NASA balloon: Aircraft ranging, data and voice experiment (May 1, 1972)

8/2/2019 NASA balloon: Aircraft ranging, data and voice experiment (May 1, 1972)

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FINAL R E P O R TN A S A B A L L O O N - A I R C R A F TR A N G I N G , D A TA A N D VO I C E E XP E RI ME NT

S. WishnaC. HambyD. Reed

M AY 1972


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A B S T R A C T

During September 1971 at Aire-Sur-l'Adour, France, N A S A conducteda series of tests to evaluateatL-band,the ranging, voice, and data communica-tions concepts proposed for the air traffic control experiment of the Applica-tions Technology Satellite-F. The ground station facilities, balloon platformsand the aircraft were supplied by the European Space Research Organization.One ground simulationand two aircraft flights at low elevation angles wereconducted. Even under high interferenceconditions good performance wasobtained for both voice communications and side tone ranging. High bit errorsoccurred in the data channels resulting in false commands. As a result of theexperience gained in operatingthe equipment in an aircraft environment severalrecommendationswere made for improving the equipment performance.


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A C K N O W L E D G E M E N T S

The authors wish to express their appreciation to the European SpaceResearch Organization and in particular to Mr. Dennis Brown, program manager,and his project team without whose help this experiment could not have beenconducted. Steve Martin, Bell Aerospace Project Manager, is also to be com-mended for the field assistance provided NASA during the tests,and for theoutstanding effort provided by his project group in getting the experimentalequipments to Europe on time.

We also wish to express our gratitude to the Prince Georges CountyBoard of Education (Maryland); to teachers Charles Thompson, Mary Kiefer,and Jack Renner; and to the many volunteer students for helping process thevoice tapes. Last but not least we wish to express our thanks to the follow-ing personnel of the Space Applications and Technology Directorate: toMr. Ralph E. Taylor for reviewing the original manuscript and making construc-tive suggestions; to Mr. William B . Risley for processing the data; and toMr. Alton E. Jones, Walter K . Allen, Dr. James C. Morakis, Samuel Gubinand George Orr for their encouragement, help and guidance.


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TABLE O F C O N T E N T S

1. I N T R O D U C T I O N . . .2. S U M M A R Y A N D CONCLUSIONS . .

2.1 Experimental Objectives . . .2.2 P L A CE Equipment Modifications.

3. E XP E RI ME NT D E S C R I P T I O N . . . .3.1 Balloon Experiment Equipment

3.1.1 Ground Station. .3.1.2 Balloon . . . .3.1.3 Aircraft.

3.2 Test Program . . . . ..3.2.1 Equipment Checkout3.2.2 Pre-Flight Tests3.2.3 In-Flight Experiment .

3.3 Experimental Data .4. E V A L U A T I O N O F E X P E R I M E N T A L D A T A . . .

4.1 Carrier/Noise and High Rate Data Errors .4.2 Side Tone Ranging .4.3 Voice Intelligibility . . . .4.4 Command/Control Data Channel

References:Appendix A - Coordinate Transformation of Radar Data

Page1-12-1

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1. I N T R O D U C T I O NA n effective air traffic control system must provide voice and data

communications as well as surveillance capability. A s part of the evolutionof such a system, the National Aeronautics and Space Administration (NASA)has made extensive investigations in cooperation with many foreign and do-mestic agencies and private corporations, e.g., the Department of Transpor-tation ( D O T ) and the European Space Research Organization ( E S R O ) , to capita-lize on the advantages associatedwith a satellite based system. Thesestudies have led to a comprehensive air traffic control experimentcalled P L A CE(Position Location and Aircraft Communication Equipment). The experiment willbegin in 1973 with the Applications Technology Satellite-F (A TS-F ) and isdesigned to further evaluate the P L A C E concepts.

During the fall of 1971, E S R O conducted a series of balloon-aircrafttests (referenceL )to provide preliminary data defining the performance of asatellite based L-band air traffic control system wherein the satellite was sim-ulated by the balloon. E S R O invited N A SA to participate in this test program,and as a result, N A SA conducted a series of tests to evaluate some of the airtraffic control concepts proposed for the P L A CE A T S - F experiment.

The surveillance, data, and voice communication system demonstratedduring the balloon-aircraft test program are limited to concept demonstration, theacquisitionof limited scientific data, and the preliminary evaluation of techno-logical approaches. Therefore, the experiment described herein should not beconsidered as a N A S A recommendation of any specific air traffic control system.

The specific objectives of the experimentwere:o Provide a preliminary evaluation of the P L A CEAT C concept prior to A T S - F launch* Demonstrate the concepts of continuous and time-division multiplex two-way tracking of aircraft by comparing

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*

experimentally acquired ranging data with radar-determinedtrajectories of the aircraft.* Evaluate duplex, NarrowBand, Frequency Modulation( N B F M ) voice channels* Evaluate duplex data channels* Demonstrate aircraft command and control utilizingthe data channel as envisioned for the P L A C E experiment* Obtain operational experience with the P LAC Etransceiver in an aircraft environment* Develop techniques for coordination of airborne andground facilities* Evaluate test procedures which determine system

performance* Collect sample data similar to that for the P LAC EexperimentDirected toward these objectives, this report presents the resultsof the aircraft-balloon experiment.

