TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics)
TIMED is a NASA exploratory mission, managed and operated by JHU/APL (Johns Hopkins University/Applied Physics Laboratory) for NASA. TIMED is the first mission in NASA's STP (Solar Terrestrial Probes) program. The primary objective is to investigate and understand the energetics of the Mesosphere and Lower-Thermosphere/Ionosphere (MLTI), the region in the Earth's atmosphere from about 60 to 180 km in altitude. The measurements of TIMED will provide data defining the basic states of the MLTI region and its thermal balance. Specific missions goals are: 1) 2) 3) 4) 5) 6) 7) 8)
• To determine the temperature, density and solar wind structure in the MLTI region, including the seasonal and latitudinal variations
• To determine the relative importance of the various radiative, chemical, electrodynamical, and dynamical sources or sinks of energy for an understanding of the thermal structure of the MLTI.
Areas of applications: The MLTI (Mesosphere and Lower-Thermosphere/Ionosphere) is a very poorly understood region of the atmosphere. The results of the TIMED mission will enable the scientific community to establish the first quantitative MLTI baseline and will serve as a basis for future investigations.
Figure 1: Illustration of TIMED observation region in Earth's atmosphere (image credit: JHU/APL)
The TIMED S/C structure is an aluminum and framework/honeycomb design with a MIL-STD-1553B data bus. The nadir-pointing S/C uses three-axis stabilization; it is controlled with reaction wheels and torque rods, and uses a ring-laser gyro and two star trackers for attitude estimation. Attitude knowledge is 0.03º (3 σ, all stellar), attitude control is 0.5º (3σ, three-axis momentum-bias); mission design life = 2 years; S/C mass = 660 kg, power = 426 W (solar panels, 50 Ah Ni-H2 battery). 9) 10)
Figure 2: Isometric drawing of the deployed TIMED spacecraft (image credit: JHU/APL)
The ADCS (Attitude Determination and Control Subsystem) employs its own MIL-STD-1553B data bus. This decouples the ADCS from the rest of the S/C subsystems, permitting for the easy implementation of autonomy and safing algorithms. A complete loss of attitude due to a failure of the attitude 1553 bus controller can be recognized by the C&DH processor, which can switch to the redundant AIU (Attitude Interface Unit).
During normal operations, TIMED is required to maintain a nadir-pointing attitude with the +z axis in the direction of the Earth’s geocentric center, the +y axis in the direction of orbit normal, and the +x axis generally in the ram or wake direction.
The Sun beta-angle, defined as the angle between the Sun and orbit plane, dictates the x-axis direction as follows. SABER contains a thermal cooler for instrument temperature regulation and requires the spacecraft to avoid pointing the +y vehicle axis toward the Sun. Orbit precession thus necessitates a π-radian rotation, called the “yaw maneuver,” about the z-axis approximately once every sixty days. In addition to the nadir pointing science mode, a power and thermally safe attitude mode exists in which the –y side of the spacecraft is pointed at the Sun and the solar arrays are rotated to face the Sun (Ref. 28).
Figure 3: The spacecraft block diagram (image credit: JHU/APL)
IEM (Integrated Electronics Module):
TIMED has been designed with a significant amount of onboard autonomy, as it is run with a low-cost mission operations concept. The spacecraft with redundant subsystems features an IEM that contains RF and digital subsystems in a common card cage. The cards within the IEM communicate over a PCI (Peripheral Component Interconnect) parallel data bus. Two identical IEM modules are used, both communicating to other spacecraft subsystems and the instruments over a redundant 1553 serial data bus. The IEM collects housekeeping data from other cards and subsystems through a 100 kHz twisted-pair housekeeping (I2C) bus. Three spacecraft subsystems and supporting power-conditioning electronics, all implemented on nine plug-in cards, are integrated into a single chassis to form the IEM consisting of: 11) 12) 13)
Figure 4: Block diagram of the IEM (Integrated Electronics Module), image credit: JHU/APL)
Figure 5: Photo of IEM unit with (a) and without (b) the front cover (image credit: JHU/APL)
Legend to Figure 5: The size of IEM is 18 cm x 33 cm x 27 cm, the mass is 11.4 kg, and the peak power consumption is 53.3 W (when the downlink amplifier is on).
C&DH (Command & Data Handling): The C&DH subsystem is implemented on three cards—processor, solid-state recorder (SSR), and C&T (Command and Telemetry) interface. It also has elements on two other cards: a CCD (Critical Command Decoder) on the uplink card and a downlink formatter on the downlink card, both part of the telecommunications subsystem. The C&DH features are:
- Processor and 1553 bus controller (MIL-STD-1553B, redundant). A dual-processor-based computer system is used (Synova Mongoose-V 32-bit RISC processors with 2 MByte of RAM and 4 MByte of Flash EEPROM)
- SSR (Solid State Recorder), 2.5 Gbit capacity
- Command and telemetry interface
- Downlink data formatter and PCI bus interface are implemented on the downlink card
- Critical Command Decoder (CCD) is implemented on the uplink card.
