INTRODUCTION
Purpose and Scope
The Project Data Management Plan (PDMP) for the THEMIS Explorer Mission outlines the comprehensive processes involved in managing scientific data from its collection on the spacecraft to its production, distribution, access, and archiving It encompasses a wide array of ground-based measurements, including imager and magnetometer data collected by 20 Ground Based Observatories (GBOs) located in Alaska and Canada, as well as 10 Education and Public Outreach (E/PO) magnetometers positioned throughout the northern continental United States.
Applicable Documents
1 THM-SYS-102 THEMIS Command Format Specification
2 THM-SYS-115 THEMISTelemetry Data Format Specification
3 THM-SYS-116 THEMISTelemetry Data Packet Format Specification
4 THM-SYS-114 THEMIS Radio Frequency Interface Control Document
5 THM-SYS-013 THEMIS Mission Operations Plan
6 THM-SYS-018 THEMIS Launch and Early Orbit Operations Plan
7 THM-SYS-019 THEMIS Contingency Plan
PROJECT OVERVIEW
Science Objectives
The THEMIS project aims to investigate the initiation and large-scale development of magnetospheric substorms, which are instabilities in the magnetic flux and plasma circulation within the solar wind magnetospheric system, closely associated with auroral eruptions in Earth's polar ionosphere Understanding these substorm instabilities is vital for advancements in space science, basic plasma physics, and space weather, as highlighted by the National Research Council (NRC) as a key strategic question in space physics For the first time, THEMIS will identify the precise timing and location of substorm onset within the magnetosphere and track their macroscopic evolution This will be achieved by synchronizing well-established plasma particle and field signatures across multiple sites in Earth's magnetotail, while concurrently monitoring substorm onset through a comprehensive network of ground observatories.
Mission Summary
The THEMIS mission's scientific goals are accomplished through five space probes, designated P1 to P5, which operate in High Earth Orbits (HEO) These probes share comparable perigee altitudes ranging from 1.16 to 1.5 Earth radii (Re), while their apogee altitudes differ Notably, P1 features a specific apogee height.
The orbital configuration of the probes includes P1 at approximately 30 Re, P2 at around 20 Re, and P3 to P5 at about 12 Re, with orbital periods of roughly 4, 2, and 1 day, respectively This arrangement facilitates multi-point conjunctions at apogee, enabling simultaneous measurements of substorm signatures across extensive distances along the magnetotail, while also streamlining ground communications and scheduling The conjunctions are meticulously coordinated with ground-based observatories during a primary observing season that lasts four months each year, centered around mid-February, as part of a two-year baseline mission A store-and-forward data flow system efficiently collects key plasma and fields data during substorm events, utilizing automated science operations for simplicity.
Ground observations will be conducted by 20 Ground Based Observatories (GBO) across Alaska and Canada, utilizing All Sky Imagers (ASI) and ground magnetometers (GMAG) to monitor auroral light and ionospheric currents This setup aims to accurately determine the timing, location, and evolution of auroral substorm manifestations Additionally, a network of 10 Education and Public Outreach (E/PO) Ground Magnetometers will be established in schools at sub-auroral latitudes in the U.S to enhance public engagement and education on this phenomenon.
Launch Vehicle: Delta II Eastern Range
Injection: 1.1x12Re, 9 degrees inclination Date: October, 2006
Space Segment Spacecraft: 5 spinning probes with fuel for orbit/attitude adjust
Instruments: 3-Axis E-Field and B-Field, 3-D Ion and electron particle detectors
The satellite operates on orbit periods of 1, 2, and 4 days, with its spin axis oriented to the ecliptic normal The ground segment consists of 20 Ground Based Observatories (GBO) located in Alaska (4 sites) and Canada (16 sites), equipped with All Sky Imagers (ASI) and Ground Magnetometers (GMAG) Additionally, 10 GMAGs are strategically placed in schools across the Northern Latitude U.S The operational phases include Integration and Testing (I&T), Launch and Early Operations (L&EO) lasting two months, targeted campaigns from December to March, and the eventual de-orbiting of the satellite.
Lifetime: 2 yearsTable 1 THEMIS Mission Summary
PROBE DESCRIPTION
Overview
THEMIS utilizes five identical space probes (P1, P2, P3, P4, and P5) operating in coordinated orbits, each comprising a probe bus and an instrument suite The probe bus includes essential subsystems such as Structural/Mechanical, Thermal, Power, RF and Communications, Command and Data Handling, and Guidance Navigation & Control (GN&C), which features an Attitude Control Subsystem (ACS) and a Reaction Control Subsystem (RCS) The Bus Avionics Unit (BAU) houses the electronics for Power, CDHS, ACS, and RCS Communication is facilitated through a low-rate S-band system employing a store-and-forward strategy near perigee, supported by a General Dynamics ColdFire processor for data handling and fault detection The power system consists of body-mounted solar panels and a small battery managed by a direct energy transfer controller The probes are spin-stabilized, with the ACS utilizing a fault-tolerant hydrazine system for orbit and attitude control, while ground-based attitude determination is conducted by the Flight Dynamics Center at UCB All maneuver sequences are meticulously planned, validated through a spacecraft simulator, and executed during real-time communications with ground control.
Subsystem Descriptions
3.2.1 RF and Communications Subsystem (RFCS).
The RFCS employs a NASA-standard 5-Watt S-Band transponder for command and telemetry communications, utilizing a single cylindrical FAST-like antenna with a toroidal gain pattern This transponder enables precise two-way Doppler ranging for accurate orbit determination, with all probes operating on the same frequency pair for telemetry and commanding Communication is conducted with one probe at a time, adhering to standard CCSDS protocols Downlink telemetry rates are adjustable to enhance probe monitoring and data recovery based on probe range, with a nominal rate of 524.288 kbps and an expected data volume of 480 - 640 Mbits during science dumps The command uplink rate is consistently set at 1 kbps.
