8.3 Onboard Avionics

Wiyan Wong

8.3.1 Overview:

We can think of the onboard avionics system as the brain of the spacecraft. It consists of the software and electronics that gather and manage information and control the state of the spacecraft. This section discusses the following components within the avionics system:

  1. Commanding and Data Handling
  2. Attitude and Navigation Control
  3. Telecommunication Electronics
  4. Crew Interface
  5. Audio/Video
  6. Timing Unit

Historically space missions rely on a large ground staff. These people monitor the spacecraft closely to ensure proper system operations. Whenever a problem occurs, an army of engineers attempts to solve the issue. However employing such a big group of people is costly. In addition, a Mars mission will encounter communication delays and blackouts, preventing constant monitoring from Earth. Therefore, the spacecraft must maintain operations autonomously even in the presence of subsystem failure. The reduction of cost and risk is the primary reason we implement spacecraft autonomy.

8.3.2Adjustable, Human-Centered Autonomous System1

It is impossible to predict every possible occurrence on a manned mission to Mars, therefore it is critical to support human intervention. A human-centered autonomous system must recognize people as intelligent agents. In addition, the system must be designed to operate independently while able to adjust to different levels of human control. Implementing such a system, we can increase the safety of the crew and the equipment; reliability, efficiency, and science capability, while decreasing mission operation costs.

To achieve this type of autonomy, there are several requirements:

  • The system must be able to react to environmental changes. For example, if the temperature in the spacecraft is too high, the system needs to cool it down. This process is done using sensors and effectors.
  • Actions need to execute routinely. The system’s sequencer carries out the actions and makes sure they are done properly. It also makes sure there are no conflicts amongst the subsystems.
  • The system must be able to plan and schedule activities. Intelligent planning software plans activities for the crew to maintain efficient mission operation. Sample activities include maintenance of the spacecraft and exercising.
  • The system needs to diagnose failure, and if necessary repair or reconfigure itself. This is comparable to the immune system of a living creature.
  • The crew and the intelligent planning software use simulations of physical systems to predict outcomes of specific actions. This allows the crew and the planner to examine different options during the mission.
  • To ensure the capability of human intervention, a user interface is needed.

8.3.3 Habitation Module (Hab)

Avionics Architecture

The Hab employs a distributed avionics architecture [Fig. 8.3.1]. Each of the subsystems within the avionics uses a separate bus. In the center of the architecture is the commanding and data-handling unit (CDH). The commanding and data-handling unit executes instructions to control and monitor each subsystem. Most subsystems consist of sensors, controllers, and effectors. Sensors gather information; the controllers apply information to control laws, which then react using the effectors. For example, the sensors gather information on the attitude of the spacecraft. Once the attitude is determined the system will apply control law to change its orientation or pointing direction by firing thrusters.

Fig. 8.3.1 Avionics Architecture for Separate Bus Configuration.2

Commanding and Data Handling

The CDH is a unit that manages other subsystems, and performs data processing for science experiments on the surface of Mars. The CDH consist of both software and hardware.

Software. Four different groups of software are onboard the Hab: control system software, system management software, mission data software, and the operating system itself.3

Control system software manages activities such as navigation and control. The software takes information from sensors, calculates the required actions, and executes necessary changes. This group of software is vital for events such as take-off, landing, and aerobraking. System management software provides fault detection, correction, event scheduling, and event planning. This is required for spacecraft autonomy. Mission data-processing software manipulates and compacts data for storage. The Operating system manages computer resources, controls the processors, monitors system parameters, and updates computer memory. The operating system is the overseer for all applications.

Hardware. In transit to Mars, the spacecraft will travel through space radiation. This demands that the onboard electronics be radiation hardened. In addition, an autonomous spacecraft requires a high performance computer system exceeding 100 Mflops and perhaps approaching 1 Gflop.4

An example of a CDH system is the PulseTM.5 PulseTM is a low cost, fault tolerant, and highly reliable system developed by Honeywell. Within the PulseTM unit there are three single board computers. Each computer uses a radiation hardened processor based on the Motorola licensed PowerPC 603e. The computers are designed to meet the environmental requirements of low earth orbits, however PulseTM can be modified to suit different environments.

