NASA/TP--2000-210047

ACTS Ka-Band Earth Stations: Technology,

Performance, and Lessons Learned

Richard C. Reinhart

Glenn Research Center, Cleveland, Ohio

Steven J. Struharik

COMSAT Laboratories, Clarksburg, Maryland

John J. Diamond

Analex Corporation, Brook Park, Ohio

David Stewart

GTE Technology Organization, Cleveland, Ohio

May 2000
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NASA/TP--2000-210047

ACTS Ka-Band Earth Stations: Technology,

Performance, and Lessons Learned

Richard C. Reinhart

Glenn Research Center, Cleveland, Ohio

Steven J. Struharik

COMSAT Laboratories, Clarksburg, Maryland

John J. Diamond

Analex Corporation, Brook Park, Ohio

David Stewart

GTE Technology Organization, Cleveland, Ohio

Prepared for the

Sixth Ka-Band Utilization Conference

sponsored by the Istituto Internazionale delle Comunicazion

Cleveland, Ohio, May 31--June 2, 2000

National Aeronautics and

Space Administration

Glenn Research Center

May 2000
Acknowledgments

The authors wish to thank the many people and organizations who have contributed to the work described herein.

Throughout the ground-station development and operations, many technical and nontechnical contributions

were made. We would like to acknowledge the feedback from the experimenter community to improve

station operations, the industry partners who advanced Ka-band technologies, and the earth station

teams and all support personnel who implemented the experiments program.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Price Code: A03
ACTS Ka-Band Earth Stations:

Technology, Performance, and Lessons Learned

Richard C. Reinhart

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

richard.c.reinhart@grc.nasa.gov

Steven J. Struharik

COMSAT Laboratories,

Clarksburg, Maryland 20871

John J. Diamond

Analex Corporation

Brook Park, Ohio 44142

David Stewart

GTE Technology Organization

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44 135

Summary

The Advanced Communications Technology Satellite

(ACTS) Project invested heavily in prototype Ka-band satellite

ground terminals to conduct an experiments program with

ACTS. The A(_TS experiments program proposed to validate

Ka-band satellite and ground-station technology, demonstrate

future telecommunication services, demonstrate commercial

viability and market acceptability of these new services, evaluate system networking and processing technology, and characterize Ka-band propagation effects, including development of

techniques to mitigate signal fading.

This paper will present a summary of the fixed ground

terminals developed by the NASA Glenn Research Center and

its industry partners, emphasizing the technology and performance of the terminals and the lessons learned throughout their

6-year operation, including the inclined orbit phase-ofoperations. An overview of the Ka-band technology and components developed for the ACTS ground stations is presented.

Next, the performance of the ground-station technology and its

evolution during the ACTS campaign are discussed to illustrate

the technical tradeoffs made during the program and highlight

technical advances by industry to support the ACTS experiments program and terminal operations. Finally, lessons learned

during the development and operation of the user terminals are

discussed for consideration of commercial adoption into future

Ka-band systems.

The fixed ground stations used for experiments by government,

academic, and commercial entities used reflector-based offsetfed antenna systems with antennas ranging in size from 0.35 to

3.4 m in diameter. Gateway earth stations included two systems

referred to as the NASA Ground Station (NGS) and the Link

Evaluation Terminal (LET). The NGS provides tracking, telemetry, and control (TT&C) and Time Division Multiple Access

(TDMA) network control functions. The LET supports technology verification and high-data-rate (HDR) experiments.

The ground stations successfully demonstrated many services and applications at Ka band in three different modes-ofoperation: circuit-switched TDMA using the satellite onboard

processor, satellite-switched TDMA (SS-TDMA) applications

using the onboard Microwave Switch Matrix (MSM), and

conventional transponder (bent-pipe) operation. Data rates

ranged from 4.8 kbps up to 622 Mbps. Experiments included (a)

a low rate (4.8 to 100 kbps) remote data acquisition and control

using small earth stations, (b) moderate rate (l to 45 Mbps)

experiments with full duplex voice and video conferencing and

both full duplex and asymmetric data rate protocol and network

evaluation using midsize ground stations, and {c) link characterization experiments and HDR ( 155 to 622 Mbps) terrestrial

and satellite interoperability application experiments conducted

by a consortium of experimenters using the large transportable

ground stations.

Overview of Ground Stations

The ground stations developed by NASA Glenn Research

Center (GRC) and its industry partners included five different

NASA/TP-2000-210047 1
sizeterminals, eachwithuniquecapabilitiesdesignedtomeet

asetofapplicationsforexperimentsanddemonstration atKa

band.Thelargeground-stationfacilitiesof theACTS/GRC

programaretheNASAGroundStation(NGS)andtheLink

EvaluationTerminal(LET).TheNGSservesas(a)ACTS

primarytrackingtelemetryandcontrol(TT&C)station,(b)a

BasebandProcessor (BBPI/VerySmallApertureTerminal

(VSAT)TimeDivisionMultipleAccess(TDMA)network

referencestation,(c)twoVSATtrafficterminals, and(d)a

backupfacilityto theLETfor MicrowaveSwitchMatrix

(MSM)experiment operations. TheLETservesas(a)ahubfor

theUltraSmallApertureTerminal(USAT)starnetworkexperiments, (b)ahigh-data-rate (HDR)terminal intheGigabit

Network,(c)anexperimenter stationusedtoconducttechnologyverificationexperiments suchaswidebanddispersionand

antennawetting,(d)anon-orbittestbedforACTSspacecraft

characterization measurements icludingfrequencyresponse

andmultibeamantennacharacterization, ad(e)a backup

facilityfortheTT&CfunctionofACTS.

Thefamilyofexperimenter ortransportable groundstations

includesVSAT,USAT,andtheHDRterminal.Eachterminal

wasoriginallydesignedto supportcertainapplications. As

advancesintechnologyoccurredoverthecourseoftheprogram,eachterminal demonstrated moreadvancedapplications,

furtherdemonstrating thecapabilitiesofgroundstationsatKa

band.TheVSATandACTSTDMAnetworkwereinitially

usedfor1.544-Mbpsvideoconferencing adteleconferencing

basedon64-kbpschannelsandfadecompensation algorithm

evaluation. Overthecourseof the program the terminals were

deployed for protocol and network experiments, propagation

data collection and analysis, TDMA network availability experiments, and technology experiments to evaluate the satellite

multibeam antenna performance. The USAT ground stations

were originally designed to demonstrate supervisory control

and data acquisition for remote electric utility stations and used

kbps data rates• Advances, primarily in solid-state amplifier

technology, enabled the stations to demonstrate > 1.544-Mbps

videoconferencing, highly asymmetric (1.544/45 Mbps) product

NAME MODE ANTENNA

(m)

NGS BBP

VSAT BBP

USAT MSM

HDR MSM

LET MSM

I-IPA

(Watt)

5.5 200

1.2, 2.4 12

0.6, t.2 .25, 1.0, 2.0

3.4 120

4.7 100

EIRP G/T

((mW) (dB/K)

78 30

60, 66 16-18

35-51 15, 21

76 28

78 27

and content distribution applications, and 2-Mbps full mesh

TDMA/FDMA Internet Protocol (IP) and asynchronous transfer mode (ATM) network experiments. The HDR station was

first deployed to demonstrate interconnectivity of

supercomputers to conduct interactive computer modeling at

high data rates. Over the duration of the program the stations

were redeployed to demonstrate high-rate commercial IP augmentation, optimized file transfer protocol (FTP), and technology verification experiments and characterizations.

ACTS has two modes-of-operation: MSM and BBP. The

MSM mode-of-operation functions as a bent-pipe memoryless

repeater with frequency translation. A microwave switch at the

satellite Intermediate Frequency (IF) connects uplink antennas

to appropriate downlink antennas for static point-to-point connections. The MSM may also be programmed using a repeating

uplink/downlink connection sequence for an SS-TDMA network for HDR operations. The BBP provides onboard storage

and routing of baseband signals for the VSAT TDMA network.

Signals from each transmitting VSAT are demodulated by the

BBP aboard the satellite and routed to the appropriate receiving

station by illuminating the appropriate spot beams over the

respective transmitting and receiving stations. 1 The satellite

burst time plan dynamically updates based on orderwire requests from stations entering the network or if existing stations

• . . "_

change their bandwidth reqmrement.- Table 1 identifies each

ground station, its primary mode-of-operation, and typical link

characteristics. The LET, HDR, and USAT terminals operate in

the MSM mode, and each uses various modulation schemes.

Because the transponders use hard-limiting amplifiers, higher

order modulation schemes above QPSK were not used. The

NGS and VSAT operate in the BBP mode of ACTS. The

modems on both the spacecraft and ground stations employ

Serial Minimum Shift Keying (SMSK) modulation.