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S U M M A R Y A N D C O N C L U S I O N SA ll experimentalobjectives of the N A S A / E S R O balloon-aircraftexperi-

ment conducted during September of 1971 were attained. Furthermore, as adirect result of this experiment, several modificationshave been incorporatedinto the equipment that should significantly enhance the P LAC E A T S - F experi-ment.2.1 Experimental Results

Essentially, the objectives of the N A S A / E S R O balloon-aircraftexperi-ment fall into two categories. First, four basic A TC functions to be performedby the P LAC E equipment were to be evaluated. These are (1) high rate (1200bps) data, (2) side tone ranging for aircraft surveillance, (3) voice commun-ication, and, (4) low rate (600 bps) data to control multiple access and pro-vide commands. The second objective was to gain operational experienceusing P LAC E hardware in an A TC test environment.High Rage (1200 bps) Data

The performance of the 1200 bps data channel was evaluated bymeans of the bit errors accummulated in the received data from a PN datatransmissionduring the experiment. Specifically, distributions of bit errorswere time correlatedwith carrier-to-noise density values. These correlations,then, provided a basis for comparisonwith the theoretical performance of thischannel. The result of these comparisons clearly indicate the high rate datachannel format must be designed to be less susceptible to interference, if biterror rates less than 10 - 3 to 10 - 4 are to be realized.Tone Ranging

The precision and accuracy of the P LAC E side tone ranging channelwere evaluated by a direct comparison of experimentallydetermined side tonerange with precision radar tracking data. These comparisons show excellent per-formance as evidenced by precision of a few hundreds of meters even duringthose time periods when considerable interference was evident in other experi-mental data.

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Voice CommunicationBoth simplex and duplex operation of the voice channel were accomp-

lished during several flight periods. Under all circumstances includingperiodsof significant interference, excellent communications (in the context of A T Crequirements) were maintained. This performance was verified by the excellentintelligibilityscores obtained from the test words transmittedover the voicechannel and recorded on-board the aircraft.

A minimum mean intelligibilityof 62 % was obtained during the mostsevere interferencewhile mean intelligibilitieson the order of 95 % were ob-tained for strong signal, non-interference conditions. Of the words listenersfailed to identify, few were actually missing. Because of its large dynamic range,the NBFM voice channel maintained lock even under severe fading conditions.Command and Control

The command and aircraft interrogation control was provided by a600 bps data channel. Although a simple paritycheck is utilized in thischannel to inhibit command/control errors, three such errors did occur duringa period of approximatelyone hour. This frequency of error suggests a needfor more sophisticatedcoding.Interference

The experiment serves to emphasize the need for carefullymonitoringthe Radio-Frequency Interference (R FI ) environment. The downlink L-band spec-trum was monitored at the ground station during all of the flights and considerableinterferencewas intermittentlypresent. Because of the lack of measurementfacilities neither the source or level of interferences could be fixed with anydegree of confidence. Most of the interference is believed to have originatedin the UHF band suggesting for any future balloon flights that this frequency bandbe avoided for the ground station-balloonlink.

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P LAC E Equipment ModificationsA s a direct result of the balloon-aircraftexperiment, several modifi-

cations have been incorporated into the P LAC E equipment. These shouldmeasureably improve the performance of the P L A C E equipment during the A T S - Fexperiment.A GC Modifications

A n important result of the balloon aircraft experimentwas the indicatedpresence of rapid changes in carrier-to-noise ratio. T o improve the performanceof the AGC, the time constant of the A GC has been shortened from one secondto 10 milliseconds.Loss of Lock and Search

Another aspect of rapid carrier-to-noise fluctuation is the possibilityof short lived loss-of-lock. During the experiment, temporary loss-of-lockin the data channels resulted in an immediate entry of the equipment into atwo-minute cycle search mode. T o eliminate this conditions and thereby re-duce susceptance to these transients, a ten second delay has been addedafter loss-of-lock before entry into the search mode. Additionally,the duration of the search period has been reduced from two minutes to 45seconds.PS K Modulation

The occurrence of loss-of-lock frequently resulted in significantdataerrors, because PS K reacquisition occurred in the wrong sense. Thesense correctioncapability of the equipment did not function rapidly enoughto alleviate this problem under low signal-to-noise conditions. T o eliminatethis condition the PS K modulation system has been changed to a differentiallycoherent phase shift keying ( D C P S K ) system. This will limit errors resultingfrom reacquisition transients to no more than one bit.

