The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on the New Horizons Mission Ralph L. McNutt, Jr.1,6, Stefano A. Livi2, Reid S. Gurnee1, Matthew E. Hill1, Kim A. Cooper1, G. Bruce Andrews1, Edwin P. Keath3, Stamatios M. Krimigis1,4, Donald G. Mitchell1, Barry Tossman3, Fran Bagenal5, John D. Boldt1, Walter Bradley3, William S. Devereux1, George C. Ho1, Stephen E. Jaskulek1, Thomas W. LeFevere1, Horace Malcom1, Geoffrey A. Marcus1, John R. Hayes1, G. Ty Moore1, Bruce D. Williams, Paul Wilson IV3, L. E. Brown1, M. Kusterer1, J. Vandegriff1 1 The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA 2Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228, USA 3The Johns Hopkins University Applied Physics Laboratory, retired 4 Academy of Athens, 28 Panapistimiou, 10679 Athens, Greece 5The University of Colorado, Boulder, CO 80309, USA 6443-778-5435, 443-778-0386, Ralph.mcnutt@jhuapl.edu The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) comprises the hardware and accompanying science investigation on the New Horizons spacecraft to measure pick-up ions from Pluto's outgassing atmosphere. To the extent that Pluto retains its characteristics similar to those of a "heavy comet" as detected in stellar occultations since the early 1980s, these measurements will characterize the neutral atmosphere of Pluto while providing a consistency check on the atmospheric escape rate at the encounter epoch with that deduced from the atmospheric structure at lower altitudes by the ALICE, REX, and SWAP experiments on New Horizons. In addition, PEPSSI will characterize any extended ionosphere and solar wind interaction while also characterizing the energetic particle environment of Pluto, Charon, and their associated system. First proposed for development for the Pluto Express mission in September 1993, what became the PEPSSI instrument went through a number of development stages to meet the requirements of such an instrument for a mission to Pluto while minimizing the required spacecraft resources. The PEPSSI instrument provides for measurements of ions (with compositional information) and electrons from 10s of keV to ~1 MeV in a 120deg. x 12deg. fan-shaped beam in six sectors for 1.5 kg and ~2.5 W. Keywords: New Horizons, PEPSSI, Pluto, Energetic particle instrument Abbreviations 1PPS - One Pulse Per Second ADC - Analog-to-digital converter APL - Applied Physics Laboratory ASIC - Application specific integrated circuit C&DH - Command and Data Handling CCSDS - Consultative Committee for Space Data Systems CFD - Constant Fraction Discriminator CSA - Charge Sensitive Amplifier eV- Electron Volt FITS - Flexible Image Transport System FOV - Field of View FWHM - Full Width Half Maximum GSE - Ground support equipment GSFC - Goddard Space Flight Center HDU - Header Data Unit HV - High Voltage Section of HVPS HVPS - High Voltage Power Supply (HV and Bias Supply Sections) IEM - Integrated Electronics Module IGSE - Instrument Ground Support Equipment ICD - Interface Control Document ITF - Instrument Transfer Frame LED - Leading Edge Discriminator MCP - Micro-channel plate MIDL - Mission Independent Data Layer MDM - Master Data Manager MET - Mission Elapsed Time MOI - Moment of inertia NA - Not applicable NASA - National Aeronautics and Space Administration NH - New Horizons ns - nanosecond = 10-9 s PDS - Planetary Data System PEPSSI - Pluto Energetic Particle Spectrometer Science Investigation PFF - Pluto Fast Flyby PHA - Pulse height analysis ps - picosecond = 10-12 s psi - Pounds per square inch RTG - Radioisotope Thermoelectric Generator SQL - Structured Query Language SSD - Solid-state detector SSR - Solid-state recorder STP - Supplemented Telemetry Packet SwRI - Southwest Research Institute TDC - Time-to-digital chip TOF - Time of flight TRIO - Temperature remote input/output T-V Thermal-vacuum UART - Universal asynchronous receive and transmit 1.0 INTRODUCTION.............................................................. 1.1 BEGINNINGS ............................................................... 1.2 PREVIOUS SIMILAR INSTRUMENTATION ......................................... 2.0 SCIENTIFIC BACKGROUND AND OBJECTIVES...................................... 2.1 THE INTERACTION OF PLUTO WITH THE SOLAR WIND.............................. 2.2 PEPSSI SCIENCE OBJECTIVES ................................................ 2.3 MEASUREMENT REQUIREMENTS.................................................. 2.3.1 Measurement Ranges ..................................................... 2.3.2 Derived Instrument Specifications ...................................... 2.3.3 Measurement Resolution Requirements .................................... 2.3.4 Platform Requirements .................................................. 2.3.5 Time Resolution Requirement ............................................ 2.3.6 Calibration Requirements ............................................... 3.0 TECHNICAL DESCRIPTION..................................................... 3.1 INSTRUMENT OVERVIEW....................................................... 3.1.1 Differences from EPS on MESSENGER....................................... 3.2 MECHANICAL DESIGN ........................................................ 3.2.1 Dimensions and Mounting................................................. 3.2.2 Mass Properties ........................................................ 3.2.3 Deployable Cover ....................................................... 3.2.4 Instrument Purge........................................................ 3.2.5 Handling Requirements .................................................. 3.2.6 Transportation and Storage ............................................. 3.2.7 Vacuum and Outgassing Requirement....................................... 3.3 DETECTORS AND ELECTRONICS ................................................ 3.3.1 Energy Measure ......................................................... 3.3.2 Time-of-Flight Measure.................................................. 3.3.3 Electronics ............................................................ 3.3.4 Operation............................................................... 3.3.5 Electrical Interface ................................................... 3.4 TELEMETERED DATA PRODUCTS ................................................ 3.4.1 Proton and Electron Energy Spectra...................................... 3.4.2 Heavy Ion Energy Spectra................................................ 3.4.3 TOF-Only Velocity Spectra .............................................. 3.4.4 Singles-Event Data (for Event Validity Check)........................... 3.4.5 PHA (Pulse Height Analysis) Event Data ................................. 3.4.6 Non-Packetized Housekeeping Data ....................................... 3.4.7 Quick look Diagnostic Data ............................................. 3.4.8 Instrument Data Rate Summary ........................................... 3.4.9 Calibration ............................................................ 3.4.10 Memory Image Dump ..................................................... 3.5 COMMANDS ................................................................. 3.5.1 Energy Commands ........................................................ 3.5.2 HVPS Commands .......................................................... 3.5.3 TOF Commands ........................................................... 3.5.4 Process Control Commands ............................................... 3.5 TELEMETRY AND COMMAND FORMAT ............................................. 3.5.1 Data Rate and Volume.................................................... 3.5.2 Telemetry Formatting ................................................... 3.5.3 Command Formatting...................................................... 3.5.4 MET Time Message........................................................ 3.6 INSTRUMENT ENVIRONMENTAL DESIGN REQUIREMENTS ............................. 3.6.1 Thermal Interface....................................................... 3.6.2 PEPSSI Thermal Design Requirements...................................... 3.6.3 Radiation Shielding Requirements........................................ 3.6.4 Electrostatic Requirements.............................................. 3.7 TEST REQUIREMENTS......................................................... 3.7.1 Vibration............................................................... 3.7.2 Thermal................................................................. 3.7.3 Acoustics and Shock..................................................... 3.7.4 EMI / EMC .............................................................. 4.0 PERFORMANCE .............................................................. 4.1 DATA CONVERSION TO PHYSICAL UNITS ........................................ 4.1.2 Flux, Differential Intensity and Phase Space Density ................... 4.1.3 Definition of Sensor Transfer Function and Geometric Factor............. 4.1.4 Goals of the PEPSSI Characterization and Calibration Efforts............ 4.2 SIMULATIONS .............................................................. 4.2.1 Geometric Factor ....................................................... 4.2.2 Ion Measurements ....................................................... 4.2.3 Electron Measurements .................................................. 4.3 PEPSSI FLIGHT UNIT CALIBRATION ........................................... 4.3.1 The JHU/APL Calibration Facility........................................ 4.3.2 Test Set Up............................................................. 4.3.3 Ground Calibration ..................................................... 4.3.4 Spacecraft Integration and Test Calibrations ........................... 4.3.5 Summary of PEPSSI Flight Model Calibrations............................. 4.4 PEPSSI ENGINEERING MODEL CALIBRATIONS..................................... 4.4.1 Original PEPSSI Foils................................................... 4.4.2 PEPSSI Start and EPS Stop Foil.......................................... 4.4.3 Engineering Model Plans ................................................ 4.5 IN-FLIGHT CALIBRATION .................................................... 4.5.1 Flight Performance ..................................................... 4.5.2 Jupiter Flyby........................................................... 5.0 OPERATIONS AND SCIENCE.................................................... 5.1 INSTRUMENT OPERATIONS .................................................... 5.1.1 Cover Release In Space ................................................. 