The ACTS ground-station program enabled the commercial

industry to make advancements in the design and performance

of traveling wave tube amplifiers (TWTA's), low-noise amplitiers (LNA's), high-rate modems, solid-state power amplifiers

(SSPA'sL and high-power frequency doublers (HPFD's). In

BURST

RATES

U/L: 27.5 or 110

D/L: 110

U/L: 27.5

D/L: 110

Up to 2.5 Msps

Up to 696

Up to 696

DATA RATES

(Mbps)

64 kbps to multiple

T1 &T2

1.792 (max)at 64

kbns increments

U/L: low kbps to 8

Mbps

D/L: up tO 45 Mbos

311 or

622 Mbps

low kpbs to 622

Mbps

Table 1 ACTS Ground Station Summary

MODULATION

SMSK

SMSK

BPSK, QPSK,

CDMA

O-BPSK (OC-3)

0-QPSK(OC-12)

BPSK, QPSK,

SMSK

2 NASA/TP-2000-210047
addition, althoughoutsidethescopeofthispaper,thedevelopmentoftheonboardprocessingTDMA network enabled various satellite and ground-station technologies not addressed

here. The implementation of commercial terrestrial interfaces

laid a foundation for terrestrial and satellite interoperability

experiments, commercial protocol research and augmentation,

and other technical advancements. The radiofrequency (RF)

technologies developed for the ACTS ground terminals, with a

few exceptions, were mainly extensions of technologies developed at lower frequency bands. Still, the design challenges

were quite real and required significant engineering effort to

overcome. The final result was high-quality ground-station

equipment that continued to perform well beyond the intended

operational life of the system.

Ground-Station Technologies

Transmitter

Development and operation of the 30-GHz Ka-band traveling wave tube amplifiers proved challenging in the early years

of ACTS. The NGS and LET were the first to procure 30-GHz

TWTA's. The NGS and LET began ground-station integration

and testing in the late 1980' s and early 1990's. Four 54-W units

were initially purchased for use in the NGS, followed by three

60-W units for use in the LET, all from the same vendor. The

TWT designs used in the LET and NGS units were similar. The

TWT's were linear beam devices with a helix type slow-wave

microwave circuit.

These first "I"WTA' s experienced several problems. The first

anomaly, a spontaneous shutdown of the TWTA protection

circuitry, was reported in 1992. 3 Testing revealed random

output power spikes ( 1- to 3-dB RF power fluctuations) present

at certain frequencies. Naturally, the random power spikes

contributed to loss of data bits and resultant increase in bit error

rate (BER). The effect would appear in the 10-8 BER range.

Strong spikes resulted in the spontaneous shutdown of the

TWTA. Test data and analysis provided to the vendor contributed to modifications in TWTA design to minimize the effect

of the power spikes. Although TWTA's in general experience

RF fluctuations, the low BER experiments conducted in the

ACTS program were more susceptible to these types of component characteristics. Other problems that occurred were high

helix current shutoff and an inability to power a unit on. Over

time the latter problem was attributed to the operational procedure of the TWTA.

The operations procedure at the start of the program was to

allow the TWTA to run without RF drive during short periods

of inactivity and turn off the TWTA during longer periods of

inactivity (e.g., overnight). Regularly turning the unit off and

on or leaving the unit turned off for short periods of time tended

to create gas in the vacuum envelope of the tube. This resulted

in a high helix current causing the helix protection circuitry to

power the unit off or prevent the unit from turning on. It is also

believed that this operational practice led to shortened TWTA

life. These practices and the technical problems mentioned

resulted in regular occurrences of TWTA malfunctions and

lengthy repair cycles. The TWTA's were routinely sent to the

vendor to repair high-voltage components or to degas the unit.

Due to the sporadic shutoffs and operational difficulties, the

original TWTA's were never run to end-of-life and were

replaced by a next-generation TWT from the same vendor (for

LET) and new TWTA's from other vendors for both NGS and

LET. The original TWTA's had 7000 to 10 000 hr before

removal from service.

The TWTA operations procedure for both LET and NGS was

changed such that energized tubes were never permitted to idle

for long periods of time without high voltage applied. Also,

they were always operated with an applied carrier, either

radiated or transmitted into a load. Particular attention was also

paid to maintaining proper cooling. This resulted in a very

stable operating environment, significantly extending TWTA

service life.

The combination of change in operational procedures and

improvements made in the second-generation TWTA's used in

the LET increased TWTA service life. A new TWTA (from the

original vendor) was removed from service with over 20 000 hr

of operation. The helix current remained stable and only

increased near end-of-life, as expected. The LET TWTA from

the second vendor provided 100-W output power using a

120-W tube. Internal components, primarily the output isolator. TWTA also employed a helix-type structure tube and

power supply in a single housing. The second vendor" s TWTA

has been in service at LET for over 22 000 hr as of this writing.

Replacement TWTA's for both the NGS and LET were

procured from new and different vendors than the original

TWTA's. The NGS tubes are capable of 200-W output power,

but internal component losses and biasing result in a saturated

power level of 150 W at the amplifier output. The slow-wave

structure of the tube is a coupled cavity, that is, the tube does not

have an actual helix but rather an interdigital delay line that

performs the same function as a helix. The circuit was developed as a better way of realizing the slow-wave structure, given

the high power level and the stringent size and accuracy

requirements in the millimeter wave range. This design is not

as broadband as tubes using actual helices. The NGS TWTA's

use a split mount design with separate units for the TWT and

power supply.

The design life of the second-generation NGS TWT collectors and electron guns are 20 000 to 30 000 hr. However,

original tubes delivered with the new TWTA's lasted only

10 000 to 12 000 hr with one premature failure at 2700 hr. The

failures occurred for various reasons including materials used

in the tube, mechanical shock, and the precision tolerances

required in the tube structure at Ka band. Another factor was the

stability of the delay line inside the tube. The attenuation of

each section must be stable over the operating temperature

NASA/TP-2000-210047 3
rangetoavoidproblemswithgainripple.Asexperiencewas

gainedwithtubeoperationitheNGSandwithotherusers,the

vendorexaminedtubefailuresandmadeimprovements inthe

tubedesignandmaterialsused.Thedesignchangesresultedin

similartubedesignlifeandachievedamuchlongeroperating

life--closerto30000hr.

TheVSATtransmitter usedKu-bandTWT'soperatingat

14.6GHzcombinedwithanHPFD.TheTWT,powersupply,

andfaultcircuitswereenclosedin theIntermediatePower

Amplifier(IPAenclosure). Theunitproduced45-dBmoutput

witha -5-dBminputsignal.TheIPAdrivestheHPFDto

producethefinaloutputfrequencyof29.2GHzat40.5dBm.

TheIPAdutycyclewas25percentundernormaloperating

conditionsinTDMAburstmode.ACpowerisappliedtothe

unitatalltimesandmonitoredonanhourmeter.Theaverage

lifeofeachIPAisabout26000hror3years.TheIPApower

suppliesweretypicallythefirsttofail.Repairingthepower

supplyunitswouldextendtheTWTlifetoabout30000hr.

WiththedecisiontoextendtheACTSprograminlate1995,

aftertheinitialexperiment phase(2years),mostVSATTWT's

andHPFD'swereintheirthirdyear.DuetotheIPAreaching

end-of-lifeanddesignissueswiththeHPFD,atradeoffwas

evaluated: whethertostaywiththeIPAandHPFDdesignoruse

new30-GHzSSPA'scapableof therequired10-to 12-W

outputpower.TheSSPA'swereavailablefroma limited

supplyof vendors.However,theoutputpowerspecification

wasdifficulttomeetandsomemanufacturers couldnotguaranteeareliableproduct.Becausethechangeintechnologies

wouldincursubstantial costandincreaserisk,theproject

stayedwiththeIPA/HPFDdesignandreplacedtheIPAentirely

andredesignedtheHPFD's.

ThefirstHPFD'susedin operationwerenotdesignedto

operatewithacontinuouswave(CW)signalapplied.Priorto

launch,theprojectidentifiedtheneedtocharacterize andtest

theHPFD'swithaCWsignalduringoperations. A second

problemwiththeoriginalHPFD'swasthattheyoperatedat

hightemperatures, andseveralunitsfailedafteronlyafew

monthsduetodiodefailures.EachHPFDhadfourdiodesthat

producedtheoutputpowerandfrequencydoubling.Thenew

unitsproducedweredesignedfor CW, operated at lower

temperature, and used a different diode biasing design. Other

design changes included a new balanced diode assembly and

additional heat sinks applied to each individual diode. Reduced

VSWR made the diode less susceptible to standing waves and

voltage spikes. The entire fleet of VSAT's (19) was upgraded

with new HPFD's as the original units continued to fail.

Although the new HPFD design was more reliable than the

original, the units were still the source of many VSAT failures.

Experience and testing with the VSAT's determined that the

IPA produced voltage spikes that degraded the HPFD's over

time. The IPA/HPFD redesign significantly improved the system availability of the VSAT network. Although the technology used required regular terminal maintenance, the reliability

data of both the IPA and HPFD enabled system engineers to

plan repairs and service each station before unexpected failures

occurred and reduced troubleshooting time to minimize impact

to the experiments program.

The USAT transmitters employed low-power SSPA' s. In the

early 1990's, cost and availability limited these discrete component amplifiers to 0.25 W. Combined with a single-stage

upconverter from 800 MHz to 30 GHz, the resultant package

was a small 5- by 3- by l-in. upconverter/amplifier integrated

near the feed of the antenna to minimize loss. As solid-state

technology matured, a second version of the block upconverter

was produced in 1997. The second generation used 1-W Monolithic Microwave Integrated Circuit (MMIC) amplifiers with a

two-stage upconverter (70 MHz to 30 GHz), yet only a small

increase in size resulted. The new units measured 5 by 4 by 1 in.

as shown in figure I. In addition, 2-W units were also produced

in the same size package by combining two 1-W MMIC chips.

