The first in a new family of instruments for aeromagnetic compensation and data acquisition, RMS Instruments' AARC500 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 AARC500 offers the ultimate in compensation, delivering unparalleled performance, accuracy, consistency and reliability.

The result of many years of R&D on aeromagnetic compensation by RMS Instruments, and collaborations with the Flight Research Laboratory of the National Research Council of Canada, the AARC500 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

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' AARC500 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 magnetism, induced magnetism and Eddy currents, and also eliminates heading errors introduced by 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”.

 

 

 

AARC500 Compensation

 

 

Calibration and solution

The AARC500 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 AARC500 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 AARC500­ 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 ancialliry 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 AARC500 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 compensation (as much as 10x lower residual errors for gradients), and simplified calibration procedures. Other novel approaches are continuously being developed and can be readily incorporated into the system thanks to its flexible architecture.

 

 

System Description

 

 

RMS Instruments' new compensation 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 to the system, the proprietary counter and synchronization hardware deliver outstanding performance with negligible noise and temperature drift, while maintaining synchronization.  

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 other three events are free for any other use. Accuracy is determined by the primary sampling period (i.e., as good as 781 μsec).   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 an AARC500 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 in real-time, and output data is available (a) at up to 40 Hz, 115.2 kbps via a serial port, (b) for recording on a USB Flash drive, (c) for display as waveforms on the built-in 6.5" VGA colour display, and (d) for output to a GR33A chart recorder. In addition to compensated and uncompensated data, serial output includes also the 3-axis vector magnetometer values, a fiducial number, the fourth difference for each magnetometer, and event input tags from the Front End.

 

 

The Host software offers optional additional filtering, with user-selectable bandwidths. It includes also facilities to carry out spectral analysis on collected data, and optional internal data recording. The software allows configuration of the various operating parameters through an easy-to-use graphical user interface.

 

 

Comprehensive statistical information is provided to assess the quality of the calibration/solution. The information, readily accessible on the display, includes the Improvement Ratio (IR), a standard measure of the effectiveness of the compensation. The AARC500 will typically achieve IRs in the range of 10 – 20 for total fields in large and magnetically complex aircraft. For gradients, figures in the range of 20 – 100 are typical, with  close to 10X better performance possible when using adaptive compensation.

 

It must be noted that the improvement offered by the AARC500 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.

 

 

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 AARC500 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 AARC500 accepts decoupled Larmor signals on BNC connectors on the unit's rear panel. An optional decoupler module is available, to separate the Larmor outputs of the magnetometers from 28-Volt power.

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AARC500 SPECIFICATIONS

Inputs:   one to eight high-sensitivity magnetometers Cs:  70 kHz – 350 kHz K:    140 kHz – 700 kHz He:  560 kHz – 2.8 MHz Magnetic Field Range:           20,000 – 100,000 nT Front End: Time base: 100 MHz, TCXO Resolution: 0.32 pT  [1] System noise: < 0.1 pT [1] Sampling rate: 160, 640, 800 or 1280 Hz – user-selectable [2] Transfer function: 1.6-Hz BW, 3.25-Hz BW, 9.8-Hz BW or raw data – user-selectable [2] Compensation Performance: IR (total field): 10 – 20, typical IR (gradient): 20 – 100, typical (up to 10X further improvement possible with adaptive compensation) Compensation Accuracy: σ ≈ 20 pT for entire aircraft flight envelope, 0 – 1 Hz Optional Filter (Host Subsystem): User-selectable in range 0.4 – 3.0 Hz BW Calibration Duration: 6 – 8 minutes, typical Vector Magnetometer: 3-axis fluxgate          Oversampling, self-calibrating, 16-bit A/D converter Data Output & Recording: Rate: 10, 20, 40 Hz, or external-trigger – user-selectable [2] Serial port: up to 115.2 kbps, ASCII or Binary USB Flash drive Chart recorder Display Event Inputs/GPS Synch.: Four latched event inputs TTL-levels, edge-sensitive Event input tags included with output data Accuracy: per Front End sampling period (down to 781 μs) Raw Data Logging: [2] At Front End sampling rate 1-MB buffer Ex.: 41666 records for 4 mags.  Display: 6.5" colour TFT digital LCD VGA resolution (640 x 480) Antiglare surface treatment Backlight: CCFL, 2 replaceable tubes Luminance: 400 nits

Mouse:      Silicone-rubber actuators      Pressure-controlled operation      No moving parts Front Panel Indicators, I/O: 8 LEDs for mag. input status 2 LEDs for Front End status PS/2 keyboard Two USB 2.0 Rear Panel I/O: 1 – 8 mag. inputs (BNC) Event input (DE-9P) Fluxgate mag. (DE-9S) Front End analog (DB-25P) Data output (RS232, DE-9P) Remote ctl. (RS232, DE-9P) Two USB 2.0 10/100/1000Base-TX Ethernet (RJ45) Chart recorder (paral., DB-25S) Analog RGB (DE-15S) Power: +28 VDC ± 6 VDC, 2.75 A For each input at RMS4880 Mag. Power/Decoupler Mod.: 0.5 A typ.; 1.0 A at turn-on. ENVIRONMENTAL: Operating Temperature: 0 – 50ºC Storage Temperature: -20 – 55ºC Relative Humidity: 0 to 99%, non-condensing Altitude: 0 to 6,000 m (0 to 20,000 ft) Size (W x H x D): 483 x 133 x 381 mm (19 x 5.25 x 15 in) Weight: 8.2 Kg  (18 lb) Notes: [1]  Over a 625-msec integration period, 1.6-Hz bandwidth. [2]  Requires AARC500 Advanced Functions Option.  Without it, system defaults to 640-Hz F.E. sampling, 1.6-Hz BW.   Specifications subject to change without notice [Oct/2005]