TacSat-3 (Tactical Satellite-3)
TacSat-3 is a follow-up US minisatellite technology demonstration mission within the ORS (Operational Responsive Space) program of DoD, representing a partnership between three military service branches. The partners include NRL (Naval Research Laboratory), AFRL (Air Force Research Laboratory), DARPA (Defense Advanced Research Projects Agency), the Army Space and Missile Defense Center, and the USAF Space and Missile Systems Center. AFRL is leading the TacSat-3 team- serving as the project integrator. The TacSat-3 mission was selected for specific capabilities to meet user needs, and to demonstrate those capabilities within cost and schedule constraints.
The overall objectives are to demonstrate a responsive system, namely the operation of three payloads in space: the ARTEMIS hyperspectral imager (HSI), the Ocean Data Telemetry Microsatellite Link (ODTML), the SPA (Spacecraft Plug-n-play Avionics) package - and to provide the observation data in a timely manner. 1) 2) 3) 4) 5) 6) 7) 8)
ORS (Operational Responsive Space) - some background:
ORS is a DoD-sponsored program and joint initiative managed by OFT (Office of Force Transformation) which started in 2002. All four services are involved in developing the concept and participating in the experimentation. The US. Air Force is leading the effort. OFT is seeking to develop new revolutionary operational concepts and technologies for the conduct of military operations. The overall objective is to develop spacecraft and payloads that can be manufactured and put into space faster then conventional systems, making them available to operational commanders who can task them based on specific needs. ORS principles are based on affordable concepts in flexible and integrated architectures - implying a networking capability to an existing infrastructure in the ground segment. 9) 10) 11)
This vision led to an initial approach involving a series of smallsats named TacSat (Tactical Satellite). The TacSat-1 definition started in May of 2003 with the goal to select key elements needed for a deployable ORS experimentation.
At the core of the TacSat experiments is the guiding principle of operational experimentation that forces the S&T/R&D (Science & Technology / Research & Development) and the operational communities together to effectively and rapidly channel promising new technologies into new capabilities and ConOps (Concept of Operations).
The need for a standardized spacecraft bus has been identified as necessary for an ORS system. Hence, the OFT and AFRL have undertaken a four phase initiative to develop and test bus standards and then transition them for acquisition. The standardization effort involves a fairly broad base, multiple government laboratories, academia and industry [MIT/LL, AFRL, NRL, JHU/APL, SMC (Space and Missile systems Command), and many industry as well as university participants] as illustrated in Figure 1.
Figure 1: Overview of standard bus phases 1 through 4 on the ORS scheme (image credit: JHU/APL)
TacSat-3 is a minisatellite using a standardized modular bus designed and built by Swales Aerospace of Beltsville, MD (contract award in early June 2006). The bus is referred to as ORSMB (Operationally Responsive Space Modular Bus). A major objective is to provide an implementation of bus standards with priority on demonstrating avionics standards - to fit into a network of space assets.
The goal is to move toward an adaptable modular bus development process that leverages plug-and-play standards and interfaces and incorporates the latest bus component technologies, supporting multiple payloads and orbital mission profiles. The modular bus development process must address the entire production chain: rapid design, fabrication, integration and test that mix and match off-the-shelf, low-cost proven bus components to meet a variety of payload and orbital mission requirements. A requirement calls for a demonstration of the common plug-and-play electrical and software interfaces between bus components and between the bus and payload.
The spacecraft is 3-axis stabilized. The spacecraft mass is < 400 kg. The spacecraft design life is one year, the goal is to obtain 3 years of operations.
Note: In the spring of 2007, Swales Aerospace of Beltsville, MD (an employee-owned company) was purchased by Alliant Techsystems Inc. (ATK) with corporate headquarters in Edina, Minnesota. Hence, former Swales Aerospace is now known as ATK Spacecraft Systems and Services of Beltsville, MD.
Figure 2: Photo of the TacSat-3 bus during integration (image credit: AFRL)
Figure 3: Artist's rendition of the TacSat-3 spacecraft (image credit: AFRL)
Figure 4: Overview of modular bus development scheme (image credit: AFRL)
System overview: The TacSat-3 system configuration is illustrated in Figure 5. It consists of the ARTEMIS sensor (instrument), an ARTEMIS Sensor Processor, a CDL (Common Data Link) communications package, a SCP (Satellite Communications Package) experiment, the SAE (Satellite Avionics Experiment), and a spacecraft bus. The overall system includes a CDL ground station, a fielded warfighter, a TGS (Tactical Ground Station), and the AFSCN (Air Force Satellite Control Network).