5.1.2 Idle State (HVPS Disabled) ............................................. 5.1.3 HVPS Activation......................................................... 5.1.4 Science Mode Operation ................................................. 5.1.5 Power Down Operation.................................................... 5.2 DATA AND DATA ARCHIVING .................................................. 5.2.1 Level 1................................................................. 5.2.2 Level 2................................................................. 5.2.3 Level 3................................................................. 5.2.4 MIDL.................................................................... 6.0 CONCLUSION................................................................ ACKNOWLEDGEMENTS.............................................................. REFERENCES.................................................................... 1.0 Introduction The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) is one of seven scientific instruments/experiments (Weaver et al. 2007) on board the New Horizons spacecraft (Fountain et al. 2007), now on its way to Pluto (Stern 2007). While it is doubtful that Pluto has an intrinsic magnetic field and magnetospshere that accelerates charged particles to high energies, Pluto does have (or has had in the very recent past) a substantial atmosphere (Brosch 1995; Elliot et al. 2003; Elliot et al. 1989; Elliot et al. 2007; Sicardy et al. 2003) that is escaping into the solar wind in a comet-like interaction (Bagenal et al. 1997; Bagenal & McNutt 1989; Delamere & Bagenal 2004; Kecskemety & Cravens 1993; Krasnopolsky 1999; McNutt 1989; Tian & Toon 2005; Trafton et al. 1997). Measured interactions at comets show that the outgassing cometary neutral atoms and molecules charge-exchange with the solar wind and are accelerated in the process (Coates et al. 1993a; Coates et al. 1993b; Galeev 1987; Galeev et al. 1985; Huddleston et al. 1993; Mendis et al. 1986; Motschmann & Glassmeier 1993; Neugebauer 1990). By measuring the in situ energetic particle population, identifying those ions from the emitting body, and noting their variation with distance to the emitting body, the outgassing source strength may be deduced (Gloeckler et al. 1986); it is also important to note that energization beyond what one would naively expect from pick-up alone is also observed at comets (McKenna- Lawlor et al. 1986; Richardson et al. 1986; Sanderson et al. 1986; Somogyi et al. 1986). For example, shock acceleration of particles at Venus can elevate some of the particles to substantial (~100 keV) energies (Williams et al. 1991). Making these measurements to determine the "outer boundary" of the influence of Pluto's atmosphere is the primary objective of the PEPSSI instrument. The extent of the interaction with the solar wind will be determined by comparing the PEPSSI measurements with those obtained of the solar wind by the SWAP instrument (McComas et al. 2007). A deduced atmospheric profile from the surface to the edge of Pluto's atmosphere will be assembled from combining PEPSSI, the New Horizons ultraviolet imaging spectrometer Alice (Stern et al. 2007) and the New Horizons radio experiment (REX) (Tyler et al. 2007) measurements. PEPSSI combines energy and time-of-flight measurements in a low-mass (1490g), lowpower (2.3 W) unit. At the same time, the instrument has a relatively large total geometric factor of ~0.1 cm2 ster and enables directional information of the particle distribution across a ~12deg. x 160deg. swath in six ~25deg.-wide angular bins. The instrument also discriminates between electrons and ions without the use of (relatively) heavy permanent magnets (or power-hungry electromagnets). While the use of silicon solid state detectors (SSDs) for measuring energetic particles date back to the near-beginning of the space program, the additional species discrimination made possible by including time-of-flight measurements (thus independently, but simultaneously, measuring both an ion energy and speed) is a relatively newer development. Further, packaging of this capability into such a compact instrument providing a variety of engineering and programmatic challenges. This story leading to PEPSSI is of sufficient import that we summarize it here. 1.1 Beginnings During the 1990s, the Pluto Fast Flyby (PFF) had evolved into the Pluto Express (PE) mission but continued to be under active consideration as the next outer planets mission by the Solar System Exploration Division (Stern 2007). As early as 26 Feb. 1993, the Space Physics Subcommittee adopted a recommendation that noted during the Pluto flyby itself "Data on fields and particles are essential to the understanding of atmospheric scavenging, surface darkening, and to important inferences of internal structure." They noted that in addition, en route to Pluto, fields and particles instruments could provide unique new information on the solar wind and a baseline for termination shock studies with minimal impact on mission resources. They further noted that the data would be central to the investigation of heliospheric structure, as the Pluto flyby trajectory would be almost aligned with the symmetry axis of the interstellar wind and could a gap between the trajectories of the Pioneer and Voyager spacecraft. A Science Definition Team (SDT) for Space Physics Objectives for the Pluto Fast Flyby Mission (SPOPFFM) (Neugebauer et al. 1993) was established by Dr. George Withbroe to establish science objectives for both cruise (heliospheric) science as well as particles and fields science during a Pluto-Charon flyby, establish a conceptual science payload, and report out from NASA's Space Physics Division on the prospects for scientific contributions that would accrue to the Pluto mission from such additions. The SDT noted that any such instrumentation must fit into a very small mass and power profile for the mission concept under discussion at that time; nominal values of 1 kg and 1 W for all fields and particles investigations (as a package) were discussed. Combinations of miniaturized plasma, suprathermal particle and energetic particles sensors were considered along with a plasma wave instrument and a magnetometer. Although a realistic choice of instrumentation had to be based upon maximizing the science return while minimizing the impact on the already tightly constrained spacecraft resources, the overriding constraints of low mass and low power remain were recognized as the principal design drivers. These initiatives in the space physics community, along with a good showing of space physics topics at the Pluto/Charon meeting in Flagstaff in July 1993 led to the inclusion of Dr. Ralph McNutt on the Science Definition Team for what was then Pluto Express, chaired by Professor Jonathan Lunine. Meetings of the SDT were held in Tucson 5-7 Apr 1995 and 21-23 Jun 1995. The SDT report (Lunine et al. 1995), while documenting the potentially unique interaction between Pluto's escaping atmosphere and the solar wind, noted that there were no in situ instruments for making such measurements included as part of the baseline mission, although such a payload was mentioned as part of a potential international collaboration. Subsequent evolution of the idea into what finally became New Horizons is discussed in the introductory paper in this volume. Suffice it to say, the push for instrumentation on a dedicated Pluto mission to directly measure the in situ environment there began its concerted push toward what became the PEPSSI and SWAP (McComas et al. 2007) investigations began in 1992. The rest is history. 1.2 Previous Similar Instrumentation PEPSSI combines time-of-flight (TOF) and total-energy measurements in six angular sectors packaged into a compact arrangement (Fig. 1). Unlike plasma instruments incorporating a TOF system at lower energies, no pre-acceleration, high-voltages (>/= 10s of kilovolts, kV) are used in these devices. Fig. 1. PEPSSI mounted on the New Horizons spacecraft via its complex-angle bracket in the clean room at APL prior to being covered with its thermal blanket. The first such instrument developed at APL was the Ion Composition Telescope (ICT) on the Firewheel satellite (1105 kg) that was launched aboard the second Ariane test flight by the European Space Agency (ESA) (De Amicis 1988). The ICT was a combined tineof- flight (TOF) and total energy ion spectrometer to operate over the range > 15 keV/nuc to >500 keV/nuc (the satellite also carried four lithium canisters and eight barium canisters that would release their contents in space). The ICT mass was 8.5 kg, power 5 W, and nominal data rate 180 bps. The satellite was launched 23 May 1980; but a fuel system fault resulted in failure of all four first stage engines, and the vehicle fell into the South Atlantic after launching from Kourou, French Guiana. The design was the basis for the Medium-Energy Particle analyzer (MEPA) instrument on the AMPTE CCE (Active Magnetosphere Particle Tracer Experiment Charge Composition Explorer) spacecraft (McEntire et al. 1985). MEPA was designed to measure spectra and composition of magnetospheric particles from ~10 keV per nucleon (oxygen) to ~6 MeV. A similar time-of-flight scheme was incorporated into the Energetic Particles Detector (EPD) on the Galileo spacecraft (Williams et al. 1992) and into the the Magnetospheric Imaging Instrument (MIMI) now operating at Saturn as part of the Cassini spacecraft payload (Krimigis et al. 2004). By including a "front-end" electrostatic analyzer and post-acceleration (after that analysis section), the technique can also be applied to lower-energy particles (Gloeckler et al. 1985). Typically, sweeping magnets have been used to eliminate electron signals in configurations without a "front end" electrostatic analyzer. On the Ulysses Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies (HI-SCALE) experiment both a sweeping magnet and foil spectrometer technique are employed (Lanzerotti et al. 1992). In the latter, an aluminized parylene foil is used to accept electrons and reject ions at one of the detectors. This is the same approach used on PEPSSI as well as on the energetic Particle Spectrometer (EPS) on the MESSENGER spacecraft. This approach saves the mass and complexity of a separate electron detector, or of a magnet. A similar approach has been selected for comparable instruments selected for inclusion on the Juno New Frontiers mission to Jupiter and the Radiation Belt Storm Probes mission at Earth. The MESSENGER EPS and New Horizons