William T. Brandon
Neil R. Helm
Robert K. Kwan
Edward R. Miller
The European Space Agency (ESA) has been, and continues to be, very active in the development of mobile satellite communications technology. It has produced very impressive results which have placed the European industry in a highly competitive position with that of the United States. The ESA program has followed a consistent and logical course since the late 1970s, addressing increasingly sophisticated technology development of both the space and ground segments.
Technology for first generation mobile communications satellites was developed by ESA during the late 1970s and resulted in the launch of the Maritime European Community Satellite-A (MARECS-A) in 1981 and MARECS-B2 in 1984. These two satellites were the most powerful elements of the Inmarsat system until the launch and commissioning of the first of the Inmarsat II series of spacecraft in late 1990. Both satellites were designed for maritime mobile satellite communications applications, but as a result of field trials, market surveys, and payload technology studies, ESA identified the land mobile market as the most important element of the mobile satellite services (MSS). To meet this need ESA funded a technology development program, referred to as the ARAMIS program, which focused on the development of key components. These include a breadboard of a large (2.2 m diameter) multi-beam phased array antenna, high efficiency linear L-band high power amplifiers (HPAs), high power diplexers, intermediate frequency (IF) filtering using surface acoustic wave (SAW) technology, and microwave monolithic integrated circuit (MMIC) IF beam forming. Some of this technology has been integrated into the European Mobile System (EMS) payload which will be flown aboard the ITALSAT F2 spacecraft to be launched in late 1994. Refined technology will be integrated into a more sophisticated payload, the L-Band Land Mobile (LLM) payload, to be flown aboard the Advanced Relay and Technology Mission (ARTEMIS) spacecraft planned for launch in 1995.
Elements of this technology have found their way into advanced commercial mobile satellite systems such as the Inmarsat III communications payload presently under development by Matra Marconi Space (MMS) in the UK. In addition, ESA has been studying the Advanced Research in Communications using Highly Inclined orbits for Mobile applications and other Experiments and Demonstrations using European Satellites (ARCHIMEDES) system, consisting of a constellation of four spacecraft operating in non-geostationary orbits (non-GEO) for direct satellite audio broadcast (DAB) and land-mobile (LMSS) applications. This system would be placed in elliptical orbit to enable high elevation angle operation over Europe to reduce shadowing and multipath problems. In addition, long-term developments are taking place in the area of large antenna structures and on-board processing techniques to enable enhanced future land mobile systems in the late 1990s.
Ground segment technology development has been carried out since 1983 with the PRODAT and Mobile Satellite Business Network (MSBN) programs. These efforts have included propagation studies and component development such as modems and development and field tests of mobile networks and transfer of the technology to industry. The following sections review in greater detail present and planned ESA experimental mobile satellite communications technology development, as well as the Inmarsat III communications payload, which contains significant European technology. Some of the mobile satellite payloads, in particular the ARTEMIS LLM payload and the ARCHIMEDES system, are described in other sections of this report and will not be discussed in depth in here.
European Mobile System (EMS). The EMS aboard the ITALSAT F2 spacecraft is shown in Figure 3.1. The EMS will provide two-way voice and data communications between fixed earth stations (FES) and land mobiles located anywhere in Europe and, possibly, North Africa (Perrotta 1990; Rogard 1992; Ananasso 1992; Jayasuriya 1990; Fraise 1992; Campbell 1992; Alenia Spazio 1991). Links between satellite and mobiles will be at L-band and the feeder link will be at Ku-band. For Europe, an edge of coverage gain of about 26 dB can be achieved with a single beam, for a satellite located around 20 degrees E. Another key element is that at such an orbital location it is possible to reuse the entire L-band spectrum. Characteristics of the EMS payload are given in Table3.1.
