Captions for figures and tables which could not be reproduced are included as they appear in the original document, though they may have been repositioned to enhance readability. Scanned images of all figures and tables are also provided. These documents may be found in GIF89A format files which employ the following naming convention: FIG##.GIF Figure ## FIG_P##.GIF Figure from page ## TAB##.GIF Table ## In addition, the following conventions have been used in this document: 1. ~ represents approximately (e.g. ~= approximately equal to, etc.) 2. >= and <= represent greater than or equal to, and less than or equal to respectively 3. * represents multiplication 4. non-ASCII characters are written out descriptively and enclosed in square brackets [] (e.g. [pi] = the lower case Greek letter Pi) The following materials have been excerpted from: Armstrong, T.P., and Sahi, R. "Ulysses HISCALE Data Analysis Handbook," Dept. of Physics and Astronomy, University of Kansas, 1996. Pgs. 2-3, 8, 24-5, 106, 178-85, 231-2. Copyright 1996 The University of Kansas. All rights reserved. Reprinted with permission from The University of Kansas (UK). Such permission of UK does not in any way imply UK endorsement of any PDS product or service. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from UK. By choosing to view this document, you agree to all provisions of the copyright laws protecting it. (following excerpt: [ARMSTRONG&SAHI1996], pp. 2-3) Scientific Objectives 1.1 Scientific Objectives The Heliosphere Instrument for Spectral, Composition, and Anisotropy at Low Energies (HI-SCALE) is designed to obtain measurements of interplanetary ions and electrons. The ions and electrons are detected by five separate solid-state detector telescopes oriented to give essentially complete pitch-angle coverage from the spinning spacecraft. Ion elemental abundances are determined by a [Delta]E Vs E telescope using a 5 mm front detector element in a three-element telescope. Inflight calibration is provided by radioactive sources mounted on telescope covers which can be closed. Ion and electron spectral information is determined using both broad-energy-range rate channels and a pulse-height analyzer for more detailed spectra. The instrument weighs 5.775 kg and uses 4.0 W of power. Objectives for the HI-SCALE experiment have been refined by taking into explicit consideration the most current theoretical and modeling efforts in heliosphere physics. The launch of Ulysses in October 1990 occurred during the peak of solar activity in solar cycle 22. HI-SCALE will make significant contributions to ISPM science during this interval and during the prime polar passes, which will occur near solar maximum. Main objectives of the experiment include: - The use of low energy solar particle fluxes as probes of the morphological changes in coronal and interplanetary magnetic field structures as a function of helioatitude. - The study of solar-flare processes via the diagnostics provided by electrons of both relativistic and nonrelativistic energies and by nonrelativistic ions measured over all helioaltitudes. - The achievement of increased insight into the basic astrophysical question of solar elemental abundances by means of measurements of the chemical composition of low-energy nuclei emitted from the Sun in the active region band and at high helioaltitudes, and comparison with similar measurements made in the ecliptic plane. - The investigation of the interplanetary propagation of low-energy solar particles by measurements of the anisotropy and composition parameters as functions of heliographic latitude. - Comparison of nonrelativistic electron events with on-board radio measurements, providing diagnostics of outward-propagating wave-particle interactions at different latitudes in the interplanetary plasma. - The study of physical processes that can produce particle energization within the interplanetary medium off the ecliptic, and the comparison of identified processes to those mechanisms previously identified in the ecliptic plane. - The measurement of the quiet-time low-energy particle populations in the interplanetary medium, and the possible separation of the solar, galactic, and planetary magnetosphere components by their different heliolatitude variations. - The application of the new knowledge gained from Ulysses investigations of the global dynamics and structure of the heliosphere to attain a better understanding of the influence of solar activity on the terrestrial environment and its technological systems. List of References and Source Documents 1.2 List of References and Source Documents 1) Armstrong T.P., Hawkins S.E., Shelley G., HISCALE Data Analysis Handbook. JHU Applied Physics Laboratory, 1991. 2) Armstrong T.P., Passbands & Efficiencies for LEMS, LEFS and WART Channels, University of Kansas, May 1992. 3) Buckely J., Geometric Factor Study for the Deflected Electrons of HISCALE, University of Kansas, April 1993. 4) Curtis D.W., ISPM / Ulysses LAN Data Processing Software Status, Dec. 1986. 5) Fisk L.A., Axford W.I., Proc. Symp. Study of the Sun and Interplanetary Medium in Three Dimensions, NASA/Goddard Space Flight Center, October 1976. 6) Fort D.E., CD Analog Crosstalk, August 1984. 7) Gold R.E., LAN Accelerator Calibration Test Plan, August 1991. 8) Guynn D.R. Jr., LAN Experiment Data System Performance and Interface Specification, June 1992. 9) Kohl J.W., Crawford J.H., Calibration of Solar Polar Energy Model LAN-2B Detector Head at GSFC Low Energy Accelerator, July 1986. 10) Lanzerotti L.J., The ISPM Experiment for Spectral. Composition and Anisotropy Measurements of charged particles at Low Energies. Astronomy & Astrophysics Supplementary Series, January 1992. 11) Maclennan C. G., User's Guide to LAN 360, AT&T Bell Laboratories, Aug. '92. 12) Maclennan C. G., Armstrong T.P., LAN Cover-Close Calibrations, AT&T Bell Laboratories, May 1995. 