ONBOARD PROCESSING

Overview

Onboard processing (OBP) can provide greatly improved performance and efficiency over non-processing satellite systems. It can be used advantageously in four places in a communications satellite:

  1. Intermediate Frequency (IF) and radio frequency (rf) communications signal switching
  2. support processing
  3. phased array antenna control and beam forming
  4. baseband processing and switching

IF and rf switching is generally the simplest, requiring the least amount of processing power. It involves electronically controlled rf/if switches, usually in a matrix format, that can be controlled statically or dynamically, and has been used commercially for some time.

Support processing has traditionally been associated with control of the satellite bus and includes such functions as attitude control, power management and telemetry, and tracking and control (TT&C). Most of these functions can be handled by general purpose onboard computer systems.

Phased array antennas with many independently steerable beams require a large number of radiating elements with individual phase (and amplitude) control for each beam. This signal control can be implemented with analog circuits (for a small number of beams) or digitally. This requires substantial digital processing, perhaps more than with the baseband processing and switching system. Phased array antennas are used on the Iridium and Globalstar satellites.

Baseband processing and switching involves functions similar to those performed in terrestrial local area networks and telephone switches. In addition, demodulation, demultiplexing, error detection and correction, switching, congestion control and notification, buffering, remultiplexing, and modulation and network synchronization must be performed. Most of these functions require specialized processors in order to be size/mass/power efficient. This is especially true for packet switched systems with a large number of earth stations—particularly if the system is required to handle multiple user rates.


Fig. 3.7. Onboard processing system design.

To date, few commercial onboard processing satellites have been flown. Next to ITALSAT, the first experimental satellite with major communications processing, ACTS has logged several productive years of experiments. ACTS has two basic onboard processing packages, a circuit switched baseband package and an rf satellite matrix switch. Both systems are controlled via ground commands, rather than via onboard autonomous control. Compared to future systems (proposed and under development) these satellites appear rather simple. Iridium is one of the first commercial onboard processing satellites. It utilizes multibeam phased array antennas, onboard processing and intersatellite links.

Future satellite systems are being discussed and planned for both Ka-band (Teledesic, Skybridge, Astrolink, etc.) and V-band (Expressway, Cyberpath, M-Star, etc.). These systems will require many advanced onboard architectures for fault tolerance, autonomous control and reconfigurability. In addition, these systems will utilize packet switching techniques and intersatellite links as part of the communication payload. Advanced modulation and coding technologies using block coding and concatenated convolutional and block coding will also be required with link qualities approaching that of fiber—bit error rates of 10-10 or better.

Progress in Onboard Processing

Digital technologies continue to improve at a rate of approximately 2 times in performance every 18 months in a combination of speed, processing power, or density (a derivation of Moore's Law, the number of transistors that can fit on a chip doubles every 18 months).

Onboard computers are being developed by a number of companies such as Saab-Ericson and Honeywell (32 bit computer with 16 and 32 bit instruction sets, radiation hardened to 1 Mrad total dose).

Application specific integrated circuits (ASICs) are increasingly being used onboard spacecraft, to reduce mass, size, power consumption and at the same time increase reliability. The drawback of using ASICs is the development cost, risk and schedule, and therefore particular attention is necessary when ASIC design is being performed to improve speed, power, and density. For instance, Altera and Xylinx currently have 100k gate 3.3 volt programmable logic devices (PLDs) and expect to introduce 2000 gate field programmable gate array (FPGA) by 1999. UTMC Microelectronics Systems has 16 micron 200K (400K by 1998) gate arrays and can provide circuits capable of withstanding 100 krads total dose and Single Event Upset (SEU) at 10-10 errors/bit day and 150 MHz clock rates. Devices are available in both 5 V and 3.3 V. UTMC also produces radiation hardened SRAMs, dual port RAMs, and FPGAs. Actel has low SEU devices that are latch-up immune up to 300 krads. These circuits are now being utilized in spacecraft. Iridium uses over 13 different 100k gate ASICS. As more of these devices are used, confidence should grow.