EMS Payload Characteristics
The EMS payload consists of two transponders, one for the forward link from the fixed ground station at Ku-band to the mobile terminals at L-band, and the other for the return link from the mobile terminals at L-band to the fixed earth station at Ku-band. Figure 3.2 gives a functional block diagram of the payload. The key technology elements of the payload are the L-band front-end and the IF processor. The L-band antenna has a 4-element off-set feed, illuminating a 2.5 x 1.2 m elliptical reflector deployed from the east panel of the spacecraft. The antenna is used for both transmit and receive functions and, because of the method of driving the elements, can produce either right-hand or left-hand circular polarization (RHCP or LHCP). The beam coverage from a 20 degrees E GEO location is shown in Figure 3.3. Four diplexers separate the transmit and receive signals, and four individually redundant low-noise amplifiers (LNAs) are incorporated in the diplexers. One of the critical design aspects of such a common transmit/receive front-end is the requirement to have an acceptably low level of passive intermodulation products (PIMs) generated by the broad-band transmit signal at frequencies in the receive band. By having four separate transmission paths from the input of the L-band amplifier modules to the antenna feed elements, the PIM performance requirement for the individual items of hardware is relaxed by some 42 dB for the critical seventh-order product. Another advantage of this front-end configuration is that the number of single-point failure possibilities is reduced; failure of a diplexer, cable or feed element will cause only a degradation of the radio frequency (RF) performance and a drop in capacity, but not a complete failure.
Figure 3.1. ITALSAT F2 With EMS Payload
Figure 3.2. EMS Payload Block Diagram
Figure 3.3. EMS Beam Pattern
The IF processor equipment is a single unit containing the channel filtering functions for both the forward and the return transponders. The transponders are essentially transparent. The forward IF processor converts the input signal down to an IF of about 150 MHz. SAW devices then filter out the three bands (4, 4 and 3 MHz). The resulting signal is then upconverted to L-band again before passing through the change-over switch which selects the polarization sense of the L-band transmission. The return IF processor performs essentially the same process as the Forward IF processor, except that each 4 MHz channel is further sub-divided into four channels of nominally 1 MHz (actually 0.85 MHz). Thus selective use of the sub-channels can be applied to ease the task of frequency coordination with other systems using the same band. The potential transponder loading caused by broad-beam mobile transmitters operating through another satellite can thus be minimized. The possibility of selecting the opposite hand of L-band polarization to that used by other systems can be used to give an additional degree of protection from interference. Full 2-for-1 redundancy is provided by a duplication of the channelization hardware in the unit. Redundancy switching is carried out by switching power supplies to the channelization circuits concerned and by operating change-over switches. Thus it is possible, by using the nominally redundant circuitry, to transmit and receive simultaneously in some channels with one hand of L-band polarization, and with the opposite hand of polarization in other channels. The forward and return EMS bands and services are given in Tables 3.2. and 3.3.
ARTEMIS. The major ARTEMIS mobile communications mission is the L-band (1.5/1.6 GHz) land mobile (LLM) communications mission demonstration. The LLM will utilize a large spacecraft reflector to provide spot beams and demonstrate frequency reuse. This system is described in detail in Chapter 5.
ARCHIMEDES. ARCHIMEDES is a high earth orbit (HEO) satellite system conceived by ESA to provide broadcast and mobile service. At the present time the primary mission of ARCHIMEDES is DAB and the secondary mission is LMSS. The ARCHIMEDES satellite system consists of a constellation of four satellites in 12 hour Molnyia orbits. Each satellite hovers for six hours per day over Europe at an elevation angle of greater than 70 degrees. By spacing the satellites in orbit planes 90 degrees apart, full 24 hour per day coverage is provided. By using HEO orbits, a high elevation angle line-of-sight path between the mobile user and satellite can be maintained even at northerly latitudes where signal fade and blockage due to low elevation angle would disrupt transmissions to and from a GEO satellite. ARCHIMEDES is described in more detail in Chapter 5.
EMS Forward Transponder Characteristics
EMS Retun Transponder Characteristics
Inmarsat III. Inmarsat contracted for its third generation spacecraft in January 1991 following an international competitive procurement and a technology validation program to increase confidence in certain key technology areas (Howell 1992; Haugli 1990; Subramaniam 1992). These satellites are now under construction, with a target first launch in 1994; the prime contractor is GE-Astro (now part of Martin Marietta), while the communications payload is being built by MMS in Portsmouth, U.K. The payload has a number of innovative features to greatly improve efficiency and increase capacity, both for exiting mobile services and for a new generations of digital earth stations, which have smaller antennas. Inmarsat III will provide a six-fold increase in capacity over Inmarsat II and will enable full utilization of the L-band mobile satellite communications frequency allocations.
