TIDE- Instrument Description

Measurement Modes:
With TIDE's capability to monitor multiple directions and mass species simultaneously, only the ion energy needs to be swept. Since TIDE has only one swept measurement, all its operating modes consist of repetitive sweeping of the mirror/RPA potentials in an uplinkable pattern coordinated with the spin sweep of the seven sectors in spacecraft azimuth with a period of 6 sec. TIDE has two sampling modes. The "standard" sampling mode provides for 32 energy steps per sweep with 32 sweeps per 6-sec spin, dividing the spin into 11.25 degrees sectors. A phase-locked-loop maintains precise spin sectoring during each spin, and provides for the phase to be adjusted by up to 22.5 degrees of spin in 256 steps. The fundamental organization of the TIDE data consists of a triplet of arrays: First, the species array has dimensions: energy, azimuth angle, polar angle, and mass, with 16 bits of accumulation depth. Second, the singles array has a similar structure, but with four additional rates in addition to sectors 1-7: STOPS, StartConverts, TimeOuts, and Resets. Finally, the direct event array stores full mass spectral information (8-bit address) for each of the 7 sectors, without regard to spin or energy, and with 32 bits of depth to accommodate accumulations of durations up to several minutes. The data reside in the spin data acquisition memory which includes two pairs of identical banks (64 k x 16 bits each) that collect data during alternating spins for the species data and singles/direct events, respectively. While data are being acquired in one pair of memory banks, processing is performed on previously acquired data, resident in the alternate pair of memory banks. During processing, specified data sectors can be copied, summed, or differenced with selected portions of bulk dynamic RAM memory (384 kB) that serves as the main working memory of the Data Processor. A schematic showing the basic functional elements of the TIDE data flow is shown in Figure 16.

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Figure 16. TIDE data flow chart.

In view of the large size of the TIDE data "images" (Species: 32 energies x 32 azimuths x 7 polar-angle channels x 5 masses x 16 bits = 70kB, Singles: 11 x 32 x 32 x 16 bits = 22 kB, and Direct Events: 256 x 7 x 32 bits = 7 kB; Total: 99 kB), and the rate of generation of images (once per 6-sec spin), the intrinsic data output of TIDE may be seen to be ~ 105 Bytes/spin or ~130 kbps. The allocated telemetry rate is 3000 Bytes/spin or 4000 bps, so it is clear that tradeoffs must be made between temporal resolution and comprehensiveness of data reporting. TIDE has two data handling modes, which provide considerable flexibility for trades in favor of the one or another aspect of the data set. In addition, onboard data processing techniques such as data compression and moment computation are used to maximize the effectiveness of the available data rate.

Data Decimation:
The following methods implement the necessary trades by reducing the content of one or another aspect of the data to be telemetered:

Floating Point Representation:
TIDE count accumulations are reduced from 16 bits to 8 bits (or 32 bits to 16 bits in the case of direct event data) by implementing a special form of floating point representation with a resolution of ~3% and a range of 216, conserving scarce telemetry and limiting the reporting precision to a level commensurate with the overall measurement accuracy. The mapping of 16-bit internal data acquisition accumulator words to encoded bytes is handled in hardware by a memory lookup table.

Azimuth Collapse:
To make the angular sampling which occurs near the spin axis directions in the TIDE sampling scheme more commensurate with the angular response of the instrument, data from the polar angle sectors can be increasingly collapsed as they approach the spacecraft spin axis. This yields a collapse factor of 1.9 without significant loss of detail in the data. To provide additional collapse at the cost of real loss of angular resolution, an additional collapse option combines alternate bins for a collapse factor of 3.8. Another collapse option simply combines 32 spin rows into two rows for laboratory test and calibration purposes where spin sectoring is not needed. Only the second of the two spin rows is reported, so that the first half of each "spin" period may be used to make adjustments to the laboratory setup in real time without invalidating the entire spin of data

Energy Array Collapse:
In cases where additional collapse is desired, an energy collapse is provided in which adjacent energy columns of data are combined, for a collapse factor of 2.

Priority Ordering:
The first form of prioritization of the TIDE data is the drawing of a distinction between the reporting of species data and direct events. Since the purpose of direct event reporting is to provide information about species which may not be sampled among the top priority species data, it is intrinsically of lower priority. It is therefore accumulated over a longer time scale of 32 spins (a superspin, see below). This requires larger accumulation registers, but drastically reduces the data rate requirement for this information, down to 56 words (long 4 Byte) per spin (of 3000).

