Galileo Plasma Instrumentation

Inflight Operation of the Instrument

The operating modes of the plasma instrument are designed to accommodate the diverse plasmas in the Jovian magnetosphere. We provide here a brief introduction to these capabilities. The instrument cycle time is 243 s and is subdivided into 12 equal intervals, or instrument spin modes. Each spin mode is a separate instrument operations and data collection cycle. The duration of a spin mode is typically one rotation period for the spacecraft spinning section, 18.3 to 19.8 seconds. By ground command the plasma instrument can be configured to sample a combination of a given set of sensors, a range of energy passbands, a range of mass channels, and a set of angular sectors as the fields-of-view rotate. The operations of analyzers A and B can be programmed independently. Limitations on the operation of these analyzers are imposed by the minimum dwell time for the energy passbands and mass channels of 8.3 ms, a service time of 1 ms for the processing of the contents of a count accumulator, and the telemetry rate allocated to the instrument of 612 bit · s^-1 (72 sensor samples · s^-1 plus overhead). Each sample of sensor responses is quasi-logarithmically compressed into an 8-bit word. Internal buffers can allow rapid bursts of < 1500 measurements to be trickled into the telemetry stream.

Consider the measurement cycle time of the plasma instrument if onboard software were not available to improve the operational efficiency. If all energy passbands, mass channels, and sensors were sampled in each of 16 angular sectors, then the time for this complete plasma measurement (1.3 × 10⁶ samples) would be 5.1 hours. Such instrument operation is ineffective and wasteful of the capabilities for obtaining plasma parameters, e.g., individual 64-point energy or mass spectra in 0.5 s. Thus the spin modes are each designed to obtain a specific type of plasma measurement during one spacecraft rotation, e.g., a three-dimensional velocity distribution, high angular and energy resolutions of an ion beam, and the mass composition of an ion beam. A spin mode is constructed of nested control loops. These loops control (1) the number of angular sectors sampled during a spacecraft rotation, (2) the number of energy passbands or mass channels in a sector, (3) the duration of an energy passband or mass channel, (4) the readout of the selected sensors, (5) the sequence of energy passbands, and (6) the sequence of mass channels. Four sequencing tables are used to determine the operation of the instrument during a spin mode: (1) sensor, (2) mass channel, (3) energy passband, and (4) angular sector. The angular sectors are referenced to a fixed position on the celestial sphere by means of information from the spacecraft attitude control system. Instrument software is available for five basic types of spin modes. Default values for the sequence tables are also included in read-only memory in the instrument processor in lieu of values from ground commands. We briefly illustrate below the capabilities of the various spin modes.

Spin mode 1. Survey of positive ion and electron velocity distributions. All electron, ion, and integral ion sensors (spectrometers) are sampled. The number of angular sectors, the energy range, and the number of energy passbands are selected by ground command. The product of the numbers of passbands and angular sectors is 64. For example, the responses of all of the above sensors for 64 passbands sampled in a single angular sector of 45° can be telemetered each spacecraft spin period. Alternately 16 passbands (every fourth passband) in each of four 90°-sectors can be telemetered during a single rotation period in order to obtain the principal features of the three-dimensional velocity distributions of positive ions and electrons once each 20.3 s.
Spin mode 2. Determination of the velocity distribution of a positive ion beam. Electron and ion sensors corresponding to those nearest the direction of the ion beam are selected. These sensors and the spacecraft rotation angle for the beam are determined with measurements from a preceding spin mode. For example, during one spacecraft rotation, energy passbands 8 through 23 can be sampled with three sensors for positive ions and two or three sensors for electrons for each of five contiguous 22.5°-sectors in the direction of the beam. Two electron sensors are used for analyzers A and B, with the exception of three for analyzer B if the beam is nearly perpendicular to the spacecraft spin axis. Again angular size of the sectors and the number of energy passbands can be selected by ground command.
Spin mode 3. Survey of ion composition. Mass spectrometers 1 and 2 are sampled for a selected range of gap magnetic fields. During one spacecraft rotation a single energy passband of the electrostatic analyzer is used and the gap magnetic fields are incremented over a selected series of values. Thus for a given energy passband and a single spacecraft rotation it is possible to sample the entire M/Q range in 64 current steps in each of four 90°-sectors.
Spin mode 4. Survey of ion composition. This spin mode is identical to spin mode 3 with the exception that mass spectrometer 3 replaces 1.
Spin mode 5. Determination of the composition of an ion beam. The mass spectrometer with direction of field-of-view nearest to that of the ion beam is chosen on the basis of previous measurements. The energy passband and angular sectors for the ion beam are similarly identified. For example, during one spacecraft rotation in the plasma sheet or torus of the Jovian magnetosphere full coverage of the M/Q range in 64 channels can be sampled in each of five contiguous 22.5°-sectors.

The instrument cycles for analyzers A and B are each selected as a sequence of 12 spin modes. The order of the spin modes and their operating parameters such as energy and mass ranges, angular resolution, etc., are controlled by the sequence tables. As an example, a sequence of spin modes during an instrument cycle for analyzer A can be 1, 1, 2, 1, 1, 5, 1, 1, 4, 1, 1, 3. Thus the various operating modes of the plasma instrument can be implemented and cycled automatically with minimal demand for command uplinks to the Galileo spacecraft. Major command sequences are used to restructure the spin modes and their sequencing for special events such as the close encounters with the Galilean satellites and the exploratory survey into the distant magnetotail.