Prof. Bezvesilnaya E. N., Tkachuk A. G.

National technical university of Ukraine “Kyiv Polytechnic Institute”,

Zhytomyr State Technological University, Ukraine

THE STRUCTURE AND CHARACTERISTICS OF

AIRBORNE GRAVIMETRIC SYSTEM

The indication of gravity anomalies from aircraft requires a combination of several instrumentation components, each of which is designed for the role of measurement or signal processing. The aggregate assemblage of these components constitutes an airborne gravimetric system. Subsets of this assemblage of components which relate system outputs to inputs will be termed the subsystems of the airborne gravimetric system. The present task is therefore to determine the number, function, and accuracy of the subsystems which make up an airborne gravimetric system.

The airborne gravimetric system consists of five functional subsystems for:

Å specific force measurement;

Å geometric stabilization;

Å terrestrial navigation;

Å altimetry;

Å computation.

In determining the accuracy required of such a system we have to recall that the only use for global gravity data is the computation of geoids’ heights and deflections of the vertical. Overall system accuracy should then be evaluated in terms of the resulting accuracy in these computations. Measurement accuracies on the order of ±1 to 3 mGal may ultimately be required.

In order to carry out a gravity survey from a moving vehicle, some means of stabilizing the gravimeter along a reference direction is required. Since it is ultimately necessary to deduce the specific force in the direction of the local geographic vertical, the direct instrumentation of the vertical provides the most desirable measurement environment. Instrumentation of the vertical on a moving base requires however, a rather complex subsystem using grade inertial components, and involves real time computation using precise navigation data. The drawbacks of complexity are reduced somewhat by the fact that such a stabilization system can also serve as the heart of a geographic inertial navigator.

As an alternate to stabilization along the vertical, the gravimeter may be allowed to track the apparent vertical, provided the proper compensation term is added to* the gravimeter output. This term, known as the Browne correction, has not been applied completely 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 gravimetric 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, sometimes called ethos 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 indistinguishability 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 mGal 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 mGal at 3 arc-minutes verticality error.

The airborne gravimetric system capable of measurement accuracy of the order 3 mGal, 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

LITERATURE:

1. Bezvesіl'na O.M., 2001, Vy`miryuvannya pry`skoren. Kyiv, Ly`bid.

2. Samotokin B.B., Meleshko V.V., Stepankovskiy Yu.V., 1986, Navigatsionnye pribory i sistemy. Kiev, Vysshaya shkola.

3. Bezvesіl'na O.M., Tkachuk A.G., 2013, P’єzoelektrichnij gravіmetr avіacіjnoї gravіmetrichnoї sistemi: monografіja, Zhitomir, ZSTU.

4. Bezvesilnaya E.N. Gravimeter of aviation gravimetric system / E.N. Bezvesilnaya, А.G. Tkachuk, K.S. Kozko // The advanced science journal (USA). – 2013. – №4. – P. 41–46.