The NEAR Spacecraft RF Telecommunications System - The Johns ...

THE NEAR SPACECRAFT RF TELECOMMUNICATIONS SYSTEMThe NEAR Spacecraft RF Telecommunications SystemRobert S. Bokulic, M. Katherine E. Flaherty, J. Robert Jensen, and Thomas R. McKnightAn X-band telecommunications system developed for the Near Earth AsteroidRendezvous (NEAR) spacecraft represents an unmatched combination of performance,innovation, and cost-effectiveness for a deep space mission. It centers about tworedundant X-band transponder systems that provide the command, telemetry, andtracking functions. Despite a tight development schedule, a significant amount of newtechnology has been used in the system. Included in the design are the most recentdevelopments in transponder hardware, an X-band solid-state power amplifier (a deepspace “first”), and efficient microstrip patch antennas. During spacecraft emergencies,a microstrip fanbeam antenna is used as part of a unique Earth acquisition algorithm.Postlaunch measurements have verified that in-flight performance closely matchespredicted performance.(Keywords: Deep space transponders, Fanbeam antenna, NEAR, Patch antenna, Solidstatepower amplifier, Telecommunications, X-band.)INTRODUCTIONThe telecommunications system design for a typicaldeep space probe is driven by mission design. Not onlydoes the distance between the spacecraft and Earthvary, but the geometrical relationships among thespacecraft, Earth, Sun, and destination object(s) alsovary, making the antenna pattern requirements highlydependent on mission design.The Near Earth Asteroid Rendezvous (NEAR)mission profile calls for a cruise period of 3 years,including a solar conjunction in February 1997, a flybyof the main belt asteroid Mathilde in June 1997, amajor trajectory correction maneuver in July 1997,and an Earth swingby in January 1998. Having accomplishedthese milestones, NEAR will rendezvous withthe asteroid 433 Eros in January 1999 and eventuallygo into orbit around it.The spacecraft is designed for simplicity by configuringthe high-gain antenna (HGA) and solar panelsso that they are nongimbaled and pointed along thesame axis. This arrangement is made possible becausethe mission trajectory design keeps the Sun–probe–Earth (SPE) angle within 40 o for most of the mission(Fig. 1).The NEAR telecommunications system had to simultaneouslysatisfy the goals of low power, lowweight, low cost, and an extremely short deliveryschedule (27 months from start to launch). The primaryrequirement was to provide a science data returnJOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 213

R. S. BOKULIC ET AL.Earth range (AU)321100001996 1997 1998 1999 2000YearFigure 1. Earth range and Sun–probe–Earth (SPE) angle for theNEAR mission.of at least 85 megabits/day from the asteroid. Thisequates to a downlink data rate of at least 2.9 kbps,assuming one 8-h deep space network (DSN) pass perday. The telecommunications system also had to befully redundant and provide a dependable commandlink and a high-quality Doppler tracking capability.The DSN 34-m high-efficiency and beam waveguideantennas were baselined for all phases of the missionexcept critical periods and emergencies, during whichthe 70-m dishes would be used.TELECOMMUNICATIONS SYSTEMDESIGNFor the NEAR telecommunications system (Fig. 2),the X-band frequency region (7.2-GHz uplink/8.4-GHz80604020SPE angle (deg)downlink) was chosen over S-band to maximize thedata rate and tracking capabilities and to minimize thesize of the HGA feed. Redundant, state-of-the-arttransponder systems (discussed in the next section) arecentral to the design. These systems are connected toseveral antenna types to provide a variety of coveragepatterns for the mission (Fig. 3).The HGA is a 1.5-m-dia. dish intended to supplythe science data return at the asteroid. Its pencil beamgives coverage whenever the spacecraft is pointedtoward Earth. The fanbeam antenna has a mediumgaincapability for portions of the mission when thedistance to the Earth is large and the HGA cannot bepointed earthward. It has an important role in therecovery of the spacecraft during emergency situations.The low-gain antennas (LGAs) supply hemisphericalcoverage in the forward and aft directions for portionsof the mission when the spacecraft is relatively nearEarth. To save weight and minimize mechanical complexity,coaxial cabling is used for all RF interconnectionsinstead of waveguide.To minimize complexity, the mission uses a select setof bit rates: two uplink data rates (125 bps for normaloperations and 7.8 bps for emergency operations) andeight downlink data rates (six between 1.1 and 26.5kbps for normal operations using the HGA and two at39.4 and 9.9 bps for cruise operations and safe-moderecovery using the fanbeam antenna). Once the spacecraftreaches the asteroid, the downlink data rate willvary from 4.4 to 8.8 kbps (Fig. 4). Occasional use ofthe 70-m DSN antennas will permit data dumping atCommandsTelemetryNo. 