The most significant technologies in the communications payload are the spot beam reconfigurability (that is, the ability to reconfigure both spot beam coverage and the proportion of RF power dedicated to each of the spot beams as well as the global beam) and reconfigurable channelization. The beam reconfigurability is achieved by use of multi-matrix distributed power amplifiers. Although matrix power amplifiers (MPAs) have been used in spacecraft before, for Inmarsat III a high amount of beam-to-beam isolation is required for frequency reuse and inter-system frequency coordination, resulting in challenging design requirements. Beam direction reconfiguration will be achieved by switchable beam forming networks (BFNs) and a rotatable feed assembly. Reconfiguration of the complex channelization is implemented in the IF processor. This processor has commandable synthesizers driving SAW filters. Adjacent SAW filters are bandwidth switchable, so that two filters can be combined to form one larger filter, recovering the guard-band.
Inmarsat III also has a C- to C-band channel to permit system internal administrative interconnection without using valuable L-band resources and an L to L-band channel for direct mobile to mobile communications under special circumstances (e.g., search and rescue operations) without requiring a double hop connection. The system also includes a navigation payload repeater, which transmits in the radio navigation L-band frequency allocation and can support a new emerging service category for Inmarsat and its signatories: radio determination, particularly radio navigation, and various hybrid communications/positioning related services including provision of a global navigation satellite system (GNSS) integrity channel, wide area differential corrections, and possibly active radio determination (positioning by two-way ranging).
The transmitted L-band EIRP for the communications transponder is generated by an offset reflector antenna system using a 22 element cup-helix feed array, in conjunction with 22 solid state power amplifiers (SSPAs) configured as an MPA generating around 440 W RF.
The Inmarsat III payload provides forward and return communications links between land earth stations operating at C-band and a range of differing mobile terminal types distributed globally and operating at L-band frequencies. The mobile terminals are serviced by a combination of global beam and up to six simultaneous spot beams. These spot beams provide a 6 dB gain improvement over current global coverage beams and improved spectrum utilization efficiency through frequency reuse. Five spacecraft GEO locations are defined with spot beam locations optimized for projected high traffic areas, for example. These spot beam locations are reconfigurable in orbit when the satellites are repositioned as shown in Figure 3.4. The traffic distribution between the spot and global beams will have a daily variation with time zone, and will change as new services are introduced during the satellite lifetime. The payload traffic capacity can therefore be flexibly redistributed between the beams on a per channel basis. The Inmarsat III transponder characteristics are given in Table 3.4.
EUTELSAT EUTELTRACS. This system has recently been introduced into Europe and operates via EUTELSAT space segment at Ku-band primarily assigned to the fixed satellite service (FSS). Following initial trials of the system carried out by European posts, telephone and telegraph entities (PTTs) to evaluate system performance, EUTELSAT entered the pre-operational demonstration phase for a six month period beginning in January 1992. Initial indications are that the system performs satisfactorily in a LMSS environment. This system employs a relatively narrow beamwidth mobile antenna coupled with spread spectrum transmissions to enable it to coexist with other Ku-band FSSs without causing harmful interference. The latter issue, the question of frequency compatibility, is currently under study in some European administrations.
Figure 3.4. Two Possible Inmarsat III Beam Configurations
Inmarsat III Transponder Characteristics
LOCSTAR. The LOCSTAR organization in Europe has proposed the implementation of a radio determination satellite services (RDSS) system to offer, in addition to the RDSS, a messaging service which utilizes the system's low-speed data transmission capability. The proposed system is to operate at L-band where primary allocations exist in some of the European countries. It is clear, therefore, that successful frequency coordination on a pan-European basis is required for the implementation of the service.
Inmarsat Project 21. The objective of Inmarsat Project 21 is to introduce affordable, personal mobile satellite communications (Inmarsat 1992). Inmarsat plans Project 21 to be an evolutionary program which will build on existing Inmarsat technology and assets and will result in a hand-held satellite phone called Inmarsat-P before the end of the decade. Inmarsat believes that a significant market for this product will exist because of (1) the mobile communications explosion, (2) the limitations of terrestrial services, (3) the miniaturization of mobile satellite equipment, (4) a favorable regulatory climate, (5) accelerating market growth, and (6) developing country applications. In mid-1992 Inmarsat had requested its signatories to carry out market studies to define requirements for Project 21. It was planned that these requirements would be generated in late 1992. Inmarsat has also funded a number of studies with U.S., Japanese and European companies to study system architectures for Project 21. Architectures under study include low earth orbit (LEO), GEO and LEO/GEO constellations. Project 21 is planned to be compatible with terrestrial cellular phone systems and it is thought by Inmarsat that the European GSM cellular standard will become the global standard. Project 21 is in the early development stage but potentially it could become coupled with one of the presently proposed L-band LEO communications satellite systems.