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AlterationsA s a result of operational experience, three further alterations have

First, a manual override has been installed in the equipment todisablement of automatic search. Second, frequent loss-of-lockoccurredthe experimentbecause of signal transients created by discrete step atten-

These have, therefore, been replaced with continuously variable controls,and data channel switching controls were changed from knob to pointer

The last alteration was implemented to improve readability of equipmentpositions.

In conclusion , it should be noted that only one minor equipment fail-e occurred during the experimental period of nearly two weeks. This was a

of the power amplifiermonitoring circuitry.

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3. E X P E R I M E N T D E S C R I P T I O NThe major five subsystems employed in the experiment were:* Ground station* Balloon platform* Radar tracking station* Aircraft Data processing centerThe ground station was co-located with the balloon launch facility

at Aire-sur-l'Adour, France and contained the equipment to: (1) generate andtransmit all of the necessary data, voice, ranging and command signals,(2 ) receive and record the return ranging, data and voice signals, (3 ) monitorballoon transmissions, and (4 ) receive telemetry data from the balloon.

The balloon platform contained a telemetry link, a radar beacon, anda transponder system. The telemetry link provided balloon power output, tem-perature and altitude information. The radar beacon provided an enhancedsignal return level to the radar. The duplex transponder received U HF signalsfrom the ground station and retransmitted them to the aircraft on L-band, alsoL-band signals received from the aircraft were converted to U H F and retrans-mitted to the ground station.

Precision radars, European versions of the FPS-16, were located atBiscarrasse-and Hourton, France and tracked the aircraft and balloon duringthe experiment. These provided digital printouts of aircraft and balloon positionsas functions of time.

The aircraft utilized the breadboard model transceiver built for theATS-F experiment and modified for this program. This transceiver receivedmodulated L-band carriers (ranging and low rate data, high rate data, voice)and demodulated these carriers to produce range tones, voice and data. Out-put range tones and aircraft-generated voice and data were modulated ontothree L-band carriers for transmission to the balloon.

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The data processing was accomplishedat NASAs Goddard SpaceFlight Center at Greenbelt, Maryland.

The ranging, data, and voice systemis designed to service 250aircraft by means of time division multiple access. The ground stationcon-tinuously transmits a 600-bps data stream and range tones, on a low-rate dataand surveillance carrier. The 600-bps data contain a series of 8-bit words(with one bit parity) that activates the transceiver logic and internal circuits,selects the time slot for transmission, selects the number of transmissionsper frame, and provides voice, data, and emergency channel status indication.To minimize command and control errors, the aircraft transceiverwill decodeonly the low-rate data during the epoch slot, the 10 status slots, and theassigned transceiver slot. The durationof each time slot is 200 msec.

High-rate data channel and voice channel access will (for the P L A C EATS-F experiment) be upon request, with immediate access capability foremergencies. A voice and high-rate data transmission is sent from the inter-rogated aircraft simultaneouslywith the ranging and low-rate data. However,the balloon-aircraft experimentused only one ranging and low-rate datachannel, one voice channel, and one high-rate data channel.

The high-rate data are generated at a 1,200 bps rate,and differentiallycoherent phase shift key (DCPSK) modulate a carrier. During theexperiment, a pseudo-randomcode data transmissionwas used on this channelto determine bit error statistics.

A n adaptive narrowband frequency modulation ( A N B F M ) technique wasused for the duplex voice communications. This techniqueachieves goodqualityvoice reception under low-carrier-to-noise conditions by employingoptimized voice processing of both input and output speech and by using aunique adaptive demodulationconcept (reduces demodulator bandwidth ascarrier-to-noisedecreases). The systemachieves a 95 % modified rhyme test( M R T ) voice intelligibility at a carrier-to-noise power density level of 46 dB-Hz,

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but will exhibit thresholding at extremely lowcarrier-to-noise levels. Toeliminate this thresholding effect, the A N B F M for P L A CE was designed to givean 85% M RT word intelligibility at a carrier-to-noise level of 46 dB-Hz. Theelimination of thresholding is considered extremely important if the voicesystem is subjected to severe fading, since this will minimize the number ofcomplete dropouts.

The transmission spectrum is shown in Figure 3-1. Except for thedifference in carrier and subcarrier frequencies, this spectrum is identical forall links. The transmitted signal consists of three carriers: Fcl, Fc2 , andFc3 ; Fcl contains the ranging and low-rate data information, Fc2 contains thehigh rate data information, and Fc3 contains the voice transmission. Theranging function is provided by two individual ranging tones (8575 Hz and7350 Hz), double sideband amplitude modulated onto F l . The low-rate dataare generated at a 600-bps rate and phase-shift-key (PSK) modulated onto aquadrature component of the same carrier Fcl. Sufficient carrier power isreinserted to enable carrier lockon at the transceiver. Table 3-1 presents thefrequency assignments of the transmitted carriers for each link of the experiment.

