Chp 5: Data Relay Satellite (DRS)

DATA RELAY SATELLITE (DRS)
Introduction
DRS will be an essential component of the ESA In-OrbitInfrastructure (IOI) planned to become operational in the second halfof the 1990s; it will provide services similar to the NASA TDRSS (RI4-1and 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 FlyingLaboratory, Polar Platform and COLUMBUS Attached Laboratory), HERMES,EURECA, SPOT and other platforms operating between 400 and 800 kmaltitude. The DRS space segment includes two operational satellites inGEO stationed at 44 degrees W (DRSS-W) and 59 E (DRSS-E), to ensurewide orbital coverage of LEO spacecraft and a good coverage of Europe.The system is designed to avoid imposing severe requirements on theuser on-board communication payloads. The DRS orbit configuration isshown in Figure5.13.
System level studies of the critical technologies were performedunder the Data Relay Preparatory Program (DRPP), approved in late 1986.In 1989 the European Ministerial Council approved the execution of theData Relay and Technology Mission (DRTM) Program, associating twoprogram elements: ARTEMIS and the operational DRS. The launch dates ofthe primary users are planned to be 1998 for the Polar Platform, 2002for HERMES and 2003 for the COLUMBUS Free Flyer; therefore, the DRSSshould be operational for the launch of the first Polar Platform in1998. However, the other major primary users will require the serviceonly after the year 2000 (see Table5.6). In orderto avoid the full deployment of the two DRSs dedicated to the firstPolar Platform (PPF #1), ESA decided, in late 1991, to use the ARTEMISsatellite in an operational mode with DRS after 1998. ARTEMIS will beused for tests and in-orbit demonstration of the data relay service, byusing 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 missionARTEMIS will be located at around 6 degrees E. In 1998 the ARTEMISsatellite will be moved to a DRS orbital position (59 degrees E) andwill provide an operational data relay service to the Polar Platform,together with the first DRS satellite. The second DRS satellite willeventually be launched when required by increased traffic. The userdata rate requirements are given in Table5.6).
Figure 5.13. DRS Orbit Concept
The DRS satellites will provide an increased data relay capabilitycompared with ARTEMIS. The final system architecture has not beenselected, but one option consists of an optical payload and twoS/K-band IOL accesses. A second option consists of one S/K-band accessplus the S-band multiple access phased array originally planned forARTEMIS.
DRS Primary User Requirements

Each DRS will be equipped with repeaters providing almost continuouscommunication links between user space terminals (UST) on-board LEOspacecraft and user earth terminals (UET). UETs may be located almostanywhere in Europe and, with some restriction, outside Europe. The DRSis conceived as a decentralized service in which data will be receivedby a number of earth terminals. This contrasts with the NASA TDRSSwhich has only one earth terminal. The feeder link frequency band willbe at Ka-band (20/30 GHz). For the IOL between the DRS and the spaceusers, 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, responsiblefor the monitoring and control of the entire system, and theoperational control center, responsible for the monitoring and controlof the DRS. It also includes three TT&C stations and three remoteranging terminals for the DRS position determination. The groundsegment also includes two ESA earth terminals dedicated to serve HERMESand COLUMBUS.
DRS Communication Payload
Frequency Plans and Coverage. In order to avoid interferencewith other services, the frequency band for communications between theDRSs and the ground is Ka-band (27.5 to 30.0 GHz) in the forward linkand K-band (17.7 to 20.2 GHz) in the return link. For the IOL betweenDRS and the space users, the frequency bands used are:
S-band, from 2.025 to 2.110 GHz in the forward direction, and 2.200 to 2.290 GHz in the return direction
K-band, from 23.120 to 23.550 GHz in the forward direction and Ka-band 25.250 to 27.500 GHz in the return direction
The optical band, using the 800 nm window in both directions
Both DRS satellites are required to provide almost permanent andfull connectivity between a UST and a ground station that can belocated anywhere in a large portion of Europe. Figure5.14 shows the specified European coverage zone, as delimited by apolygon with vertexes located at Fucino, Madrid, Liverpool, Oslo, Malmoand Vienna. To enhance system flexibility and connectivity, a steerablespot beam antenna is foreseen on-board DRS, serving locations outsideEurope that see DRS with more than 5 degree elevation. Broadcastingover such a large zone conflicts with the requirement to separate thesatellites as far as possible, in order to maximize the LEO spacecraftcoverage. Satellite separation is even more constrained by the highatmospheric attenuation that is experienced at low elevation angles inthe 20/30 GHz frequency band foreseen for the feeder link. Thesatellite locations at 44 degrees W (DRSS-W) and at 59 degrees E(DRSS-E) are the result of a compromise between the feeder link designfor both the ground stations and the DRS antenna and transmitters, andthe minimization of the zone of exclusion.
Communication Requirements. Tables5.7and5.8 show the transmission modes in both forwardand return links for a single active channel. The forward link is lessdemanding 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 remotesensing payloads leading to a demanding service (up to 150 Mbits/sec).The high data rates together with the need for minimizing the powerrequirements on the UST led to using coded links (convolutional r =1/2, K = 7) at the expense of a bandwidth doubling. The S-band linkscarry command, telemetry and low rate data. Spread spectrum modulationis used at S-band to comply with the power flux density regulationsconstraints. The optical channel is regenerative, and uses opticalfrequencies in the IOL in forward and return binary pulse positionmodulation (2-PPM). In the feeder link (20/30 GHZ) binary phase shiftkeyed (BPSK) modulation is used.
SKDR Payload RF Requirements. Table5.9presents the feeder link RF requirements. The feeder link requirementsare specified over several locations within Europe; here only a subsetis reported. The positions 44 degrees W and 59 degrees E refer to theDRS satellite, while the position 59 degrees E and 6-19 degrees E arerelevant to ARTEMIS.
The aim of the link design has been to have a uniform power fluxdensity over any location within Europe, thus compensating for thevariations in the path length and atmospheric attenuation withdifferent G/T and EIRP requirements. Table5.10shows the IOL RF requirements. The EIRP is the maximum required, butthe satellite must be able to deliver lower levels as a function of theservice data rates.
Figure 5.14. Coverage of the DRS Feeder Link
Forward Link Transmission Modes

