PROCEEDINGS OF TIHE IRE

APPE1NDIX I

If the ioni trap is in the ioniized gas flow, the currenltmeasured is

I = Neirr2 Vaa'f(Otrap)where e=electron charge, r=radius of the grid surfaceof the ion trap, a=a tranisparency coefficient, V= sput-niik's speed, Otrap = trap grid potential with regard to thesurrounldinig medium.

APPENNDIX 11The retardinig potenitial fret can be determinied from

the relationi

2

where (= electroni charge, m =mass of iolns which aretrapped, V-=sputnik's speed.

Automatic Sweep-Frequency IonosphereRecorder, Model C-4*

J. N. BROWNt, MEMBER, IRE

Summary-An improved sweep-frequency ionosphere recorderwas needed to carry out the expanded program of observationplanned for the IGY. The resulting equipment included improve-ments in output power, receiver sensitivity, purity of output signal,and other details, over earlier equipments. The basic principles ofoperation are described and the transceiver-type of circuitry ex-plained. The operating characteristics are: frequency range, 1-25mc; output power, 10-30 kw peak pulse power; pulse length, 50 Msec;repetition rate, variable from 10-70 pulses per second. An elaboratecontrol and programming system is provided which can be used tooperate the equipment on an unattended automatic basis. The "vir-tual height" of the ionospheric layers, as a function of frequency, is re-corded on photographic film by two cameras-a 35-mm uniformly-moving-film unit, and a frame-by-frame 16-mm camera, which pro-duces time-lapse motion pictures of the changes which occur. A briefhistory of the development of the sweep-frequency recorder tech-nique is given.

PURPOSE OF TIE EQuIPMENTT---HE Model C-4 ionosphere recorder is initenided

primarily to measure the "virtual lheight" of thelayers of the ioniosphere by meanis of pulse echoes

at vertical incidenice. As the operatinig frequency isswept, the records also yield the critical frequenicies ofthe layers. The data are produced oII photographicallyrecorded graphs of virtual height of the ioniosphericlayers as a function of frequency, the frequenicy ranigecovered being from 1-25 mc. The equipmenit is auto-matic in operationi, requiring attentioni onily for chanig-inig film and for routine maintenance.With modification, this equipmenit has also been used

to perform types of experiments other than vertical-inicidence measurements. These experiments include ob-lique-incidence backscatter measurements and oblique-incidence, two-station, synchronized-pulse experi-

* Original manuscript received by the IRE, November 14, 1958.t Appl. Sci. Corp. of Princeton, Princeton, N. J.

menits to examine the finie structure of pulse trainisreceived from a distanit station. In the litter case, theexperimenit requires the use of two Model C-4 recordersthat are svnchronized both in pulse repetitioni rate anidin the radio frequency beinig swept.

EARLIER E(utIP,mENT

T he pulse method of ioniosphere soundinig that is inconmmoni use in the medium and high-frequency spec-trum dates back to the original experimenits of Breit anidTuvel at the Carnegie Institutioni of Washinigtoni, D. C.These original experiments were performecd oni selectedfixed frequenicies anid gave data in the form of ioino-spheric layer-height chaniges againlst time. Rapidchanges of the layer height at a giveni frequency withchanlginig time, and the obvious existenice of more thanione layer, pointed out the desirability of varying thetranismittinig and receivinig freq uencies simultanieouslyanid of viewuing the spectrum oni a more revealing Iano-ramic basis.

Ihe early sweep-frequenicy equipmienits that probedthe ioniosphere were mechainically ganged unlits thattracked the various tranismitter tanik circuits as thefrequency of operationi was chaniged, anid the receiverfrequenicy had to be varied to corresponid with that ofthe tranismitter. Some very awkward anid ofteni ingeni-ious methods were devised to accomplish the task. Thefirst major improvemenit oni this scheme was the use ofthe "transceiver" priniciple, whereini the tranismitter andreceiver tuning frequenicies were derived from a commonoscillator by a heterodyne method.2 This basic scheme is

G. Breit and M. A. Tuve, "A test for the existence of the con-duicting layer," Phys. Rev., vol. 28, pp. 554-575; September, 1926.2 T. R. Gilliland, "Field equipment for ionospheric measure-

m-ients," J. Res. NBS, vol. 26, pp. 377-381; May, 1941.

