DATA ACQUISITION  & 

ADAPTIVE AEROMAGNETIC REAL-TIME 

COMPENSATION

The RMS Instruments' DAARC500 offers the ultimate in aeromagnetic compensation, together with comprehensive and flexible data acquisition and recording.  Powerful, versatile and rugged, yet compact and light, the DAARC500 is ideally suited to airborne and mobile geophysical and environmental survey applications.

Aeromagnetic compensation in the DAARC500 has its roots in the AADCII, for many years the de facto standard in aeromagnetic compensation in the geophysical exploration industry throughout the world.  The result of many years of R&D by RMS Instruments, and collaborations with the Flight Research Laboratory of the National Research Council of Canada, the DAARC500 continues the AADCII tradition of consistently producing outstanding data in a cost effective manner.

The system is built on the foundation of state-of-the art, very reliable hardware and firmware, and sophisticated and robust compensation algorithms that have been proven in a multitude of installations.  Consistent with compensation, data acquisition is delivered with unparalleled performance, accuracy and reliability.

Aeromagnetic Compensation

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' DAARC500 Data Acquisition System & Adaptive Aeromagnetic Real-Time 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 modern high sensitivity magnetometers can be utilized.  The compensation accounts for the effects of permanent and induced magnetism, Eddy currents, and heading errors from the sensors.

The need for real-time compensation

The magnetic signature of typical survey aircraft is extremely prone to change in-flight.  Something as simple as switching-on 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".

DAARC500 – Compensation

Calibration and solution

The DAARC500 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 or for further analysis and comparison with other solutions.

With the DAARC500 there is no need for any post-flight software.  The system uses the full 360-degree pattern to obtain a robust solution.  If necessary, in the event full 360-degree signal acquisition is not possible, the DAARC500 allows calibration for each active zone, and a corresponding solution.  Furthermore, any set of such partial calibrations can also be readily combined to produce a single robust solution for all of the sensor's active zones.

Compensation – total fields & gradients

In compensation mode measured values of up to 8 total field high-sensitivity magnetometers and gradients are corrected in real-time using one of the solutions previously obtained.  Compensated and uncompensated values along with the 3-axis vector magnetometer values and other ancillary data, are available in real-time for output to a data acquisition system, for recording on Flash media, and for monitoring on the built-in display or other peripheral devices.

Adaptive compensation

The DAARC500 incorporates sophisticated adaptive signal processing techniques that allow the system to continuously "learn" from input signals, and adapt the solution coefficients for optimum compensation.  This can lead to improved band-passed and gradient compensation  (up to several times lower residual errors), and simplified calibration procedures.  Other novel approaches are continuously being developed and can be readily incorporated into the system thanks to its flexible architecture.

DAARC500 – Data Acquisition

Comprehensive and flexible data acquisition and recording complement the aeromagnetic compensation functions in the DAARC500.

External instruments and sensors with digital (serial) and analog outputs can be connected directly to the DAARC500.  The system provides eight high-speed, isolated, serial (RS232) inputs and outputs, and sixteen differential analog inputs.

Flexible serial protocols (ASCII and Binary) and practically unlimited buffering space, allow easy interfacing to most devices.

All data sampling, including magnetometers and compensation output, is at rates based on the same time base.

Data are recorded with time and event tags that allow accurate synchronization to GPS receivers.

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

System Description

RMS Instruments' new compensation and data acquisition technology is based on a flexible architecture that incorporates dual 32-bit processors.  It includes state-of-the-art COTS (industrial-grade) electronics, and a new proprietary magnetometer interface module.

Front End subsystem

The Front End is based on a high-performance, low-power, superscalar, RISC PowerPC processor.  Signals from the 3-axis fluxgate magnetometer are processed using a high-resolution (16-bit) A/D converter.  The magnetometer interface, most critical for high-performance compensation, uses the latest in analog and digital electronics to provide excellent accuracy and synchronization for up to four total-field magnetometer sensors in one card.  The system may be configured with one or two magnetometer interface modules;  real-time compensation (total-field and gradients) for up to 8 magnetometers opens the door to exciting new applications in sensor array processing.

The magnetometer interface uses a very stable, temperature-compensated 100-MHz crystal oscillator time base.  As additional magnetometers are added, the proprietary counter and synchronization hardware deliver outstanding performance with negligible noise and temperature drift.