PEPSSI instruments have been known as "the hockey puck" due to the cylindrical aperture/detector/TOF section that has about the same proportions and size as a regulation ice hockey puck. The original work on such a miniaturized sensor was begun at APL under internal research and development funds. Following the Space Physics SDT report, a proposal was submitted 7 Sep 1993 in response to NRA-93-OSS-01 (Space Physics Supporting Research and Technology and Suborbital Program) entitled "An Integrated Compact Particle Detector for a Mission to Pluto." Rated "Very Good" overall, the proposal was not funded (The proposal could not respond to NRA 93-OSSA-5 issued to explore the instrumentation for the strawman payload on the Pluto Fast Flyby mission because a particle instrument was not included in the strawman payload). The next year, a revised proposal was submitted to the Planetary Instrument Definition and Development Program (PIDDP) under NASA NRA 94-OSS-11. The proposal "A Compact Particle Detector" was submitted 6 Oct 1994 and targeted to a PFF new start authorization in FY98. Requested funding was for 1 Jan 1995 through 31 Dec 1997. In the abstract of the proposal are found the words: "In accord with the recommendations of the Science Definition Team for Space Physics Objectives for the Pluto Fast Flyby Mission (SDTSPOPFFM - M. Neugebauer, chair) that met August 1993, the instrument concept to be developed is a Suprathermal Particles Sensor that would be one part of a Combined Particle Sensor. The CPS was recommended as the highest priority particles and fields instrument for PFF." The same basic proposal as to the Space Physics Division was used; but with a block diagram and the idea of a fan of six solid state detectors (SSDs) added. The instrument was called the Pluto Ion Composition Analyzer or PICA. The technical specifications were <0.5 kg, <0.5 W; particles ~15 to 20 keV to ~3 MeV including ion spectra and composition, electrons, and neutrals. Possible applications presented were Pluto Express (PFF was now history), Solar Probe, Interstellar probe, and Jupiter MEASURE. Development work under this program led to the inclusion of the instrument in the first MESSENGER proposal in the discovery program under NASA AO-96-OSS-02. The NASA proposal system transitioned over to the Research Opportunities in Space Science (ROSS) system as a broad agency announcement as defined in FAR 6.102 (d) (2) with the 1998 release on 5 February 1998. In this new scheme, the PIDDP program was element A.3.5 with a due date of 3 Aug 1998. A new PIDDP proposal entitled "Miniaturized Energetic Particle Development" was submitted to NASA 31 Jul 1998. The basic design was to be carried forward from the previous PIDDP work - the energy chip design was characterized as "almost complete" for a single channel per chip - although this never materialized as flight hardware for a variety of reasons. Other electronics development topics were also pursued. Little from this effort was available for incorporation into the MESSENGER flight hardware given the timing of MESSENGER's selection for flight (McNutt et al. 2006), but some improvements were possible for PEPSSI due to work under this second grant prior to the selection of New Horizons for flight (Andrews et al. 1998; McNutt et al. 1996). As had been predicted in the initial community 1992 work, the most difficult aspect was the development of a scientifically useful energetic particle instrument with the small mass and power limits that had to be met to allow for inclusion on a Pluto mission. 2.0 Scientific Background and Objectives 2.1 The Interaction of Pluto with the Solar Wind The interaction of the various planets of the solar system with the solar wind goes back to the initial forays outside of the Earth's magnetosphere in the early 1960s. As part of the studies that eventually led to the Voyager program (Dryer et al. 1973) made an initial scoping study of what a solar wind interaction with Pluto might look like. They noted that for the anticipated scale lengths a kinetic approach was more proper for Pluto as for Mercury and that for a vanishingly small ionospheric scale height at Pluto, a long, induced magnetotail is, nonetheless, expected. Anticipating the then-future Voyager encounter with Neptune and its large moon Triton (McNutt 1982-4) wrote: "The outermost known planet in the solar system, Pluto, is similar in size and spectral signature to Triton [77]. Pluto apparently has a tenuous methane atmosphere [78] and, like the other medium-sized moons in the solar system, probably has no intrinsic magnetization. As there are no plans for a spacecraft flyby of Pluto in this century, Voyager observations of Triton and its interaction with the solar wind and/or a Neptunian magnetosphere, will, for the foreseeable future, provide our best guesses for the interaction of Pluto with its plasma environment." Here references [77] and [78] refer to (Morrison et al. 1982) and (Fink et al. 1980), respectively. There is no more work in the published literature for several years. On 17 Jan 1989 F. Bagenal sent an E-mail message to R. McNutt: "Alan Stern is organizing an AGU session this spring on the Pluto-Charon system. He asked me if anyone had thought about Pluto's magnetosphere. He said he could not find anything in the published literature. He urged me to submit an abstract. Well, I guess I could do a simple standoff calculation using the latest atmospheric measurements (which Alan has made, I think). Have you thought about Pluto lately? I remember you kept going on about Pluto in the good old days. Perhaps we could through something together next week - unless you have already done so!!!" This began a collaboration between Bagenal and McNutt on Pluto's escaping atmosphere and how it would interact with the solar wind. Current thinking is that the interaction of Pluto with the solar wind is something between that of an unmagetized planet, such as Venus and that of a comet, depending upon the strength of the atmospheric outflow. If Pluto were to have even a weak intrinsic magnetization, then the interaction would be more akin to that of the magnetized planets due to the weak solar wind ram pressure at ~30 AU and beyond. Estimates of the overall outgassing rate of the atmosphere Q0 are between ~10^27 and 10^28 molecules s-1. Such rates have effects ranging from just shielding the surface from the solar wind to producing a well-formed magnetosphere encompassing the the orbit of Charon (Fig. 2). Detailed discussions of past and current thinking about the interaction is given by (Bagenal et al. 1997) and (McComas et al. 2007), respectively, and more detailed simulations have been carried out as well (Delamere & Bagenal 2004; Harnett et al. 2005). Fig. 2. Schematic of expected interaction of Pluto with the solar wind for the strong-interaction limit. Based upon scalings from cometary interactions, the first detection of pickup ions from Pluto are expected as early as ~20 hours prior to closest approach. 2.2 PEPSSI Science Objectives The PEPSSI sensor is designed to perform in situ measurements of the mass, energy spectra, and distributions of moderately energetic particles in the near-Pluto environment and in the Pluto-interaction region. The instrument measures particle velocity and energy, derives particle mass, and discriminates between electrons, protons, alphas, and carbonnitrogen- oxygen (CNO - taken as a closely-spaced group in atomic weight), and heavier ions. The direction of particles is also discerned. PEPSSI objectives, within the context of New Horizons science mission group objectives include: Group 1 Objective. A group 1 objective is characterization of the neutral atmosphere of Pluto and its escape rate. To support this objective, PEPSSI will detect heavy ions and measure associated energy spectra and spatial variation along the trajectory. By analogy with cometary measurements, these measurements will be used to determine the neutral particle escape rate, which along with UV spectral measurements made in the upper atmosphere (Stern et al. 2007), will be used to put together a fully self-consistent model of Pluto's upper atmosphere to satisfy this group 1 objective. Group 2 Objectives. A group 2 science objective is characterization of Pluto's ionosphere and interaction with the solar wind. This characterization will be aided by PEPSSI measurements of the spatial extent and composition of pickup ions; these measurements are complementary to those that will be made by SWAP (McComas et al. 2007). Group 3 Objectives. A group 3 science objective is characterization of the energetic particle environment of Pluto and Charon. This will require measurement of the spatial extent and velocity-space distributions of energetic ions (e.g., H+, N+, and N2 +). The PEPSSI instrument will make the required energetic ion measurements. 2.3 Measurement Requirements 2.3.1 Measurement Ranges Energy thresholds and energy ranges depend upon the energy measurement mode, i.e, TOF-only, energy (SSD) only, or coincidence measurements through the entire system. The ranges as a function of species and the mode are given in Table 1. Table 1. 2.3.2 Derived Instrument Specifications 2.3.2.1 Mass Resolution (Mass Uncertainty). Particle mass is derived from energy and TOF measurements. The uncertainty in the derived mass, i.e., the mass resolution, is determined by (a) energy measurement resolution, (b) TOF measurement resolution, (c) particle mass, and (d) calibration accuracy. For Energy-plus-TOF measurements, the mass resolution for three species of particles (spanning light, medium, and heavy mass) is specified as <2 atomic mass units (AMU) for H+ (25 keV to 1 MeV), < 5 AMU for C+/N+/O+ (60 keV to 1 MeV), and < 15 AMU for Fe+ (60 keV to 1 MeV). For TOF-Only measurements, the means to ascertain particle mass is less precise, and this requirement, with respect to mass resolution, is to distinguish between H+ and CNO group particles. To support derivation of species mass, for particle energies in the 700 eV to 1 MeV range, PEPSSI was specified to be capable of measuring particle TOF over a range of 1 to 320 ns. 2.3.2.2 Species Mass Range. The PEPSSI instrument is constrained in downlink capability from Pluto as well as in the mass and power available for the instrument. Hence, prudent choices had to be made to meet all of the constraints while still enabling the collection of appropriate data from the vicinity of Pluto and its transmission to Earth following the flyby. Species resolution for the various energy spectra is limited by the counting statistics and the physical size of the detector that limits the TOF drift space. To enable the discrimination of solar wind particles (primarily protons and alpha particles, i.e. doubly-ionized helium nuclei) from pickup particles from Pluto, including atomic "debris" as well as ionized molecules of nitrogen, methane, and carbon monoxide, and allow for discovery science within the confines of the requirements, energy spectra are output for proton events, electron events, CNO events, heavy particle events (> 24 AMU, typified by Fe). 