296 Febru.ary

Brown: Automatic Sweep-Frequency Ionosphere Recorder, Model C-4

still in use in the current equipment. However, when thesystem was first used, the transmitter tank circuits werestill mechanically ganged and tracked to the drivingfrequency.The next obvious step was to eliminate the necessity

of tracking the various stages of the transmitter. Broad-band amplifiers to cover the range of 1-25 mc were de-vised by Sulzer' and useful peak pulse power of 5-10 kwwas obtained by this technique. It was the developmentof high-perveance pulse tubes which made this methodpractical. Various experimental models of the equip-ment were developed during the years following WorldWar II. The Model C-i recorder was built by theCentral Radio Propagation Laboratory of the UnitedStates National Bureau of Standards and became theelectrical prototype of the commercially producedModels C-2 and C-3.4 The last of the Model C-3 record-ers was manufactured in 1949. Since that time, theseequipments have continued to be used extensively bythe world-wide network of ionosphere sounding stationsoperated by the Bureau of Standards and the UnitedStates Army Signal Propagation Agency.

THE IGY PROGRAM

When plans were being formulated for the 1957-1958International Geophysical Year program, it was desiredthat the world-wide ionosphere sounding program beextended and the program of observation be increased.The sounding equipment then in use was almost tenyears old and was insufficient in quantity to equip themany additional stations planned. The IonosphericPhysics Panel of the United States IGY Committee,decided that a new, modernized version of the sweep-frequency sounder should be procured in sufficientnumbers to equip properly the additional stationsneeded and to modernize some of the stations alreadyin existence. The older equipment had certain undesira-ble features, including spurious emissions that needed tobe eliminated. The Ionospheric Physics Panel drew upa set of specifications that were to be the requirementsfor the new equipment, and asked the National Bureauof Standards to undertake its procurement. Invitationsto bid were sent out, and Barker and Williamson, Inc.,of Bristol, Pa., was the successful bidder. Time was veryshort, but a successful schedule was met that yielded apreproduction model in one year and 21 additionalunits in the eight months that followed. The originalorder of 14 recorders for the United States IGY pro-gram was augmented by the requirements of variousUnited States government and military research agen-cies as well as by foreign governments, including theUSSR, which have taken part in the IGY. A secondproduction run of 11 recorders brought the total to 33Model C-4 recorders that were built by Barker andWilliamson

I P. G. Sulzer, "lonosphere measuring equipment," Electronics,vol. 22, pp. 137-141; Julv, 1946.

'J. M. Carroll, "Automatic ionosphere recorder," Electronics,vol. 28, pp. 128 131; May, 1952.

Fig. I-Model C-4 ionosphere recorder.

2

IIII

Fig. 2-Block diagranm of Model C-4 transmiiitterand receiver circuits.

DESCRIPTtON OF THE NEW EQUIP-MENT

The TransmitterFig. 1 is a general view of the main rack of equipment

that comprises the Model C-4 recorder. Not shown inthe photograph is the large pendulum-actuated IBMmaster clock which controls the elaborate automaticprogramming system that is a part of the over-allequipment.

Fig. 2 is a block diagram of the complete transmitter-receiver system, and shows how it is associated with thefilm recording methods used. A brief description of thesystem follows. The basic frequency control circuit isthe variable-frequency oscillator (VFO) which operates

1959 297

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I X.

PROCEEDINGS OF THE IRE

between the frequenicies of 31 and 55 mc. The VFO isfollowed by a buffer-amplifier which is tracked over thesame frequency range. Considering first the transmittersignal generation, the variable 31-55-mc signal is hetero-dyned with a fixed-frequency pulsed signal at 30 mc.This 30-mc signal is derived from a 10--mc crystal oscil-lator and pulsed tripler and amplifier stages. Leakageof the 10-mc crystal signal is held to a very low level toprevent spurious receiver and transmitter signals. Theheterodyne difference between the VFO signal and the30-mc signal is extracted from the mixer (balancedmodulator) stage thus producinig a variable, pulsed1-25-megacycle signal. This signal is then amplified ina chain of broad-band amplifiers. The higher level stagesare all simultaneously gated on for the duration of thetransmitted pulse and then turnied off for the intervalbetween pulses. This allows a very high duty factor tobe used without exceeding the tube dissipations.The actual transmitter circuitry following the crystal-