Polarization Left-Hand Circular Polarized (LHCP)
Modulation Downlink – BPSK (4 highest data rates)
PCM/PSK/PM (6 lowest data rates) Uplink - PCM/PSK/PM
Encoding Downlink – Reed-Solomon + Rate-1/2 Convolution
Compression Scheme (VC3 only) Differencing and truncation or Huffman (TBR)
Data Volume per Orbit per Probe 480 - 640 Mbits
Table 2 RF and Communications Subsystem Summary
3.2.2 Guidance Navigation and Control (GN&C)
The GN&C subsystem includes the Attitude Control Subsystem (ACS) and the Reaction Control
The Attitude Control System (ACS) employs a thruster interface guided by ground-processed estimation and command algorithms, ensuring on-board limit and time-out protection Attitude data is collected from a Miniature Spinning Sun Sensor (MSSS) and a Fluxgate Magnetometer (FGM), sampled at 10 Hz, and sent to the ground for standard 3-axis post-processing estimation Before any commands are uploaded, ground-generated thruster command sequences undergo rigorous testing in a high-fidelity probe simulator Additionally, two single-axis gyros, positioned transverse to the spin plane, offer short-term attitude verification prior to orbital maneuvers The on-board protection logic continuously monitors the sun aspect angle and spin period, comparing them to a pre-uploaded ground-commanded reference; if any thresholds are exceeded, the maneuver is automatically terminated.
The RCS features two fuel tanks, a pressurization tank, a pyro valve, two latch valves, fuel lines, and filters, along with four 5-N thrusters Of these thrusters, two are oriented axially (+Z) for primary orbit placement and attitude control, while the other two are tangentially oriented for spin adjustments and minor side thrusting to fine-tune the orbit The minimum duration for thruster pulses is 50 milliseconds.
3.2.3 Command and Data Handling Subsystem (CDHS).
The Command and Data Handling System (CDHS) offers real-time and stored command capabilities for bus subsystems and instruments, efficiently collecting, formatting, and transmitting data to the ground It also ensures engineering data storage, distributes time to the Instrument Data Processing Unit (IDPU), and incorporates autonomous fault protection features to maintain the probe's health and safety These essential functions are executed through flight software and hardware located in the Bus Assembly Unit (BAU).
The Command and Data Handling System (CDHS) receives uplink commands from the Radio Frequency Command System (RFCS) at a fixed rate of 100 bps, utilizing CCSDS telecommand protocols that ensure the correct and sequential delivery of variable-length command packets These command transfer frames are authenticated, enhancing security The CDHS can execute hardware commands that allow for critical operations, such as hardware reconfiguration, without processor involvement Additionally, it supports stored command capabilities through Absolute Time Sequence (ATS) and Relative Time Sequence (RTS) loads, enabling control of the probe and its instruments even when not in contact with a ground station.
The CDHS (Command and Data Handling System) processes various types of engineering and science data, including real-time and playback data, sourced from the IDPU (Instrument Data Processing Unit) It collects and packetizes engineering data from the bus subsystem and instruments, delivering it to the RFCS (Radio Frequency Communication System) in real-time (VC0) or storing it locally in the BAU (Buffering and Archiving Unit) for later playback (VC1) Additionally, the CDHS routes both real-time (VC2) and stored (VC3) science data to the RFCS for downlink The telemetry format utilized by THEMIS adheres to CCSDS (Consultative Committee for Space Data Systems) standards, incorporating concatenated rate-1/2 convolutional and Reed-Solomon coding for effective error correction Furthermore, VC3 packet data is compressed to optimize storage and transmission efficiency.
0 Real-time Engineering Data (Probe and Instruments)
1 Stored Engineering Data (Probe and Instruments)
The Power Subsystem features a Direct Energy Transfer (DET) system that connects the battery and solar array directly to the power bus This solar array is composed of eight panels, strategically placed with four on each side and two on each deck.
At nominal attitudes, the side panels generate approximately 59 Watts EOL, while the top and bottom panels contribute 21 Watts EOL After accounting for battery recharging, increased eclipse heater power, and power control efficiencies, the minimum available load power is 41.7 Watts, which comfortably exceeds the required load power of 29.2 Watts An 11.8 A-hr, 28V Lithium-Ion battery effectively balances eclipse and peak transient loads, such as transmitter operation Additionally, thermal management systems utilizing heaters and thermistors maintain the battery temperature within a range of -5 to +25 degrees Celsius.
The probe features a lower deck that serves as the main mounting surface for various instruments and components, while the upper deck, corner, and side panels enclose the internal cavity Solar cells are mounted on the exterior surfaces of the side panels, and key instruments like the FGM, SCM, ESA, SST, sun sensor, and thruster brackets are strategically placed on the corner panels to ensure an unobstructed Field of View (FOV) Additionally, the mechanical and thermal designs incorporate a low conductance composite structure, effectively isolating the body-mounted solar panels to minimize thermal energy fluctuations during both full-sun and shadow operations.
The BAU features a SMEX-Lite heritage uplink/downlink communications card, a processor card identical to the IDPU processor card, and a Direct Energy Transfer (DET) power control card, all rooted in SMEX-Lite and EO-1 heritage The flight software, developed in C-language, is based on previous SMEX mission modules and operates on the heritage CMX-RTX Real-Time Operating System (RTOS) Housekeeping data is stored in the local bus memory, while science data is retained in the IDPU During ground station contacts, the BAU transmits housekeeping data directly and retrieves science data from the IDPU memory, merging both into the telemetry stream in a bent pipe flow, akin to the FAST implementation.
3.2.7 Probe Carrier Configuration and Launch.
THEMIS will utilize a standard Delta sequence to inject the Probe Carrier Assembly (PCA) directly into the target insertion orbit, with the PCA remaining attached to the third stage until after burnout and yo-yo despin The probes will separate independently, triggered by built-in sequence timers and ELV separation signals to prevent single point failures, while ground command can also initiate separation if needed To ensure safety, multiple hardware and software timers are implemented to guard against premature probe separation.
INSTRUMENT DESCRIPTIONS
Overview
Each probe is equipped with instruments designed to measure both DC and AC electric and magnetic fields, along with the energies and distributions of electrons and ions For a comprehensive overview of the instrument data quantities, data rates, and data volume, please refer to Appendices A, B, and C Additionally, Figure 3 illustrates the configuration and placement of these instruments within the probes.
Fluxgate Magnetometer
A tri-axial fluxgate magnetometer will measure the 3D ambient magnetic field in the frequency bandwidth from DC to 64 Hz (Nyquist).
1) Measure DC and low frequency perturbations of the magnetic field
2) Time wave and structure propagation between probes
3) Provide information on plasma currents based on instantaneous magnetic field differences on two or more probes, separated by >0.2 Re.
