DE-1/RIMS consists of four instrument assemblies interconnected to form one experiment. Three of the assemblies are sensor heads and one is the central electronics assembly. The three heads are labeled according to the mounting axis on the DE-1 spacecraft: Radial, +Z, and -Z. The radial sensor views perpendicular to the spacecraft spin axis, while the ąZ sensors on the ends of the spacecraft view parallel and anti-parallel to the spin axis. The central electronics assembly (CEA) provides the spacecraft interface, all data processing, command decoding, and complete timing control of the entire RIMS experiment.
Each sensor head consists of an RPA followed by a magnetic mass analyzer with two separate exit slits corresponding to two mass/charge ranges in the ratio 1:4. The total mass/charge range covered is 1 to 40 amu/Z. Surrounding and attached to the entrance of the sensor head is a 20 cm circular aperture plate. The plate is connected to a relay which can, on command, connect to either spacecraft chassis ground or to the aperture potential power supply output. The aperture potential power supply can be set by spacecraft minor mode A command to 0, -2, -4, or -8 V to partially compensate for a non-zero positive spacecraft potential.
The retarding grid of the RPA is connected to the retarding potential power supply through a shielded conductor. The retarding potential may be set to any one of 1024 linearly spaced steps from 0 to +51.125 V with a resolution of 0.05 V. The RPA collector plate is connected through a co-axial cable to a 5-decade logarithmic amplifier. The reference voltage for the front end of the log amp is the output of the aperture potential power supply. The output of the log amp is converted, by command from the CEA, to a 10-bit digital word using an analog/digital (A/D) converter. The digital word is held in a holding register until the CEA is ready to read and process the data.
Those ions passing into the Ion Mass Spectrometer (IMS) are sorted according to their mass to charge ratio. The proper combination of ion accelerating voltage, applied magnetic field strength, and ion beam gyro radius in the applied magnetic field determines the mass/charge of the ion focused on each collector slit. Varying the ion accelerating voltage varies the ion mass/charge detected. Ions of mass/charge 1 to 8 amu/Z and 4 to 32 amu/Z can be focused on the low and high mass slits, respectively. Ions exiting the collector slits are counted by channel electron multiplier (CEM) detectors. The ion mass/charge range is also programmable by a minor mode command. Any 32 of 4096 voltage steps may be selected. All 32 steps may be the same, in which case the mass/charge analyzer will be locked onto a given pair of mass/charge peaks having the ratio 1:4. The two-channel IMS uses CEMs as detectors. The two CEMs in each sensor head are powered by a single multiplier high-voltage power supply which can be set, by spacecraft minor mode A command, to any one of four voltages, -1200, -2100, -2400, and -2800 V. The IMS accelerating voltage comes from a swept high voltage power supply that can be addressed to any one of 4096 linearly spaced steps between 0 and -2250 V.
The output of each detector is connected to a charge sensitive pulse amplifier whose output is sent to a level detector. The discrimination level is set by spacecraft minor mode A command. Pulses from the discriminator are coupled to an 18-stage binary counter. Four bits of the counter are located in each head, and the other 14 bits of each accumulator are located in the CEA. The CEA contains the circuitry for compressing each accumulator output into a 10-bit base 2-floating point number (6-bit mantissa and 4-bit exponent) for output into the telemetry buffer.