diode balanced modulator and low-pass filter consists ofa broad-band unkeyed amplifier and a second pulsedstage, both using 6AG7 tubes. This drives a parallel-connected type 3E29/829B stage which is also pulse-keyed. The following intermediate power amplifier(IPA) drives a push-pull connected power amplifier(PA). The IPA tube and both PA tubes are Eimac 41PR60-A high-current, pulse amplifiers. The PA stagemerits comment in that it is driven from an unbalancedsource with resistance-capacitance coupling and pro-duces balanced, push-pull output. Balanced output isattained by cathode coupling betweeni the two tubes ofthe PA. The grid of one PA tube is driven from theplate of the preceding (IPA) stage, and the grid of theremaining PA tube is grounded for radio frequLencies bymeans of a bvpass capacitor. A choke, bifilar-wound tosupply heater voltage, is the common cathode im-pedance for the two tubes. The voltage at the plate ofthe grid-driven PA tube is reversed in phase with respectto the driving signal, while the voltage at the grounded-grid, or cathode-driven tube plate, remains in phase. Theresulting push-pull output is coupled to the transmittingantenna.

In operation, this circuit configuration has an excel-lent, natural, self-balaincing action that is very tolerantof misadjustmient of the keying pulses applied to theindividual grids. The transmitter circuit is very stableand free of parasitic oscillations. The transmitter outputpower varies over the band, from a value of approxi-mately 30 kw peak pulse power at the low frequency endto a value of 10 kw at the higher frequencies. The highoutput power at the low frequency end of the operatingspectrum tenids to compensate for the decreased radia-tion efficiency of the broad-band terminated delta an-tenna.

The Receiver

Again referring to Fig. 2, the receiver portion of therecorder gets its first-conversion-oscillator injection

signal from the same VFO that supplies the transmittersignial. The first receiver IF stage operates at 30 mc,which is always the difference between the 1-25-mcoperating frequency and the varying 31-55-mc VFOfrequency. The input circuit of the balanced mixer stage(a type 6J6 tube) is untuned and contains a carefullymade broad-band transformer which excludes the inl-phase component of nearby strong signials and passesonly out-of-phase or push-pull signals. The 30 mc IFstage has a bandwidth of approximately 150 kc. Funida-mental 30-mc signals that appear on the antenna areatteniuated by a 25-mc low-pass filter in the aniteinna cir-cuit plus a group of 30-mc trap circuits.The final receiver selectivity is accomplished after a

second coniversion- from 30 mc to 1.4 mc. The 28.6-mccrystal oscillator feeds the injectioni voltage into thesecond mixer stage (a type 6U8 for both oscillator andmixer). The 1.4-mc IF strip consists of two stages (a6CL6 and a 6146) with three double-tunied IF trans-formers which were made of high-Q toroids with capaci-tive couplinig. The IF bandwidth was adjusted for 25 kcto accommodate the 50-microseconid transmitter pulselength. Note that a small tranismittinig tube is used asthe last IF amplifier so that large dynamic-ranige signalscan be handled without serious overload. hliis preven-tsstrong interfering signials from overloadinig the last IFstage and cutting off the pulse-echo signials. The videodetector is conventionial and is arraniged so that it canlfurnish an automatic gain control voltage also to helpprevent signal overload in the IF stages. Following de-tection, the video signials are severely limited andcoupled to the oscilloscope inidicating circuits through acathode follower stage. The receiver has proven to bevery satisfactory in design and has a pulse-signal sensi-tivity of better than one microvolt for an easily recog-nizable pulse in the background noise.

Pulse and Time-Base Generator

The specifications for the Model C-4 equipmenit im-posed a very strict tolerance on the accuracy of the cali-brating height markers placed on the film records. Inthe earlier C-2 and C-3 equipments, a gated, free-running oscillator was used as a source of height marks.A separate piece of crystal-controlled calibrating equip-ment was necessary to check the accuracy of the markeroscillator. It appeared to be an obvious solution to usea crystal oscillator as the marker generator source to in-sure the permanent accuracy of the system. A 3000-cpsquartz crystal is used as a marker generator to furnish50-km height markers and a binary couniter furnishes1500-cps signals for the 100-km marks. Sin-ce 3000-cpscrystals cannot be gated at the start of each oscilloscopesweep, it is necessary to synchronize the basic trans-mitter pulse rate with some submultiple of the standardfrequency. Thus the transmitter pulse rate and film-display height markers are derived from a very stable(0.01 per cent) temperature-controlled source. Thenecessary oscilloscope sweep and gate waveforms are de-

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Brown: Automatic Sweep-Frequency Ionosphere Recorder, Model C-4

rived in a conventional manner. The transmitter de-layed-trigger pulse is derived by use of a very stablephantastron circuit. This allows the transmitter signalto be delayed until after the start of the oscilloscopesweep and to be carefully aligned with a reference "zero"height marker. The transmitter pulse rate can be set atany frequency between 10 and 70 cps that is a sub-harmonic of the 1500-cps crystal-derived signal.

