Note: Descriptions are shown in the official language in which they were submitted.
~28~3
20104-8325
RADIO DIR~CTION-FINDING USING TIME OF ARRIVAL MEASUREHENTS
The invention relates to a method and to a system for
determining the direction of incidence of electromagnetic ~ave
signals from a distant source by determining the difference
between the time of arrival (ToA) of the leading edge of wave
siynals received respectively from the source at the two elements
of at leas~ one pair of mutually-spaced wave-receiving elements.
The invention furthex relates to a method and to a
system for determining the direction of incidence of
electromagnetic wave signals from a dis~ant source using both ToA
and phase-difference measurements.
ToA (also known as Time Difference of Arrival, TDOA)
direction-finding (DF) with a long baseline, i.e. wherein the
wave-recelving elements are spaced miles apart, is known from, for
example, the article "Passive Direction Finding and Signal
Location" by A.R. Baron et al, Microwave Journal, September 1982,
pages 59-76~ see particularly page 59 and pages 66-70. A major
disadvantage of ToA DF using a long baseline in many practical
situations is that if, as is normally the case, the direction of
incidence is to be determined over a substantial ran~e of
directionæ, there is a substantial interval of time over which
signals from the same source may arrive at one wave-receiving
element of the pair relative to the other, the difference between
the ToAs depending on the position of the source relative to the
pair of elements. If there is a plurality of sources from which
signals may be received, for example sources emitting pulsed
signals with a subs~antial pulse repetition frequency (PRF), then
"" ~LZI 3~93
20104-8325
there is a significant probability that the signals whose ToAs at
the two elements are compared come from difierent sources rather
than the same source; the greater the spacing between the
elements, and consequently the longer the above-mentioned period,
the greater is the probability. It is then necessary to compare
one or more characteristic parameters of the
~``` gL21~14~3
2 PHB 33274
signals received at the two elements, for example frequency, PRF or
pulse length, to ascerta;n whether they come from the same or
d;fferent sources. Not only does th;s requ;re substant;al further
equ;pment, but ;t substant;ally ;ncreases the t;me taken to
ascerta;n the d;rect;on of ;nc;dence of the s;gnals.
ToA DF us;ng a short basel;ne, for example 24 feet, ;s known
from US patent 3 936 83~. The use of a short basel;ne has the
advantage ~although there ;s no ment;on of ;t ;n the US patent) that
the above-ment;oned ;nterval with;n wh;ch s;gnals from the same
source can arrive at the two elements is so small that there ;s a
h;gh probab;lity ;n pract;cal s;tuat;ons that s;gnals from a
d;fferent source will not arr;ve in that period~ However, the use
of a short basel;ne imposes the d;ff;culty of determ;ning time
differences of the order of tens of nanoseconds or less. The
above-ment;oned US patent proposes a system wherein a capac;tor is
charged at a fast, linear rate from a constant-current source,
charg;ng be;ng started by the arr;val of a pulse signal at one
element and stopped by the arrival of a pulse signal at the other
element; the time d;fference ;s then effectively multipl;ed by
transferr;ng the voltage on the capac;tor to a further capac;tor
wh;ch is d;scharged at a much slower constant rate. However, the
~ c;rcu;try disclosed for performing these funct;ons would not ;n
; practice be suitable for the very short t;me d;fferences ;nvolved.For example, the current from the constant-current source could not
be sw;tched between zero and ;ts full value ;n a t;me wh;ch ;s short
compared w;th the t;me d;fference ;nvolved. Moreover, F;gure 5 of
the US patent, wh;ch ;s a graph of a count representat;ve of the
measured t;me difference against time delay ~actual t;me
d;fference), shows a predom;nantly substantially rect;linear
variation from about 2000 nanoseconds down to about 150 nanoseconds;
at th;s po;nt, there ;s an abrupt change of slope, with what appears
to be a hypothet;cal extrapolat;on to the or;g;n of the graph. Th;s
;nd;cates that the c;rcu;t would not in fact operate as ;ntended for
the t;me differences of 0-50 nanoseconds that would actually need to
be measured~
-` 12~ 93
20104-8325
It is an object of the invention to provide an improved
method and system for short-baseline ToA DF. It is a further
object of the invention to provide an improved timing circuit.
According to a first aspect of the invention, a method
of de~ermining the direction of incidence of electromagnetic wave
signals from a distant source from the time of arrival of the
leading edge of the wave signals comprlses:
receiving said signals at a plurality of mutually spaced
wave-receiving elements,
detecting the respective instantaneous amplitude of the
signal received a~ each element,
measuring the times at which the detected amplitudes of wave
signals received respectively at at least two of said elements
first exceed a minimal threshold value such that signals can be
satlsfactorily distinguished from noise and which threshold value
is substantially less than the minimum peak value of signals whose
direction of incidence is to be determined by said method, the
time being measured ln suah a manner that the measured time is
generally unaffected by multipa~h propagatlon,
determining the difference between the measured times in
respect of one palr or of a plurali~y of pairs of said elements,
wherein the two elemen~s of said one pair or of each of at least
two of said plurality of pairs are sufficiently close together
tha~ the length of the interval of time within which signals from
: the same source must arrive at ~he two elements is so short that
there is a high probability in operation that no signals from
another source will arrive in that interval, and
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20104~8325
deriving a representation of the direction of incidence from
the time difference(s) utilising the relationship
cos ~ - c ~t~d
where is the angle between the direction of incidence of the
signals and the line joining the two elements of a said pair, d i5
~he distance between those two elements, ~ t is the time
difference between the signals at that pair of elements first
exceeding the minimum threshold value, and c is the free-space
velocity of electromagnetic waves.
According to a second aspect of the invention, a system
for determining the direction of incidence of electromagnetic wave
signals from a distant source from the time of arrival of the
leading edge of the wave signals, comprises:
a plurality of mutually spaced wave-receiving elements,
means for detecting the respective instantaneous amplitude of
the signal received at each element,
means for measuring the times at which the detected
amplitudes of wave signals reaeived respectively at at least to of
said elements first exceed a minlmal threshold value such that
signals can be satisfactorily distingulshed from noise and which
threshold value is substantially less than the minimum peak value
of signals ~hose direction of incidence is to be de~ermined by
said method, the time being measured in such a manner that the
measured time is generally unaffected by multipath propagation,
means for determining the difference between the measured
times in respect of one pair or of a plurality of pairs of said
elements, wherein the two elements of said one pair or of each of
.
2010~-8325
at least two of said plurality of pairs are suificiently close
together that the length of the interval of time within which
signals from the same source must arrive at the two elements i5 SO
short that there is a high probability in operation that no
signals from another source will arrive in that interval, and
means ~or derivin~ a representation of the direction of
incidence from the time difference(s) utilising the relationship
cosc~ - c S t~d
where c~ is ~he angle between the direction of incidence of the
I.0 signals and the line joining the two elements of a said pair, d is
the distance between those two elements, S t is the time
difference between the signals at that pair of elements first
exceeding the minimum threshold value, and c is the free-space
velocity of electromagnetic waves.
