On-Board Satellite Controller using ARM Based Microcontroller

Satellite Subsystems

Irrespective of the intended application, is it a communications satellite or a weather satellite or even an Earth observation satellite, different subsystems comprising a typical satellite include the following:

1. Mechanical structure
2. Propulsion
3. Thermal control
4. Power supply
5. Tracking, telemetry and command
6. Attitude and orbit control
7. Payload
8. Antennas

The structural subsystem provides the framework for mounting other subsystems of the satellite and also an interface between the satellite and the launch vehicle.

The propulsion subsystem is used to provide the thrusts required to impart the necessary velocity changes to execute all the maneuvers during the lifetime of the satellite. This would include major maneuvers required to move the satellite from its transfer orbit to the geostationary orbit in the case of geostationary satellites and also the smaller maneuvers needed throughout the lifespan of the satellite, such as those required for station keeping.

The thermal control subsystem is essential to maintain the satellite platform within its operating temperature limits for the type of equipment on board the satellite. It also ensures a reasonable temperature distribution throughout the satellite structure, which is essential to retain dimensional stability and maintain the alignment of certain critical equipments.

The primary function of the power supply subsystem is to collect the solar energy, transform it to electrical power with the help of arrays of solar cells and distribute electrical power to other components and subsystems of the satellite. In addition, the satellite also has batteries, which provide standby electrical power during eclipse periods, during other emergency situations and also during the launch phase of the satellite when the solar arrays are not yet functional.

The telemetry, tracking and command (IT &C) subsystem monitors and controls the satellite right from the lift-off stage to the end of its operational life in space. The tracking part of the subsystem determines the position of the spacecraft and follows its travel using angle, range and velocity information. The telemetry part gathers information on the health of various subsystems of the satellite encodes this information and then transmits it. The command element receives and executes remote control commands to effect changes to the platform functions, configuration, position and velocity.

The attitude and orbit control subsystem performs two primary functions. It controls the orbital path, which is required to ensure that the satellite is in the correct location in space to provide the intended services. It also provides attitude control, which is essential to prevent the satellite from tumbling in space and also to ensure that the antennae remain pointed at a fixed point on the Earth’s surface.

The payload subsystem is that part of the satellite that carries the desired instrumentation required for performing its intended function and is therefore the most important subsystem of any satellite. The nature of the payload on any satellite depends upon its mission. The basic payload in the case of a communication satellite is the transponder, which acts as a receiver, amplifier and transmitter. In the case of a weather forecasting satellite, a radiometer is the most important payload. High resolution cameras, multispectral scanners and thematic mappers are the main payloads on board a remote sensing satellite. Scientific satellites have a variety of payloads depending upon the mission. These include telescopes, spectrographs, plasma detectors, magnetometers, spectrometers and so on.

Antennas are used for both receiving signals from ground stations as well as for transmitting signals towards them. There are a variety of antennas available for use on board a satellite. The final choice depends mainly upon the frequency of operation and required gain. Typical antenna types used on satellites include hom antennas, centre-fed and offset-fed parabolic reflectors and lens antennas.

Tracking, Telemetry, and Command Subsystem

According to Maini and Agrawal (2007), for satellite controller, we have to consider tracking, telemetry and command subsystem. The tracking, telemetry and command (TT&C) subsystem monitors and controls the satellite right from the lift-off stage to the end of its operational life in space. The tracking part of the subsystem determines the position of the spacecraft and follows its travel using angle, range and velocity information. The telemetry part gathers information on the health of various subsystems of the satellite. It encodes this information and then transmits the same towards the earth station. The command element receives and executes remote control commands from the control centre on Earth to effect changes to the platform functions, configuration, position and velocity. The TT&C subsystem is therefore very important, not only during orbital injection and the positioning phase but also throughout the operational life of the satellite.

Figure 1 shows the block schematic arrangement of the basic TT&C subsystem. Tracking, as mentioned earlier, is used to determine the orbital parameters of the satellite on a regular basis. This helps in maintaining the satellite in the desired orbit and in providing look-angle information to the Earth stations. Angle tracking can, for instance, be used to determine the azimuth and elevation angle from the Earth station. The time interval measurement technique can be used for the purpose of ranging by sending a signal via the command link and getting a return via the telemetry link. The rate of change of range can be determined either by measuring the phase shift of the return signal as compared to that of the transmitted signal or by using a pseudorandom code modulation and the correlation between transmitted and received signals.

During the orbital injection and positioning phase, the telemetry link is primarily used by the tracking system to establish a satellite-to-Earth station communications channel. After the satellite is put into the desired slot in its intended orbit, its primary function is to monitor the health of various other subsystems on board the satellite. It gathers data from a variety of sensors and then transmits that data to the Earth station. The data include a variety of electrical and nonelectrical parameters. The sensor output could be analogue or digital. Wherever necessary, the analogue output is digitized. With the modulation signal as digital, various signals are multiplexed using the time division multiplexing (TDM) technique. Since the bit rates involved in telemetry signals are low, a smaller receiver bandwidth with a good signal-to-noise ratio is used at the Earth station.

