the wild d13s distomat - Krcmar Surveyors Ltd.

TECHNICAL REPORT
THE WILD
D13S
DISTOMAT
BY T. P. JONES, O.L.S.
The above instrum ent is an electrooptical distance m easuring device and a
com puterised control unit. It is a product
of W ild H eerbrugg of H eerbrugg, Switzer­
land and the Societe d ’Etudes Recherches
et C onstructions E lectroniques (SER C EL)
of N antes, France. It is distributed in
this country by W ild-Leitz C anada Ltd.,
and N orm an W ade Co. Ltd.
The hollow axis of the control unit
fits in any Wild tribrach; the top of it
takes a W ild T l, T 16 or T2 theodolite.
The distancer, with counterw eight, fas­
tens to the telescope of the theodolite.
The telescopes of the T l and T 16 will
transit with this attachm ent.
The distancer m easures a distance
completely autom atically to 2 000 metres
in approxim ately ten seconds. Signal
balancing and calibration are taken care
of by the control unit. Its accuracy is
claim ed to be plus or minus 5mm plus
5ppm. D istances may be displayed in
feet or metres at the discretion of the
operator.
The control unit, besides m onitor­
ing the m easuring cycle, com putes hori­
zontal distances, height differences and
co-ordinate differences (latitudes and de­
partures) from the keyed-in data. Angles
may be keyed into the com puter in the
sexagesimal or centesim al systems.
W hat follows is a short description
of the electronics in the E .D .M ., and
some rem arks on such instrum ents in
general.
When such a subject arises, the first
thought that occurs is “T o w hat extent
should I, a land surveyor, qualify myself
in the field of m iniature circuitry and
solid state electronics?”
I have used com plicated optical
instrum ents for years, and have never
felt the lack of an ability to grind out a
new objective lens for a first-order theo­
dolite. In the unlikely event that I will
ever need to replace such a lens, I will
buy one from an expert lens m aker, and
probably have it installed and aligned
8
by another expert under laboratory con­
ditions. I can recognise m alfunctions in
my optical instrum ents, and can identify
non-working parts which I can repair
myself, or have repaired by others, as
the circum stances dictate.
I think the same philosophy should
govern our abilities with E.D .M . equip­
ment. I do not believe th at we should go
to the extreme quoted by B om ford in
“G eodesy” that ‘geodesy is regarded as
beginning when the waves leave the in­
strum ent and ceasing when they retu rn .’
But by the same token, we should not
expect ourselves or newly qualified m em ­
bers of our A ssociation to be on a par
with graduate electronic engineers.
ELECTRO-M AGNETIC W AVES
The basis of the electronic distancer
of the DI3S, as most others, is the speed
of propagation of electro-m agnetic waves.
Light can be said to be a m ixture of
electro-m agnetic waves of slightly dif­
ferent wave lengths.
The speed of these waves in vacuum
is know n to w ithin very precise limits.
In air, their speed is affected by baro­
m etric pressure, tem perature an d hum idi­
ty. Some waves are affected m ore by one
factor than another.
T he availability of w avelengths for
distance m easurem ent is restricted some­
w hat to two bands or “w indow s” ; those
from 0.3 m icrom etre to approxim ately
one m icrom etre (light waves and infra-red
waves in the electro-optical instrum ents)
and from about one to ten centim etres
(so-called radio waves in the micro-wave
instrum ents).
In these regions no strong absorp­
tion bands exist. E xtrem ely short waves,
such as gam m a rays or x-rays, cannot
be used because they are so readily ab-
THE ONTARIO LAND SURVEYOR, SUMMER 1977
sorbed by the atm osphere, and
would be too short. Longer
too difficult to focus, and
because of the divergence of
cause problems.
their range
waves are
reflections,
the beam,
The advantages of the micro-waves
are their long range, and their ability to
penetrate fog and haze. They are, how ­
ever, more susceptible to tem perature
and humidity than the shorter light waves,
and cannot give results to the sam e de­
gree of accuracy over shorter distances.