These aspects of the invention involve the recognition
that in contrast to other methods and systems ~or direction-
finding such as in~erferometry, ToA DF can provide a good degree
of immunity to multlpath propagation, provided that the time of
arrival is measured
4a
~28[)~93
5 PHB 33274
1 early on the leading edge of the s;gnal and ;n such a manner that
the measured t;me ;s not substant;ally affected by mult;path
propagation wh;ch involves more than a short add;t;or,al path length
for the ;nd;rect, delayed signal. If the ToA ;s not measured early
on the lead;ng edge of a s;gnal, multipath can s;gn;f;cantly d;stort
the shape of the lead;ng edge as the s;gnal approaches its peak
value~ leading to a substantial error ;n a measured t;me difference
and hence in the der;ved d;rect;on of ;nc;dence. As w;ll be
described below, the ToA may be measured ;n respect of a threshold
wh;ch is well below the m;nimum peak ampl;tude and in such a manner
that the measured time is unaffected by variations in signal
ampl;tude w;thin a period of, for example, not more than about 10
nanoseconds after the threshold is first exceeded, so that the
system is immune to mult;path propagation which ;nvolves more than
about three metres add;t;onal path length, as w;ll generally be the
case for a distant source.
The above-mentioned US patent pays particular attention to
attempting to elim;nate the effects of pulse amplitude on ToA
measurement by us;ng a so-called Normalizer, but makes no reference
to the poss;ble effects of mult;path. In the f;rst of two
Normal;zer processes described with reference to Figure 7 of the
patent, the value of the s;gnal ampl;tude at wh;ch the s;gnal ;s
t;med ;s dependent on the rate of ;ncrease of the ampl;tude, and
since th;s value would necessar;ly have to be at a m;n;mal threshold
for sat;sfactory d;st;nct;on of rece;ved s;gnals from no;se when the
rate of ;ncrease ;s at ;ts lowest acceptable value, the amplitude
value at wh;ch the s;gnal ;s t;med w;ll generally be above the
m;n;mal threshold value and hence not as early on the lead;ng edge
as ;t m;ght be~ In the second Normalizer process descr;bed w;th
reference to Figure 8 of the patent, a second threshold well above a
first threshold ;s used. The Normal;zer processes are therefore
inherently more suscept;ble to multipath wh;ch affects the shape of
the lead;ng edge of the s;gnalO Moreover, ;n v;ew of the relat;vely
slow-act;ng circu;try descr;bed ;n the patent, the Normal;zers would
necessar;ly requ;re the t;med po;nt on a pulse to be well after the
~ ~28~493
6 PHB 33274
1 start of the pulse. The present ;nvent;on ;nvolves the recogn;t;on
that ;n pract;ce, it is more des;rable to obv;ate the effect of most
; mult;path propagation: t;m;ng errors due to multipath w;ll generally
be worse than errors due to d;fferent s;gnal ampl;tudes. Moreover,
by mak;ng the ToA measurement early on the lead;ng edge,
;naccuracies due to different signal amplitudes may ;n any case be
reduced.
Ow;ng to the difficulty of measur;ng very short time
differences accurately~ there ;s ;n pract;ce l;kely to be a
significant uncertainty ;n the difference between measured ToAs, and
therefore ToA DF with a short basel;ne ;s l;kely not to be very
accurate, although a s;ngle ToA d;fference measurement can g;ve an
unamb;guous ind;cat;on of d;rect;on .
~etter accuracy ;s achievable w;th interferometersO
Direction-finding using interferometers ;s well known. The
d;fference ;n phase between s;gnals rece;ved respect;vely from a
d;stant source at two mutually spaced wave-rece;ving elements is
representat;ve of the angle between the d;rect;on of ;nc;dence of
the s;gnals and the l;ne jo;n;ng the two elements. The greater the
spac;ng between the elements, the more accurately the angle of
;nc;dence can be determ;ned, but the smaller ;s the unamb;guous
range of directions of ;nc;dence. To resolve ambiguity, a
rect;linear array of wave-rece;v;ng elements ;s used to prov;de a
series of pairs of elements w;th progress;vely smaller spac;ngs; the
w;dest-spaced pa;r prov;des an accurate but amb;guous
representat;on~ and the closest-spaced pa;r prov;des a coarse but
unamb;guous representation. With successive spacings in a suitable
rat;o, the ambigu;ty ;n the phase measurement on the w;dest-spaced
pa;r can be resolved by reference to the successively more
closely-spaced pa;rs of the ser;es. However, to prov;de good
accuracy may requ;re a substantial amount of equ;pment s;nce each
element ;s associated w;th a respect;ve receiver, and with N
elements, at least (N-1) phase discr;minators are needed. Such a
system may consequently be expensive.
It ;s also well known to determine the direction of ;nc;dence
8~493
7 PHB 33274
of rad;o ~aves by compar;ng the ampLitudes of the signals received
by t~o adjacent similar antennae whose respect;ve main-beam axes are
;nclined to one another. If the d;rection of inc;dence lies between
the axes, the rat;o of the amplitudes is representat;ve of the angle
between the direct;on of ;ncidence and either of the axes. This
arrangement provides an unambiguous but reLatively ;naccurate
measurement of d;rect;on.
Accord;ng to a th;rd aspect of the ;nvent;on, a method of
determ;ning the direct;on of ;ncidence of electromagnetic wave
s;gnaLs from a distant source compr;ses:-
rece;v;ng sa;d signals at each of a plural;ty of mutually spaced
wave-receiv;ng elements,
measur;ng the phase d;fference between the signals rece;ved
respect;vely at the two elements of one pa;r of sa;d elements or the
respect;ve phase d;fferences between the signals rece;ved
respectively at the two elements of each of a plurality of
substant;ally collinear pairs of said elements with different
respective spacings, wherein the phase measurement on said one pair
or on the closest-spaced of said pairs is amb;guous ;n the operating
range of directions of incidence and the operating frequency range,
determining by a method embodying the first aspect of the
invention the approximate direction of incidence of said signals
from the times of arrival of the leading edges of corresponding wave
signals received respectively at two of sa;d plurality of elements,
the l;ne jo;ning wh;ch two elements is parallel to and substantially
coincident ~ith the line joining said one pair or said plurality of
substantially collinear pairs, wherein the range of possible values
of the actual time difference due to the uncertainty in the
d;fference between the measured t;mes corresponds to a range of
angles of ;ncidence whose magnitude is not greater than the
magn;tude of the range of angles of ;nc;dence corresponding to the
unambiguous range of phase difference measurement on said one pair
or said closest-spaced pair, and
resolving the ambiguity in said ambiguous phase measurement by
comparing the possible directions represented thereby with the
~~28~1493
8 PHB 33Z74
1 approx;mate direct;on determ;ned from the d;fference between the
measured t;mes.
