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Whitepaper: Spread Spectrum Time-Domain Reﬂ ectometry www.T3Innovation.com

SPREAD-SPECTRUM

TIME-DOMAIN

REFLECTOMETRY

SSTDR

WHITEPAPER:

MADE IN

USA

Whitepaper: Spread Spectrum Time-Domain Reectometry www.T3Innovation.com

IEEE SENSORS JOURNAL, VOL. 5, NO. 6, DECEMBER 2005 1469

Analysis of Spread Spectrum Time Domain

Reﬂectometry for Wire Fault Location

Paul Smith, Member, IEEE , Senior Member, IEEE , Member, IEEE, Cynthia Furse , and Jacob Gunther

Abstract—Spread spectrum time domain reﬂectometry (SSTDR)

and sequence time domain reﬂectometry have been demonstrated

to be effective technologies for locating intermittent faults on air-

craft wires carrying typical signals in ﬂight. This paper examines

the parameters that control the accuracy, latency, and signal to

noise ratio for these methods. Both test methods are shown to be

effective for wires carrying ACpower signals, and SSTDR is shown

to be particularly effective at testing wires carrying digital sig-

nals such as Mil-Std 1553 data. Results are demonstrated for both

controlled and uncontrolled impedance cables. The low test signal

levels and high noise immunity of these test methods make them

well suited to test for intermittent wiring failures such as open cir-

cuits, short circuits, and arcs on cables in aircraft in ﬂight.

Index Terms—Aging wire detection, arc detection, sequence time

domain reﬂectometry (STDR), spread spectrum time domain re-

ﬂectometry (SSTDR), time domain reﬂectometry (TDR), wire fault

detection.

I. INTRODUCTION

FOR MANY years, wiring has been treated as a system that

could be installed and expected to work for the life of an

aircraft [1]. As aircraft age far beyond their original expected

life span, this attitude is rapidly changing. Aircraft wiring prob-

lems have recently been identiﬁed as the likely cause of several

tragic mishaps [2] and hundreds of thousands of lost mission

hours [3]. Modern commercial aircraft typically have more than

100 km of wire [2]. Much of this wire is routed behind panels or

wrapped in special protective jackets, and is not accessible even

during heavy maintenance when most of the panels are removed.

Among the most difﬁcult wiring problems to resolve are those

that involve intermittent faults [4]. Vibration that causes wires

with breached insulation to touch each other or the airframe,

pins, splices, or corroded connections to pull loose, or “wet

arc faults” where water drips on wires with breached insulation

causing intermittent line loads. Once on the ground these faults

Manuscript received April 10, 2004; revised September 14, 2004. This work

was supported in part by the Utah Center of Excellence for Smart Sensors and in

part by the National Science Foundation under Contract 0097490. The associate

editor coordinating the review of this paper and approving it for publication was

Prof. Michael Pishko.

P. Smith is with VP Technology, LiveWire Test Labs., Inc., Salt Lake City,

UT 84117 USA (e-mail: psmith@livewiretest.com).

C. Furse is with the Department of Electrical and Computer Engineering, Uni-

versity of Utah, Salt Lake City, UT 84112 USA, and also with VP Technology,

LiveWire Test Labs., Inc., Salt Lake City, UT 84117 USA (e-mail: cfurse@

ece.utah.edu).

J. Gunther is with the Department of Electrical and Computer Engineering,

Utah State University, Logan, UT 84322-4120 USA (e-mail: jake@ece.usu.

edu).

Digital Object Identiﬁer 10.1109/JSEN.2005.858964

often cannot be replicated or located. During the few millisec-

onds it is active, the intermittent fault is a signiﬁcant impedance

mismatch that can be detected, rather than the tiny mismatch

observed when it is inactive. A wire testing method that could

test the wires continually, including while the plane is in ﬂight

would, therefore, have a tremendous advantage over conven-

tional static test methods.

Another important reason to test wires that are live and in

ﬂight is to enable arc fault circuit breaker technology [5] that is

being developed to reduce the danger of ﬁre due to intermittent

short circuits. Unlike traditional thermal circuit breakers, these

new circuit breakers trip on noise caused by arcs rather than re-

quiring large currents. The problem is that locating the tiny fault

after the breaker has tripped is extremely difﬁcult, perhaps im-

possible. Locating the fault before the breaker trips could enable

maintenance action.

This paper describes and analyzes one such method, based

on spread spectrum communication techniques that can do just

that. This method is accurate to within a few centimeters for

wires carrying 400-Hz aircraft signals as well as MilStd 1553

data bus signals. Results are presented on both controlled and

uncontrolled impedance cables up to 23 m long.

Early research on spread spectrum time domain reﬂectometry

(SSTDR) [6] has considered fault location tests on high voltage

power wires. Sequence time domain reﬂectometry (STDR) [7]

has been studied and used to test twisted pairs for use in commu-

nications. More recently, it has been demonstrated for location

of intermittent faults such as those on aircraft wiring [18]. These

test methods could be used as part of a smart wiring system

[2], and could provide continuous testing of wires on aircraft

in ﬂight, with automatic reporting of fault locations to facilitate

quick wiring repairs. This could be done by integrating the elec-

tronics into either the circuit breaker or into “connector savers”

throughout the system. In order for this to be feasible, the pro-

totype system that has been described here is being redesigned

as a custom ASIC, which should cost on the order of $10–$20

per unit in bulk. This paper focuses on the analysis of SSTDR

and STDR. Parameters required for these methods to function

as potential test methods on wires carrying 400-Hz ACor high

speed digital data such as Mil-Std 1553 are discussed. This anal-

ysis is critical to determine the system tradeoffs between speed,

accuracy, code length/system complexity, etc. This ideal anal-

ysis provides information on the expected accuracy, which is

veriﬁed with tests of near-ideal lossless controlled impedance

coax. The effect of realistic noncontrolled impedance cable is

also evaluated, and sources of error within a realistic system are

discussed.

