Figure 1. A Ku-band reconfigurable antenna system (Alenia Spazio)
Satellite communications is both the largest and the fastest-growing sector of space applications. In the field of television broadcasting, for example, there are already more than 22 million analogue satellite TV receivers installed in Europe alone, and the advent of digital TV will greatly increase the number of people who will want satellite TV. The introduction of new services, such as hand-held communications via constellations of low-earth-orbiting satellites, is imminent, and high-speed multimedia communications as part of the Global Information Infrastructure will become a reality by the end of the century. To meet the rapidly growing demand of today's and tomorrow's services, satellite communications systems and applications must evolve just as quickly.
ESA's ASTP and successor ASTE programmes are providing many of the advanced technologies and equipment needed for today's and tomorrow's communications satellite systems. Like the ASTP, the ASTE programme not only covers satellite subsystem and equipment developments, but also addresses service developments and the demonstration of end-to-end user applications. For most of these activities inter-operability with the current terrestrial communications infrastructure is a key requirement, in order to ensure the optimum functionality for the Global Information Infrastructure.
ESA is continuing to look towards the future and in 1994 it started the Advanced Research in Telecommunications Systems (ARTES) Programme, which has two primary objectives:
The ARTES programme has, at present, 12 elements, ranging from studies and investigations to the development and demonstration of new satellite payloads and associated services (see inset). Additional elements will be defined as the need for new developments or activities arises.
One of the most exciting areas within the ARTES Programme is the development of Advanced Systems and Telecommunications Equipment (ASTE). Within the ASTE Programme, i.e. ARTES Element 5, European and Canadian firms are designing and developing state-of-the-art hardware and software to meet the requirements of future communication satellites poised to enter the World market.
This article gives an overview of the ASTE Programme (and its predecessor, ASTP) and highlights some of its successful developments.
ESA's supporting satellite communications programmes actually have a long history: they began in the early 1970s when a Supporting Technology Programme (STP) was created to develop technology for the Orbital Test Satellite (OTS). The STP was then followed by a series of four-year Advanced Systems and Technology Programmes (ASTP) which provided the framework for the European Communications Satellites (ECS) and Maritime Communications Satellites (Marecs), and prepared industry for a number of non-ESA satellite procurements.
ASTP-4, the fourth programme in the series, was then followed in 1994 by ASTE, an on-going programme, which has received subscriptions to date of about 72 million ECU. The emphasis of the ASTE Programme is on new experimental missions and services, and novel systems and equipment. Since the start of the Programme, many areas have been investigated, including:
Many of the studies and developments undertaken under the ASTE Programme are on the verge of becoming commercial products or service offerings, ready for the World satellite communications market.
On ESA's behalf, European industry has developed advanced on-board antenna systems for telecommunications and broad-casting satellites. The activities supported include the development of reflectors, feed systems and direct radiating arrays. A number of engineering/qualification models of new antenna systems have been built under the ASTP Programme. With that experience, industry in turn has been able to develop the expertise required to bid for contracts for operational satellite systems.
Some of the highlights to date are:
Figure 1. A Ku-band reconfigurable antenna system (Alenia
Spazio)
Figure 2. A Ku-band reconfigurable antenna system (CASA)
Shaped reflector systems are currently dominating the Ku-band fixed satellite service. Mobile and Intelsat C-band systems already use flexible beam-forming networks in the antenna feed system. The requirement for such systems is expected to grow and extend to other frequencies, such as the Ku- and Ka-bands. Under ASTP, key elements have been developed to respond to those specific requirements. They include high-power analogue and digital beam-forming networks. Two analogue high-power beam-forming systems are shown as examples. They were parallel developments, undertaken to engineering-model standard, of Ku-band beam-forming networks and the requisite variable power dividers and variable phase shifters. The multi-chip module developed within ASTP for digital beam-forming has recently been selected for flight on the EAST satellite proposed by Matra Marconi Space (UK).