Table 3-2 summarizes the link calculations for the balloon-to-aircraftand aircraft-to-balloon links. The critical link with respect to power limitationswas the balloon-aircraft link. The ground-to-balloon links were not powerlimited, primarily because of the lower path loss of the UH F transmission andthe use of a high-gain antenna at the ground station. The aircraft-to-balloonlink had approximately 17 dB more transmitterpower than the balloon-to-aircraft link. The values used for the link parameters were nominal worstcase; e.g., the minimum anticipated antenna gains were used and the pathloss was computed for the maximum anticipated slant range of 240 statute miles.Table 3-2 shows that for an experimental system a total power density require-ment of 49 dB-Hz (voice: 46 dB-Hz; High-rate data: 42.8 dB-Hz; and low-rate data and range tones: 42.0 dB-Hz) could be achieved. These levelscorrespond to anticipated satellite air traffic control requirements.

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TABLE 3-1FREQUENCY ALLOCATION

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Link FC1 F C 2 F C 3M Hz M H z MHzGround-to-balloon 444.1250 444.0750 444.0375444.0000' 443.9625

443.9250 443.8875Balloon-to-aircraft 1550.3550 1550.4050 1550.4425

1550.4800 1550.51751550.5550 1550.5925

Aircraft-to-balloon 1651.3750 1651.4250 1651.46251651.5000 1651.53751651.5750 1651.6125

Balloon-to-ground 400.7750 400.7250 400. 6875400.6500 400.6125400.5750 400.5375


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Balloon Experiment EquipmentThe N A S A / E S R O Balloon experiment was designed to make maximum

use of existing P L A CE equipments and designs as well as available E S ROground, balloon and aircraft equipment. Figure 3-2, the system block diagram,shows within the dashed lines the NASA equipments that were located in theground station and the aircraft. Table 3-3 contains a list of the equipmentsused, includinga summary of equipment functions and identificationof NASAand E S R O supplied items.3.1.1 Ground Station

The NASA ground station was located in the E S R O / E S T E C (EuropeanSpace Research and Technology Center) ground station at Aire-Sur-l'Adour,France. The ground station employed two switchable UHF antennas for com-municating with the balloon, one antenna covering elevation angles from thezenith to 450 and the othercovering angles from 450 downward. The higherangle coverage was provided by a fixed mounted Archimedes spiral with anominal gain of 7 dB. The low angle antenna was a four-unit corner reflectorwith a nominal gain of 18 dB, fixed in elevation but movable in azimuth.These antennas were used for both transmit and receive by employinga diplexerbetween the antennas and the ground station power amplifier and receiver.

The U HF receiver was a N E M S Clark 1037C with a crystal-controlledRFT 102A tuner head. The receiver 3 0 MHz intermediate frequency was con-verted to 70 MHz as an input to the ground modem. Another channel of thesame converter was used to provide 444 MHz to the power amplifier from the60 M Hz output of the ground modem. The ground station power amplifier,operating Class C, produced an output power level of 15W from the 10 mw,444 MHz input signal.

The modem units employed in this experimentwere essentially PLACEequipments with minor modifications in channel frequency spacing.

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T A B L E 3-3E Q U I P M E N T LIST

B A L L O O N - A I R C R A F T RA N GIN G, DATA AN D V OICE EXP ERIMENTLocation Item

Ground Station UH F Low Gain AntennaUH F High Gain AntennaDiplexerUH F Power AmplifierUH F Driver AmplifierFrequency ConverterRange Tone GeneratorEpoch and Data Test GeneratorGround Modem

Phase MeterA/D ConverterPrinterTelemetry Recorder

Data Test SetPaperRecorderT!me Code Gene-atorL-Band ReceiverU HF ReceiverL-Band AntennaSpectrum AnalyzerVoice Tape UnitMicrophone/HeadsetL-Band LowGain AntennaL-Band High Gain AntennaP LACE R/T Unit

P LACE H.V. Power SupplyP LACE Airborne ModemFrequency ConverterEvent Indicator UnitTelemetry Recorder

Paper Recorder

Titge Code GeneratorMicrophone/HeadsetBalloon Package

Radar Facility

SourceESROESROESROESROESRONAS ANAS ANASA

NASA

N A SANAS AESROESRO

ESROESRO

ESROESROE S R ONASAESRO

ESRONASAESROESRONASA

NASANASA

NASANASA

ESRO

ESRO

ESRO

NASAESRO

.Typc/Model

HP 230 ABell ModelNo.Bell Model No.Bell ModelNo.Bell Model No.

Dranetz Model No. 202AH P 5245L, HP 5265AHP 5050BPhillips Analog 7

Fredrick Model 60 0Brush M K 260Datum Model 9310Nems Clark 1037GRF Head RF T1040 ModifiedNems Clark 1037GRF Head RFTI02APolarad CA-L

SonyT C 366T E L E X C S - 6 1BoeingDioscuresBell No.