CDL is the DoD standard wideband communications waveform for ISR (Intelligence Surveillance & Reconnaissance) in airborne platforms (the CDL manufacturer is L-3 Communications, Salt Lake City, UT). A CDL system in space brings tactical ISR data directly into existing theater ground stations, allowing for responsive tasking and collection. Utilizing CDL in space provides the benefit of using the existing in-theater CDL ground infrastructure for tactical communications. CDL is a full-duplex, jam resistant spread spectrum, point-to-point digital link. The uplink operates at 200 kbit/s - and possibly up to 45 Mbit/s. The downlink can operate at 10.71 -45 Mbit/s, 137 Mbit/s, or 274 Mbit/s. The CDL program establishes data link standards and specifications identifying compatibility and interoperability requirements between collection platforms and surface terminals across user organizations. 12)
Figure 5: System diagram of TacSat-3 (image credit: AFRL)
The prime functions of the ARTEMIS Sensor Processor (SP), a stand-alone version from the ARTEMIS sensor, is to provide the control functions of the sensor, power switching, collecting state of health data from ARTEMIS, and storing ARTEMIS source data. Additionally, a fundamental capability of the SP is to autonomously process data cubes from ARTEMIS and produce tactically relevant data for dissemination directly to the fielded warfighter. These products primarily are in the form of text products along with some imagery dependent upon the dissemination method.
Figure 6: Illustration of the deployed TacSat-3 spacecraft (image credit: AFRL)
Concept of Operations (ConOps): TacSat-3 operations provides two modes of mission support: routine and tactical.
- The routine mode is used for collecting HSI (Hyperspectral Imaging) data outside of the assigned theater of operation.
- The tactical mode is reserved for anytime the spacecraft can collect over an assigned theater of operation.
The tactical mode is driven by one requirement: to demonstrate responsive delivery of decision-quality information to operational and tactical commanders by enabling tactical tasking and data delivery. The delivery latency of the decision-quality information must be less than 30 minutes at a maximum with a goal of less than 10 minutes.
RF communications: The second generation CDL (Common Data Link) provides data rates of up to 274 Mbit/s (downlink and uplink in X-band) - in addition to lower data rates for potential ROVER (Remote Operated Video Enhanced Receiver) connectivity. The spacecraft is monitored and controlled by a mission operations center located at Kirtland AFB, NM. The TT&C function is provided in S-band.
SCP (Satellite Communications Payload) is an additional transmission link provided by ONR (Office of Naval Research). The objective of SCP is to provide a UHF link for the ODTML (Ocean Data Telemetry MicroSat Link) services and to function as an internal S&F (Store & Forward) subsystem. The downlink of tactical data products via UHF services is considered as more user-friendly since a far greater number of fielded UHF receivers are available in the ground segment than CDL ground stations. 13)
Launch: The TacSat-3 spacecraft was launched on May 19, 2009 on a Minotaur-1 vehicle of OSC (Orbital Sciences Corporation) from the commercial MARS (Mid-Atlantic Regional Spaceport) facility at Wallops Island, VA, USA. 14)
Secondary payloads on this flight were the CubeSats: PharmaSat-1 (~5 kg) a nanosat of NASA/ARC, AeroCube-3 of the Aerospace Corporation of El Segundo, CA, HawkSat-1 of the Hawk Institute for Space Sciences, Pocomoke City, MD, and CP-6 (CalPoly-6) of California Polytechnic State University, San Luis Obispo.
Orbit: Near-circular LEO, altitude of about 425 km, inclination of about 40º, period = 93.6 minutes.
· On April 30, 2012, TacSat-3 reentered Earth's atmosphere, nearly three years after its May 2009 launch. TacSat-3 was designed for six months of operation, with a goal of one year. Not only did it outlive its design life, it also surpassed its original mission requirements and goals as an experimental spacecraft, and was successfully transitioned to operational status in 2010. 15)
The spacecraft is a pioneer of the emerging ORS (Operationally Responsive Space) program, which was designed to meet the growing need of U.S. forces for flexible, affordable and responsive satellite systems.
