IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-22, NO. 4, DECEMBER 1973

The sensitivity of the circuit to harnonic distortions hasbeen tested by admixing 1 percent third harmonic to the inputsignal and observing the output deviations for the worst casephase relationship. The result, 4@106 peak-to-peak, wasexpected to be in that order of magnitude, since the accuracyof the resistors in the synchronous rectifier is only 1 10-3 .Finally, the line-frequency interference has been tested by

operating the circuit at a stable frequency of 50 Hz. Theoutput fluctuations in this case, due to the line-frequencydeviations from 50 Hz, were less than 2- 10-6 peak-to-peak.

CONCLUSIONSIt has been shown that a purely electronic ac-to-dc transfer

can be performed which is sufficiently stable for precision ap-

plications. Since the transfer is largely insensitive to low-orderharmonic distortions of the sine wave, it has been applied in acontrol circuit for stabilizing the rms output voltage of aRC oscillator. The highly stable relationship between therms voltage and the dc reference established by the controlcircuit makes it possible to calibrate this sine-wave source. Anadditional advantage is that the stability of the output sinewave is largely independent of the quality of the oscillator andof a power amplifier included in the control loop.

AcKNOWLEDGMENTThe author wishes to thank G. Herrgoss for his technical

assistance during this work and Dr. H. D. Hahlbohm forreviewing the manuscript.

Optoelectronic Electricity Meter forHigh-Voltage Lines

ANDREAS BRAUN, MEMBER, IEEE, AND JORN ZINKERNAGEL

Abstract-A new method has been developed for measuring the poweron high-voltage lines. It involves determining the power directly on thehigh-voltage line and transmitting the measured values to the groundpotential side by the aid of infrared light pulies. Two electromechani-cal counters at ground side, one for each direction of energy flow, re-cord the transported electrical energy. The measuring error of the de-vice is of the order of 0.2 percent. The small number of activeelectronic components should result in satisfactory reliability.

I. INTRODUCTIONPOWER metering on high-voltage lines is normally done

with a voltage transformer and a current transformer,both insulated for the full line voltage. Their secondary wind-ings are connected to an electricity meter. At very high linevoltages both transformers are bulky and very expensive. Onthe other hand, this method has proved its reliability for a longtime.In order to replace this conventional power metering device

by an electronic system, the following aspects should be con-sidered.

Manuscript received May 1, 1973; revised July 9, 1973. This paperwas presented at the 1973 Electrical and Electronic Measurement andTest Instrument Conference (EEMTIC), Ottawa, Ont., Canada, May15-17.The authors are with the Physikalisch-Technische Bundesanstalt,

Braunschweig, Germany.

1) The accuracy should be at least as good as with systemsnow in use. Furthermore, the electronic system should be ex-pected to produce much better accuracies in later stages ofdevelopment.2) The insulation problems should be significantly reduced

so as to yield substantial cost reductions at very high line volt-ages.

3) The continuous operating time without maintenanceshould be of the order of some years.4) In case of failure, the electronic components should be

easily replaceable and only standard and readily available partsshould be used.

5) The design should be applicable to commercially manu-factured apparatus. Complex calibration techniques should beavoided.Electronic electricity meters using optical telemetry for high

voltage line measurements have not yet been described in theliterature. But there are some papers concerning electroniccurrent transformers [1] -[3], which contribute enough infor-mation to enable the development of an optoelectronic elec-tricity meter. Other papers providing background informationon optical telemetry are listed also [4] -[7] .The optoelectronic electricity meter to be descnbed has the

following features:

1) rated voltage 1 0/v'3kV;

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BRAUN AND ZINKERNAGEL: OPTOELECTRONIC METER FOR HV LINES

2) rated current 200 A;3) power factor > 0.16;4) error 0.2 percent at power factor > 0.6;5) line derived power supply for the high-voltage side elec-

tronic components.Without major alterations the system will operate with line

voltages up to 750 kV and line currents up to 1200 A. In itspresent state it is a laboratory model. It has not been designedfor outdoor use mainly because of its mechanical construction.But these limitations are not serious, and it is not difficult tomake a new mechanical design for field testing.The measuring method involves determining the energy

transferred via the high-voltage line directly at the high-voltageside. A power-to-frequency converter delivers pulses, the fre-quency of which is proportional to the power to be measured.A light emitting diode converts the electrical pulses into near-infrared pulses which are transferred to the ground potentialside by means of a flexible light guide. A photodiode amplifierreconverts the infrared pulses into electrical pulses, which arethen counted by electromechanical counters.The only path of information from the high-voltage side to