The GNS (GPS Navigation subsystem) resides on two cards. One card contains a receiver consisting of an RF downconverter, an oscillator, a synthesizer, and a 12-channel GTA (GPS tracking ASIC) chip.
RF communications: The scalable architecture of the S-band RF transceiver system is part of the IEM. The transceiver consists of two RF cards, each of size 15 cm x 22 cm. The IEM approach provides the integration of functionalities that are normally part of the system (inclusion of a real-time critical command decoder). The downlink card outputs a CCSDS-compatible downlink signal and can support data rates up to 4 Mbit/s. The downlink card also includes an on-board data framer and Reed-Solomon encoder.
- Downlink S-band channel consisting of Reed-Solomon, convolutional and CRC encoding, frequency synthesizer, vector modulator, and S-band power amplifier
- Uplink S-band channel consisting of a downconverter, frequency synthesizer, AGC control, command bit detector/synchronizer and an experimental non-coherent Doppler tracking system
- The transmission protocol is CCSDS (Consultative Committee for Space Data Systems) compliant
The G&C (Guidance and Control) hardware architecture, consists of two star trackers, redundant 3-axis IRUs (Inertial Reference Units), redundant 3-axis magnetometers, redundant Sun sensors on each of the +y and -y sides, three electromagnetic torquer bars (EMTs) containing redundant coils, four reaction wheel assemblies, and a pair of redundant processors. One processor, the AIU (Attitude Interface Unit), interfaces the G&C system with the rest of the spacecraft and provides logic for safe mode pointing and momentum management. The other processor, the AFC (Attitude Flight Computer), provides the logic for the nadir pointing mode (Ref. 28).
Figure 6: Block diagram of the TIMED G&C subsystem (image credit: JHU/APL)
TIMED also carries a redundant GNS (Guidance & Navigation System) developed at JHU/APL. GNS is the spacecraft's navigation and timekeeping system. It is designed to autonomously provide position, velocity, time, sun vector, and defined orbital event notifications (e.g., terminator crossings and SAA (South Atlantic Anomaly) region encounters in real time. GNS consists of: 14)
- A GNS dual processor, one processor for tracking the received GPS signals, the other for producing navigation results and other functions
- GPS receiver and digital signal processing ASIC that form 12 channels for tracking GPS signals. A pre-amplifier, external to the IEM and located close to the GPS antenna, supplies the incoming GPS signals to the receiver
- In addition (outside of IEM), GNS employs a zenith-oriented antenna.
The GNS was designed for the hostile radiation environment of space. It is latch-up immune, has a very low SEU (Single-Event-Upset) rate and, except for the computer memory, is hardened to >300 krad (Si). The core electronics can sustain total dose radiation in excess of 1 Mrad (Si). The system has extensive command and telemetry capability and provides access to raw and intermediate data products.
The GNS receive antenna is located on the spacecraft’s optical bench on the zenith-pointing surface. The pre-amplifier, consisting of pre-select filters and a low noise amplifier, is located just underneath the optical bench and generates the dominant component of the system thermal noise power. The remainder of the system, composed of an RF downconverter, a baseband signal processing subsystem, and a dual microprocessor, is located on two Stretch-SEM-E circuit boards and housed in the TIMED IEM.
The GNS has five major elements: (1) GNS antenna, (2) RF subsystem, (3) baseband electronics subsystem, (4) dual-processor subsystem, and (5) system software.
Figure 7: Simplified GNS block diagram (image credit: JHU/APL)
Noncoherent navigation: In addition, the transceiver system cards provide the capability for highly accurate Doppler tracking, using a two-way noncoherent technique. The APL-developed technique obviates the need for coherency between the uplink's carrier tracking oscillator and the downlink carrier. In this technique, the uplink carrier signal is received and compared with the receiver's onboard reference oscillator. This operation results in a set of phase comparison counts, placed in the telemetry and transmitted to the ground. There, the ground station continues tracking the S/C as if it were a coherent transponder. The uplink and downlink error counts are compared with the telemetered phase comparison counts. Tracking accuracies to 0.1 mm/s (velocity error) are achieved.