Figure 1. Block schematic arrangement of the basic TT&C subsystem (Maini & Agrawal, 2007)

The command element is used to receive, verify and execute remote control commands from the satellite control centre. The functions performed by the command element include controlling certain functions during the orbital injection and positioning phase; including firing the apogee boost motor and extending solar panels, etc., during the launch phase. When in orbit, it is used to control certain onboard equipment status including transponder switching, antenna pointing control, battery reconditioning, etc. The control commands received by the command element on the satellite are first stored on the satellite and then retransmitted back to the control station via a telemetry link for verification. After the commands are verified on the ground, a command execution signal is then sent to the satellite to initiate intended action.

Key Characteristic of On-Board (Orbit) Control

On-board control offers several technical capabilities. All of the orbital elements of the spacecraft are controlled automatically, including specifically:

• Period (and, therefore, the semi major axis)
• Eccentricity
• Argument of Perigee
• In-track phase (i.e., mean and true anomaly vs. time)
• Longitude of the ascending node
• Node drift rate (and, therefore, the inclination)

This means that the spacecraft follows a fully predictable orbit pattern, such that:

• The position of the spacecraft at all future times is known as far in advance as desirable
• The ground track (or inertial track) of the spacecraft can be made to follow a predefined pattern which can be changed at the convenience of the user.

The process for computing future positions is sufficiently simple that it can be included in virtually any ground-based equipment that uses a general purpose microprocessor. There is a longer planning horizon for all future activities:

• Payload planning.
• Maneuver planning to achieve desired future coverage.
• Dealing with the potential problems of RF or physical interference with other spacecraft or debris.

Disturbance torques is much lower than with more traditional orbit control processes the size and responsiveness of control actuators can be reduced. Restrictions on the timing of station keeping maneuvers can be reduced or eliminated (Gurevich and Wertz, 2001).

ARM Processor as the Main Controller

The on board satellite controller is embedded system. The embedded systems can control many different devices, from small sensors found on a production line, to the real-time control systems used on a NASA space probe. All these devices use a combination of software and hardware components. Each component is chosen for efficiency and, if applicable, is designed for future extension and expansion.

Embedded systems typically use a microprocessor, combined with other hardware and software, to solve a specific computing problem. Microprocessors range from simple (by today’s standards) 8-bit microcontrollers to the world’s fastest and most sophisticated 64-bit microprocessors (Andrews, 2005). In this paper we introduce ARM processor (visit http://www.arm.com for more information about ARM Processors) which can be used as a main controller.

Acording to Sloss, et al (2004), there are a number of physical features that have driven the ARM processor design. First, portable embedded systems require some form of battery power. The ARM processor has been specifically designed to be small to reduce power consumption and extend battery operation-essential for applications such as mobile phones and personal digital assistants (PDAs) or on board satellite controller.

Figure 2. An example of an ARM-based embedded device, a microcontroller (Sloss, 2004)

Figure 2 show a typical embedded device based on an ARM core. Each box represents a feature or function. The lines connecting the boxes are the buses carrying data. We can separate the device into four main hardware components:

• The ARM processor controls the embedded device. Different versions of the ARM processor are available to suit the desired operating characteristics. An ARM processor comprises a core (the execution engine that processes instructions and manipulates data) plus the surrounding components that interface it with a bus. These components can include memory management and caches.
• Controllers coordinate important functional blocks of the system. Two commonly found controllers are interrupt and memory controllers.
• The peripherals provide all the input-output capability external to the chip and are responsible for the uniqueness of the embedded device.
• A bus is used to communicate between different parts of the device.

According to these features, we can use ARM as a main controller in on board satellite controller. First time we have to design the overal system, what kind and how many of block function involved. From this design we an choose one from many type of ARM processor which suitable for on board satellite controller (and of course for on board data handling (OBDH) too).

ARM has designed a number of processors that are grouped into different families according to the core they use. The families are based on the ARM7, ARM9, ARM10, and ARM11 cores. The postfix numbers 7, 9, 10, and 11 indicate different core designs. The ascending number equates to an increase in performance and sophistication. ARM8 was developed but was soon superseded.

Table 1 shows a rough comparison of attributes between the ARM7, ARM9, ARM10, and ARM11 cores. The numbers quoted can vary greatly and are directly dependent upon the type and geometry of the manufacturing process, which has a direct effect on the frequency (MHz) and power consumption (watts).

Within each ARM family, there are a number of variations of memory management, cache, and TCM processor extensions. ARM continues to expand both the number of families available and the different variations within each family.