Intellectual property (IP) and core logic (CL) are third party designs of specific, generic, complex functions that can be licensed and are widely utilized today. These designs may be hardwired for specific ASICs (cell based) or software based using a hardware description language (HDL) such as Verilog of VHDL. Cores are available for a vast array of functions such as network interfaces (ATM, ethernet, etc.), digital filters, coding, compression, and MPEG-2 to name a few. Utilizing IP/CL allows companies to concentrate on the overall system design without having to design and maintain individual complex functions.

The following section describes some of the ongoing onboard processing activities in Canada, Europe, Japan, Korea, and the United States. The information was obtained from a combination of company reports, site reports, the World Wide Web, as well as experience, visits and contacts with a number of experts in the field.

In Canada, Spar concentrates on demultiplexers, demodulators, and uplink access scheduling. A breadboard of a fast packet switch has been constructed but so far there are no plans for ASIC development that would be required for flight. The processor output would interface to a standard ATM switch. Spar is also leading the system engineering work for Canada Advanced Satellite Program, including analysis of packet switching in a mobile multimedia environment.

COM DEV built the analog onboard processing unit for Inmarsat-3. It performs traffic management, switching, and routing, and is said to be the most complex electronic hardware yet flown on a commercial spacecraft. The system is reconfigurable from the ground. There are two processors per payload. The system operates statically—it does not do TDMA switching—but all COM DEV products are capable of switching at sub-microsecond speeds. An Inmarsat-3 processor contains 168 SAW filters and a total of 70,000 components. COM DEV makes SAW filters as components and as part of subassemblies, primarily if frequency converters for digital payloads. A significant new product is BEAMLINK, a complete channelizer with solid state switch matrices for subchannel connectivity. It can connect any of 37 input channels to any one of 8 antenna beams.

In Europe, Alcatel Telecom plays a leading-edge role in the development of information superhighway technology, including broadband ATM, switching systems and SDH transmission, which are compatible with the Internet. Alcatel has plans to develop SkyBridge, a constellation of 64 LEO satellites with onboard ATM capability complementing that of ground infrastructures. In addition to Internet access, the SkyBridge system will provide bandwidth- on- demand for other types of high-speed data communications, at speeds up to 60 Mbps. Alcatel has the design and fabrication capabilities to provide OBP technologies including onboard computers, switching and routing, and phased array antenna controls.

Although Alcatel is fully capable of implementing space based onboard processing for commercial applications, the company has developed a "Switchboard-In-The-Sky" satellite network concept that performs switching in intelligent ground terminals using ATM technologies. This concept is known as "Cadenza." With Cadenza, ATM subscriber modules are plugged into a backplane whose plugs and sockets are antenna dishes. The backplane's traditional copper tracks (in ground-based applications) are replaced, in Cadenza, by radio links, and the green fiberglass printed circuit board is replaced by—literally —the sky.

Alcatel Microelectronics, formerly Alcatel Mietec, located in San Francisco, California, markets intellectual property (IP) and "system-on-chip" (SOC) application specific standard products for wireline and wireless access solutions worldwide. Alcatel Microelectronics will market Alcatel IP and design services to original equipment manufacturers building highly integrated communications products. Alcatel Microelectronics has direct access to the IP developed by over 10,000 telecommunications systems designers at its parent organization, Alcatel. Besides this strong IP base in communications, Alcatel Microelectronics has also licensed IP from leading vendors worldwide. Alcatel Microelectronics emphasizes advanced methodologies to manage, develop and assemble its IP, using architectural templates that will support "plug and play" design. These hardware/software co-design techniques allow the company to integrate its IP portfolio in new SOC solutions with continuously shortened design cycles. The company will employ its own advanced mixed signal manufacturing technology for designs requiring the highest level of analog and digital functionality on a single chip.