Figure 7: Reentry trajectory of TacSat-3 (image credit: The Aerospace Corporation) 16)
· The SAE (Spacecraft Avionics Experiment) was developed and flown as a secondary experiment on the AFRL TacSat-3 spacecraft as the first on-orbit demonstration of the SPA (Space Plug-and-Play Architecture ) capability. On numerous occasions during SAE development and test, the modular, reconfigurable SPA interface provided significant cost and schedule savings. SAE consisted of a SPA-USB (SPA-U) network with multiple plug-and-play components with the objective of demonstrating as much core avionics and spacecraft operation as possible in a small secondary payload package. To this end, most elements of a GN&C (Guidance, Navigation, and Control) system were incorporated, to include a sun sensor, rate sensor, temperature sensors, and an AC coupled interconnect experiment. SAE also provided operational backups for TacSat-3 to include a Surrey SGR-05 GPS receiver and a backup high speed SPA-S (SPA-SpaceWire) link between the hyperspectral image processor and the C&DH for payload data transfer. SAE used built-in self test functions to verify on-orbit performance autonomously. Both the SPA-U and SPA-S interfaces operated as designed. One of the SPA-U experiments had a SEU (Single Event Upset) detector on board and no SEUs were observed during experiment operations. SPA-S demonstrated data transfers at 75 Mbps rate with no bit errors observed on orbit. SAE was developed for AFRL by the Space Systems Group at Sierra Nevada Corporation. 17)
· In late February 2012, satellite control authority of TacSat-3 was transferred to the SMC (Space and Missile Systems Center). The SMC has started end-of-life testing prior to disposing of the vehicle. The reason: TacSat-3 has continued to lose altitude. The satellite will eventually fall from orbit and burn up in Earth's atmosphere. 18)
- The satellite demonstrated a high level of flexibility by supporting both combat operations and humanitarian missions worldwide, and its extended life span was attributed to the outstanding abilities of its manufacturer as well as the operators who kept it flying. The satellite assisted also in recovery operations following the devastating Japanese earthquake and subsequent nuclear meltdown of 2011. - Aside from the operational success, the TacSat-3 spacecraft was used to prepare Team 8-Ball for the arrival of ORS (Operationally Responsive Space-1), a USAF minisatellite of TacSat-3 heritage (launched on June 30, 2011) which began to provide operating services in 2011 (Ref. 18).
- TacSat-3 demonstrated the versatility of its hyperspectral imaging payload time and time again. The payload's ability to delineate between natural and man-made materials proved instrumental in providing intelligence on everything from detecting camouflage to locating certain types of crop growth. 19)
· The TacSat-3 mission is operating nominally in 2011 - marking its two-year anniversary on-orbit in May 2011. The spacecraft has far exceeded expectations in both its superior imaging performance and in its operational service life. The ARTEMIS imager continues to provide valuable information to combatant commanders. 20) 21)
- Operations of the satellite have progressed to the point where the payload is generating about 100 hypercube data products each month. The time needed to compare the hypercubes to a catalog of known materials and provide useful information for troops on the ground has been reduced dramatically since the beginning of the program. A data product that used to take a full day to exploit can now be processed in a few hours. Today, all of the processing and analysis of the ARTEMIS raw data is done in the United States and then the information is relayed to the troops elsewhere in the world. Raytheon engineers are working on a software upgrade for the spacecraft that will allow it to do a limited amount of automated exploitation and send the virtual "data chip" directly to the users in the field. 22)
- All of the SNC PnP electronics, i.e., IPDR Generation 1, hardware performed nominally after over a year in orbit. This included SpaceWire, analog and digital interfaces, rate sensor and sun sensor. All of the SPA-U PnP electronics hardware performed nominally after over a year in orbit. This included the XScale USB Host, and two ASIMs. 23)
· The TacSat-3 mission is operating nominally in the early fall of 2010.
· After the handover of the experimental TacSat-3 mission from AFRL/SMC to the Air Force Space Command at Peterson AFB (June 2010), the AFRL development team reflected on the various aspects of the TacSat-3 mission in the summer of 2010. Key insights into hyperspectral imaging capabilities were obtained in the mission so far. 24) 25) 26)
Lessons learned from the development, I&T (Integration & Test phase), and experimental operations of TacSat-3 and ARTEMIS will be carried forth to future AFRL flight experiments. As technology advances, it demonstrates the utility of small spacecraft to make meaningful impacts in support of national defense.
Figure 8: Sample of an ARTEMIS image of the Kilauea volcano, Hawaii, modified for public release using three of 400+ spectral bands to create a rendering from detailed spectral data (image credit: AFRL)
· In May 2010, TacSat-3 completed its one-year experimental mission. On June 12, 2010, TacSat-3 transitioned from an experimental demonstration to an operational asset when spacecraft control authority officially was transferred from AFRL/SMC to the Air Force Space Command at Peterson AFB, CO, USA. The mission achieved a number of milestones such as proving the capability of transmitting processed data to a ground station within 10 minutes of call up; also, use of PnP technology in the avionics experiment, downloading information obtained from ocean-based buoys to a ground station with the ONR-sponsored Satellite Communications Package (ODTML). The small satellite has been able to assist with the earthquake relief efforts in Haiti and Chile and now the team looks forward to its new role in the operational arena. 27) 28) 29) 30) 31)
· As of March 2010, TacSat-3 so far has collected some 1,600 hyperspectral images and has been working its way through 90 different experiments. Data from the satellite have been used in operational missions. In April 2010, the Pentagon will decide whether to hand the spacecraft over to the combatant commands to use full-time in May or June 2010. The performance of the system has been verified. All systems are go from an engineering perspective. 32)
· During the first month after launch, the program has accomplished early on-orbit checkout and anticipates completing calibration procedures by the end of June 2009. The following items represent some accomplishments during the first month in orbit: 33)
- Within two hours after launch, the spacecraft's solar arrays initiated power to operate key components, and controllers operated the satellite
- About 48 hours following liftoff, program officials verified the functionality of the primary payload, the ARTEMIS (Advanced Responsive Tactically-Effective Military Imaging Spectrometer), and sensor processing.
