AADCII

6877-1 Goreway Drive, Mississauga, Ontario, Canada L4V 1L9

Tel:  (905) 677-5533, Fax:  (905) 677-5030, e-mail:  rms@rmsinst.com

 

Based on a sophisticated compensation algorithm, which is extremely robust	Compensates pitch, roll, yaw and heading effects in real-time
Fully digital and automatic - no compensation coils required	Excellent quality control tool - especially with gradiometer systems
Compatible with most high sensitivity magnetometers (Cesium, Helium and Potassium)	Solutions may be stored in non-volatile memory and easily recalled when required
Solution obtained immediately when calibration is terminated	Provides compensated, uncompensated and aircraft attitude data
High resolution magnetometer processor 0.001 nT	Fast calibration - typically 6 to 8 minutes
Accepts one to four high sensitivity magnetometer inputs	Compensates the total field of each sensor as well as the gradients
High MTBF and low operating costs	Lightweight, easily installed, and easy to use

The quality of the data collected in aeromagnetic surveys is largely dependent on the quality of compensation. Despite the outstanding sensitivity of modern magnetometers, in the absence of good compensation anomaly signals, which fall off as the third or fourth power of distance, can be completely masked out by the interference of the nearby magnetics of the aircraft.

The aircraft's magnetic interference is related to its motions about its principal axes. A mathematical model may be built to accurately represent the aircraft's magnetic signature. Careful optimization and implementation of this model, within the framework of sophisticated hardware and firmware technologies, can lead to real-time compensation that effectively eliminates the aircraft's magnetic interference.

The RMS INSTRUMENTS' AADCII Automatic Aeromagnetic Digital Compensator provides real-time compensation of local magnetic interference for inboard magnetometer systems in fixed wing aircraft and helicopters, to the point where the full resolution of current high sensitivity magnetometers can be utilized.  The compensation accounts for the effects of permanent magnetism, induced magnetism and Eddy currents, and also eliminates the heading errors introduced by the sensors.  Calibration in the aircraft takes only six to eight minutes.  Extremely accurate, consistent and reliable, the AADCII has been for many years the de facto standard in the geophysical exploration industry throughout the world.

The result of many years of research and development on aeromagnetic compensation by RMS Instruments and the National Research Council of Canada, the AADCII has consistently produced outstanding data in a cost effective manner. The system is built on the foundation of extremely reliable hardware and firmware, and a sophisticated and robust compensation algorithm that has been proven in a multitude of installations ranging from single-sensor to four-sensor gradiometers.

The AADCII is built around a multiprocessor architecture.  Each of the (up to four) Larmor frequency signals from the high sensitivity magnetometers is handled by a dedicated processor. Oversampling, very precise counter circuitry, and excellent synchronization are behind the outstanding performance of these processors.  Their outputs are input to the main Microcomputer, which implements the compensation algorithm and runs the system software.

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 magnetometer interface uses a very stable 100 MHz crystal oscillator time base. When additional magnetometer sensors are added to the system the counters use the same time base, eliminating drift and maintaining synchronization.  The signals from the magnetometers are digitized faithfully, without aliasing or phase distortion.  The AADCII provides a default bandwidth of DC to 0.9 Hz, the range of most interest to the geophysicist, but other bandwidth choices are user selectable.

The AADCII is self-calibrating, making it very simple to operate. After an initial calibration flight of only a few minutes, the system is immediately providing compensated data.  A new calibration is required only if the magnetic configuration of the aircraft has been altered.

The AADCII uses a 3-axis fluxgate magnetometer to monitor the aircraft's position and motion with respect to the ambient magnetic field while flying a set of standard maneuvers of rolls, pitches and yaws in the cardinal headings.  During the calibration mode of approximately 6-8 minutes, the positional data together with the magnetometer sensor(s) readings are utilized by a sophisticated model to arrive at a solution of approximately 30 terms.

