ARCHIMEDES

Introduction

ARCHIMEDES is a high earth orbit (HEO) satellite system conceiv ed by ESA to provide European mobile radio services (Taylor 1992; Stuart 1992; British Aerospace 1990). At the present time, the primary mission of ARCHIMEDES is direct satellite audio broadcast (DAB); the secondary mission is LMSS. The ARCHIMEDES satel lite system consists of a constellation of four satellites in 12 hour Molnyia orbits as illustrated in Figure 5.18. Each satellite hovers for six hours per day over Europe at an elevation angle of greater than 70 degrees. By spaci ng the satellites in orbit planes 90 degrees apart, full 24-hour 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 a nd blockage will disrupt transmissions to and from a GEO satellite.


Figure 5.18. ARCHIMEDES Orbit Configuration

Many European national studies such as the British T-SAT , the French SYCOMORES and the German LOOPUS were crystallized by ESA into studies of the ARCHIMEDES system by British Aerospace, Stevenage, UK. Support for the system was ratified by the ESA Council of Ministers Meeting held in Munich (November 1991) an d has been stated in the long term plan. This support is planned to be in the form of sponsoring the non-recurring development costs of the first generation system. This ESA support would provide the following:

This sub stantial support to the development of a new commercial satellite system for Europe is similar to that given by ESA to the formation of EUTELSAT. ESA plans for the first spacecraft to be launched in 1997. ESA will only commit to this plan if a commercia l consortium can be formed to provide the remainder of the first generation system. Commercial partners, therefore, are required to sponsor the recurring development costs. Looking beyond the first generation system, subsequent generations could be expa nded to cover other regions to build ARCHIMEDES into a global, mobile satellite system.

Services

Direct Satellite Audio Broadcast. The total EIRP of each ARCHIMEDES satellite is 62.5 dBW. The G/T of the individual L-band beams is +8 d B/K and the total bandwidth is approximately 50 MHz. It is claimed that the EIRP per Hz of this system has a far higher performance than any other satellite system. This performance gives the ARCHIMEDES two major advantages over its competitors: (1) th e cost per channel is low, and (2) the high EIRP and G/T allow very low cost mobile terminals to be used.

Link analysis of the DAB system has assumed a Ku-band feeder link from uplink stations in eight channel blocks. The link margin assigned to the fading channel has been set at 3 dB at the edge of coverage to guarantee worst case availability of 99 percent in any hour. At the center of coverage, and with the satellite directly overhead, the link margin is over 9 dB in practice. It is clear then t hat the availability will be better than 99 percent overall. With these figures the required HPA output power per stereo channel is 6.4 W. The net effect of these factors shows a minimum 8 dB advantage of HEO over GEO. The cost of a channel in the HEO system should, therefore, be about 8 dB cheaper than the cost of a GEO channel. The communications system has been designed to provide as many DAB channels as possible with reasonable economic constraints on spacecraft antenna size. A frequency allocati on has been assumed in the region of 1.5 GHz, and six 2.8 degree beams are arranged to provide multinational coverage of Europe. With greater than 99 percent availability, 96 stereo channels of 256 kbits/sec each are provided over Europe.

The transmi ssion scheme employs a multipath resistant COFDM scheme. The provision of eight channels in each 2 Mbits/sec COFDM block provides a range of ISDN-rate program distribution options from studio to feeder station. The system provides 12 such 8-channel bloc ks with flexibility to switch blocks to any of six beams providing multi-national coverage. Assuming the available payload power to be 2,500 W and the DAB transmission scheme to be COFDM/MUSICAM, the required RF per channel is 5.9 W giving a system capac ity of 96 DAB channels. State-of-the-art bit-rate-reduction schemes (e.g., MUSICAM) allow the compression of the transmission data rate from about 1.5 Mbits/sec to about 220 kbits/sec. The COFDM technique takes additive advantage of the power in the mul tipath signals and is bandwidth efficient achieved by digitally multiplexing several stereo channels into a single COFDM block (typically eight stereo channels).

As an illustrative example the link budgets for DAB have been calculated. These assume t he transmission scheme is based on the COFDM/MUSICAM standard as proposed by the European Broadcast Union. The link budget shows that the required satellite RF power per stereo channel is 5.9 W. For this link budget the optimum HPA output backoff was ca lculated to be 3.5 dB. At this output backoff the efficiency of the amplifiers in converting DC power to RF power is 23 percent. The spacecraft can supply approximately 2,500 W of DC power to the amplifiers after all other power requirements have been m et. Therefore the capacity of the ARCHIMEDES system is about 96 stereo channels.

Land Mobile Satellite Service. The costs of an initial operational system may be shared with an LMSS payload. This combined mission would require an additional 5 m receive antenna, with both services making use of the common transmit antenna. With appropriate use of power control and voice activation each stereo broadcast channel is convertible to about 120 voice channels. Flexibility is provided to transfer R F power to the LMSS application according to market demand. This can provide up to 12,000 voice channels or a lower number of two-way 64 kbits/sec channels. The ARCHIMEDES system provides two grades of service for LMSS: a near-toll-quality service conn ecting mobile terminals to the Public Switched Telephone Network and a private service connecting mobiles to small private earth stations located at the users' premises. Two types of mobile terminals are available, also two earth stations for the two app lications, the major difference between these being the reduced size and cost of the private earth stations. The mobile segment operates at L-band and the fixed segment at Ku-band.

Orbit Configuration and Antenna Coverage

A MOLNYIA system has been selected on the basis of lowest cost for a European application. In this system four spacecraft are placed in highly elliptic orbits inclined at approximately 63 degrees and with a 12 hour period. Each spacecraft is placed in a separate orbital pl ane so that the four orbital planes are spaced at 90 degrees. Each of the spacecraft is active for six hours over the desired European coverage region, returning 24 hours later.