13) Simmnett G.M., Gold R.E., The RTG Background in the LAN Experiment and the Shielding Required to Control It. April 1980. 14) Software Interface Specification, Common Data File, Jet Propulsion Laboratory, April 1993. 15) Software Interface Specification, Supplementary Experimental Data Record, JPL, February 1992. 16) Townsend J.P. Jr., Ulysses Pulse Height Analysis documentation, JHU, Applied Physics Laboratory, July 1990. 18) Tappin S.J., HISCALE IDL display System (IDL-HS), University of Birmingham, June 1992 19) Tappin S.J., HSIO- HiScale I/O Library, June 1991. 20) Ximenez de Ferran S., Ulysses Spin Reference Pulse, May 1985. Data File Descriptions (following excerpt: [ARMSTRONG&SAHI1996], pp. 8) CHAPTER 2.0 HISCALE DATA FILE DESCRIPTIONS The HISCALE archive records (level 1) are generated from the Experimental Data Record (EDR) (level 0). The contents of the HISCALE archive record are given in section 2.2 below including the record header, RATE block (count rate), PHA (Pulse Height Analysis) block, and MFSA (M and F spectra accumulation) block. Each record has a record header, and either one of the RATE block, PHA block, or MFSA block. The RATE block archive file contains only the RATE records (record header + RATE block), and each RATE archive file contains one day of RATE records. The name of the file is ULAyyddd.RAT where yy is the last two digits of the year, and ddd is the Julian date. The PHA and MFSA records are put together in one file, ULAyyddd.PHA. ULAyyddd.PHA also contains one day of MFSA, and PHA records. Full time resolution is maintained in the first processing step (level 0 to level 1). The procedure called ARCGEN, decommutates, decompresses and reformats the HISCALE date and writes RATE records, MSFA records, and PHA records to output files. The RATE records then serve as input to a procedure LANAVG which forms time averages. MAGMERGE, applied to AVG or RATE RECORDS, inserts the magnetic field, computes and inserts the pitch angle information. Finally, the AVG records which are written with all data collapsed into the spin one position of the 10 spins reserved for repetitions can be COMPRESSED (10:1) into more compact records. Compatibility of data and record representations across different computer systems is accomplished with an I/O package called LANIO. The archive files are generated at the University of Kansas on a VAX/VMS system. However, since the HISCALE team is comprised of VAX/VMS and Unix users, special I/O routines, HSIO and HSTRANS have been used to facilitate the conversion of HISCALE archive data from the VAX/VMS to Unix system. The diagram below shows the use of HISO and HSTRANS. [FIG_P8] The archive record created with HSIO consists of: 1. A record header of 8 bytes which contains the record size of the file and the number of bytes of data in the record. 2. The archive data, which is a combination of Rate, PHA, and MFSA records. 3. Padding to the record size, with zero bytes. The record size of the file is the maximum record size passed to or returned from HSIO plus the size of the header(8 bytes). In the case of VMS systems, the file is a fixed-record-length file with this record size. On Unix systems the record structure is software imposed (hence the reference to pseudo-records). The format allows quick access to the data, but since all records are of the same form and size there are no hidden bytes in the file and utilities such as the VMS APPEND command can be used to concatenate files (following excerpt: [ARMSTRONG&SAHI1996], pp. 24-25) Data File Descriptions 2.1.2 EDR Data Block Format Specifications Data Overview EDRGEN accepts frame synchronized telemetry data in the Science Data Format. EDRGEN extracts the appropriate words from the telemetry minor frame and places them into the defined locations in the data blocks of each individual science record. Filler and spare bits in the EDR formats are binary zeroes unless otherwise noted. Each experiment record contains standard SFDU primary and secondary headers, an ISPM unique tertiary header, and an instrument unique science data block reflecting the instrument record structure. Each record will also contain a complete set of engineering and housekeeping for the time period covered by that record. Table 2-12 defines an EDR record format. EDR tapes contain collections of these records separated by inter-record gaps. The Radio Science and Gravitational Wave experimenters occasionally request engineering EDRs which contain only engineering and ground monitor records. The following subsections define EDR processing, record build-up, and the EDR data block formats for each instrument. The "DS#" designations are generic and represent digital science words for the particular instrument. These designations are the same as those shown in the telemetry format diagram found in the Experiment Appendices of the ESA Functional Requirements for ISPM Data Processing ("green blocks"). Table 2-12 ISPM EDR Record Format ------------------------------------------------------------------ |SFDU HEADERS | ENGINEERING AND SCIENCE DATA | ------------------------------------------------------------------ 2.1.3 EDR Processing EDRGEN extract science, science instrument housekeeping, and spacecraft engineering data from each minor frame of the Science Data Format EDRGEN then builds individual experiment data records by collecting all the minor frames of data of a given type corresponding to 8 major frames ("formats") or 256 minor frames (except 4 major frames/128 minor frames for STO). This data, combined with the time of the first minor frame in the record, the corresponding spacecraft event time, data presence indicator flags, GCF error flags, and other related parameters (reference Section 4) are written as one experiment data record. An accumulation of summary data shall be performed and a copy of the complete summary for a given period shall be provided to the PI with the EDR. Spacecraft clocks and Counters The on-board