Bell No._Bell No.-

BellNo._Bell No.-Phillips Analog 7

Brush M K 220

E E C O Model 1125ATELEX CS-61

ESRO3-9

FunctionTransmit/Receive Antenna - 4 unit cornerTransmit/ReceiveAntcnna - Archlmeds SpiralTransmit/Receive Channel SeparationClass C Power AmplificationLinear AmplifierConverts transmit 60 MhTz to 44 4 MHkz andreceive 30 M Hz to 70 M HzGenerates 8575 Hz and 7350 Hz tones60 0 bps command and control, 1200 bps, testsignal, 1 minute time markerDemodulatesvoice, data, and range tones,modulates voice, data, and range tones andconverts to 60 M HzMeasures the phase angle of the received rangtones relative to the transmitted tonesConverts analog phase angle reading to BC DreadoutAngle readout of range tone phase s h i f tRecord on separate channels received 5575 Hztone, received 7350 Hz tone, received 60 0 bpsreceived 1200 bps, transmitted 60 0 bps, timecode dataGenerate PN sequencesProvide analog carrier plus noise and analognoise recordSupply serial and B CD readout ofclockReception of balloon L-band transmissionReception of balloon UH F transmissionReceive AntennaAdjust power levels and monitorballoon L-bandtransmissionPlay voice tapes into systemVoice communicationsTransmit Receive AntennaTransmit Receive AntennaReceives and transmits in L-band, out-uts andinputs at IF. A lowpower L-band interfaceis provided.Provides D.C. voltages to S P A unitIdentical to ground station modemexcept for a600 bps decoder and activating circuitryConverts 70 M Hz IF to 10 M Hz IFOutputs analog voltage as a function of thenumber of input eventsRecord on separate channels received voice,received 60 0 bips, bit error count. time code,carrier plus noise, and noiseRecord output of event indicator, and AG C

Supply sertal time readoutVoice CommunicationsConsists of the followingprinciple sutsystemstransponder, antenna systems, radar bcacon,altirmeter, balloon telemetry system, and powersupply.Provide aircraft and balloon azimuth andelevation positiondata.

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The ground modem, consisting of a modulator section and a demodu-lator section, was identical to the aircraft modem except for the lack of thedecoder and activation circuitry. The modulator section consists of the narrow-band FM voice modulator, DCPSK high rate data modulator, and a low-rate dataP S K modulator which has a reinstated carrier in phase quadrature with the rangetones. The modulator outputs are summed and the composite a signal frequencytranslated to 60 MHz. The modem circuit provides control for (1) selecting oneof three data channels (2) selecting one of three voice channels and, (3) adjust-ing the output power level of each modulator.

The inputs to the ground modem consist of two range tones, low-ratedata, high-rate data and voice. The range tones are generated continuouslyby means of a range tone generatorwhich consists of two oscillaotrs at 8575 Hzand 7350 Hz, respectively.

The low-rate data, which also performs the command and control func-tion, is generatedat 600 bps by the Epoch and Data Test Generatorand provides:

* A n epoch sync signal generated each minute,* Aircraft address, slot position and rate of interrogation,* Status signals on the use of the three voice channels,

the high-rate data channels and emergency,* Transmission control of the aircraft transceiver.

The Epoch and Data Test Generator provide a coded 24-bit phrase comprisingthree 8-bit words at selected times relative to a fixed epoch code. The 24-bitcode is set manually by 24 switches during normal operation; however, in thisexperiment 10 of the command and control sequenceswere wired to a 10-positionstepping switch. The stepping switch operated at one step per minute totransmitautomaticallya new command/control function to the aircraft.

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The high-rate data input (1,200 bps) was obtained from a Frederick600 Data Test Set providing 2047 bits per frame of pseudo-random code. Thevoice was obtained by playing back on a commercial quality tape recorder atape containing Modified Rhyme Test words, Phonetically Balanced words,Speech Communication Index Meter ( S C I M ) signals, and typicalair trafficcontrol messages.

The phase shifts of the received range tones were measured by a phasemeter, the Dranatz Sampling Vector Computer System Series 202. The referencesfor these measurements were the tones generated by the Range Tone Generator.The phase meter output, a voltage analog of phase angle, was converted to B C Dand recorded using the HP 5050B Digital Recorder. Also, time was recorded tothe nearest tenth of a second as provided by the time code generator in theground station.

The telemetry recorder, a Phillips Analog 7, was used to record theEpoch and Data Test Generator 600 bps output, NASA 36-bit serial code fromthe time code generator, the ground modem demodulated range tones, low-ratedata and high-rate data. A strip-chart recorder was used to record the voltageanalogs of carrier plus noise and noise from the ground modem.

The ground station also employed an L-band receiver and a spectrumanalyzer to monitor the balloon transmissions. This permitted ground stationpersonnel to set the relative power levels of the ranging, data and voicechannels by adjusting the modulator drives in the ground modem. Ground sta-tion personnel also used this equipment to monitor the spectral purity of theL-band signal received by the aircraft.3.1.2 Balloon

The balloon and associated equipment were supplied by E S R O . Theballoon equipment consisted of the transponder (see Figure 3-3 block diagram),a vertically polarized U HF antenna, a right-hand circularly polarized L-bandantenna, a telemetry transmitterand a primary power source.

