signal down to the ground station asking for the user ID and password. If a valid ID and password are received, the 386 sends a signal telling the ground station that they are connected. The server then takes all the e-mails for that user from memory and sends them over the spread along with header information describing the size and type of message. It then waits for an acknowledgement signal from the ground indicating the full file was received. If the packet size received matches the size sent, it is assumed the email was error free. This is a safe assumption due to the 32-bit CRC check performed by the spread radios. If the two sizes do not match, then the server continues to resend the entire packet until they do.

After downloading has completed successfully, the satellite server begins uploading e-mails from the ground. The same bit testing and correction methods are used as in the downloading, and files are resent if corrupt. The 386 then takes the messages and stores them in the correct destination folder for the given user space. Once the satellite passes over the main ground station, at Taylor University, all pending e-mails are downloaded and sent over the Internet. Also, all external e-mails from the Internet are uploaded and stored in the correct folders. A visual representation of this can be seen in Fig. 8 below.

Fig. 8—Simplified picture showing how the TUSAT1 email system will work.

Due to power restrictions, the 386 e-mail server as well as other subsystems on Taylor's satellite (including the radios and various scientific instruments) will need to be powered down. Pinky will control all of this as well as handle battery cycling. It will also control power allotment with a secondary microchip that it will control through the use of our onboard IIC bus. This secondary microchip will have 15-17 digital pins, which it will use to switch several NEC DIP solid state relays controlling the power flow throughout the satellite. Pinky will make all power priority decisions using data it will collect relating the current and voltage needed by all systems as well as that available in the system. It will collect this data through the use of amperage and voltage probes placed on the same line as the batteries and solar cells (see Fig. 6). All power control decisions are fully automated, but are also modifiable from the ground during those periods when the satellite is over Taylor.

The main purpose of Brain is to collect scientific data for later evaluation at Taylor. Our main type of scientific data is through our onboard plasma probe. This device will report plasma density and temperature, which will be stored temporarily on off chip memory through the IIC bus. Once the 256 KBytes of off chip memory storage fills, the data will be passed through Brain to the 386, which will store the data until it is transmitted to earth.

The other major type of scientific data we are collecting is from the magnetometer. This device will give us information on the direction and intensity of the magnetic fields in the x, y and z axis. This data will be stored in off chip memory for transmission to earth and used to calculate our rotation relative the earth.

Another major system the microchips control is the real time clock. This device allows us to keep track of the actual time of day by tracking the months, days, hours, minutes and seconds. This information will be stored along with all scientific and diagnostic data in order to mark when the data was taken.