7.5.1 SOM Hardware

Asterius uses a distributed bus architecture. Figure 7 shows a schematic of the bus and the various nodes. For redundancy, there are two buses. All components can communicate through both buses, eliminating the possibility of single point failure.

The different components that communicate through the bus are classified into five types based on function: interfaces to other components, housekeeping devices, on-board computers, memory storage devices, housekeeping devices, and the timers.

7.5.1.1 Interfaces to Other Components

Most of the navigation and scientific instruments on Asterius do not connect directly to the bus. Many will be off-the-shelf instruments that communicate using their own protocol. The interface is the device which is compatible with the bus protocol on one end, and the instrument's protocol on the other.

Many of the instruments use a standard RS-232 serial line interface. Other instruments, especially the high-data-rate instruments, use faster interfaces, such as parallel-line interfaces. Custom built instruments may have their interfaces built directly into their electronics. Components such as actuator motors and rockets are interfaced to the bus by the kinetic controller and monitor unit (KCM unit). The KCM unit sends high-level discrete commands to activate the engines and motors, and then collects telemetry on their status.

The interface to the communications subsystem is more complex than the other interfaces. It must validate and decode the uplinked information before transmitting on the bus. It must also fix simple bit errors. Although the on-board computer could provide this functionality, the communications interface provides it so that Asterius can be controlled even if the computer becomes inoperable for some reason.

7.5.1.2 Housekeeping Devices

Various devices monitor Asterius' health. Such housekeeping devices include thermostats, ammeters, strain gages, and fuel gages. They are located throughout the spacecraft. Generally, they collect analog data which is converted to digital data by their bus interface.

  

Table 10: Computer Resources Estimate: Functional Breakdown

Function	Code	Data	Throughput
	(Kwords)	(Kwords)	(KIPS)
Command Processing	1.5	6	11.5
Telemetry Processing	1.5	3.75	4.5
Autonomy	22.5	15	30
Fault Monitors	6	1.5	22.5
Fault Correction	3	15	7.5
Power Management	1.8	0.75	7.5
Thermal Control	1.2	2.25	4.5
Rate Gyro	1.2	0.75	13.5
Sun Sensor	0.75	0.15	1.5
Star Tracker	3	22.5	3
Kinematic Integration	3	0.3	22.5
Error Determination	1.5	0.15	18
Thruster Control	0.9	0.6	1.8
Reaction Wheel Control	1.5	0.45	7.5
Ephemeris	5.25	3.75	6
EuropaCam	2	4	3
Acoustics	2	25	40
Seismometer	4.5	3.5	10
Dust Detector	2	2	1.5
Energetic Particle Detector	2	2	1.5
Metastable Ionization Det.	1.5	4.5	0.5
Magnetometer	0.3	0.15	1.5
MicroWISPCam	3	4	2
Compression	8	2	10
Gages	1.5	4	3
OS Executive	7	4	120
OS Kernel	16	8	-
OS I/O	4	1.4	400
OS Built-in-tests	1.4	0.8	0.5
OS Utilities	2.4	0.4	-

7.5.1.3 On-Board Computers

Asterius carries an on-board computer. It is responsible for command and data processing, attitude control, flight operations, and payload operation.

The computer's baseline throughput is 3.05 million instructions per second, and its baseline memory is 24 megabits (about 15 million 16-bit words). A functional breakdown yielded these numbers. Table 10 lists the memory and throughput estimations for various functions. Because of our ignorance, we can expect the total resources required by launch to be double what the functional breakdown yields. In addition, 30%-50% of the computer's resources should be reserved for spare capacity [3, p. 624]. Therefore, the values yielded by the functional breakdown were multiplied by 4 to obtain the baseline estimate.

7.5.1.4 Memory Storage Device

Asterius uses two solid-state recorders as its baseline memory storage devices. The solid-state recorder was chosen because of its significant benefits over magnetic tape: random access, variable-speed access, and lack of moving parts [9]. We felt that these benefits outweighed the solid-state recorders' susceptibility to radiation, which is a problem near Jupiter.

The memory requirement of the recorders are determined by the data rate. The period of Europa's rotation is 3.55 days. Asterius can transmit data for a maximum of half that time. Assuming that Asterius can be tracked 40% of the time, and given a data rate of 50 kbps, the total number of bits sent per Europa rotation is 6.134 billion. Assuming that one-third of the data transmitted is collected while Asterius is turned away from the Earth, the required memory to store this data is 2.0448 billion bits. Each recorder would hold one gigabit.

7.5.1.5 Timer

The timer on Asterius consists of two oscillators: a long-term oscillator which has low granularity (on the order of a second), but drifts less than one second every ten years. The short-term oscillator has much higher granularity, on the order of microseconds. It is periodically synchronized to the long-term oscillator.

The timer works by raising an interrupt in the on-board computer at regular intervals. (As seen in Figure 7, the timer has a direct connection to the computer: it does not use the bus.) The length of the interval is programmable.

For error recovery, a watchdog circuit is built into the timer. When the timer interrupts the computer, it expects the computer to return an acknowledgment signal. If the timer sends a certain number of interrupt signals without acknowledgment, the watchdog circuit attempts to reset the computer.