To achieve 1 Gflop performance the Hab must use 17 PulseTM units. However, the newest PowerPC processor available commercially (PowerPC 750) is capable of 1 Gflop per processor. PowerPC 603e was first available commercially in 1997 and today it is available for space applications. Therefore, a radiation hardened version of PowerPC 750 should be available for space applications in 2005.

PulseTM provides a basis for estimating the mass, power consumption, and reliability of the CDH for the Mars mission.

Table 8.3.1 Specification of PulseTM Commanding and data-handling unit

Specifications
Weight / 24.9 Kg
Dimension / 47.7 cm (L) x 29.8 cm (W) x 27.2 cm (H)
Volume / 38771.8 cm3
Power Consumption / 500 watts
Reliability / 0.995 for 10 year with a 30°C baseplate temperature

The hardware also includes a storage unit to store key mission data. The crew uses memory storage to store large amounts of science data on Martian surface. Since not all data are transmitted back to earth, a long duration mission such as PERForM needs a reliable and large amount of memory. We use five Solid State Mass Memory Unit onboard the Hab.

Attitude and Navigation Controls

Interplanetary navigation. Deep Space Network (DSN) tracks the Hab on the way to Mars. From Earth, DSN transmits electromagnetic signal to the Hab. Upon receiving the signal, the Hab retransmits it back to Earth. The position of the spacecraft is determined from the time lag of the two transmissions. The velocity of the spacecraft is obtained by examining the Doppler Shift of the signals.

Attitude determination. Upon arrival to Mars, we need to determine the Hab’s attitude so that it can perform maneuvers and land on the Martian surface. The attitude determination and control subsystem provides the Hab’s attitude and its orientation. The subsystem uses three different types of sensors to provide this information.

  • Sun sensors. The sun sensors reference the Sun and provide a single position vector, which allows the Hab to assess its pointing direction. Normally a sun sensor is 0.5 to 2 kg and requires 3 watts of power. The Hab needs one sun sensor, but is equipped with two for redundancy.
  • Star trackers. Star trackers assess the orientation of the spacecraft about the Sun vector. The star trackers use telescopes to identify patterns of stars. Star trackers typically have masses from 3 to 7 kg, and require 5 to 20 W.4 The Hab needs one star tracker, but is equipped with two for redundancy.
  • Inertial Measurement Unit (IMU). The IMU measures the changes in attitude. It contains gyroscopes and accelerometers that measure angular rates and linear accelerations of the spacecraft. The IMU allows the determination of attitude when information from either the sun sensors or the star trackers is absent. IMU’s have a mass of about 50 kg and require up to 200 watts of power.

Attitude Control. The spacecraft uses three-axis control to achieve stability. This technique requires the use of devices to produce torque in any or all of the three principle axes of the spacecraft. The Hab employs reaction wheels and thrusters to attain this. Four reaction wheels are onboard the Hab, three of which are placed orthogonal to each other. The redundant fourth is placed at an equal angle to the others, in order to eliminate single point failure. 6

Once the Hab enters the Martian atmosphere, a radar altimeter aids landing. The radar altimeter measures absolute altitude from the spacecraft to the nearest terrain feature within the beamwidth of the antennas. This examines the terrains ahead of the Hab and allows it to perform maneuvers if necessary. In addition, the Mars Launch Vehicle (MLV), already on Martian surface, serves as a localizer beacon. It emits signals to aid the Hab in landing within its 10 km target radius.

Telecommunication Electronics

Telecommunication electronics onboard the Hab performs 4 operations: signal conditioning, modulation, coding, and data packaging. Signal processing is the amplification of signals. The system must amplify the weak signals received by the antenna, at which point they can be processed without causing the degradation of signal quality by noise in electronic circuits.6 Downlink transmission signals are also amplified to a required output level. Modulation is the process by which an input signal manipulates the characteristics of a radio frequency carrier. These characteristics are amplitude, phase, frequency, and polarization.7 Modulation is done because the original signal frequency is unsuitable for direct transmission of radio waves.6 In the coding process, the system detects and corrects errors in signals. Coding allows the system to correctly interpret the received information. Data packaging is the process that organizes data for transmission. This includes the process of encrypting the information.