The unit features two orthogonal ring core elements of varying diameters, secured with a bobbin and mounted on a 2-meter double-hinge carbon epoxy boom Its electronics include driver and control circuits housed within the IDPU, enabling efficient digital excitation, data acquisition, feedback, and compensation for low power consumption Additionally, the low noise level facilitates straightforward inter-calibration with the search-coil magnetometer at approximately 10 Hz frequencies.
To achieve a 1 nT absolute accuracy requirement, it is crucial to ensure that two independent probes yield identical readings under stable medium conditions Calibration data will be collected at 32 Hz near apogee and perigee to assess zero levels, gains, and sensor orientation for individual probes During the early phase of the mission, magnetometers on all five probes will undergo inter-calibration in current-free or low current density regions of the magnetosphere In the second year, high-rate magnetometer data from probes P3, P4, and P5 will be gathered outside burst-mode triggers for inter-calibration of their relative orientation and offsets The validity of the divergence-free assumption will be evaluated to confirm the current-free approximation, and if this assumption is not satisfied, time-tagged data from probes in the same region will be compared for trend recognition through long-term averaging.
During the L&EO phase, ground command will deploy the magnetometer booms once the FGM and SCM are operational Initially, the FGM data rate is configured to 32 Hz, and the FGM data is monitored in real-time through the telemetry stream (VC2) Subsequently, the release mechanisms for both the prime and secondary booms are activated in order Finally, the rotation of the FGM axis is confirmed during the deployment process.
Electrostatic Analyzers (ESA)
The Electrostatic Analyzers will measure thermal ions and electrons in the range 5 eV - 30 keV
1) Plasma moments to within 10%, at high time resolution (10s or better) for inter-probe timing studies.
2) Instantaneous differences in velocity and ion pressure between probes, to estimate the scale size of transport, the size and strength of flow vortices and the pressure gradient.
3) Distribution functions of ions and electrons, to ascertain the presence of free energy sources.
The ion and electron ESA feature a 180-degree look direction in elevation, divided into eight 22.5-degree bins, with measurements taken over 4π steradian once per spin Particle selection occurs in E/q through a sweeping potential applied in 32 steps, executed 32 times per spin across 32 azimuths, between outer (0 kV) and inner (~3 kV) concentric spheres arranged in a Chevron configuration On-board moment, pitch angle, and averaging computations are performed by the IDPU, utilizing FGM and SST data to ensure accuracy when peak flux exceeds the plasma instrument's energy range Despite onboard averaging, the ESAs produce nearly 3 kbytes of data each spin, necessitating onboard moment calculations for spin period data Three-dimensional distributions are transmitted at a lower cadence, except during event bursts, which include spin period distributions.
To achieve the science requirement of 10% accuracy in moment computation, independent calibration of the ESAs is essential However, by inter-calibrating hour-long averages of routinely collected particle distributions during quiet-time probe conjunctions, it is anticipated that the accuracy will exceed that achieved through independent ESA calibration.
An automated calibration procedure performs a complete angle/energy calibration of an instrument stack in less than 1 day Calibration determines:
1) Analyzer constant, uniformity of energy/angle response
Absolute geometric factor values are determined from computer simulations and calibrations with a Ni 63 beta source.
During the L&EO phase, the ESA entrance aperture covers will be detached, a process directed from the ground using a Shaped Metal Alloy (SMA) device for cover release.
Solid State Telescope (SST)
The Solid State Telescope (SST) provides extensive coverage of approximately 3π steradians to measure the angular distribution of super thermal ions and electrons Each SST probe is equipped with two identical telescope pairs, similar to those used on the WIND spacecraft, ensuring consistent and reliable data collection.
1) Perform remote sensing of the tail-ward moving current distribution boundary (at P3, P4, P5)
2) Measure the time-of-arrival of super thermal ions and electrons (30-300 keV, at 10s resolution or better) during injections, and ascertain the Rx onset time (P1, P2).
The SSTs feature a dual-telescope design equipped with double-ended sensors, as illustrated in Figure 6 Each sensor consists of three stacked, fully depleted, passivated, ion-implanted silicon detectors, with the central detector measuring 600 µm in thickness and the outer detectors at 300 µm These four sensors are capable of measuring ions and electrons from opposing sides of their detectors The configuration of the telescope pairs allows one set of sensor apertures to be oriented above the spin plane at angles of 25 and 55 degrees, while the other set is positioned below the spin plane at angles of -25 and -55 degrees.
Figure 6 The Solid State Telescope (SST)
Absolute calibration points are established by observing the highest energy of stopped protons and using coincident pairs or triplets of detectors to monitor the minimum ionizing energy of penetrating particles This methodology has achieved excellent alignment between SST and ESA flux measurements on the WIND satellite, resulting in an absolute flux uncertainty of less than 10% Additionally, inter-probe calibration will occur during periods of low plasma sheet activity when flux anisotropy is minimal.
The SST attenuator, managed by the IDPU, activates when a probe nears the radiation belts, responding to measured flux levels The design incorporates approximately 10 minutes of hysteresis to ensure effective operation.
Search Coil Magnetometer
The SCM measures the 3D magnetic field in the frequency bandwidth from 1 Hz to 4 kHz It will extend with appropriate sensitivity the measurements of the FGM beyond the 1 Hz range.
The scientific requirements stem from the necessity to measure cross-field current disruption waves with high sensitivity, specifically less than 1 pT/√Hz at 10 Hz These measurements must be conducted at distances from Earth of at least 8 Earth radii, focusing on frequencies around 0.1 fLH to 60 Hz.
The SCM quantifies the changes in magnetic flux through three orthogonal high permeability metal rods, achieving a unit sensitivity of 0.5 pT/Hz at 10 Hz To maintain phase stability, a flux feedback loop is utilized.
The signals from the three sensors are pre-amplified and then processed together with the EFI data at the IDPU.
Figure 7 The Search Coil Magnetometer (SCM)
Absolute amplitude and phase calibration is achieved using calibration coils that generate a known AC pseudo-random noise across a range of discrete frequencies, effectively covering a bandwidth from 10 Hz to 4 kHz The initiation of the calibration process is controlled by the IDPU, following a predetermined sequence.
Electric Field Instrument (EFI)
The Electric Field Instrument (EFI) is designed to measure the three-dimensional electric field across a frequency range from DC to 300 KHz The EFI experiment features four spin-plane spherical sensors, each suspended on a 20 mm deployable cable that extends 20 meters from the probe center Additionally, two axial tubular sensors, each measuring 1 meter in length, are mounted on a 4-meter-long stacer element.
