TECHNICAL NOTES - CT1

The RMS INSTRUMENTS' Automatic Aeromagnetic Digital Compensator, model AADC, has been in active use since 1983. Continuing research and development in this and other areas has resulted in the AADCII, a unique, extremely productive and reliable instrument.

Although the AADCII has proven its superiority worldwide, in many different installations, the careful geophysicist will always wish to evaluate all the alternatives available. When doing so, some fundamental aspects must not be overlooked.

The most widely used compensation model requires that in order to arrive at the optimum solution, all of the flight data collected during the calibration period be obtained in one complete maneuver. This means the data collected from all four headings including the turns, must be used. Anything less than this will result in a sub-optimal solution and poor compensation.

A system that requires that one land and use a separate program (and perhaps an additional computer) in order to arrive at a solution, can hardly be considered 'automatic'. If calibration data has been collected in separate segments, i.e., individual headings, DC level shifts will occur with different headings, and correction factors will have to be entered manually - once again, hardly automatic compensation! Furthermore, a solution arrived at in this manner will be less than optimum, and not very robust: performance will vary with changes in location, and will lack consistency when trying to achieve the same result over a fixed location when approached from different headings. In an attempt to overcome these problems, some systems use an altimeter term in the solution, and compensate for the vertical gradient during calibration; this, however, is practically meaningless unless the vertical gradient is being measured with a true vertical gradiometer system when on survey. The approach has in fact been abandoned by many systems, including for example, the U.S. Navy.

The RMS INSTRUMENTS' AADCII Automatic Compensator provides an extremely robust solution immediately after a 6 to 8 minute calibration flight. In addition to the sophisticated compensation algorithm, the front-end counter uses a 100MHz crystal that provides a very stable, accurate, and high resolution time base over a wide temperature range. The 3-axis fluxgate magnetometer provides a very precise position reference; signals are processed using a high resolution (16-bit) A/D converter, together with oversampling and anti-aliasing techniques.

The AADCII is truly automatic - a solution is available immediately after the calibration maneuvers are completed. There is no need for any kind of post-flight software. The system uses the full 360º pattern to obtain a robust solution. If necessary, in the event full 360º signal acquisition is not possible, the AADCII allows calibration for each active zone, and a corresponding solution. In addition, all of these 'partial' calibrations can then be combined to produce a single solution. In any event, there will always be a solution available, automatically, whenever the sensor is in its active zone.

In 1990, RMS INSTRUMENTS produced the world's first four-sensor airborne real-time compensator. The following year we were able to compensate a helicopter in real-time, also with great success - another world's first!

For performance evaluation, the AADCII provides industry standard measures such as Improvement Ratio, and Standard Deviation (of compensated and uncompensated data). The Figure of Merit (FOM) is readily obtained from the real-time chart, as produced on the GR33A Chart Recorder.

The AADCII remains a unique instrument, capable of satisfying the very demanding requirements of automatic aeromagnetic real-time compensation in geophysical exploration.

AEROMAGNETIC COMPENSATION SUMMARY

When a magnetometer is mounted on an aircraft in a fixed or strapped down manner such as in a tail stinger, disturbances to the magnetic field are introduced by the aircraft itself maneuvering in the earth's magnetic field which will interfere with the readings of the sensor. This interference can be compensated for.

The main sources of interference are;

• the permanent and induced magnetism of the aircraft,
• magnetic fields from the aircraft electrical system,
• Eddy currents due to the conductive aircraft itself maneuvering through the earth's magnetic field.

• limited operating angle,
• orientation angle error,
• a heading error caused by any magnetic inclusions within the sensor itself.

The magnetic interference caused by the aircraft maneuvering in the earth's magnetic field is significant and must be removed from the data otherwise false anomalies will occur and a distorted presentation will result. This interference is not constant and varies with maneuvering on each heading.

Aircraft that are selected for magnetometer work are generally magnetically 'cleaned' before being used. A magnetometer survey is performed on the aircraft itself giving a magnetic profile and is used as an aid in placing the sensor. Depending on the severity of the interference, certain magnetic bolts, control cables and iron ballast may have to be replaced with non-magnetic materials to eliminate or reduce their effects on the sensor.