In support of the current and future satellites, ESA has expended great effort in the development and improvement of RF equipment for satellite payloads, including power amplifiers, low-noise amplifiers, up- and down-coverters, switch matrices, filters, multiplexers and local oscillators. Key goals for these developments are: improvement of electrical performances (e.g. efficiency of power amplifiers), improvement of reliability, miniaturisation, cost reduction, and reduced time-to-market.
Some of the highlights of the developments undertaken are:
Important development work has been performed within the ASTP Programme in the area of travelling-wave-tube amplifiers (TWTAs). FIAR's 135 W TWTA is under manufacture and will be flown on the Eutelsat HotBird-4 satellite at the end of 1997. The development of a fully integrated microwave power module is planned in the current phase of the ASTE Programme.
Figure 3. A Travelling-Wave-Tube Amplifier (FIAR)
Ground terminals
Eighty percent of the
satellite communications market is in the ground segment - in
other words, the sale of satellite terminal equipment and the
provision of value-added services. The space segment itself -
satellite production, launch and operations accounts for the
other 20 percent. In addition, the ground segment is the fastest-growing
and most competitive sector.
Within the ASTP and ASTE Programmes, therefore, the focus has been on the development of satellite terminals, networks and associated applications for broadcasting, fixed and mobile satellite services.
In the broadcasting area, ESA has emphasised the development of digital transmission systems, based on digital satellite television and on future satellite audio broadcasting channels.
In the fixed satellite service area, novel ground-terminal systems and associated applications have been studied, designed and developed. The focus has been on addressing applications such as digital satellite newsgathering, video conferencing, portable personal communications, and low-data-rate data collection and remote control. High-speed satellite links for the backup of fibre-optic transmissions and the connection of terrestrial network nodes is another highlight of the ASTP and ASTE Programmes. Several of these developments are already at an advanced stage and about to reach the market.
For mobile satellite services, ground terminals and networks have been developed for operation with the new-generation regional European mobile satellite payloads: EMS (the European Land Mobile System) which was launched in August this year on the Italian satellite, Italsat-F2, and LLM (L-band Land Mobile) which will fly on ESA's Artemis satellite. Small terminals - either vehicle-mounted or suitcase-style - have been developed for land-mobile communications.
Several of these developments have been extremely successful. Some examples of the new technologies are:
Figure 4a. Arcanet (Indra Espacio)
Figure 4b. TSAT 2000 (TSAT)
Figure 4c. Pico terminal (MPR, Joanneum Research)
Figure 4d. PRODAT terminal (FIAR)
Figure 4e. MSBN mobile terminal (FIAR, VTT, Ylinen)
Figure 4f. Digital SDH satellite modem (Newtec)
satellite modem (Newtec)
Electrical power systems
Electrical power
is one of the satellite's most fundamental requirements, given
that loss of onboard power equates to the loss of the space
mission.
The onboard power-system requirements have increased significantly for communication satellites as the payload power levels have been rising and satellite lifetimes have been increasing over the last three decades, reaching up to 15 years for today's commercial communications satellites.
To ensure that European industry is prepared for the changing requirements, ESA has undertaken work in all areas related to the satellite's electrical power system: solar arrays including solar cells, batteries, and power control and distribution.
Two of the most successful developments undertaken within ASTP and ASTE are the integrated power-conditioning unit and advanced rigid four- and five-panel solar arrays.
Figure 5. An integrated power conditioning unit (Alcatel ETCA)
Figure 6. Advanced rigid solar arrays (Fokker Space)
Attitude and orbit control systems
Until
a few years ago, requirements for attitude and orbit control
systems (AOCS) for communications satellites were focussed on
geostationary satellites. Low, medium and high Earth orbits (LEO,
MEO and HEO) have only recently been considered for
communications satellite missions. ESA, for example, has been
studying a HEO satellite concept, called Archimedes, which would
primarily be used for digital audio broadcasting. Such missions
impose different constraints to the traditional geostationary
(GEO) mission, particularly where onboard autonomy and low-cost,
low-complexity AOCS solutions are concerned.