Determine the time of onset at 8-10 Re by measuring:
1) The plasma pure convection motion, i.e., without the effects of diamagnetic drifts that ESA measurements are subject to.
2) The low frequency (T~1min) wave mode and pointing flux.
The inner probes will determine the axial component independently from the axial boom measurement and provide both a method for calibration of the axial measurement and a backup solution.
Boom Electronics, situated at the EFI housing, is responsible for stub and guard voltage control as well as sphere biasing Signal processing is conducted within the IDPU alongside the SCM, capturing routine waveforms at 32 samples per second and burst waveforms ranging from 128 to 8192 samples per second Low-frequency spectral processing (below 8 kHz) is performed in the DSP, mirroring the SCM's approach The wire booms are deployed with near real-time monitoring during a release and spin-up sequence that lasts 1-2 hours per probe Throughout the science and sphere-release phases, the mission's total EFI deployment duration is under 10 days, alternating between various THEMIS probes.
Figure 8 The Electric Field Instrument (EFI)
The calibration of the individual probe achieves an impressive absolute DC measurement accuracy of 0.1 mV/m, which is less than 10% of the expected field value during rapid flows Enhanced measurement reliability will be established through inter-spacecraft calibration during periods of minimal activity.
The EFI necessitates operational commands for managing boom deployment and adjustments, alongside scientific commands to regulate sensor bias voltages, data sampling rates, filter settings, and control of spectral resolution.
As in previous missions, a typical mode can be specified with ~200 commands valid over a typical operational period of ~1 month once deployment and checkout phases have been completed.
1) Deployment alternatively extending the wire boom pair in predetermined increments During radial wire boom deployment and at each stop, sphere potentials are monitored in order to characterize probe charging affects, plasma environment, and EFI status After the radial boom deployment, the axial booms are each deployed to their final lengths using one initiator event per boom.
2) Checkout: Assuming nominal potential measurements and probe spin rate, the checkout phase begins with final adjustments in wire boom lengths to verify that each pair deployed symmetrically relative to the probe body These occur in near real-time sessions, monitoring the release and spin-up sequence, each lasting 1-2 hours/probe Alternating between different THEMIS probes in science data collection and sphere-release phase, mission-total EFI deployment lasts < 10 days After boom deployment, an EFI early-checkout phase begins in which the photo-currents are characterized and the guards, stubs, and bias adjusted accordingly, requiring a new command load roughly once per week, per probe Science quality data are returned during this phase which lasts ~1 month.
3) During the nominal science phase, the EFI is configured roughly once each month through a command sequence.
Instrument Data Processing Unit (IDPU)
The Instrument Data Processing Unit (IDPU) is essential to the instrument package, serving as the central hub for power supply, function control, and command reception It collects, processes, and stores both housekeeping and scientific data, facilitating communication with the probe bus electronics Acting as the interface between the instrument sensors and the probe's Bus Assembly Unit (BAU), the IDPU is equipped with an 8085 processor and 256 MB of memory dedicated to storing scientific data.
The IDPU is responsible for collecting, compressing, and storing instrument data, transmitting it to the ground at downlink rates of 524.288 kbps or 1,048.576 kbps upon command It utilizes a 1 kbps, COP-1 compliant command uplink protocol, with commands relayed by the bus processor Instrument data is continuously provided to the IDPU based on the system mode, which can include survey, particle burst, or wave burst operations The data format consists of a 24-bit structure, comprising an 8-bit application process identifier (APID) followed by 16 bits of data The processor handles data compression and packetization before storing it in IDPU memory, while the IDPU-to-bus C&DH telemetry requires a 1 Mbps serial data stream Additionally, the IDPU can prioritize and mix engineering and science frames based on operational preferences during downlink.
In nominal operation, the IDPU sends instrument housekeeping packets to the probe-BAU, which are integrated with its data into CCSDS frames for downlink transmission Additionally, stored science data is transmitted separately after the engineering data over a high-speed link to the BAU when commanded from the ground.
The IDPU monitors instrument science data by utilizing predefined measurement quantities to assess overall data rates It employs a command upload table to direct instrument quantities into a trigger buffer, guided by a trigger APID list Real-time evaluations of individual measurements or weighted combinations are conducted, comparing them against preset thresholds to determine instrument rates for surveys, particle bursts, or wave bursts.
Mode definition tables are extensive macros utilized by the IDPU to configure instruments for specific regions of space The IDPU is programmed with various mode definitions that can be activated through ATS commands or onboard triggering logic With 32 macros of 512 bytes each, a complete reload necessitates 16 kbytes of data At system startup, the IDPU FSW calculates ESA and SST Moment Tables, which are then loaded into the corresponding circuitry For contingency operations, these tables can also be directly loaded from the ground Additionally, EFI biasing, FGM, and SCM parameter modes are compact and included within the mode definitions.
Ground Observations
Two networks of instruments will conduct ground observations and measurements of the aurora and Earth's magnetic field, with the first network consisting of 20 Ground Based Observatories (GBOs) strategically located throughout Alaska.
(4) and Canada (16) The GBO's will contain All Sky Imagers (ASI) and Ground Magnetometers
The second network consists of 10 Education and Public Outreach (E/PO) GMAGs situated in schools at sub-auroral latitudes across the U.S., as illustrated in Figure 9.
The THEMIS mission captures substorm signatures both on the ground and in space with a remarkable time resolution of under 30 seconds To address the complexities of substorms, THEMIS employs a comprehensive strategy that includes monitoring the night side auroral oval using fast, low-cost white-light All Sky Imagers (ASI) with exposure times of less than one second, alongside high-time resolution Ground Magnetometers (GMAGS) operating at one-second intervals These advanced instruments generate detailed auroral images and precise Earth magnetic field measurements Additionally, Ground-Based Observatories (GBO) contribute vital health and safety data to assess site conditions The ASIs are provided by the University of Colorado Boulder (UCB), while the GMAGs are supplied by the University of California, Los Angeles (UCLA).
Figure 9 Ground Based Observatory (GBO) Locations
The GBOs will utilize existing ground-based networks' infrastructure and instrumentation Each of the 20 locations will feature a UCB ASI, while only 10 will include a UCLA GMAG Additionally, two sites in Alaska will depend on magnetometers that are provided and managed by external sources.