The light-waves are not so depen­
dent upon tem perature and hum idity,
have a very high accuracy over short
distances and their transm itters consum e
little power. They do not penetrate fog
and haze too well. This disadvantage is
being overcome with the introduction of
lasers; the G eodim eter 8, for example,
could m easure up to 40 miles.
The D I3S has a carrier wave in
the infra-red range, having a wavelength
of 885 nanom etres. This is practically
a light wave, of course, and as such is
ridiculously short - about one hundred
thousandth of a m illim etre. By itself
it cannot be used to m easure anything.
In the D I3S the opportunity has
been taken to provide an eleven position
“scale factor” switch to vary this variable
pow er source (see block diagram ). T he
atm ospheric corrections, sea level cor­
rection, and even the projection scale
factor, if it is not too big, can be applied
autom atically to the distance. G raphs
for using the switch are provided by the
Com pany.
T he m easuring frequencies, there­
fore, are transm itted to the am plifier
which supplies the exciting voltage for
the G aA s diode.
T he radiation is bundled, or focused,
by a 35 m m objective, reflected by a
specially designed ‘prism -target’ com bina­
tion at the far end of the line, and at
the receiver, focused onto an avalanche
detector by another 35 m m objective.
T he wavelength chosen by the designer-m anufacturer for any particular
instrum ent depends to some extent upon
w hat sort of w ork the instrum ent will
be required to do, its accuracy, its range,
and of course the state of the science
of electronics at the tim e it is designed.
T he same econom ic principles apply for
distance m easuring instrum ents as hither­
to applied for angle m easuring instru­
m ents. D isregard of these principles will
result in an inefficient and expensive
hybrid.
The approxim ate m odulation fre­
quencies of some instrum ents are listed
below (one m egaH ertz indicates a fre­
quency of one million cycles per second) >
The m odulation wave is of a m uch
lower frequency, and consequently of a
much greater length. It is the actual
m easuring frequency.
500 M H z
M ekom eter ____
W ild D I60 ___________ 150 M H z
Tellurom eter M R A -4 ___ 75 M H z
G eodim eter 8 __________ 30 M H z
Kern D M 500 __________ 15 M H z
Reg E lta 1 4 _____________ 15 M H z
W ild D I10 _____________ 15 M H z
W ild D I3S _____________ 7.5 M H z
There is no definite position on
this radiation that can be noted as it
leaves the transm itter, and noted again
as it is received at the detector. T he
radiation has to be m arked with a signal,
or m odulated.
THE M O DU LATIO N W AVELENG TH
This can be done in several ways.
By pulsing, or interrupting, the carrier
wave, by m odulating the frequency of
the carrier wave, or by m odulating the
am plitude of the carrier wave. The last
one is m ainly used, gives the best results,
and is used by the DI3S.
CAQR.1EQ
W AVE:
CAQfZJER
A M P C -I T u p E .
P IG
M O O U L /^ T E -O
I
T he carrier wave can be m odulated
indirectly or directly. It can first be
generated and then m odulated afterw ards,
just before it leaves the instrum ent, i.e.
indirect, or the m odulation frequency can
be applied directly to the radiation, in
this case the gallium arsenide diode
generating the infra-red waves.
THE ONTARIO LAND SURVEYOR, SUMMER 1977
DI3S
BLOCK
DIAGRAM
9
N ote the high frequency used by
the M ekom eter. This is an extrem ely
accurate instrum ent (one fifth of a milli­
m etre - 0.2 - plus or minus one part per
million up to a m axim um range of three
kilom etres) developed by the N ational
Physical L aboratory in London, England
and m anufactured by Kern, of A arau,
Switzerland. It is used for laying out
nuclear m achines in pow er stations and
things like that. The Surveys and M ap­
ping Branch of our Federal D epartm ent
of Energy, Mines and Resources use it
for m easuring their series of base lines
across the country. A separate report
will be found in this issue on the installa­
tion, m easurem ent, use and location of
a reference line in M etro T oronto by
R alph Smith, O.L.S.
