ABSTRACT
  ========
  The magnetometer instrument onboard the Venus Express orbiter
  is described. This instrument consists of two tri-axial fluxgate
  magnetometers. The nominal 500 earth days (2 Venus years) orbiting
  Venus Express mission will lead to a detailed understanding of the
  interaction between Venus' atmosphere and the solar wind. In
  addition it will provide the magnetic field data for any combined
  field, particle and  wave studies such as lightning and planetary
  ion pickup processes, map with high time resolution the magnetic
  properties in the magnetosheath, magnetic barrier, the ionosphere,
  and the magnetotail and identify the plasma boundaries between the
  various plasma regions.


  INSTRUMENT DESCRIPTION
  ======================
  The VEX magnetometer MAG measures the 3D magnetic field in the
  frequency bandwidth from DC to 128 Hz. It consists of two triaxial
  fluxgate sensors (MAGOS and MAGIS). The dual sensor configuration
  was chosen for a better monitoring of the stray magnetic fields
  produced from other S/C units. The electronics box comprises two
  sensor electronics boards, the DPU board and the DC/DC converter.
  MAGOS will be mounted to the tip of a deployable boom whereas the
  inboard sensor (MAGIS) will be directly attached to the +Z panel of
  the spacecraft.

  Well-known statistical methods, using time series of solar wind
  measurements, are used to determine the offset field at the
  magnetic field sensors (including instrument offset and S/C DC
  field). Since major parts of the orbits around Venus are in the
  solar wind, these data can be used for statistical offset
  determination. If a too high offset (caused by DC field disturbance)
  is detected, it is reduced by using the 12-bit compensation DACs.
  The maximum possible compensation field is +/-10,000 nT along each
  of the all together 6 sensor axes. A front to end health check is
  possible by command which generates artificial fields using the
  12-bit compensation DACs (Calibration Modes 2-4). Boom deployment
  at the beginning of the cruise phase is essential for a proper MAG
  in-flight calibration.


    TABLE  I
    Main Instrument Characteristics

    MASS
    Inboard Sensor, Harness and MLI                    0.357 kg
    Outboard Sensor, Harness and MLI                   0.355 kg
    Electronics Box and Harness                        1.060 kg
    Boom, Hinge, Lock and MLI                          0.536 kg
    Total                                              2.308 kg

    SCIENCE MODES
    Instrument Mode           Sensors                  Data Rate
    Solar wind                Outboard and Inboard         1 Hz
    Pericenter                Outboard and Inboard        32 Hz
    Burst                     Outboard and Inboard       128 Hz

    POWER
    All modes                           4.25 W (+/-10%) maximum

    NORMAL DOWNLINK DATA RATES
    Solar Wind                              104 bits per second
    Pericenter                             3328 bits per second

    DYNAMIC RANGE, RESOLUTION
    low                                      +/-32.8nT,   1.0pT
    default                                 +/-262.1nT,   8.0pT
    high                                   +/-8288.6nT, 128.0pt

  DATA Sampling in-flight
  -----------------------
 CRUISE phase:
MAG was the first instrument to be commissioned on Venus Express, 10
days after launch, and its boom deployed. Afterwards, it remained ON
during the commissioning of all the other instruments, to enable
registration and characterisation of the magnetic disturbances
generated during payload operation.
During the CRUISE phase, only 'Solar wind mode' is default and 1 Hz
data rate is transmitted.

Instrument Mode Sensors active  Data Rate  Nominal operation in cruise
 phase
Solar wind 1     OS and IS      1 Hz    always active
  (SW1)

 Nominal science modes in nominal orbit around Venus
(= after start of 'nominal mission' 14-05-2006):
MAG is operating continuously in orbit around Venus (which is in
principle 24 hr per 24 hr orbit around Venus) and mostly in an
autonomous mode, requiring little or no commanding. Higher data-rates
are started only after start of the nominal mission in orbit around
Venus (14 May 2006).
During a typical science orbit, MAG is switched to 'Pericenter mode'
one hour before pericenter, and then back to 'Solar wind mode' one
hour after pericenter.
The instrument is commanded to the high resolution 'Burst mode' one
minute before pericenter for duration of 2 min in order to detect
lightning.