the low-voltage side is provided by the electrically neutral lightguide. Consequently, difficulties related to insulation prob-lems are very much reduced.No electronic measuring system for power line application,

either commercially available or in the state of laboratory in-vestigation, has been able to prove its reliability over a signifi-cant period of time. But the electronic circuitry, incorporatedin the examples given in literature and in this electronic powermeter, has shown its importance and reliability when used forother kinds of electronic measurement apparatus. Therefore,we feel confident that in the future electronic systems will beused with advantage for power line measurements as well.

II. BLOCK DIAGRAM

Fig. 1 shows a simplified block diagram of the system. With-out loading the capacitive voltage divider, a voltage-followeramplifier delivers a small voltage, of the order of a few volts,which is linearly related to the voltage in the high-voltage line.At the output of the current transformer exists a voltagewhich is proportional to the line current within 0.02 percentand 0.03'.The power-to-frequency converter delivers a square-wave

signal, the frequency of which is proportional to the linepower within 0.2 percent. A logic circuit automatically sensesthe direction of energy flow and supplies a "forward" or"backward" signal tol the encoder. The latter circuit convertsthe output pulses of the power-to-frequency converter intopulses having the same frequency, but of pulse durations of100 or 700 Ms, respectively, according to the direction ofenergy flow.The light emitting diode (LED) converts the electrical pulses

of 100 ,us, or 700 ,us into near-infrared pulses of the samelength. A flexible light guide transfers the infrared pulses tothe ground potential side.A photodiode amplifier delivers 100- or 700-Ms electrical

pulses to the decoder. This circuit classifies the received pulsesaccording to their length and channels them to one of the two

LOW-VOLTAGE SIDELIGHT GUIDE .

)= t~~~~~ECODER COtUNTER

Fig. 1. Block diagram.

electromechanical counters. The same counting circuit is in-stalled at the high-voltage side for checking purposes.The dc operating voltages for the electronic components at

the high-voltage side are supplied by a transformer and stabi-lized dc power supplies. The primary current of this trans-former is the capacitive current drawn by a high-voltagecapacitor.A metallic cover serves primarily as a screen in order to avoid

an influence of capacitive stray currents on the capacitivevoltage divider. In addition, it protects the entire high-voltageside apparatus against environmental factors.

III. HIGH-VOLTAGE SIDE COMPONENTSA. Voltage and Current TransformerA pure capacitive voltage divider is used as a voltage trans-

former. Since the low voltage of the voltage divider is neededat the high-voltage side, the capacitive voltage divider is in-verted in comparison with a regular capacitive voltage divider.The capacitor with the higher capacitance is located at thehigh-voltage side, and the capacitor with the lower capacitancehas one electrode connected to ground. Fig. 2 shows a sec-tional drawing of a compressed gas capacitor CG together withthe upper voltage divider mica capacitor CM.Because of the rather small capacitance of CG, an arrange-

ment like this cannot tolerate the parallel capacitance Cp, sinceCp depends upon environmental conditions which are not pre-dictable. In order to eliminate Cp, a metallic cover H is em-ployed, which functions as a screen, the stray capacitance CEof which will not disturb the voltage divider ratio CMICG. Thecapacitance CN between the cover and the upper electrode ofCG certainly adds to CM, but CN is very small compared withCM and, what is more important, is of constant value and canbe calibrated together with CM.The spark guard S is tied to the laminated paper body P of

the compressed gas capacitor by means of paraffin wax Pa inorder not to allow surface leakage currents, represented by Rpand Rp, to disturb the voltage divider ratio.This ratio has been chosen to be I 10/\/3 kV: 5/x3 V. With

CG = 103.9 pF, the upper capacitor takes a value of CM =2.286 ,uF. Even with line voltages of 200-percent rated volt-age, a value which might occur with 120-percent rated voltageand a simultaneous short circuit at one of the other two wires,

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, DECEMBER 1973

lu(t)

/~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. .....

1,33 5 6.67"1, t[msl

i(t)/,p 60° " \

Fig. 3. Forward-backward recognition: phase relationship. Solid line:line voltage. Dashed line: line current (phase shift: +600). Dottedline: line current (phase shift: -60°).