F or a detailed description of the
M ekom eter, please refer to the July, 1968
issue of our O .L.S. Q uarterly.
The lengths of the m odulation
waves produced vary indirectly with the
frequencies. A ccording to the definition
of propagation, this is the product of
w avelength and frequency, which is
300 000 k m /sec. approx. T he wavelength
used in the DI3S, therefore, if you haven’t
figured it out already, is 40 metres.
T he com pensation, or nulling, m eth­
od, using a phase detector is another.
T he null balancing between the alter­
nating current of the em itted and that
of the reflected, m easuring wave is ob­
served on a galvanom eter and the value
of their m utual phase shift is read, usually
on a three-digit counter. T he TeUurom eter M R A -3 and the M odel 6 G eodi­
m eter are examples.
Phase
angle
Fig. 3
Figure 3 shows the situation with
the reflector set 16 m etres in front of
the instrum ent, and the corresponding
phase angle. Figure 4 has the reflector
at 36 m etres in front of the instrum ent.
The corresponding phase angle, of course,
is identical with that in Figure 3. F o r
longer lines, unam biguous determ ination
of distance requires an additional m eas­
urem ent with another frequency. This
second, or. course, frequency today is
usually chosen to be one hundredth of
the fine frequency. In the case of the
DI3S, this is 74.927 kH z, producing a
wavelength of 4 000 metres, and a “m eas­
uring scale” of 2 000 metres.
Phase
angle
Fig. 4
The business of com paring the
phase of the em itted, or reference, signal
with that of the reflected, or measuring,
signal has been attained in several ways in
the past.
Fig.
2
Rem em bering that as the light travels
the distance to be m easured twice between
the transm itter and the detector (both
located in the sam e instrum ent) via
the reflector at the end of the line, the
“measuring scale” for the single distance
is only one half of the m odulation wave­
length, in this case 20 metres.
D istance in m ost E .D .M .’s is deter­
m ined by com paring the phase of the
em itted light with that of the reflected
light. Owing to the periodicity of the
phase, if the fine m easuring frequency
alone were used, an unam biguous result
w ould be obtainable only up to 20 m etres
with the DI3S.
10
One is the direct reading of the
phase angle on a circle of a cathode
ray tube graduated in either units of
seconds of the wave travelling tim e or
in m etric units of the wavelength. T he
pioneering Tellurom eter models M R A -1
and M R A -2 are examples of this type
of equipm ent.
SI P R E F IX E S
P refix
M u lt ip ly in g Factor
1 0<)() 000 000
1 000 000
1 0 00
1
0
0 000
0.0 0 0 0 00
0.000 0 00 000
000 000
000 ooo
(KM) 000
0(H) 00 0
1 000
1
000
000
000
000
ooo
000
100
10
0.1
0 01
0 001
0.000 001
0 00 000 001
000 000 001
000 000 (MM
000 000 001
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=
=
=
=
=
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=
=
=
=
=
=
=
I 0 ,H
10IS
I 0 '2
10f*
|0«
10s
I0 2
10'
10’
10 2
10
10*
10
10 12
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1 0 '"
1
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T he industry is now com ing around
very quickly to autom atic electronic m eas­
urem ent, in w hich the phase shift is m eas­
ured digitally in w hat is called a phase­
m eter.
A ssum ing th at the fine m easurem ent
series in the D I3S is in progress, then
two signals are at the disposition of
the internal electronics in the instrum ent.
O ne has the phase-position of the refer­
ence signal at its departure, and the
other one the phase-position of the m eas­
uring signal at its return.
Even though the frequency of these
signals at 7 492 700 cycles per second
its drastically low er th an th at of the
frequency of the carrier wave, it is still
a bit too fast to m easure the phase dif­
ference w ithout some m odification. So
the two signals are transposed into a low­
er frequency range. This is accom plished
by mixing the tw o signals w ith the fre­
quency of a V C X O (voltage controlled
crystal oscillator) which is 1544.89 H z
higher than the m easuring frequency,
and then filtering out the m easuring
frequency.
