Note: Descriptions are shown in the official language in which they were submitted.
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REDUCING THE DRIFT RATE OF AIRBORNE GRAVITY MEASUREMENT SYSTEMS
Introduction
This invention relates to apparatus for improving the accuracy of airborne
measurement
equipment, in particular equipment used for measuring terrestrial physical
parameters for
the purpose of geological and geophysical surveys.
Background
The accuracy of airborne electronic equipment such as accelerometers is
important if
reliable data is to be obtained during measurement. Airborne surveys are
preferable to land
based surveys in many locations, particularly in rough terrain, or terrain not
well serviced by
roads and other terrestrial transport facilities. Also, where it is desired to
survey a large
geographical area, the advantages of an airborne survey are immediately
apparent in that
the area can be surveyed relatively rapidly, as compared with on the ground
surveys.
However, any survey needs to have an acceptable level of accuracy in order to
be useful.
Airborne surveys, whilst being advantageous for the reasons outlined above,
are complex to
undertake and yet still achieve an acceptable level of accuracy because the
platform (in
other words the aircraft) on which the measurement equipment is mounted is not
stable.
The aircraft is in continuous motion, accelerates and decelerates, changes
altitude, and so
forth. Also, ambient conditions such as wind, temperature and moisture vary as
the aircraft
moves over the terrain being surveyed. A typical survey might, for example,
last a number of
hours, and the temperature variation from the start of a flight to the end of
the flight might
vary significantly, particularly in inhospitable areas where many such surveys
are conducted.
When ambient atmospheric conditions in which an aircraft is operating vary
over the course
of a day, or from day to day, the measurements obtained by the equipment on
board that
aircraft may drift, that is, two measurements taken at the same location, but
spaced apart in
time, will vary. Measurement drift is an acknowledged problem, and there have
been
various suggested solutions to counteract the problem. For example, US Patent
Application
No 2008/0092653 deals with reducing or eliminating the effect of water vapour
on relative
gravimeters.
Drift is a complex problem to deal with in that drift is often not linear
where a particular
ambient parameter has changed over the course of time. It is also not
particularly easy to
determine whether or not drift has in fact taken place, or the degree of drift
that has
occurred. In many survey situations, there is no absolute reference point for
the equipment,
so whilst the equipment would appear to be operating perfectly, the fact that
drift has
occurred is not apparent, and neither is the degree of drift. In some surveys
the survey team
may attempt to estimate drift by flying, at the start of a survey session,
over a particular
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location on the ground, take measurements, and then at the end of the survey
fly over the
same location, take the same measurements, and then the difference will be the
drift.
However, this method of correcting for drift is only really applicable for
short duration
surveys, say 30 minutes or less. That is not satisfactory when a survey is due
to last for a
much longer period, say 5 hours. Also, as mentioned above, drift is not
necessarily linear,
and therefore the scaled correction may in fact alter some measurements much
more than
required, and alter others far less than required.
It is an object of the present invention to provide means for improving the
accuracy of
airborne survey equipment.
Summary of the Invention
According to the invention there is provided a temperature control device
comprising: a
thermally insulated housing for mounting to an airborne vehicle and within
which electronic
survey equipment is located in use, and temperature control means for
maintaining the
temperature within the housing at a selected value.
Preferably the housing has control apparatus mounted thereto for selecting the
temperature within the housing. The temperature control means may be capable
of raising
and/or lowering the temperature within the housing relative to ambient
temperature.
Preferably the temperature within the housing is maintained at a value of
between 10 C and
25 C, at the option of the operator thereof.
The electronic survey equipment may be a gravity meter, and the airborne
vehicle may be a
fixed wing aircraft or a helicopter.
These and further features of the invention will be apparent from the
description of a
preferred embodiment thereof, given below by way of example. In the
description
reference is made to the accompanying drawings, but the specific features
shown in the
drawings should not be construed as limiting on the ambit of the invention.
Brief Description of the Drawings
Fig 1 shows a schematic illustration of a fixed wing aircraft used for
geological survey which
is fitted with a temperature control device according to the invention;
Fig 2 shows a cross-sectional side view through a temperature control device
according to
the invention;
Fig 3 shows a perspective view of a gravity meter of the type to be fitted
within the
temperature control device;
Fig 4 is a graph showing the difference between the pre and post flight
readings; and
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Figs 5 to 7 show graphs indicating the improved results obtained by keeping
the equipment
at constant temperature within a temperature controlled cabinet according to
the
invention.