INDICATING AND RECORDING SYSTEM

Normally, the C-4 recorder operates with the 35-mmuniformly-moving-film camera photographing one of thetwo oscilloscope indicators. The other oscilloscope isused for monitoring the received signals as well as tocheck the various circuit functions. Fig. 3 shows atypical height vs frequency plot recorded on the 35-mmcamera. The oscilloscope display for this method of re-cording is an intensity modulated horizontal line. Thevideo pulse signals modulate the intensity downward sothat a pulse echo signal will appear as a dark spot onthe oscilloscope trace. The vertical motion of the filmprovides the frequency base on the record of Fig. 3. Theheight markers (100-km intervals) are also applied asintensity modulated information. The frequency mark-ers appear as blanked oscilloscope sweeps and are ap-plied automatically by generating blanking signals ob-tained by mixing the VFO signal with harmonics of a1-mc crystal oscillator.

Fig. 3-Typical daytime height vs frequency recording madeon Model C-4 ionosphere recorder.

The oscilloscope tube is an electrostatically deflectedtube, a Dumont 5ATP-7 selected for its high light-output and high definition of trace. The C-2 and C-3equipments used electromagnetically deflected indica-tors and had the usual nonlinear sweep problems.The date-time information is recorded on the film by

momentarily flashing a set of small lights which illumi-nates a modified IBM date-time printer unit.

Time-lapse 16-mm movies are also possible with theC-4. With this technique, a single frame of 16-mm filmis exposed every 15 seconds. The 16-mm exposure ismade by displaying the intensity-modulated A-scanpattern vertically on the face of the oscilloscope tubeand moving the line horizontally as the frequency is

being swept throughout the 1-25-mc range. This is ac-complished by deriving a sweep voltage from a linearpotentiometer driven by the same shaft that sweeps theVFO frequency. At the end of the sweep, the oscilloscopetrace is blanked and the 16-mm camera is advanced oneframe in readiness for the next sweep exposure. Notethat no shutter is used on the 16-mm camera since thecamera is advanced during the oscilloscope blankingperiod. If a long series of these 16-mm pictures is taken,and then projected at the normal motion picture rateof 16 frames per second, a time-lapse motion picture isproduced showing in the span of a few minutes the di-urnal variations of the various ionosl)heric layers.

CONTROL AND PROGRAMMING SYSTEM

An elaborate control system is provided for uinat-tended, automatic operation of the equipment. Thespecifications required that the equipment have pro-visions for quickly changing the methods of operationaccording to preset programs. As a result, an IBMmaster program clock mechanism is used to actuate thevarious receiver and transmitter circuits according tothe desired operating program. The system permits theautomatic selection of any one of three separate re-ceiver-gain settings at any or every one-minute intervalthroughout a 24-hour day. Since the ionospheric absorp-tioin varies greatly from night to day, the variable-gainfeature is a necessity for properly recording the echopatterns in the presence of noise and extraneous signals.

Program pins are placed in the master wheel of theIBM clock to control the operation. There are actuallythree pins for each minute throughout the 24-hour day.Each of the three pins represents one of the three possi-ble receiver gain settings that can be adjusted from thefront panel. Modifications to any elaborate preset pro-gram of operation may be made by switch selection fromthe front panel.The IBM master clock has a manufacturer's guar-

anteed stability of 2 second per day. The spring mecha-nism, which drives the invar pendulum, is wound elec-trically to insure that a constant driving force is avail-able. If the ac source of power should fail, the clock hasa reserve of approximately 12 hours of operation in thefully wound condition. However, when the ac power fails,a changeover relay transfers the complete control sys-tem to a set of 24-volt batteries, which activate theclock and master program wheel until normal power isrestored.A selection of sweep times is available. The 1-25-mc

spectrum can be swept in 15, 30, or 120 seconds. Themotor-drive-gear ratios of both the VFO and 35-mmcamera must be changed to give the desired sweep rate.The spectrum can be swept in either a linear or logarith-mic manner. Selection of the properly shaped drive camfor the VFO determines the manner in which the spec-trum will be covered. Special cam shapes may also beused to examine certain parts of the frequency spectrumin detail.