In support of communications satellite programmes in such new orbits, as well as those in the conventional GEO orbit, ESA has devoted considerable effort to AOCS-related areas, including the development of sensors, attitude and orbit actuators, and the associated onboard data processing.
Two highlights of such ASTP and ASTE developments are the improved infrared Earth sensor developed by Officine Galileo and the magnetic-bearing momentum wheel developed by Teldix Bosch Telecom.
Figure 7. A magnetic-bearing momentum wheel (Teldix Bosch
Telecom)
Satellite operations
Satellite operations
are a major driver of the overall cost of a satellite system
mission. Traditionally, satellite integration and testing, in-orbit
commissioning and normal operations have been very complex
procedures per-formed by large teams supported by special high-performance
computers. Within ASTP and ASTE, ESA has undertaken
a number of activities to simplify these procedures and reduce
costs. Because of the highly competitive nature of communications
satellite services, the cost element is particularly important.
One focus of ESA's activities has been the development of a low-cost, stand-alone simulation, testing and operations tool based on the new worldwide CCSDS (Consultative Committee for Space Data Systems) telemetry and telecommand standard. It includes the development of a front-end work station that converts the packetised CCSDS space-to-ground link to baseband, a telemetry and telecommand graphic display unit, and a satellite simulator that interfaces with the CCSDS work station and simulates all basic satellite functions.
The application of artificial-intelligence-based systems to satellite operations has also been studied. It could assist spacecraft operators in coping with the high complexity onboard the satellite and enhance their ability to detect and resolve anomalies.
Another example of a successful ASTP development is the low-cost satellite simulator for next-generation satellites, which is a very useful tool for satellite, subsystem and pay-load testing during satellite integration and operations.
Data-relay services for space users
A
number of low-Earth-orbiting (LEO) satellites will be launched
within the next decade, including Earth-observation, science,
manned and military-reconnaissance missions. Conventionally, data
generated by the LEO instruments is stored onboard the satellite
- a far from failsafe method - and transferred in high-data-rate
bursts to strategically placed ground stations during the short
periods when the satellite is in view.
A more attractive alternative is to transmit the data in real time to the ground via a data-relay satellite in geostationary orbit. This offers the advantage of prompt data transfer to the users, eases RF downlink requirements, and considerably simplifies the ground segment by reducing the number of ground stations needed to service the system. Optical terminals offer potential advantages of lower mass and power consumption, and smaller antenna size and footprint compared to microwave alternatives for such inter-orbit links.
ESA has programmes underway to place data-relay satellites in GEO within the next decade. The first to fly will be Artemis in 1999/2000 an experimental satellite for testing and operating these new telecommunications services. It will carry a revolutionary laser data-relay payload called SILEX (Semi-conductor Laser Inter-satellite Link Experiment). Using laser communications, SILEX will be able to receive data from LEO satellites (initially the French Spot-4 satellite) and relay it in the Ka-band feeder link to user ground stations at very high data rates. Being bi-directional, it will also relay commands from the Control Centre to a LEO spacecraft via Artemis.
Japan is also preparing a data-relay user spacecraft, OICETS, a LEO satellite that will carry a laser terminal for communicating with the SILEX terminal on Artemis.
ESA has also been looking further ahead than the SILEX programme, into the future of optical inter-satellite communications. Three directions have been taken within the ASTP and ASTE Programmes:
Figure 8. A Small Optical User Terminal (Matra Marconi Space,
CAL, SPAR, Spacebel)
In support of RF data-relay services, both S-band and Ka-band user terminals are being developed within the ASTP and ASTE Programmes for use onboard LEO satellites. An experimental S-band terminal will be flown on Spot-4 to transmit its telemetry data to ground using ESA's Artemis and the Japanese Comets data-relay satellites.