T he figures go like this:m easuring frequency = 7 492 700 Hz
V C X O F = 7 494 244.89 H z
low frequency = 1 544.89 H z
and to be m entioned in a m om ent,
counting frequency = 15 448 90 0 Hz
W hile this transposition is going on,
the difference in phase between the two
signals is fully preserved.
T he two low frequencies are now
applied to the phasem eter. W hen the
reference wave passes through zero vol­
tage, a gate is opened and a counter
begins to run, counting the impulses of
the above counting frequency, until the
gate is closed again by th e zero passage
of the incom ing wave.
A m ore detailed description of this
function, with diagram , will be found
with the account of the Zeiss E lta 14
in the Spring ’77 issue of the Quarterly.
Y ou will see th at the choice of these
frequencies enables a phase cycle to be
divided into ten thousand parts. Thus
each pulse represents 2 mm as the fine
m easurem ent is being carried out.
A counter chain then adds up the
value of one thousand phase measure-
THE ONTARIO LAND SURVEYOR, SUMMER 1977
ments of this type, and at the same time
averages them out. This averaging pro­
cess reduces the scatter of individual
measurements due to signal noise, or
interference.
Five hundred coarse m easurem ents
follow the fine m easuring procedure.
This resolution of the phase angle
to such a fine degree is a fairly significant
step forward in the developm ent of these
instruments. To my knowledge, the pre­
vious best phase resolution was to one
thousand. W ith a “m easuring scale” of
ten metres, this yielded one centim etre
for each unit of m easurem ent.
W ith this advanced capability, SERCEL accepted the unit of 2 mm, and a
“measuring scale” of 20 metres. Thus
the D I3S enjoys not only a ‘finer’ m eas­
uring system, but with no change in the
rest of the circuitry doubles the unam big­
uous m easurem ent distance from the usual
1 000 m etres to 2 000 metres.
The whole distance-m easuring pro­
cess, from pressing the start switch to
reading the slope distance on the L .E .D .
display, takes about ten seconds. The
C om pany suggests that during this time
the instrum entm an, instead of standing
around adm iring the scenery, reads the
horizontal and vertical circles of w hatever
theodolite is being used.
All data, w hether read from the
display or the theodolite or com puted on
the control unit, m ust be recorded by
hand.
T oday we hear a lot about recording
the m easured data. The m ain argum ent
of the advocates of autom atic recording
is that it elim inates reading and booking
errors that we surveyors are alleged to
m ake all the time. However, it is inter­
esting to note from the recent experiences
of com puter centres, which handle both
handw ritten notes and autom atically re­
corded field data, that the form er m eth­
ods of reading and booking are surpris­
ingly reliable. It has even been establish­
ed that the level of errors in surveys
undertaken with self-recording instru­
ments is definitely higher than with the
form er m ethods.
A utom atic recording is supposed to
lighten the task of the instrum entm an,
because it is only necessary to point
and then trigger the m easuring/recording
cycle. H ow ever, before pressing the trig­
ger, the operator m ust set in an address
code th at has to be selected from a list
containing up to fifty code numbers.
These cover both station and target iden­
tification as well as being the key to
control the subsequent com putation in
the com puter centre.
If the autom ated system is to w ork
efficiently, it is essential that the entry
of code num bers, which m ay sometimes
run up to twelve figures, is done very
carefully. This requires far m ore con­
centration on the p art of the observer
than merely reading the instrum ent, and
is certainly not easy under field condi­
tions, especially in bad w eather or in
heavy traffic.
A utom ated systems, therefore, would
seem to be m ore suitable for those sur­
veys in which hundreds of detail points
of m ore or less the same type can be
picked up from only a few instrum ent
stations, so that the feeding-in of coded
inform ation becomes routine.
Technical data for the foregoing was
supplied by Messrs. Doug Peden, W ildLeitz C anada L td., and D ave Butler,
K ern Instrum ents of Canada, Ltd., and
the diagram s by R oger G rant, Surveys
and M apping, City H all, Ottawa. T heir
assistance is greatly appreciated.