Accord;ng to a fourth aspect of the ;nvent;on, a system for
determin;ng the direct;on of ;nc;dence of electromagnetic wave
signals from a distant source comprises:-
a plural;ty of mutually spaced wave-rece;ving elements,
means for measur;ng the phase difference between the s;gnals
received respectively at the two elements of one pair of said
elements or the respective phase differences between the signals
received respectively at the two elements of each of a plurality of
substant;ally coll;near pa;rs of sa;d elements ~;th d;fferent
respect;ve spacings, where;n the phase measurement on said one pa;r
or on the closest-spaced of said pa;rs is amb;guous in the operat;ng
range of directions of incidence and the operat;ng frequency range,
means, comprising a system embodying the second aspect of the
invention, for determin;ng the approx;mate d;rect;on of inc;dence of
sa;d s;gnals from the times of arrival of the lead;ng edges of
correspond;ng signals rece;ved respect;vely at two of sa;d plural;ty
of elements, the l;ne jo;n;ng wh;ch two elements ;s parallel to and
substant;ally co;nc;dent ~ith the l;ne jo;ning sa;d one pa;r or sa;d
plurality of substant;ally collinear pa;rs, where;n the range of
poss;ble values of the actual t;me d;fference due to the uncertainty
in the d;fference between the measured times corresponds to a range
of angles of incidence whose magn;tude is not greater than the
magn;tude of the range of angles of ;ncidence correspond;ng to the
unamb;guous range of phase d;fference measurement on sa;d one pa;r
or sa;d closest-spaced pair, and
means for resolving the amb;gu;ty ;n said amb;guous phase
measurement by comparing the possible d;rections represented
thereby w;th the approx;mate direct;on determined from the
difference between the measured t;mes.
~y using a ToA DF arrangement to resolve ambiguity in the phase
measurement on the s;ngle pa;r or the closest-spaced pa;r of
elements of an ;nterferometer DF arrangement, the need for one or
more ;nterferometer channels wh;ch would merely be needed for
-` 12~30493
9 PHB 33274
1 resolving ambiguity but ~hich would not increase the accuracy of the
direction-finding is avoided, and the combined arrangements may be
cheaper and simpler than a purely interferometric one.
To determine directions of incidence over a ~ide range of
directions~ particularly d;rections which are not restricted
substantiaLly to a plane including the elements, a system embody;ng
the fourth aspect of the invention may comprise phase-d;fference
measur;ng means, approximate-direction-determing means and
ambiguity-resolving means operable ;n respect of a f;rst pa;r or a
f;rst plural;ty of substant;ally coll;near pairs of the elements and
of a second pair or a second plurality of substantially collinear
pairs of the elements to derive first and second unambiguous phase
measurements, wherein the line joining the elements of said first
pa;r or said f;rst plurality of pa;rs and the line joining the
elements of said second pa;r or said second plurality of pairs are
substantially coplanar and ;ncl;ned to one another, sa;d f;rst and
second phase measurements being representative of the angle d
between the direct;on of ;nc;dence and the l;ne joining the elements
of the respective pair(s), and further comprising means for deriving
a representation of the angle 0 and/or a representat;on of the angle
~, where 0 ;s the angle between the d;rect;on of ;nc;dence
projected ;nto the plane of the l;nes and the normal to a respect;ve
one of sa;d l;ne ;n said plane and where ~ ;s the angle between
the d;rect;on of ;nc;dence and sa;d plane, from the f;rst and second
unamb;guous phase measurements ut;l;s;ng the relat;onsh;p
s;n (90 degrees - ~ ) = s;n ~ cos ~ .
For s;mpl;city, said lines may be mutually perpendicular.
In a particularly s;mple system, the
approx;mate-direction-determining means may be operable in respect
of the times of arrival at a common element and at each of t~o
elements respectively on the two l;nes, and furthermore the
phase-difference measuring means may be operable to measure the
phase differences bet~een said common element and each of two
elements respectively on the t~o lines.
As an alternat;ve, a system may comprise three or more mutually
Z80493
PHB 33274
1 ;ncl;ned successively adjacent pairs or plurality of pairs of
elements, means for measuring the ampl;tude of wave signals received
at one or more elements of each of said three or more pairs or
plural;ty of pa;rs, and means for select;ng as sa;d f;rst pair or
plural;ty of pa;rs one of sa;d three or more pa;rs or plurality of
pa;rs ;n respect of ~hich the ampl;tude ;s at least as great as the
ampl;tude in respect of each of the remain;ng pa;rs or plurality of
pairs and as sa;d second pa;r or plurality of pa;rs a pair or
plural;ty of pa;rs adjacent sa;d first pair or plural;ty of pa;rs ;n
respect of wh;ch the ampl;tude ;s at least as great as the
ampl;tude in respect of any other adjacent pair or plurality of
pairs. Suitably, such a system comprises four mutually orthogonal
pa;rs or plural;ty of pa;rs of elements~
A method embody;ng the f;rst aspect of the ;nvent;on may
;nvolve using three substantially coplanar but substantially
non-collinear elements to form at least two said pairs, and deriving
a representation of the angle 3 and/or a representat;on of the angle
utilising the relat;onsh;p
s;n (90-c~) = sin 0 cos ~
;n respect of each of sa;d at least two pairs, ~herein 0 is the
angle between the d;rect;on of ;nc;dence projected ;nto the plane of
the three elements and the normal ;n sa;d plane to the l;ne jo;n;ng
the two elements of a sa;d pa;r, and ~ ;s the angle between the
d;rect;on of inc;dence and sa;d plane~ D;rect;ons of ;nc;dence ;n
three d;mens;ons may thus be determ;ned from ToA measurements at
three elements.
The method may further compr;se determ;n;ng a parameter
representat;ve of the rate of ;ncrease of the detected amplitude of
the respect;ve s;gnal received at at least one of the elements ;n
the reg;on of sa;d threshold value, and determining the direction of
;nc;dence of rece;ved s;gnals only if sa;d parameter sat;~f;es a
criterion representing a m;nimum rate of ;ncrease ;n sa;d reg;on.
S;gnals whose d;rection of incidence cannot be determined with
sat;sfactory accuracy can thereby be d;scarded. Such a method may
compr;se measuring the time at wh;ch the detected amplitude first
~2~ 493
20104-8325
exceeds an adjacent further threshold value, wherein said
parameter is the difference between the measured times in respect
of the two threshold values, and wherein said criterion is that
said parameter does not exceed a maximum value. Alternatively,
such a method may comprise di~ferentiating the increasing detected
amplitude at least in said region, wherein said parameter is the
rate of increase in detacted amplitude derived by differentiation,
and wherein said criterion is that said parameter exceeds a
minimum value.
A system embodying the second aspect of the invention
may comprise means for performing optional features of a method
embodying the first aspect o~ the invention, as set forth in
~laims 7 to 10.
An embodiment of the means ~or measuring the times may
comprise a clock pulse generator, a tapped delay device having a
plurality of n mutually spaced taps, a latch coupled to the delay
device for latching any signal on each o~ the n taps, and a
decoding device coupled to the latch for producing a time
representation from the signal(s) latched from the n taps, is
characterized in that an input signal to be timed is coupled to
the input of the delay device, in that the clock pulse generator
is normally operable to clock the latch, in that the circuit
comprlses inhibiting means responsive to the presence of a signal
on at least one of the n taps when the latch is clocked to inhibit
further clocking of the latch, and in that the decoding device is
operable to produce a representation of the interval between the
time that said input signal reaches the tap nearest the input o~
~IL2~0~93
20104-8325
the delay device and the preceding clock pulse.
