INSTRUMENT_HOST_ID = MGS INSTRUMENT_ID = MAG INSTRUMENT_NAME = Magnetometer INSTRUMENT_TYPE = Fluxgate Magnetometer Instrument Overview =================== The Mars Global Surveyor magnetic field instrument consists of dual, triaxial fluxgate magnetometers, capable of measuring fields between +/- 4 nT and +/- 65536 nT. Automated range switching allows the instrument to maintain maximum digital resolution over a wide range of field strengths. The description and ASCII drawings of the instrument mounting and frames are derived from the SPICE instrument kernel version 1.2 dated Sept. 16, 1998. Please review the published material for a complete description of the instrument. ============================================================================ Scientific Objectives ===================== The primary objective of the Mars Global Surveyor (MGS) magnetic field experiment is to establish the nature of the magnetic field of Mars. This includes determining whether or not Mars has a global field of internal origin indicating either present or past dynamo field generation. The existence of an internally generated field would place significant constraints on the composition, thermal state, and dynamics of the interior of the planet. Even if dynamo activity ceased as the planet cooled, there is evidence that dynamo activity existed in the past. Remanent magnetization has been observed in meteorites that are widely believed to have originated on Mars. An important objective of the magnetic field investigation is to identify and characterize crustal remanent magnetization. Magnetic anomaly maps, together with geological data will provide a history of Martian magnetism and crustal evolution. Previous missions to Mars have determined that if there is a global magnetic field at Mars it must be small. Remanent magnetic fields are also likely to be small. In order to accurately measure magnetic fields of Martian origin, the nature of the solar wind and interplanetary magnetic field interactions with Mars must be well determined. =========================================================================== Calibration ============ The flight software incorporated into the on-board data processor includes diagnostic and self-calibration routines. On-board calibration sequences provide the currents required for determination of the gain of each axis sensor for the various dynamic ranges, as well as for determination of electronic offsets by reversal of the polarity of the signals processed by the magnetometer electronics. In addition, spacecraft maneuvers will be performed that will allow the spacecraft field and sensor offsets to be determined independently. ============================================================================ Operational Considerations ========================== The magnetometer power consumption is 375 mW in zero field and increases with the magnitude of the measured field up to 420 mW. The typical rms noise level in the sensors is 0.006 nT over a 10 Hz bandwidth. The zero-level stability is less than 0.15 nT over the range of -40 to +60 degrees Centigrade and for durations up to a year. The upper range of 65,536 nT allows the instrument to be operated in the Earth's magnetic field without special shields or field cancellation magnets. ============================================================================ Detectors ========= The MGS magnetometer experiment consists of two, fully redundant, fluxgate magnetometers. There are two sensor triads, two sets of electronics, and two power converter packages. Either sensor triad can be connected to either electronics package. Only one of the two systems is powered at any time. The other system is powered off and maintained in a standby state for redundancy. The detectors are constructed using the ring core geometry, which has been shown to have excellent performance characteristics in terms of long-term zero-level stability and drive power requirements. The magnetic material used to manufacture the sensors in an advanced molybdenum-permalloy alloy developed for low-noise, high-stability applications. The MGS spacecraft does not have a magnetometer boom to mount the sensors on as the original Mars Observer spacecraft did. Instead, the two sensor packages are mounted on the solar panel arrays. The following diagram shows dimensions required for determination of locations of the MAG sensors relative to the s/c center: -Y MAG yoke gimbal s/c gimbal yoke +Y MAG | | | | | | | | 3.817m | 0.729m|0.669m | 0.669m| 0.729m| 3.817m | | or | or | or | or | or | or | | 150.285in | 28.7in|26.33in|26.33in| 28.7in| 150.285in| |<---------->|<----->|<----->|<----->|<----->|<---------->| | | | ___|____ | | | | | V / | /| V | | | __________|_ _____ /____|__/ | ____|__ _________|__ V / V// | | | | | / V // V / / -Y Solar // | | | | | / // +Y Solar / @ Array /@ @--| | | ---@ @/ Array @ ----- / // / | | | | | // / ^ /___________//______/ | | | | |_____//___________/ |0.934m |____V__|/ |36.77in / \ V /__@__\ ----------- The orientations of the instrument frames of the +Y and -Y MAG sensors relative to the corresponding solar array frames are shown below: -Y Solar Array frame +Y Solar Array frame +Z +X +Z | / | | / | | / | +Y _______|/ |_______ +Y / / +X / -Y MAG Sensor frame +Y MAG Sensor frame _______ +Y _______ +Y /| /| / | / | / | / | / | / | +Z +X +Z +X This schema shows that +Y MAG is +90 degrees rotated about Y axis relative to the +Y solar array and -Y MAG sensor is -90 degrees rotated about Y axis and after that +180 degrees rotated about the new position of Z axis relative to the -Y solar array. Frames diagram -------------- The following diagrams shows the frames defines for the MGS spacecraft, solar arrays and MAG sensors: +Z | | *--- +Y +X / S/C body FR +Z (MGS_SPACECRAFT) +Z | +X | | |/ | | +Y ---* | *--- +Y | +X / -Y Gimbal FR | +Y Gimbal FR (MGS_RIGHT_ | (MGS_LEFT_ +Z +Z SOLAR_ARRAY) | SOLAR_ARRAY) | | +X | | | | |/ | | | *--- +Y +Y ---* | | | +X / -Y Yoke FR | | | +Y Yoke FR (MGS_+Y_SOLAR | | | (MGS_+Y_SOLAR _ARRAY) | | | _ARRAY) *--- +Y | | | | | *--- +Y /| | | | | | /| +Z | | | | | | +Z | +X | | | | | +X | | V | | -Y MAG sensor FR | | ________ | | +Y MAG sensor FR (MGS_MAG_-Y_SENSOR) | V / /| V | (MGS_MAG_+Y_SENSOR) | __________|_ _____ /_______/ | ____|__ _________|__ V / V// | | | | / V // V / / -Y Solar // | | | | / // +Y Solar / @ Array /@ @--| | ---@ @/ Array @ / // / | | | | // / /___________//_____/ | | | |_____//___________/ |_______|/ 'MGS_SPACECRAFT' frame is the frame associated with the MGS spacecraft main bus. This frame is defined in an MGS SCLK file created by mgs_scet2sclk program at LMA. Orientation of this frame is provided in the CK files produced by the ATTREC program at LMA. 