STPSat-1 (Space Test Program Satellite-1)
STPSat-1 is the first STP demonstration minisatellite of DoD built to specifically exploit the new ESPA (EELV Secondary Payload Adapter) multi-mission launch capability (see the ESPA description under the STP-1 header). The spacecraft of AFRL (Air Force Research Laboratory) at Kirtland AFB hosts three experiments to demonstrate new technologies for future space applications.
Figure 1: Illustration of the deployed STPSat-1 spacecraft (image credit: AeroAstro)
The low-cost STPSat-1 minisatellite was built, integrated and tested by Comtech-AeroAstro Inc. of Ashburn, VA, as prime contractor (partners include Northrop Grumman TASC, and Avidyne). The S/C consists of two modules, the avionics module and the payload module. The majority of the bus avionics are contained in a single box, the IEM (Integrated Electronics Module). 1) 2) 3)
The S/C design utilizes the AeroAstro SpaceFrame, a modular structural bus design concept. The spacecraft platform is 3-axis stabilized using a star tracker to determine the S/C attitude. Attitude knowledge of 0.03º is required for proper pointing of the SHIMMER instrument. S/C control is momentum-biased about the pitch axis. A momentum wheel is used to maintain a nadir-pointing attitude, while two smaller reaction wheels provide off-axis adjustments and special attitudes as needed. 4)
The ADACS (Attitude Determination and Control Subsystem) uses GPS position knowledge in the control loop to provide inertial-to-local attitude transformations for pointing control. Additionally, GPS is the primary data source for onboard orbit determination and for all timing functions. The ADACS configuration permits the S/C to point the SHIMMER FOV to a tangent point of 21.5º below the orbital normal. Special attitudes are needed to calibrate SHIMMER (involving moon pointing). S/C power of 171 W (average) @ 28 V is provided by triple-junction GaAs solar cells. Lithium-ion batteries (3 parallel cells) are used for eclipse-phase operations. 5)
Table 1: Overview of STPSat-1 spacecraft parameters
Figure 2: Block diagram of STPSat-1 (image credit: AeroAstro)
Figure 3: Photo of the STPSat-1 spacecraft (image credit: AeroAstro)
Launch: A launch of STPSat-1 took place on March 9 (UT), 2007 on an Atlas-5-401 vehicle from the Cape Canaveral Air Force Station. Spacecraft separation occurred at T+56 minutes and 41 seconds into the flight.
STPSat-1 is a secondary payload on the STP-1 mission of DoD. The primary payload on this flight is OE (Orbital Express). The other secondary payloads are: CFESat, MidSTAR-1, and FalconSat-3.
Note: The STP-1 mission had to deal with the deployment of five satellites into two orbital planes at two different altitudes. The Orbital Express (prime payload) and MidSTAR-1 spacecraft were deployed in the first orbital plane at an altitude of 492 km and an inclination of 46º. After two more centaur burns, the remaining ESPA payloads, STPSat-1, CFESat, and FalconSat-3, were inserted into the second orbital plane at an altitude of 560 km and an inclination of 35.4º. 6)
Orbit: The shared orbit for all secondary payloads is circular, altitude = 560 km, inclination = 35.4º.
Figure 4: Artist's view of the deployed STPSat-1 in orbit (image credit: Comtech-AA)
• The STPSat-1, built for DoD STP (Space Test Program) and operated by the DoD STP for the first year, then transitioned to NRL for the last 16 months, was decommissioned on October 7, 2009 after completing almost 2 ½ years of successful on-orbit operations. The satellite's two payloads, both designed and built by the Naval Research Laboratory (NRL), provided unique measurements of middle atmospheric hydroxyl, polar mesospheric clouds and the low latitude ionosphere. 7)
• In March 2009, STPSat-1 completed its second year on orbit. For the first year on-orbit, the project was sponsored by the DoD Space Test Program. The second year on-orbit has been sponsored by the Navy Research Laboratory. 8) 9)
• The spacecraft is fully functional on-orbit in 2008, providing SHIMMER and CITRIS experimenters with data as part of its one-year mission. In its first year of mission operations, STPSat-1 has successfully achieved mission and experiment objectives, and at this writing continues to provide valuable science data from SHIMMER and CITRIS. The mission overcame several anomalies by leveraging robust spacecraft design capabilities and software features, and implementing creative operations approaches. 10)
• In March 2008, STPSat-1 completed one year in orbit. 11)
Both SHIMMER and CITRIS performed flawlessly during the nominal one year mission of STPSat-1. After one year however, STP's charter required that the fiscal responsibility for the satellite to be transferred to another agency in order for it to continue operations. In collaboration among the NRL Space Science Division, the NRL Plasma Physics Division and the NRL Spacecraft Engineering Department, NRL demonstrated a novel, highly automated, low cost approach to running the satellite out of its Blossom Point facility near LaPlata, Maryland in southern Charles County. With the success of this demonstration, the STP transferred ownership of STPSat-1 to NRL on June 1, 2008 (Ref. 7).