Satellite navigation
At present,
satellite navigation is based on the US military Global
Positioning System (GPS) infrastructure and its Russian
counterpart GLONASS. Ultimately, a global navigation satellite
system must come, at least partially, under civil institutional
control. It is of strategic importance that Europe participate
in such a system, as this will allow independent access for
services such as air-traffic navigation and surveillance, and a
variety of services for maritime and land applications, such as
an Intelligent Highway System.
Figure 9. A multistandard receiver for satellite navigation
(Sextant Avionique, Thomson CSF, CIR, GMV, Univ. of Leeds)
ESA, the European Union and EuroControl (the European ari-traffic-control organisation) are cooperating in the deployment of an initial European overlay, the Global Navigation Satellite System (GNSS-1), that will complement the GPS system. This system will be composed of a network of ground stations, and control and processing facilities as well as geostationary navigation transponders, in order to provide GNSS-1 users with enhanced satellite-navigation performance over the European region. To this end, additional ranging signals, integrity information and wide-area differential corrections will be broadcast to the users.
ESA has already started work on the GNSS-1 ground system within ASTP and ASTE. A multi-standard receiver demonstrator has been developed for use in field trials to support the definition phase of the system.
In addition, other activities are planned within the ASTE Programme which will include advanced applications of navigation receivers onboard satellites. One particularly promising application is the use of GNSS receivers for the attitude and orbit control of LEO satellites, which has tremendous market potential in view of the proposed LEO satellite constellations for future global personal and multimedia communications.
The achievements of the ASTP and ASTE Programmes have been displayed at various exhibitions, including the ESA stands at Telecom '95 in Geneva and at the AIAA 16th International Communications Satellite Systems Conference and Exhibition in Washington DC in February this year. A special brochure was produced for this event explaining ESA's satellite communications support programmes and giving a catalogue for the items exhibited (ESA BR-115, available from ESA Publications Division).
Figure 10. Full-scale model of the Artemis satellite on
display at Telecom '95 in Geneva
The ASTE Programme is managed by the ASTE Programme Office within the ESA Telecommunications Directorate. The present three-year phase of ASTE includes some 85 activities, with an average contract value of 700 kECU. For the administration and management of these ASTP-4 and ASTE activities, a modern information management tool has been developed. The system contains information on both proposed and approved activities, and on their administrative progress. It is being used for online consulting, as a reporting tool, and for project control of the ASTP-4 and ASTE activities.
The first phase of the ASTE Programme has been extended to mid-1997. The second overlaps with the first, covering the period from early 1997 to 1999, and the work plan for 1997 has already been approved. The overall envelope of the ASTE Completion Phase is 100 MECU and the first subscriptions to the programme have already been received.
The ASTE Programme will continue to prepare European industry for the upcoming opportunities of advanced satellite communications. Emphasis will be put on the development of prototype systems in order to reduce the time-to-market and increase European industry's competitiveness in the fierce marketplace of satellite communications. The Programme will focus on the integration of satellite communications systems with advanced applications providing users with complete end-to-end solutions. Activities are grouped into the different satellite communications service areas: Fixed and Broadcast, Navigation and Advanced Mobile Satellite Services, and General Communications Services. In order to reflect the importance of support activities for the Global Information Infrastructure (GII), a new area of activities has been introduced focussing on this domain. Inter-satellite cross-links will be part of the GII and related activities will therefore be undertaken in support of the GII for the provision of future multi-media services via satellite.
The ASTE has continued the success of the long series of ASTP Programmes. This has been recognised by the industrial Telecommunications Advisory Groups and by Delegations through the approval of the second phase of ASTE and the early subscriptions that have already been received. It is an essential element in the successful development of future satellite communications services and applications in Europe and Canada.
ESA Bulletin Nr. 88.