The Geophysical Institute at the University of Alaska, Fairbanks, collaborates with Canadian Geospace Monitoring (CGSM) and Natural Resources Canada (NRCAN) to operate eight additional magnetometer sites in Canada Table 2 provides details on the types of magnetometers used at each location, along with their geographic coordinates.
No Site Abbrev Location Latitude Longitude GMAG type
1 Gakona GAK USA 62.4 214.8 GI & GPS5
2 Fort Yukon FYU USA 67 199.6 GI & GPS1
5 Inuvik INUV Canada 68.3 226.7 CGSM&GPS4
6 White Horse WHOR Canada 60.7 224.9 GMAG7
7 Lac de Gras LGRA Canada 64.6 250 GMAG2
8 Fort Simpson FSIM Canada 61.8 238.8 CGSM&GPS3
9 Prince George PGEO Canada 53.9 237.4 GMAG-Proto
10 Rankin Inlet RANK Canada 62.8 267.9 CGSM&GPS6
11 Fort Smith FSMI Canada 60 248.1 CGSM&GPS6
12 Athabasca ATHA Canada 54.7 246.7 NRCan&GPS2
13 Gillam GILL Canada 56.4 265.4 CGSM&GPS9
14 The Pas TPAS Canada 54 259 GMAG3
15 Pinawa PINA Canada 50.3 264 CGSM&GPS7
16 PBQ PBQ Canada 55.3 292.3 NRCan&GPS10
20 Goose Bay GBAY Canada 53.3 299.6 GMAG8
The GBO's key components include the Computer System Enclosure (CSE), which features an external insulated environmental enclosure and an internal rack mount This rack mount houses essential equipment such as the system computer, hot-swappable hard drive, GMAG interface electronics, Power Control Unit (PCU), CD10X Datalogger with battery, ASI power supply, and an Uninterruptible Power Supply (UPS).
The CSE is designed to function efficiently in external temperatures ranging from -50 degrees Celsius to 40 degrees Celsius It utilizes a solid-state air conditioner for dust-free cooling and incorporates small space heaters for effective heating.
11) It provides access for external cables, maintenance, hot swapping of hard drives
Figure 11 GBO CSE Heating and Cooling
The Power Control Unit (PCU) regulates both temperature and power for instruments, ensuring that the temperature in the CSE and ASI is consistently maintained at 20 degrees Celsius, with a permissible variation of +/- 10 degrees Celsius Additionally, the PCU facilitates a smooth shutdown of the system computer in case of power loss or temperature fluctuations.
The CR10X Datalogger is an advanced controller designed to replace traditional thermostats, offering user-friendly programming and robust data logging capabilities It features both analog and digital I/O for seamless integration with system computers and remains continuously operational and accessible for reprogramming via the internet or Iridium modem With an impressive operating temperature range of -55°C to 85°C and low power consumption, the CR10X can function for extended periods on battery power, making it an efficient choice for various applications.
Remote access to the GBO’s will typically be through an internet connection made possible by one of the following:
• Hardwired using a local LAN connection
• Telesat HSi (Canada) or Starband 480 (Alaska) o Can provide fixed IP address o Minimum 10 kbps uplink rate
• Backup connection via an Iridium modem o 2400 bps
Figure 13 Prime Data Communications Link
Figure 14 Backup Data Communications Link
The ASI design incorporates commercially available components and is derived from the heritage ASI systems utilized at AGO sites in Antarctica Key elements of this design feature a Charged Coupled Device (CCD) camera paired with an all-sky (fisheye) lens, ensuring comprehensive coverage and effective imaging.
Figure 15 All Sky Imager (ASI)
The ASI is housed in a heated environmental enclosure topped with a polycarbonate/acrylic dome (Figure
The enclosure features a hermetically sealed design with a nitrogen purge, ensuring optimal performance in external ambient temperatures ranging from -50°C to 40°C It maintains a stable internal temperature of 20°C, with a tolerance of +/- 10°C.
Figure 16 ASI Mounting and Enclosure
An internal sun shield will protect the ASI UV radiation damage during non-operation periods (figure 17).
Ten Ground-Based Observatories (GBOs), including two in Alaska and eight in Canada, will be equipped with Ground Magnetometers (GMAGs) developed by UCLA Also known as Fluxgate Magnetometers, these compact and energy-efficient units feature a durable all-weather sensor design The GMAG processor card, which facilitates data retrieval and firmware uploads via a USB interface, is housed within the Central Support Equipment (CSE).
The sensor features a dynamic range of +/-72KnT at a resolution of 0.01nT, generating two vectors per second Each vector includes three measurements of magnetic field strength along the Bx, By, and Bz axes The expected data output is 86.4 kbytes per hour, supplemented by around 100 bytes per hour for housekeeping and log data Raw magnetometer data is recorded in its original format, with calibration applied during product compilation for distribution Data recovery is facilitated through ASI data.
The GBO records around 10 kbytes of health and safety data every five minutes, equating to 100 kbytes per hour Additionally, critical information is captured at a significantly lower rate of approximately 20 bps, resulting in a data volume of 10 kbytes per hour.
4.8.2 E/PO Ground Magnetometers (E/PO-GMAGS)
UCLA plans to construct and install 10 additional GMAGs in selected K-12 schools situated in sub-auroral latitudes across the U.S This initiative, managed by the UCB Education and Public Outreach (E/PO) group, aims to enhance inquiry-based and theme-based learning while encouraging active student involvement.
GROUND DATA SYSTEM (GDS) DESCRIPTION
Overview
The THEMIS Ground Data System (GDS) is composed of several key functional segments, including Ground Stations (GS) for communication with orbiting probes, a Mission Operations Center (MOC) for managing probe telemetry and command control, a Flight Dynamics Center (FDC) for determining probe orbit and attitude, and a Science Operations Center (SOC) responsible for instrument data collection, processing, archiving, distribution, and command generation for instrument operations All these components, including the Berkeley Ground Station (BGS), are located at the Space Sciences Laboratory on the University of California, Berkeley campus, each playing a crucial role in the overall mission operations.