Two challenges of these higher power units were achieving

good component yield and hand selecting the highest power

chips for integration. This resulted in good overall performance

and stable operation over temperature with losses of only 1 to

1.5 dB at 80 °C. Solid-state amplifiers in the 4- to 10-W range

were also available in the late 1990's, but cost and integration

prohibited their use. The goal of the USAT was a modest data

rate (>1.544 Mbps) with small packaging, which was achieved

with the 1- and 2-W units. As successful as these units were in

size, operation, and performance, advances are still needed in

device yield and unit production and integration to make these

products affordable for the mass market.

The HDR station also employed 30-GHz high-power

TWTA's. The units were a split-mount design with a 12-m

cable to remotely control and supply power to the TWT. The

TWTA was mounted on the boom of the antenna near the feed

with the power supply and preamplifier rack mounted inside the

HDR equipment trailer. Like the LET TWT, the HDR TWT

was designed around a helix-type slow-wave microwave circuit

with an output of 100 to 120 W and >I-GHz bandwidth. Unlike

other terminals, the HDR TWTA remained powered down for

Figure I USAT 1-Watt upconverter/amplifler module.

4 NASA/TP-2000-210047
Station

NGS

VSAT

USAT

HDR

LET

Technology

54WattSingle

Chassis

150 Watt

TWTA-split

mount

12 Watt KuBand TWT,

HPFD

.25-2 Watt

Solid State/

MMIC

120 Watt

TWTA-split

mount

60 Watt TWTA

Single Chassis

100 Watt

TWTA Single

Chassis

Initial Issues

Sporadic shutdowns

RF power transients

Low reliability

TWT failure at

10,000-12,000 hours

Cooling fans

HPFD reliability

Solid state and

MMIC cost and

availability

Low reliability

- Environmental

effects

Sporadic shutdowns

RF power transients

Low reliability

None

Advancements / Lessons Learned

Improved gun insulation and insulation application

procedure.

Modified tube design and operational changes led to

>30,000-hour tube life with minimal service.

New HPFD diode and heat sink design

Technology gaps remain for affordable Ka-band

10-12 Watt SSPA.

Single integrated block upconverter/amplifier

Advancements in MMIC made devices more

available.

Reduction in cost & integration required enabling

consumer terminals.

Identified potential high voltage interface issues of

outdoor installed split mount TWTAs.

Environmental concerns remain for precision

tolerances required for Ka-band TWTA operation.

Improved gun insulation and insulation application

procedure.

Operational changes led to >22,000-hour tube life

with minimal service.

Table 2 Transmitter Technology Summary

the majority of the time, as recommended by the TWTA vendor

(different from NGS and LET), despite the experience gained

in the NGS and LET operations. TWTA operations without

high voltage applied were limited to only a few minutes.

The TWTA used in the beginning of the project had an

extremely high failure rate. The high failure rate was attributed to

the following: (a) water penetrating the outdoor high-voltage

connectors and shorting out the power supply; (b) high-voltage

shotting out to chassis due to faulty potting; and (c) temperature

fluctuations causing the TWT to defocus. In addition to the failure

rates, turnaround time on defective units was excessive. In mid1997, a different vendor was selected to provide TWTA for the

HDR project. TWTA performance and reliability improved.

Table 2 summarizes the transmitter technology used in each

ground station. A number of different approaches were taken

with each station because of experiment and application requirements. Design changes in TWTA material and integration, changes to TWTA operation procedures, and advancements

by industry in solid-state and MMIC amplifiers were highlights

of this technology area.

Receiver

Receiver technology experienced modest advancements during the ACTS program. The majority of LNA and low-noise

converters (LNC's) were built to specifications often at or

slightly in advance of state-of-the-art at the time. Most of the

devices used, with the exception of the VSAT LNC, were

technically good devices lasting for many years of service.

The NGS LNA employed quarter-micron HEMT FET, which

were the state-of-the-art in the late 1980' s when the station was

built. Two sets of LNA are used: one at 20 GHz for the TDMA

downlink and telemetry beacons and one at 27.5 GHz for the

transmit band propagation beacon.

The 20-GHz LNA subassemblies are implemented in a

cascade of two stages, each with a gain of 30 dB and a noise

figure of 3.0 dB. The station employs three such LNA subassemblies in a redundancy configuration of three for two. The

total system noise temperature referenced to the antenna output

is 570 K. This value includes an antenna temperature of 123 K

and an effective system electronics temperature, including

waveguide loss, of 447 K.

The 27.5-GHz LNA also employ two stages each with a gain of

20 dB and a noise figure of 4.5 dB. The total system noise

temperature, referenced to the antenna output, is 1687 K. This

value includes an antenna temperature of 121 K and an effective

system electronics temperature, including waveguide loss, of

1566 K.

The NGS LNA have been in nearly continuous operation,

either in test facilities or on the air, since 1991. They have

proven to be a robust technology. Few failures occurred over

their 9 years of operation, most explained by environmental

incidents, that is, periods of high ambient temperature due to

NASA/TP-2000-210047 5
facilityHVACfailuresorinadvertent signaloverload,rather

thandevicefailure.Periodicmaintenancechecksindicated

littleornodegradationindeviceperformanceovertheperiod

ofservice.

TheoriginalLET20-GHzreceiver/downconverter useda

four-stageHEMTLNAfollowedbyanMMICmixerand

amplifierstagestodownconvert thesignalto anIF of 3 to

4GHz.Therewerethreesuchunitsproducedunderaproof-ofconcept(POC)developmenteffort. The LNA and

downconverter hadacombinednoisefigureofapproximately

4to5.5dBwithanominal gainof25dB.WhilethePOCunits

exhibitedexcessivegainslopeacrossthefull I-GHzdownlink

band,theyexhibitedonlymodest gainslopeacrosstheiroperationalband(300MHz)andwerequiteusableforapplications

withinthatbandwidth. Consequently,heywereincorporated

intotheinitialstationbuildupandusedforrelativelynarrow

bandsignalapplicationsaanefficientuseofexistinghardware

andacost-savingsmeasureearlyin theprogram.However,

becauseoftheirgainslopeacrossthefullband,thePOCunits

provedunusableforwidebandapplicationssuchason-orbit

linkandspacecraft characterization experimentsoftheentire

transponder bandwidth, andemergingHDRmodemtechnologyrequiringlargerbandwidthswithminimumgainslope.To

accommodate thesewiderbandwidthapplications, it wasnecessarytoreplacethePOCunitswithreceiverstailoredforthat

purpose.

TheLETreceiverswerereplacedwithcommerciallyavailableLNA'sandanin-house-designed downconverter. The

LNA'sselectedwereidenticalto thoseusedin theHDR

stations. TheLNAexhibitedanoisefigureof3.5dBand50dB

ofgainwithaminimalgainslopeacrosstheI-GHzband.The

LNAusedinHDRandLETprovedreliableovertheproject.

Manyhavebeeninservicesince1995withlittleornoperformancedegradation. Thefewfailuresthatdidoccurweremainly

theresultofphysical damagetotheamplifier.

TheVSATstationsencountered significant challengesinthe

receiverdesignandperformance. TheVSATLNCisathreestageHEMTLNA,RFbandpassfilter,mixer,siliconMMICIF

amplifier,voltageregulator, andfaultcircuit.Developedinthe

early1990's,thereceiversexhibitedawiderangeofperformance. Thereceivergainwasspecifiedat45dB,andthenoise

figurewasspecifiedat5dB.Theperformance fromunittounit

differedsignificantly. Eachunitwashandtunedatthefactory

asaresultoftheperformance. Thetotalreceivergain(multiple

stages) ranged from 40 to 52 dB and the noise figure varied from

3 to 9 dB. These variations required that the downlink of each

station be adjustedto ensure like performance and operation

compared to other VSAT's. Because the receiver performance

varied so much between units, the entire receiver characteristics drastically changed when different receivers were installed

due to failure or service. This lengthened the service time at a

particular station to allow time to make necessary adjustments

to account for differences in the receivers.

The first-generation USAT ground stations that were used

combined LNA and single-stage block downconverters from 29

GHz to 70 MHz. Performance, small size, and low cost were the

primary goals of the USAT low-noise downconverters (LND).

The original LND exhibited a 25- to 28-dB gain with a 4- to 4.5-

dB noise figure over a 40-MHz bandwidth. 4 The units measured

4.25 by 2.25 by 0.4 in. The LND could operate with 40-MHz

bandwidth increments from 19.7 to 20.4 GHz by varying the

downlink oscillator. The first-generation LND failed after 3 years

of operation. Coincidentally, four of five units failed within the

same month after varying scenarios of operation. The fifth unit

failed a short time later. Each LND had a similar amount of

operation time but was often located in different geographical and

temperature locations. No particular operational parameter was

identified as being the cause of these failures.

Improvements made by industry during the 1990's resulted

in improved performance for the second-generation LND.

With a similar size package, the new LND's had a 32-dB gain

with a 2- to 2.5-dB noise figure over a 50-MHz bandwidth. A

total of 15 units were purchased in 1997 with all units exhibiting

similar gain and noise figure performance. These units also

covered the 19.7- to 20.04-GHz spectrum but had larger receive

bandwidths of 50 MHz (compared to 40 MHz). With the

improved noise figure, these units raised the available data rate

between small ground stations making them more adaptable to

a variety of applications. 5

Antenna

All the antennas used with the transportable experimenter

ground stations were offset fed parabolic reflectors. The USAT

and VSAT reflectors were actually Ku-band reflectors, which

proved adequate for operation at Ka band. However, propagation experiments conducted on the antennas revealed that wet

antenna effects of Ku-band antennas operating at Ka band

resulted in a greater loss due to the rainwater and the thickness

of the Ku-band dielectric. Minimizing the dielectric thickness

at Ka band is needed to reduce loss in the presence of water on

the reflector. 6 As much as 2 to 5 dB is lost due to wet antenna

and feed radome effects.