The ground track of each MOLNYIA spacecraft is shown in Figure 5.19, which shows the apogee dwell over Europe and also the second apogee over the Pacific. In Figure 5.20 is shown the high elevation angles which are achieved by this four satellite configuration over Northern Europ e. The inner 70 degree contour encloses cities from Reykjavik to Moscow. The outer 60 degree contour encloses all of Mediterranean Europe and Eastern Europe as far as the Ural mountains.

The classic spacecraft configuration in Molnyia orbit is with sun-pointing arrays fixed to the spacecraft body and a steerable antenna focused on the earth. For ARCHIMEDES a configuration was selected where the antennas are mounted on the earth-pointing face of the spacecraft, and a combination of rotation about th e boresight axis together with solar array drive mechanisms are used to keep the arrays pointing towards the sun. Since the spacecraft must rotate to keep the solar arrays sun-pointing, the antennas must produce a beam pattern which rotates to counter th is motion and maintain a fixed footprint on the earth. An array-fed imaging system with a 5 m reflector has been adopted as a baseline. An edge of coverage gain of 36 dB is provided in each beam. Some additional advantage is possible through power cont rol on a beam basis, but it is clear from the beam geometry that there is little chance of any frequency reuse between beams.


Figure 5.19. ARCHIMEDES Single Satellite Grou nd Track


Figure 5.20. ARCHIMEDES Multi-Beam Arrangement

Ground Segment

Feeder Link Terminals. Fixed earth stations for HEO applications differ from conv entional GEO equipment only in that they must be able to track the satellite in elevation and azimuth. An earth station design incorporating this tracking capability was produced by Telespazio during the first ARCHIMEDES study. The ARCHIMEDES system des ign cancels out most of the errors resulting from Doppler shift at the fixed earth station, which adjusts its transmit frequency accordingly. This leaves only a small resultant error at the mobile terminal due to the variation in Doppler shift across the coverage area, which increases the size of the guard bands required between channels. The requirement to hand over communications services between satellites occurs regularly during the normal service period. The solution adopted has both satellites op erating simultaneously on different channels. At the beginning of the handover period no calls are accepted on the old (off-duty) satellite and new calls are gradually allocated to the new satellite, transponder by transponder, until all calls have been transferred.

A master control station controls the allocation of LMSS capacity on the spacecraft and logs calls for billing purposes. It is anticipated that this function would be co-located with the general housekeeping TT&C activities in a larg e central earth station. A demand assignment scheme is operated for both public and private services to make the most efficient use of the spacecraft channels. Voice channels are allocated on a single channel per carrier system, with users who request a ccess to the system being allocated a pair of frequency slots in the forward and return directions. At the end of each call these frequencies are returned to the pool of frequencies that can be allocated to future users. During the study a set number of capacity options to provide 200, 1,000 and 5,000 voice channels were analyzed. In order to satisfy the capacity requirements an optimization of the number of antenna beams was carried out, resulting in the selection of 1, 7 and 19 beam solutions. Vocod ed voice channels are used, operating at 9.6 kbits/sec offset QPSK with an overall link quality of 45 dBHz for the public service. The private service operates with 4.8 kbits/sec voice at 42 dBHz link quality. FDMA is adopted for its low cost and inhere nt system flexibility. Voice activation is used both in forward and return directions as a means of economizing on satellite RF power, and mobile link power control is also used, giving a significant power saving.

Mobile Terminals. The perfor mance of the L-band mobile terminal is critical to the system. This must be compatible with mass production techniques and simple fitting to the vehicle. Studies have assumed a microwave integrated circuit patch construction flat-mounted on the roof of the car. This is possible because of the high elevation angles available through the use of ARCHIMEDES. The electrical performance is designed to provide baseline coverage to 70 degrees in elevation. The terminal G/T is -1.4 dB/K at edge of coverage. In order to effect an economic receiver design with the same low cost mass production goals, a differential detection scheme is required with an Eb/No of 8.5 dB. Mobile terminals require a directional antenna. There is no need for steerable arrays; desi gns ranging from a simple helix element, to a short backfire element, to a patch array produce relatively high antenna gains at low manufacturing cost.

Technology Challenges

The chosen aperture size results from a cost optimization which provides enough channels per spacecraft for an economic system while avoiding the risk and expense of technologically challenging reflector diameters. Such deployable 5 m reflectors were planned for demonstration on the ARTEMIS LLM payload, but have been replaced with smaller 3.3 m reflectors. Elsewhere in the world they are regarded as mature or even "old" technology. Reflectors of this size were built in the 1970s for the U.S. TDRSS program.

ARCHIMEDES Spacecraft

The spacecraft design can be based on standard GEO platforms, but with modifications to the attitude and orbit control subsystem. Figure 5.21 shows the basic configuration of the spacecraft. The spacecraft can accommodate a relatively large comm unications payload and supply it with 2,500 W of DC power. The spacecraft are three-axis stabilized and use earth pointing which requires rotation of the spacecraft body around the boresight. The spot beams remain fixed with respect to their designated coverage areas by electronic steering using an imaging phased array antenna. Use of a dedicated vehicle is feasible. The Delta, Atlas, and Long March 2 launch systems are compatible, but the Russian Vostock launcher (sometimes called the MOLNYIA rocket) is designed for injection into inclined orbit, and from its 60 degree launch site at Baikonur offers the most cost-effective solution.


Figure 5.21. ARCHIMEDES Spacecraft with 5 m Antennas


Published: July 1993; WTEC Hyper-Librarian