Ground Stations
The Berkeley Ground Station (BGS) serves as the primary ground station for THEMIS, utilizing advanced front-end processors for essential tasks such as bit synchronization, Viterbi decoding, and frame synchronization Real-time engineering and science data streams are directly routed to the Mission Operations Center (MOC) for continuous state-of-health monitoring and control Additionally, all telemetry data received at the ground station is stored in separate files for each virtual channel and is automatically transferred to the MOC and Science Operations Center (SOC) via FTP after support is completed.
Commanding of probes is carried out from ITOS workstations in the MOC, adhering to standard CCSDS procedures Commands are segmented into Command Load Transmission Units (CLTUs) and sent to front-end processors through secure TCP/IP connections The command data stream is transmitted in real-time at a rate of 1 kbps, modulated using BPSK onto a 16-kHz subcarrier, which is then PM modulated onto the RF carrier with a modulation index of 1.0 rad The CCSDS COP-1 protocol ensures command reception verification on the probe ITOS monitors the Command Link Control Word (CLCW) in each telemetry frame to track command verification status, automatically retransmitting any unverified commands.
Each probe is equipped with a coherent STDN-compatible transponder, enabling precise two-way Doppler ranging for accurate orbit determination All probes operate on the same frequency pair for telemetry and command functions, and communications are conducted with one probe at a time.
5.2.2 Secondary and Backup Ground Stations
NASA/GN stations, including WGS 11-m, MILA 9-m, AGO 9-m, and HBK 10-m, will provide secondary ground station support for the mission During the Launch and Early Orbit (L&EO) phase, TDRSS support will be utilized to monitor the probe's release, assist with maneuver operations, and facilitate recovery from any anomalous conditions.
Real-time telemetry and command data are transmitted between the ground station and the Mission Operations Center (MOC) using a T1 line Telemetry data stored on the ground is transferred to both the MOC and the Space Operations Center (SOC) through the open Internet During Launch and Early Orbit (L&EO) operations, the TDRSS S-Band Single Access (SSA) mode facilitates low data rate communications with each probe when they are within range of a TDRS spacecraft.
Pass schedule requests from the MOC are sent to the relevant scheduling offices for each ground station network Once confirmed, these schedules facilitate mission planning and are transmitted via secure network links Tracking data from all ground stations is relayed to the FDC for orbit determination, which generates updated ephemeris products Additionally, attitude sensor data from the probes is routed through the MOC to the FDC for ground-based attitude solutions After verification, these ephemeris products and attitude solutions are utilized to plan orbit maneuvers.
The ground station telemetry file transfer protocols and file formats are listed in reference document [4] The file naming convention is listed below:
• FACILITY.PROBE_BUS_NAME.TLM_VCN.YYYY_DDD_HHMMSS.dat
• BGS.THEMIS_A.TLM_VC0.2007_028_060312.dat
• WGS.THEMIS_B.TLM_VC1.2007_029_012031.dat
• MIL.THEMIS_C.TLM_VC2.2007_038_102319.dat
• AGO.THEMIS_D.TLM_VC3.2007_032_151745.dat
• HBK.THEMIS_E.TLM_VC6.2007_034_234512.dat
• WSC.THEMIS_A.TLM_VC0.2006_301_001834.dat
Mission Operations Center (MOC)
The THEMIS Mission Operations Center (MOC) is responsible for mission planning, commanding, and monitoring the health of five probes The UCB Flight Operations Team (FOT) will manage the recovery of scientific and engineering data, as well as data trending and anomaly resolution.
The operational phases of the probes encompass Pre-Launch, Launch and Early Orbit (L&EO), Nominal Science Operations, Maneuvers, and End-of-Mission During Pre-Launch and L&EO, the probes are powered with their receivers active, and all deployable appendages are stowed during the Probe Carrier dispense operation Upon release, the probes are passively spin-stabilized, and command and telemetry operations begin, initiated by ground control using unique identification codes The In-orbit Checkout (IOC) phase verifies key instrument functions and collects calibration data for each probe configuration Although each probe is self-sufficient, the Flight Dynamics Controller (FDC) will manage orbit and attitude determination through the Berkeley Flight Dynamics System (BFDS), with maneuvers executed during ground station contact The design features a passively spin-stabilized control scheme, body-mounted solar panels, and near omni-directional communications, ensuring fail-safe operations without the need for maneuvers.
5.3.1.2 Pre-launch, Launch and Early Orbit (L&EO) Operations
Pre-launch operations encompass comprehensive data flow tests, rehearsals, and full mission simulations to ensure seamless integration of all GDS elements During the launch, the Delta II rocket places the PCA into the designated orbit, facilitating the release of probes Command and control authority shifts from the launch vehicle controllers to the Mission Operations Center (MOC) at UCB, where ground station contacts monitor the probes in a round-robin manner to assess their health and gather telemetry data for orbit and attitude determination Once orbits are confirmed, the MOC uplinks the initial command loads to each probe, followed by the deployment of the FGM/SCM magnetometer booms and powering up all scientific instruments After verifying the functionality of the probes, the final constellation IDs (P1, P2, P3, P4, and P5) are assigned based on test results and magnetic signatures, enabling mission redundancy and a strategy for probe replacement to mitigate issues with any science instruments.
The final orbit injection process begins with re-spinning all probes to a rate of 20 rpm, including the calibration of tangential thrusters The orbits of the probes are then adjusted through discrete pairs of apogee and perigee maneuvers using axial thrusters, followed by precise orbit and attitude determination to calibrate the thrusters Real-time verification of thruster performance is conducted via telemetry data from RCS temperature sensors, tank pressure gauges, and attitude sensors Once the probes reach their designated mission orbits, they deploy the radial EFI wire booms, followed by the simultaneous deployment of the axial EFI booms Calibration measurements of the probe potentials are incorporated into this deployment sequence, and the spin rate is ultimately fine-tuned to 20 rpm for all probes.
Preparation for the conjunction season marks the start of normal operations, during which daily communications with each probe are established through the primary ground station (BGS) This process ensures the monitoring of probe health and safety, facilitates the recovery of stored engineering data, and gathers tracking information essential for accurate orbit determination.
Science data is stored on-board probes and transmitted to ground stations during 15-30 minute passes near perigee Data transmission is triggered by time sequence commands, part of an Absolute Time Sequence (ATS) load, which is generated for each probe using the Mission Planning System (MPS) ATS loads are uploaded multiple times weekly, covering at least 8 days for P1 and 4 days for P2, while P3, P4, and P5 receive 3 days of data Ground stations send real-time health and safety data (VC0) and a subset of science data (VC2) to the Mission Operations Center (MOC) for instrument performance monitoring Additionally, stored engineering (VC1) and science (VC3) data are saved and delivered to the MOC and Science Operations Center (SOC) after each pass.