'MGS_LEFT_SOLAR_ARRAY' and 'MGS_RIGHT_SOLAR_ARRAY' frames are associated with the +Y and -Y solar array gimbals respectively. These frames are defined in an MGS SCLK file created by the mgs_scet2sclk program at LMA. Orientation of these frames is provided in the CK files produced by the MGSSCK program at LMA. Note that there are no separate frames defined for inboard ('elevation') and outboard ('azimuth') gimbals for each solar array. Instead each pair of gimbals is considered as a single gimbal having two degrees of rotation. These frame can be considered as 'nominal' solar array position frames since they specify gimbal orientation and do not take into account any additional rotations/transformation that can (did) occur due to incomplete deployment of an array. 'MGS_+Y_SOLAR_ARRAY' and 'MGS_-Y_SOLAR_ARRAY' frames are associated with the +Y and -Y solar array yokes respectively. These frames are 'fixed offset' frames whose orientation is specified by a set of Euler angles relative to the corresponding frames associated with gimbals. Defining these frames was required because of -Y Solar array deployment failure, which introduced an additional rotation in the yoke for that panel. For the +Y panel this frame is the same as the gimbal frame. 'MGS_MAG_+Y_SENSOR' and 'MGS_MAG_-Y_SENSOR' frames are associated with +Y and -Y MAG sensors. These frames are fixed offset frames whose orientation is specified by a set of Euler angles relative to the corresponding yoke frames. ============================================================================ Electronics =========== Signals from the sensors are first processed by the analog electronics and then by the digital processing unit (DPU). Analog data are anti-alias filtered and then sent to a twelve bit (12-bit) successive approximation analog to digital (A/D) converter that is controlled by a microprocessor. Variable time resolution data are derived from the basic measurements and the spacecraft telemetry mode. The microprocessor activates the automatic gain control logic in the electronics. If the magnitude of the measured vector component falls within upper or lower guard bands (256 data numbers), then the range (scale factor) is incremented or decremented to maintain maximum digital resolution. Range adjustments change the dynamic range and digital quantization by a factor of four. Range Field Strength Quantization -------------------------------------------- 0 +/- 4 nT .002 nT 1 +/- 16 nT .008 nT 2 +/- 64 nT .032 nT 3 +/- 256 nT .128 nT 4 +/- 1024 nT .512 nT 5 +/- 4096 nT 2.048 nT 6 +/- 16384 nT 8.192 nT 7 +/- 65536 nT 32.768 nT The DPU unit's primary function is to acquire the magnetic field data and package it with instrument state and housekeeping data in a form that can be picked up and transmitted to the ground by the Payload Data System (PDS). The system consists of a master executive program that is resident in ROM. The DPU uses the 80C86 microprocessor and associated memory and peripheral devices. Default parameter tables used for data processing are stored in ROM but can be modified by ground command. Parameters such as sensor zero levels, alignment matrices, scale factors, etc. are expected to be updated periodically under normal operating conditions. RAM memory is used to double buffer data while packets are being created and accessed by the PDS. Double buffering allows a completed packet to be read out while a new packet is being created without access conflicts between the instrument and the PDS. Data collection and processing routines are interrupt driven by a real time interrupt (RTI) signal provided eight times per second by the onboard PDS. The clock is multiplied four times (32 Hz) and is the fundamental timing signal for all processes in the instrument. Data compression techniques are used to maximize data return within the bandwidth allocated to the experiment. Raw magnetometer data are averaged and then 6-bit differenced between adjacent averages. The differences are combined with periodic 12-bit 'full-words' and formatted into data packets. If the differences exceed the dynamic range of six bits, the system folds the values (modulo 64) over rather than saturating. This allows the reconstruction of rapidly varying fields that would otherwise be lost. If the number of folded differences exceeds a predetermined value, the DPU left shifts the differencing scheme by the least significant bit doubling the dynamic range. There is a maximum of two left shifts permitted. The data return sample rate is linked to the spacecraft data rate. The instrument has three data rate allocations. Both the rate of primary (compressed) samples and secondary (full-word) samples varies with the data rate. Data Rate Primary Values Secondary Values (bits/sec) (samples/sec) (samples/sec) ----------------------------------------------------------- 324 8 1/6 648 16 1/3 1296 32 2/3in ============================================================================ References ========== Acuna, M. A., J. E. P. Connerney, P. Wasilewski, R. P. Lin, K. A. Anderson, C. W. Carlson, J. McFadden, D. W. Curtis, H. Reme, A. Cros, J. L. Medale, J. A. Sauvaud, C. d'Uston, S. J. Bauer, P. Cloutier, M. Mayhew, and N. F. Ness, Mars Observer Magnetic Fields Investigation, J. Geophys. Res., 97, 7799-7814, 1992. Bogard, D.D., and P. Johnson, Martian gases in an Antarctic meteorite?, Science, 221, 651, 1983. doi.org/10.1126/science.221.4611.651 Luhmann, J.G., Space plasma physics research progress 1987-90: Mars, Venus and Mercury, Rev. Geophys., 29, suppl., 965-975, 1991.