spacecraft clock (SCLK) is a 32-bit counter that increments once every 2 seconds. However, the SCLK is output in the telemetry only once every 32 minor frames (1 major frame). Thus, the relationship between data rate, number of minor frames (MF), time, and SCLK is as follows: ------------------------------------------------------------------- | Data Rate | #MF | #Seconds (N) | SCLK Increment | |----------------+--------------+----------------+----------------| | 1024 bps | 32 | 32 | 16 | |----------------+--------------+----------------+----------------| | 512 bps | 32 | 64 | 32 | |----------------+--------------+----------------+----------------| | 256 bps | 32 | 128 | 64 | |----------------+--------------+----------------+----------------| | 128 bps | 32 | 256 | 128 | ------------------------------------------------------------------- In addition, Word 63 in each minor frame contains an 8-bit counter that counts from 0-31 and is used to synchronize ground decommutation. Word 127 of each of minor frame contains a 5-bit auxiliary MF counter counting 0-31 and is used to identify major frame boundaries in real time as well as playback. Extraction Procedure The reference to "n" seconds pinpoints 2 and 3 below refer to the table of the previous section. The extraction procedure is applicable to all real-time science records. All science records correspond to an N second Ulysses telemetry major frame. Experiment data is extracted from the telemetry formats using SCLK count, ERT, and the minor frame counters to control the extraction process. The 1. Start of a new major frame: If the current real-time minor frame counter is ZNO, the minor frame contains the SCLK count. Save this new SCLK Count and the current ERT as references. Start building a new EDR record with the current minor frame. 2. Subsequent minor frames of the same major frame: If the current real-time minor frame counter is not zero and the ERT of the current minor frame is greater than the ERT of the previous minor frame "0" by less than N seconds, then continue building the current EDR record. If the current real-time minor frame counter indicates a skip greater than one count, locate the data by the real-time minor frame counter, update the current EDR record, and flag the filler blocks. 3. Subsequent minor frames of different records: If the current real-time minor frame counter is not ZNO and the ERT of the current minor frame is greater than the ERT of the previous minor frame by 8N seconds (4N for STO records) or more, then flag the remainder of the current EDR record with filler and output the current EDR record. Begin updating a new EDR record with the current minor frame flag filler data blocks as needed. 4. Real-time minor frame counter regression: If the current non-zero real-time minor frame counter indicates a decrement, flag the remainder of the current minor frame; flag filler data blocks as needed. Interspersed playback data extraction is handled in similar fashion. After real- time and playback data are separated, the extraction procedure is the same as for real-time telemetry. However, the auxiliary minor frame counter is used to identify minor frame order. Telemetry Bit Rate Change When the telemetry bit rate changes, the current EDR record shall be closed out and completed with filler bits to maintain standard record lengths. A new EDR record shall be started with the first telemetry frame at the new bit rate. The data shall be positioned in the output record according to clock values, preceded by filler data if required. (following excerpt: [ARMSTRONG&SAHI1996], pp. 105-106) Chapter 4.0 INSTRUMENT DETAILS 4.1 System Block Diagrams and General Features The HI-SCALE experiment consists of five apertures in two telescope assemblies mounted by means of two stub arms on a box containing all of the instrument electronics. The entire instrument is mounted on one corner of the spacecraft to give unimpeded fields of view to the telescopes. The HI-SCALE instrument viewing cones are shown in Figure 4.1. The entire system weighs 5.775 kg and consumes 4.0 W. An additional 7.5 W are available for heaters used in the telescopes and for thermal control of the log amplifiers and activation of the cover closure mechanisms. The measurements required to fulfill the heliospheric science objectives cannot be made with a single charged-particle sensor. To attain the lowest energy of response over a wide variety of particle species with appropriate geometrical factors and angular resolution, HI-SCALE utilizes three distinct silicon solid-state detector systems. These are Low-Energy Magnetic/Foil Spectrometers (LEMS/LEFS) and the Composition Aperture (CA). The CA system is sometimes referred to as the "WART" owing to its appearance. The LEMS/LEFS systems provide pulse-height-analyzed single-detector measurements with active anticoincidence. The CA system uses a multiparameter detection technique to provide measurements of ion composition in an energy range similar to those of LEMS/LEFS. The five separate detector systems are contained within two mechanical structures LAN2A and LAN2B as shown in Figure 4.2. The individual telescopes are referred to as LEFS 60, LEFS 150, LEMS 30, LEMS 120, and CA 60, where the number indicates the inclination of the telescope axis with respect to the spacecraft spin axis. The opening angles and spin-axis orientations of the telescopes are shown schematically in Figure 4.1. The MF, M'F, and BC detector pairs (Table 1) are identical (for ease of replacement); each consists of two equal 200 mm thick, totally depleted silicon surface barrier detectors. The D detector is a thin (5 mm) silicon detector of the epitaxial type. In the LEMS30 and LEMS120 telescopes, electrons with energies below ~ 300 keV are swept away from detectors M and M' with a rare-earth magnet. The geometric factor for the ions measured by detectors M and M' is ~ 0.48 cm^2 sr. In the LEFS60 and LEFS150 