Detailed Description of a Preferred Embodiment
As shown in Figure 1, a fixed wing aircraft 10 is depicted which has sensing
equipment 12 on
board. Typically, a fixed wing aircraft or helicopter carrying out geological
surveys will have a
range of different sensing equipment on board so multiple aspects of the
terrain over which
the aircraft is traversing can be surveyed simultaneously. One of the items of
survey
equipment is a gravity meter 16 which is carried on board the helicopter
within a
temperature control cabinet 18, diagrammatically shown in Figure 1, but shown
in more
detail in Figure 2.
As shown in Figure 2, the temperature control cabinet 18 has an internal
volume of
approximately 100 to 150 litres. The cabinet 18 is essentially a sealed unit
formed of
thermally insulated walls 20, which may conveniently be formed of inner and
outer skins
comprised of Kevlar, or similar high strength materials having a suitable high
quality
insulation material 21 sandwiched between them. The gravity meter 16 is shown
in Figure 2
by dotted lines, spaced away from the walls 20.
The cabinet 18 includes an inspection port 22 which is openable to provide
access to the
gravity meter 16. The cabinet is mounted on an aluminium base 24 which in use
is mounted
to the floor of the aircraft 10.
Refrigeration is provided to the interior of the cabinet by means of a
refrigeration unit 26.
The refrigeration unit 26 comprises a compressor unit 28, a condenser 30, and
an
evaporator 32. The refrigeration unit is essentially mounted to the outer wall
of the cabinet
18, and cooling air is pumped into the interior of the cabinet by means of
fans 33 through
ports 34. The compressor and condenser selected for the operational unit was a
WAECO
Cold Machine 94 unit, coupled to a WAECO VD-16 evaporator unit. The power
inputs
required for these two units are approximately 60 watts respectively. The Cold
Machine 94
unit comprises a fully hermetic high-performance compressor with AEO
electronics and
integrated low-voltage protection, low-voltage cut-off, a brushless DC fan,
fin condenser,
and self-sealing valve couplings. These units are well capable of maintaining
the
temperature of a 150 litre insulated cabinet at the temperature required for
operation of
the invention.
Temperature sensors 36 monitor the interior temperature of the cabinet, and
are used to
control the refrigeration unit to ensure the temperature of the interior of
the cabinet is kept
at a selected value. A temperature selection switch 38 is used by the operator
to select the
temperature at which the interior of the cabinet is to be maintained. That
temperature will
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typically be about 15 C. The temperature in the cabinet can be read from a
temperature
gauge 37.
The refrigeration unit will typically be powered, either directly or
indirectly, using the
aircraft power supply system.
The invention is not limited to use with gravity meters, or gravity meters 16
of the type
shown in Figure 3. The gravity meter shown in Figure 3 is illustrative of the
type of
equipment that could benefit from the invention, and the following description
illustrates
the improvement in survey results that use of the invention is able to
achieve.
Traditionally the drift allowed in a gravity survey or survey specification is
less than 0.5mGal
per hour. This value was recognised as typical because this is the sort of
drift one can
realistically expect from most of the gravity systems, although some airborne
gravity
manufactures quote more accurate results, these results would have been
obtained under
ideal conditions, that is, constant temperature and movement conditions in a
laboratory.
The GT systems of the type illustrated in Figure 3 have four thermally
insulated chambers
nested one within the other. The ambient temperature however seeps through the
four
chambers affecting the various components. Each component behaves differently
as the
temperature drifts. For example the gyro required to maintain platform
stability has a
certain drift with temperature change, the accelerometers used to measure the
platform
accelerations drift with temperature change, and obviously the most sensitive
unit, the
actual gravity sensor drifts with temperature change.
In practice, if drift exceeds a predefined specification then the complete
flight will be
rejected. However, it can be difficult to even estimate whether drift has
occurred, and if so,
what the extent of that drift has been.