During regular operations, the orbits of P1, P2, and P5 are fine-tuned 2 to 4 times a year to enhance conjunctions, typically using side thrusting for short-duration adjustments Additionally, P1 and P2 undergo annual orbit maneuvers to counteract lunar perturbations affecting inclination, which helps minimize long shadow periods and maximizes scientific conjunction time These extended burns for P1 and P2 are conducted outside the primary science season and utilize axial thrusting.
The Berkeley Emergency & Anomaly Response System (BEARS) is designed to alert FOT members when probe or instrument telemetry exceeds set limits or when other critical GDS conditions arise It efficiently analyzes log files generated by ITOS during real-time operations and reviews stored engineering data to automatically identify any yellow or red telemetry limit violations Additionally, BEARS responds to email warnings from various GDS components Upon detecting a limit violation or GDS anomaly, on-call FOT members receive immediate notifications through 2-way email pagers to promptly address and resolve the issue.
After the second year of operations the probes will be positioned for re-entry course.
Flight Dynamics Center (FDC)
The FDC plays a crucial role in managing orbit dynamics and maneuver functions, which include generating ephemeris and mission planning products, determining orbits, calibrating ACS sensors, assessing attitude, planning maneuvers, and analyzing and calibrating thruster performance.
Four key software tools are essential for generating ephemeris and mission planning products, determining orbit and attitude, and executing maneuver planning These include the Goddard Trajectory Determination System (GTDS), the General Maneuver Program (GMAN), and the Multi-mission Spin Axis Stabilized Spacecraft (MSASS) attitude determination systems, all developed at NASA's Goddard Space Flight Center (GSFC) Additionally, SatTrack serves as a Commercial-off-the-Shelf (COTS) solution Probe conjunction analysis is performed using a combination of GTDS and a proprietary Interactive Data Language (IDL) software library developed in-house at SSL.
The four major functions of these software tools are described below:
GTDS conducts high-precision orbit propagation and determination for the THEMIS probes by utilizing two-way Doppler tracking data in UTDF format, collected from ground stations This data is gathered during regular science transmissions at distances of 30,000 km or less, as well as during additional orbital passes GTDS updates the state vectors for the five probes, generating an updated ephemeris and new mission planning products These updated vectors are then distributed to ground stations to optimize acquisition angles for future passes Additionally, routine NORAD orbit determination using radar tracking data serves as a backup for the primary orbit determination process.
SatTrack produces mission planning products using GTDS ephemeris output, which encompasses ground station view periods, link access periods, and eclipse entry and exit times, along with other critical orbit events essential for the Mission Planning System (MPS) Additionally, the SatTrack software suite facilitates the distribution of real-time event messages to various ground system components, including ITOS and BGS, within a fully autonomous client/server network environment.
The ground-based attitude determination of probes employs the MSASS system to process raw telemetry sensor data, transforming it into vectors aligned with the spacecraft's body coordinates Each probe is equipped with a suite of attitude sensors, including a V-slit sun sensor, two mini-gyros, and a dual-use three-axis FGM, which is crucial for cross-calibrating the other sensors during the near-Earth phase of their orbits Reference vectors for converting from the body frame to the inertial frame are sourced from the spacecraft's ephemeris, as well as solar, lunar, and planetary data, alongside the latest International Geophysical Reference Model (IGRF) of Earth's magnetic field Utilizing these inputs, the MSASS estimator accurately calculates the inertial attitude vector for each probe at any given moment.
The GMAN tool is essential for maneuver planning, utilizing probe propulsion and current target state vectors to create an optimized mission profile that incorporates spin-axis reorientation and orbit adjustment maneuvers, along with coast periods between thruster firings A typical scenario involves reorienting the probe from its mission attitude to the orbit maneuver attitude, executing the orbit maneuver, and then returning to the nominal attitude To enhance accuracy, attitude reorientation maneuvers are ideally performed near perigee, leveraging magnetometer data for independent attitude confirmation before the orbit maneuver Orbit maneuvers are strategically conducted near perigee and apogee to facilitate mission orbit insertion and ongoing orbit maintenance, with planning carried out at the FDC in collaboration with GSFC/FDAB.
Flight Operations Team (FOT)
The UCB operations personnel oversee the MOC, FDC, and BGS systems, ensuring efficient control and maintenance Within the MOC, the Flight Operations Team (FOT) utilizes the Integrated Test and Operations System (ITOS) for probe command, control, and Health and Safety (H&S) monitoring By employing ITOS from Integration and Testing (I&T) through on-orbit operations, FOT members receive early training in bus and instrument operations, which promotes a seamless transition to standard operational procedures.
Science Operations Center (SOC)
The THEMIS Science Operations Center (SOC) oversees the collection, processing, archiving, and distribution of data from Probe and GBO instruments, while also planning and generating operational commands Collaborating closely with the co-located Mission Operations Center (MOC), the SOC ensures efficient transfer and processing of telemetry data and maintains proper control and configuration of the instruments Additionally, the SOC will coordinate with the GBO team to facilitate similar operations.
Overview
The following sections detail the flow of THEMIS Project Data from probe and GBO instrument collection through ground processing and product production and availability
Figure 20 THEMIS SOC Data Flow
Probe Instrument Data
Figures 14 and 15 outline the key processing steps and timeline related to the probe instrument data and the resulting products Refer to these figures for a clearer understanding as you read the subsequent sections.
Figure 21 Instrument Data Collection and Processing Timeline
Data collection on the probe initiates the flow, with rates influenced by its orbital position and local plasma conditions For specific collection rates, refer to Appendix B Engineering data is stored in VC1 format, while science data is recorded as VC3.
During ground station contacts near perigee, stored engineering and science data (VC1 and VC3) are transmitted to Earth within a 15 to 30-minute window Once data is collected on the spacecraft, it may take up to 8 days to recover a complete set of data for a full orbit This duration accounts for the probe with the longest orbital period (P1, which lasts 4 days), the frequency of instrument data transmissions per orbit (1 transmission for P1), and the maximum number of transmissions required to retrieve the entire orbit (up to 2 transmissions for P1).