telescopes, a thin (~ 0.35 mg/cm^2) aluminized parylene foil (described in Section 4.2.4.1) prevents ions (~< 350 keV) from reaching the F and F detector, while electrons (~> 30 keV) can penetrate the foil with little energy loss. Both telescopes have a geometrical factor of - 0.48 cm^2 sr. In the LEMS 30 telescope the magnetically-deflected electrons are counted by a separate detector B. Section 4.7 in the following pages gives the calculated electron trajectories and resultant geometrical factors (-0.14 cm^2 sr). The CA telescope uses detectors D and C as show in Figure 4.3 as a [Delta]E vs. E telescope, with the B detector operating in anticoincidence. The geometrical factor for the DC'B combination is 0.24 cm^2 sr. The logic and discrete energy channels for the CA telescope are described in Section 4.4. In addition, the HISCALE data system (Section 4.5) uses a rotating priority scheme to identify those individual particle events whose energy loss signals in detectors D and C are pulse-height- analyzed. The pulse-height-analyzer system is also used to obtain 32 channel energy spectra (referred to as MFSA) of the singles rates in the LEMS and LEFS telescopes. The singles rates in the individual detectors are monitored and telemetered in a multiplexed mode. These rates are used as engineering monitors of the instrument performance and can also be used, if required, to make pulse-pileup corrections to the coincidence rates. Figure 4.4 shows now the principal features of the HISCALE system interact. (following excerpt: [ARMSTRONG&SAHI1996], pp. 178-185) Data Processing Data from the MFSA and the rate accumulators is summed and logarithmically compressed. Data from the CA PHA is sorted by a priority schedule during slow telemetry modes. The data -1 and data -2 flags, which call the rate and CA processing routine and the MFSA summation routine respectively, are set at the end of each valid sector of data accumulation. The data -3 flag, which calls the MFSA compression and output routine, is set at the end of each data collection cycle. The functions of each processing routine are described in this section. Rate Data Processing At the end of each sector, 2 but data from each of 48 channels is summed with the contents of a summation buffer. At times appropriate for each channel, the results are compressed to 8-bit counts by an algorithm described in space format channel and sector is the zeroed. Accumulation Periods Rate data is collected over one of five periods for a given telemetry mode. The periods are the following: a) One sector duration of one spin, for channels P'[1] - P'[5] and E'[1]-E'[4]. b) Two continuos sectors duration of one spin, for channels P[1]-P[4] and E[1]-E[4]. c) One sector duration for each spin of a spin pair, for channels P'[6]-P'[8], E'[5], FP'[6], FP'[6], FP'[7], D[2], D[2]a, D[3], D[4], and W[1]-W[8]. d) Two continuos sectors duration for each spin of a spin pair, for channels P[5]-P[8], E[5], FP[6], FP[7], and DE[1]-DE[4]. e) The entire duration of a spin pair, for the multiplexed channels S[1] and S[2]. The accumulations over sector pairs combine the sums over sectors 1 and 2, 3 and 4, 5 and 6, and 7 and 8, where sector one starts a commendable duration after the occurrence of the sun pulse. During slow telemetry rates, the same method applies, except that data of periods a and b are summed once a spin group (2n spins, where n is the mode ID), and data of periods c, d, and e are collected over a pair of spin groups. MFSA Schedule The MFSA schedule bits define the detector and sectors used to collect MFSA according to a preset schedule. Note that these lines refer to the schedule currently in use: the data contained in the first format was collected during the previous cycle and used the previous MFSA schedule. SECTORS COMPOSING DATA GROUP -------------------------------------------------------------------- |Schedule|MSB LSB|Detectors| A | B | C | D | |--------+-------+---------+---------+---------+---------+---------| | 1 | 000 | M | 1,2 | 3,4 | 5,6 | 7,8 | |--------+-------+---------+---------+---------+---------+---------| | 2 | 001 | F' | 1 | 3 | 5 | 7 | |--------+-------+---------+---------+---------+---------+---------| | 3 | 010 | M' | 1 | 3 | 5 | 7 | |--------+-------+---------+---------+---------+---------+---------| | 4 | 011 | F' | 2 | 4 | 6 | 8 | |--------+-------+---------+---------+---------+---------+---------| | 5 | 100 | M' | 2 | 4 | 6 | 8 | |--------+-------+---------+---------+---------+---------+---------| | 6 | 101 | F' | 1 | 3 | 5 | 7 | |--------+-------+---------+---------+---------+---------+---------| | 7 | 110 | F | 1,2 | 3,4 | 5,6 | 7,8 | |--------+-------+---------+---------+---------+---------+---------| | 8 | 111 | F' | 2 | 4 | 6 | 8 | -------------------------------------------------------------------- MFSA Data Processing MFSA data is accumulated according to one of the 8 schedules. Each schedule is active for the duration of the data collection cycle, regardless of the telemetry mode. At the end of the appropriate sectors, as indicated by the schedule, 8-bit data from each of 32 channels is summed into one of four, 16-bit x 32 buffers. This routine may also be called when an overflow of one of the 8-bit counts occurs in the MFSA hardware. An overflow of the 16-bit summation is telltaled in the status trailer, indicating the data group and the spin group for which the overflow occurred. After an overflow occurs, summation for all data groups stops for future sectors until the end of the data cycle. The number of the current MFSA schedule is telltaled in the status preamble. MFSA Downtime The data system processes interrupts from the MFSA in addition to the regularly scheduled loads. As more interrupts occur, a greater part of the accumulation time is unused while the MFSA waits for the data system to notice and read it. The effect of these delays is tabulated below, for given rates into a single energy