Fig. 2. Capacitive voltage divider (refer to text for explanation).

the divider output voltage will not exceed the linear range ofthe following amplifier. This impedance converter amplifier isnot only adjustable with respect to gain but also with respectto phase shift. Therefore the output voltage of this amplifieris linearly related to the line voltage.The current transformer has an additional winding for error

compensation and delivers an output of 10 V for its rated cur-

rent at its burden. Its amplitude error has been measured to be0.015 percent and its phase displacement amounts to 0.03'.The insulation of its secondary winding against the high-volt-age line is that of a low-voltage transformer.

B. Power-to-Frequency Converter

The power-to-frequency converter used in this measuring de-vice has been described in detail [8] . Since it is not used withthe original input voltage of 220 V, but with 5/NJ37V, the in-put circuit has been altered to work with input voltages of thismagnitude. In addition, the original system operated only forone direction of energy flow. Consequently, a second altera-tion was required in the logic circuit of the converter.The converter makes use of the following principle. The in-

stantaneous voltages at the voltage part of the converter are

converted by means of a voltage-to-time converter into square-wave pulses, the time duration of which is proportional to theinstantaneous voltage. These pulses control field-effect-tran-sistor switches which allow the voltage measured at the burdenof the current transformer to drive a current into an integrat-ing circuit only for the duration of each pulse.The output voltage of the integrating circuit is compared

with two voltage limits, an upper and a lower limit. When theoutput voltage reaches these limits, a bistable multivibratorchanges its state. The resulting square wave at the output ofthe bistable multivibrator serves primarily as the power-pro-

portional output pulse of the converter. In addition it is fedback into earlier logic stages of the converter and controls thefield-effect-transistor switches. During one state of the bi-stable multivibrator the integrating circuit accepts only nega-

tive current amplitudes. Positive current amplitudes are evalu-ated during the other state of the multivibrator. This

Fig. 4. Forward-backward recognition: block diagram.

procedure results in a significant reduction of the errors arisingfrom offset voltages in the operational amplifiers used. Detailsconcerning this method are given in the literature [8].Additional logic circuits accept the "forward" or "back-

ward" signal (see next section) and cause the power-to-fre-quency converter to work with either direction of energy flow.The output frequency of the converter with rated power(rated voltage, rated current, and a power factor of 1.0) is ofthe order of 10 Hz.

C. "Forward"- "Backward" RecognitionThe "forward" or "backward" signals needed by the power-

to-frequency converter are produced as follows.If the current in the high-voltage line leads or lags the voltage

by not more than 600, the current will have positive instanta-neous amplitudes during a time interval of 3.33-6.67 ms afterthe positive zero crossing of the voltage (Fig. 3). This indicatesdelivery of energy ("forward" direction of energy flow). Ifthe current amplitude appears to be negative during the timeinterval mentioned, the phase shift between voltage and cur-rent will be somewhere between 1200 and 2400 indicatingbackward flow of energy.The circuit which accomplishes this is shown in Fig. 4. Be-

tween 4.5 ms and 5.5 ms following each positive zero crossingof the line voltage (corresponding to - 810 < ep < + 810), apositive current amplitude will set a bistable multivibrator tothe "forward" state, while a negative current amplitude willresult in "backward" state of the multivibrator. Each positivezero crossing of the line voltage initiates a monostable multi-vibrator with an output pulse duration of 4.5 ms. The trailingedge of this pulse initiates a second monostable multivibratorof 1 ms output pulse duration during which time the currentamplitude is checked for its polarity.

D. "Forward"- "Backward " EncodingThis circuit delivers output pulses, the frequency of which

equals the output pulses of the power-to-frequency converter.The duration time of these pulses is either 100ls in case of

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BRAUN AND ZINKERNAGEL: OPTOELECTRONIC METER FOR HV LINES

HVL

TS|||rl|| =5V5V20V= V

TS T

1 17 8 zxlo 8xzxlo

Fig. 5. High voltage side power supply. CS: supply capacitor. HVL:high voltage line. TS: supply transformer. TI: intermediatetransformer.

energy delivery and 700 ,us if the energy is being obtained.These short pulses are then supplied to the LED amplifier.