ESA has had a mobile satellite communications ground segment development activity in place for almost ten years. This program has included market studies, system architecture design, terminal development, field trials and technology transfer to European industry of two L-band systems called PRODAT and the Mobile Satellite Business Network (MSBN) (ESA brochure; Jongejans 1992; Rogard 1989; Roederer 1992). In addition, the program has supported focused ground segment technology in modems, mobile antennas and MMIC development for mobile applications. System architecture studies have included GEO, LEO and intermediate orbit systems, such as ARCHIMEDES, terrestrial (GSM)/satellite hybrid systems and 20/30 GHz aeronautical systems.
The PRODAT system is a low data rate message handling system designed to serve land, maritime and aeronautical mobile satellite communication with a single system. The program is being carried out in several phases. Phase One was an analytical phase and was completed in 1985. In 1987 the first tests and demonstrations started, using first the MARECS satellite and later other Inmarsat satellites. During these tests several thousands of messages were exchanged, including communications with land mobile terminals on trucks, maritime terminals and aeronautical terminals. Following these tests PRODAT Phase Two was initiated, in which a redesign of the hub station was carried out and the mobile terminal simplified. Industrialization of the system was begun and was to be completed in late 1992. The forward link uses a binary phase shift keyed/time division multiplexed (BPSK/TDM) structure, with a rate of 1.5 kbits/sec. The return link is based on code division multiple access (CDMA) with 35 codes at 300 bits/sec with all terminals operating at the same frequency. The low rate terminals utilize an omnidirectional antenna (1 dB gain), with a G/T of -24 dB/K, a class C HPA with 10 to 12 W EIRP. The system uses forward error correction. The next phase of this program will be the development of EUROPHONE, which will utilize the EMS payload aboard ITALSAT F2 to provide low rate data transmission to small terminals (G/T = -23 dB/K) and a voice/data service at 9.6 kbits/sec for terminals with enhanced characteristics.
The Mobile Satellite Business Network (MSBN) is a second system under development and will utilize the ITALSAT F2 EMS payload. This system will enable random access with reservation for a voice at 6.4 kbits/sec or random access for short messages at 2.4 kbits/sec. The basic configuration of the MSBN system will consist of a star network with one dedicated channel. The design will be capable of evolving toward a multi-hub network, a shared network and a multi-channel star network configuration. The system will enable a user to have direct access to the satellite through very small aperture terminals (VSAT) hub stations operating at Ku-band. These terminals will have an EIRP of 45 dBW/channel and G/T of 20 dB/K, which corresponds to an antenna diameter of about 1.8 m, and a 1 W amplifier. The mobile earth stations are characterized by an EIRP of 22 dBW and a G/T of -13 dB/K, derived from the PRODAT terminal with a directive antenna providing about 11 dB gain.
Mobile antenna technology under development includes low gain antennas for use in air, sea and land applications. Development activities also include a maritime pyramidal electronically steered antenna with five facets and a gain of 7 dB. For land mobile applications a two-element mechanically steered array with 11 dB gain and a low profile array with printed yagis and low profile motor have been developed.
ESA believes that mobile satellite communications will be an important segment of the satellite communications market and has been aggressively pursuing the development of technology in this area. This area is being given much higher priority within ESA than within NASA. The ESA program includes the development and launch of a series of L-band land mobile communications payloads and a vigorous mobile ground terminal development program. The NASA MSAT-X program was the only comparable payload development program, but because of funding limitations no space hardware was developed and this program has been decreased to essentially zero funding level. The NASA MSAT-X program and the ACTS Mobile Terminal program led to the development of L-band and Ka-band mobile terminal technology and demonstrations but these programs have been very limited in scope in comparison with the ESA programs. Mobile satellite communications programs are being pursued vigorously by U.S. private industry, however. Examples of these systems are the AMSC GEO mobile communications satellite system scheduled for launch in 1994, a number of L-band LEO systems, such as the Motorola IRIDIUM, TRW ODESSEY and SS/Loral GLOBESTAR, and VHF/UHF LEO systems such as the Orbital Sciences Orbcomm system. The large infusion of government funding into European aerospace industry in the mobile communications sector will have long range impacts on the competitive balance between U.S. and European industry, and will create a clear disadvantage for American industry without NASA support.