The drift may theoretically be calculated by doing a pre-run data collection
flight (20
minutes) and then a post run data collection flight (20 minutes) with the
aircraft on an
identical spot (marked out on the ground with tape or paint). The drift is
then applied in a
linear fashion over the complete flight. As mentioned previously, this drift
compensation is
really only accurate for short flights 30 minutes to 1 hour which are not cost
effective. Also
the drift is not linear in reality and over longer flights up to 5 hours,
although the total
estimated drift at the end of the flight could be with in 0.5mGal per hour
limit, it is the
variable drift due to ambient temperature change that is the problem. There
are other
means of trying to estimate this drift, such as looking at cross-over
intersections between
traverse and control lines (tie lines) in conjunction with the linear drift.
This method is also
unsatisfactory because there could be noise due to other external forces such
as turbulence
or flight altitude or direction change.
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Figure 4 is a graph that shows the difference between the pre and post flight
readings for
seven flights with the refrigeration cabinet 16 installed and seven flights
without the cabinet
16 installed, the sample flights chosen was with similar ambient temperature
fluctuations.
The graph depicts the total drift for a flight. The results with the cabinet
installed are far
more constant and predictable than those without the cabinet installed. As
described above
it is very difficult to define or estimate the total drift on a system, with
the refrigerated
cabinet installed the drift is far more constant and definable.
All current airborne gravimeters rely on a level platform to which the gravity
measuring
device is attached. The system normally consists of a three axis gyro
stabilized inertial
platform the gravity sensing unit is held level within 10 arc seconds by a
"Schuler Tuned
Inertial Platform" it makes use of accelerometers and gyroscopes and a number
of complex
feedback loops.
A maximum angle in the X axis or Y axis of 4.5 arc minutes or 0.075 will
result in an error
reading of approximately 1 mGal. In real time while on line and collecting
data errors are as
high as 1 to 2 Arc minutes but are normally post processed out to the required
accuracy of
arc seconds (0.002777777778').
As the accuracy of the platform depends on various sub-systems of the Schuler
tuned
platform system as shown in Figure 3
1. Torque Motors X and Y TM x, y axis
2. DTG Dynamically Tuned Gyro
3. ASx,y,z Angular sensor X, Y, Z axis
4. AC x, y, z Horizontal Accelerometers
5. FOG Fibre Optic Gyro z axis
All of these different items of equipment are in turn affected by temperature
changes. If all
items are maintained at a constant temperature the gravimeter will operate to
a far greater
degree of accuracy.
The inner most chamber houses the gravity sensing unit (GSU) see Figure 3. The
GSU is in a
hermetically sealed container thus it cannot dissipate heat easily and is most
affected by
temperature changes (12mGal/Degree C) thus the ambient temperature play a
large role on
the stability of the GSU. The inner most heating chamber is kept at a constant
temperature
of 60 C.
Prior to post flight processing the data interpreter will typically use a flag
of 2 arc minutes or
0.0333334684 to determine the accuracy of the platform during flight.
After post flight processing the data interpreter will typically set a flag at
5 arc seconds or
0.0013888889 as a pass indicator or an indicator of good stable platform and
thus good
gravity data.
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The applicant extracted the results of the flag in the X and Y axis prior to
post processing on
a line by line basis for 52 lines from two projects, one with the refrigerated
cabinet installed
and one without the refrigerated cabinet installed. The mean of the standard
deviations
down the line as well as the mean of all 52 lines in the X and Y axes were
plotted against
each other to show the improvement to the misalignment of the platform when
the
refrigerated cabinet was installed. The improvement was approximately 22%
across the
board.
Figure 7 shows the effects of ambient temperature variations on the GSU. The
profile above
shows the change in ambient temperature (top curve, Celsius), cause a very
small change in
temperature in the inner GSU chamber (middle curve, Celsius) this small change
of
temperature cause significant change in the output reading in mGal of the
Gravity Meter
(lower curve).
The GT system operating range is between -10 to +40 degree's. Surveys are
often carried
out in areas where the ambient is above 40 degree (ambient in the present case
is the cabin
of the aircraft, and this heats up much faster than the outside air
temperature).
The graphs shown in Figures 6 to 8 clearly show the improved results obtained
by
maintaining the equipment in a refrigerated temperature controlled environment
within the
aircraft.
Clearly the cabinet does not need to be in the form indicated in the drawings,
and the
refrigeration unit could be completely separate from the cabinet, with cooled
air piped into
the cabinet, for example.