E. LED and Flexible Light GuideThe LED converts the electrical input pulses into near-in-

frared (X = 910 nm) pulses. Although this radiation is not"light", because it is not visible to the human eye, the wordLED is still in use even for infrared radiation emitting semi-conductor devices and is employed here for this reason.In order to obtain higher radiant intensities and therefore

increase, the signal-to-noise ratio, use is made of the short dutycycle of the transmitted pulses. During these short pulses, acurrent of 1.2 A flows through the LED compared with themaximum allowable steady current of 70 mA for this particu-lar device.A good connection between the flexible light guide (active

diameter approximately 1 mm) and the LED at one end andthe photodiode at the other end is easily achieved, since thesephoto devices do not use small lenses to cover the radiatingand sensitive areas, but a short piece of light guide.

F. Power SupplySince this optoelectronic transformer makes use of inte-

grated circuits and transistors at the high-voltage side, the nec-essary dc voltages must be available there.The circuit is shown in Fig. 5. A high-voltage capacitor CS

(360 pF) draws a current from the high-voltage line HVL.This current flows through the primary winding of the supplytransformer TS (l0/\/3 kV: 100/V3 V). The secondary volt-age of TS is fed to a second transformer TI having four second-ary windings. Three of them supply stabilized dc power sup-plies to generate +15 V, -15 V, and +5 V. The fourthwinding is loaded with a chain of 16 Zener diodes. At about60 percent of the rated line voltage, the avalanche voltage ofthe Zener diodes will be reached. Increasing the line voltageabove that value will merely increase the current through theZener diodes, thereby limiting the voltages at the remainingthree windings. The latter voltages are of sufficient magnitudeto maintain operation of the electronic circuits between 40and 200 percent of rated line voltage.

IV. Low-VOLTAGE SIDE COMPONENTSA. Receiving AmplifierThe infrared pulses received at the ground potential side are

converted into electrical pulses by means of a photodiode.

Fig. 6. Forward-backward evaluation. RA: receiving amplifier.

This photodiode operates in the reverse biased mode and drivesa photocurrent of approximately 10 MA through a 100-kQ re-sistor. The resulting voltage of 1 V drives a comparator circuitat the output of which a pulse of +5 V appears when an in-frared pulse is being received, and zero in the absence of anyinput signal.The signal-to-noise ratio of the information path is about

70 dB using a light guide of 5-m length. The device worksequally well even with a light guide of 10-m length. Thereforethis optoelectronic electricity meter is suitable for use withrated line voltages in excess of 1000 kV.

B. "Forward"- "Backward" EvaluationThis circuit (Fig. 6) automatically senses whether an incom-

ing pulse has a duration of 100 Ms (according to "forward"direction of energy flow) or 700 Ms ("backward" direction).Each pulse from the receiving amplifier RA starts a mono-

stable multivibrator, the output pulse duration of which hasbeen chosen to be 400 ps. If the trailing edge of the receivedpulse appears ahead of the trailing edge of the multivibrator,the remaining part of the output pulse of the latter will serveas a counting pulse for the "forward" energy direction. Sinceonly the 100 ps pulse is shorter than the multivibrator outputpulse, a 300 ps "forward" pulse results in this case.

If, on the other hand, the multivibrator resets at a time whenthe input pulse is still present, the remaining part of this pulseserves as a counting pulse for the "backward" direction. Be-cause received pulses longer than 400 ps exist only in case of700-ps impulses, counting pulses of 300-ps duration are chan-neled into the "backward" direction.

C. Electromechanical CountersThe "forward" or "backward" counting pulses are amplified

in order to drive associated step motors, which in turn drivethe last wheel of 6-digit mechanical counters.The two counters show the amount of energy transferred

through the high-voltage line. One counter corresponds to theamount of delivered energy while the other one records theamount of electrical energy obtained.

V. EXPERIMENTAL SET-UPThe low-voltage side of the capacitive voltage divider consists

of a compressed gas capacitor CG which also serves to supportthe high-voltage side components (Fig. 7). These componentsare arranged in three decks which are separated by laminatedpaper disks. The bottom side of the lower disk has beenplated with Permalloy PY.The lowest deck contains the upper voltage divider mica

capacitor CM. The second deck contains the supply trans-

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, DECEMBER 1973

E EASUREO POWER-REFEtENCE POWER °00REFERENCE POWER

I 'L"""[]Fig. 7. Experimental set-up. CG: compressed gas capacitor. CM: mica

capacitor. H: metallic cover. HVL: high-voltage line. L: flexiblelight guide. PY: Permalloy plate. REC: electronic components low-voltage side. SEC: electronic components high-voltage side. TC: cur-rent transformer. TI: intermediate transformer. TS: supply trans-former.