The probe will transmit real-time engineering data (VC0) and a portion of the science data (VC2), controlled by pre-loaded commands from the Flight Operations Team (FOT) or ground commands Ground stations facilitate the real-time transfer of VC0 and VC2 to the Mission Operations Center (MOC) All VCs are stored in files and are sent to the MOC and Science Operations Center (SOC) about one hour after ground station contact concludes.
VC2 will contain a subset of the instrument science data in real-time, which will be useful during
Integration and Testing (I&T) and Launch and Early Orbit (L&EO) operations focus on thorough testing and configuration of instruments for standard operations The VC2 system provides the Flight Operations Team (FOT) with a reliable method to monitor daily instrument performance Data from VC0 and VC2 will be subjected to limit checks by ITOS and archived for post-pass processing The limit checking results are documented and forwarded to the BEARS system for error detection and to notify personnel of any issues.
Approximately one hour after ground station contact, VC1 and VC3 files are sent via FTP to the SOC, with VC1 also being forwarded to the MOC for automatic processing by an ITOS workstation to check back-orbit limits Log files are sent to BEARS for error detection and operator notifications Upon receipt of the VC files, the SOC begins autonomous data processing, which includes quality checks and statistics generation to identify any data gaps within the files or between previous transmissions These data files will support instrument operations during integration and testing (I&T) and the commissioning phase of the L&EO instrument The VC files are stored on a hard drive and backed up onto DVD for accessibility.
6.2.4 Level Zero Processing – Time T1+2Hrs
After the initial quality checks and statistics generation, data enters Level Zero Processing, which involves sorting by Application (Packet) Identifier and arranging packets in chronological order This process creates 24-hour Level Zero (L0) data files that encompass all engineering and scientific data collected, including slow survey, fast survey, and burst data These L0 files are stored locally, archived onto DVDs, and made accessible for data analysis tools.
After the creation or update of a Level 0 (L0) file, it is transformed into Common Data Format (CDF) or Level 1 (L1) data Level 1 CDF files include raw data from all instruments at the highest temporal resolution By utilizing instrument calibration data, raw data is converted into physical quantities, which provide particle moments, distributions, and field quantities.
Upon the completion of CDF processing, instrument diagnostic plots are generated within four hours Daily, the THEMIS Operations Scientist (Tohban) reviews these plots to ensure the proper operation and calibration of each instrument This review encompasses checks on overall data quality, analysis of housekeeping data trends such as detector efficiencies and offsets, and the identification and documentation of significant geophysical events.
6.2.7 Browse/Key Parameter (K0) Data Creation – T1+24Hrs
The completion of CDF processing initiates the generation of Browse or Key Parameter (K0) Data, which will be publicly accessible on the UCB website approximately 24 hours after the instrument data is first received at the SOC This space-based instrument key parameter data is provided in physical units and at a resolution of 3 seconds.
• 3-D magnetic field, DC waveforms and AC spectrograms.
• 3-D electric field, DC waveforms and AC spectrograms
• Core (10eV-40 keV) ion density and velocity moments, temperature and pressure tensor, energy spectrogram.
• Core (5 eV-30 keV) electron temperature and pressure tensor, energy spectrogram.
• Energetic ion and electron fluxes.
• Indications when burst data has been collected
Browse data are essential for monitoring large-scale particle and field dynamics, allowing users to select specific time periods of interest Generated through IDL scripts by instrument investigators, these data are accessible in both GIF plots and CDF files While they provide valuable insights, it's important to note that they are not routinely verified for accuracy and may be revised as new data or updates to calibration and orbit data become available Additionally, key parameter data related to GBO and E/PO ground-based observations will also be produced.
Around one month after receiving data at the Science Operations Center (SOC), the initial key parameter set (K1) is transmitted to the Sun-Earth Connection Active Archive (SECAA) and the National Space Science Data Center (NSSDC) through the internet This delivery includes essential updates on data content, calibration adjustments, and orbital information.
At approximately 6 months (T1+6months) after data reception at the SOC, the definitive key parameter set (K2) is delivered to SECAA and NSSDC.
GBO Data
The University of Calgary (UC) will act as the main data distribution hub for the UCB All Sky Imager (ASI) and UCLA Ground Magnetometer (GMAG) data, along with GBO health and safety (H&S) data Meanwhile, the University of Alberta (UA) will retrieve and process the CGSM and NRCan magnetometer data, ensuring it is available for download.
Figure 22 Ground Based Data Flow
The ASI will generate full PNG images at a rate of one image every five seconds during nighttime, with each image consisting of 256x256 16-bit pixel values Thanks to PNG compression, the file size is reduced to approximately 90 Kbytes per frame, resulting in a data rate of 150 kbps or 60 Mbytes per hour, equating to 720 images per hour For Stream 2, the anticipated uncompressed data volume is between 220 to 290 gigabytes per year per site.
Thumbnail images in PGM format are created from full images, featuring a resolution of 20x20 pixels with 8-bit values, plus approximately 50 bytes of header information, totaling around 450 bytes With GZIP compression, the size is reduced to 270 bytes per frame, achieving a 60% reduction At a frame rate of 5 seconds, this results in a data rate of 430 bps, equating to 190 Kbytes per hour.
Stream 1 and 2 are stored locally on a hard drive connected to the system computer
The GMAG will generate 2 mag vectors every second Each vector consists of three quantities: Bx, By, and
Bz, which are measurements of the magnetic field strength along each axis The data output is expected to be 86.4 Kbytes/hour.
GBO data collection is summarized in the table below
ASI 1 image every 5 seconds Stream 1: “Thumbnail” low resolution image 20x20 8- bit values (pixels) plus header information (roughly 50 bytes) for a total of ~450 bytes PGM Format.
Stream 2: Raw image frames 256x256 16-bit values
GMAG 2 mag vectors every second Each vector consists of three quantities: Bx, By, and Bz.
6.3.2 Thumbnail Image Recovery by UC – T0+1min
Stream 1 (thumbnail frames) is transmitted to UC daily and complies with the needs of THEMIS to determine the substorm onset to better than 0.5 hours in MLT Primary means of stream 1 data retrieval is through the Internet provider Telesat HIs using a typical TCP/IP connection, with the expected throughput rate of 50 kbps Stream 1 should arrive at UC within a few seconds of acquisition and be available for download after review and movement to central storage (~5minutes) A fraction of the high-resolution data (Stream 2) will be recovered with Stream 1 (