Alenia (Aerospazio Division) in Italy has wide experience building onboard processing satellite equipment including ASIC Components. Alenia is building Skyplex, a digital TV system that uses technology developed from the ESA OBP work. Skyplex combines six Ku-band digital TV uplinks in the satellite to form a Ku-band DVB/MPEG type downlink. This equipment was scheduled to fly on EUTELSAT's Hotbird 4 in early 1998. This onboard processor has 33 MHz bandwidth, 6.8 kg mass, 24 x 25 x 18 cm dimensions, and uses 43 W of prime power. It contains down- and upconverters, demodulates 6 digital carriers (2 Mbits); and combines the data streams for retransmission. An improved model with up to 18 channels is under development.

ESA requires ASIC manufacturers to utilize the VHSIC hardware description language (VHDL) for use in all phases of the creation of electronic systems to minimize development risks and avoid finding that "unpleasant surprises" late in the development cycle, from ensuring that the correct specification is established, to using a design methodology and IC technology suitable for high-reliability designs. This design methodology also enables reuse of ASICs for similar applications, if sufficient care has been taken during the development. ASIC technologies commonly used for space applications include: ABB Hafo (S) 1.2 µm and 2.0 µm CMOS/SOS (silicon on sapphire); GPS (U.K.) 1.5 µm CMOS/SOS; TEMIC/MHS 0.8 µm and 0.6 µm CMOS; TCS 1.0 µm and 0.8 µm CMOS SOI (silicon on insulator). A list of some of the components developed for ESTEC can be obtained from the following web site: http://www.estec.esa.nl/wsmwww/components/ supportlist.html.

Saab Ericson Space is jointly owned by the Saab and Ericson Groups, which offer world class aerospace and telecommunications/computer technologies. Saab Ericson Space develops and manufactures a large variety of spacecraft equipment including onboard computers and data handling systems. For instance, Saab Ericson Space supplied the computers for the first four SPOT earth observation satellites. The development of new computers for SPOT-5 and the forthcoming large European satellite for environmental monitoring, Envisat, is in progress. Saab Ericson Space has developed its own fault tolerant microprocessor, THOR, specially designed to suit space computer applications requiring high reliability, long life and low sensitivity to cosmic radiation. THOR is also adapted to the ADA programming language, has been successfully tested in space, and will be used in the next Swedish satellite project, ODIN, to control satellite positioning in orbit. The company also led the industrial team that developed the space version of the SPARC microprocessor, ERC32. Products based on ERC32 are now available.

Telespazio is involved in specific programs and projects for telecommunications that include onboard processing. The ITALSAT Program is supported by Telespazio both operationally and as a participant. ITALSAT is a multibeam (six beams using two antennas) digital system that operates in the 30/20 GHz (uplink/downlink) frequency bands, providing onboard switching of signals using a baseband matrix switch. ITALSATITALSAT F1 was launched in January 1991. ITALSAT F2, launched in 1996, provides multibeam capabilities at 30/20 GHz as well as ISDN capability. Program objectives are:

In onboard switching technology, Telespazio is the prime contractor to ESA on Phase B activities for an advanced satellite system known as OBP. The Phase B activities encompass two main efforts: (1) to define the system and develop system specifications, and (2) to develop a laboratory model of the OBP that will include an engineering model of the onboard baseband switch matrix. The major technical innovations of the OBP package are as follows:

In Japan, CRL is planning ETS-VIII. This satellite is in the design stage and is expected to be a three ton GEO satellite that will be used primarily for studies of multimedia mobile communications between a base station and small, mobile terminals. Some onboard processing is anticipated including an onboard switch and computer system utilizing radiation hardened gate arrays with an SRAM based memory. This system will be ground controlled and include a 1 Mbps packet switch system. Non-packetized voice and packetized data will be handled by separate switches.

CRL is also proposing a K/Ka-band "Gigabit Satellite" which will address gigabit (1.2 - 1.5 Gbps), very high data rate (155 Mbps) and broadband multimedia (1.5 - 155 Mbps) users. The gigabit links will use SS/TDMA while the others use SCPC/TDM uplinks and TDM/TDMA downlinks with onboard ATM switching.