Said interval may be represented as zero for the case in
which the input signal has reached the tap furthest from the input
when the latch is clocked, and other intervals represented
accordingly.
A timing circuit embodying the third aspect of the
invention may be contrasted with the timing circuit disclosed in
GB 2 132 043 A and EP 113 935 A, in which the clock pulse
generator is coupled to the input of the delay line, and the input
signal to be timed is used ~o latch the latch.
lla
~;; 8~ 3
12 PHB 33274
1 Su;tably, the circuit further comprises a counter for counting
the pulses of the clock pulse generator, ~herein the inh;bit;ng
means are further operable to inh;bit further count;ng of the clock
pulses, the outputs of the decod;ng device and the counter being
concatenated.
In order to be able to produce representat;ons of intervals
over the majority of the per;od of the clock pulse generator, the
period of the clock pulse generator may be not substantially less
than the t;me delay bet~een the tap nearest to and the tap furthest
from the input of the delay device. ~uitably, sa;d per;od ;s
substant;ally equal to sa;d time delay.
To make good use of the delay dev;ce and to prov;de
representations of integral multiples of a fraction of the period of
the clock pulse generator, the time delay between each adjacent pair
of the n taps may be the same, being equal to T, and the period of
the clock pulse generator be nT.
Where the circuit ;s to be used to t;me the beg;nning of
signals ~hich persist for at least the per;od of the clock pulse
generator, the ;nh;bit;ng means may be respons;ve to the presence of
a s;gnal on the tap nearest the ;nput of the delay device when the
latch ;s clockedR Th;s helps to d;st;ngu;sh true signals to be
t;med from no;se ;n the case where a s;gnal ;s present on a tap
beyond the tap nearest the ;nput of the delay dev;ce ~hen the latch
;s clocked, since such a s;gnal m;ght be due to noise. As a further
safeguard aga;nst false measurements due for example to no;se, the
decoding device may be operable not to produce said time
representat;on unless when the latch is clocked a s;gnal ;s present
on each of the n taps between the ;nput of the delay device and the
tap furthest from the ;nput of the delay dev;ce on ~hich a signal is
present~
It has been found that attempting to operate a timing circuit
of the k;nd d;sclosed ;n the above-ment;oned GB and EP publ;shed
appl;cations to measure time to a resolution of about 1 nanosecond
produces difficulties in synchron;sing the fine count produced by
the decoding device and the coarse count produced by the counter. A
` ~80~93
20104-8325
timing circuit embodying the third aspect of the present invention
can be both simpler and more reliable. Furthermore, it has been
found advantageous to use the threshold crossing merely to feed an
input signal to the delay device, rather than to use it to control
gates: the latter is liable in practice to produce undesired
di~tortion of the signal.
Embodiments of the invention will now be described, by
way of example, with reference to the accompanying drawings, in
which:
Figure 1 is a block diagram of a ToA DF system embodying
the inven~ion and comprising one pair of wave-receiving elements;
Flgure 2 is a block diagram of a timing circuit suitable
for UBe in a ToA DF system embodying the invention;
Figure 3 is a block diagram of a ToA DF system embodying
the invention and comprising three collinear wave-receiving
elements;
Figure 4 illustrates the disposition of three non-
collinear wave-receiving elements for an omnidirectional ToA DF
system embodying the invention:
Figure S illustrates schematically processing to
calculate an angle using the elements of Figure ~;
Figure 6 is a schematic diagram of a simple DF system of
a kind using both ToA and phase-difference measurements;
Figure 7 is a schematic diagram of a particularly simple
omnidirectional DF system of this kind;
Figure 8 illustrates the disposition of ToA and
inter~erometer antenna arrays for a more complex omnidirectional
13
~28049:~
20104-8325
DF system of this kind, and
Figure 9 illustrates processing for a DF system using
the arrays of Figure 8.
Figure 1 shows a simple system comprising one pair of
wave-receiving elements. The system comprises two similar
channels A and B respectively. Each of the channels comprises in
succession an antenna AN'r, an RF amplifier RFA, a detector D, a
video amplifier VA, and a timing circuit TC. The antennae may be
omnidirectional, or may be directional with their axes
substantially parallel. The
13a
28~g3
14 PHB 33274
spacing d between the antennae is chosen to be sufficiently small
that the length of the interval of t;me with;n wh;ch s;gnals from
the same source must arr;ve at the two elements is so short that
there ;s a h;gh probab;l;ty in operation that no signals from
another source ~;ll arr;ve ;n that ;nterval. If s;gnals may be
received from any direct;on, the length of the interval is t~ice the
time taken for electromagnetic waves to travel the distance d (;n
free space): the limits of this interval are set by the possibil;ty
of s;gnals being ;ncident along the line joining the antennae ;n one
sense or the other~ ;.e. from left or r;ght ;n Figure 17 The
probability of no s;gnals arr;v;ng from another source ;n that
;nterval ~ill depend on the number of sources from which s;gnals can
be rece;ved, the frequency with ~hich they em;t fresh s;gnals, and
the durat;on of the s;gnals. What probability is sufficiently high
w;ll depend on what proport;on of ;ncorrect representat;ons of
d;rection, due to measurements hav;ng been made ;n the two channels
on signals from d;fferent sources~ ;s cons;dered acceptable. For
typ;cal opera~;onal situat;ons of the number of sources em;tt;ng
pulsed signals, their PRF and pulse length, a separation d of the
order of 50 feet, giv;ng an ;nterval length of about 100
nanoseconds, is considered to give an acceptably high probabil;ty.
When an RF s;gnal ;s ;ncident on the antenna of one of the channels,
the RF signal is amplified and de~ected, and the time at which the
instantaneous detected amplitude of the signal, after further
ampl;ficat;on, f;rst exceeds a threshold value is measured. This
threshold value is chosen to be substantially less than the min;mum
peak value of signals whose direction of incidence is to be
determined, as ~;ll be explained in greater detail belo~; the
threshold w;ll usually be much less than the typical peak value.
The timing circu;ts operate w;th a common clock (CLOCK3. A
calculating un;t CALC determines the difference ~ t between the
measured t;mes and provides therefrom a representation of the
direction of incidence, for example of the angle ~ between the
incident signals and the line joining the antennae, utilising the
relationship
9~
15 PHB 33274
cos o< = c ~t/d
where c is the free-space velocity of electromagnetic waves.
The time at wh;ch the threshold value is first exceeded is
measured in each timing circuit TC in such a manner that the
measured time ;s unaffected by multipath propagat;on ~here the
; delayed s;gnaL has been reflected ~rom a surface not virtually
coincident with the direct path between a distant source and the
antenna.
