CANadian Magnetic Observatory System (CANMOS)

Introduction

The CANMOS (CANadian Magnetic Observatory System) is a modular PC-based data-acquisition system that serves as the automated portion of the Canadian Magnetic Observatories, operated by the Geological Survey of Canada (GSC). CANMOS uses commercially-available magnetometers that provide digital RS232 signals to the central processor PC. Output devices are also connected to the central processor by RS232 links. The concept of modularity applies not only to the hardware, but also to the design of the software running on the central processor.

The Software

The essence of the CANMOS is, in fact, the central software. This software performs the control functions to coordinate the magnetometer measurement cycles; it cleans, filters and formats the data into several data streams to be written onto hard disk, onto removable disk, and to be sent to the satellite transmitter, or relayed via modem on standard telephone links. The software performs numerous quality checks on the data, and on the correct operation of the magnetometers. An extensive set of error messages is available to the technical staff at network headquarters, who can interrogate the CANMOS by telephone and perform a number of diagnostic tests.

The Hardware

The central processor is a 386-type PC, with several additional cards, including multi-RS232 serial ports, a bus-clock, and watchdogs. The heart of the CANMOS is a "bus-clock", a specially-designed plug-in card in the PC that carries a highly accurate temperature-compensated crystal oscillator, resettable from an external source. CANMOS operation is supervised by two "watchdog" circuits that, on failure to receive specific signals, cause the system to cycle power to the PC and all external devices. The triaxial magnetometer currently used in CANMOS is the digital-output ring-core fluxgate magnetometer produced by Narod Geophysics. For this application as an observatory variometer, the firmware in the Narod magnetometer has been modified by the GSC to permit accurate calibration of the individual bias steps used in cancelling the total vector components. A novel feature of the CANMOS design is the GSC-designed tilt-correcting suspension for the fluxgate triaxial sensors. This double bifilar suspension (Pendulously suspended magnetometer sensors, by D.F. Trigg and D.G. Olson, Reviews of Scientific Instruments, 61 (10), pp.2632-2636, October 1990) has a tilt reduction ratio of greater than 10,000:1. The proton magnetometer currently in use in the CANMOS is an Overhauser magnetometer Model 9BB, produced by GEM Systems. The CANMOS can be accessed via telephone modem at speeds up to 14400 baud. CANMOS relays the data in near real-time (data bursts every 12 minutes) to network headquarters via GOES satellite, using a Valcom transmitter and a yagi antenna. The complete system is powered from an Un-interruptible Power Supply connected to the local power utility. During power failures, the UPS can run the CANMOS for about 18 hours, using four 12 volt deep-discharge UPS batteries. Temperature control is important, and a CANMOS installation has the fluxgate magnetometer sensors in an insulated, temperature-controlled enclosure, with a non-inductive, non-magnetic heating element. The fluxgate electronics are mounted in a separate enclosure with a temperature sensor, a heating element and a thermoelectric cooling device. These arrangements are designed to maintain the sensor assembly and the electronics at constant, though different, temperatures.

The Data

The fluxgate magnetometer samples the magnetic field at 8 Hz. The three component data streams are despiked and filtered using a rectangular filter over 9 data points, and resampled at 1 Hz. These 1 Hz data streams can be made available at one of the RS232 ports on the central PC and are stored on hard disk. The 1 Hz data are further filtered using a 49 point Gaussian filter, and resampled at intervals of 5 seconds. The proton magnetometer samples the field at 5 second intervals. The four 5-second data streams can be made available at an RS232 port. These data also are stored on the hard disk, and can be downloaded to a removable medium. The four 5-second data streams are filtered using a 19 point Gaussian filter (as listed in the INTERMAGNET Technical Reference Manual) and resampled at 1 minute intervals. The 1-minute data are stored on the hard disk and can be downloaded to a removable medium. To meet INTERMAGNET recommendations, the four 1-minute data streams are passed to the transmitter to be relayed in bursts at 12 minute intervals via a GOES satellite to network headquarters in Ottawa and to other INTERMAGNET data nodes. All data streams can be accessed via modem, although this is normally reserved for diagnostic or backup purposes. Summary data, in the form of hourly means and hourly ranges, can be obtained via modem. In addition to the magnetic field data, CANMOS records temperature data: the fluxgate sensor temperature, the fluxgate electronics temperature, the ambient enclosure temperature, and the outside temperature. These data are recorded on the hard disk, and downloaded with the magnetic data.

Future Enhancements

The 1 Hz data will be stored on hard disk for periodic downloading to a large capacity removable medium. Access to the CANMOS using higher speed modems will be incorporated.