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3.1.3 AircraftThe aircraft employed both medium gain and high gain L-band antennas.

The medium-gainantenna, manufactured by the Diamond Company, is a slotted,cross-dipole having a nominal gain of 5 dB. This antenna was flush mountedon one side of the aircraft near the tail assembly. The high gain antenna,manufactured by Elecma of France, is a fixed-array with a nominal 10 dB gain.The Elecma antenna is built as two switchable sections located in the front ofthe aircraft, with one section facing upward to the right and the other facingupward to the left.

The aircraft transceiverused in this experiment was the breadboardmodel built for the ATS-PLACE experimentwith minor modifications to providecompatibilitywith the balloon transponder frequencies. The breadboard trans-ceiver consistsof three separate units: receive/transmit (R/T), modem andhigh-voltage power supply. Figure 3-4 is a block diagram of this transceiver.

The R/T unit consists of an L-band diplexer, receiver, down-converter,up-converter and power amplifier. The incoming 1550.48 MHz signal from theantenna is channeled to the receiver input port by the diplexer. The diplexeris bypassed when using the high gain antenna. The receiverdown-converteramplifies the received signal and converts it to a 70 MHz intermediate frequencywhich is then fed to the modem unit. A 60 MHz modulated signal from themodem unit is delivered to the up-converter power amplifierwhich converts thissignal to a 1650.50 MHz L-band signal. The L-band signal is delivered tothe antenna through the diplexer.

TheHigh Voltage Power Supply provides all voltages necessary for theR/T unit. The modem contains its own internal power supply.

The Modem Unit in the aircraft is identical to the ground stationmodem except for the addition of the decoder andactivation circuitry. The70 MHz IF from the R/T unit is demodulated by three demodulators; the rangetone/low-rate data demodulator, the high-rate data demodulator, and the voice

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demodulator. The demodulated low-rate data and tones are separated and thelow-rate data delivered to the modem decoder which provides internal controland event indicatoractivation.

Modem outputs provided are baseband low-rate data, high-rate data,tones and voice. These outputs are recorded on-board the aircraft. F or thisexperiment they are also delivered to the modem unit modulators. The high-ratedata modulator and voice modulator can each transmit on any one of threeselectable carrier frequencies. The modulator outputs are summed and up-converted to 60 M Hz for delivery to the R/T Unit. During this experiment onlysimplex data and voice channel evaluations were conducted because of spacerestrictions on-board the aircraft, precluding full duplex operations.

The decoder decodes the low-rate data from which it identifies theaircraft address, synchronizes the system internal clock and shifts into anappropriate register the assigned time slot. When the clock timecoincides with the time stored in the register, the aircraft automatically trans-mits for 200 msec the range tones and low-rate data. During 10 specified200 msec slots per minute, the aircraft receives voice channel; data and;emergency status information. The decoder provides seven outputs indicatingthe status of; the three voice channels, the three high-rate data channelsand emergency. For this test, to minimize recorder channel utilization, allseven outputs were summed in the event indicator. This voltage sum was thenrecorded on one channel of the paper recorder and indicates the number ofexecuted commands.

The 60 MHz IF of the R/T unit was converted to 10 M H z to providethe necessary interface with E S R O equipment. A Frederick 600 N data test setwas not used on-board the aircraft for bit error measurements because of spacelimitations on-board the aircraft. The high-rate data was recorded and retrans-mitted. The time code generator provided a time reference for coordinating therecorded data.

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The Phillips Analog 7 telemetry recorder was used to record voice,low-rate data, high-rate data, carrier plus noise analog, noise analog, andN A S A time code. A strip-chart recorder was employed to record the receivedsignal A G C and the event indicator-outputvoltage.

More detailed descriptions of the P L A C E equipmentsused in thisexperiment are available in references ( A ) , (B), and (C). A descriptionof themodificationmade to the P L A C E units used in this experiment may be obtainedfrom reference (D).3.2 Test Program

The experiment test plan, reference (E), consisted of three testphases:

e Equipment checkout at the manufacturersplant* Pre-flight tests at the ground station at Aire-

Sur-l'Adour, France, and

* Flight tests involving voice communications, datatransmission, CW and gated range tones.

3.2.1 Equipment CheckoutThe equipment checkout was made to insure that all equipments supplied

for the experiment were operating within specifications. The pre-flight testserved to verify that equipment installationand calibration had been properlyperformed, while the flight operations were used to collect the necessary datato meet experiment requirements.

The equipment checkout consisted of individual equipment tests plusan R F loop back-to-back test. Figures 3.5 and 3.6 define these tests whichconfirmed adherence to design specifications.