The HDR station was the only experimenter station antenna

in the ACTS program made up of individual panels. The USAT

and VSAT reflectors (0.6, 1.2, and 2.4 m) were all one-piece

structures. The 3.4-m HDR antenna was made up of four

individual panels. Station performance validated that using this

type of sectioned reflector is a viable alternative for Ka-hand

operation compared to a single piece reflector. Surface tolerance and feed alignment were adequate for operation.

Both the NGS and LET antennas (see fig. 2) employ

Cassegrain-type feed systems. The LET antenna uses the more

conventional configuration in which the feedhorn extends from

the apex of a 4.7-m dish and couples the reflector optics to

6 NASA/TP-2000-210047
thathedeicerheatingsystemdidcausethermal distortionof

the antenna structure, which reduced antenna gain. Modification of the heating system reduced the effect although it is still

observable particularly at high heat settings.

These heating effects will vary with antenna design, but the

experiences gained point out the need for careful attention

during the design process to all potential sources of mechanical

deformation, to maintain the close surface tolerances required

of Ka-band antenna reflectors. The potential usefulness of

specific design features to mitigate water and icing effects on

the antenna structure should be noted, for example, through the

use of shields or other devices to reduce water accumulation on

particularly sensitive areas of the antenna.

Figure 2 LET (left) and NGS (right) antennas.

transmit and receive equipment located in a hub assembly

immediately behind the reflector. The NGS employs a beam

waveguide system that couples an aperture at the apex of a

5.5-m dish to a feedhorn located at the base of the antenna. The

feedhorn then connects to transmit and receive equipment

located indoors in a room directly beneath the antenna.

The NGS feed network follows the feedhorn and consists of

a half-wave polarizer and orthogonal mode transducer (OMT)

connected by rotary joints. This network allows the antenna to

receive and transmit in dual orthogonal linear polarizations

with two 20-GHz receive ports and two 30-GHz transmit ports.

The 27.505-GHz transmit band satellite beacon is also received

at one transmitport and is separated from the transmit signal at

that port by a multiplexer and filter assembly. The antenna

polarization angle is adjustable through 360 ° by rotating the

polarizer to match the antenna polarization to that of the

satellite signals.

The LET antenna feed network is a combined corrugated

horn and OMT assembly. Waveguides for both transmit and

receive extend into the antenna hub. The assembly mounting

plate has four slots along its radius, corresponding to bolts on

the antenna structure, to allow polarization adjustments upon

installation. Polarization is adjusted by physically rotating the

combined feed assembly until peak signal is reached. Polarization adjustment of LET is an iterative process because of the

rigid waveguide connections inside the hub. Flexible and rotary

joint waveguide would ease the procedure, but it remains a

physically challenging task.

Early in system operations a correlation was noted between

a decrease in signal level and operation of the NGS antenna

deicing system. 7 At the time it was thought that the observed

effect could be a combination of antenna wetness from melting

snow and thermal distortion of the antenna reflector. Further

investigation confirmed both theories. Antenna wetting effects

at Ka band have been investigated by several people and have

been quantified as an additional source of link degradation. 8

Also, subsequent maintenance of the NGS antenna revealed

Modems

The combination of ACTS and the extensive ground-station

program enabled a number of advances in modem technology

and the demonstration of various modem implementations.

From custom ground- and onboard-processing TDMA network medium-rate burst modems to SS-TDMA high-rate burst

modems to the use of commercial off-the-shelf low-rate continuous wave modems and ground-based TDMA/FDMA network low-rate modems, the ACTS program enabled flexible

network architectures and system configurations.

The VSAT and NGS modems supported TDMA/FDMA

operation of the spacecraft onboard processor and the associated TDMA network. The spacecraft and ground-station modems were the same design and operated in burst mode at i 10

or 27.5 Mbps using SMSK modulation. The VSAT modems

were limited to 27.5 Mbps on the uplink. As part of the network

fade mitigation design, the modems could also be operated at

one-half their normal burst rates, 55 or 13.75 Mbps, with

1/2 rate Forward Error Correction (FEC) coding, to achieve a

10-dB increase in system uplink and downlink margins.

The NGS and VSAT modem design proved to be stable and

well executed, and modem performance was generally excellent over the course of the program. Maintenance problems

were infrequent and usually involved power supplies, oscillators, or cooling fans, which are normal failures associated with

age and facility environmental upsets, Minor wiring problems

also occurred because the units were custom-built. Only one

design idiosyncrasy was noted--sensitivity to certain data

patterns, for example, long strings of l's or O's. This was

present in both the spacecraft and ground modems, and was

obviated by locking out certain data combinations.

Because of the burst mode operation and the data rates

involved, fault diagnosis was more challenging, particularly in

the case of intermittent failures. However, the modems were

delivered with special test equipment designed to exercise the

three different modem pairs (ll0-Mbps uplink, 27.5-Mbps

uplink, and 110-Mbps downlink). The modem special test

equipment proved to be a valuable tool in stand-alone modem

NASA/TP-2000- 210047 7
faultdiagnosis. However,it wasalsoincorporatedintothe

station test equipment suite so that the modems could be

exercised over the station transmit and receive paths to verify

overall station performance. Regular BER measurements were

performed as part of station periodic maintenance as a check on

both the modems and the amplitude and group delay characteristics of the station transmit and receive equipment.

The HDR burst modem is a dual-mode device capable of

operating in offset binary phase-shift keying (OBPSK) or offset

quadrature phase-shift keying (OQPSK) modulation. The symbol rate of the burst modem is 348 MS/s providing data rates of

311 Mb/s in OBPSK or 622 Mb/s in OQPSK. Of the six

modems designed and built for the HDR program, three remained in service and fully functional to the end of the inclined

orbit program operating for over 2 years without repair. Although the modems performed soundly, it is believed the

addition of adaptive equalizers to the demodulator frontend to

compensate for the satellite nonlinear transponder and changes

in the system due to aging would have enhanced performance.

The companion to the HDR burst modem was the Digital

Terminal (DT). The DT interfaced the high-rate terrestrial

Synchronous Optical Network (SONET) interface to the burst

modem. The DT also managed/controlled the TDMA network

scheduling, the Reed-Solomon FEC encoding/decoding, bit

scrambling, and the operator interface. The network processor

board of the digital terminal depicted in figure 3 is one of six

boards that make up the digital terminal. The DT and HDR burst

modems were custom-made equipment and the DT is arguably

the most sophisticated component of the HDR station. Due to

the complexity of the equipment, its interfaces, and operational

issues, a large portion of the problems associated with the HDR

stations were attributed to the DT.

The USAT ground stations were designed to support a wide

variety of applications. Designed with a 70-MHz IF interface,

the station could be used with a variety of commercial modems

employing standard 70-MHz interfaces. Configured with a

I-W transmitter and a 0.6-m antenna, the USAT could support

2 to 4 Mbps between stations with adequate margin. Configured with a 1.2-m reflector, experiments were conducted using

6 to 8 Mbps. Higher data rates could be achieved by employing

Reed-Solomon coding or reducing the available link margin on

clear days. The modems allowed the stations to offer a variety

of serial and data interfaces such as RS-232, RS-449, DS- 1/E 1,

IP, ATM, Frame Relay, ISDN, DS3, and HiSSi (high-speed

serial interface). All the interfaces mentioned were used with

the USAT ground stations for testing or experiment applications during the program.

Because the USAT stations used commercial modems and

operated in bent-pipe mode, various modulation schemes were

used. Most experiments used either QPSK or BPSK; however,

CDMA was also used on one occasion. Although the USAT

station did not prohibit higher order modulation schemes, the

hard limiting amplifiers onboard ACTS degraded the performance of these schemes, and they were therefore not used. 9

The USAT also demonstrated a ground-based TDMA/FDMA

mesh network using commercial products. A four-node fullmesh USAT network was configured and demonstrated. Burst

rates of 5 Mbps using QPSK modulation yielded nearly 2 Mbps

per node with 1/2 rate Viterbi and Reed-Solomon coding.

ATM, IP, and Frame Relay networks operated simultaneously

between stations.

The LET also provided modem flexibility, A set of variable

rate burst modems from 1 to 220 Mbps operating at 3-GHz IF

were built into the station. The modems were part of a POC

program in the late 1980's. The modems were similar to the

VSAT and NGS modems employing SMSK modulation. A

custom digital interface was developed inhouse to handle

satellite tracking and timing and included a custom BER test

capability. The modems were used to test and characterize the

station prior to satellite launch. Shortly after launch a 70 MHz

interface was designed for the LET to allow commercial

modems used by the USAT stations to operate with LET. The

commercial modems handled all satellite-tracking functions,

and standard BER test sets were used. The POC SMSK modems were considered for HDR capability prior to the HDR

concept in the early 1990's. The drawback of the custom

capability was the lack of user interfaces to conduct applications or experiments using the modems. Although the use of the

modems was limited to system characterization BER measurements, they still provided valuable data on the performance of

the ground station and subsystems and were used to conduct

high-rate interference experiments with a companion ground

station at GRC with identical modems, l0

Figure 3 DT network processor board.