Return Link Transmission Modes

Feeder Link G/T and EIRP Requirements

RF IOL EIRP and G/T Requirements

SKDR Payload on DRSS. The DRS capability will be increased incomparison with ARTEMIS, by providing the DRS with two S/K-band IOLaccesses rather than one. Capability will also be increased by adding asteerable feeder link antenna, which will establish a bi-directionalspot beam between the DRS and any point of the visible earth. Thisallows the RF connection with countries outside Europe (U.S., Japan,Kourou in the French Guyana). The return link service will provide upto five simultaneously operating channels in the feeder link, allowingthe 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 channelsfor PPF #2). The DRS SKDR payload selection has not been finalized butit is expected to be based on the design of the SKDR payload onARTEMIS. The system described here is largely based on the hardwareprocured for ARTEMIS, but with five channels rather than four forgreater redundancy. A block diagram of the system is shown in Figure5.15. Two RF front ends serve the two feeder linkantennas (European and steerable). The power dividers feed four tunablefrequency converters, two of which are common to the paths coming fromthe two feeder-link antennas. A new element in the block diagram is areconfigurable microwave switching matrix (RSM), based on a powerdivider/combiner architecture, connecting the 5.5 GHz signal to theproper transmission chain. The S-band IOL section consists of threechains in 2/3 redundancy. Each chain provides the down conversion toS-band and amplification (30 W SSPA).
The Ka-band IOL section comprises four transmitting chains. Twochains are used for transmission of the communication signals towardstwo IOL users, while a third chain is used to broadcast a beaconsignal. The tunable frequency converters feed a 30 W TWTA. The returnrepeater has two IOL antennas of a design similar that to be flown onARTEMIS. When operating at S-band, the output of one antenna feeds thediplexer and a receiving section, in 2/3 redundancy, which amplifiesthe signal and converts it to the 5.5 GHz IF. The K-band IOL section isfed by the front ends behind the two IOL antennas. These in turn feedtwo 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 GHzRSM has six input ports connected to the K-band IOL section, two portsconnected to the S-band IOL section and a port fed by the 5.5 GHz BPSKmodulator, which receives the baseband 50 Mbits/sec signal from theoptical terminal. The nine RSM outputs are connected to seven returntransmitting chains, while two outputs feed the tracking receivers. Theoutput chains work in 5:7 redundancy, converting the 5.5 GHz signals toa selectable frequency in the 20 GHz frequency band.
After a ring redundancy network, the signals (up to fivesimultaneously) feed the output multiplexers and the EFLA. Two transferswitches derive two channels driving the second output multiplexers andthe steerable feeder link antenna (SFLA). The EFLA is conceptuallysimilar 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 feedand beam steering achieved by reflector (0.7 m diameter) steering. Thedesign of the IOL antennas and of the antenna pointing subsystem willbe similar to those on ARTEMIS.

Figure 5.15. DRS SKDR Payload Block Diagram (option with 2 singleaccess S/Ka Band IOL antennas)
Optical Communications Package
The optical communications package is expected to be based onSILEX.
Satellite Platform. The mass of the payload is expected to bearound 205 kg. Because of the major interest of Italy in the ARTEMISand DRS programs, an enhanced ITALSAT platform has been baselined asshown in Figure5.16. The ARTEMIS satellite systemis designed to demonstrate new technologies and, because of this,includes an ion propulsion system. The possibility of using eitherchemical bipropellant or ion propulsion for DRS has not been completelyeliminated.

Figure 5.16. DRS Spacecraft Configuration
Satellite Configuration. The optical terminal will be mountedon the spacecraft on the +Z face, together with the EFLA and SFLAs. A2.85 m aperture IOL antenna will be stowed on one East/West wall duringlaunch and deployed once in orbit. This antenna will operate at boththe S-band and 23/26 GHz IOL frequencies. The antenna will bemechanically steered during tracking of a LEO spacecraft. An S-bandmultiple access phased array will be stowed on the other East/West wallduring launch and deployed through 90 degrees when in orbit. In stowedconfiguration, DRS can be accommodated as an upper passenger in a dualARIANE-4 launch. ARTEMIS, however, is intended to be launched on one ofthe ARIANE-5 APEX qualification flights so it is anticipated that DRSwill be qualified for an environment which encompasses both ARIANE-4and -5 launch conditions.
Conclusions
Recent ESA directives approved by the European Council of Ministersin the fall of 1991 modified the mission objectives of the two elementsof the DRTM program. A K-band IOL data relay capability has been nowintroduced 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 fromthe results of the early phase studies of DRS. Among the possibleoptions for DRS is a payload architecture reusing the hardware designedand manufactured for the ARTEMIS satellite. It is expected that theARTEMIS C/D phase will start late in 1992 with the satellite to belaunched in 1995, after construction and testing. The first DRS couldthen be launched two years later, in 1997, to provide full data relayservice to the Polar Mission.

Published: July 1993;WTECHyper-Librarian