An ECL tem;tter-coupled logic) circuit arrangement, su;table
for the tim;ng circuit TC of Figure 1 and capable of measuring to
an accuracy of 1 nanosecond, is shown in Figure 2. The detected and
amplified video signal from the video amplif;er VA is applied to a
very fast comparator COMP ~hose output changes from a log;c "O" to
"1" when the input signal exceeds a threshold voltage VT. The
t5 comparator output s;gnal ;s fed to the input of a tapped delay line
TDL having 8 consecutive taps separated by 1 nanosecond intervals.
The taps are connected to respective ;nputs of an 8-b;t latch
LATCH. The latch is clocked at 8 nanosecond ;ntervals by a 125 MHz
clock CLOCK v;a a gate G1, the clock signal also being supplied via
a further gate G2 to a synchronous counter CNTR ~hich prov;des a
coarse measurement of time. The outputs, labelled 0-7, of the latch
are fed to a decoding circuit DCDR; the presence o~ a signal on at
least one output, in this case the first output, O, ;s also used to
control the gates G1 and G2~ the output being connected thereto by a
fast ~eedback loop. The outputs of the counter CNTR and the decoder
DCDR are concatenated to give a representation of the t;me at wh;ch
the output of the comparator COMP changed from O to 1.
In operation, the gates G1 and G2 are normally open. The
counter CNTR measures time in 8-nanosecond un;ts, up to a maximum
time at least as long as the above-mentioned interval, determ;ned
by the antenna spacing d, within which signals from a distant
source must be received by both antennae. The latch is s;m;larly
clocked at 8-nanosecond intervals, but while the comparator output
is 0, there are no signals from the tapped delay line, and the
3~ latch outputs rema;n at zero. ~hen the comparator output changes
80g~93
16 PHB 33Z74
1 to 1 (which in the case of a true received signal as opposed to
noise uill normally pers;st for longer than the period of the
clock), the signal travels along the delay line changing successive
tap outputs from 0 to 1. When the latch is next clocked, the series
of ones and remaining zeroes is held ;n the latch. The presence of
a "1" on the first output, 0, of the latch closes the gates G1 and
G2, preventing further clocking of the latch and the counter. The
output of the latch w;ll be one of the following codes:
1ûOOOOOO
'11 000000
11 100~00
11110000
111110nO
11111100
11111110
1111~111
The first of these codes represents the most recent ToA and the
last the earliest ToA since the latch was last clocked; the first
ind;cates that 7 nanoseconds should be added to the time represented
by the counter CNTR while the last requ;res zero add;t;on. The
decoder DCDR transforms that latch output to binary digits which are
concatenated with the counter output~
The threshold the cross;ng of wh;ch ;s timed ;s, as prev;ously
explained, set at a low value in order largely to avo;d errors due
to multipath. Setting the threshold at a level substant;ally below
the m;n;mum peak level of signals whose direction of incidence is to
be determined, for example 10 dB below the minimum peak level, also
provides the advantage of tending to alleviate timing errors ~hich
would occur if the signal amplitude crossed the threshold at a slo~
r~ rate because the amplitude ~ reaching its peak level. The
lowest level at which the threshold can be set will depend on the
noise level in the system: ;f the threshold is set too close to the
noise level, the accuracy of t;ming will be degraded by the random
fluctuation in the amplitude of desired s;gnal plus noise, and a
positive-going threshold crossing may even be caused by noise
- ~ 213~9C93
17 PHB 33274 r
1 alone.
The decoder ;s ;n th;s embod;ment arranged to accept only the
above-mentioned codes. It consequently accepts only s;gnals whose
amplitude remains above threshold long enough to produce a
continuous succession of ones in the latch, and rejects any other
pattern of zeros and ones which might result from trigger;ng of the
comparator by noise spikes or from a received signal w;th a slow
rate of ;ncrease of amplitude.
It will be seen that provided the amplitude remains above
threshold long enough for this state to be latched, the
measurement of ToA will be unaffected by subsequent variat;ons in
ampl;tude, ;n part;cular var;at;ons due to a delayed mult;path
signal wh;ch arr;ves in phase oppos;tion to the or;g;nal
d;rect-path signal and causes the amplitude to fall below
t5 threshold~ The max;mum period taken to latch the above-threshold
state is in this embodiment the length of the delay line, ;.e. 8
nanoseconds. This time may be much shorter than the t;me taken for
the amplitude to reach typical peak value.
A delay not less than and substantially equal to the t;me
taken for electromagnetic waves to travel the d;stance d may be
;ncluded in one channel before the timing c;rcuit so that time
differences are measured with respect to the time of arrival of a
signal in the other channel. Suitably, a time "windw" is used to
prevent unnecessary computation on time differences which are too
large for the signaLs to have come from the same source. Where the
above-mentioned delay is included in one channel, this window may
be de~ined as beg;nn;ng w;th a time difference of zero and end;ng
with a time d;fference not less than and substant;ally equal to
tw;ce the t;me taken for electromagnet;c ~aves to travel the
distance d. The use of a time w;ndow also prov;des some protection
against random noise signals wh;ch cause the detected amplitude to
exceed the threshold from causing false measurements.
At least one, and preferably each, of the channels in the
system of Figure 1 may comprise a signal validat;ng c;rcuit ~not
shown) to ascertain the rate of increase of the amplitude of the
4~3
18 PHB 33274
1 s;gnal in that channel in the region of the threshold vaLue, and to
cause the system not t-o determine the direction of inc;dence unless
the rate satisfies a criterion of m;nimum slope. For th;s purpose,
the output of the video amplif;er VA may be supplied to a further
timing circuit (not sho~n) wh;ch measures the t;me at wh;ch the
signal amplitude first exceeds an adjacent further threshold value.
The difference between the times measured by the two circuits ;n a
~hannel may be determined, and the direction of ;nc;dence determ;ned
only ;f the difference does not exceed a max;mum value~
0 Alternatively, the ampl;tude ;ncrease may be different;ated
and the d;rection o~ incidence determined only if the rate of
;ncrease of a~plitude derived by differentiation exceeds a minimum
value.
As a further way of distinguishing signals coming from a
distant source from no;se, at least one of the channels may
comprise means (not shown) for determining the peak amplitude of a
signal wh;ch causes the threshold to be exceeded, and for inhibiting
the determ;nation of the d;rect;on of incidence unless the peak
ampl;tude is substantially greater than the threshold.
It is cons;dered that a su;table criter;on of minimum slope
may be that the steepness of the rising edge should be
predom;nantly controlled by the v;deo bandw;dth of the system. Th;s
;nter al;a has the effect of reducing the dependence of the measured
t;me on the rate of increase of the RF ampl;tude and hence tends to
ach;eve the same object as the Normal;zers ;n the above-mentioned US
nfltent. It may be des;rable for the v;deo bandwidth to be
switchable between a broadband value and a narrowband value. The
broadband value may allow more accurate t;m;ng of steeply-r;sing
lead;ng edges, but the narrowband value may enable acceptable
results to be obtained with more slowly-r;s;ng edges, since it may
reduce the no;se level in the v;deo c;rcuit and allow the threshold
to be set to a lower value and hence to a relat;vely steeper part of
the lead;ng edge.