channel: ------------------------------------------------------------------- | Rate Per Channel (Events/Second) | % of Time Not Accumulating | |----------------------------------+------------------------------| | 50 | 0.6 | |----------------------------------+------------------------------| | 100 | 0.6 | |----------------------------------+------------------------------| | 250 | 1.0 | |----------------------------------+------------------------------| | 500 | 1.4 | |----------------------------------+------------------------------| | 1000 | 2.8 | |----------------------------------+------------------------------| | 2000 | 6.1 | |----------------------------------+------------------------------| | 3000 | 10.2 | |----------------------------------+------------------------------| | 3880 | 12.4 | ------------------------------------------------------------------- CA PHA Data Processing The data system sequences the priority schedule provided to the CA PHA. The CA PHA, designed by C.R. Tielens, uses this information to select which events will be retained as data, based on the table shown in [FIG_P239]. For example, if all four event types occur while using schedule 11 (S1, SO), then data for the O and Fe events will be retained and provided to the data system at the end of the sector. The data system notes that the event of the lowest priority was type O, and for the corresponding sector of the next spin group assigns the appropriate schedule bits. The schedules for the 8 sectors of a spin are independent of each other. Slow telemetry Rates During slower telemetry rates, the schedule for each sector is maintained for all the spins of a spin group. During these times, the number of spins within a format has increased but the amount of data to be output must be constant. So, at the end of each sector, the data system chooses two events to be retained from the four available: Two events from the last time plus two new ones ( i.e., the sorting is done sequentially, rather than collecting data for each spin of a group and then sorting). The two events to be retained in this case are the two of the highest priority, based on the current schedule. So within a spin group, the two highest priority events are retained for output; of these, the lowest priority event determines the schedule for the next spin group. The priority schedule was designed by R.E. Gold using computer simulations of probable event scenarios, and attempts to provide even sampling rates for all data types. Log Compression Routine This routine compresses 16-bit (MFSA) and 24-bit (rate) data to an 8-bit word, composed of a 4-bit exponent followed by a 4-bit mantissa. The value of the data is (Mantissa + 16) x 2^(Exponent-1), except that when the exponent is zero, then the value equals the mantissa. The word precision of the data is +1 2^4 = +1/16 = +6%, -0%. -0 -0 Table of Values The binary and decimal value ranges for each exponent are shown below: ---------------------------------------------------------------- | Exponent | Binary Range | Decimal Range | |------------------+---------------------+---------------------| | 0 | 000000-00000F | 0-15 | |------------------+---------------------+---------------------| | 1 | 000010-0001F | 16-31 | |------------------+---------------------+---------------------| | 2 | 000020-00003F | 32-63 | |------------------+---------------------+---------------------| | 3 | 000040-00007F | 64-127 | |------------------+---------------------+---------------------| | 4 | 000080-0000FF | 128-225 | |------------------+---------------------+---------------------| | 5 | 000100-0001FF | 256-511 | |------------------+---------------------+---------------------| | 6 | 000200-0003FF | 512-1023 | |------------------+---------------------+---------------------| | 7 | 000400-007FF | 1024-2047 | |------------------+---------------------+---------------------| | 8 | 000800-000FFF | 2048-4093 | |------------------+---------------------+---------------------| | 9 | 001000-001FFF | 4096-8191 | |------------------+---------------------+---------------------| | A | 002000-003FFF | 8192-16383 | |------------------+---------------------+---------------------| | B | 004000-007FFF | 16384-32767 | |------------------+---------------------+---------------------| | C | 008000-00FFFF | 32768-65535 | |------------------+---------------------+---------------------| | D | 010000-01FFFF | 65536-131071 | |------------------+---------------------+---------------------| | E | 020000-03FFFF | 131072-262143 | |------------------+---------------------+---------------------| | F | 040000-07FFFF | 262144-524287 | ---------------------------------------------------------------- Overflow MFSA data is limited to 16 bits and, therefore, does not exceed the limits of the log compression routine. Rate data counts may be as high as 223-1. The MS Byte of rate data with be treated modulo (220 -1), so that the modulus may be added to the indicated value to obtain the actual value, when indicated by the time history of that channel. A graphical depiction of the result is shown in figure 13. Timing The basic timing sequence of the data system is as follows: Data is accumulated, stored, and processed over various numbers of spins and sectors, and output a short time later to the S/C telemetry channel. This simple scenario is complicated by many factors. There are four S/C telemetry modes, implying four bit rates for each telemetry mode. In addition, the telemetry mode can change at the beginning of any S/C format, not necessarily at the beginning of the LAN Data format. The input timing sequence is also complex. The range of and transitions in spin rate must be accounted for. The experiment provides information about the spatial origin of events by treating each spin as eight sectors of approximately equal duration. Two sectoring modes are provided. During sun sectoring, each spin is defined as starting some fractional spin after