DATA RELAY SATELLITE (DRS)

Introduction

DRS will be an essential component of the ESA In-Orbit Infrastructure (IOI) planned to become operational in the second half of the 1990s; it will provide services similar to the NASA TDRSS (RI4-1 and RI4-2). The IOI will give autonomy to Europe in communication, control and monitoring of a variety of manned and unmanned spacecraft. Primary DRS users are the COLUMBUS elements (COLUMBUS Free Flying Laboratory, Polar Platform and COLUMBUS Attached Laboratory), HERMES, EURECA, SPOT and other platforms operating between 400 and 800 km altitude. The DRS space segment includes two operational satellites in GEO stationed at 44 degrees W (DRSS-W) and 59 E (DRSS-E), to ensure wide orbital coverage of LEO spacecraft and a good coverage of Europe. The system is designed to avoid imposing severe requirements on the user on-board communication payloads. The DRS orbit configuration is shown in Figure 5.13.

System level studies of the critical technologies were performed under the Data Relay Preparatory Program (DRPP), approved in late 1986. In 1989 the European Ministerial Council approved the execution of the Data Relay and Technology Mission (DRTM) Program, associating two program elements: ARTEMIS and the operational DRS. The launch dates of the primary users are planned to be 1998 for the Polar Platform, 2002 for HERMES and 2003 for the COLUMBUS Free Flyer; therefore, the DRSS should be operational for the launch of the first Polar Platform in 1998. However, the other major primary users will require the service only after the year 2000 (see Table 5.6). In order to avoid the full deployment of the two DRSs dedicated to the first Polar Platform (PPF #1), ESA decided, in late 1991, to use the ARTEMIS satellite in an operational mode with DRS after 1998. ARTEMIS will be used for tests and in-orbit demonstration of the data relay service, by using SPOT-4 for the optical link and other LEO users (e.g., EURECA), from the launch date (1995) to 1998. During this phase of the mission ARTEMIS will be located at around 6 degrees E. In 1998 the ARTEMIS satellite will be moved to a DRS orbital position (59 degrees E) and will provide an operational data relay service to the Polar Platform, together with the first DRS satellite. The second DRS satellite will eventually be launched when required by increased traffic. The user data rate requirements are given in Table5.6).


Figure 5.13. DRS Orbit Concept

The DRS satellites will provide an increased data relay capability compared with ARTEMIS. The final system architecture has not been selected, but one option consists of an optical payload and two S/K-band IOL accesses. A second option consists of one S/K-band access plus the S-band multiple access phased array originally planned for ARTEMIS.

Table 5.6
DRS Primary User Requirements

Each DRS will be equipped with repeaters providing almost continuous communication links between user space terminals (UST) on-board LEO spacecraft and user earth terminals (UET). UETs may be located almost anywhere in Europe and, with some restriction, outside Europe. The DRS is conceived as a decentralized service in which data will be received by a number of earth terminals. This contrasts with the NASA TDRSS which has only one earth terminal. The feeder link frequency band will be at Ka-band (20/30 GHz). For the IOL between the DRS and the space users, three different frequency bands will be used: S-band (2 GHz), Ka-band (23/26 GHz) and the optical band (800 nm).

The ground segment includes the mission control center, responsible for the monitoring and control of the entire system, and the operational control center, responsible for the monitoring and control of the DRS. It also includes three TT&C stations and three remote ranging terminals for the DRS position determination. The ground segment also includes two ESA earth terminals dedicated to serve HERMES and COLUMBUS.

