ACE/MAG Project
MAG Instrument


Except for minor modifications, the ACE/MAG instrument consisting of a set of twin sensors and of an electronics control unit is the flightspare from the instrument currently flying on the WIND spacecraft launched in November of 1994, 

The ACE/MAG instrument will measure the local interplanetary magneticfield (IMF) direction and magnitude and establish the large scale structure and fluctuation characteristics of the IMF at 1 AU upstream of Earth as a function of time throughout the mission. This experiment will provide: 

  • continuous data at 3,4 or 6 vectors/sec, and
  • snapshot memory data and Fast Fourier Transform data (FFT) based on 24 vectors/sec. acquired on board, working synchronously with blocks of 512 samples (FFT only) each.
These measurements will be precise, accurate, and ultra sensitive. The basic instrument is a twin triaxial fluxgate magnetometer system. Each of two identical sensors is on booms that extend past the end of diametrically opposite solar panels. The digital processing unit utilizes a 12 bit A/D converter to easily resolve small amplitude fluctuations of the field, and is microprocessor controlled. It also incorporates a dedicated FFT processor developed around high performance DSP integrated circuits, which produces a 32 channel logarithmic spectrum for each axis, synthesized from a "raw" 256 point linear spectrum. All components of the power spectral matrices corresponding to the 32 estimates are transmitted to the ground once every 80 seconds, providing power and phase information together with the corresponding snapshot memory time series data. As in previous instruments developed at GSFC, high reliability is obtained by the use of fully redundant systems and extremely conservative designs. The intrinsic zero drift of the sensors is expected to be below 0.1 nT over periods of up to 6 months. Electrical "flippers" designed to simulate a 180 degree mechanical rotation of the sensors, will be used to monitor the zero level drift associated with aging of electronic components. The use of advanced statistical techniques for estimating absolute zero levels is also planned. The instruments feature a very wide dynamic range of measurements capability, from ± 4 nT up to ± 65,536 nT per axis in eight discrete ranges; all ranges can be activated either by command or, most commonly, automatically. The upper range permits end to end testing in the Earth's magnetic field without the need for special field cancellation coils or magnetic shields. Table 1 summarizes the principal instrument characteristics. 

Table 1
Summary of Instrument Characteristics

Instrument type:
Twin, triaxial fluxgate magnetometers (boom mounted)
Dynamic ranges (8):
±4 nT; ±16 nT; ±64 nT; ±256 nT; ±1024nT; ±4096 nT; ±16,384 nT; ±65,536 nT
Digital Resolution (12-bit):
±0.001 nT; ±0.004 nT; ±0.016 nT; ±0.0625nT; ±.25nT; ±1.0 nT; ±4.0 nT; ±16.0 nT
12 Hz
Sensor noise level:
< 0.006 nT RMS., 0-10 Hz
Sampling rate:
24 vector samples/s in snapshot memory and 3,4 or 6 vector samples/s standard
Signal Processing:
FFT Processor, 32 logarithmically spaced channels, 0 to 15 Hz. Full spectral matrices generated every 80 seconds for four time series (Bx, By, Bz, |B|)
FFT Windows/Filters:
Full despin of spin plane components, 10% cosine taper, Hanning window, first difference filter
FFT Dynamic range:
72 dB, *-Law log-compressed, 13-bit normalized to 7-bit with sign
Sensitivity threshold:
~0.5 x 10-3 nT/*Hz in Range 0
Snapshot memory capacity:
256 Kbits
Trigger modes (3):
Overall Magnitude Ratio, Directional max.-min. peak to peak change, Spectral increase across frequency band (RMS.)
Telemetry Modes:
Three, selectable by command
Sensors (2): 450 g. total 
Electronics (redundant): 2100 g. total
Power Consumption:
2.4 watts, electronics - regulated 28 Volts ± 2% 
1.0 watts , heaters - unregulated 28 Volts

ACE/MAG Project Home Page
Original HTML Author: Jacques L'Heureux -
Current Author: Charles W. Smith -