- During the first 2.5 days of the mission, the ARTEMIS sensor produced a high-resolution image, the satellite successfully communicated to a ground station via a high-bandwidth data link and operators discovered TacSat-3 had 50% more power than originally planned for due to the solar panels' efficiency.
- Following cool down in the initial week, the ARTEMIS also conducted a hyperspectral image collect.
- In week two after launch, controllers initiated ARTEMIS focus operations and validated the spacecraft's autonomous software.
- Between week 2 and 3 after liftoff, the satellite proved its tactical mode by collecting and processing hyperspectral imagery, downloaded a tactical product within a single, 10-minute pass, and validated that the secondary payloads, the Office of Naval Research Space Communications Package and the AFRL Space Avionics Experiment, were performing as required.
Sensor complement: (ARTEMIS, ODTML, SAE)
Building on the experiences with TacSat-1 and -2, TacSat-3 is the first spacecraft of the series to have gone through a formal payload selection process with AFSPC (Air Force Space Command)) and Coordinating Commands (COCOMs) and Services.
ARTEMIS (Advanced Responsive Tactically Effective Military Imaging Spectrometer):
ARTEMIS is a hyperspectral imager (HSI), funded by AFRL with additional funding by the US Army, designed and developed at Raytheon Space and Airborne Systems of El Segundo, CA, using COTS components extensively (ARTEMIS contract award in 2005). There is also a collaboration on the imaging spectrometer from NASA/JPL. The main objectives are:
· To demonstrate tactically significant hyperspectral imagery collection and processing sufficient to meet militarily relevant detection thresholds
· For a single-pass opportunity, the time period from a specified target collect to delivery of a processed product to the warfighter level must occur within 10 minutes (threshold: 30 min).
The instrument consists of a telescope (35 cm diameter), an Offner imaging spectrometer known as HSI (Hyperspectral Imager), a HRI (High Resolution Imager) and a real-time processor referred to as HSIP (Hyperspectral Imaging Processor). ARTEMIS provides HSI observations in the visible and SWIR (Short Wave Infrared) region as well as panchromatic data. The spectral range coverage is from 0.4 -2.5 µm.
The front-end telescope is a standard Ritchey-Chrétien form, a two-mirror design that feeds both the HSI and HRI subsystems; the design is telecentric as is required to meet the spectral and spatial uniformity goals of the imaging spectrometer (heritage of TacSat-2). Additionally the secondary mirror has a built-in focus mechanism for on-orbit optimization. 34) 35) 36) 37) 38)
Also housed within the telescope is a relative calibration source referred to as the OBHM (On-Board Health Monitor). The OBHM provides flood source illumination to the spectrometer entrance slit for the purpose of spectral, spatial, and radiometric performance trending.
Figure 9: Illustration of the ARTEMIS telescope (image credit: AFRL)
The HSI is of the basic Offner form consisting of two powered reflecting surfaces comprising the primary and tertiary elements. The secondary mirror is replaced by a curved grating for dispersion and is the limiting stop of the system. This form has the merit of being simple, compact, and both spatially and spectrally uniform. Spatial and spectral uniformity is critical to the operational performance of imaging spectrometers as it enables robust exploitation of data products. Spectral sampling is at 5 nm intervals. Additionally the design has < 5% spatial and spectral non-uniformity. The precision entrance slit is reticulated with small apertures at the top and bottom to aid in alignment and testing.
The dual-angle blaze grating was selected largely due to its superior performance in reducing the effect of obscurations at the grating stop. The grating is designed to optimize the SNR (Signal-to-Noise Ratio).
Table 1: Use of COTS/tactical grade hardware with minimal redundancy
Figure 10: Schematic elements of the Offner spectrometer (image credit: AFRL)
The HSI (Hyperspectral Imager) design also relies upon a single substrate-removed HgCdTe focal plane array (FPA) that extends its sensitivity into the blue wavelengths to cover the full spectral range (VNIR and SWIR). This single focal plane eliminates the co-registration issues associated with multi-FPA systems. The quantum efficiency of the FPA is better than 70% at all wavelengths and the array is equipped with a three-zone blocking filter for order sorting. A significant requirement for geolocation accuracy across the full spectrum of each spatial pixel is spectral co-alignment.