Power conditioning electronics:
- Power from the spacecraft's switched bus (unregulated), nominally +28 VDC (Volt Direct Current), is converted to regulated and filtered voltages for the C&DH and GNS cards within the IEM and for the RIU (Remote Interface Units) external to the IEM
- Power from the spacecraft's switched and unswitched busses is converted to regulated and filtered voltages for the RF communications subsystem. 15)
The TIMED onboard Autonomy System has two basic functions: 1) to ensure fault protection and safing of the spacecraft, and 2) to perform a limited set of routine operations. The main requirements of fault protection and safing are to detect failed or improperly functioning spacecraft components and to autonomously replace them in the operational configuration with properly functioning counterparts. 16)
The IEM concept was to incorporate multiple spacecraft subsystems into a singe chassis, thereby conserving critical spacecraft resources at a reduced cost. The IEM was implemented on TIMED including C&DH, GNS, RF telecommunications, and IEM power conditioning (Ref. 13).
Launch: A launch of TIMED took place on Dec. 7, 2001 aboard a Delta-2 launcher from VAFB, CA (co-manifest with Jason-1).
Orbit: Circular orbit, altitude = 625 km; inclination = 74.1º with a 720º per year nodal regression (this means the local time of the equator crossing varies from dawn to dusk in three months).
Figure 8: Artist's rendering of the TIMED spacecraft (image credit: NASA and APL)
Table 1: Downlink card characteristics
TIMED uses an operations concept called “event-based” commanding, which eliminates the need for repetitive time-tagged commands. The TIMED GNS (GPS Navigation System) enables this mode of operation. Using the GNS, which consists of a GPS receiver and orbit propagator, the spacecraft knows its position and velocity at any given time and can predict events such as terminator crossings and passages over auroral zones and ground stations.
Mission operations are performed at JHU/APL in Laurel, MD. The payload operations concept requires each PI to perform his own instrument operations from his home site. A combination of onboard GPS processing and the use of Internet move the data from the APL ground station to each investigator's home site. By separating instrument operations from all S/C system activities, the instrument teams are able to control all of the instrument modes, operations and science data return at their own choice without explicit interactions or approvals by the S/C project team. In addition, the Internet is used by each investigator to control his instrument directly (the packetized messages are integrated into the uplink command structure in an automated fashion).
TIMED mission status:
• 2014: The TIMED spacecraft is operating nominally (in its 13th year on orbit) and is currently collecting data on one of the last frontiers in Earth's atmosphere. 17)
• The 2013 Senior Review revealed abundant evidence that Heliophysics extended missions are providing scientific value well beyond the realm of heliophysics. Measurements from the current constellation of extended missions support not only Heliophysics science objectives and the overarching goals of the SMD (Science Mission Directorate) but also specific goals of the other three divisions within SMD. 18)
- Heliophysics extended missions focussing on the physics and chemistry of the upper atmosphere — AIM (Aeronomy of Ice in the Mesosphere) and TIMED — are resolving the solar-terrestrial impacts on Earth's climate, which are larger than hitherto thought. The measurements provide a means of determining the impacts of energetic solar particles on the chemistry of the mesosphere and lower thermosphere, which affect stratospheric ozone and the circulation of the lower atmosphere. These results offer new insights into the sources of change in the Earth system, a primary science theme of NASA's Earth Science Division.
- TIMED, along with AIM, is the terrestrial anchor of NASA's HSO (Heliophysics System Observatory), and as such provides the majority of the “to Earth” link fo the “from Sun to Earth” connection. TIMED data provide for improved understanding of the fundamental processes of the space environment, which os the primary Heliophysics Research Objective.
- Spacecraft/instrument health status: The spacecraft and instruments are showing their age but appear to be capable of supporting the PSGs (Prioritized Science Goals) as planned. The SABER instrument appears to be in the best health and is the primary instrument for the extended mission. The GUVI instrument can no longer scan across the limb and is used in spectrographic mode at 30º off nadir. The TIDI performance has improved from what it was during the early orbit, but the PSGs do not strongly depend on it. The SEE instrument has only a 3% duty cycle (Ref. 18).
• In 2013, the TIMED spacecraft and its payload are operating nominally (in its 12 year on orbit). 19)
• The TIMED spacecraft is operating nominally in 2012. 20)
- In early March 2012, the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on TIMED measured the impact of the huge solar flare (CME) on Earth's upper atmosphere. The upper atmosphere heated up, and huge spikes occurred in infrared emission from nitric oxide and carbon dioxide. 21)
Figure 9: A surge of infrared radiation from nitric oxide molecules on March 8-10, 2012, signals the biggest upper-atmospheric heating event in seven years (image credit: NASA)
• On Dec. 7, 2011, the TIMED team celebrated 10 years of on-orbit operations - collecting data over almost an entire solar cycle. - Since TIMED was launched, APL has been responsible for project science leadership, spacecraft mission operations and sustaining engineering, as well as public outreach and education initiatives. 22) 23)
By studying daily, seasonal and yearly variations on a global scale, with the comprehensive data set collected by TIMED, the project is able to get a full picture of the driving forces behind the observed atmospheric changes.