the sun-S/C meridian is passed. The duration of each sector is defined by a fixed count of a clock derived from the previous two detections of the sun crossing. When the period between sun crossings (the spin rate) is outside some fixed limits, the time sectoring mode is active. A pseudo-sector of commendable duration is created, and accumulations then occur over this new period. The experiment may be commanded to either use only time sectoring or to automatically switch between time sectoring and sun sectoring based on spin duration. This prevents processing data which is generated too quickly for accurate sampling, due to delays of program execution time, or too slowly to fit the data format. During times of minor spin rate variation, we attempt to maintain nearly constant the lag between the occurrence of the sun pulse and the beginning of the first sector, as well as the duration of each sector. The lag between the sun-S/C meridian crossing and the beginning of a spin is also commendable. The lag minimizes the number of sectors for which data is strongly affected by the sun. Timing delays, variations in spin rate, and sectoring modes for each spin group are telemetered in the status trailer of the data stream and in the digital housekeeping (DHK) stream. This section describes the input and output timing sequences. In general, the process for the fastest telemetry mode is described, and then details for slower modes are defined. Output Timing Initialization The data system maintains a set of pointers which are moved through the output data queue addresses as each frame of data is output. These pointers are initialized at power-up and at the occurrence of the four-format pulse. Since power-up is assumed to be independent of telemetry timing, the output data is skewed until the first subsequent occurrence of the four format pulse. Digital housekeeping data and the analog housekeeping channel assignments are also invalid until the four format pulse occurs. Note that although the digital science and housekeeping output channels are then synchronized, data is not yet valid. The relationship within the queue of the input pointers to the output pointers will not be guaranteed correct until the next data collection cycle starts. So analog housekeeping data becomes valid upon the first four-format pulse occurrence after power-up, and digital science and housekeeping data becomes valid upon the second four-format pulse occurrence after power-up. Telemetry Mode Changes Changes of the S/C telemetry mode do not cause frames or formats to be dropped, so the output pointers are not affected. However, the input sequence becomes completely meaningless, so the synchronization procedure must occur again. Therefore, after the mode changes, data is invalid until the second subsequent occurrences of the four-format pulse (counting any which occurred at the same time as the mode change. Figure 14 shows two potential sequences of events and the corresponding status of data validity.) When the mode changes, the mode change bit is set and appears immediately afterward in the status preamble. This bit is reset at the next occurrence of the format at pulse, so it appears only once for each change of mode. The mode ID bits, however, are telemetered for each format of data. Slow Telemetry Modes The output format is identical to figures 3-5 regardless of S/C telemetry mode; only the interval over which data is accumulated, changes. Input Timing, Spins A spin group consists of 1,2,4, or 8 consecutive spins, depending on the S/C telemetry mode; 1 spin during the fastest mode and 8 spins during the slowest. A data collection cycle, or cycle, consists of 10 consecutive spin groups, and therefore, 10, 20, 40, or 80 spins. There is a one-to-one correspondence between data collected during a cycle and data output during a four-format, with minor exceptions. Cycle Staring Slot A data queue is used, rather than saving data for an entire input cycle, in order to minimize the memory requirements. Therefore, some minimum delay is required between inputting and outputting data. The delay is ensured by starting the data input cycle within a fixed window in time prior to the S/C 4-format pulse. Computer simulations of the input and output process were run under conditions of varying spin rate to determine the minimum delay and maximum data buffer required. It was determined that the starting window should extent from 1-3/4 to 3/4 maximum spin group durations prior to the four format pulse. Skipped Spins When the spin rate is faster than the slowest allowable rate, the time required to input data for 10 spin groups will be less man the 4-format period. The end of the data collection cycle will slip backward in time relative to the cycle starting slot, and eventually will end prior to the cycle starting slot. When this happens, spins will be skipped for data collection purposes until one ends within the cycle starting slot. Note that spins, not spin groups, will be skipped. This feature allows sampling during the full desired spin count, particularly after initialization, and helps avoid dropped spins. Dropped Spins At times it will be necessary to drop spins from the end of the data collection process, that is, to collect data over a number of spins less than 10 x 2n. This situation may occur during initialization or when the S/C spin rate is very close to either limit of the allowable rates for sun sectoring. The final spin count for each collection cycle is telemetered in the status trailer, and allows for a count of fifteen less than the expected number. Cycle Start Under conditions similar to those causing dropped spins, it will be necessary to start the data cycle with the second spin group rather than the first. This condition is also minor in the status trailer. Note that the status trailer telltales the conditions which have most recently occurred, which are the end of cycle M and the start