Figure 11: Illustration of the dual-band imaging spectrometer of ARTEMIS (image credit: Raytheon) 39)
The HRI (High Resolution Imager) is adapted from off-the-shelf hardware for simplicity and cost savings. It consists of a modified Dalsa Piranha 2 line scan CCD camera. The model selected has 4096 pixels (7 µm square pixels), dual output, 1 kHz frame rate, and a 10 bit digitization. The camera has been modified to survive the space environment.
Figure 12: Summary of the COTS HRI (image credit: Raytheon Space & Airborne Systems)
Table 2: Some parameters of the ARTEMIS instrumentation
Note: The military nature of the program does not reveal any specifics as to the performance parameters of the hyperspectral imager - such as resolutions, swath widths, etc.
Two sets of cryocooler electronics were flown with a simple relay switch between them. The redundancy is used to reduce the risk exposure of radiation effects on the primary mission. However, as of the summer of 2010, the redundancy has not been used so far (Ref. 24).
Figure 13: Tactical cryocooler and redundant cryocooler electronics (image credit: AFRL)
ARTEMIS calibration: The spectral imagery acquired will make use of solar radiation-based calibration methods for both the pre-flight and on-orbit radiometric and spectral calibration. The pre-flight calibration relies on a unique experiment in which ARTEMIS viewed two panels illuminated by the sun. The first panel was specially-coated to provide a set of known absorption features for the spectral calibration of ARTEMIS. A second panel with relatively flat spectral reflectance and measured reflectance characteristics was used for the absolute radiometric calibration of ARTEMIS.. 40)
At-aperture radiance from the panel while illuminated by the sun was determined through direct measurement using a wellcalibrated transfer radiometer. The multispectral measurements from the radiometer were converted to hyperspectral data via measurements from a field-portable spectrometer. A secondary prediction of at-aperture radiance was also provided through a SRBC (Solar Radiation-based Calibration). The SRBC approach relies on the determination of the spectral solar irradiance incident on the panel combined with the known BRDF (Bi-directional Reflectance Distribution Function) to give the reflected spectral radiance. The solar irradiance is found by transmitting a given solar irradiance model through the atmosphere using atmospheric transmittance as a function of wavelength found from solar radiometer measurements.
A correction for skylight illumination on the panel is obtained from measurements of the panel while shaded from the sun. Error analysis of the spectral characterization shows the center wavelength of ARTEMIS bands can be determined to better than 1 nm. The absolute uncertainty of the solar-based radiometric calibration is less than 3% in spectral regions not affected by strong absorption.
ARTEMIS makes use of an onboard health monitor to evaluate short-term changes in radiometric response of the sensor but will rely on vicarious methods for the long-term evaluation of the sensor's absolute radiometric response. The on-orbit vicarious radiometric calibration will be carried out using the reflectance-based method. This approach relies on measurements of surface reflectance of selected ground sites and atmospheric optical properties that are used as input to a radiative transfer code. The measurements and radiative transfer code provide results at 1 nm spectral intervals for the entire spectral range of ARTEMIS.
Figure 14: Artist's view of the TacSat-3 spacecraft taking hyperspectral imagery - i.e. an "image cube" (image credit: AFRL)
Demonstration/experimentation with tactical tasking and downlink of HSI data:
The ARTEMIS instrumentation includes a co-aligned panchromatic high resolution imager (HRI) and an onboard processor, referred to as HSIP (Hyperspectral Imager Processor) or simply as ARTEMIS processor. The Artimis processor uniquely separates payload data management such as storage, processing, and control separate from an integrated sensor and processor. This allows for the ARTEMIS and Sensor Processor combination to be hosted on a more generic platform such as a modular bus by adapting only a piece (the SP) as required for future concepts.
The modular, scalable processor architecture has been designed and developed (by SEAKR Engineering Inc., Centennial, CO, and software developed by Space Computer Corporation of Los Angeles, CA) to enable high performance reconfigurable processing for space applications. HSIP is providing real-time hyperspectral image processing, image storage, payload control and power switching, and flexible plug-and-play interfaces such as SPAs. 41) 42) 43) 44) 45) 46)
This combination will enable the tactical user to request a hyperspectral analysis of a given region for specific objects. The imager will collect the full hyperspectral data cube of the region plus a panchromatic image of the area. The on-board processor will analyze the region for objects of interest and will then geo-rectify the location of these objects with the panchromatic image. The tactical user will then be provided with a panchromatic image of the region with icons on the image that represent locations of the objects of interest.