The TIDE species data are also routinely priority ordered from the most important species to least important species. The ordering is controlled by a default table in ROM, but may alternatively be controlled by means of an uplinked table. The START singles rates (seven of them) may be treated as a distinct, composite mass species and introduced into the telemetry as such. By default, the START singles data is the highest priority mass species in the priority ordering. The other four singles rates, STOPS, StartConverts, TimeOuts, and Resets, are reported only after reduction to spin moments, as described below.

In-Flight Data Processing: In order to minimize the impact of data decimation, the following in-flight processing techniques are used:

Bulk Data Compression:
Collapsed arrays of floating point represented data are further compressed by a lossless encoding technique similar to those used for telecommunications of computer files. This eliminates data with little information content and provides reductions in the total data volume by a factor ranging from approximately 2 to several, depending on the information content of the specific data. The resulting data volume depends in general upon the nature of the data collected, but this uncertainty is handled by truncation or by allowing as much time as is needed for reporting. The result is the transmission of more data content per spin (less truncation), or a reduced number of spins between complete reports, respectively.

Moments:
Moments computed each spin provide an important measure of variability from spin to spin when full distributions can only be sent at less frequent intervals. In addition, moments of the singles rates provide useful diagnostic information concerning the operation of the instrument detectors and electronics.

Selected simple moments of the species and singles, angle and energy distributions, are computed from the raw data arrays and reported on a single spin basis. A short history of these moments is updated by the Data Processor each spin and used to judge the variability of the data for purposes of triggering a high time resolution mode. The criterion for the switch is that the standard deviation within the current record of selected moments exceeds an uplinkable threshold.

Owing to limited computational speed, even with two 32-bit processors, the moments computed are total events, simple mean Azimuth and Polar Angle bin numbers, and effective widths for Azimuth and Polar Angle. The energy distribution is weighted to provide the energy of maximum phase space density and the effective width of the phase space density distribution. It is, of course, planned to develop the full physical moments from the downlinked data.

Dead-Time Correction:
The finite dead time of the TOF analysis circuits, and the desirability of operating with large counting rates to maximize the statistical significance of the TIDE data, indicate that TIDE requires a dead-time correction onboard to support the computation of accurate collapses and moments. Therefore, it is necessary to perform an onboard dead-time correction of all accumulations based upon the overall rate of TOF events. This is done on the basis of the current START CONVERT rate, by means of a look up table stored in DPU memory.

Operating Modes:

Operation and Commanding:
The strategies for reducing TIDE data volume involve a number of options, the selection of which defines a TIDE operating mode. In addition, the basic partitioning of energy and angle sampling can be modified so as to gain fine angular resolution at the cost of reduced time resolution. The mode structure is summarized in Table 4. In the following paragraphs, the major mode options are described:

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Table 4. TIDE operating mode summary

Calibration Mode:
During testing and calibration on the ground, there is no "spin" dependence of the data. A "calibration mode" has been developed for use in this context. In this mode, the normal 32 spin sectors are summed into two sectors, one of which is discarded to allow for settling of changes made in the test setup, while the latter of the two is reported. This reduces the data volume by a factor of 32, removing over-sampling that is inappropriate at the time of calibration, and making it possible for all of the collected data to be reported within a single "spin," as defined by the simulated spacecraft spin clock provided by the GSE. In addition, a unique additional data product is defined for this mode containing all singles rate data, which is plotted in real time to facilitate management of detector and TOF high voltages during testing.

Truncate (High Time Resolution) Mode:
Data are truncated to the amount which can be sent in a single spin of telemetry, but with priority ordering to insure that the most important data are sent. To maximize the number of species reported on in this mode, maximum collapses are used for both energy and azimuth angle. Since each species requires approximately 940 Bytes at this level of collapse, while 3000 Bytes can be telemetered per spin, only the three highest priority species can be monitored in this mode. By default, the three species reported are START singles, Mass 1 (H+), Mass 2 (He++). However, this designation can be modified by uplink of an alternate priority table.