Front End sampling rates are user-selectable, up to 1280 Hz.  Finely tuned, user-selectable transfer functions deliver outstanding anti-aliasing characteristics.  Data output is at rates up to 40 Hz.

The high-speed raw data, at the primary Front End sampling rates, is optionally made available to the user.  This allows in-depth frequency domain analysis and troubleshooting of installations.

Data output by the Front End includes event input tags, which encode the time of occurrence of four event inputs.  One of them will typically be used for data synchronization to the PPS signal from a GPS receiver.

The Front End offers a resolution of 0.32 pT, and internal system noise better than  σ = 0.1 pT, over a 625-msec integration period.  For comparison, the figure aside shows system noise in a DAARC500 configured with identical sampling rates and transfer function as an AADCII – system noise has been reduced by a factor of close to 2.

Host subsystem

The host subsystem is built around one of the latest Pentium processors available.  The application software resides in Flash memory, as does the operating system, QNX 6.3 (or later) – a deterministic and very reliable real-time operating system.

Data received from the Front End is processed (i.e., compensated) in real-time, and output data is available (a) at up to 40 Hz, 115.2 kbps via a serial port, (b) for recording in Flash media or a hard drive – more on this below – (c) for display as waveforms on the built-in 6.5" VGA colour display, and (d) for output to a GR33A chart recorder.  I

It must be noted that the improvement offered by the DAARC500 is achieved over and above any passive compensation of the magnetometer installation.  For example, with a magnetically "clean" installation, or if passive compensation has been achieved to 0.45 nT, a conservative IR of 15 will yield system performance of 0.03 nT.

Recording media options include embedded 1-GB Flash memory, a Flash disk connected through any of the USB ports available, and an optional (internal) hard disk.

Analog and serial inputs are sampled at the same rate as magnetometer data, or a submultiple of it.  The 16 analog inputs are digitized using a 16-bit, self-calibrating A/D converter.  Data is recorded in a file as a sequence of blocks with leading time and event tags that allow easy correlation with other data, and with the PPS signal from a GPS receiver.

Up to 8 isolated (RS232) serial ports are supported, with bit rates up to 115.2 kbps.  Through the user interface the format expected of incoming data can be readily specified.  Data from all active channels are arranged into blocks with leading time and event tags, and these are recorded in a file on the USB Flash drive.

Other outstanding features of the Host subsystem include:

• Integrated graphics controller and TFT support (DVI-I).  Allows simultaneous output to the built-in 6.5" colour display, and to any other external display connected via the analog RGB interface.
• Remote control via a serial (COM) port.
• Extensive general-purpose I/O: full-duplex 10/100/1000Base-TX Ethernet, USB 2.0 (4x), PS/2 keyboard.

User interface, front panel

The user interface to the DAARC500 is via the 6.5" colour display (TFT LCD) and mouse on the unit's front panel.  With full VGA resolution, anti-reflective surface treatment on the front polarizer, and 400-nits luminance backlight, the display offers outstanding readability under all conditions, including high-brightness outdoor environments.  Connectors for a PS/2 keyboard and two USB 2.0 interfaces are also available on the front panel.  LED indicators show the status of each of the magnetometer inputs, and of the Front End subsystem.

An analog RGB connector on the rear panel allows an additional display/monitor to be used simultaneously with the built-in display.

Optional decoupler

In its standard form the DAARC500 accepts decoupled Larmor signals on BNC connectors on the unit's rear panel.  An optional decoupler module is available for four or eight inputs (on TNC connectors).  The decoupler separates the Larmor outputs of the magnetometers from 28-Volt power.

DAARC500 SPECIFICATIONS

[1]  Over a 625-msec integration period, 1.6-Hz bandwidth.

[2]  Requires DAARC500 Advanced Functions Option.  Without it the system defaults to 640-Hz Front End sampling, 1.6-Hz BW.

[3[  The 1-A current rating at turn-on is as per Geometrics G-822A-type sensors.  The RMS4880 Magnetometer Power/Decoupler Module in the DAARC500 can handle up to 4 (RMS4880-1) or 8 (RMS4880-2) such sensors.  For other manufacturer's sensors current ratings may be different. Consult RMS Instruments.

Specifications subject to change without notice [Sept/2006]