ARTEMIS processor: The interfaces between components within the ARTEMIS processor include power plane, cPCI (Compact Peripheral Component Interconnect) and SpaceWire for command and data handling instructions (C&DH), and high speed serial to support fast data processing. The ARTEMIS processor interacts with other spacecraft components via several external interfaces including SV 28 V power, SpaceWire and RS422 for spacecraft C&DH, GigE (Gigabit Ethernet) for uplink commands and downlink data, and sensor connections that can interface via a wide array of standards using adaptable mezzanine connectors. Several custom interfaces have been developed and interfaced through the adaptable sensor connector including LVDS camera links for a Hyperspectral Imager (HSI) and a High Resolution Imager (HRI), other digital interfaces for a mass data storage board, and a focus mechanism, as well as an analog input for a position sensor.
The ARTEMIS processor system features four types of boards: a power supply, a UPS (Universal Power Switch), a G4-SBC (G4-based Single-Board Computer), and a RA-RCC (Responsive Avionics Reconfigurable Computer) board. The power supply receives the space vehicle 28 V power and outputs regulated +3.3 V, +5 V, +15 V, and -15 V as required by the other boards in the system. Power switching commands are passed from the G4-SBC to the RA-RCC via the cPCI bus. The RA-RCC then passes the commands to the UPS via the SEAKR Serial Bus (Figure 15).
Figure 15: Main components of the ARTEMIS processor architecture (image credit: SEAKR Engineering Inc.)
The G4-SBC manages external interfaces to the spacecraft and up/down links, controls system configuration and orchestrates data processing. The primary functions of the RA-RCC are to control the payload sensor functionality, perform on-board processing of the sensor data, and to control power switching of the sensors and nonvolatile mass data storage (Figure 16). The board contains four FPGAs including one Actel RTAX2000 and three Xilinx V4 LX160 coprocessors (COPs). The radiation tolerant Actel provides a PCI-to-PCI bridge between the back plane and the local PCI bus interconnecting three COP FPGAs.
Each COP controls and has access to a 256 MB bank of Reed-Solomon EDAC protected DDR2-SDRAM available for processing applications. The COPs also interface to adaptable high-speed mezzanine interfaces that can connect them to sensors, or additional memory or system interconnections. At present, the ARTEMIS processor connects to a 16 GB bank of Reed-Solomon EDAC protected NAND Flash, two camera links for camera control and receipt of the high rate image data, current drive for opto-isolator control lines for the focus mechanism stepper motor controller, and analog position telemetry from the focus mechanism via mezzanine cards. Also, custom LVDS I/O interfaces are implemented between the COPs to improve inter-processor communication. Due to the flexibility of FPGAs, all of these interfaces may be enabled or disabled as required by the mission without changing board hardware layouts. These adaptable interfaces provide a cost-effective way to customize the ARTEMIS core components for a wide range of missions.
Figure 16: Schematic view of the RA-RCC architecture (image credit: SEAKR Engineering Inc.)
International participation in ARTEMIS: The United Kingdom, Canada, and Australia plan to participate in the TacSat-3 experiments to gain better understanding of the value of spaceborne HSI sensors. The Air Force International Affairs Office is coordinating collaboration which will be done under existing agreements. Specific activities are still in the planning stages, but potentially the Australia DSTO (Defense Science and Technology Organization) will use the UHF tactical communications ground station similar to Army Ground Station and demonstrate tasking and direct downlink over ground sites in Australia. Canada will support the Australian experiments using tools developed under multiple airborne experiments. The UK DSTL (Defense Science and Technology Laboratory) is looking at MOD (Ministry of Defense) ground sites for tasking with the end objective being to explore, and demonstrate the potential military applications of a small hyperspectral satellite. The international collaboration will provide lessons learned about the practical problems of operating a small satellite that are potentially applicable to future systems (Ref. 10).
ODTML (Ocean Data Telemetry Microsatellite Link):
ODTML, a next generation data collection system, is provided by ONR (Office of Naval Research). The objective is to collect data from sea-based buoys and to transmit the information back to a ground station. Current methods of obtaining data from these terminals (i.e., Service Argos and airborne platforms) are considered costly, time consuming, and/or inefficient. 47) 48)
ODTML is a transponder concept developed and demonstrated by Praxis Inc. of Alexandria, VA. It uses COTS (Commercial-off-the-Shelf) technologies to improve the data collection and dissemination process for remote sensors. Some capabilities of ODTML are:
· ODTML uses a two-way delay-tolerant messaging capability to provide 'Internet-like' services on global basis to support ocean platform monitoring and provide near real time situational data to web-based maps and other data analysis systems.
· ODTML uses higher bandwidths and lower power than the current Argos DCS (Data Collection System)
- > 50 kbit per node per day
- < 0.1 Joule per bit transmitted.