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MECHANICAL CONSIDERATIONSThe Model C-4 equipment was designed with ease of

serviceability in mind. Each chassis unit can be re-moved from the main frame by unfastening the retain-ing screws holding its front panel to the frame, and slid-ing the unit out. Most of the unlits (where weight per-mits) are equipped with roller-type slides for ease ofremoval or partial withdrawal for examinationi anid tubereplacement. Each chassis unlit is furnished with an ex-tension cable which permits total removal and testoperation on a work bench. The transmitter panel is theonly part of the equipment which cannot be removedfrom the frame for service, and this is unnecessary sinceall the transmitter components are readily availablefrom the rear of the main frame.

OPERATION OF THE EQUIPMENT

Since operation of the equipment was, for the mostpart, to take place in isolated parts of the world, pro-visions were taken to furnish at least a 100 per centcomplement of spare electrical parts. Complete filmprocessing equipment was also shipped as part of theequipment.

There are several operational advantages of the newequipment over earlier equipments. One of the mostnoticeable is the reduction of interference to niearbycommunication facilities. Since it uses pulse transmis-sion, there is inevitably a certain amount of interference,particularly at the time when the sweeping frequencypasses through that in use by the other service. How-ever, the signal purity is considerably improved by theabsence of parasitic oscillation and spuriotus leakage

signals. The increased power and receiver sensitivitygreatly increase the number of ionospheric phenomenathat are observed. Physically, the new equipment issmaller than the C-2 or C-3 because the earlier unlits hada large separate cabinet of ac power-regulatinig equip-ment, which is now an integral part of the C-4 mainframe unit. The regulator used is a servo-conitrolled,auto-transformer type maniufactured by the GenieralRadio Company.

Fig. 3 is a reproduction of a ty pical daytime 35-mmfilm record made on the C-4 equipment. In this record,frequency scale is logarithmic. From this form of datapresentation, it is possible to extract not only verticalincidence informationi but oblique incidenice iniforma-tion by the use of transparent drawings of transmissioncurves used as overlays.

ACKNOWLEDGMENT

While most of the electrical anid mechanical design ofthe Model C-4 equipment was that of the Barker andWilliamson engineers, it should be acknowledged thata major portion of the transmitter and receiver circuitrywas contributed by J. M. Watts, of the Central RadioPropagation Laboratory. The development of thisequipment to such a strict delivery schedule was possi-ble only through the closest cooperation of many peopleat both CRPL and B&W.The author would like to express his appreciation to

Barker and Williamson, Inc., for permission to publishthis article and for their cooperation in furnishing photo-graphic material.

The IGY Three-Frequency Backscatter Sounder*A. M. PETERSON t, MEMBER, IRE, R. D. EGAN t, MEMBER, IRE, AND D. S. PRATTt

Summary-Design criteria and operating characteristics of theIGY Fixed-Frequency Backscatter Sounder are outlined. This three-frequency (12, 18, and 30 mc) rotating-antenna pulse sounder wasdesigned at Stanford University for oblique-incidence ionospherestudies. During the IGY a network of these sounders has beenoperated at thirteen stations in polar, temperate, and equatorial re-gions. Data reduction and publication of data summaries have beencarried out at Stanford using semiautomatic punch card methods.Preliminary analysis of data has yielded new information on sporadic-E, magnetic field aligned irregularities, large-scale traveling dis-turbances in the F region, and the effects of ionospheric tilts onlong-distance propagation.

* Original manuscript received by the IRE, December 19, 1958.t Radio Propagation Lab., Stanford University, Stanford, Calif.

I NTRODUCTION

ACKSCATTER sounding of the ioniosphere is anloblique-incidence, pulse radar method for thestudy of the regular layers, sporadic-E, and a

variety of irregularities which occur. The method per-mits the surveillance of a large region of the ionospherefrom a single location. A large percentage of the echoesobtained by a backscatter sounder are "ground back-scatter echoes" which result from normal ionosphericpropagation to remote points oni the earth's surface an-dscattering by the groun-d of energy back over the ioIno-spheric path to the transmitter. Other echoes observedin backscatter sounding arise by scatterinig from irregu-

Fe-bruary300