F;gure 3 depicts a modification of the system of F;gure 1
comprising three coplanar and collinearly d;sposed antennae L~ M, N
"
93
20104-g325
reæpectively in respective channels each channel being the same as
each of the channels shown in Figure 1. The spacings of each of
the two pairs of elements LM and MN are equal (each being d) and
each satisfy the above-mentioned criterion that the spacing is
sufficiently small that the length of the interval of time within
which signals from the same source must arrive at the two elements
of a palr is so short that there is a high probability in
operation that no signals will arrive from another source in that
interval; the spacing 2d between antennae L and N may however be
too large to satisfy khis criterion. Nevertheless, the difference
between the times of arrival of signals at antennae L and N may be
used to provide a more accurate representation of the direction of
incidence than could be provided by the system of Figure 1 if one
or more steps are taken to reduce the posslbility that the time
difference measured on one of the pairs of antennae LM, MN does
not relate to the same source as the time difference measured on
the other pair. For example, as indicated in Figure 3, the time
differences measured in relation to each pair of antennae, tLM and
tMN respectively, may be compared, and only if their values are
equal to within a small tolerance is the direction of incidence
determined from the difference between the times of arrival at
antennae L and N; the probability that signals from different
sources should result in substantially equal time differences
being measured between the elements of the pairs LM and ~N is
small, and even if the signals should have come from different
sources, the resultant error in the indicated direction of
incidence in relation to the source from which signals were first
19
`- ~2~ 93
20104-8325
received at one pair of antennae will be small.
The systems so far described provide only a
representation of the direction of incidence that defines an angle
to the line joining a pair of wave-receiving elements, and hence
the surface of a cone whose axis is said line. Where sources are
known to lie substantially in a single plane including said line
and where the wave-receiving elements are directional, this may be
sufflcient (although it should be borne in mind that the accuracy
with which c~ can be determined decreases as ~ decreases from 90
degrees to 0); however, when signals may be received from each
side of said line, and particularly when sources are not
restricted to a single plane, it is desirable to perform
measurements on at least one further wave-receiving element which
is not collinear with the one pair of elements. Figure 4 depicts
the deposition of three substantially coplanar but not collinear
elements A, B, C respectively, forming an arbitrarily-shaped
trian~le. The spacings AB, BC, CA each satisfy the above-
mentioned criterion of being sufficiently small. By measuring the
times of arrival of slgnals at each elemen~, the direction of
lncidence may ~e determined for the general case of distant
sources in 3-dimensional space as follows.
Let the spacings AB, BC, CA be dl, d2, d3 respectively.
A
Let the angles CAB and ABC be m and n respectively. Let the
length of the perpendicular from C onto AB be a, and the distance
from A to the intersection of said perpendicular with AB be b, so
that a - d3 sin m and h - d3 cos m. (Thus b is negative if m ~ 90
degrees.~ Let the angle between the direction of incidence and
2 !3(~ 3
20104-8325
the normal to AB in the plane of AB be ~ (so that ~ =
~90 deyrees-~) and coscX = sin ~), the angle between the normal to
AB in the plane of ABC and the direction of incidence projected
into that plane be ~ (typically the azimuth angle), and the angle
between the direction of incidence and the plane o~ ABC be ~
~typically the alevation angle). Let the times oi arrival at A,
B, C be tA tB~ tc respectively.
Then
sin ~ = C(tA-tB)/~
Now sin ~ a sin ~ CQS
Writin~ x = c(t -t )/d
y = c(t -t )/d
z = c(tB-tc)/d2,
we may put x - sin ~ cos
and analogously y = sin ~-m) cos
z ~ sin [~-(180-n)] cos
-sin (~+n) cos ~.
20a
-` ~LZ80493
21 PHB 33274 -
1 El;minat;ng ~ from either of two pairs of these equat;ons,
one obta;ns
tan ~ = x sin m/~x cos m - y]
and tan 3 = -x sin n/~x cos n + z].
These expressions have two-fold amb;guity. To distinguish between
-90 degrees < 0 ~ 90 degrees and 90 degrees ~ 0 < 270 degrees, one
may note that the denominator of for example the first expression
for tan ~ may be expanded as
sin ~ cos ~ cos m - sin(~-m) cos
or cos ~ cos ~ sin m.
Thus the denom;nator is pos;t;ve for -90 degrees ~ O ~ 90 degrees
and negat;ve for 90 degrees ~ ~ < 270 degrees.
Re-writ;ng the first express;on for tan ~ in terms of the
t;mes of arr;val and mult;plying the numerator and denom;nator by
~5 d1d3
tan O ~ (tA-t~)d3 sin m/C(tA-t~)d3 cos m -(tA-tC)d
= - a (tA-tg)/~d1(tA-tC) ~ b~tA tB)J
= - a (tA-tB)/~(d1-b)(tA-tc)+b(t~-tc)~-
Writ;ng
P = a(tA tB)
a = (d1-b) ~tA~tc
R ~ b (tB-tC),
one obta;ns
tan O a - P/~Q+R)
or ~ = - Arc tan CP/(Q+R)~
Figure 5 illustrates schematically the processing to calculate O
according to this expression. The differences tA-tp, tB-tC,
tA-tC are formed from the measured times tA~ t~, tc and
then scaled to produce the quantities P, Q, R. The quantities ~ and
R are summed and d;vided into P; the angle whose tangent is equal to
minus the quotient is then determ;ned, for example from a look-up
table in a PROM~ to obtain an ambiguous value aamb f ~. The sign
of (Q+R) is also checked; if (Q+R)> O, a quantity 00 = 180 degrees
is produced, otherw;se ~0 = - ~amb and ~0 are summed to produce an
unambiguous value of 0. Having calculated ~, the angle ~ may be
L2~3~9L9~
22 PHB 33274
1 calculated by subst;tut;ng ~ ;n, for example, the expression
x = sin ~ cos ~ .
Alternatively, ~ may be calculated without needing to calculate
by elim;nating 0 from a pair of the expressions for x, y, ~.
The calculations and processing may be simplified for
particular cases. For an equilateral triangle of side d,
a = ~3d/2 and
b = d~2,
so that the time differences need only be scaled by factors which
are independent of du ~lternatively, if m = 90 degrees,
a = d3 and
b = 0~
so that the quantity R is zero. If d1 = d3, the scaling factors are
again independent of the actual value of the spac;ng.
An omnidirectional direction-finding system may comprise four
receiv;ng eLements d;sposed at the corners of a parallelogram, or
more especially a rectangle and more part;cularly still a square.
The d;rection of inc;dence may be calculated from the times of
arrival of the earliest-received three s;gnals wh;ch are of
acceptable qual;ty. This allows for the possibil;ty that s;gnals
rece;ved at one of the four elements may have been degraded by, for
example, an obstruction in the region of the elements.
Comparison of the equations which can be derived from the above
two expressions for tan ~ in terms of x and y and of x and z
respectively to relate the error in 0 to errors in x, y and z show
that the error is not dependent on wh;ch expression is used. The
choice of which baselines are considered as primary and secondary
baselines for determ;ning ~ is therefore not signif;cant.