• February 2011: The TIMED spacecraft is operating nominally and is currently collecting data on one of the last frontiers in Earth's atmosphere. 24)
• On October 1, 2010, NASA extended the TIMED mission through 2014. This is its fourth extension since the original 2-year mission began in January 2002. TIMED will focus this time on a problem that has long puzzled scientists: differentiating between human-induced and naturally occurring changes in this atmospheric region. This extension also allows TIMED to continue collecting data for longer than a full 11-year solar cycle. 25) 26)
• The TIMED spacecraft and its sensor complement are “operating nominally” in June 2010. 27)
• The spacecraft and its sensor complement are operating nominally as of 2009 (the TIMED spacecraft is currently in its eighth year of operation, six years beyond the planned two year mission).
Over the course of this time degradation or outright failures have occurred with a star tracker, the gyros, a reaction wheel, and sun sensors. As a result, various changes have been made to the attitude estimation and control algorithms and autonomy rules to allow continued operation under these anomalous conditions. 28)
• In late 2009, the observations from TIMED show a dramatic cooling in the upper atmosphere that correlates with the declining activity of the current solar cycle. For the first time, researchers can show a timely link between the Sun and the climate of Earth's thermosphere, the region above 100 km, an essential step in making accurate predictions of climate change in the high atmosphere. This finding also correlates with a fundamental prediction of climate change theory that says the upper atmosphere will cool in response to increasing carbon dioxide. 29)
The TIMED measurements show a decrease in the amount of ultraviolet radiation emitted by the Sun. In addition, the amount of infrared radiation emitted from the upper atmosphere by nitric oxide molecules has decreased by nearly a factor of 10 since early 2002. These observations imply that the upper atmosphere has cooled substantially since then. The research team expects the atmosphere to heat up again as solar activity starts to pick up in the next year.
• NASA approved a third mission extension for completion in January 2013.
Figure 10: TIMED image of a major geomagnetic storm that occurred in April 2006 (image credit: JHU/APL)
Legend to Figure 10: The image of the geomagnetic storm is superimposed over an Earth image. Throughout its five years in operation, TIMED has served as a catalyst for a greater understanding of our thermosphere and ionosphere. In coordination with a network of space- and ground-based systems, TIMED has provided the first view of the mesosphere, ionosphere and thermosphere as a coupled system throughout a range of solar activity levels.
• On Dec. 2006, the TIMED spacecraft was 5 years in orbit. 30)
• In May 2006, NASA announced a second extension of the TIMED mission for 4 years with operations and data analysis continuing through 2010. 31)
• In January 2004, after the end of the nominal design life of 2 years, NASA extended the TIMED mission for another three years of operations and data analysis.
• In mid-2003, “first light” results from TIMED’s suite of four instruments are presented. 32)
The excellent data set collected by UARS has given the MLTI research community a first glimpse at the MLTI dynamic structure. The project team has learned that its mean background winds have a clear seasonal structure and its daily global pattern is highly variable both spatially and temporally, dominated by the presence of tides and large-/small-scale waves.
TIMED has provided the first simultaneous measurements of MLTI density, winds, and temperature structures, critical elements for a detailed understanding of MLTI momentum balance. It has also provided the first simultaneous measurements of MLTI energy inputs and radiative energy outputs that are needed to quantitatively understand the MLTI energy balance. Most importantly, TIMED observations allow the Sun-Earth Connection community, for the first time, to investigate the MLTI response to various types of external energy inputs.
• Science operations officially began on January 22, 2002 - about two months after launch.
Sensor complement: (GUVI, SABER,SEE, TIDI)
Table 2: Summary of TIMED instrument measurements 33)
GUVI (Global Ultraviolet Imager):
Instrument PI: A. B. Christensen, The Aerospace Corporation. The instrument is of SSUSI heritage (flown on the DMSP F-16 S/C) and a joint collaboration between JHU/APL and The Aerospace Corporation, El Segundo, CA. The objective is to monitor three general regions: the daytime low- to mid-latitude thermosphere, the nighttime low- to mid-latitude ionosphere, and the high-latitude auroral zone. The goal is to obtain a detailed quantitative and predictive understanding of auroral phenomena. The instrument consists of the following elements: a scan mirror feeding a parabolic telescope and Rowland circle spectrograph, with a wedge-and-strip detector at the focal plane.