1No. 2CommanddetectorunitTelemetryconditioningunitX-banddeep spacetransponder no. 1ReceiverExciterDiplexerSolid-statepower amplifier,5 WLow-gainantenna(aft)RHCFanbeamantenna,beamwidth= 8° 40°Right-handcircularpolarization(RHC)TransferswitchRHCHigh-gainantenna,1.5-m dishCommandsTelemetryNo. 1No. 2CommanddetectorunitTelemetryconditioningunitX-banddeep spacetransponder no. 2ReceiverExciterPowerconverterunitDiplexerSolid-statepower amplifier,5 WLeft-handcircularpolarization(LHC)RHCLow-gainantenna(forward)LHCPowerconverterunitFigure 2. Block diagram of the NEAR telecommunications system (switches shown in cruise configuration).214 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)

z axisHigh-gain antenna±1° about nadirTHE NEAR SPACECRAFT RF TELECOMMUNICATIONS SYSTEMwith a Reed-Solomon 8-bit (255,223) block code. TheNEAR telecommunications system takes advantageof the newly deployed Block V digital receivers inthe DSN.Fanbeam antennacoverage to 40°from nadirFigure 3. NEAR spacecraft antenna coverage.Forward low-gain antenna±90° about nadirAft low-gain antenna±90° about zenith17.6 and 26.5 kbps. Two convolutional codes are incorporatedinto NEAR: a rate 1/2, constraint length 7code for cruise and emergency operations and a powerfulrate 1/6, constraint length 15 code for asteroidoperations. In all cases, the symbols are concatenatedTRANSPONDER SYSTEMTECHNOLOGYAs shown in Fig. 2, the Motorola transponders areused for command reception, telemetry transmission,and coherent tracking. These units were developed forthe Cassini program (NASA’s mission to Saturn)under sponsorship from the Jet Propulsion Laboratory(JPL) and are being flown for the first time on NEAR.To condense the packaging and enhance performance,surface mount technology is used extensively in thetransponders, along with technologies such as dielectricresonator oscillators, surface acoustic wave oscillators,and high electron mobility transistors. Thetransponders are easier to produce than previous designsowing to a significantly reduced number ofunique hybrid designs and RF modules.The RF output of the transponder is amplified to a5-W level by an APL-developed solid-state poweramplifier (SSPA, Fig. 5). The use of solid-state X-bandpower amplification is a first for a deep space mission,breaking with the traditional traveling-wave-tubeamplifier approach. The unit incorporates metal-semiconductorfield-effect transistor technology with openloopgain compensation and is powered by an externalpower converter unit.Baseband data conditioning is accomplished by twocomponents: the command detector unit (CDU) andtelemetry conditioning unit (TCU). The CDU designwas developed by JPL for the Cassini program. ExactDownlink data rate (kbps)3020100Arrival1-dB margin26.5 kbpsArrival + 1 year17.6 kbps2-dB margin3-dB margin8.8 kbps4.4 kbps2.9 kbps1.1 kbps1999.0 1999.5 2000.0YearFigure 4. NEAR downlink data rate capability at the asteroidassuming use of the rate 1/6, constraint length 15 convolutionalcode and a 34-m ground antenna. These data hold for 90% ofweather conditions. Ground antenna elevation angle = 20 o .Figure 5. The NEAR X-band power amplifier. This unit representsthe first use of solid-state power amplification at X-band ona deep space mission. (Photograph courtesy of designers RoySloan and John Penn.)JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 215

R. S. BOKULIC ET AL.copies of that design, which incorporates uplink subcarrierdemodulation, bit detection, and synchronizationfunctions onto a single application-specific integratedcircuit chip, are being flown on NEAR. The TCU wasbuilt for NEAR by APL and is used to set both thedownlink mode (direct or subcarrier) and the modulationindex in each mode. In the direct mode, symbolsare sent directly to the transponder for modulationonto the carrier at a phase modulation index of 1.2 rad.In the subcarrier mode, low-rate symbols are modulatedonto a 23.4375-kHz square-wave subcarrier beforebeing sent to the transponder for modulation onto thecarrier at a phase modulation index of 0.9 rad. Thisindex was selected to optimize performance at the 9.9-bps downlink bit rate. Most digital functions of theTCU are incorporated onto a field-programmable gatearray. (Table 1 gives a power and weight breakdown forthe NEAR telecommunications system.)ANTENNA TECHNOLOGIESTo minimize weight, the 1.5-m HGA reflector isconstructed with a graphite-resin material on a Nomexhoneycomb core. The feed is a choke ring horn witha septum polarizer to provide dual-frequency, dualpolarizationcapability. The overallefficiency (effective area/physicalarea) of the antenna is 63% at 7.2GHz and 58% at 8.4 GHz. Colocatedon