The DE spacecraft utilizes a pulse code modulation data system with 128, 8-bit words at a main frame rate of 16 frames per second. Note that the RIMS instrument is assigned 26 of these 8-bit telemetry words. One 8-bit word is reserved for status information, five groups of 5 8-bit telemetry words each are used for measurements where each group is considered a telemetry channel by the processor. With this convention, each telemetry channel can contain 4 each 10-bit instrument words. Each data sample represents a period of 1/4 minor frame (1/64th of a second, or 15.625 msec.) Each RIMS sensor head has three data outputs, the RPA and high and low mass IMS channels, and therefore outputs 9 measurements each 1/4 frame for a total of 36, 10-bit instrument words per telemetry minor frame. Since the 40-bit telemetry channel can contain only 4, 10-bit instrument words, only 20 of the 36 instrument words may be loaded into the telemetry channels each telemetry frame (4 instrument words x 5 telemetry channels). The telemetry channel assignments, number of words assigned to each, and their content are as follows:
| Channel Assignment | Number of Words | Content |
|---|---|---|
| A | 1 | Housekeeping and status |
| B | 5 | Radial IMS low-mass channel accumulation |
| C | 5 | Radial IMS high-mass channel accumulation |
| D | 5 | Alternates each 1/4 frame between +Z IMS low & high-mass accumulation |
| E | 5 | Alternates each 1/4 frame between -Z IMS low & high-mass accumulation |
| F | 5 | Alternates each 1/4 frame between radial, +Z, and -Z RPA (electrometer) accumulation |
The instrument cycle is 32 data samples from each data source. The RIMS completes one instrument cycle each 0.5 sec. Each data sample represents a period of 15.625 msec. This interval consists of a measurement interval and a data processing interval. During the measurement interval, the IMS accumulators are active for a period of 12 msec. The 3.625 msec data processing interval is used to process the data accumulated during the measurement interval and to establish the mass/charge voltage and retarding voltage for the next integration period.
The RIMS instrument continued to return usable data until the end of the DE mission in March 1991. The radial head RPA failed on 81/329 19:50:18. It worked again between 82/045 23:30 and 82-046 06:15 before failing again. Sometime before 82/046 the -Z head aperture bias mechanism failed. At launch the -Z head was found to be biased -2 V with respect to the other two heads. The anomaly disappeared on day 73 or 74 of 1982. The Z heads show spin variation primarily at low altitudes in cold, dense plasmas at high spacecraft velocities. For a more detailed and complete account of RIMS performance characteristics, see Olsen et al., DE-1/RIMS Operational Characteristics, NASA TM-86527, 1985.
The RIMS instrument data was provided by GSFC to ES53/MSFC on 1600 BPI magnetic tapes. The RIMS data was extracted and combined with orbit/attitude and magnetic field data to produce mission analysis files (MAF) and summary spectrograms. There is usually one MAF file per pass (RIMS instrument on/off time). Each MAF file data record consists of 8 seconds of RIMS data (512 samples per channel, each sample 1/64 sec measurement), in time order. One MAF record is 5624 bytes in size. The first 100 locations (16-bit words) of this record contain timing, orbit/attitude, B-field, and instrument mode information. Since the instrument mode takes some logic to decode, subroutines are available that perform the decoding of the instrument flags, determination of RPA/IMS settings for any sample through the 512, and decommutation of the D and E channels (Z-head data).
Data quality is, in general, excellent. Data are not always recorded at altitudes down to the DE-1 perigee of 675 km. At the lower altitudes, the plasma density can be quite high and the RIMS instrument is generally shut off to protect the particle detecting CEMs from excessive counting rates. However, on occasion, saturation of the detecting CEMs may be seen in the radial head data during low altitude-high density passes.
Detection of very low-energy ions is made difficult by the presence of floating spacecraft potentials. Analysis of RIMS data by Olsen [1989] established a density-potential relationship, such that the satellite accumulates positive charge on entering density regions below 1000 cm-3, rising slowly to about +1 V at 100 cm-3, and about +5 V at 10 cm-3. By preventing measurement of the coldest plasma components, this potential can mask out an isotropic background plasma or, when the potential is sufficiently high, can give the appearance that no cold plasma exists in a region. This effect is known to be a concern in the plasmapause and trough region, where densities are of the order 7 to 70 cm-3 and the spacecraft may reach potentials of +2 to +5 V [Olsen, 1989]. At altitudes above the ionosphere in the polar regions, densities again can be sufficiently low that observation of ions of classical polar wind speed is possible only during negative aperture bias instrument modes [Nagai et al., 1984].
The DE-1/RIMS team has attempted to compensate for the spacecraft charging by the addition of a voltage bias plate at the entrance aperture of the instrument. The diameter of the external aperture plane is 20 cm, which may be compared to the spacecraft dimension of one meter in height and 1/4 meter in diameter. The instrument retarding voltages are referenced to this aperture plane, so that to the RIMS detector the effect of the bias is similar to a change in spacecraft potential, though sheath effects can complicate the interpretation of the resulting data.
Space Plasma Physics Branch
Space Sciences Lab
Marshall Space Flight Center
NASA
Last Updated: September, 1998