Hardware for the telecommunication system is contained in a single package. We estimate a package mass of 30 kg. An extra package is onboard the Hab for redundancy, totaling 60 kg.

Crew Interface

For a human-centered system, crewmembers onboard the Hab need to monitor the status of the spacecraft, and if necessary override instructions implemented by the CDH. We need a number of devices to achieve this. 15 Liquid Crystal Displays (LCD) present all vital status of the spacecraft to onboard crewmembers. Each has a mass of 4 kg. The crew uses human-input devices to provide instructions to the spacecraft, such as keyboards, mouse, and hand controller.

Timing Unit

Two master timing units are used to keep current time onboard the Hab. The spacecraft needs a reference time for the execution and logging of events. One of the timing devices tracks Earth time, while the other tracks the Martian time. The crew lives in Martian time, however a reference Earth time is needed when the spacecraft is interfacing with ground control.

Audio and Video

Audio and video equipment consist of speakers, microphones, video cameras, and data storage. Onboard crewmembers use these to communicate with the ground staff. Video cameras are also used for documentations of events. In the control room contains a stationary camera, speakers, and microphones. Two additional portable handheld cameras are available for documentation elsewhere in the Hab.

Others Subsystems

Other subsystems onboard the Hab includes the power subsystem, mechanism control, and environmental control. The power subsystem is discussed in section 8.2. Mechanical devices such as the tether system are discussed in section 2.2, and parachute deployment device is discussed in section 1.3. Within the life support subsystem there are devices such as fire detection sensors, water handling devices, and atmospheric controls. Life support subsystem is discussed in section 5.2.

Wires and Connectors

A significant amount of mass consists of wires and connectors. The precise mass of these materials is impossible to estimate since it depends on the specific design and location of each subsystem. Instead, examining the space shuttle allows for a rough estimation of 3 tonnes.

Hab

Fig. 8.3.2 Basic avionics architecture for the Hab

Table 8.3.2 Summary of the power and mass requirement for the Hab

Units / mass total [tonnes] / power total [watt]
Computer system / 3 / 0.100 / 1500
Memory storage / 5 / 0.100 / 120
Communication electronics / 2 / 0.100 / 500
Cameras / 4 / 0.005 / 20
Speakers / 2 / 0.005 / 0
Mic / 2 / 0.000 / 0
Audio/video storage / 2 / 0.500 / 120
Display / 15 / 0.070 / 450
Timing units / 2 / 0.010
Connectors/wires / 2 / 3.000 / 0
Reaction wheels / 4 / 0.500
Sun sensors / 2 / 0.020 / 60
Inertial mearsument unit / 3 / 0.070 / 30
Star trackers / 2 / 0.020
Software / N/A / 0.000 / 0
Misc / n/a / 0.500 / 1000
Total / 5.000 / 3800

8.3.4 Earth Return Assembly (ERA)

The ERA consist of the Earth Return Vehicle (ERV), Mars Launch Vehicle (MLV), Crew Transfer Vehicle (CTV), and Mars Garage (MG). Its operations are not identical to the Hab, however it employs similar systems.

ERV

The ERV’s avionics is almost identical to the Hab. However, the ERV will not land on the Mars, therefore no radar altimeter is onboard. We still employ attitude determining sensors such as sun sensors, IMU’s, and star trackers. The ERV performs the duties of a satellite to aid navigation and communication while the crew is on the Martian surface. The ERV needs to receive signals from the Mars surface and transmit them back to earth. It must also use electronics to help track rovers. When it is time to return to earth, the ERV will dock with the CTV. We need sensors, controller, and effectors for this process.

MLV/CTV

Upon reaching Mars, the MLV separates from the ERV and descends down to the surface. Because of this, it requires its own CDH unit and attitude determining sensors and controls. Since both MLV and CTV lands on Mars, the two systems share the same CDH. The CDH system is housed inside the CTV. The MLV employs the same attitude determining sensors as the Hab. The MLV has a communication device that communicates with the ERV. This is essential for the monitoring of the in-situ propellant production process. In addition, the communication device sends signals to the Hab while the Hab descends to the surface. This will help the Hab land within the MLV’s 10 km radius. We also use mechanism sensors for in-situ propellant production and the parachute system.

