gravimeter output. This term, known as the Browne correction, has not been applied
completly in the airborne measurements reported to date. Stabilization along the
apparent vertical also places a greater load on any gravimeter output filtering scheme
due to the presence of components of short term horizontal acceleration in the
gravimeter output.
An airborne gravimetry system may be thought of as the instrumentation of a
single dynamic equation, relating the outputs of the required subsystem to the indicated
gravity anomaly. As this equation shows, the indicated gravity anomalies are obtained
by compensating the output of a specific force sensor (gravimeter) which is stabilized
along a vertical or apparent vertical axis. Four types of compensation term appear in
equation, 1) vertical accelerations of the aircraft, 2) Coriolis and centrifugal force
corrections, 3) free air gravity reduction terms, and 4) the computed reference value of
gravity at sea level. If an apparent vertical stabilization system is used, the Browne
correction must also be applied. All but the first of these compensation terms can be
easily computed from the outputs of the previously specified subsystems. The first
term, aircraft vertical acceleration, is more difficult to deal with, because it cannot be
measured directly due to the indicting - usability of gravitational and inertial
accelerations. There remains the possibility of double differentiation of altitude data,
separation by filtering and combination of these techniques, all of which will be
considered.
Compensation error due a given velocity measurement error varies with both
aircraft heading and latitude, the minimum sensitivity for any latitude occurring on a
due west heading.
For a given specific force sensor uncertainty, the minimum system uncertainty
results when the sensor is physically stabilized along the z axis (vertical axis) of an
instrumented local geographic coordinate frame. Errors in the 2 axis alignment of such
a frame result in 1.20 mGl error for each arc minute of misalignment due to projection
of horizontal Coriolis forces along the measurement axis, and a smaller second order
error which reaches 0,4 mGl at 3 arc-minutes verticality error.
We see that an airborne
gravimetric
system capable of measurement accuracy of
the order 3 mGl, must be capable of nominal subsystem accuracies as follows
velocity
no heading restriction
0,18 knot
no westerly headings
0,4 knot
latitude
0,5 mile
verticality
1 arc minute
sea-level altitude
10 feet
specific force measurement
1 mGal
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