of cycle M+1. Data Validity If spins have been dropped, the rate and CA data for those spins is invalid. The rate common to a spin group which has lost some spins must be multiplied accordingly. The interval of MFSA data collection may or may not have been shortened: this cannot be determined. If the cycle started with spin group 2, the rate and CA data for group 1 and the rate data common to groups 1 and 2 is invalid. The MFSA data is then accumulated over the duration of 9 spin groups rather than 10. Changes in S/C Spin Rate When using the sun sectoring mode the data system relays the origin of events to the angle relative to the sun line. During time sectoring, the pseudo-sectors generated can be related to the four format pulse, and their duration is commendable. In either event, the S/C will time-tag epochs of the magnetic field zero-crossing, and ground programming will attempt to relate the magnetic field to the origin of events. The steady state spin period of the S/C is nominally 11.76 to 12.24 seconds; there will also be periodic transient effects. A requirement was derived for the data system to accommodate a range of 11.17 to 12.275 seconds while using sun sectoring, and to use time sectoring if the period falls outside of that range. The data system can also be commanded to maintain time sectoring regardless of spin period. This feature would be used in the event of failure or erratic operation of the S/C sun sensor. The sectoring mode may change at any time as long as auto sectoring is enabled. Telemetering Sector Mode Changes The sectoring mode used for each spin group is telemetered in bytes 17 and 18 of the status trailer. These bits are set at the end of the first spin of each group. Therefore, any mode change first appears for the next group. During the fastest telemetry mode, the contents of trailer byte 17 being (0011 1111) indicated that spin groups 1 and 2 (of 1 to 10) used sun sectoring and groups 3 through 8 used time sectoring. During slower telemetry modes, the same contents indicate the same result for groups 1 and 3 through 8; however, group 2 may consist entirely of sun sectored spins, the corresponding data validity bit in byte 15 of the trailer will be zero; other wise it will be a one. Similar statements apply to sectoring mode changes from time to sun. Even if the validity bit is zero, the e last spin of the group prior to the mode change may have been truncated at an unknown time, and data for mat prior group is invalid. Input Timing, Sectors Time Sectoring When time sectoring is used, sectors are defined as count of the high frequency clock The duration of these sectors then varies only by command. This duration can take on values ranging from 1.414 to 1.589 seconds in 0.017 second increments. (Spin periods from 11.286 to 12.714 seconds in 0.413 second increments). It is necessary to specify a time sector duration each time automatic mode switching is enabled or disabled. Time sectoring spins are generated independently of all epochs except the cycle starting slot, but can be related to the four format pulse. Sun sectoring When sun sectoring is used, a spin is defined as starting some commendable interval after the sun pulse epoch. Each sector thereafter normally lasts the duration of 2048 counts of the 16384 x Spin Clock. The S/C adjusts the rate of the 16384 x Spin Clock at each sun pulse epoch based on the interval since the previous epoch, so the 16384 x Spin Clock rate lags the spin rate by one spin at all times. During times of varying spin rate the sun pulse can therefore occur more or fewer than 16384 counts after the previous epoch. Timing Requirements Based on the above scenario, two requirements were derived for data system operation during sun sectoring. The lag from the sun pulse epoch to the start of a new spin is to be maintained constant in terms of counts of the (varying) 16384 x Sin clock. Also, the duration of each sector is to be maintained constant as far as possible. Timing conditions relating desired sector boundaries to the expected and the actual sun pulse epoch are shown in Figure 15 for a commendable lag of less than one sector duration, and in figure 16 for a lag of more than one but less than two sector duration. At each sun pulse or sector end epoch, the data measures the time relative to the previous sun pulse epoch and adjusts the state of internal flags to provide for future during operations. Telemetering Changes in Spin Rate Ant decrease in spin rate can cause truncation or elimination of sector 8. In order to properly process the counts for each sector and relate them to the orientation of the S/C, the changes in spin rate are telemetered in the digital housekeeping channels. Telemetering Delays in Sector Flag Processing Short delays may infrequently occur between a sector flag epoch and processing data for that sector. These delays are telemetered in the status trailer. Interpretation of Data Validity When a change of sectoring mode id indicated in the status trailer, rate and CA data from the last spin group under the old mode is invalid. CA data is valid during the previous and the next spin group. If the new mode starts with an odd spin group, all of the rate data for the new group is valid, and that rate data which is summed over two spin groups interval (labeled 'C' & 'D' and 'S' in the flowcharts) for the previous spin group is invalid. If the new mode starts with an even spin group, that rate data summed over only one spin group interval ('A & B') is valid for the new group, and the C & D and S data for the spin group pair is valid but was summed during one spin group only. (following excerpt: [ARMSTRONG&SAHI1996], pp. 231-232) 5.3 Backgrounds Document Reference: Dr. S. J. Tappin; Version 7 Oct., 1993 Early in the mission, GMS made a preliminary determination of the HI-SCALE instrument backgrounds based on two relatively short periods in early 1991 when the counting rates in most detector channels appeared to close to the instrument background levels. The dates which he used were 12 Jan., 1991 for electrons and Feb. 1991 for ions. It now seems appropriate to re-assess these background levels for a number of reasons: 1. In the most recent stages of the mission there have been many more periods during which the fluxes were at, or close to, background so that the determination will be easier and we can be more certain that we are really at background. (Also it should be possible to get values for those channels which could not be measured in the first determination. 