The CTV descents to the Mars surface together with the MLV, however CTV is the only section that rendezvouses with the ERV. It is also the only section that will enter back into earth’s atmosphere. The CTV requires its own set of attitude sensors and control. It also employs the same type of attitude determining sensors and controls as the Hab. The CTV includes manual control devices such as a hand controller to manually dock if necessary. Small communication devices are needed to communicate with the ERV for rendezvous. The CTV has mechanical devices for docking.

Table 8.3.3 Summary of the power and mass requirement for the ERA

Units / mass total [tonnes] / power total [Watts]
ERV
CDH / 3 / 0.1 / 1500
Data storage / 5 / 0.1 / 120
Communication package / 2 / 0.1 / 500
Camera / 4 / 0.005 / 20
Speakers / 2 / 0.005 / 0
Mic / 2 / 0 / 0
Audio/video storage / 2 / 0.5 / 120
Display / 15 / 0.07 / 450
Timing units / 2 / 0.01
Connectors/wires / 2 / 3 / 0
Reaction wheels / 4 / 0.5 / 100
Sun sensors / 2 / 0.02 / 60
Inertial mearsument unit / 3 / 0.07 / 30
Star trackers / 2 / 0.02 / 10
Misc / n/a / 0.5 / 1000
MLV
Communication / 1 / 0.05 / 100
Sun sensors / 2 / 0.02 / 60
Inertial mearsument unit / 3 / 0.07 / 30
Star trackers / 2 / 0.02 / 10
Reaction wheels / 4 / 0.3
CTV
CDH / 2 / 0.07 / 800
Communications package / 1 / 0.05 / 50
Displays / 3 / 0.01 / 90
Data storage / 1 / 0.02 / 20
Sun sensors / 2 / 0.02 / 60
Inertial mearsument unit / 3 / 0.07 / 30
Star trackers / 2 / 0.02 / 10
Reaction Wheels / 4 / 0.2
Total / 5.92 / 5170

Fig. 8.3.2 Basic avionics architecture for the ERA.

8.3.5 Conclusion

The avionics onboard the Hab and the ERA must operate autonomously. More specifically, the systems must be centered around the crew. Spacecraft autonomy increases reliably while decreases mission cost.

We need to include the avionics system in the mission’s mass and power budget. We estimate the Hab to weight 5 tonnes and consumes about 4 kW power. The ERA requires about 5 kW of power and weights about 6 tonnes.

Acknowledgements

I would like to thank all those who helped me on this report, especially A. Nusawardhana for helping me understand the functions of avionics; Dave Hansen for providing valuable information on computers; and Brad Engels for helping me on the writing process.

References

  1. Bonasso, R.P., Dorais, G. A, KortenKamp, D., Pell, B., Schreckenghost, D., “Adjustable Autonomy for Human-Centered Autonomous System on Mars,” Proceedings of the First International Conference of the Mars Society, Aug. 1998.
  2. Smith, J. F., “Spacecraft Avionics”, Dec. 1998.
  3. Hansen, L. J., Pollocak, C. H., “Spacecraft Computer Systems,” Space Analysis and Design, 2nd ed., Microcosm, Inc. Torrance, 1991, pp.603-635
  4. Kogge, P., Sterling, T., “New Approaches to Spaceborne Computing,” Proceedings of the 1998 IEEE Aerospace Conference, Vol. 1, Mar. 1998. P22-34.
  5. Elias, M., “Development of a Low Cost, Fault Tolerent, and Highly Reliable Command and Data Handling Computer (PulseTM),” Proceedings of 2000 IEEE Aerospace Conference, Vol. 5, Montana, Mar. 2000, pp. 251-261.
  6. Smith H., “Telecommunications,” Spacecraft Systems Engineering, 2nd ed., Wiley, New York, 1996, pp. 369-421.
  7. Davies, R. S., “Communication Architecture,” Space Analysis and Design, 2nd ed., Microcosm, Inc. Torrance, 1991 pp. 503-552.
  8. “ Shuttle Reference Manual,” Mar. 2001.

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