DRS Communication Payload

Frequency Plans and Coverage. In order to avoid interference with other services, the frequency band for communications between the DRSs and the ground is Ka-band (27.5 to 30.0 GHz) in the forward link and K-band (17.7 to 20.2 GHz) in the return link. For the IOL between DRS and the space users, the frequency bands used are:

Both DRS satellites are required to provide almost permanent and full connectivity between a UST and a ground station that can be located anywhere in a large portion of Europe. Figure 5.14 shows the specified European coverage zone, as delimited by a polygon with vertexes located at Fucino, Madrid, Liverpool, Oslo, Malmo and Vienna. To enhance system flexibility and connectivity, a steerable spot beam antenna is foreseen on-board DRS, serving locations outside Europe that see DRS with more than 5 degree elevation. Broadcasting over such a large zone conflicts with the requirement to separate the satellites as far as possible, in order to maximize the LEO spacecraft coverage. Satellite separation is even more constrained by the high atmospheric attenuation that is experienced at low elevation angles in the 20/30 GHz frequency band foreseen for the feeder link. The satellite locations at 44 degrees W (DRSS-W) and at 59 degrees E (DRSS-E) are the result of a compromise between the feeder link design for both the ground stations and the DRS antenna and transmitters, and the minimization of the zone of exclusion.

Communication Requirements. Tables 5.7 and 5.8 show the transmission modes in both forward and return links for a single active channel. The forward link is less demanding than the return link, being required to transmit commands, audio and high definition video channels up to 25 Mbits/sec at Ka-band. The return link relays data from on-board experiments and from remote sensing payloads leading to a demanding service (up to 150 Mbits/sec). The high data rates together with the need for minimizing the power requirements on the UST led to using coded links (convolutional r = 1/2, K = 7) at the expense of a bandwidth doubling. The S-band links carry command, telemetry and low rate data. Spread spectrum modulation is used at S-band to comply with the power flux density regulations constraints. The optical channel is regenerative, and uses optical frequencies in the IOL in forward and return binary pulse position modulation (2-PPM). In the feeder link (20/30 GHZ) binary phase shift keyed (BPSK) modulation is used.

SKDR Payload RF Requirements. Table 5.9 presents the feeder link RF requirements. The feeder link requirements are specified over several locations within Europe; here only a subset is reported. The positions 44 degrees W and 59 degrees E refer to the DRS satellite, while the position 59 degrees E and 6-19 degrees E are relevant to ARTEMIS.

The aim of the link design has been to have a uniform power flux density over any location within Europe, thus compensating for the variations in the path length and atmospheric attenuation with different G/T and EIRP requirements. Table 5.10 shows the IOL RF requirements. The EIRP is the maximum required, but the satellite must be able to deliver lower levels as a function of the service data rates.


Figure 5.14. Coverage of the DRS Feeder Link

Table 5.7
Forward Link Transmission Modes

Table 5.8
Return Link Transmission Modes

Table 5.9
Feeder Link G/T and EIRP Requirements

Table 5.10
RF IOL EIRP and G/T Requirements

SKDR Payload on DRSS. The DRS capability will be increased in comparison with ARTEMIS, by providing the DRS with two S/K-band IOL accesses rather than one. Capability will also be increased by adding a steerable feeder link antenna, which will establish a bi-directional spot beam between the DRS and any point of the visible earth. This allows the RF connection with countries outside Europe (U.S., Japan, Kourou in the French Guyana). The return link service will provide up to five simultaneously operating channels in the feeder link, allowing the transmission to ground of data coming from the two polar platforms (2 x 100 Mbits/sec channels for PPF #1 and 3 x 100 Mbits/sec channels for PPF #2). The DRS SKDR payload selection has not been finalized but it is expected to be based on the design of the SKDR payload on ARTEMIS. The system described here is largely based on the hardware procured for ARTEMIS, but with five channels rather than four for greater redundancy. A block diagram of the system is shown in Figure 5.15. Two RF front ends serve the two feeder link antennas (European and steerable). The power dividers feed four tunable frequency converters, two of which are common to the paths coming from the two feeder-link antennas. A new element in the block diagram is a reconfigurable microwave switching matrix (RSM), based on a power divider/combiner architecture, connecting the 5.5 GHz signal to the proper transmission chain. The S-band IOL section consists of three chains in 2/3 redundancy. Each chain provides the down conversion to S-band and amplification (30 W SSPA).