The solution is a comprehensive mathematical model that accurately describes the magnetic interference of the moving aircraft. The solution is calculated instantly, upon termination of the calibration maneuvers. It is immediately available for use in compensation mode (see below) or for further analysis and comparison with other solutions previously obtained and stored in memory.

With the AADCII there is no need for any kind of additional 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. Furthermore, any set of such partial calibrations can also be combined, once again immediately, to produce a single robust solution for all of the sensor's active zones.

The AADCII is an excellent instrument for gradiometer work — in addition to total-field compensation for up to four sensors, it also provides real-time compensation for the various gradients. Proprietary counter and synchronization hardware deliver outstanding performance with negligible noise and temperature drift.

WORLD’S FIRST! Cessna Titan 404 with a four sensor triaxial gradiometer system installed.  Photo courtesy of Poseidon Geophysics (Pty) Ltd., Botswana

Compensation Mode

The AADCII automatically enters this mode at power-up and measured values of the total field and gradient (if more than one sensor are installed) are corrected for the aircraft interference using the last solution selected. Compensated and uncompensated values of the high sensitivity magnetometers along with the 3-axis vector magnetometer readings are available for display on the front panel, output via an RS232 serial port, as well as for monitoring on the RMS Instruments’ GR33A graphic recorder.

The magnetic signature of typical survey aircraft is extremely prone to change in-flight. Even the simple switching-on of a light in the cockpit may cause a significant DC-shift in the compensated data. Detecting these changes while monitoring uncompensated data is practically impossible. State-of-the-art aeromagnetic surveying requires real-time monitoring of compensated data, so that problems are identified immediately and are promptly corrected. Relying solely on post-flight compensation is akin to “flying blind”.

The AADCII can output the data at up to 20 times per second (user selectable) with a bandwidth of  0.9, 1.8, or 3.2  Hz.  The output also contains a fiducial number, a clock value and the 4th difference for each magnetometer.  The output is user definable with respect to data, serial parameters and data type (ASCII or binary).

A standard terminal output is supplied allowing the AADCII to be controlled from a remote location or computer.  All of the front panel displays and setup menus are transmitted for the operator's convenience.

Comprehensive statistical information is provided to assess the quality of the calibration and allow the operator to evaluate system performance. The information, readily accessible on the display, includes the Improvement ratio  (IR), a standard measure of the effectiveness of the compensation. The IR is calculated as the ratio of the standard deviations of the signals before and after compensation, and routinely values in excess of 10 – 20 are achieved in large and magnetically complex aircraft.

Resolution of high sensitivity magnetometers realized

Improvement offered by the AADCII is achieved over and above any passive compensation of the magnetometer installation.  If the user has a magnetically “clean” installation or has achieved passive compensation to 0.45 nT, they can expect, with a conservative IR of 15, to achieve system performance of 0.03 nT.

Cost Effective

The AADCII allows the user to mount a magnetometer sensor or sensors inboard an aircraft allowing the user to avoid towed bird systems which are less sensitive, require higher maintenance and are less reliable.  The AADCII also permits the use of gradiometer installations of two, three and four magnetometers, providing data for total field, horizontal, vertical and longitudinal gradients. 

The AADCII is an excellent quality control tool since it provides the compensated data immediately.  With the aircraft interference removed the user can observe in real-time that the data is meeting the desired criteria.  This eliminates the delay and uncertainties inherent in post-flight compensation techniques, which must also be very cautiously implemented, especially in gradiometer systems, to avoid undesired phase shifts that will distort the data.

Actual data from a Convair 580 aircraft using the AADCII and the DGR33A Data Acquisition System.

Bandpassed uncompensated and compensated data for a full calibration flight (8 minutes). The uncompensated waveform clearly shows the aircraft interference on the four headings. (Waveforms are offset for clarity.) Performance indicators: σuncomp = 0.5502 nT,  σcomp = 0.0282 nT,  IR = 19.5 Mean value = 56188.507 nT

AADCII SPECIFICATIONS