the feed assembly arethe forward LGA and a magnetometer.The presence of the magnetometerrequired careful selectionof materials for the feedassembly and coaxial cables, includingnonmagnetic RF connectorsmade with beryllium copper.The LGAs provide hemisphericalcoverage along the forwardand aft spacecraft directions andare extremely lightweight at 90 geach. The antennas are constructedas dual-frequency stacked microstrippatches on Rogers thermosetmicrowave material (TMM).This substrate material is highlystable over temperature and providesa relatively hard, yet machinable,surface. Dual polarization ismade possible through separatestacked patches on the same substrate(Fig. 6). Each LGA yields apeak gain (+6 dBic) and beamwidthcomparable to a horn antennaat a fraction of the weight. Theforward LGA is located on theHGA feed to minimize reflections from the spacecraft.Interestingly, a null in its pattern occurs at about 15 ooff boresight owing to backlobe radiation that is reflectedand focused by the HGA reflector.The fanbeam antenna has proven useful for manyscenarios, especially recovery from emergency situations.It incorporates two microstrip patch arrays on asingle substrate to give dual-frequency, right-handcircular polarization capability (Fig. 7) and provideswide-plane coverage out to 40 o from the spacecraft’s zaxis, with a narrow-plane 3-dB beamwidth of about 8 o .The peak uplink and downlink gains are 18.1 and 18.8dBic, respectively. This antenna is also built on RogersTMM substrate material. A slip-pin arrangement wasdesigned to prevent thermal working of the RF connector/microstripinterface. The fanbeam antenna’sdevelopment schedule was accelerated considerablyusing a tabletop circuit milling machine instead ofchemical etching for iterating the design; each designiteration took only 4 to 8 h to perform. The fanbeamantenna weighs 465 g, including an aluminum backingfor the substrate material. It has been a workhorsefor the mission, providing low bit-rate downlinkcommunications without having to slew the HGAtoward Earth.Table 1. Power and weight breakdown for the NEAR telecommunicationssystem.Nominalbus power WeightComponent (W) (kg) NotesX-band transponder 9.1 a 4.0CDU 1.0 0.4 Power number includes 79%efficient power converter intransponderTCU 3.0 0.8SSPA 34.0 0.8 Power number includes 80%efficient external powerconverter unitExternal powerconverter unitfor SSPA 0 1.3Diplexer 0 0.1Coaxial switchassembly 0 0.6 Includes all five switchesHGA 0 6.3 Excludes magnetometer andLGAFanbeam antenna 0 0.5LGA 0 0.1Note: Values are per unit unless otherwise noted.a Combined exciter and receiver power of 2.5 and 6.6 W, respectively.216 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)

THE NEAR SPACECRAFT RF TELECOMMUNICATIONS SYSTEMmission resulted in the Earth direction falling near anull in the fanbeam antenna pattern. In this case, thepredictions were less accurate because prelaunch measurementsof the antenna pattern (made on a mockupof the spacecraft) were less representative of the actualin-flight pattern near the pattern null.Figure 6. The NEAR LGA mounted at the end of the HGA feed. Theantenna incorporates two dual-frequency stacked patches, one foreach polarization. (Photograph courtesy of designer Allan Jablon.)IN-FLIGHT PERFORMANCEThe in-flight performance of NEAR’s telecommunicationssystem has closely matched prelaunch predictions.The special nature of a deep space proberequires careful monitoring of RF signal strengths andspacecraft oscillator frequencies. Uplink and downlinksignal strengths are tracked weekly, and the transponderreceiver and auxiliary oscillator frequenciesare tracked monthly.Figures 8 and 9 show a portion of the signal strengthhistory when the spacecraft was receiving andtransmitting through the HGA and fanbeam antenna,respectively. Experience has shown that the measuredsignal levels are usually within 1 dB of predicted values.The most noteworthy exception occurred duringmission days 22 through 38 when the geometry of theSUN-SAFE MODE RECOVERYOne of the more challenging aspects of the telecommunicationssystem design was the recovery procedurefor the spacecraft under emergency conditions. In theevent of a serious anomaly (e.g., low bus voltage), thespacecraft is autonomously pointed at the Sun andbegins a 2 o /min rotation about the spacecraft–Sun axis(the z axis) until contact with the Earth is made.Because of the large distances involved (up to 3.2 AU)and the relatively low transmitter power, a mediumgainantenna must be used to establish Earth communications.The fanbeam antenna provides a radiationpattern that extends from the z axis to approximately40 o off the z axis. As the spacecraft rotates, a beaconsignal emitted by the fanbeam will eventually sweepthrough the Earth direction and be detected on