GUVI is a FUV scanning imaging spectrograph providing horizon-to-horizon images in five selectable bands or “colors.” The colors chosen are: H 121.6 nm, O 130.4 nm, O 135.6 nm, and N2 Lyman-Birge-Hopefield (LBH) bands at 140 to 150 nm and 165 to 180 nm. GUVI uses a scan mirror to sweep its FOV of 11.78º through an arc of up to 140º (scan from 80º to -60º) in the cross-track direction. This FOV is mapped via an f/3 Rowland circle spectrograph (with a toroidal grating) into 14 spatial and 160 spectral “pixels.” The scan sweep time is 22 s. The detector is a microchannel plate (MCP) intensified wedge-and-strip anode which provides a 2-D readout. 34)
The SIS (Scanning Imaging Spectrograph) of GUVI consists of a cross-track scanning mirror at the input to the telescope (a 75 mm focal-length off-axis parabola system with a 25 x 50 mm clear aperture) and a Rowland circle spectrograph. Two 2-D photon-counting detectors are located at the focal plane of the spectrograph. The operating detector is selected by a “pop-up” mirror that is moved into or out of the optical path to direct radiation from the grating onto one of two detectors. The detectors employ a position-sensitive anode to determine the photon event location.
Figure 11: Schematic view of the GUVI instrument and its components (image credit: JHU/APL)
Figure 12: Illustration of the GUVI instrument (image credit: JHU/APL)
Figure 13: Observation concept of the GUVI instrument (image credit: JHU/APL)
GUVI has both cross-track disk and limb scan modes; it measures the UV radiation emitted at altitudes generally above 150 km up to 520 km by limb-scanning and imaging to determine during daytime conditions the concentrations of N2, O2, O, and the temperature; infers the fluxes of precipitating auroral particles. Instrument mass=19.2 kg, power=24 W; swath width of about 300 km; spectral coverage: 110 - 180 nm; look direction: cross-track scan (disk + limb); data rate = 8.0 kbit/s.
The GUVI observations are compared with data collected by the ground stations for “ground truthing.” GUVI products include maps of the auroral oval, the characteristic energy and flux of the electrons which excite it, F-region ionospheric electron density profiles, and dayside neutral composition information.
Table 3: GUVI environmental parameters measured by different colors
SABER (Sounding of the Atmosphere using Broadband Emission Radiometry):
Instrument PI: J. M. Russell, Hampton University; instrument builder: SDL of Utah State University, Logan, UT. SABER (heritage of LIMS, SAMS, CIRRIS, ATMOS, HALOE, CLAES, ISAMS, and SME) is a 10-channel radiometer with the objective to measure the IR emissions emitted at altitudes generally below 100 km by limb scanning to drive vertical distributions of temperature and concentrations of energetically important species (O3, H2O, NO, NO2 CO, CO2) as well as radiative energy loss. The telescope is an on-axis Cassegrain design (rejection of stray light outside IFOV). The telescope and baffle assembly are cooled to 240 K by a dedicated radiator. The focal plane assembly, consisting of a filter array, a detector array, and a Lyot stop, is cooled to 75 K by a miniature cryogenic refrigerator. Instrument drifts due to changes in telescope and focal plane base temperatures are corrected by an in-flight calibration system. The detector arrays are: five HgCdTe (photoconductive mode of operation), two InSB (photovoltaic mode of operation), and three InGaAs (photovoltaic mode of operation). Instrument mass=61.7 kg, power = 64 W, the data rate is 4 kbit/s. 35) 36)
Spectral coverage: discrete ranges from 1.27 - 17 µm (7865 cm-1 to 650 cm-1). A scan depressing angle of -20.0º to -13.9º from the S/C horizontal is used; vertical resolution of 2 km. SABER scans up and down the Earth's horizon once every 58 seconds, collecting data over an altitude range from about 180 km down to the Earth's surface (vertical profiles of elemental constituents) and temperature. Based on the thermal emission characteristics of its measured CO2 15.4 µm emission, SABER provides atmospheric temperature measurements critically needed to study atmospheric momentum and energy balance. In addition, SABER measures the vertical distributions of molecular constituents (e.g., ozone, water vapor) that are important for their direct role in solar photon energy absorption and their indirect role in chemical reactions involving energetically important chemical species.
Table 4: Overview of SABER measurements and applications
Figure 14: Illustration of SABER (image credit: NASA/LaRC, JHU/APL)
SABER is the first spaceborne instrument to provide systematic global measurements of atmospheric density and temperature as well as key atmospheric heating and cooling rates.
SEE (Solar EUV Experiment):
Instrument PI: T. N. Woods; SEE was built by LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado, Boulder, CO. The solar sensors in SEE are EGS (EUV Grating Spectrometer) and XPS (XuV Photometer System). The objective is to measure the absolute fluxes of solar UV, EUV, and XUV radiation and to determine the rates of energy deposition, dissociation and ionization. 37)
EGS features a Rowland-circle grating design; detector: 64 x 1024 MCP/CODACON (microchannel plates with coded anode position array); spectral range of 25 to 200 nm; resolution of 0.4 nm (0.17 nm per anode); FOV = 6º x 12º.