As an alternative to a clock common to the t;ming c;rcu;ts of
all the channels as depicted in Figures 1 and 3, each channel may
have a respective accurate clock and the clocks be kept in
synchronism via a lo~-bandwidth link. The time measurements and any
other other data may be passed to a central processor and control
unit via, for example, an optical fibre l;nk~
The principle of using both ToA and phase-difference
. . ., ,~
~80~93
23 PH~ 33274 -
measurements will be explained with reference to Figure 6. A ToA DF
system comprises two antennae ANT1, ANT2 separated by a distance L1,
and a measuring and calculat;ng un;t MCtTOA) wh;ch determines the
difference ~t between the respective times of arrival of the
lead;ng edge of an RF s;gnal at the two antennae. The an~le c~
between the direction of incidence of the signal and the line
joining the two antennae (the baseline of the system) is given by
the equation
cos o~ = c ~t/L1 ~13
where c is the free-space velocity of electromagnet;c waves. An
interferometer DF system comprises a rectiLinear array of antennae,
in this case three antennae ANT3, ANT4, ANTs, so disposed that the
l;ne joining the antennae (the baseline of the system) is parallel
to and substant;ally coincident ~;th the baseline of the ToA DF
system (in order that the two systems should measure the same angle
o~in respect of signals from a distant source). The w;dest-spaced
pa;r of antennae of the interferometer system are separated by a
distance L2, and the closest-spaced pa;r by a distance L3; the
accuracy w;th wh;ch the angle o~ can be determined by the
interferometer system depends on the value of L2, and the
unamb;guous range of coverage depends on the value of L3. The
;nterferometer system comprises a measur;ng and amb;gu;ty resolut;on
un;t MAR~IR) wh;ch measures on each of a plural;ty of pairs of the
antennae of different respect;ve spac;ngs, from the w;dest-spaced
to the closest-spaced, the phase d;fference between RF signals
rece;ved respect;vely at the two antennae of the pa;r, ;n this case
the phase d;fference 035 and ~45 between ANT3 and ANTs and between
ANT4 and ANTs respect;vely; the phase measurements may be performed
after convert;ng the RF signals to an intermediate frequency tIF).
Since the measurement of phase is restr;cted to a range of 2 ~, the
measured phases 0 are amb;guous. The actual unamb;guous phase
d;fferences may be denoted ~ where
= ~ + 2k~
where k is an integer. The unit MAR(IR) in known manner resolves
the ambigu;ty in ~35 as far as possible by reference to ~45 (see for
B~)~93
- 24 PH~ 33274
1 example Ge 1 337 099). Now
cos o~ = C~3s/2 ~fL2 ~2)
and
cos o~ = C~4s~2 ~fL3 (3)
where f is the frequency of the signals and ~35, ~45 are the
unambiguous phases. The unamb;guous range of coveraQe of the
interferometer system may be obta;ned by ;nsert;ng ;n equat;on 3
values of ~45 separated by 2 n.
Comb;n;ng equations 1 and 3,
~45 = 2 ~f/~t L3tL1 (4)
Now ;n order sat;sfactorily to be able to resolve the rema;n;ng
amb;gu;ty ;n the measured phases using the d;rect;on of ;ncidence
determ;ned by the ToA DF system, the range of possible values of the
actual difference between the ToAs of s;gnals at ANT1 and ANT2 due
to uncerta;nty ;n the measured time difference should correspond to
a range of bC whose magn;tude ;s not greater than the magn;tude of
the range of oC correspond;ng to the range of ~45 wh;ch can be
determ;ned unamb;guously from 045, ;.e. 2 ~. Thus ;f the
uncerta;nty ;n ~t is ~ t, so that the range of possible values
of the actual t;me difference is ~ t + ~ t, we obta;n from
equat;on 4
2 7r ,~ 2 7r f .2 ~) t ~ L3 /L1
or
L1/L3 ~ 2~ t f~ ~6)
Relat;onsh;p 6 def;nes the m;n;mum value of the rat;o of the ToA
system basel;ne~ L1, to the closest spac;ng of the ;nterferometer
system, L3, wh;ch w;ll enable sat;sfactory amb;gu;ty resolut;on w;th
a g;ven uncerta;nty ~ t ;n the measured time difference, at the
h;ghest frequency of operation. (The unambiguous angular coverage
of the interferometer system ;ncreases as the frequency decreases,
whereas the uncerta;nty ;n the angle measured by the ToA system ;s
;ndependent of frequency.)
The outputs of un;ts MC(TOA) and MAR(IR) are fed to a
calculat;ng un;t CALC wh;ch compares the value of 035, in ~h;ch
ambigu;ty has been resolved as far as poss;ble by reference to a45,
~80~3
25 PHB 33274
w;th an approximate but unambiguous value of ~35 derived from ~t in
accordance w;th the equat;on
~35 = 2 ~f~ t L2/L1 (7)
~hich is obta;ned by combining equations 1 and 2; the ambiguity is
resolved in kno~n manner. A representat;on of ~( ;s then calculated
from the accurate value of ~ 5 derived from g35. The accuracy of
th;s value of ~ ;s g;ven by differentiating equation 2:
~ G~= -c ~035/(2 ~fL2 s;no~ ) (8)
where ~ o( ;s the uncertainty in the calculated value of ~ and
~ 035 is the poss;ble error ;n the measured phase d;fference 035.
If, for example, ~ 035 is 30 degrees and L2 = 0.66 metres, then
from equat;on 8, S~ ;s ~25 degrees at 12 GHz and 0.5 degrees at 6
GHz~ I~ the uncertainty S t in the time difference is 2
nanoseconds, and taking L3 = 0.33 metres, then from relationship 6,
L1 should be not less than 16 metres for operat;on up to 12 GHz.
If the frequency of the RF s;gnals is not previously known~ the
compos;te To~l;nterferometer system should comprise means for
measuring the frequency.
Resolut;on of ambi~u;ty ;n the ;nterferometer system by
reference to the ToA system ;s particularly s;mple because both
systems determine the direction of incidence with reference to the
angle oC ~h;ch define a cone the axis of which is the baseline of
the respective system, the baselines of the two systems being
parallel and substant;ally co;nc;dent. ~y contrast, for example, an
amplitude-comparison DF system locates the d;rection of ;nc;dence
substant;ally in a plane normal to the plane of the main-beam axes
of the antennae, wh;ch means that such a system is not readily
compat;ble ~;th an interferometer DF system.