The Ka-band IOL section comprises four transmitting chains. Two chains are used for transmission of the communication signals towards two IOL users, while a third chain is used to broadcast a beacon signal. The tunable frequency converters feed a 30 W TWTA. The return repeater has two IOL antennas of a design similar that to be flown on ARTEMIS. When operating at S-band, the output of one antenna feeds the diplexer and a receiving section, in 2/3 redundancy, which amplifies the signal and converts it to the 5.5 GHz IF. The K-band IOL section is fed by the front ends behind the two IOL antennas. These in turn feed two 1:4 power dividers which drive six tunable frequency converters, two of which are shared among the two IOL accesses. The 9 x 9 5.5 GHz RSM has six input ports connected to the K-band IOL section, two ports connected to the S-band IOL section and a port fed by the 5.5 GHz BPSK modulator, which receives the baseband 50 Mbits/sec signal from the optical terminal. The nine RSM outputs are connected to seven return transmitting chains, while two outputs feed the tracking receivers. The output chains work in 5:7 redundancy, converting the 5.5 GHz signals to a selectable frequency in the 20 GHz frequency band.

After a ring redundancy network, the signals (up to five simultaneously) feed the output multiplexers and the EFLA. Two transfer switches derive two channels driving the second output multiplexers and the steerable feeder link antenna (SFLA). The EFLA is conceptually similar to that on ARTEMIS (single offset reflector with two feeds), but optimized for the DRS positions (44 degrees W and 59 degrees E). The SFLA design is based on a single offset geometry, with fixed feed and beam steering achieved by reflector (0.7 m diameter) steering. The design of the IOL antennas and of the antenna pointing subsystem will be similar to those on ARTEMIS.


Figure 5.15. DRS SKDR Payload Block Diagram (option with 2 single access S/Ka Band IOL antennas)

Optical Communications Package

The optical communications package is expected to be based on SILEX.

Satellite Platform. The mass of the payload is expected to be around 205 kg. Because of the major interest of Italy in the ARTEMIS and DRS programs, an enhanced ITALSAT platform has been baselined as shown in Figure 5.16. The ARTEMIS satellite system is designed to demonstrate new technologies and, because of this, includes an ion propulsion system. The possibility of using either chemical bipropellant or ion propulsion for DRS has not been completely eliminated.


Figure 5.16. DRS Spacecraft Configuration

Satellite Configuration. The optical terminal will be mounted on the spacecraft on the +Z face, together with the EFLA and SFLAs. A 2.85 m aperture IOL antenna will be stowed on one East/West wall during launch and deployed once in orbit. This antenna will operate at both the S-band and 23/26 GHz IOL frequencies. The antenna will be mechanically steered during tracking of a LEO spacecraft. An S-band multiple access phased array will be stowed on the other East/West wall during launch and deployed through 90 degrees when in orbit. In stowed configuration, DRS can be accommodated as an upper passenger in a dual ARIANE-4 launch. ARTEMIS, however, is intended to be launched on one of the ARIANE-5 APEX qualification flights so it is anticipated that DRS will be qualified for an environment which encompasses both ARIANE-4 and -5 launch conditions.

Conclusions

Recent ESA directives approved by the European Council of Ministers in the fall of 1991 modified the mission objectives of the two elements of the DRTM program. A K-band IOL data relay capability has been now introduced in the ARTEMIS, in addition to the optical and S-band IOLs. The new S/K-band data relay payload of ARTEMIS is largely derived from the results of the early phase studies of DRS. Among the possible options for DRS is a payload architecture reusing the hardware designed and manufactured for the ARTEMIS satellite. It is expected that the ARTEMIS C/D phase will start late in 1992 with the satellite to be launched in 1995, after construction and testing. The first DRS could then be launched two years later, in 1997, to provide full data relay service to the Polar Mission.


Published: July 1993; WTEC Hyper-Librarian