XPS consists of a set of nine XUV silicon photodiodes (thin film coatings on diodes) as detectors, designed to measure the full-disk solar soft X-ray spectral irradiance at several fixed spectral wavelengths. It provides solar irradiance measurements from 0.1 to 35 nm, with each photometer having a spectral bandpass of 5 to 10 nm. FOV = 20º in diameter. An additional filtered photometer is a bare X-ray ultraviolet photodiode with Acton Lyman-alpha filters for a redundant measurement of the important Lyman-alpha irradiance.
Instrument mass = 26 kg, power=16 W (average). Look direction: solar pointing; data rate = 210 bit/s. 38)
Figure 15: Illustration of the SEE instrument (image credit: LASP)
TIDI (TIMED Doppler Interferometer):
Instrument PI: T. L. Killeen, UCAR (University Corporation for Atmospheric Research), Boulder, CO. Objective: investigation of the dynamics and energetics of MLTI. TIDI measures the VIS/NIR emissions emitted at altitudes between 60 and 300 km by limb scanning techniques to determine the temperature and horizontal winds with the use of the Doppler effect. The instrument makes also density measurements, mostly on the day side of the orbit.
Figure 16: Schematic layout of the TIDI showing 2 for 4 telescopes (image credit: University of Michigan)
TIDI comprises three subsystems: four identical telescopes (off-axis Gregorian type, aperture 7.5 cm, f/2.2 FOV = 2.5º horizontal x 0.05º vertical), a Fabry-Perot interferometer (fixed-gap single etalon) with a CCD detector (passively cooled), and an electronics box. TIDI views emissions from OI 557.7 nm, OI 630.0 nm, OI 732.0 nm, O2 (0-0), O2 (0-1), Na D, OI 844.6 nm, and OH to determine Doppler wind and brightness temperature of the atmospheric airglow emission lines at a very high spectral resolution throughout the altitude range. TIDI obtains these scans simultaneously in four orthogonal, azimuthal directions: two at 45º forward but on either side of the spacecraft's velocity vector and two at 45º rearward of the spacecraft. 39) 40)
Figure 17: Illustration of the TIDI telescope (image credit: UCAR)
Each vertical scan consists of individual views of the limb at 2.5º (horizontal, along the limb) x 0.05º (vertical, normal to the limb) angular resolution or at 125.0 km x 2.5 km spatial resolution. The altitude step size ranges from 2.5 km in the mesosphere to 25.0 km in the thermosphere. Each up/down acquisition cycle takes about 100-200 s to complete, resulting in a nominal horizontal spacing between profiles of approximately 750 km along the orbit track.
Instrument mass=42 kg; power=19.3 W; spectral coverage: selected lines between 550 - 900 nm; look direction: limb scan; a scan depression angle of 23.2º to - 16.8º from the S/C horizontal is used; data rate = 2.336 kbit/s.
Figure 18: Schematic view of TIDI optics configuration (image credit: UCAR)
Figure 19: Illustration of TIDI views (image credit: UCAR)
The TIMED operations system includes the MOC (Mission Operations Center), MDC (Mission Data Center), and the SCF (Satellite Communications Facility) at APL, four POCs (Payload Operations Centers) located at facilities across the country, and the distributed SDS (Science Data System). 41)
Mission operations and instrument data distribution are the responsibility of the Laboratory (APL). These functions are performed at the TIMED Mission Operations Center (MOC) on the APL campus. The instrument teams operate their instruments directly from the POCs, located at the institutions where the instruments were developed.
The TIMED spacecraft was designed with a high degree of autonomy to enable inexpensive mission operations using a small Mission Operations Team consisting of eight members. One key to making this possible is a decoupled instrument operations approach. TIMED is the first APL mission to be operated in this manner. The strategy is based on the concept that the spacecraft is the “bus” and the instruments are the “passengers.”
The decoupled operations approach is further illustrated in Figure 20. The mission data flow is in two paths. The outer path, indicated in green, represents the science data flow of commands and telemetry and shows the principal activities of the POCs. All of the processes in the spacecraft, as well as the mission operations ground system portion of this path, are automated. The inner path of bus activity is the mission operations data flow, which is generated and processed independent of the science data flow. Thus the Mission Operations Team operates the spacecraft bus while the instrument teams operate their instruments. No personnel effort is expended to merge the two sets of activities, thereby saving a great deal of operational costs.
Figure 20: Overview of the TIMED end-to-end system (image credit: JHU/APL)
Legend to Figure 20: The outer path (green) shows the flow of instrument data, both commands and telemetry. The inner path shows the flow of spacecraft bus commands and telemetry. The left side represents spacecraft processes, while the right side represents ground processes.