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FIGURE 3-6. PRE-FLIGHT EQUIPMENT T E S T S

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3.2.2 Pre-Flight TestsThe pre-flight tests were conducted on 9 September 1971 utilizing:

the ground station at Aire Sure l'Adour, France; an aircraft parked at Pau Airport;and the balloon transponder located on a near-by mountain top, known as Pic-Du-Midi. This is the highest accessiblepoint having line-of-sight visibilityfor both the airport and the ground station, the elevation angle being about5 degrees from the ground station. Data recorded during this testwere:

* Analog of carrier power level plus noise* Analog of noise powero Range tone signals* Phase angle of returned range tone signals Bit error count on 1200 bps data channel.

3 .2 .3 In-Flight ExperimentFigure 3- 7 illustrates the flight test configuration. During each seg-

ment of a flight test the aircraft was flown on a nearly circular course, at aconstant elevation angle relative to the balloon. Tests were planned for anglesof 5, 10, 15 and 20 degrees; however, because of logistic constraints, testswere run only at 5 and 10 degrees.

Tests initially planned consisted of a CW ranging and a multiple-access period with each period containing four 45-minute segments, one ateach elevation angle. Figures 3-8 and 3-9 show the operational time plotsassociated with the two test periods. Each flight segment of each period wasto utilize both the low gain and high gain antenna for approximately equal timeintervals.

Allowing time for aircraft manuevering between flight segments, it wasestimated that a complete test could be concluded in four hours. Accordingly,three flights of appropriate duration were scheduled to insure the collectionof sufficient data. Adverse weather conditions forced the cancellation of one

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these was a CW mode wherein the tone was continuously transmitted forseveral minutes at a time. The other mode was a pulse mode wherein the toneswere transmitted for 200 millisecond intervals.

The lower part of Figure 3-11 presents two types of data. One typeof data is the range between the aircraft and balloon as derived from the radardata during the experiment. The second type of data indicates regions duringwhich all or none of the desired data were acquired; in particular, those regionswherein the aircraft receiver could not achieve or maintain lock and thoseregions wherein radar data was not provided. The regions wherein no radardata was available are evident by the gaps in the aircraft to balloon range curve.The regions during which the receiver was not in lock are indicated by thecross-hatched intervals.

Significant periods of time during which the aircraft receiver was notin lock are evident in Figure 3-1.1. There are two different types of these gapswhich should be noted. One type of data gap corresponds to those time intervalsbetween the planned flight segments. These regions correspond to periods ofaircraft maneuvering wherein neither radar data nor aircraft receiver lock wasanticipated or planned.

The second type of data gap (loss of lock) occurs during the plannedtest period of nearly constant range. There are regions wherein external inter-ference to the system was evident.

However, the duration of these periods is being magnified by theequipmentwhich automatically enters a two-minute search mode should amomentary signal drop out occur. A s noted in Section 2.2, this problemhas been significantly alleviated by modifications to inhibit immediate entryinto a search mode as well as a speed up in the search process itself.

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E V A L U A T IO N O F E X P E R IME N T A L DATAThe experimental data pertaining to the radar supported flight of

13/14 September 1971 has been evaluated. The intent of this evaluation isto determine ranging accuracy, error rate experienced by the 1200 bps and600 bps data, and voice intelligibility. The lack of radar support for the flightof 17 September precludes the use of this data for ranging evaluation.

The presence of interference in the balloon-aircraft experimentaldata is indicated repeatedly in the ensuing evaluations. Unfortunately, thetype of data gathered during the experiment does not permit anything moreconcrete than speculation as to its source, magnitude, and time dependence.4.1 Carrier-to-Noise Ratio and High-Rate Data Error

One of the primary objectives of the experiment is to evaluate theperformance of the 1200 bps data channel. This was achieved by means of atime-correlated measurement of bit error rate, and by comparingmeasurederror distributions to theoretical performance for the corresponding levels ofcarrier-to-noise ratio measured, for the different flight segments.

The carrier-to-noise ratio measurements of the received signal wereobtained by measuring the rms voltage at the "in-phase" and "out-of-phase"detector sections of the transceiver's Costas type demodulatorfor the 1200bps data channel. When the Costas tracking loop is in lock for a D C P S Ksignal in the presence of Gaussian noise, the in-phase section containssignal-plus-noise, whereas the out-of-phase section contains only noise.The ratio of the rms voltage at the in-phase section to rms voltage from theout-of-phase section is then a direct indication of the received carrier-to-noiseratio. Because there is a fixed power relationship set at the ground stationbetween the voice, data and surveillance channels, the carrier-to-noise ratio,C/N, for each of these channels can readily be computed from the 1200 bpsdata channel. The readout of C/N from the transceiver'smodem unit wascalibrated in the laboratory using all three modulated carriers with added whitenoise.

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The instrumentation employed for measuring rms voltage in the Costasloop had a time constant on the order of one second. Such a long time con-stant, although smoothing out noise variations, precludes detection of rapidcarrier-to-noise changes. In this regard, it should be emphasized that thecalibration of the C/N was based only upon direct carrier and Gaussian typenoise being present. With interference present the values obtained for thesignal carrier-to-noise ratio would be in error, and evidence of such interfer-ence was noted.