2. There is a possibility that the background levels are changing as the RTG becomes dirtier and fuel is used. This needs to be included in any automatic background removal. 3. Recent spectral plots have shown clear evidence of distortion due to background levels, thus indicating a need to include some kind of background subtractions. Since a casual examination of any time-series plot for the period from the second half of 1992 to the present time shows that all channels spend long periods at their background levels, it becomes relatively simple to determine the background level. The time series for the whole mission to the end of August 1993 is shown following the text of this document. The time-range selected to compute the backgrounds for all channels was from Day 196 (July 14) 1992 to Day 150 (May 30) 1993, during which time there was a regular series of CIRs with low count rates between. For each channel the time-series was accumulated for hourly averages of the counting rates (using PLOT RATES) and then a histogram was plotted of frequency of occurrence against count rate. For those channels with very low background level for several days at a time during the inter-event periods, there is a little cause to worry about the hourly averages not giving a true measure of the background rate. From these histograms the background was estimated as the end of the monatomic rise in frequency as the count-rate increase from zero. (Note that this is not always the modal count rate as some low-energy channels have multiple maxima and the first is not always the largest.) The plots used are summarized In table 5.3.1 Table 5.3.1: Type of Average and resolution of background determination as a function of estimated background --------------------------------------------------------------------- | Range | Average | Plot Max | Resolution | Accuracy | |-------------+------------+-------------+-------------+------------| |>2 c/s | Hourly | 10 | 0.02 | 0.05 | |-------------+------------+-------------+-------------+------------| |0.2-2 | Hourly | 2 | 0.01 | 0.01 | |-------------+------------+-------------+-------------+------------| |0.02-0.2 | 6-Hour | 0.2 | 0.001 | 0.001 | |-------------+------------+-------------+-------------+------------| |0.001-0.01 | Daily | 0.01 | 0.0001 | 0.0001 | |-------------+------------+-------------+-------------+------------| |0.00004-0.001| Daily | 0.005 | 0.00001 | 0.00001 | |-------------+------------+-------------+-------------+------------| |<0.00004 | Daily | 0.00005 | 0.000001 | n/a | --------------------------------------------------------------------- In order to make some son of determination of the way in which the background has changed over the course of the mission so far, count-rate histograms have been produced for selected channels dividing the first 1000 days of the mission mro 10 100-day blocks. The results of the background determination are shown in the following pages as the count-rate histogram plots for the various channels, along with the time variation histograms for channels DE2, P5 and E'1. Most channels showed a well defined background level which could be measured. However for the high WART channels the level was less than 1 count/day so for these an upper limit is presented (it would be possible to estimate a level from the ratio of the 0,1 and 2 count days but the levels are so low it hardly seems worthwhile since it is not very meaningful to subtract a background if there is a less than 1 count per averaging period). The results are presented in Tables 5.3.2 to 5.3.7 along with the earlier determinations where these are available. It seems possible from this that the lower electron channels have shown an increase in background count rate of 30-50% over the approximately two years between the determinations, while these electron channels have shown a decrease. Although some of the 100-day histograms do not show well-defined background levels; those which do, show that rather than varying steadily through the mission the background underwent a significant shift in the vicinity of Jupiter and have been otherwise relatively constant. This is not too surprising as the background are dominated by emissions from the RTG scattered from the spacecraft fuel, and a significant amount of fuel was used for course maneuvers as Ulysses approached Jupiter. The best option at present for setting a background subtraction level therefore seems to be to have two levels one for before encounter and the other after. It seems therefore appropriate to add a further column to the IDF.DAT file to accommodate the two background levels. A sample IDF.DAT follows the tables at the end of this document, in this file, the two levels are the old GMS determination pre-counter and the values from this document afterwards. In the cases where the level was not determined earlier I've used one of zero, the post-encounter value or the value from the other corresponding head, in any released version a proper determination of these levels will be needed. The BGC line in the file gives the time to change from one set of levels to the other, currently noon during the switch-off. The two time-series plots show the 6-hour averaged data for three selected channels before and after subtraction of the backgrounds. Points lying less than 2s above the background are omitted.