Mission operations: Unlike previous spacecraft ground systems, the TIMED ground system design was not driven by the spacecraft design. Instead, the end-to-end system design for both spacecraft and ground was driven by the desire to reduce operational costs by easing spacecraft and instrument operations. This led to a highly autonomous system that uses distributed processes communicating over local and wide area networks. Figure 21 is a diagram of the major components of the TIMED ground system.
The TIMED MOC is located on the APL campus near the SCF (Satellite Communications Facility). The MOC houses the computer systems used to operate the spacecraft, process and store the mission data, and serve the science community.
Figure 21: Simplified block diagram of the TIMED ground system (JHU/APL)
The MDC (Mission Data Center) was developed to support the TIMED mission concept of operations in which geographically dispersed POCs operate their instruments remotely and independent of spacecraft bus operations. To support this approach, a data system was developed that provides completely automated online data archival and delivery functionality. The MDC provides the POCs and MOC with several essential services including:
• The capability for real-time monitoring of spacecraft and instrument telemetry during ground station passes
• On-demand playback of spacecraft and instrument data
• Short- and long-term data archival
• The production of supporting mission data products.
The Router and the Archive Server are the two main software components that make up the MDC’s Telemetry Server. These workhorses provide the data delivery and archival functionality.
SDS (Science Data System): An important goal of the TIMED mission is to quickly create and disseminate processed atmospheric science data to the scientific community, K–12 educators, and the general public in addition to the TIMED program elements. The objective is to produce an initial version of routine science products, available to all TIMED users, within 54 hours after telemetry acquisition on the ground.
The TIMED ground system includes a distributed SDS. The SDS is composed of the TIMED MDC and those portions of the POCs involved with the processing and distribution of science data products (Figure 22). As in the typical space mission science center, the SDS is responsible for the acquisition, generation, distribution, and archiving of science data necessary to support the TIMED mission. Unlike a traditional mission science center; however, these functions of the SDS are distributed over its component facilities. Supporting its goal of disseminating science products, the SDS uses a Web site as its common user interface and relies on standard protocols of FTP and Web document transfers across the Internet.
Figure 22: Schematic configuration of the SDS (image credit: JHU/APL)
Legend to Figure 22: The TIMED SDS includes the data archive and distribution functions of the MDC as well as the science data processing portion of the POCs. It provides the data interface to other external collaborators and users. Blue lines represent MDC and POC data input; red lines represent their data output.
1) Information provided by G. E. Cameron and by K. J. Heffernan of JHU/APL
3) D. Y. Kusnierkiewicz, “A description of the TIMED spacecraft,” American Institute of Physics (AIP) Conference Proceedings, 387, Part One, pp. 115-121, 1997
4) R. S. Bokulic, et al., “A Highly Integrated S-Band Transceiver System with Two-Way Doppler Tracking Capability,” Proceedings of AIAA/USU Conference on Small Satellites, 1997, pp. 1-8
6) TIMED Mission Guide, Dec. 2006, URL: http://www.timed.jhuapl.edu/WWW/common/content/pdfs/missionGuide.pdf
7) David Y. Kusnierkiewicz, “TIMED Mission System Engineering and System Architecture,” JHU/APL Technical Digest, Vol. 24, No 2, 2003, URL: http://www.jhuapl.edu/techdigest/TD/td2402/Kusnierkiewicz2.pdf
10) David Y. Kusnierkiewicz, “An Overview of the TIMED Spacecraft,” JHU/APL Technical Digest, Vol. 24, No 2, 2003, URL: http://www.jhuapl.edu/techdigest/TD/td2402/Kusnierkiewicz1.pdf
11) A. A. Chacos, P. A. Stadter, W. S. Devereux, “Autonomous Navigation and Crosslink Communication Systems for Space Applications,” JHU/APL Technical Digest, Vol. 22, No 2, 2001, pp. 135-143