When the composite DF system is required only to determine the
direction of inc;dence of signals from sources on one s;de of the
common baseline, and part;cularly when sources can be assumed to be
substant;ally ;n an s;ngle plane including the common baseline,
calculation of the angle OCmay be suffic;ent to locate the
direction of incidence~ The system may in that case use directional
antennae which are relat;vely 7nsens;t;ve to s;gnals from the other
~X3~93
26 PHB 33274 '
1 s;de of the baseline. Where sources l;e substantially ;n said plane
but may be on either side of the basel;ne, omnid;rect;onal antennae
being used, this ambigu;ty may be resolved by comparing the times of
arrival of signals at the t~o antennae of the above-described ToA
system and at a third antenna coplanar but not collinear ~ith the
first two~
It may be noted from equation 8 above that the accuracy is
greatest ~hen o( = 90 degrees and decreases as o~ decreases to~ards
zero. It may therefore be desirable to use a second composite
~ system w;th a common baseline coplanar with but inclined to that of
the first system, for example at 90 degrees, to achieve improved
accuracy for small values of C~. Such an arrangement may also be
used when the direct;on of incidence is not restricted to a singLe
plane. It may then be des;red to determ;ne the angle 9 and/or the
angle ~ ~here
s;n ~ = sin ~ cos ~ (9)
where ~ is the angle between the direction of incidence and the
normal to one of the common basel;nes ;n the plane ;nclud;ng the
direct;on of ;nc;dence ~so that ~ = t90 degrees -C~ ) and
s;n ~ = cos 0C), ~ ;s the angle between the d;rection of ;nc;dence
projected ;nto the plane of the basel;nes and the normal to the
relevant basel;ne ;n that plane, and ~ ;s the angle between the
direction of inc;dence and the plane of the basel;nes. Typically,
the plane of the basel;nes ;s hor;zontal, so that ~ ;s bear;ng and
~ is elevation.
F;gure 7 ;s a schemat;c d;agram of a part;cularly s;mple
omnid;rect;onal DF system us;ng two compos;te ToA/interferometer
arrangements ~ith coplanar mutually orthogonal common baselines.
The system comprises an array of seven antennae ANT21-ANT27 ~ith
omnidirect;onal responses ;n the plane of th~ arrays, as denoted by
a c;rcular symbol. Four antennae, ANT21-ANT2~, are located at the
corners of a square; a fifth antenna, ANT2s~ ;s located at the
centre of the square, and the remain;ng antennae, ANT26 and ANT27,
are respectively d;sposed colL;nearly ~;th the d;agonaLs of the
squares~ equ;d;stant from the centre of the square. A f;rst ToA
~8~ 3
27 PHB 33274
system comprises antennae ANT26 and ANT2s, and the assoc;ated first
interferometer system compr;ses antennae ANT21, ANT2s and ANT23.
The second ToA system comprises antennae ANT27 and ANT2s, and the
assoc;ated second ;nterferometer system compr;ses antennae ANT22,
ANT2s and ANT24. The ToA systems comprise respective measurement
ar7d calculation units MC(TOA)1, MC(TOA)2 (although the units may be
integrated to the extent that onLy a single ToA measurement is
required ;n respect of antenna ANT2s), and the ;nterferometer
systems comprise respective measurement and ambigu;ty resolution
units MAR(IR)1~ MAR(IR~2. The outputs of the assoc;ated ToA and
;nterferometer systems are fed to respective calculating units
CALC1, C~LC2 which in th;s embod;ment only determine the respèctive
unambiguous phase measurements ~ 2~ referred to the widest-spaced
pair of antennae of the respective interferometer, that represent
the angles ~1~ DC2 between the direction of ;ncidence of RF signals
and the respective common baseline. Having regard to equations 2
and 9 above, one may write
~1 = 2 ~fL2 sin ~ cos ~ /c (10)
and analoguously
~2 = -2 ~fL2 cos ~ cos ~ /c~ t11)
Solving these simultaneous equations~ one obtains
tan 0 = -~ 2 t12)
cos ~ 12+~22)1/2 (c/2 ~fLz). ~13)
The unabiguous phase angles ~ 2 are supplied to a further
calculating unit CALC 3 ~hich calculates ~ and/or ~ in accordance
with the above equat;ons~
While the system of Figure 7 is particularly simpLe, it does
have the disadvantage that some of the antennae are liable to
obstruct signals to others of the antennae when the direct;on of
incidence is at only a small angle ( ~) to the plane of the antenna
array~ This ;s liable to affect the accuracy of the overall system,
since the interferometers are more susceptible to errors due, for
example, to mult;path. Thus~ when the plane ;s horizontal, the
system is best su;ted to measur;ng direct;ons of ;nc;dence at
substantial angles of elevat;ons. F;gure 8 ;llustrates an
8[)~9~
28 PHB 33274
1 alternative antenna arrangement wh;ch ;s better su;ted for small
values of ~ as well as larger values. Omn;d;rect;onal coverage ;s
;n th;s case prov;ded by four mutually perpend;cular nterferometer
systems, IR1-IR4, d;sposed about a common central po;nt and each
compr;s;ng three un;formly spaced antennae wh;ch ;n th;s case each
have a substantial response over approximately 180 degrees ;n the
plane of the antenna array (as denoted in F;gure 8 by a sem;circular
symbol). ~ach of two ToA systems aga;n compr;ses a pa;r of
omn;direct;onal antennae wh;ch ;n th;s embod;ment are d;sposed on
oppos;te s;des of the common central point. The respective
measurement and amb;guity resolut;on units of the ;nterferometers,
MAR(IR)1-MAR(IR)4~ in this embodiment compr;se means for measuring
the amplitude of the s;gnal received at at least one of the antennae
of the respect;ve array. The measured amplitudes, A1-A4, and the
amb;guous measured phases, 01-04~ (in which ambiguity has been
resolved as far as possible by reference to the closest-spaced pa;r
of antennae of the ;nterferometer) from the four ;nterferometers, as
well as the time d;fferences measured by the two ToA systems,
~t1 and ~tz, are then processed as will now be descr;bed w;th
reference to Figure ~
The amplitudes A1-A4 are suppl;ed to an ampl;tude compar;son
and select;on control un;t ACS, and the phases 01-04 are fed to a
phase selector un;t PS. The un;t ACS compares the ampl;tudes and
selects for further processing two adjacent ;nterferometers, denoted
A and B~ At one of these, the ampl;tude ;s at least as great as at
each of the rema;n;ng ;nterferometers, and at the other, the
amplitude is at least as great as the ampl;tude at the other
adjacent interferometer; the basel;nes of ;nterferometers A and B
are respectively parallel to those of the f;rst and second ToA
systems. The phase selector unit PS accordingly selects the
amb;guous phases from those two ;nterferometers, 0A and ~, and
suppl;es them to a phase calculat;ng un;t CALC0 wh;ch also rece;ves
the values of ~tl and at2 measured by the two ToA systems. The
un;t CALC0 separately resolves the ambiguity ;n 0A and ~ by
reference to ~t1 and ~t2 respectively, as explained above w;th
~Z~)493
29 PHB 33274
1 reference to Figure 1, and produces unambiguous phase angles ~A
and ~B~ These are suppl;ed to a d;rectionaL angle calculating
unit, CALC~, ~ which also receives an indication of A and B from the
unit ACS. The unit CALC0, ~ calculates the value of O and/or ~ as
explained above w;th reference to F;gure 2, also tak;ng ;nto account
;n calculat;ng ~ ~hich two ;nterferometers the phase measurements
have been derived from, so as to add an appropriate ;ntegral
mult;ple of 90 degrees.