Whitepaper: Spread Spectrum Time-Domain Reectometry www.T3Innovation.com

1470 IEEE SENSORS JOURNAL, VOL. 5, NO. 6, DECEMBER 2005

Fig. 1. S/SSTDR circuit diagram.

II. CURRENT WIRE T TEST ECHNOLOGY

There are several test technologies that can be used to pin-

point the location of wiring faults. Some of the most publicized

methods are: time domain re ectometry (TDR) [8], standingﬂ

wave reﬂ ﬂectometry (SWR) [12], frequency domain re ectom-

etry (FDR) [13], impedance spectroscopy [14], high voltage,

inert gas [15], resistance measurements, and capacitance mea-

surements. At the present time, these test methods cannot re-

liably distinguish small faults such as intermittent failures on

noncontrolled impedance cables without the use of high voltage.

In addition, the signal levels required to reliably perform these

tests may interfere with aircraft operation if applied while the

aircraft is in use [4]. Another test method is needed that can test

in the noisy environment of aircraft wiring, and that can be used

to pinpoint the location of intermittent faults such as momentary

open circuits, short circuits, and arcs.

III. SPREAD PECTRUM IRE ESTINGS W T

Spread spectrum signals, both in baseband (STDR) [7] and

modulated (SSTDR) [6], are detectable through cross correla-

tion, even though they may be buried in noise. The ability to

pick out the signal is due to processing gain, which for direct

sequence spread spectrum (DSSS) can be expressed as

where is the bandwidth of the spread-spectrum signal, is

the duration of one entire STDR/SSTDR sequence (considering

the entire sequence equal to one bit in communication-system

terms), is the duration of a PN code chip, is the chip rate

in chips per second, and is the symbol rate, which in this

case is the number of full sequences per second [16].

Because of this processing gain, it is reasonable to assume

that a spread spectrum test system could operate correctly in a

noisy environment with 400-Hz 115-V ACor digital data on the

wires. The test system could be designed such that it would not

be damaged by or interfere with any of the signals already on

the wires. For the analysis that follows, the digital data on the

wires will be assumed to be Mil-Std 1553, a standard aircraft

communication data bus that speci es a 1 Mbit/second data rate,ﬁ

a 2.25 20 V RMS signal level, normally operates on low-loss–

(3 dB/100 m) 70- shielded twisted pair cable, and allows for a

SNR of 17.5 dB [17].

The block diagram of the STDR/SSTDR block is shown in

Fig. 1. A sine wave generator (operating at 30 100 MHz) creates–

the master system clock. Its output is converted to a square wave

via a shaper, and the resulting square wave drives a pseudo-noise

digital sequence generator (PN Gen). To use SSTDR, the sine

wave is multiplied by the output of the PN generator, generating

a DSSS binary phase shift keyed (BPSK) signal. To use STDR,

the output of the PN generator is not mixed with the sine wave.

The test signal is injected into the cable. The total signal from

the cable (including any digital data or ACsignals on the cable,

and any re ections observable at the receiver) is fed into a cor-ﬂ

relator circuit along with a reference signal. The received signal

and the reference signal are multiplied, and the result is fed to

an integrator. The output of the integrator is sampled with an

analog-to-digital converter (ADC). A full correlation can be col-

lected by repeatedly adjusting the phase offset between the two

signal branches and sampling the correlator output. The loca-

tion of the various peaks in the full correlation indicates the lo-

cation of impedance discontinuities such as open circuits, short

circuits, and arcs (intermittent shorts). Test data indicate that this

test method can resolve faults in a noisy environment to within

1/10th to 1/100th the length of a PN code chip on the cable, de-

pending on the noise level, cable length, and type of cable [4].

IV. STDR/SSTDR ANALYSIS

The operation of STDR/SSTDR depends on the fact that por-

tions of electrical signals are re ected at discontinuities in theﬂ

characteristic impedance of the cable. A spread spectrum signal

shown in Fig. 2 is injected onto the wires, and as with TDR,

the re ected signal will be inverted for a short circuit and willﬂ

be right-side-up for an open circuit [8]. The observed re ectedﬂ

signal is correlated with a copy of the injected signal. The shape

of the correlation peaks is shown in Fig. 3. In this gure, theﬁ

modulating frequency is the same as the chip rate. Note that the

sidelobes in the correlation peak are sinusoids of the same am-

plitude as the off-peak autocorrelation of the ML code. This is

due to the selected modulation frequency and synchronization.

Use of a different modulation frequency or different synchro-

nization will yield a different correlation pattern that may have

higher side lobes [4]. Different PN sequences also have different

peak shapes, as shown in Fig. 4 for ML and gold codes.

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