theground. With knowledge of the rotation rate, groundcontrollers can then send a “stop rotation” commandone revolution later. When the spacecraft roll isstopped, the downlink is modulated with low-rate data(39.4 or 9.9 bps), and troubleshooting can commence.For portions of the mission when the SPE angle isgreater than 40 o , the Earth range is sufficiently low thatcommanding can be done through the forward LGA,regardless of rotation phase.During the weekend of 17 and 18 August 1996, thescenario just described was tested when the spacecraftwent into Sun-safe mode. This occurred at an SPEangle of 34 o and an Earth range of1.3 AU. The downlink beacon wasobserved for about 12 min every 3h as it emerged from the receivernoise floor (170 dBm), peaked at159 dBm, then disappeared backinto the noise floor. A stop rotationcommand was sent successfully,with the peak of the fanbeampattern oriented closely towardEarth.Figure 7. The NEAR fanbeam antenna. The series-fed elements provide improvedefficiency over corporate-fed elements. The uplink and downlink arrays are combined witha microstrip diplexer. (Photograph courtesy of designer Jeffrey Sinsky.)SUBSYSTEM TEST ANDCHARACTERIZATIONThe design goals for the RFground support equipment (GSE)were to provide (1) a capable andcost-effective way to test the RFtelecommunications system duringsubsystem integration and (2) RFJOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 217

R. S. BOKULIC ET AL.Signal strength (dBm)–80–100–120–140Predicted uplinkMeasured uplinkPredicted downlinkMeasured downlinkrequiring a complicated decoder. The GSE also allowsfor Reed-Solomon decoding, thus yielding bit error rate(BER) measurements of the concatenated-codeddownlink data. Uplink BER testing is accomplishedwith a software BER tester (developed by Dan Minarikof APL and Orbital Sciences Corp.). This tester countsand updates errors on a frame-by-frame basis, giving theuser continuous insight into the performance at lowdata rates.–160320 340 360 380 400Mission dayFigure 8. Uplink and downlink signal strengths for the NEARspacecraft while transmitting and receiving through the HGA.Predicted and measured values are shown as a function ofmission time.Signal strength (dBm)–80–100–120–140–160Mission dayPredicted uplinkMeasured uplinkPredicted downlinkMeasured downlink–1800 40 80 120 160 200 240Figure 9. Uplink and downlink signal strengths for the NEARspacecraft while transmitting and receiving through the fanbeamantenna. Predicted and measured values are shown as a functionof mission time.communications to the spacecraft during spacecraftintegration and test activities. A significant challengeof the design was to ensure that the equipment couldreceive deep space signaling formats without incurringprohibitive costs. The resulting design is a combinationof off-the-shelf and special-purpose equipmentwith technical innovations in several key areas.The RF GSE provides uplink and downlink RFinterfaces to the flight subsystem. A split-channel receiversystem, developed by Microdyne, supplies carrierthreshold performance that is superior to traditionaltelemetry receiver designs. In addition, the RF GSEcan decode the rate 1/6, constraint length 15 convolutionalcoding employed by NEAR. The decoderdesign, which was realized on a single VME 6U card,performs code inversion at high signal levels. This ismade possible by synchronizing a local encoder withthe received encoded frame sync word and performinghypothesis testing on correlations between the localencoder output and the input symbols (developed byMark Simpson and Ed Mengel of APL). This innovationpermits testing at the spacecraft level withoutFUTURE TECHNOLOGYDEVELOPMENTDevelopment of the NEAR telecommunicationssystem has led to the identification of several key areaswhere technology improvements can enhance theperformance of deep space missions. For example,room exists for substantial integration and simplificationof the transponder. APL is pursuing an approachto break the unit into simpler transmit and receivefunctions that can be integrated onto cards in anintegrated electronics module. This approach is beingused for the RF system for NASA’s TIMED (Thermosphere-Ionosphere-MesosphereEnergetics and Dynamics)spacecraft currently being designed at APL.Portions of the command and data handling system areintegrated onto the cards along with the RF circuitry(e.g., the command receiver card also contains a realtimecommand decoder). For applications that requireDoppler tracking, the cards use a highly accuratenoncoherent navigation technique recently developedat APL. This innovation provides accuracy suitable fordeep space missions (< 0.1 mm/s) and is compatiblewith existing ground station assets.Room also exists for substantial improvements inSSPA efficiency. The overall efficiency (RF outputpower divided by 28-V bus input power) of X-bandSSPAs using today’s GaAs field-effect transistor (FET)technology is typically 15 to 25%. The goal over thenext 5 years is to double this range, thus potentiallydoubling the science return of future missions. Theseefficiency values can be improved through the use ofheterostructure FET, heterojunction bipolar, andpseudomorphic high electron mobility transistors.Finally, NEAR has shown that microstrip antennatechnology provides a lightweight, low-cost alternativeto conventional antennas for deep space missions.Further technological advances, such as aperturefeeding of the elements, will make large arrays moreefficient and easier to fabricate. Such advances willopen the door to the use of this technology for electronicallysteered HGAs.ACKNOWLEDGMENTS: The NEAR program is sponsored by the NASAOffice of Space Science. The authors thank all the members of the RF EngineeringGroup (formerly the Microwave and RF Systems Group) at APL who worked sohard on this challenging project. This work was supported under contract N00039-95-C-0002 with the U.S. Navy.218 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)

THE NEAR SPACECRAFT RF TELECOMMUNICATIONS SYSTEMTHE AUTHORSROBERT S. BOKULIC received his B.S.E.E. degree from Virginia PolytechnicInstitute in 1982 and his M.S.E.E. degree from The Johns Hopkins Universityin 1985. He joined the APL Space Department in 1982 and has been involvedin RF telecommunications systems ever since. He is currently the lead engineerresponsible for development of an S-band telecommunications system forNASA’s TIMED spacecraft program. Prior to that, Mr. Bokulic was the leadengineer responsible for the telecommunications system on the NEAR spacecraft.His experience includes work in areas such as link analysis, noise statistics,spectral analysis, phaselock loops, microwave hardware design, and linkverification testing. In addition to his work at APL, Mr. Bokulic is an instructorof communication systems courses in The Johns Hopkins University WhitingSchool of Engineering and in APL’s Career Enhancement Program. He is amember of the APL Principal Professional Staff and the IEEE. His e-mail addressis robert.bokulic@jhuapl.edu.M. KATHERINE E. FLAHERTY received a B.S.E.E. in 1992 and an M.S.E.E.in 1994, both from the University of Maryland. Her thesis work involved thedesign of high-power microwave amplifiers. She began her career at APL in1994 as a satellite communications engineer in the Space Department’s RFEngineering Group. Ms. Flaherty worked extensively on the integration and testof the NEAR satellite and now provides engineering support for the MissionOperations Team in the role of NEAR telecommunications analyst. She was theRF system engineer for AGRE, a project where an RF beacon was integratedonto a Russian rocket and tracked by the Midcourse Space Experiment.Currently, she is designing a new X-band deep space receiver. She teachesa science class to middle school students and is active in the outreachprograms of the Society of Women Engineers. Her e-mail address iskate.flaherty@jhuapl.edu.J. ROBERT JENSEN received a B.A. from Cornell College in Mt. Vernon,Iowa, in 1973 and Ph.D. in physical chemistry from the University ofWisconsin, Madison, in 1978. He joined the Submarine Technology Departmentof APL in 1978 and worked on a variety of nonacoustic detectionproblems, principally involving radar performance analysis, signal processingalgorithms, and rough surface scattering. In 1989, he joined the APL SpaceDepartment and has participated in TOPEX altimeter preflight testing, thedevelopment and testing of algorithms for the beacon receiver on the MSXsatellite, the NEAR telecommunications system, and other satellite programsincluding the development of new concepts for radar altimetry. He is a memberof the APL Principal Professional Staff, IEEE, and AGU. His e-mail address isbob.jensen@jhuapl.edu.THOMAS R. McKNIGHT received a B.S. in electrical engineering from theUniversity of Maryland in 1985 and an M.S. in electrical engineering from TheJohns Hopkins University in 1989. Before joining APL, he designed electronichardware for radio proximity fuzes at the Harry Diamond Laboratories. He beganhis career at APL in 1985 as a design engineer developing Global PositioningSystem receiver hardware and software. More recently, Mr. McKnight hasworked on various aspects of spacecraft telecommunications systems and on thedevelopment of hardware, software, and algorithms for an S-band monopulsereceiver. Prior to this, he developed signal processing algorithms for thedetection of speech in noise and for the analysis of laser radar vibration sensorsignal returns. His e-mail address is thomas.mcknight@jhuapl.edu.JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 219