2.3.2.3 Sensitivity and Geometric Factor Requirements. With PEPSSI mounted on the spacecraft, including installation of the RTG power source, and with the PEPPSI covers closed, the background ion particle count rate was specified not to exceed one particle per second. Expected fluxes at Pluto are relatively low (~100 events per second), so to stay within low power limits of operation, the PEPSSI instrument was specified to be capable of processing at least 103 particle events per second, where this event rate is applicable to the total of all classes of measurements, i.e., Energy-plus-TOF, Energy-Only, and TOFOnly. The PEPSSI instrument has the potential to measure particle events at a much higher rate; this rate should be established once the analysis of data from the Jupiter flyby is fully analyzed. 2.3.2.4 Geometric Factor. Expected count rates at Pluto are unknown but expected to be low. Hence, the geometric factor was required to be as large as possible, consistent with the targeted low mass of the instrument of ~1 kg. On the basis of these trades, The PEPSSI geometric factors, for electron and ion detection, were specified to meet or exceed the values given in Table 2. The geometric factors for electron and ion detection are different because of the difference in numbers of ion and electron detectors. The values that follow apply to the entire aperture acceptance angle of 160deg. by 12deg., i.e. the geometric factor per "pixel" is less. 2.3.2.5 Integration Interval. Nominally, energy-plus-TOF measurements, used to determine particle species and associated energy spectra, are integrated over a 10-second interval (based upon consideration of spacecraft speed, and hence spatial resolution, telemetry rates and data volume playback during the Pluto encounter). TOF measurements, used to determine particle velocity distribution, are integrated over the identical time interval. By command, the integration interval may be adjusted from 1 to 7200 seconds. 2.3.3 Measurement Resolution Requirements These specifications flow, in turn, to the next level of implementation requirements that drove the system design and implementation. 2.3.3.1 Energy Resolution. As a goal, instrument energy measurement resolution, which includes the effects of all noise sources including analog-to digital converter (ADC) quantization noise, was 5 keV full-width at half maximum (FWHM) or less. As a requirement, instrument energy measurement resolution for ions was ~7 keV or better, for electrons 8 keV or better; ADC quantization is equivalent to 1 keV. The Energy resolution is determined to a large degree by the performance of the energy-peak detector chips for the given SSD detector capacitance and leakage currents. The energy resolution of the energy-peak detector chips is a trade-off between power dissipation, mainly in the charge sensitive amplifier, and integration time of the shaper. Both parameters are fixed on the energy board with two resistors (common for all 12 channels) as described in more detail in the electronics section. The overall energy resolution is also determined by the digital noise (mainly the ADC noise) on the energy board. The clock frequency of the temperature remote input/output (TRIO) application specific integrated circuit (ASIC) chip for the ADC can be set comparatively low, to 150KHz, and, given the very low digital power dissipation of the chip (1 mw digital power), the system noise is within the specification levels. 2.3.3.2 TOF Resolution. Time measurement resolution, which includes the effects of time jitter, time walk, path dispersion, and time quantization, are specified as 1 ns FWHM or less. The time resolution is dependent on the constant fraction discriminator (CFD) ASIC chip time walk and time jitter and the time jitter of the time to digital converter (TDC) ASIC chip. The CFD time walk is ~200-300ps for a 1:100 input energy dynamic range; the time jitter is in the range 50-100ps depending on the power dissipation. The time jitter of the TDC chip is ~50ps. Thus the overall electronics time resolution is < 500ps. The optimum is achieved with proper delay line selection and optimization of the power dissipation within the power budget limits. 2.3.4 Platform Requirements For proper interpretation of the PEPSSI data, the instrument alignment with respect to the spacecraft and the knowledge of the spacecraft attitude are both required to 1.5deg.. For deciphering the measurements to be made at Pluto during the Pluto flyby, knowledge of spacecraft distance to Pluto is required to an accuracy of 225 km. This knowledge of position figure is equal to roughly 1/10 the diameter of Pluto, and is achieved with the spacecraft timing system and navigation of the spacecraft at the system level. 2.3.5 Time Resolution Requirement 2.3.5.1 Timetagging Requirements. For proper interpretation and processing of science data, knowledge of the data intervals over which PEPSSI science data is collected must be known to within to +/-1 second of spacecraft mission elapsed time (MET). To this end, data packets sent from PEPSSI to the command and data handling (C&DH) system are time tagged with MET time to 1-second resolution (Fountain et al. 2007). Based on spacecraft time-keeping requirements, knowledge of MET time relative to UTC time to ~10 ms accuracy is known after the fact. 2.3.5.2 Science Data Synchronization. PEPSSI science data (species energy spectra, velocity, and pulse-height analysis (PHA)) are collected over fixed time intervals. These data intervals are synchronized by, and time aligned with one pulse-per-second (1PPS) timing epochs input from the spacecraft that are coincident with the MET one-second time increments. 2.3.6 Calibration Requirements 2.3.6.1 Required Ground Calibration. Calibration of the PEPSSI instrument is required in order to meet instrument performance specifications. Ground calibration tests were planned using both linear accelerator facilities at APL and Van De Graaff facilities located at GSFC. These facilities provide a calibrated source of electrons and a variety of ions (e.g., protons, helium, CNO, iron) at a variety of energies. Time and availability issues reduced the calibration plan to using the APL facility only. Calibration was performed with the PEPSSI instrument in vacuum at a temperature of 25 deg.C for each sector. Calibration was performed prior to installation on the spacecraft. A calibrated alpha-particle source is installed in the collimate assembly and described below. This source was used to during thermal vacuum tests to verify instrument performance has not changed. 2.3.6.2 In-flight Calibration Characterization. Following launch, the PEPSSI instrument calibration must be characterized during initial instrument test and prior to all major data collecting operations. To characterize the instrument, background particle data was collected and downlinked to the PEPSSI payload operations center. Archived energy and TOF measurement data in the PHA data packet, as well as particle species data, have been processed to evaluate instrument calibration. To correct for some drift in measurements, new 'look-up' tables have been configured and uplinked. Further characterization is occurring during the close approach to Jupiter, using that planet's magnetospheric plasma as a calibration source. The characterization will be monitored during the yearly checkouts during the cruise to Pluto. The installed alpha-particle calibration source will also be monitored to look for any changes in instrument performance prior to arrival in Pluto-space in 2015. 2.3.6.3 Energy Board Temperature Monitor. Energy measurements vary slightly as a function of energy board component temperature. A temperature sensor is installed on the energy board, and this temperature data is telemetered in instrument housekeeping telemetry. Energy board temperature informations allow the instrument ground processing software to account for any slight errors in energy measurement due to energy-board temperature variation. 3.0 Technical Description 3.1 Instrument Overview Given the requirements, as well as ongoing lessons from the near-concurrent development of the EPS detector for the MESSENGER mission (Andrews et al. 2007), the required functionality was achieved within the prescribed constraints. These properties are given in Table 3 and Table 4. Some of the numbers are still being refined based upon the final analysis of instrument performance during the New Horizons flyby of Jupiter (28 Feb 2007). PEPSSI consists of a collimator and sensor assembly, referred to as the sensor module, mounted atop an electronic board stack (Fig. 3; Fig. 4). The electronic stack consists of six metal-framed electronic boards. The stack is a cube measuring approximately 10 cm on a side. The sensor module is approximately 2 cm high and protrudes out from the stack an additional 6 cm. A simplified block diagram of the PEPSSI instrument is shown in Fig. 5. The corresponding simplified schematic for operation is shown in Fig. 6. As shown in the block diagram, the sensor module includes a time-of-flight (TOF) section about 6 cm long feeding a solid-state Si detector (SSD) array. The SSD array, connected to the energy board, measures particle energy. Secondary electrons, generated by ions passing through the entry and exit foils, are detected to measure ion TOF. Event energy and TOF measurements are combined to derive mass and to identify particle species. Table 3. PEPSSI Performance Parameters While PEPSSI uses the same type of energy and time-of-flight measurement scheme as the previous such instruments mentioned above, it also employs multiple (six) angular apertures in a swath in order to provide angular information without relying upon either a mechanical scanning mechanism or the rotation of the spacecraft. The PEPSSI acceptance angle is fan-like and measures 160deg. by 12deg. with six 25deg. segments. Each segment is separated by a 2deg. gap. Its total ion geometric figure is greater than 0.1 cm2 sr. Particle direction is determined by the particular 25deg. sector in which it is detected. The