In the United States, Aerospace Corporation studied the financial and technical tradeoffs involved in OBP systems. Advances in waveforms and desire for future flexibility must be balanced with the difficulty of changing waveforms with a processing satellite repeater, and there is a need for increasing data rates and system capacity. Aerospace Corporation performed a study for the European Space Agency on the communications payload for the ICO system to determine if an all digital processing repeater for about 5,000 voice circuits was feasible. The study identified several alternative architectures and how they scaled with the number of circuits. The conclusion was that the approach of using an FF polyphase filter bank with narrowband channelization implementable with available chips resulted in the minimum power and was attractive.

NASA's Goddard Space Flight Center has extensive experience in information systems that can distribute large amounts of data directly from a spacecraft's onboard scientific instruments to data archive centers and/or scientists in the field. Goddard has funded the development of numerous onboard processors.

Several satellite programs ongoing at Hughes Space Communications (HSC) include onboard processing; among these are the 12 satellites for ICO Global Communications (London) that use phased array antennas with digital beam forming in addition to baseband communications processing. HSC anticipates the density and the layout of radiation hardened chips to reach the levels of today's standard CMOS in a few years. In addition, work in the industry is expected to extend CMOS chips to 8 million gates; extend InP devices to operate at 200 GHz; and enable the production of new SiGe HBT devices. OBP technology development for phase array antennas and their associated processors are considered very important for future communications satellites.

L-3 Communications Systems-West is actively engaged in unique technology development for specialized airborne antennas, wideband spread spectrum, multiplexers, modems, and command/control hardware. Of special note is L-3 development of image compression and ASICs for modulator functions. The modem ASICs are highly programmable to accommodate multiple data rates and modulation types. L-3 is completing development of an ASIC that will accommodate BPSK, QPSK and 8 PSK modulation with or without direct sequence spreading and data rates up to 140 Mbps.

Motorola is one of the most active companies in onboard processing. It is currently launching the Iridium satellites that constitute the first operational commercial use of onboard processing. The Iridium system has onboard demodulators, switching, and routing as well and orbital location control. The onboard processor has been constructed using 178 very large scale integrated circuits (VLSIs) designed specifically for the project. It includes 512 demodulators, with closed loops (via control channels to the hand-held units). For the Iridium satellite, each user shares 45 ms transmit and 45 ms receive frames in channels that have a bandwidth up to 31.5 kHz spaced 41.67 kHz apart. All users are synchronized so that they all transmit and all receive in the same time windows, alternatively. Motorola has extensive plans to develop further commercial satellite systems that require extensive OBP technologies with an order of magnitude greater capability than the Iridium system.

NASA Lewis Research Center (LeRC) was responsible for the Advanced Communication Technology Satellite (ACTS), which is currently in operation. ACTS has an onboard circuit switch and utilizes TDMA uplinks at 27.5 Mbps and TDM downlinks at 220 Mbps. The switch is controlled from the ground. ACTS also has a satellite matrix switch (SMS) onboard that performs wideband if switching controlled from a master ground station. The SMS can be programmed to perform dynamic switching in a cyclical manner. In addition, the Digital Communications Technology Branch at LeRC has funded development of numerous OBP technologies such as advanced modems and codecs, fast-packet switches, and multi-channel demultiplexer/demodulators. In addition, the branch has performed numerous studies related to specific satellite network architectures that would utilize onboard processing.

Teledesic is building a global, broadband "Internet-in-the-Sky." Using a constellation of several hundred LEO satellites (288 plus spares), Teledesic's network will provide worldwide, "fiber-like" access to telecommunications services such as broadband Internet access, videoconferencing, high-quality voice and other digital data needs. The Teledesic satellites require substantial amounts of OBP for phase array antenna control, switching and routing, modulation and coding, orbital location control and intelligent power distribution. Teledesic has toured the industrial world in order to identify companies that can supply systems and subsystems.


Published: December 1998; WTEC Hyper-Librarian