Table 1. Revision History (Sloss, et al. 2004)

Samsung S3C2440 ARM9 Based Microcontroller

Besides ARM itself, many vendors mad theirs microcontroller based on ARM Core, for instance is Samsung. Let’s take a look Samsung S3C2440 ARM9 Based Microcontroller.

The S3C2440A is developed with ARM920T core, 0.13um CMOS standard cells and a memory complier. Its low power, simple, elegant and fully static design is particularly suitable for cost- and power-sensitive applications. It adopts a new bus architecture known as Advanced Micro controller Bus Architecture (AMBA).

The S3C2440A offers outstanding features with its CPU core, a 16/32-bit ARM920T RISC processor designed by Advanced RISC Machines, Ltd. The ARM920T implements MMU, AMBA BUS, and Harvard cache architecture with separate 16KB instruction and 16KB data caches, each with an 8-word line length.

By providing a complete set of common system peripherals, the S3C2440A minimizes overall system costs and eliminates the need to configure additional components. The integrated on-chip functions that are described in this document include (Samsung Electronics, 2004):

• Around 1.2V internal, 1.8V/2.5V/3.3V memory, 3.3V external I/O microprocessor with 16KB I-Cache/16KB D-Cache/MMU.
• External memory controller (SDRAM Control and Chip Select logic)
• LCD controller (up to 4K color STN and 256K color TFT) with LCD-dedicated DMA
• 4-ch DMA controllers with external request pins
• 3-ch UARTs (IrDA1.0, 64-Byte Tx FIFO, and 64-Byte Rx FIFO)
• 2-ch SPIs
• IIC bus interface (multi-master support)
• IIS Audio CODEC interface
• AC’97 CODEC interface
• SD Host interface version 1.0 & MMC Protocol version 2.11 compatible
• 2-ch USB Host controller / 1-ch USB Device controller (version 1.1)
• 4-ch PWM timers / 1-ch Internal timer / Watch Dog Timer
• 8-ch 10-bit ADC and Touch screen interface
• RTC with calendar function
• Camera interface (Max. 4096 x 4096 pixels input support, 2048 x 2048 pixel input support for scaling)
• 130 General Purpose I/O ports / 24-ch external interrupt source
• Power control: Normal, Slow, Idle and Sleep mode
• On-chip clock generator with PLL

Figure 3 shows the S3C2440 Block diagram. Its clearly shown, that this microcontroller is rich in features and accomodate many embedded system application, also it can be used as main controller in on board satellite controller. Because this microcontroler based on ARM9 processor, which support for embedded operating system, we can develop specific operating system used for satellite controller. Using recently communication technology, we can remote access to the satellite (using telnet, etcs).

Figure 3. S3C2440 Block Diagram (Samsung Electronics, 2004)

For learning and also developing embedded system applications, there are many development board sell commercially or DIY (do-it-youself). One of the low cost many features development board already we used is Mini2440, as shown in Figure 4, its only 10cm x 10cm in size and equipped with 3,5″ LCD Touch Screen, as shown in Figure 5.

For more information according to S3C2440 and Mini2440 pelase visit this link:

• Samsung S3C2440, part-1 (Click)
• My First Experince using Samsung S3C2440, part-1 (Click)
• My First Experince using Samsung S3C2440, part-2 (Click)
• My First Experince using Samsung S3C2440, part-3 (Click)

So its time to develop on board satellite controller using this microcontroler, or you can develop with other kind or type microcontroler or microprocessor. As shown in Figure 6, this S3C2440 can be used in Flight Computer Module, and also in other modules which need it.

Figure 4. Mini2440 - S3C2440 Microcontroller based Development Board

Figure 5. Mini2440 - S3C2440 Microcontroller based Development Board with LCD Touchscreen

Figure 6. Example of Command and Data Handling Architecture

References

• Andrews, J.R., 2005, Co-verification of Hardware and Software for ARM SoC Design, Elsevier Inc.
• Gurevich, G. and Wetrz, J., 2001, Autonomous On-board Orbit Control: Flight Results and Cost Reduction, JHU/APL Symposium on Autonomous Ground Systems for 2001 and Beyond, April 25-27, 2001, Laurel, Maryland
• Maini, A.K. and Agrawal, V., 2007, Satellite Technology: Principles and Applications, John Wiley & Sons, Ltd.
• Samsung Electronics, 2004, S3C2440A 32-Bit CMOS Microcontroller User’s Manual, Revision 1, Samsung Electronics Co. Ltd., Korea.
• Sloss, A.N., Symes, D., and Wright, C., 2004, ARM System Developer’s Guide: Designing and Optimizing System Software, Morgan Kauffman Publisher.

Thank You!

Tags: arm, Mikrokontroler, obdh, on board data handling, s3c2440, satelit, sattelite