angular resolution of the instrument was required to determine the incoming direction to better than 40deg. for the ions in each of the six sectors, i.e., within a 25deg. by 12deg. window. Electron particle direction is limited to sector 1, sector 3, and sector 6. Table 4. Final PEPSSI flight parameters and assessment against requirements The entrance apertures for the axially symmetric time-of-flight (TOF) section are 6 mm wide. Each aperature is covered by a 50Angstrom aluminum/350Angstrom polyimide/50Angstrom aluminum foil. These foils reduce the TOF UV Lyman-alpha photon background. The exit apertures are covered by a 50Angstrom palladium/500Angstrom polyimide/50Angstrom palladium foil. Both entrance and exit aperture foils are mounted on a high-transmittance stainless steel grid supported on a stainless steel frame. Fig. 3. Expanded view of the PEPSSI instrument showing, left to right: sunshade, collimator assembly (with acoustic doors open - flight configuration), sensor assembly, and electronics boards stack. Fig. 4. Assembled view of the sensor, again in flight configuration. Fig. 5. Simplified block diagram of the PEPSSI instrument. Flow of information from the sensor to the spacecraft interface is shown. Fig. 6. Simplified operation of the PEPSSI sensor (1 of 6 shown). Incoming ions and electrons pass through the collimator assembly hitting the "start" foil and ejecting secondary electrons. The start electrons hit the start anode flagging the time-of-flight start. The incoming particles cross the 6-cm drift space and then hit the stop foil, again ejecting secondary electrons that hit the stop anode. The incoming particle then hits the SSD where its total energy is recorded. The electronics detect and classify both nominal events and valid "triples" (that have valid start, stop, and energy signals within appropriate electronic windows such that the incoming particle can be fully categorized. 3.1.1 Differences from EPS on MESSENGER The MESSENGER EPS unit was the first flight unit of this basic design. PEPSSI presented different requirements with respect to available power and maximum count rates. While larger rates were anticipated for the encounter with Jupiter and could be used to good advantage for some calibration activities, the requirements were always focused at the conditions at Pluto. In addition, the launch of New Horizons over two years after that of MESSENGER allowed for some upgrades to be included in the PEPSSI unit. The combination of lower expected counting rates and the need to save power led to the implementation of (1) lower-current MCPs, (2) a new low-voltage power supply (LVPS) with a complete custom converter design to save power, (3) a complete redesign of the energy borad using new application-specific integrated circuit (ASIC) chips to save power, and (4) a redesign of the time-of-flight (TOF) board with different amplifiers and delay lines to achieve higher gain. With the need for long-term reliability during the cruise to Pluto, PEPSSI added a high - voltage (HV) safing system, which is always active. This system samples the HV current 1000 times per second, and shuts down the supply if the HV current exceeds a threshold. A different HVPS driver is used on PEPPSI that provides improved performance (not incorporated on PEPSSI due to the MESSENGER launch schedule). The PEPSSI unit also incorporates a buffer amplifier and a filter, neither of which are present in the EPS unit. The EPS instrument has 24 detectors (six large for ions, six large for electrons and the same counts with small ones), whereas PEPSSI uses only 12, and on its detectors ties the inside one to the outside one, rather than all being independent, as on EPS which has six processing chains for ions and six for electrons, with large or small detectors selected by ground command. The front - end collimator is more open on PEPSSI than on EPS to increase the geometric factor and PEPSSI also has an added sunshade to be able to look back near the solar direction (EPS is mounted well away from the Sun-looking directin on the MESSENGER spacecraft to keep the temperature down and avoid spurious signals from the eleven Sun solar luminosity at Mercury's perihelion. EPS is thermally isolated from the MESSENGER spacecraft with a separate thermal radiator to reject internal heat to space, while PEPSSI has no radiator, but is quasithermally coupled to the spacecraft mounting bracket (conductively isolated but radiatively coupled). Furthermore, the EPS thermal design uses heaters and mechanical themostats to keep the instrument from getting too cold, while PEPSSI relys on the spacecraft to keep it just warm enough to not need heater power. 3.2 Mechanical Design Top-level mechanical design requirements and design factors of safety are in accord with those used throughout the New Horizons project. In particular, the PEPSSI instrument was designed to withstand a quasi-static load limit of 30 g along 3 orthogonal axes and (applicable to the primary structure design and as multiplied by the appropriate factor of safety). In addition, first mode structural frequencies were specified to be above 70 Hz in the spacecraft thrust direction and above 50 Hz in the lateral directions and the instrument designed to withstand maximum pressure rate change of 1.0 psi/sec. All of these specifications were verified during environmental tests. 3.2.1 Dimensions and Mounting The PEPSSI envelope is 19.7 cm x 14.7 cm x 21.6 cm as installed and 25.1 cm 14.7 cm x 21.6 cm following the one-time opening of the acoustic doors. The instrument itself is mounted on a bracket that provides for a complex angular offset of the field of view (FOV) of the viewing fan from any of the spacecraft decks (Fig. 1; Fig. 7; Fig. 10). The complex angular offset was employed to optimize the viewing of freshly ionized pick-up ions in the vicinity of Pluto due to charge exchange with neutrals from Pluto's atmosphere. This allows for the instrument to be mounted to the spacecraft deck while looking past the high gain antenna (HGA) while not being obscured by it (Fig. 8). Fig. 7. Cutaway view of the PEPSSI sensor showing the FOV defined by the internal sensor structure, collimator assembly, and Sun shade (top). Fig. 8. Location of PEPSSI on the New Horizons spacecraft. The lightly shaded area denotes PEPSSI Field-of-View (FOV). Alignment for PEPSSI's FOV on the observatory's top deck was determined by the design and location of its mounting bracket. Alignment control was to be kept within 1.5deg. of the Euler angle rotations defining the bracket's mounting surface. Knowledge of PEPSSI's FOV was specified to be within 1.5deg. of the observatory's coordinate system, as referenced to the orthogonal plane surfaces of the PEPSSI instrument. An error in interpretation of the one of the interface control documents (ICDs) led to a misalignment in the bracket surface as manufactured. Fig. 9 shows the planned and actual fields of view in the coordinate system of the observatory, i.e., the New Horizons spacecraft. Fig. 9. The PEPSSI FOV in angular coordinates referenced to the main deck of the spacecraft. The shaded area above the FOV traces indicates the HGA and the shaded area to the right indicates the body of the spacecraft. The red line indicates the Sun shield, the red dot the location of the Sun and the P and C (near the center) the locations of Pluto and Charon near the beginning of the mission. Fig. 10. PEPSSI mounting orientation with respect to the New Horizons deck. With this FOV PEPSSI points at an off-angle to the Sun during spacecraft downlink operations and most of Pluto flyby operations. (The HGA points towards the earth, along the +Y axis, during downlink operations). This allows PEPSSI to detect pickup ions, coming from the general direction of the Sun, while keeping the Sun out of the FOV. There are no rigid requirements to keep the Sun out of the instrument FOV; however, for normal operation the Sun should be ~ 10deg. from the instrument boresight. In the event the Sun is in the instrument FOV, no damage to the instrument will occur. However, the flood of energetic photons may cause a degradation of both energy and TOF measurements. To mitigate these affects, in the event the Sun is in the FOV, PEPSSI includes include the capability to disable by command the start and stop anode(s) of the sector(s) pointed in the sunward direction. In fact, this phenomenon with the Sun in the FOV was what led to the mapping of the actual FOV. PEPSSI is mounted to the spacecraft's top panel with four (4) #8-32 stainless-steel screws into threaded inserts on a dedicated spacecraft-designed mounting bracket. The instrument is thermally isolated from the bracket by thermal washers between the bracket and PEPSSI's mounting tabs. PEPSSI's alignment and repeatability of mounting with respect to the observatory's coordinate system was satisfied by dimensional tolerancing and position of the mounting holes in the bracket and on the spacecraft. 3.2.2 Mass Properties The mass budget for the PEPSSI instrument was 1500 grams, including 120 grams of margin. This mass included all sensor pieces as well as the mounting bolts and was worked to on a board-by-board basis (including the metal framing for the boards). The measured sensor mass at delivery to the spacecraft was 1475 g. 3.2.3 Deployable Cover The PEPSSI design includes two cover doors installed over the instrument aperture. One purpose of the instrument covers is to prevent acoustic (air pressure) damage to the thin entry/exit foils within the sensor module during launch. Another is to keep out air-born dust and contaminants during instrument ground test. During all ground operations and storage, except functional test of cover release, the aperture covers were maintained in place to prevent damage to the entry/exit foils and to prevent contamination of the MCP. The covers are crescent shaped and each covers one-half of the 160deg. aperture angle. Each cover is mounted at one end on a hinge bracket assembly that includes a Mandrel with torsion spring. In the closed position an actuator pin holds the covers in place. To open the covers, the actuator is fired to retract the retaining pin, allowing the covers to spring open. A spacecraft command must be sent to fire the actuator. Once open, the covers are maintained in the open position by torsion-spring action. To allow for ground tests (e.g., EMI tests) with the covers open, the actuator used to pin the doors closed shall be capable of at least 100 cycles of operation. Cover opening was tested in a vacuum chamber prior to flight for correct actuator function. The covers were open on the flight unit on 3 May 2006. 