Operations

The ACTS program provided the first nonmilitary Ka-band

experiment opportunity and experience for the U.S. commercial

satellite industry. The objective of operations was to conduct an

extensive experiments program to demonstrate Ka-band

8 NASA/TP-2000-210047
operationandservicesandreduceperceivedrisktoKa-band

satellitetechnologyandoperations. Theground-stationfleet

wasanintegralpartof theexperimentsprogramproviding

lessonslearnedinterminal installationandoperation. Conductingexperimentsandoperationsaroundthecountryentailed

ensuringsystemavailabilityandreliability,conductingremote

operations, monitoringstationperformance, anticipatinghardwarefailures,andprovidingtestbedcapabilitiestoverifyand

maintainstationperformance.

TheVSATandHDRprojectshadthemostsophisticated

remoteoperationscapabilityof theexperimenter terminals.

TheVSATreliedondedicateddial-upmodemaccesstoeach

earthstationtoaccessstationcomputerstocontrolandconfigurea hostof stationandTDMAnetworkparametersand

monitorstationBERperformance. TheHDRmanagement

networkwasbasedontheSimpleNetworkManagement Protocol(SNMP)betweentheHDRnetworkmanagement terminal(NMT,locatedat thecontrolcenter)andeachdigital

terminalfortheacquisitionprocess, terminalconfiguration,

status,andcontrol.Thesystemincludedanover-the-satellitesignalingnetworkandbackupterrestrial Internet(IP-based)

connectivity. TheUSATprojectusededicateddial-upaccess

to queryandcontrolthecommercial modem.USATstation

parameterswerenotmonitoredremotely.All stationsused

dial-upcontrolforcyclingACpowertostationhardware.

TheVSAThasadatathroughput capabilityof 1.792Mbps

dividedamongtheexperimenter andsystemneeds. Primarily,

theexperimenter usedthefull1.544MbpsorTI datarate(24

channelsof64kbpseach)forexperiment activitiesallowing

fourchannelsforsystemandstationtesting,monitoring, and

troubleshooting. Of thefourchannels, theTDMAnetwork

computer intheNGSusedtwo64-kbpschannelsforuplinkand

downlinkBERtesting,andonechannel wasusedforcalling

intoeachearthstationtopolltheBERdatafromthestation. The

BERdatacollectedfromeachstationwasdownloadedtoadata

baseatthenetworkcontrol center.ThisBERversusEb/Nodata

wascomparedtodailyweather mapstoanalyzerainfadeevents

andmonitorthestatusofeachearthstation.Monitoringand

recordingtheindependent uplinkanddownlinkBERofeach

VSATstationallowedTDMAnetworksystempersonnel to

observetrendsinthedataindicatingpossibleequipment degradationorfailurebeforeit affectedtheexperimenter.

Figures4and5indicatetypicaldailyBERdataobtainedfrom

aVSATstationduringoperations. Theplotsof eachfgure

startingatthetopare(a)Eb/No,(b)upfinkBER,and(c)downlink

BER.Eachparameter isplottedversustime(0to24hr,18hr

shown).Figure4indicatesastrong(>15dB)andastableEb/

Noover18hr.Thevariationsin Eb/Norepresent fadeor

spacecraft thermal events. 11Intheexample, therearenorain

fadeevents.TheuplinkBERdepictsbiterrorspresent(jagged

line,10-8inthisexample). However, thedown[inkBERindicatesanerror-freelink(solidline).SincetheEb/Nois>15dB,

andstable,thestationwouldnotexpectanyuplinkfade

degradationadshouldprovideanerror-freeup[ink.Thus,the

problemcanberemotelydiagnosedfromthecontrolcenteras

beinganuplinkproblem. ReplacingtheVSATIPAresultedin

thedataillustratedinfigure5recorded1weeklater.Again,

figure 5 illustrates a strong Eb/No, but replacing the IPA now

yields an error-free uplink. The information available from the

remote capability saved time and cost by reducing the need for

engineers and technical support to travel to individual locations

for each anomaly or problem. Much of the troubleshooting

could be completed from the operations center, which allowed

less skilled personnel to simply replace failed subsystem assemblies. Minimum onsite diagnostics were required.

Remote diagnostic capability built into the stations during

the program included an additional support computer for station monitoring and control (including FFP and station software configurations) and AGC signal strength and demodulator

lock indicators added during the inclined orbit upgrade. The

additional upgrade also allowed remote station control and

configuration using the TDMA network link. This allowed

software upgrades and monitoring without relying on dial-up

access. In addition, an external fan alarm was installed to reduce

failures due to overheated equipment. Several failures early in

the program were attributed to overheated equipment due to

failed cooling fans. Other software modifications reduced call

setup time between stations and enabled any VSAT to monitor

the operation of every VSAT in the network. This capability

enabled one person to monitor the network performance and

troubleshoot each VSAT from a single field location.

The VSAT employed 1/2 rate FEC coding to increase the

system reliability and availability during rain fade events. The

coding was enabled when the Eb/No decreased below a predefined threshold. In coded operation, the throughput of both

uplink and downlink was reduced by half. Due to the difference

in frequency, the fade of the uplink generally determined when

coding would be applied to both links. Applying coding to the

uplink and downlink paths independently would have

improved system efficiency by maintaining the maximum throughput on the individual channels until coding was required. The

ACTS propagation and experiments program conducted numerous experiments to characterize the propagation effects at both 20

and 27 GHz independently. The results of these experiments

should lead to more efficient systems in the future.

Aside from the TWTA, the HDR station was controlled

remotely. The NMT coordinated the HDR acquisition process

and data connections through the use of the SNMP. Using the

public Intemet for station control was an example of the use of

commercial technology for NASA missions. However, increasing network security concerns both by NASA (mission/

ground station safety) and experimenter sites (corporate firewall

integrity) complicated the use of the SNMP. Continued use of

Internet technologies for future remote station control will

require secure and virtual private network (VPN) technologies.

The HDR network provided long-haul point-to-point and

point-to-multipoint full-duplex SONET services at rates up to

622 Mbit/s (SONET OC-12c) with signal quality comparable

NASA/TP-2000-210047 9
8I

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Figure 4 VSAT BER data, U/L BER problem,

to that obtained with terrestrial fiber networks. Data multiplexing over the satellite was accomplished using TDMA techniques coordinated with the switching and beam-hopping

facilities provided by ACTS. Transmissions through the satellite are protected with Reed-Solomon encoding providing

virtually error-free transmission under modest fade conditions.

The user interfaces are compatible with SONET standards

performing the function of conventional SONET multiplexers

and, as such, can be readily integrated with standard SONET

fiber-based terrestrial networks. ]2

Among the most common problems encountered during

HDR operations were loss of network connectivity with the

NMT and the interface between burst modem and DT. Network

service loss was catastrophic since the NMT required

connectivity to all ground stations for network and station

acquisition and data link configuration. The burst modem/DT

interface was a custom version of the High Performance Parallel Interface (HIPPI). The interface cable had to be reseated

before/during experiments to establish good connectivity between the burst modem and DT. Possible items that may have

contributed to the problem were lack of cable supports, poor

cable design, or impedance difference between the different

logic families of the burst modem and the DT. Future designs

might consider combining portions of the interface into the

modem chassis to reduce the complexity of the interface

between systems.

Additions to the HDR system to enhance operations

would include having both the standard SS-TDMA mode and

10 NASA/TP-2000-210047
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Figure 5 VSAT BER data, error-free operation.

continuous mode (non-TDMA) modem operation and a built-in

RF calibration signal for link calibration and testing. The

continuous modem operation would have been valuable for

station characterization and debugging and provided a more

robust link for a point-to-point connection when the TDMA

network overhead was unnecessary. Also, an additional dial-up

capability to enable PPP (point-to-point protocol) connections

from the NMT to each site would have improved system

availability in the event of a network failure.

The use of SONET as the HDR terrestrial interface enabled

experimenters to use commercially available off-the-shelf

OC-3/OC-12 equipment to interface to the HDR station.

Further upgrades to the HDR station would include the ability

to monitor the status of the SONET connection between the

HDR interface and the experimenter's end-equipment from the

NMT operator interface. The operator interface provided numerous statistics on the state of the link including coded and

uncoded BER, number of unique words sent and received,

number of correctable and uncorrectable errors, but only basic

SONET status (i.e., loss of link, loss of pointer, etc.) information. Toward the end of the program, particularly when using

the OC- 12 data rate, the HDR interface would indicate an errorfree link while the SONET equipment reported SONET data

errors making the link unusable. Because the system functioned properly early in the program, degradation of the SONET

interface or timing issues between the SONET interface and

HDR modem electronics are most suspect as the cause of the

erroneous statistics. Because of the cost to redesign and repair

NASA/TP-2000-210047 11
theDTlate in the program, the systems were used"as is." Often,

resetting the SONET interface or rebooting the DT would

alleviate the problem.

From a system level, the basic USAT consisted of RF

electronics, power supply, antenna and mount, commercial

modem, and tracking controller. A support computer allowed

remote access to the tracking controller and modem. Due to the

simplicity of the stations, the USAT operations were relatively

easy compared to the other experimenter stations. The stations

were built to minimize cost and most resemble potential future

commercial terminals. Test ports and built-in diagnostic equipment were not part of the USAT design. However, the use of

commercial modems provided adequate test capability using

the built-in CW or BER test capability. Dial-up access to each

USAT allowed ACTS network operators to remotely control

the modem for test configurations or obtain receive signal

characteristics provided by the modem (receive power, BER,

AGC, frequency offset, etc). Remotely controlling the USAT

modem allowed configuration changes between continuous

wave and modulated signals, data rate. and frequency changes.