12) Ch. C. DeBoy, M. J. Reinhart, “A Flexible, Transceiver-based RF Communications System for Small Satellites,” Proceedings of the 3rd International Symposium of IAA, Berlin, April 2-6, 2002, pp. 363-366
13) Paul C. Marth, “TIMED Integrated Electronics Module (IEM),” Johns Hopkins APL Technical Digest, Vol. 24, No 2, 2003, pp. 194-200, URL: http://www.jhuapl.edu/techdigest/TD/td2402/Marth.pdf
14) William S. Devereux, Mark S. Asher, Robert J. Heins, Albert A. Chacos, Thomas L. Kusterer, Lloyd A. Linstrom, “TIMED GPS Navigation System (GNS): Design, Implementation, and Performance Assessment,” Johns Hopkins APL Technical Digest, Vol. 24, No 2, 2003, pp. 179-193, URL: http://www.jhuapl.edu/techdigest/TD/td2402/Devereux.pdf
15) G. Dakermanji, M. Butler, “The TIMED Spacecraft Power System Extended Mission Orbital Performance,” 5th International Energy Conversion Engineering Conference and Exhibit (IECEC), June 25-27, 2007, St. Louis, Missouri, AIAA 2007-4763
16) Raymong J. Harvey, “TIMED Autonomy System,” Johns Hopkins APL Technical Digest, Vol. 24, No 2, 2003, pp. 201-208, URL: http://www.jhuapl.edu/techdigest/TD/td2402/Harvey.pdf
18) William Lotko (Chair), Doug Braun, Jim Drake, Joe Fennel, Richard R. Fisher, Joe Giacalone, Tim Horbury, Bob McCoy, Mark Moldwin, Alexei Pevtsov, John Plane, Howard Singer, Charles Swenson, “Senior Review 2013 of the Mission Operations and Data Analysis Program for the Heliophysics Extended Missions,” NASA, June 13, 2013, Submitted to: Victoria Elsbernd, Acting Director Heliophysics Division, URL: http://science.nasa.gov/media/medialibrary/2013/07/05/Helio_SR_2013_FINAL_ALL_v2.pdf
21) Michael Finneran, “NASA Measures Impact of Huge Solar Flare on Earth's Atmosphere,” Space Daily, March 27, 2012, URL: http://www.spacedaily.com/.../NASA_Measures_Impact_of_Huge_Solar_Flare_on_Earth_Atmosphere
22) “TIMED Atmospheric Spacecraft Marks 10 Years of Groundbreaking Science,” JHU/APL, Dec. 7, 2011, URL: http://www.jhuapl.edu/newscenter/pressreleases/2011/111207b.asp
23) “Ten Successful Years of Mapping the Middle Atmosphere,” NASA, Dec. 7, 2011, URL: http://www.nasa.gov/mission_pages/sunearth/news/timed-10yrs.html
24) Feb. 11, 2011: URL: http://www.timed.jhuapl.edu/WWW/overview/newsCenter/missionNews.php
25) “NASA Extends TIMED Mission for Fourth Time,” Nov. 5, 2010, URL: http://www.nasa.gov/topics/solarsystem/sunearthsystem/main/timed-extended.html
26) “APL-led Atmospheric Mission Extended for Fourth Time TIMED Will Study Upper Atmosphere during Increasing Solar Activity,” JHU/APL, Nov. 1, 2010, URL: http://www.jhuapl.edu/newscenter/pressreleases/2010/101101.asp
28) Wayne F. Dellinger, “The aging of TIMED - G&C issues of extended missions,” Proceedings of the 32nd AAS Guidance and Control Conference, Breckenridge, CO, USA, Jan. 31.- Feb. 4, 2009, AAS 09-033
29) Chris Rink, “NASA Shows Quiet Sun Means Cooling of Earth's Upper Atmosphere,” NASA/LARC, Dec. 16, 2009, URL: http://www.nasa.gov/centers/langley/news/releases/2009/09-101.html
30) “Celebrates 5-Year Anniversary,” Dec. 8, 2006, JHU/APL, URL: http://www.jhuapl.edu/newscenter/pressreleases/2006/061208.asp
31) Jeng-Hwa(Sam) Yee, Janet Kozyra, Richard Goldberg, and The TIMED Science Team. “TIMED - The Terrestrial Anchor Mission of The Sun-Solar System Connection Great Observatory,” S3C Program Senior Review Nov. 15 , 2005, URL: http://www.timed.jhuapl.edu/.../TIMED_2005_Senior_Review_Presentation.pdf
32) Elsayed R. Talaat, Jeng-Hwa Yee, Andrew B. Christensen, Timothy L. Killeen, James M. Russell III, Thomas N. Woods, “TIMED Science: First Light,” Johns Hopkins APL Technical Digest, Vol. 24, No 2, 2003, pp. 142-149, URL: http://www.jhuapl.edu/techdigest/TD/td2402/Talaat.pdf
33) J.-H. Yee, E. R. Talaat, A. B. Christensen, T. L. Killeen, J. M. Russell III, T. N. Woods, “TIMED Instruments,” Johns Hopkins APL Technical Digest, Vol. 24, No 2, 2003, pp. 156-164, URL: http://www.jhuapl.edu/techdigest/TD/td2402/Yee2.pdf
41) Elliot H. Rodberg, William P. Knopf, Paul M. Lafferty, Stuart R. Nylund, “TIMED Ground System and Mission Operations,” Johns Hopkins Technical Digest, Vol. 24, No 2, 2003, pp. 209-220, URL: http://www.jhuapl.edu/techdigest/TD/td2402/Rodberg.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.