   Inst. Mode    Sens. act. Data Rate    Nominal op. in nominal orbit
   Solar wind 1  OS and IS      1 Hz     always active, i.e. full
      (SW1)                              orbit sampling coverage,
                                         except if mode with higher
                                         data rate is active

   Pericenter 1  OS and IS     32 Hz     1 hr before and after
      (PC1)                              pericenter

   Burst         OS and IS    128 Hz     1 min before and after
      (CAL5)                             pericenter


  THE Sensors
  -----------

  Both fluxgate sensors, featuring low mass and power consumption,
  consist of two single ring-core sensors measuring the magnetic
  field in X- and Y-direction. The magnetic field in Z-direction is
  measured by a coil  surrounding both single sensors. The side length
  of the cubic shaped sensor triad is approx. 5 cm. The sensor is
  identical to the ones of Rosetta Lander and MIR instrument package
  and similar to the ones flown on Equator-S (same soft-magnetic
  ringcores made of an ultra-stable 6-81 Mo permalloy band: 2 mm
  x 20 mum). The ringcores have been tested under extreme environmental
  conditions aboard numerous space missions as well as in applied
  geophysics. The excellent low noise and stability behaviour of the
  sensor material has especially been proven aboard Equator-S.
  Because of the wide operating temperature range of the fluxgate
  sensor from -160 °C up to +120 °C, the sensor can be mounted outside
  of the temperature controlled S/C only covered by a passive
  multi-layer insulation blanket. No active heating or cooling is
  needed for the sensors.

  The sensor electronics generates an excitation AC current
  (fundamental frequency of approx. 9.6 kHz), which drives the soft
  magnetic core material deep into positive and negative saturation.
  According to the fluxgate principle, the external magnetic field
  distorts the symmetry of the magnetic flux and generates field
  proportional even harmonics of the drive frequency in the sense
  coils.

  The induced voltage in the sense coil is digitised immediately after
  the preamplifier at four times the excitation frequency. The
  front end signal processing (synchronous detection and integration
  and calculation of the feedback signals) is done by logic blocks
  within an Actel FPGA (54SX32). A feedback field increases the
  overall linearity and stability of the magnetometer. It is supplied
  to all sensor elements via 12-bit DACs (feedback DACs) and a
  separate pair of feedback coils per sensor axis. Sense and feedback
  signals are continuously transmitted to the controller (128 Hz)
  which calculates the magnetic field values (24 bits) by scaling and
  adding up the received data (k1*ADC+k2*DAC). The appropriate dynamic
  range is defined by selecting and transmitting of just 16 bit of the
  calculated 24 bits mentioned above (also a kind of data compression)
  Therefore, the range can be modified by T/C between +/- 32,8 nT and
  +/- 8,388,6 nT with a corresponding digital resolution between 1 pT
  and 128 pT. The default range/resolution will be +/- 262,1 nT/8 pT.
  During the operational phase, an artificial magnetic field of
  +/- 10,000 nT can independently be applied to each sensor via
  additional 12-bit DACs for compensation of any disturbing DC stray
  field. The digital magnetometer concept of MAG requires
  analog-to-digital conversion at a higher data rate but it shows a
  number of advantages over the more traditional analog fluxgate
  magnetometer: Early digitisation makes the sensed signal robust to
  changes of the environmental temperature and the supply voltages as
  well as insensitive to EMC. Furthermore, no range  switching is
  needed for getting the full range at full resolution, which reduces
  design complexity and facilitates data analysis.


  INSTRUMENT ELECTRONICS
  ======================

  The data processing unit (DPU) designated for the instrument controls
  the two sensors and the spacecraft interface of the experiment and
  performs internal data handling (sampling, data pre-processing,
  compression, data frame generation). The DPU is based on an Intersil
  HS-RTX2010RH rad-hard microcontroller, which was especially
  developed for space systems embedded control.
  The controlling logic for the DPU, the sensor interfaces, the address
  decoder, the clock generator, the reset logic and the instrument
  spacecraft interface (standard ESA OBDH interface) are implemented in
  an Actel 1020RH FPGA. A watch-dog circuit, which is also implemented
  in the FPGA, is supervising continuously the operation of the DPU and
  can release a cold start in case of a system crash. The DPU integrates
  128 kbytes of static RAM, 64 kbytes of program memory (PROM) and
  64 kbytes of EEPROM. After power-on, the onboard software is copied
  to RAM. After this the PROM is switched off and the instrument
  software is executed in RAM. This procedure helps decreasing the
  power consumption of the DPU. Software patches and various parameters
  can be uploaded to the instrument and stored in the EEPROM.

  The DC/DC Converter (MAGE-P) bears design heritage from the units
  developed  by Imperial College for Cassini and Rosetta. For the Venus
  Express MAG instrument, a single (non-redundant) converter is
  provided. The +28V main and redundant primary power inputs are
  therefore connected together after the on/off relays (see
  Fig. 2.2 5 MAGE-P Block Diagram). The converter provides 4 secondary
  supplies to the MAG instrument: +8 V (+8 V-D for excitation) and
  +/-8 V (+/-8 V-A) to the analogue electronics, and a separate +5 V
  (+5 V-D) digital supply. Over-current protection is implemented on
  the primary input side; in the event of an over-current the supply
  will automatically shut down. On the secondary side, the MAG
  electronics is protected by an over-voltage circuit, which will shut
  off the secondary side of the converter. The converter is stabilised
  with respect to changes in the load and changes in the input Voltage,
  and is capable of maintaining the secondary voltages within the tight
  tolerances required by the MAG electronics.