Certain effects cannot be compensated at this time, these include variable magnetic fields caused by the electrical system such as the change of current caused by the cycling of heaters etc. or the movement of control surfaces. The magnetometer sensor is placed as far away from the aircraft as possible to minimize these effects.

Interference caused by propellers or rotor blades generally have a frequency content that can be filtered out. However one should be aware of the possibility of a frequency component being folded down or aliased into the frequency range used, typically 0 to 0.9Hz.

Compensation can be very effectively performed in real time with the use of the RMS Instruments' AADCII automatic digital compensator. It can also be done in post flight processing but this is not as efficient as performing it in real time automatically. The AADCII is an excellent quality control tool providing data immediately for verification of the desired survey specifications. This eliminates the delay encountered with post flight compensation. The system provides both the compensated and uncompensated data.

In order to accomplish compensation, it is necessary to know the attitude of the aircraft within the earth's magnetic field at all times. A 3-axis fluxgate magnetometer is used for this purpose. Although the sensitivity of the fluxgate magnetometer is far less than the main sensor, it is still mounted at a location where interference from the aircraft is minimal.

The RMS Instruments' AADCII console contains all of the functional subsystems required;

• Front end precision counter and processor.
• The Larmor frequency from the magnetometer sensor electronics is applied to the AADCII console where it is oversampled, counted and processed. The data is output at 0.1 second intervals from the front end which is capable of an accuracy of 0.001 nT.
• Fluxgate input section and its processor.
• The above data is passed to the main system processor where the compensation is performed.
• The compensated and uncompensated data are available at this point for recording and monitoring.

The optimum solution will be obtained when the sensor is oriented such that a signal is present at all times over a 360 degree pattern. When this condition exists, then all interfering frequencies in the bandwidth from DC to 0.9Hz, 1.8Hz, or 3.3Hz are automatically compensated (the desired bandwidth is user selectable). This includes any inherent heading error that may be present in the sensor itself. The calibration maneuvers consist of a series of rolls, pitches and yaws in each of the cardinal headings combined with banks in the turns. These maneuvers can be completed in 6-8 minutes with the AADCII providing a solution immediately as well as statistical results for the solution. The statistics give a measure of the quality of the solution by providing the standard deviation of the uncompensated and compensated data (over approx. 4000 points), their ratio, which is referred to as the Improvement Ratio (IR), and a value indicating the degree of difficulty at arriving at the solution. Generally, the same flight pattern is repeated in the COMPENSATION or run mode and the compensated and uncompensated data can be compared immediately for the results. The system is ready for survey at this point.

When a sensor, (i.e. cesium) cannot provide a signal for the full pattern due to its limited active zones, then separate solutions are obtained for those portions of the pattern when the signal is available. For example, North and West, South and East. These two separate solutions can be selected automatically from an external computer/terminal or manually from the front panel as required. The AADCII also allows the operator to merge these two solutions, again immediately, to provide an additional single automatic solution for whenever the sensor enters its active zone. The user has the flexibility to use either method, to suit the project.

The Figure of Merit (FOM) is also used as an indicator of performance and is a summation of the twelve peak to peak values of the roll, pitch, and yaw, maneuvers in the four headings. The AADCII will produce a value of one nanoTesla consistently. The Improvement Ratio (IR) value given by the AADCII is typically 10 - 25 with compensated (standard deviation) values of 25 - 50 picoTeslas being achieved. There is no direct correlation between the FOM and the IR.

The AADCII is also a superior instrument for gradiometer work not only using two sensors but also three and four sensors. The system provides compensation for each total field sensor and their respective gradients. It is the only system capable of real time compensation on four sensors. The use of a single time base and excellent synchronization between channels eliminates temperature drift and induced noise.

The AADCII is being used successfully on all sizes of aircraft from small single engine to large four engine military ASW types. In addition, compensation has been achieved on light turbine helicopters. The AADCII has been installed successfully on aircraft without any magnetic 'cleaning', as referred to above, with excellent results.

The AADCII is the result of many years of research on compensation both by RMS Instruments and the NAE division of National Research Council of Canada. Automatic aeromagnetic compensation in real time using the AADCII is a very practical and effective way of producing excellent results efficiently.