3.2.4 Instrument Purge The PEPSSI instrument interface included a purge manifold. Once PEPSSI is assembled with the MCP, it requires constant purge with at least MIL-P-27401D Grade C (99.995% pure) nitrogen, at a flow rate of 0.1 liters/minute. The purge system was capable of being disabled for a short period of time (up to 30 minutes) while in a class 10,000 or better clean room. Records were maintained of all periods when purge has either been removed or failed. 3.2.5 Handling Requirements The PEPSSI instrument was kept under purge at all times, except during test in vacuum chambers. The PEPSSI door remained closed to protect the entry and exit foils. When open, all airflow toward the foils, including breath, was prevented. As long as the cover is closed and the unit is being purged, it could be in any normal room environment. Proper procedures and equipment to eliminate the risk of electrostatic discharge were followed and used. The collimator was not be cleaned or handled in any manner, and the cleaning of the outside cover was minimized. A high-voltage power supply (HVPS) safing plug was included and removed prior to launch. In addition, a cover release arming plug was installed prior to launch to enable cover release commands in space. 3.2.6 Transportation and Storage The PEPSSI instrument required a shipping container to provide protection during transportation and storage. The instrument was double bagged and filled with dry nitrogen for transport and constrained and padded within the container, to minimize vibration and shock during transport. 3.2.7 Vacuum and Outgassing Requirement The PEPSSI HVPS was required to be turned on to full voltage only in a vacuum environment < 3x10-6 torr. As a consequence, the HVPS must not be activated until sufficient time in vacuum has elapsed for spacecraft and instrument outgassing. A high concentration of outgassing products and particles could contaminate the MCP under high-voltage conditions, so outgas products must be at a sufficiently low level for safe turn-on of the HVPS. Prior to launch, during both instrument-level and spacecraft-level thermal-vacuum test, appropriate detectors were placed inside the test chamber to monitor the level of outgas products. During spacecraft-level testing, 24 hours of time in vacuum (at < 3x10^6 torr) was required before the HVPS may be safely turned on. During instrument-level test, the test conductor determined when it was safe to turn on the HVPS. Prior to HVPS activation, the PEPSSI instrument could be (and was) powered on for low-level engineering level checkout. At full atmosphere, during instrument and spacecraft level testing, the HVPS voltage (applied to the MCP) was kept less than 700 volts. After the New Horizons launch, the requirement was for at least two weeks to pass prior to first activation of the HVPS. The PEPSSI instrument was powered earlier (with HVPS off) to allow for initial checkout (20-22 Feb 2006) and engineering-level checks (1-2 Mar, 27 Apr, and 2 May 2006). 3.3 Detectors and Electronics Power as initially allocated to the instrument took into account the limited resources on the spacecraft. Referenced to the +30V input the peak power specifications were <2000 mW in science mode and <1750 mW for the high voltage power supply (HVPS) disabled. During development, these numbers were found to not be realizable without extensive additional development. To remain within financial resources a request was granted to raise the allowable power required by 550 mW . 3.3.1 Energy Measure Each SSD sector in the sensor module has 2 detectors. In sectors 1,3,6, one detects ions, the other electrons. The electron detectors are covered with a 1-"m Al layer, to block low-energy protons and heavier ions. The detector pairs in sectors 2,4,5 are just ion detectors. A key to the electronics and functional layout is given in Fig. 11. Energetic electrons from 25 keV to 500 keV are measured by the electron detectors. The Al layer blocks protons and ion particles with energies less than 100 keV, and particle energy levels above 100 keV are expected to be rare in the near-Pluto environment. For those rare events, where ion energy levels exceed 100 keV, coincident TOF measurements will be used to discriminate between ions and energetic electrons. Ion energy measurements using the ion detectors are combined with coincident TOF measurements to derive particle mass and identify particle species. Particle energy for protons greater than 40 keV and heavy ions (such as the CNO group) greater than 150 keV are measured, up to a maximum of 1 MeV. Lower-energy ion fluxes are measured using TOF-Only measurements; detection of micro-channel plate (MCP) pulse height provides a coarse indication of low-energy particle mass. Fig. 11. Schematic layout of PEPSSI showing the nominal FOV and detectors and rates for the various modes and channels. 3.3.2 Time-of-Flight Measure Before an ion passes through the TOF head, it is first accelerated by a 2.6-kV potential. Secondary electrons from the foils are electrostatically separated on the MCP, providing start and stop signals for TOF measurements. The segmented MCP anode, with one start anode for each of the six angular segments, determines the direction of travel, particularly for lower-energy ions that do not yield an SSD signal above threshold. A nominal 500- volt accelerating potential between the foil and the MCP surface controls the electrostatic steering of secondary electrons. The dispersion in transit time is less than 400 picoseconds (ps). 3.3.3 Electronics The six electronics boards in the electronics stack (cf. Fig. 3) each provide a specific function for the instrument (Table 5). The common board size has also been used on other APL spacecraft subsystems, notable the electronics for the (redundant) digital processing units and several of the instruments on the MESSENGER spacecraft (Gold et al. 2001). Table 5. Electronics Boards in the PEPSSI Instrument 3.3.4 Operation 3.3.4.1 Science Mode. In the science mode, three basic classes of measurements are concurrently made. The first class, called TOF-plus-Energy, uses SSD measurements in coincidence with TOF measurements to determine energetic ion particle mass (species) and associated energy spectra. Measurements are collected from six different sectors and particle direction to a particular 25deg. by 12deg. sector is determined by the start sector. The second class of measurement is referred to as TOF-Only. It is a measure of ion particle velocity based on the TOF measurement and is made if no coincident energy event is detected at the SSDs. Particle composition is categorized as light (protons), medium (mass < CNO), or heavy (mass ! CNO). These measurements occur if the ion particle energy is below the energy threshold of the SSD detector. The third class of measurements consists of Energy-Only measurements of particles where the SSD measurement event is not coincident with any TOF measurement event. These measurements include electron energy measurements, where transit time of electrons through the TOF chamber is, for practical purposes, zero, as well as ion energy measurements made when the ion fails to generate any secondary electrons (for TOF measurement). 3.3.4.2 HVPS Activation. The high voltage (HV) supply of the HVPS can only be turned on in vacuum (< 3x10-6 torr). If the HV supply is accidentally turned on during ground test in air, the MCP may be degraded. Therefore, when the PEPSSI instrument is powered up on the ground, the HVPS (consisting of HV and bias power supply) is disabled and no science data is collected. A sequence of instrument commands is required to enable the HVPS, fine-tune voltages, and tweak HVPS clock frequency for optimum efficiency. Valid science data is output after the HVPS command sequence has been executed and science data enabled. 3.3.4.3 Cover Door Open. At first power-up on orbit, the PEPSSI covers are in the closed position. A spacecraft command must be sent to fire the actuator and open the front covers. Once open, the covers remain open indefinitely, and no cover release commands are required at any subsequent turn-on of the instrument. 3.3.4.4 Test Mode. The instrument can implement a test mode that allows for data input and output through a test port instead of the C&DH communication ports. This is strictly for use during instrument software development pre-flight and is not for spacecraft level test or for use in space operations. 3.3.4.5 Calibration. Ground-based calibration is performed with the instrument in a dedicated calibration mode. In this mode, energy and TOF measurements are telemetered at a high rate. Based on an evaluation of telemetered energy and TOF data, instrument calibration tables are configured and subsequently loaded into the instrument. 3.3.5 Electrical Interface There are five electrical interfaces from PEPSSI back to the spacecraft. Only the power and command and telemetry interfaces are still of use. The others were used for ground testing, safing, and the deployment of the cover that occurred as part of the overall commissioning. The PEPSSI instrument does not require a survival heater for operation. However, PEPSSI does require a spacecraft-provided and controlled heater that can be can be powered during cruise phase when observatory power is available. The heater provides thermal margin, if required, and dissipates 1 watt when switched on. 3.3.5.1 PEPSSI Power Interface. Switched power is provided to PEPSSI at 30 volts nominal and is regulated to within +/-1 volt of nominal. Primary power is supplied to the PEPSSI support electronics through a separate dedicated connector that has no signal or secondary lines. Total power consumed by PEPSSI was initially specified to be 2 watts or less, but was later increased during the design process. There are specifications regarding input voltage characteristics (operating and survival) regarding regulation, input voltage ripple, source voltage transients, source impedance, and voltage turn-on/turn-off rates. Inrush current transients at power turn-on and power turn-off imposed on the instrument are also specified. The instrument was designed and tested to operate over and survive respective voltage operating conditions as required. 