A corresponding modem at the LET allowed full end-to-end

link analysis of remote USAT stations.

The USAT ground stations were generally used for experiments on a short duration basis. The intermittent scheduling of

each station at varying locations made it difficult to trend

station performance for any length of time. Stations were

generally bench tested periodically to ensure operation when

deployed. Station and system designers must consider the cost

and necessity of implementing remote diagnostic capability

versus station complexity. For ACTS, the complexity of the

VSAT station and TDMA network fully warranted full remote

diagnostic capability. It was found to be invaluable for system

characterization, monitoring, and analysis in an experimental

system. In the case of USAT, the design simplicity allowed the

stations to operate without routine monitoring. The limited

remote diagnostics provided by the commercial modem proved

sufficient for reliable operations.

The NGS and LET are co-located in the same facility at GRC

known as the Master Ground Station (MGS). The MGS was

staffed on a 24-hr, 7-day basis and was the control center for

ACTS Operations. It is linked by terrestrial circuits with the

Lockheed Martin ACTS Spacecraft Operations Center (ASOC),

which provides spacecraft bus monitoring and control with the

NGS providing the TT&C link to the spacecraft itself. A

redundant set of ASOC spacecraft monitoring and control

equipment is located in the MGS so that the spacecraft can also

be controlled entirely from the MGS.

MGS facilities include extensive automation and monitoring

capabilities for the NGS/LET stations, as well as for the entire

ACTS system, via satellite orderwire circuits and the terrestrial

intemet. Status and controls are available at the two station

operation consoles for station RF and antenna systems, spacecraft TT&C functions, and the ACTS BBP, HDR, and MSM

networks, including the spacecraft communications payload.

During most operations, one operator at the MGS and one at the

ASOC facility can operate the entire system. In addition to

system operations, station operators also perform dataprocessing tasks for the enormous amounts of data collected

daily by the system.

The extensive automation available could have allowed

unattended or remote operation. However, in practice it was

quickly found that while such automation was useful and good,

there is no replacement for a live operator onsite in main control

facilities. Protection of the considerable investment in spacecraft and ground system assets, and immediate service of

normal and extraordinary system needs including response to

system users, are best served with live around-the-clock staffing. While relatively uncommon, there were several significant

occasions during the 7-year mission when incidents such as

facility roof leaks and air conditioning, power, computer, and

communication equipment faults were immediately handled

by the on-duty operator, minimizing system outages and containing potentially catastrophic equipment damage. The tradeoff

and cost of fully automated and call-in processes must be

compared with the value of immediate response times when

system availability and fault repair are considered. The ACTS

system and equipment, though designed as an experimental

system with limited redundancy, experienced excellent availability throughout the entire 7-year mission. This was due in

large part to the immediate availability of onsite support.

System designers would do well to note this significant lesson

learned.

Integration

The integration of each ground station was unique to the

technology used and available during its development, cost

considerations, and common practice. Each field terminal and

gateway earth station had unique integration issues to resolve

or manage. Although the design and implementation of each

station is beyond the scope of this paper, examples of the

challenges or successes of each terminal are highlighted to

provide lessons learned for future Ka-band ground station and

system designers.

For the VSAT, the major challenge was the integration of the

electronics enclosure located at the feed of the antenna. The

enclosure contained the LNC, phase locked oscillator (PLO),

HPFD, voltage regulator, fault circuits, orthomode transducer,

feedhorn assembly, and various waveguide and filter assemblies both at 19 and 29 GHz. Experience with operating and

maintaining the station determined that the LNC was positioned too close to the HPFD. As described earlier, the HPFD

operated at high temperature, even after redesign. The LNC

was installed directly above the HPFD exposing the receiver to

excessive heat without adequate air circulation or ventilation.

The excessive heat affected the receiver noise figure and gain,

decreasing the receiver sensitivity and increasing system noise.

12 NASA/TP-2000-210047
Combinedwithinconsistent receiverperformance, theadded

heatfurthercomplicatedoperationsandadverselyaffected

performance. To improve the design, the HPFD inside the

electronics enclosure could have been installed inside the IPA,

near the TWT. In that configuration the TWT could be monitored for voltage spikes using protection circuits to protect the

HPFD input. This design change would have corrected two

problems: ( 1) heat adversely affecting the LNC and (2) additional circuitry to protect the HPFD from TWT power fluctuations (spikes). Due to implementation cost, the project used the

current configuration throughout the program. The link margins built into the systems were sufficient to conduct operations

without impact to experiments.

A second problem with the electronics enclosure was the

presence of moisture in the feedhorn and in the enclosure itself.

The feedhorn moisture was corrected by replacing the "O" ring

gaskets, using feedhorns with better window adhesive, and

pressure testing the membrane seal. The moisture inside the

enclosure was corrected by changing the feedhorns and seals,

changing the desiccant packs inside the unit, and replacing the

gasket inside the enclosure cover. The moisture inside the

enclosure damaged the various electronics inside. The HPFD

failed as moisture damaged the diodes. The LNC, PLO, and

voltage regulator performance were also affected by moisture.

The experience with the VSAT electronics enclosure illustrates

the need for careful moisture-proofing design in outdoor electronics and periodic maintenance of small but important details

such as feedhorn protective windows, O rings, and gaskets and

careful assembly.

A highlight of the NGS integration was extensive IF and RF

ioopback capability built into the NGS to permit off-the-air

performance verification of station transmit and receive paths

for both TT&C and TDMA functions. These features were

useful for testing and fault isolation, particularly when used

with the modem test equipment. Also, since the ACTS BBP is

a regenerative repeater, that is, the uplink cartier cannot be

observed on the downlink, they enabled link monitoring that

otherwise would not have been available. In practice; however,

the reliability of station equipment over the mission life was

very high, and aside from periodic maintenance checks and a

few troubleshooting episodes, the loopback features were rarely

used. However, the loopback paths were used extensively for

station characterization testing prior to launch.

The inclusion of a spacecraft simulator in the MGS station

test equipment proved to be more useful in practice than the

built-in loopback paths. The simulator is the engineering model

of the ACTS spacecraft communications package and contains

a single transponder with a baseband processor and microwave

switch matrix. The simulator is functionally equivalent to the

flight communications package. It was used as a testbed in the

development of the NGS and was then incorporated into the

station test equipment suite. Station transmit and receive paths

were connected to the simulator at both IF and RF. A VSAT

terminal located in the station was also connected to the

simulator to emulate a small TDMA network. The spacecraft

simulator allowed the NGS and VSAT to simulate BBP operations as a complete system, all within the station equipment.

Most importantly, it allowed network personnel to investigate

problems offline, with the spacecraft in use by MSM experimenters, thus, not requiring valuable spacecraft time for troubleshooting.

The integration, assembly, and maintenance of the USAT

stations, which consisted of only seven active components,

were relatively easy. The first-generation USAT (0.25-W

upconvener/amplifier) was configured using two enclosures.

The first enclosure held the IF section of both the transmit and

receive and the first-stage uplink and reference oscillators. The

upconverter/amplifier, LND, and remaining oscillators were

installed in a second enclosure, The configuration made for

easy access to all components. Heat was dissipated through the

base of the aluminum enclosure and did not affect other

components. The drawback was the number of cables required

to interconnect both enclosures (Tx, Rx, ref LO, power) since

with one enclosure mounted at the feed and the second on the

mount, the cables had to allow a small degree of movement for

station pointing and later inclined orbit satellite tracking.

The second-generation USAT used a slightly larger enclosure (fig. 6) to hold all the active components (2-W upconverter/

amplifier (base), LND (sidewall), three oscillators (two on

sidewalls, one in middle of enclosure), reference oscillator

(back wall) and IF amplifier (sidewall)). A change in the

upconverter/amplifier design to include all the uplink electronic s

in a single package reduced the number of assemblies and

interconnects. Also, the components were more tightly integrated into the enclosure. Integrating the components into the

package increased the assembly time, but reduced station

Figure 6 USAT RF enclosure.

NASA/TP-2000-210047 13
installationtimeandcomplexitybyeliminating one enclosure.

In both cases, the DC power supply used by the station was

separate from the electronics.

The LET was designed to evaluate the link between ground

station and satellite. Numerous test ports were designed into the

system such as receive, transmit (both before and after the

TWTA), loopback, calibration signal source, and all station

oscillators. Built-in test equipment provides power, frequency,

and spectrum measurement at each test port. Station modems

and built-in BER test system, described earlier, provide a

pseudorandom data source for detailed BER characterization.

Built-in Ioopback capability and RF connections to the MS M of

the satellite simulator (ACTS transponder engineering model),

described earlier, proved valuable for offline system testing,

station checkout, and fault isolation. Access to test ports on

both uplink and downlink signal characteristics proved invaluable during system anomalies and characterization experiments.

As described earlier, LET station equipment was divided

between the antenna hub (directly behind the antenna reflector)

and an area directly below the roof under the antenna. The

receiver, TWTA, and portions of the upconverter are housed in

the hub, with the remaining components and all test equipment

located inside. An environmental control system and flexible

ductwork from the building to the antenna hub provided an

appropriate temperature-controlled environment for hub

electronics. The NGS beam waveguide antenna allowed all

transmit and receive equipment to be located indoors.