220V"4

Fig. 8. Measuring set-up.

former TS, the second transformer TI, the current trans-former TC, and the Zener diodes ZD, mounted oil adequateheat sinks. The high-voltage rod HVL also runs through thesecond deck.The electronic components SEC including the stabilized dc

power supplies, are contained within a metallic box (21 X 15X 16 cm) at the upper deck. The flexible light guide L leavesthis box and passes through an opening of the metallic coverHto be guided into a second metallic box REC at the groundpotential side. This box (21 X 15 X 16 cm) contains all com-ponents of the low voltage side.

VI. EXPERIMENTAL RESULTSThe experimental results were obtained from the circuit

shown in Fig. 8. The power dependent frequency of the re-

ceived pulses was measured with a 6-digit electronic counterand compared to the reading of the class 0.1 reference watt-meter (calibrated to 0.03 percent at FS).Fig. 9 shows the dependence of the error e versus power for

0.5,0,5

0,3

0,2

0,1

0,1

0,2

0,3

0,4

0,5

- F (%)

20 40 60 d0 100 __720 K-?40o 160 1t0 200

POWERFACTOR 1.0, RATED VOLTAGE

POWERFACTOR 0.6, RATED VOLTAGE

Fig. 9. Error versus power (percent of rated power).

*eu'M.)

0.5

0.40.3

0.2

0.1C

0.1

0.2

0.3

0.40.5

_Erl/.J

POWERFACTOR 0.6, I0% RATED VOLTA6E

POWERFACTOR 1.0 IO% RATED VOLTAGE

20 4io ---iV5 1tO 00 120 160 1 0 TM) 210

---

POWERFACTOR 0.6, 200% RATED VOLTAGE

POWERFACTOR 1.0 200% RATED VOLTAGE

P(% J

Fig. 10. Error versus power (percent of rated power).

the two power factors 0.6 and 1.0, and at rated line voltage of10f\f4 kV. The maximum value of e is well below 0.2 per-

cent. Fig. 10 shows e versus power for the two power factors0.6 and 1.0, at two different line voltages of 80 percent and200 percent rated voltage, respectively. The maximum valueof error is 0.3 percent at 0.6 power factor, 200 percent ratedvoltage, and 22 percent rated power. Since these operatingconditions are rather unlikely to appear simultaneously, thiserror may be tolerated.Because the apparatus was built for laboratory investigations,

the electronic components were not selected to be insensitiveto severe temperature changes. But, nevertheless, there aretwo questions to be answered. 1) Will the system operatewithin the expected ambient temperature range of -250C to+70°C? 2) What is the measuring error expected to be withinthis temperature range?

All components used are replaceable with easily available-550C to +1250C versions. The power supply for the high-voltage side components is already designed to operate at am-bient temperatures up to +700C. For extended temperaturerange, the LED will be operated at about one tenth of the levelmentioned earlier. At 700C the radiant intensity will still besuch that the signal-to-noise ratio exceeds at least 40 dB.The variations of the measuring accuracy with temperature

depend on two different parts of the system: 1) the behaviorof the voltage divider and the current transformer, and 2) thestability of the multiplier.The instrument transformers are conservatively designed,

and their temperature dependence can be safely assumed to be

.-- I 1-. -.- -: 7==-'

pR *- .,- . - - .. .̂^#^-.

398

p('/-)

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-22, NO. 4, DECEMBER 1973

well below the system error of 0.2 percent. The stability ofthe multiplier depends on a single reference dc voltage whichcan be held stable within the desired temperature range simplyby selection of proper cbmponents.

It was mentioned earlier that the voltage divider amplifier isadjustable with respect to phase shift. This adjustment coversa few minutes of arc. The long-term stability and the indepen-dence of ambient temperature changes of this adjustment is ofthe order of a few percent of the adjustment range. Therefore,errors due to alterations of this setting are negligible.

VII. CONCLUSIONThe results obtained with the first experimental set-up using

the new optoelectronic measuring method clearly show thatthe device compares favorably with conventional energy mea-suring methods. The new system is expected to result in abetter accuracy-to-price ratio.The effectiveness of this new technique should improve with

increasing line voltage, especially for power lines using linevoltages of 220 kV and higher.Future work will emphasize the following: 1) substitution of

the compressed gas capacitor by another type of capacitor, 2)substitution of the supply transformer and supply capacitor by

a saturated current transformer, and 3) extension of the mea-suring reactive power as well.