The ODTML network system consists of the following elements:
1) "Smart sensor nodes," each containing an RF terminal, which collect the sensor data and communicate with the satellite payload. These smart sensor nodes are mounted on the sensor platforms, e.g., free-floating buoys or UGS (Unattended Ground Sensors).
2) Spacecraft Communications Payload (SCP), a microsatellite-mounted payload serving as a "router in the sky."
3) Portable ground stations, acting as gateways to transfer the sensor data from the RF link to the Internet.
4) The Internet, as the communication conduit between the users and the ocean and ground-based observing platforms.
Figure 17: Overview of the ODTML system elements (image credit: NRL)
Figure 18: Conceptual overview of ODTML elements (image credit: Praxis Inc.)
The ODTML demonstration will collect data from sea-based buoys and then will transmit the information back to a ground station.
SAE (Space Avionics Experiment):
The collection of concepts developed by AFRL to realize PnP (Plug-and-Play) space systems is collectively termed SPA (Space Plug-and-Play Avionics). These concepts include self-forming networks, machine-negotiated interfaces, encapsulation of complexity, and test bypass. The objective is to validate plug-n-play avionics capability, which involves the use of reprogrammable components to integrate the SPA experiment and the spacecraft structure. 49) 50) 51) 52) 53)
1) Encapsulation: The most fundamental concept in the SPA paradigm is that of encapsulation-hiding complexity within modular building blocks in order to simplify design. In SPA, this concept manifests itself both in the design of hardware and software. In hardware, the complex inner workings of the device are hidden from the rest of the system. Only single-point electrical connections consisting of data, power, and time synchronization are used to connect the device to the SPA network.
Software encapsulation occurs at many levels, but the greatest example is in the use of XML-based or xTEDs (eXtended Transducer Electronic DataSheets) to precisely define the interfaces between components and even "pieces of software." The goal of this architecture is the achievement of "pure" or "glueless" hardware and software modularity. "Gluelessness" is a very constrained form of modularity that allows rapid integration to occur. Instead of requiring custom electronics or software (the glue) to interface one modular block with another, each block contains everything it needs to maintain compatibility with other blocks in the system.
2) Self-forming networks: The second important SPA concept is that of self-forming networks. In SPA, every device is considered an endpoint on the network, including both traditional bus components, such as reaction wheels or torque rods, and payload components, such as imaging devices. In fact, even structures are endpoints and can be treated in the same manner as other SPA devices on the network. For example, a spacecraft structural panel may contain its own harnessing and internal routers and hubs - essentially an entire SPA sub-network in itself, but the panel is also an endpoint and can be treated as such in the larger SPA network that is the PnP spacecraft. The result is a collection of endpoints separated by hubs or routers and arranged in any order or configuration. The SPA network is created dynamically as devices are introduced.
3) Machine-negotiated interfaces: Glueless modularity and self-describing networks are achieved in the SPA architecture through the use of the third SPA concept-machine-negotiated interfaces. SPA interfaces are defined by components in their resident xTEDs and managed by the SDM (Satellite Data Model). The xTEDs contains descriptions of all commands accepted, variables produced, and data messages that can be delivered by the device. It fully describes the services and data provided by the device and represents the protocol for accessing these services and data.
SDM is a type of "middleware" that manages the SPA distributed network and makes it possible for applications and components to share data and services without needing to know addresses or specific messaging structures. The SDM allows plug-and-play-based hardware communications interfaces to devices. It also allows intelligent devices to describe their controls and data formats to the network using an electronic data sheet. SDM allows application software to query a device to discover its requirements and capabilities for both control and data. By facilitating the query of device capabilities and needs, SDM offers the capability of a glueless rapid system integration, which is capable of dynamic device interfacing without a-priori knowledge of an expected system configuration.
Some background: In 2004, an AFRL study, referred to as RSATS (Responsive Space Advanced Technology Study), revealed that Plug-and-Play technologies similar to those used in the commercial electronics industry could be used to help achieve ORS capabilities such as the rapid reconstitution and augmentation of existing space assets. The study featured an AFRL proposal to develop the AAE (Adaptive Avionics Experiment), which embraced many of the key principles behind a modern plug-and-play (PnP) approach for aerospace. The AAE focused on avionics as the area most readily transformed into a PnP system, with the following four elements as crucial: appliqué sensor network, adaptive wiring manifold, high-performance computing on-orbit, and software definable radio. The appliqué sensor network, proving to be the most useful of the four was expanded and refined to become the current SPA architecture.