3.3.5.2 PEPSSI Command and Telemetry Interface. PEPSSI implements a low-speed command and telemetry interface circuit that conforms to the EIA RS-422 standard for serial data transmission. The low-speed buses for PEPSSI each consist of three circuits: a 1 pulse/second (1PPS) sync signal, a command circuit, and a telemetry circuit. PEPSSI implements two RS-422 bi-directional universal asynchronous receive and transmit (UART) ports to the spacecraft. The first port connects to the spacecraft C&DH system located in the integrated electronics module (IEM) #1, and the second port connects to the spacecraft C&DH system located in IEM #2. Normally, only one of the two IEMs is active, and PEPSSI responds and interacts with the active IEM (Fountain et al. 2007). The serial ports are bi-directional and PEPSSI is capable of simultaneously sending and receiving at 38.4 kbaud rate with 8 data bits, no parity, 1 start bit, and 1 stop bit. The least significant bit of each byte is sent first. For multi-byte values, "big endian" format is used, where the most significant byte is sent first; the least significant bit is referred to as "b0". PEPSSI receives instrument commands and MET time from the spacecraft over this port, and sends instrument science and housekeeping data to the spacecraft. Data sent to the spacecraft is normally recorded in the SSR of the respective IEM for latter downlink to the ground station. The active IEM and PEPSSI exchange messages over the UART using the standard UART protocol to govern the lower-level aspects of the transfer. A higher-level construct, an Instrument Transfer Frame (ITF) protocol, is used for higher-level synchronization and error control. The IEM provides PEPSSI with command messages and spacecraft time messages. PEPSSI provides the IEM with TLM messages that contain instrument state data as well as science data. PEPSSI does not exchange messages with any other instrument and every message to or from PEPSSI is contained within an ITF. In addition to Transmit (to spacecraft) and Receive (from spacecraft) signal lines, each port includes the 1PPS signal input to PEPSSI. This signal provides nominal one-second timing information; command and telemetry transfer is synchronized to 1PPS epochs as illustrated in Fig. 12. Fig. 12. PEPSSI command and telemetry timing. Commands, instrument telemetry, and MET time are packed in the ITF format that is described below. This frame is defined by the time between the rising edge of any two adjacent 1PPS signals. Command and telemetry data frames are transferred as a serial 8- bit byte stream. MET time is transferred from the spacecraft to PEPSSI over each one-second interval. MET time is transferred to 1-second precision and is valid when the 1PPS signal is asserted. MET time is transferred within 0 to 950 ms following each 1PPS. PEPSSI telemetry data may be transferred anytime between 1PPS epochs except when the 1PPS rising edge occurs. All telemetry transmissions terminate 300 "s prior to the rising edge of the 1PPS. All three signals (Transmit, Receive, 1PPS) are implemented as complementary RS-422 electrical standard interfaces. PEPSSI sends a number of different science and housekeeping data products to the spacecraft. These products are packed in CCSDS telemetry packet format. A number of CCSDS telemetry packets may be included in each telemetry ITF sent to the spacecraft. 3.3.5.3 Test Port Interface. For early hardware and software development, PEPSSI implemented an RS-232 protocol test interface to the instrument processor. However this was intended only for early development and test and was not accessed at the spacecraft level of integration and test. 3.3.5.4 HVPS Safing. The PEPSSI HVPS could be safely turned on to full level only when the instrument is in vacuum. Damage to the MCP may result if high voltage power is applied at atmospheric or partial atmospheric pressures. To prevent accidental turn-on of the HVPS, the PEPSSI instrument was designed to accept a safing plug connection- installation of the safing plug in the spacecraft harness was required to disable HVPS operation. The safing plug was removed before launch to enable on-orbit HVPS operation. 3.3.5.5 PEPSSI Cover Release. The PEPSSI design includes a cover for the instrument aperture. One purpose of this cover is to protect the instrument foils during ground vibration and acoustic shock tests. Prior to launch, the cover is opened only during T-V, calibration, and special cover tests; the cover remained closed at all other times. The instrument cover is opened by spacecraft command. The spacecraft command system and associated spacecraft ground support equipment (GSE) included implementation of an arming plug. The arming plug was removed during ground system test to prevent accidental opening of the cover and then installed before launch to enable on-orbit cover release operation. The spacecraft provided a one-time activation signal to open the cover. An electrical nonexplosive shape memory alloy (SMA) pin-puller is incorporated in the instrument and was used for the release. The SMA pin-puller employs redundant circuitry. Each circuit requires a current load of between 0.5 to 2.0 amps at 30 VDC. Depending upon the operational load selected, the spacecraft power distribution unit could apply this power for a minimum of 200 msec. The pin-puller employs an auto shut-off switch removing power after it has activated, approximately 100 msec after power is applied. The cover has been successfully opened, and this feature will no longer be used. 3.4 Telemetered Data Products 3.4.1 Proton and Electron Energy Spectra In the energy spectra data for detected electron, proton, and heavy ion particles each energy bin corresponds to a particular particle energy range (defined in the software data base look-up table), and the telemetered value represents the number of particles detected in the respective energy bin. The initial look-up table that defines the energy bin coordinates for each particle was determined and loaded during pre-launch instrument calibration. Ion particle data is output for each of six sectors; electron data is output for each of three sectors. The integration interval, i.e., the time interval over which the data is collected, is nominally 10 seconds, but data integration interval may be changed by command from 1 second to 7200 seconds. For those sectors that include an electron detector (sectors 1,3,6), the number of ion events counted in each energy bin represents the number of ion particles impinging on the respective single ion detector. For those sectors that include ion detector pairs (sectors 2,4,5), the number of ion events counted in each energy bin represents the sum of ion particles impinging on the respective ion detector pair. Table 6. Proton/Electron/Heavy Ion Energy Spectra Data Products 3.4.2 Heavy Ion Energy Spectra The energy spectra data for heavy ions, output from each of six sectors, is conditional; it is output only if the particle event rate exceeds a predefined threshold (Table 7). The event rate threshold includes hysteresis, so that once heavy ion spectra data is output, the event rate must drop to a somewhat lower value for output to cease. The particle event rate threshold is programmable by instrument command and is typically set at 100 particle events per second. The data integration interval, nominally 10 seconds, is identical to the integration time interval for all other spectra type data. It may be adjusted by instrument command over a range of 1 to 7200 seconds. For those sectors that include an electron detector (sectors 1,3,6), the number of heavy ion events counted in each energy bin represents the number of ion particles impinging on the respective single ion detector. For those sectors that include ion detector pairs (sectors 2,4,5), the number of heavy ion events counted in each energy bin represents the number of ion particles impinging on the respective pair of SSD ion detectors. Table 7. Heavy Ion Energy Spectra 3.4.3 TOF-Only Velocity Spectra Particle velocity spectra data, derived from TOF-only measurements are comprised of three sets of particle velocity data corresponding to particle mass categorized as light, medium, or heavy (Table 8). The two stop-anode discriminators are employed to differentiate between light, medium, and heavy particles. The discriminator threshold settings normally are set so that light particles correspond to protons, medium particles correspond to the CNO group, and heavy particles correspond to particles with mass greater than the CNO group. Similar to heavy ion spectra, particle velocity spectra data is output only if the particle event rate exceeds a predefined threshold. As with the other products just mentioned, the data integration interval, nominally 10 seconds, is identical to the integration time interval for all other spectra type data. It may be adjusted by instrument command over a range of 1 to 7200 seconds. Table 8. TOF-Only Velocity Spectra 3.4.4 Singles-Event Data (for Event Validity Check) To allow validation of instrument science data, PEPSSI counts and telemeters a number of instrument single events (Table 9). Single-event data is collected over the same identical data integration intervals as ion-energy spectra and velocity spectra data. It is nominally 10 seconds, but may be adjusted by instrument command over a range of 1 to 7200 seconds. Table 9. Singles Event Data (Event Validity Check) 3.4.5 PHA (Pulse Height Analysis) Event Data Particles detected by PEPSSI are prioritized and ranked according to 'interest'. A table, called the PHA rotating priority structure, loaded prior to launch, defines the order in which priority is assigned. Energy and TOF measurements, collectively referred to as PHA data, are telemetered for particles with highest priority. The specific data telemetered for each PHA event are specified in Table 10. PHA event data are collected over fixed ten second intervals and output as a single block. The number of PHA events collected in any single second is limited to a maximum number that is programmed by instrument command. Typically, the limit is set to 40 PHA events per second (400 PHA events per 10 second collection interval). When low particle event rates are encountered, the PHA event rate will most likely average less than the maximum limit. Table 10. PHA Event Data 3.4.6 Non-Packetized Housekeeping Data PEPSSI telemeters non-packetized housekeeping data. These include voltage and current measurements that are monitored during activation of the instrument energy measurement system and TOF measurement system. PEPSSI non-packetized housekeeping data are placed in the instrument-defined status field of Instrument Transfer Frames sent from PEPSSI to the spacecraft C&DH system. Housekeeping data is updated at a regular 1- second rate. The total number of bits in each housekeeping frame is 56 bits, resulting in an average output data rate of 56 bps. Table 11. PEPSSI Non-Packetized Housekeep Data 3.4.7 Quick look Diagnostic Data For a quick look analysis of instrument status and measurement results, PEPSSI can output a diagnostic set of data at a fixed 300-second rate. It is intended that PEPSSI diagnostic data be downlinked as soon as possible following the near Pluto encounter, i.e., on a first-look basis. This will allow for examination of instrument performance in the quickest and most timely manner. It is estimated, over a 24-hour timeline, roughly the time duration of the near-Pluto encounter, about 480 kbits of PEPSSI quick-look data will be available for downlink. The diagnostic data is identical to the electron/proton/heavy-ion energy spectra and ratecounter data previously defined, except that it is collected over 300-second integration time intervals. In addition, 600 bits of extended housekeeping data are included. The diagnostic integration time interval is constant at 300 seconds, and the subsequent average output data rate is less than 6 bits per second. 