Because the power radiated from the balloon could not be varied, theC/N level received by the transceiver was adjusted by a variable step attenua-tor at the transceiver's receiver input port. If the predominant source of noisewas receiver noise, then this method could be used to establish a given C/Nlevel such as 49 dB-Hz.

However, during all 6-degree medium-gainantenna flights, thereceived C/N was below 49 dB-Hz. This indicated that the C/N was notbeing established by receiver noise, but by external interference. Using thehigh-gain antenna, the desired 49 dB-Hz could be established. However,during some flight sequences, C/N could not be maintained at a given fixedlevel.

T o correlate measured carrier-to-noise with bit error probability,carrier-to-noise level as recorded on the aircraft was used. Data were takenat 1-minute intervals and havebeen plotted vs time, as shown in Figures 4-1through 4-6. Similarly, the recording of bit errors as they occurred in timeat the ground station was used to determine the bit error rate during the1-minute intervals, about the points of measured carrier-to-noisedensity;these data are also shown in Figures 4-1 through 4-6.

Figures 4-7 through 4-10 provide a direct comparison between theoreticaland experimental performance. Figures 4-7 through 4-10 show a comparisonofactual and theoretical bit error distributions based upon the statisticaldistribu-tion of received carrier-to-noisedensity in the high-rate data channel. Each

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of these figures consist of three graphs - (1) a histogram and cumulative distri-bution of carrier-to-noise samples, (2 ) the variation of bit error probabilitywith carrier-to-noise density for optimum P SK signaling and for degradations of4 dB and 1.5 dB, and (3) a comparison graph, wherein the cumulative distri-bution of bit error probabilities derived from the cumulative distribution of C/Nsamples is compared with experimentally observed distributions of bit errorprobabilities. Note in the data shown in Figure 4-8, the third comparison-graph does not appear because no bit errors were observed during this flightperiod. A numerical example will aid in the explanation of these figures.

In the lower right-hand portion of Figure 4-7, the histogram andresulting cumulative distribution of C/N samples is shown. In this case,approximately 50 % of these samples were lower than 42 dB-Hz and 90 % of thesamples lie between 39 and 45 dB-Hz.

The upper right-hand portion of Figure 4-7 presents the theoreticalrelationship between C/N and bit error probability for two conditions-(1) optimuPSK signaling and (2), optimum P SK signaling with a degradation of 1.5 dB and4Combining theoretical curves with the experimentaldistribution of C/N permitsdetermination of the theoretical distribution of bit error probability shown inthe upper left hand graph. For example, at a C/N of 42 dB-Hz, assumingoptimum PSK performance, a bit error probability of about 10-6 would be antici-pated. From the distribution of C/N samples, 50 % of the samples yieldedC/N higher than 42 dB-Hz. This would be anticipated because of the highreceived carrier-to-noise level. Therefore, the theoretical, distribution of bit-6error probability shows that the error probability should be less than 10 witha probabilityof.

The experimental distributions of bit error probability noted in theupper left hand graphs were obtained by taking 1-minute intervals during eachflight segment and determining bit error probabilities from the data. Thesesamples of bit error probabilities were then transformed to the cumulativedistributions. For example, in Figure 4-7, probability of bit error being less

-4 1than 10 is about 2.4-13


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Two aspects of the data shouldbe discussed. First, the error-ratedata acquired from the ground simulationtest (Figure 4-10) perhaps withoutthe interference encounteredduring flight, shows a variation of error rate thatis comparable to theoretical. Secondly, if the interference is always presentand related to the carrier(i.e., multipath interference), then the carrier-to-interference ratio must be relatively low as evidenced by the lack of bit errorsduring the high carrier-to-noise data acquired during flight segment 2 (Figure 4-8).4.2 Side-Tone Ranging

The measurement of range between the aircraft and satellite is accom-plished by measuring the transit time of a tone transmitted from the groundstation through the balloon transponder to the aircraft transceiver, and returningby the same route. This time is determined by referencing the returning phaseof either of the two tones to the phase of the transmitted tone, and then sub-tracting the fixed phase range shifts caused by the equipment through which thesignals pass.

Figure 4-11 is a diagram of the path of a range tone from ground stationto aircraft to ground station. The primary delays are:

T1 = ground station transmitterT 2 = ground station to balloon transponderT = balloon transponder (UHF to L-band)3T4 = balloon to aircraftT 5 = aircraft to transceiverT 6 = aircraft to balloonT balloon transponder (L-band to UHF)7T8 = balloon to ground stationT9 = ground station receiver.

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NASA balloon: Aircraft ranging, data and voice experiment (May 1, 1972)

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Transcript of NASA balloon: Aircraft ranging, data and voice experiment (May 1, 1972)