In late 2004 AFRL received approval for a committee on standards (CoS) by the AIAA (American Institute for Aeronautics and Astronautics), a US national level standards development organization. This CoS included representatives from government, industry, and academia. The working group developed a series of standards, christened "Spacecraft Plug-n-Play Avionics" (SPA) to describe a spacecraft architecture based on standard interfaces to connect modular components or subsystems. These standards are based on commonly used standards in the Information Technology (IT) industry.
Figure 19: Overview of the plug-and-play avionics architecture (image credit: AFRL)
Any SPA standard, for example, must support data transport, power delivery, time synchronization, single point ground connection, and minimal "hooks" for self-description. While they are excellent standards for data transport, neither USB (Universal Serial Bus) nor SpaceWire provide all of these features. To make them into SPA standards, the CoS focused on minimally-invasive approaches. The CoS tended to overlay features as opposed to modifying the inner workings of proven standards. Instead, sensible extensions were proposed:
· Connectors with enhanced robustness
· The addition of a separate 28V power delivery
· Use of a distinguished single point ground conductor
· Addition of a one pulse per second (1PPS) signal for system-wide time synchronization
· Provision for an electronic datasheet
· The optional inclusion of a special test bypass interface to support test/debug and hardware-in-the-loop integration.
Ultimately these standards allow a spacecraft avionics architecture with distributed computer processors and sensors connected as shown in Figure 20.
Figure 20: Distributed avionics architecture based on plug-n-play standards (image credit: AFRL)
ASIM (Appliqué Sensor Interface Module). ASIM is a device as well as an enabling technology for SPA systems. One of the challenges faced by the commercial computer electronics industry in designing PnP devices was (and is) the sheer complexity of the interfaces.
The ASIM interface module was defined for existing spacecraft components. This evokes a "peel-and-stick" paradigm; permitting legacy devices to rapidly integrate into a PnP (Plug-n-Play) network. In essence, the ASIM is a smart interface chip (or multi-chip module). They are analogous to a USB interface chip in a personal computer. Just as USB chips permit the rapid integration of a mouse or keyboard into a personal computer, the ASIM permits ordinary legacy spacecraft components (such as a reaction wheel, thermometer, or camera) to be rapidly integrated into a spacecraft vehicle. The ASIM electronically stores specific information about the device (using XML) in a small nonvolatile memory. This information becomes an xTEDs (XML Transducer Electronic Datasheet). The ASIM passes xTEDs information to the spacecraft system upon request as devices are plugged into the network.
The ASIM not only acts as a SPA interface chip, but also includes other SPA-enabling features such as xTEDs, power management, time synchronization and test bypass.
Figure 21: Block diagram of a generic ASIM device (image credit: AFRL)
The aerospace industry is plagued with a vast array of incompatible interface standards. The integration of numerous components and payloads utilizing many different connection standards into a spacecraft bus is one of the more time-consuming aspects to spacecraft design, often leading to time delays and cost overruns.
However, if non-PnP components are affixed with a SPA interface, the actions of integrating components into a SPA-compliant bus are reduced to simple plugging functions, thereby vastly reducing satellite build time. One of the primary functions of the ASIM is to serve as a bridge between legacy components and a SPA network. On one side, referred to as the 'host side', the ASIM functions as a SPA device, communicating with the SPA network via the SPA-x protocol. On the other side, referred to as the 'target side', the ASIM communicates with the legacy device according to its native communications protocol.
A certain amount of time is still required to program the ASIM to communicate with the legacy device, but this overhead is a small price to pay for the time and reduction of human-induced errors saved during integration. This action could even be incorporated into the component design itself, encapsulating both the SPA nature of the ASIM and functionality of the component into one single truly SPA-compatible device. All SPA devices and structural panels could be designed in this way and stored until required for a new spacecraft. Spacecraft construction would then consist of connecting panels together to form a bus, selecting whatever components are required for the specific mission, pulling them off the shelf and plugging them into the panels.
AFRL has created two prototype versions of an ASIM, referred to as Gen 0 (Generation 0) and Gen 1 (Generation 1). Gen 0 was a "house" version of the ASIM (developed by SAIC, Albuquerque, NM) used predominately in the Responsive Space Testbed (RST) for initial exploration of SPA devices and networks. The most current version of the ASIM (Gen 1) is based on the SPA-U interface and has been made commercially available (Data Design Corp., Gaithersburg, MD). The ASIM is currently a small printed wiring board (PWB), employing an FPGA-based design serving as a "soft testbed" for further refinement.
The standards have initially focussed on interface definitions for low-data-rate components, "SPA-U" which is based on the USB 1.1 standard, and high-data-rate components SPA-S (based on the European SpaceWire standard).
Figure 22: Simplified architecture of a SPA device (image credit: AFRL)
Figure 23: Photo of the SAE flight assembly (image credit: AFRL, Ref. 17)
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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.