Complete Report Mode:
In order to report a larger number of mass species or to regain the full available resolution in energy and azimuth, or both, it is necessary to give up time resolution and report the full data set over a period of multiple spins. The full ordered data array is queued for telemetry, in general leaving a partial spin of telemetry unfilled. In order to fully utilize the available telemetry, the next spin header is introduced at mid-spin followed by the next spin record without waiting for the next spin clock pulse. This is possible because of the buffered nature of the data processing. The mean number of spins required to report a data array is, to a large extent, controlled by the level of collapse which is implemented and the character of the data itself. The Complete Report mode operations result in a time series of data arrays for a selectable number of species (from 1 to 5 plus START singles), at time intervals ranging from every spin to every several spins, depending on the collapse level. Moments of the data, computed and reported every spin, may be used to identify the existence of variability on time scales shorter than the current number of spins between distribution samples (see below).

TIDE monitors the EFI (electric field investigation) and Hydra (three dimensional energetic plasma) experiments' "burst" modes by means of dedicated lines connecting the instruments. It also monitors the reduced moment data from the most recent several spins of data. TIDE can be enabled to switch to its high time resolution mode, truncating data to permit single spin reporting, when a specified combination of these sources indicates the presence of variability in the plasma environment meeting criteria which can be defined by uplink. It dwells in the high time resolution mode for an uplinkable duration.

High Angular Resolution Mode:
As noted above, the "standard" sampling is based on 32 energy sweeps of 11.25 degrees duration each. The M/RPA optics supports a finer sampling of Spin Azimuth angle when operated at reduced sensitivity. The ultimate angular resolution of the optics is limited by the available flux, but it is thought that resolutions down to approximately 3 degrees can be supported.

A "high angular resolution" sampling mode is based on the capability to vary the sector phase from spin to spin. By stepping the phase of the sectors through a submultiple (N) of 11.25 degrees each spin for N spin reports (plus another spin to restore the phase to baseline), a factor of N finer angle sampling is supported at the cost of a loss of time resolution by a slightly larger factor. The value of N commensurate with maximum practical angular resolution is approximately 4. The spin phase would be incremented once per telemetry report so that both truncated and complete reporting would be supported by this mode.

Over Counting and Auto Sensitivity Control:
The basic strategy involves sampling the entire data acquisition memory and counting the number of saturated counting values which exceed a hierarchy of thresholds. For each energy (RPA) step of the current spin of data, if a significant number of samples exceed a particular threshold in the hierarchy, the sensitivity is reduced by a corresponding number of "stops", where the term "stop" is used by analogy with mechanical aperture systems. The next spin of data is again tested for saturation as described above and further reduction is imposed if needed, continuing until the over counting criterion is no longer met.

The flight software provides 16 sensitivity "stops". These are set at equi-log intervals in geometric factor, across over three decades, based on the measured variation of geometric factor with mirror ratio (see Figure 11). The lowest two "stops" each correspond to a complete shut-down of the instrument, first at a mirror ratio of 0.3, then at a ratio of 0.0.

When a low counting threshold is exceeded by an insufficient number of samples, the sensitivity can be increased, at a maximum rate of one "stop" per spin. The spacing between thresholds and "stops" is determined from laboratory testing of the control algorithm. The overall control strategy is best described by an analogy with the human eye. Upon encountering a bright light the eye blinks and then slowly recovers by squinting and slow reopening.

The testing process is repeated and implemented every spin. More elaborate recovery algorithms using the number of saturated counting values from previous spins in a linear scaling procedure could also be implemented if deemed necessary. However they would require additional coding and control functions, and thus they are not implemented in the baseline software scheme.

TIDE sensitivity control is implemented in flight software at two levels. At the default level, over counting in a particular energy step at any point during a spin triggers a proportionate reduction of the sensitivity level for that step in the energy sweep. Cessation of over counting does not, however result in an increase in the sensitivity for that energy step, and the instrument becomes adjusted to the largest fluxes encountered at each given energy step of the sweep. At the optional level, the instrument responds to low counting levels by returning the sensitivity to higher values at each energy step, at a maximum rate of one "stop" per spin.

Default sensitivity control is in force at all times when the instrument is not being monitored in real time, to protect against over counting. The lowest available sensitivity 'stop' is designed to totally shut down the flux to the detection system. In the event that this still fails to eliminate over counting on any given energy step, the control software causes shutdown of the High Voltage supplies, TOF first, then MCP bias. The default sensitivity control can be disabled by means of an appropriate command, which is designated as hazardous.