The LET electronics-in-hub configuration is the more economical design, but is inconvenient to maintain when work is

required on the hub electronics, particularly in bad weather. A

small platform constructed on the back of the antenna structure

helped safety and stability, but accessing electronics inside the

hub was still difficult. The NGS beam waveguide design with

all equipment indoors allows easier and more immediate access

for maintenance and adjustments and direct access to antenna

input/output flanges for measurements and tests. The design

also allows implementation of more complex transmit/receive

systems and easier configuration changes. These features are

particularly well suited to experimental operations and in-orbit

test facilities. The disadvantage is the added expense of the

beam waveguide design. Station designers will need to weigh

the above factors against complexity required and intended

service in cost-benefit analysis of future designs.

Unlike the NGS and LET, the VSAT was not equipped with

loopback capabilities either at RF or IF. Lack of any loopback

capability made station testing difficult and relied upon the

TDMA network to evaluate station performance. Nonstandard

IF of 758 MHz (transmit) and 1620 MHz (receive) prohibited

loopback testing of the modems. Further, to contain costs, the

stations were not equipped with many test points. Test points in

both transmit and receive to measure various frequencies and

power levels throughout the system would have proved valuable and allowed easier alignment, troubleshooting, and testing

of the system.

The HDR stations were developed with TDMA Control Card

loopback (circuit card only), modem digital loopback (internal to

modem only), and IF loopback (external to modem). However,

the stations did not have an RF loopback capability. The loopback

functions were extremely useful to debug and test the ground

stations to isolate problems to either the digital terminal, the burst

modem, or to the RF system. RF Ioopback capability would have

been valuable to characterize individual station performance, but

again, cost and implementation must be considered.

The electronics of the HDR station were divided between the

equipment trailer and the boom of the antenna. The upconverter

and downconverter electronics were installed in the equipment

trailer with the LNA and TWTA mounted on the boom of the

antenna. As a result, both the transmit and receive paths had

approximately 12 m of waveguide between the equipment

trailer electronics and the antenna feed electronics. The 750-MHz

bandwidth signals of the burst modems transmitted over these

long waveguide runs introduced crosstalk between the I and Q

channels of the OQPSK signal due to group delay dispersion of

the waveguide. 13 Custom waveguide equalizers with opposite

group delay were installed to mitigate the crosstalk and improve station performance.

Inclined Orbit

By deleting the final north-south stationkeeping maneuver of

standard ACTS operations, ACTS experiment operations were

extended by operating in an inclined orbit. East-west

stationkeeping was maintained due to the small amount of fuel

required to execute east-west stationkeeping maneuvers. The

resulting orbital inclination increased at a rate of about 0.8 ° per

year, which made antenna tracking a requirement for all stations. Because of their aperture size, the LET and NGS antennas

already had existing tracking systems, which were used throughout the ACTS program. Both systems operated in a steptracking mode using the 20-GHz telemetry beacon signal from

the satellite. No modifications were made to the NGS-tracking

system. However, the LET-tracking system (and antenna) had

been developed under a POC contract more than 12 years

earlier. As with the LNA, the antenna and its tracker were put

into service without modification as a cost-savings measure.

Although the tracking system functioned technically well, a

new commercial system was installed to assure better system

availability (spare parts, current electrical equipment, etc.)

throughout the remaining program.

There were several impacts to the ground-station program that

resulted from inclined orbit operation. Each field station required

new mounts and electronics to track the satellite. Also, the BBP

TDMA network and HDR network equipment required software

modifications to accommodate the increased range and range rate

variation in the inclined orbit. All modifications to the earth

stations were performed inhouse to reduce costs and to achieve

stringent pointing requirements for all stations.

14 NASA/TP-2000-210047
Becauseoftheincreasedrangeandrangeratevariationinthe

inclinedorbit,therewereimpactsontheBBPTDMAnetwork

acquisitionprocess, whichpotentiallyrestrictedthetimesat

whichproperacquisitioncouldoccur.Tracking(oftiming)was

alsoaffected, whichlateinoperationsrestrictedthetimesthat

synchronization couldbemaintained. Thechangesweregradual;

however, andoccurredaspredicted. Minorsoftwaremodificationsweremadeto boththe VSAT and NGS software to

mitigate most of these effects, but complete mitigation would

have required additional software modifications to ground

equipment. Given the projected length of inclined orbit operation, it was decided to work around the limitations when

necessary by modifying operating times and procedures. This

approach proved successful. No real limitations were encountered until very late in the program, as expected.

The HDR software system was expected to accommodate the

sate llite's increase in range rate and Doppler shift, but software

modifications were made to the acquisition process (range time

determination) algorithm. The initial round trip time (RTT)

was estimated based on the ground-station location and ephemeris data, with further RTT updates made once acquisition was

complete. After operating in inclined orbit, it was found that the

RTT software modifications functioned properly, but the increase in range rate limited the times at which an HDR station

could acquire. Station acquisition was limited to the north and

south extremes of the satellite" s orbit. Once acquired: however,

the HDR stations maintained acquisition, even at the range rate

peak, through the center of the orbit. Review and analysis of the

HDR acquisition process revealed that the most likely cause of

the problem was a restriction in the acquisition software. Due

to cost and time to correct, the software was not modified.

Instead, the operations schedule was adjusted, with no impact

to station performance.

A commercial antenna-tracking controller was modified to

support remote configuration, control, and monitoring required by the field stations. The majority of system parameters

were programmable by dial-up modem or through support

computers using the existing station remote control capability.

The most significant feature implemented in the controller was

the ability to remotely upload and download the 24-hr history

and future movement of the antenna system. The ability to

access and control this table allowed station personnel to

analyze ground-station position relative to satellite position

(download feature), program the antenna to move to predicted

positions in the case of a new installation (upload feature), or

repeat the past position for a specific time of day in the absence

of a received signal (memory track). Because of ACTS scheduling, the receive signal used by the VSAT and USAT stations

for closed-loop antenna tracking was not always available.

The memory track featured allowed the stations to use the last

known position for a particular time slot. This feature maintained suitable pointing until the station was again scheduled

for experiments at which time the position table in memory

would update.

Each field terminal used the tracking controller for inclined

orbit operations. The controller required a 0- to 10-Vdc signal

input for closed-loop tracking. The memory table provided

open-loop tracking in the absence of a downlink signal. This

feature was often used due to ACTS scheduling of particular

earth stations. Each station addressed the closed-loop signal

requirement based on its system architecture and capabilities.

The VSAT used a feedback voltage from the automatic gain

control (AGC) card present within the VSAT as input to the

tracking controller. Minimal station design was required to

utilize the AGC feedback signal for the controller. However. to

ensure an accurate AGC feedback signal, the controller was

modified to accept a contact closure signal from the VSAT

indicating demodulator lock when synchronized with the BBP.

The AGC signal was only valid in this mode.

Figure 7 illustrates a tracking signature of a VSAT 1.2-m

system compared to the NGS with both co-located at GRC. The

figure indicates accurate SAT tracking relative to the NGS

reference with a maximum difference of<0.2 ° in either azimuth

or elevation, corresponding to <1 dB pointing loss.

Although the use of commercial modems gave the USAT

flexibility in modulation and data interfaces, the lack of a

digital interface posed a challenge for inclined orbit operations. To enable the USAT to track the satellite independently

of the modem used with the station, a separate circuit was

designed to provide the required analog input signal to the

tracking controller. Because the LND filtered the beacon

signal from the satellite, the analog signal was derived from the

70-MHz-received signal at the station. A series of narrow

bandpass filters and appropriate amplifiers followed by a

microwave detector followed by a narrow band digital filter

derived the required signal. The detector was operated in the

low-sensitivity region such that the voltage variation was

approximately linear with input power. Figure 8 illustrates a

typical tracking signature of a USAT with a 0.6-m reflector

compared to that of the NGS. Both are located at GRC. The

plot illustrates the ability of the USAT to follow the NGS

pattern from which satellite range and position data is verified.

Also evident is the lack of resolution at the satellite location

extremes due to the larger beam width of the smaller USAT

aperture. Pointing error in the example is approximately 0.2 °

azimuth and 0.4 ° elevation, which contribute approximately

l-dB pointing error loss.

Because the tracking signal was derived from the received

communications signal, the USAT antenna controllers were

configured to conduct their antenna peaking at alternating

intervals so that signal variations induced by uplink antenna

tracking did not confuse the downlink antenna-tracking process ultimately causing both stations to mispoint.

The USAT-tracking mount was an easily deployable terminal with minimal impact to installation and little or no increase

in the overall terminal size once installed. Figures 9 and 10

show the USAT mount before and after inclined orbit modifications, respectively. The fixed mount shown in figure 9 shows

NASA/TP-2000-210047 15
"O

[]

40.20

40.00

39.80

39.60

39.40

39.20

39.00

38.80

38.60

38.40

38.20

38.00

37.80

37.60

37.40

37.20

37.00

205.6

NGS

--e-- VSAT

205.8 206 206.2 206.4 206.6 206.8

Azimuth, degree

Figure 7 VSAT 1.2-m inclined orbit tracking versus NGS reference.

207

==

®

1D

=-

.o

4-P

40.3

40.1

39.9

39.7

39.5

39.3

39.1

38.9

38.7

38.5

38.3

38.1

37.9

37.7

37.5

37.3

NGS

--II-- USAT

205.6 205.8 206 206.2 206.4 206.6

Azimuth, degree

Figure 8 USAT 0.6-m inclined orbit tracking versus NGS reference.

206.8 207

16 NASA/TP-2000-210047
i:i!ii=iil : : ilii i  ̧ iil

Figure 9 USAT fixed mount.

threaded rods behind the reflector used to adjust azimuth and

elevation. The RF electronics are installed in front of the

reflector. Figure 10 depicts the inclined orbit-tracking mount.