ACKNOWLEDGMENTThe authors wish to express their thanks to Dr. R. Friedl

from the PTB who first conceived the application of a power-to-frequency-converter to high-voltage line measurements.

REFERENCES

[11 B. M. Pressmann, "The TRASER system for light-coupled currentmeasurement at EHV," in Proc. Soc. Photo-Optical Instrum.Eng., vol. 21, pp. 93-1O2, 1970.

[21 A. G. Siemens, German Patent DBP 1264606, 1968.(31 F. W. Ruthloh, "Strommessung in Hochspannungsfreileitungen

mittels TrAgerfrequenz," Elektrotech. Z. Ausg. A, Dec. 1965.[4] A. Erez, "Low-frequency electrical s4pnal measurement by elec-

trooptical methods," IEEE Trans. Instrum. Meas., vol. IM-21, pp.358-360, Nov. 1972.

[51 R. E. Danxienberg and H. Katzman, "An application of opticaltelemetry to shock tibe tneasurements," Rev. Sci. Instr., vol. 40,p. 640, May 1969.

[61 M. L. McCirtney and J. H. Highfill, "An opticaly coupled ECGsystem," IEEE Trans. Instrumm Meas., vol. IM-22, pp. 13-17,Mar. 1973.

171 G. W. York et al., "System for data aquisition from high voltageterminals," Rev. Sci. Instr., vol. 43, p. 230, Feb. 1972.

[81 R. Friedl, W. Lange, and P. Seyfried, "Electronic three-phasefour-wire power-frequency converter with high accuracy over awide range of use," IEEE Trans. Instrum. Meas., vol. IM-20, pp.308-313, Nov. 1971.

Buried Marking of Point LocationsJAMES H. LOUGHEED, MEMBER, IEEE, AND D. ZIMMERMANN

Abstract-An independent system to pinpoint buried equipmentto depths of 10 ft has been developed. Its primary use will be rapid,accurate, and unambiguous location of underground utility servicesand survey points. Tle system has two components-a passive markerburied at a point of interest, and a hand-cxied electronic interrogator.The marker, which is packaged as a sturdy peg, collects and reradiateslow frequency energy when simulated by an interrogator. The pegdesign enhances pick-up range and location accuracy, and allows user

identification by frequency. The interrogator is a sequentially switchedtransmitter-receiver that generates precise energy bursts and detectsreturned sigls; optimum processing by frequency selection and crosscorrelation is used. The design of both components acknowledges anabusive field operating environment. Interrogator physical and opera-tional details received particular emphanis, resulting in a simple func-tional instrument of unconventional shape.

I. INTRODUCTIONXISTING systems for locating buried facilities requireelectrical energy to be induced or conducted into a metal-

lic path such as a cable jacket or pipe. Using various tech-

Manuscript received May 1, 1973; revised June 8, 1973. This paperwas presented at the 1973 Electrical and Electronic Measurement andTest Instrument Conference (EEMTIC), Ottawa, Ont., Canada, May 15-17.The authors are with Bell-Northern Research, Ottawa, Ont., Canada.

niques, a route may then be traced, but a point on the pathsuch as a buried shutoff valve or splice cannot be specificallylocated. Also, ambiguous results will occur due to signal split-ting if another metallic path runs parallel for any significantdistance. In contrast, our system enables utilities to fix under-ground equipment locations precisely and without ambiguityas it is not affected by altemate signal paths. Furthermore, bydistinct frequency assignment, several users may be accom-modated in one geographical area without mutual interferenceor confusion.

I1. SYSTEM DESCRIPrTONThe location system has two components, a passive peg

placed with the concealed facilities, and an active interrogatorcarried by field personnel. The interrogator trar4smits a shortburst of low-frequency energy (Fig. I (a)) to pump up a high-Qresonant circuit contained in the peg. The interrogator thenswitches to a receive mode and listens for the peg decay char-acteristics (Fig. I (b)). The cycle is repeated continuously, i.e.,when a peg is within range, the operator receives an intermit-tent audio signal which lengthens as he approaches the marker.An inductive field is responsible for coupling between the

interrogator and the marker. Frequencies in the 40- to 100-

399