3.4.8 Instrument Data Rate Summary The telemetry system of the PEPSSI instrument may be configured in a number of different ways by instrument command. The data integration interval for a number of products may be adjusted, heavy ion and TOF velocity data products may be enabled or disabled, and the maximum PHA event rate may be changed. The volume of data telemetered from PEPSSI, and the resulting average data rates, will vary accordingly. Table 12 lists PEPSSI data products and computes telemetry data rates for a number of representative telemetry system configurations. The first four products listed (proton/electron energy spectra, heavy-ion spectra, TOF velocity spectra, and ratecounter data) represents data collected over a data integration time interval that is changeable and may be set by instrument command. The remaining data products (PHA data, non-packetized housekeeping, and quick look diagnostic data) are collected and output at fixed rates. Table 12. Example PEPSSI Telemetry Data Rates As Function of Telemetry Configuration The data integration interval, for the first four products listed, is nominally set to 10 seconds and is never set any lower during in-space operation. For anticipated in-space operations, where the heavy-ion spectra and TOF velocity spectra are enabled, and the PHA limit is set to 40 PHA events per second, the maximum output data rate is about 1620 bits per second. For certain ground test operations, prior to spacecraft integration, where data is integrated over short, one-second time intervals, the output data rate averages about 3950 bits per second. If required by mission operation or spacecraft constraints, as illustrated, the instrument telemetry system may be configured to telemeter less data. The PEPSSI instrument will output four types of telemetry packets on regular time intervals: - High Priority - Medium Priority - Low Priority, and - Common Status The time intervals may be modified by instrument command, but it is expected that the default telemetry packet rates are sufficient to adequately support the Pluto encounter as well as cruise phase operations. Table 13 lists the packet types and provides current best estimates of average data output rates and 24-hour data storage volume for the Pluto encounter. The data rate and volume figures are raw values and do not include reductions due to use of the Fast data compression algorithm. The amount of reduction, in data volume and rate, depends upon the variableness of the raw data stream and therefore, is not fixed. It is anticipated the reduction of data volumes and rate will be 50% or more; therefore, the actual output data rates at the PEPSSI command and telemetry interface are expected to be on the order of 300 to 500 bps. The data rates and volumes given in the table should be considered upper bounds. The table also indicates that the 24-hour volume for High Priority data is on the order of about 1.55 Mbits. However, the Fast compression algorithm can reduce this volume by up to 50% leaving the remaining volume less than 2% of the spacecraft 24-hour 'firstlook', day one downlink capability following the Pluto encounter. Table 13. Average Data Rates and Volumes 3.4.9 Calibration For instrument calibration prior to launch, PEPSSI test software included a capability to read out raw instrument measurements. Raw measurement data was evaluated in order to configure the look-up calibration table that was subsequently loaded into the instrument software database. 3.4.10 Memory Image Dump PEPSSI implements a capability to load selected portions of processor memory, e.g., look-up tables for instrument calibration. The PEPSSI instrument includes a capability to telemeter an image of the newly loaded segment of memory, to allow load verification in the spacecraft command center. 3.5 Commands PEPSSI uses a variety of commands to provide functionality to the instrument and set various parameters that affect how the data is collected and processed before being sent to the ground. 3.5.1 Energy Commands Commands related to energy measurement are given in Table 14. These include commands to adjust energy detection thresholds of leading edge discriminators. The leading edge discriminator thresholds operate in current mode so that a threshold current essentially corresponds to an energy threshold. Table 14. PEPSSI Energy Measurement System Commands 3.5.2 HVPS Commands High-voltage PEPSSI operation requires a vacuum environment. At power turn-on, the HVPS is disabled by default. PEPSSI includes a set of HVPS and bias voltage supply commands to enable high-voltage operation, adjust high-voltage power supply clocks for optimum efficiency, and fine-tune the high-voltage outputs. A summary of these commands is given in Table 15. Table 15. PEPSSI HVPS Commands 3.5.3 TOF Commands In concert with energy threshold commands, PEPSSI implements a set of commands to adjust the thresholds of the start and stop constant fraction discriminators (CFD). With the threshold set to zero, about 5x105 electrons are required to generate a start or stop pulse respectively (and thus initiate a TOF measurement). The threshold can be adjusted upward, requiring more electrons and proportionally higher energy levels to initiate a TOF measurement event. A summary TOF command list is given in Table 16. Table 16. PEPSSI TOF Measurement System Commands 3.5.4 Process Control Commands PEPSSI implements a number of commands to manage instrument operations, including energy measurement integration times, enable/disable event pile-up checks, etc. A summary list follows in Table 17. Table 17. PEPSSI Process Control Commands 3.5 Telemetry and Command Format The C&DH system receives all telemetry from PEPSSI as non-packetized critical housekeeping data and as CCSDS packets. The CCSDS formatted packets can be common packets with standard formats, common packets with PEPSSI-specific formats, and PEPSSI-specific packets. 3.5.1 Data Rate and Volume As just noted, the raw data rate, with the instrument telemetry system configured in the preferred manner for space operations, is a maximum of about 1620 bps, representing a total raw data volume collected over 24 hours of about 140 Mega-bits. These are raw figures that apply prior to application of the Fast data compression, which may reduce data volume by up to one-half. If necessary, the instrument telemetry system may be configured for lower data rates. Telemetry data sent from the PEPSSI instrument is compressed using the Fast algorithm. Fast is an APL-developed data-compression algorithm that is imbedded in instrument common code used by a number of MESSENGER and New Horizon instruments. Fast is a lossless compression algorithm; the reduction of data volume is variable, data volume may be reduced by up to one-half. 3.5.2 Telemetry Formatting 3.5.2.1 Instrument Transfer Frame (ITF). All messages exchanged between the spacecraft C&DH and PEPSSI are transported using the ITF format. The format applies to all telemetry data sent from the instrument and to commands and MET time received from the spacecraft. The 48-bit frame header associated with this format is outlined in Table 18. Table 18. Instrument Transfer Frame Format Byte 0 is first byte transmitted/received by PEPSSI. The instrument resynchronizes to the incoming sync pattern on each ITF to limit error propagation in case of bit errors. An ITF contains a single message, (S/C time, Command, or Telemetry) and will have variable length depending on the length of the contained message. The maximum length for command messages is selected to accommodate memory load commands. The maximum length for telemetry messages is selected to provide adequate margin for the one-second timing requirements for telemetry transfer. S/C time messages are fixed length. The time messages contain information in addition to time that vary from instrument to instrument. The ITF header includes a checksum (byte 4) on the data that follows. In the case of Command and S/C Time messages transferred to PEPSSI, the C&DH computes and adds the checksum to the ITF when the telecommand packet from the spacecraft is parsed into PEPSSI commands. PEPSSI computes and adds the checksum to all telemetry messages sent to the C&DH. PEPSSI does not receive or send messages to any other instrument or subsystem other than the C&DH. 3.5.2.2 Telemetry Interface. Once per second, the PEPSSI instrument formats a telemetry message containing data in an ITF for transfer to the C&DH. This frame is defined by the time between the rising edge of any two adjacent 1PPS signals. The message includes one or more CCSDS-formatted telemetry packets. The format is shown in Table 19. Transfer of the ITF may begin anytime during the one-second frame but must be completed prior to the rising edge of the 1PPS. If there are more packets than can fit in the ITF, then the maximum number of bytes are put into the ITF, even if this means a partial packet is used. The remaining (unsent) portion of the packet is held in memory until the next ITF is formatted, whereupon it is inserted at the start of that ITF's telemetry data. If PEPSSI does not have enough telemetry to fill an ITF during that second, the size of the ITF will be smaller than the maximum allowed. 3.5.2.3 CCSDS Packetization. The header of the CCSDS-formatted packet is defined in Table 20. The length of the packet is variable and is specified in the CCSDS header. The data field of each CCSDS telemetry packet is compressed using the lossless FAST algorithm. The application proc