The reflector and RF electronics are configured in the same way

as the fixed mount. However, moving the azimuth axis-ofrotation from behind the reflector to between the reflector and

electronics improved the stability of the mount. Adjusting the

mount center-of-gravity and slightly reducing the mount height

reduced the ballast required for installation.

The inclined orbit mount required additional installation

time for adjustments and pointing to minimize the coupling of

elevation to azimuth movement inherent in the design. The

coupling of elevation movement due to azimuth occurred

because the azimuth axis-of-rotation was angled relative to the

ground compared to traditional mounts. This adjustment enabled the required degree-of-motion given the design constraints. The impact was minimal for the small inclination of

ACTS, provided the mount was properly installed.

The HDR system provided a feedback signal to the tracking

controller for closed-loop antenna tracking using the 20-GHz

telemetry beacons. The bandwidth of the receive system allowed access to the satellite beacon signal in the equipment

trailer. A system was developed inhouse to convert the received

signal to the required 0- to 10-Vetc input required by the

controller. Due to the lack of test equipment available onsite, a

spectrum analyzer was used as a beacon receiver to monitor the

beacon signal strength. Having the spectrum analyzer onsite

provided needed test equipment to troubleshoot anomalies and

conduct link characterization measurements. The spectrum

analyzer signal (beacon strength) was converted to an analog

output for the tracking controller by a computer and data

acquisition card.

Like the VSAT and USAT mounts, the HDR antennatracking mount was also designed and fabricated inhouse. The

HDR antenna was custom made by Prodelin Corporation and

consisted of a four-petal, 23.5°-offset, 3.4-m Ka-band dish

bolted to a fixed mount frame. Figure 11 shows the antenna

after the in-house modifications were complete for inclined

Figure 10 USAT inclined orbit mount.

Figure 11 HDR inclined orbit mount.

orbit operations. Although the design worked well for HDR

tracking, the initial pointing of the antenna was more difficult

compared to traditional mounts because the axis-of-rotation

(highlighted in fig. 11 ) of each actuator was offset 45 ° from

NASA/TP-2000-210047 17
traditional mounts.Theoffsetdesignpartiallybalancedthe

weight of the antenna allowing simpler modifications and

smaller actuators.

The NGS and LET step trackers provided very accurate

tracking of the spacecraft and were used to provide spacecraft

location inputs to ephemeris calculations. As discussed in

reference 7, fade effects are known to affect step tracker pointing accuracy, particularly when the fade level fluctuates during

the execution of the tracking algorithm. These effects were

observed in both the NGS and LET systems. Multiple features,

including memory track, fade level, and fade rate threshholds

are available in the trackers to set algorithm parameters so as to

mitigate these effects. Early in the program some operations

impact occurred due to antenna pointing effects. However,

parameters were adjusted to reduce the occurrence of these

effects significantly. There are several factors to be considered.

Additional experimentation would be desirable but is limited to

times of appropriate weather conditions and by the need to

minimize impact to gateway station operation. The alternative

technology of monopulse tracking is less susceptible to propagation effects, but is more expensive to implement. System

designers faced with cost-benefits tradeoffs can note that step

tracker technology has been used successfully in the ACTS

program.

Conclusion

Over the past 7 years the Advanced Communications Technology Satellite (ACTS) Program has successfully demonstrated advanced applications and services using ACTS through

experiments and demonstrations. Key to the experiments and

demonstrations were the deployment and operations of the Kaband ground stations.

Three classes of transportable earth stations ranging in antenna diameter size from 0.35 to 3.4 m provided access to

ACTS to conduct applications using data rates from 4.8 Kbps

to 622 Mbps. Ground-station operations provided many lessons learned over the years in technology development, integration, and maintenance. Two gateway earth stations located

at GRC provided (a) tracking, telemetry and control of ACTS,

(b) the reference station of the BBP Time Division Multiple

Access (TDMA) network and MSM satellite-switched TDMA

network, and (c) conducted high-data-rate interoperability experiments and link characterization experiments. The Ka-band

technologies described in this report were applied in various

ways to the field and gateway earth stations.

This report summarizes the Ka-band technologies, station

hardware integration and configuration, and lessons learned

throughout the program. Included is a discussion of the ACTS

inclined orbit operations and required modifications to the

ground stations. The ground station's inclined orbit design and

performance present various alternatives to implementing

closed-loop-tracking systems.

References

IConey, T.A.: Advanced Communications Technology Satellite (ACTS) Baseband Processor Mode (BBP) System Performance. NASA Technical Paper, NASA Lewis Research Center.

Cleveland, OH, 1996.

2Lee, Kerry D.: Design and Development of a Baseband

Processor for the Advanced Communications Technology Satellite. Proceedings of the Advanced Communication Technology Satellite Results Conference, NASA CP-10183, 1996.

3May, Brian D.: Kerczewski, Robert J.; and Svoboda, James S.:

Anomalous TWTA Output Power Spikes and Their Effect on

a Digital Satellite Communications System. NASA TM105875, 1992.

4Fijikawa, Gene; and Svoboda, James S.: Test Results of a

20 GHz, Low Noise Downconverter for USAT Applications.

NASA TM-106892, 1995.

5Reinhart, Richard C.: System Design and Applications of the

Ultra Small Aperture Terminal with the Advanced Communications Technology Satellite. NASA TM-107536, 1997.

6Acosta, Roberto J.: Special Effects: Antenna Wetting, Short

Distance Diversity, and Depolarization. Paper presented at the

Sixth Ka-Band Utilization Conference, NASA Glenn Research

Center, Cleveland, OH, June 2000.

7Struharik, S.J.; and Meadows, D.N.: The ACTS Master Ground

Station Design and Performance. Proceedings of the Advanced

Communications Technology Satellite Results Conference,

NASA CP-10183, 1996.

8Acosta, R.J.; and Johnson, S.: Six Years of ACTS Technology

Verification Experiment Program Results. Paper presented at

the Fifth Ka-Band Utilization Conference (Taormina, Italy),

Oct. 1999.

9Reinhart, Richard C., et al.: Modem Characterization Through

a Wideband, Hard-Limited Ka-Band Satellite Channel. NASA/

TM--1999-209454, 1999.

10Kerczewski, Robert K., et al.: CW Interference Effects on

High Data Rate Transmission Through the ACTS Wideband

Channel. NASA TM-107156 (AIAA Paper 96--1104), 1996.

18 NASA/TP-2000-210047
ItAcosta,RobertoJ.:AdvancedCommunication Technology

Satellite(ACTS)MultibeamAntennaAnalysisandOn-Orbit

Performance. PresentedattheSecondKa-BandUtilization

Conference(Florence, Italy),Sept.1996.

12Hoder, Douglas;andBergamo, Marcos:GigabitSatellite

NetworkforNASA'sAdvanced Communication Technology

Satellite (ACTS). Int. J. Satell. Commun., vol. 14, no. 3, 1996,

pp. 161-173.

13Kearney ' Brian; and Hoder, Douglas: High Data-Rate Ground

Station for the Acts Gigabit Satellite Network (GSN). Proceedings of the Advanced Communication Technology Satellite

Results Conference, Part 1, NASA Lewis Research Center,

Cleveland, OH, 1996.

NASA/TP-2000-210047 19
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May 2000 Technical Paper

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

ACTS Ka-Band Earth Stations: Technology, Performance, and Lessons Learned

6. AUTHOR(S)

Richard C. Reinhart, Steven J. Struharik, John J. Diamond, and David Stewart

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546- 0001

WU-315-90-81-4)0

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REPORT NUMBER

E-12250

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TP--2000-210047

11. SUPPLEMENTARY NOTES

Prepared for the Sixth Ka-Band Utilization Conference sponsored by the Istituto lnternazionale delle Comunicazion, Cleveland, Ohio,

May 3 l--June 2, 2000. Richard C. Reinhart, NASA Glenn Research Center; Steven J. Struharik, COMSAT Laboratories,

Clarksburg, Maryland 20871; John J. Diamond, Analex Corporation, 3001 Aerospace Parkway, Brook Park, Ohio 44142; and

David Stewart, GTE Technology Organization, 21000 Brookpark Road, Cleveland, Ohio 44 135. Responsible person,

Richard C. Reinhart, organization code 6130, (216) 433_588.

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13. ABSTRACT (Maximum 200 words)

The Advanced Communications Technology Satellite (ACTS) Project invested heavily in prototype Ka-band satellite

ground terminals to conduct an experiments program with ACTS. The ACTS experiments program proposed to validate

Ka-band satellite and ground-station technology, demonstrate future telecommunication services, demonstrate commercial viability and market acceptability of these new services, evaluate system networking and processing technology, and

characterize Ka-band propagation effects, including development of techniques to mitigate signal fading. This paper will

present a summary of the fixed ground terminals developed by the NASA Glenn Research Center and its industry

partners, emphasizing the technology and performance of the terminals and the lessons learned throughout their 6-year

operation, including the inclined orbit phase-of-operations. The fixed ground stations used for experiments by government,

academic, and commercial entities used reflector-based offset-fed antenna systems with antennas ranging in size from 0.35 to

3.4 m in diameter. Gateway earth stations included two systems referred to as the NASA Ground Station (NGS) and the Link

Evaluation Terminal (LET).

14. SUBJECT TERMS

Satellite communications; ACTS; Ka-Band; Ground terminal; Amplifiers; Antenna;

Gateway; Inclined orbit

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