DIGITAL DATA ACQUISITION AND PROCESSING FROM A REMOTE MAGNETIC OBSERVATORY R.C. Snare, D.J. Peters, P.J. Coleman, Jr., R.L. McPherron Department of Planetary and Space Science Institute of Geophysics and Planetary Physics University of California, Los Angeles 90024 Abstract High quality magnetic field measure- ments in the auroral zone are particularly important in present studies of magneto- spheric physics. The standard analog magnetometers are inappropriate because of limited dynamic range, sensitivity, and time resolution and inconveniences in the manipulation of the data. Remote observatories using satellite type magne- tometers and digital data acquisition sys- tems are therefore a desirable improve- ment. To complement the UCLA magnetometer on ATS-1, we have established a magnetic observatory near the ATS-1 conjugate point at Tungsten, N.W.T., Canada. The main- tenance of this station and computer processing of the data have involved a number of unexpected problems which could delay similar efforts elsewhere. There- fore, the instrumentation and data proces- sing for this observatory are described, with particular emphasis on outstanding problems. Alternatives to the use of digital incremental tape recorders as in the present system, e.g., data trans- mission via satellite link, are also discussed. Introduction The first Applications Technology Satellite placed in synchronous equatorial orbit December 6, 1966 carried a magne- tometer as part of its scientific payload. The location of the spacecraft is 1500 west longitude at the approximate inter- section of the geographic and geomagnetic 6"uatorial planes. The northern end of the dipolar geomagnetic line of force passing through the spacecraft intersects the earth near the mining village of Tungsten, Northwest Territories, Canada, Fig. 1. The existence of this village provided the opportunity to study related magnetic activity on the ground and at 6.6 Re in space. Consequently the Geo- physical Institute of the University of Alaska and the University of California at Los Angeles, with the support of the National Science Foundation, established a magnetic observatory at this location, Manuscript received February 12, 1972; revised July 19, 1972 Fig. 2. An important consideration in the choice of instrumentation for such a location is the fact that the power spec- trum of the magnetic field fluctuations observed at such high latitude is rather steep containing v riations from 103y with periods of 10 s.econds to variations of 10-2y with a 1 sec period. This broad, steep spectrum requires the use of broad dynamic range instrumentation with high resolution and good signal to noise ratio. A second consideration in selecting equipment for this station was the prim- itive conditions under which the equipment would be operated. Highly skilled oper- ators would not be present and laboratory conditions desirable for the operation of high quality instrumentation of the stan- dard geomagnetic observatory would not be available. The instrumentation selected for this station, as reported by Snare and Petersl, consisted of a three axis fluxgate magne- tometer system with an automatic offset system and two axes of induction coil equipment, see Fig. 3. Fluxgate Magnetometer The fluxgate system was a magne- tometer from the fifth Orbiting Geo- physical Observatory (OGO-5) satellite as reported by Snare and Benjamin2. This system, as is the nature of most spacecraft instrumentation, was rugged, highly reliable and required no adjust- ment or maintenance after its initial installation. The magnetometer has a dynamic range of +16y with a noise level of .05y RMS, (.OOT to 1.0 Hz bandwidth). The digital offset system produces a local field at the sensor which opposes the ambient field in order to keep the total field within the limited dynamic range of the magnetometer. The offset field generator supplies stable currents in discrete steps to an auxiliary coil wound around the magnetometer sensor. Thus, the offset field generator extends the dynamic range of the overall instrument without reducing the amplitude resolution of the magnetometer. The state of the offset field generator is recorded and represents the total field when added to the magnetometer output. 127