• On March 28, 2007, STPSat-1 successfully completed Normal Operations Readiness Review (NORR). NORR marks completion of the on-orbit checkout period. 12)
• The on-board GPS receiver, an SSTL SGR-05 device, initially performed well enough during LEOP (Launch and Early Orbit Phase) such that the transponder-based ranging by ground assets was discontinued after one week due to the high quality of GPS position and velocity data downlinked in telemetry. - During this phase, some transient hardware anomalies caused by ionizing radiation were discovered that affected both STPSat-1's star tracker and GPS. However, mitigation approaches were identified during LEOC, and later proved successful.
• The launch sequence and final orbit were near-perfect, providing further verification that the EELV Secondary Payload Adapter (ESPA) is a viable secondary launch alternative.
Sensor/experiment complement: (SHIMMER, CITRIS, MEPSI)
SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals):
SHIMMER is the primary payload of STPSat-1, designed and developed at NRL. The instrument allows to simultaneously observe the OH solar resonance fluorescence from 32 altitudes at a superior resolving power of 25,000. NRL's Space Science Division developed SHIMMER in cooperation with St. Cloud State University and the University of Wisconsin. NASA's Planetary Instrument Definition and Development Program supported the development of the monolithic interferometer. 13) 14) 15) 16) 17)
The concept of this instrument is similar to SHIMMER-Middeck, which was flown on board Shuttle flight STS-112 in October 2002 (Oct. 7-18, 2002) as a proof-of-concept demonstration. The most significant difference between the Middeck instrument and SHIMMER-STPSat-1 is that the separate interferometer components are replaced by a monolithic interferometer. For the monolithic SHS (Spatial Heterodyne Spectroscopy) interferometer, all interferometer components (beamsplitter, prisms and gratings) are optically contacted with fused silica spacers forming a single piece of glass. No adhesives are used in this bonding technique, where the contact surfaces are polished to the extent that the intermolecular forces hold the components together after making physical contact. This avoids numerous thermal and alignment problems posed by using adhesives. - SHIMMER-STPSat-1 also uses an anamorphic telescope which images points on the limb to lines parallel to the limb onto the gratings of the interferometer. This way, potential horizontal structures on the limb do not contaminate the spectral information recorded at the CCD. The CCD is a thinned, back illuminated, UV anti-reflection coated device cooled by a thermoelectric cooler.
The two main objectives of SHIMMER on STPSat-1 are to show that the new optical technique of Spatial Heterodyne Spectroscopy is valuable for long-duration UV remote sensing from space (more than a year) and to observe the seasonal evolution of middle atmospheric OH by inferring global vertical concentration profiles in the tropics and subtropics.
Figure 5: Photo of the SHIMMER instrument (image credit: NRL)
SHIMMER measures the UV emission between about 30 and 100 km altitude and 307.9 - 309.4 nm by imaging the limb with an altitude sampling of about 2.2 km. The high spectral resolution of 120 mA allows the removal of the scattered solar background from the spectra leaving the OH resonance fluorescence from which OH vertical density profiles can be inferred.
OH remains one of the least measured trace gases in the middle atmosphere. This method was pioneered by NRL's MAHRSI (Middle Atmospheric High Resolution Spectrograph Investigation) instrument in the 1990s. Note: MAHRSI was part of the CRISTA-SPAS-1 freeflyer payload of the STS-66 mission, Nov. 3-14, 1994, as well as of the CRISTA-SPAS-2 freeflyer payload of the STS-85 mission, Aug. 7-19, 1997. Compared to MAHRSI, SHIMMER is not only smaller by a factor of three in mass and volume, but it samples the atmosphere seven times faster due to its higher sensitivity.
One additional goal of SHIMMER is to observe the equatorward edge of the polar mesospheric cloud (PMC) region, around 55º latitude. Originally, PMCs were thought to be caused solely by water vapor lofted from the lower atmosphere over the summer polar region. However, MAHRSI demonstrated that water vapor exhaust injected into the upper atmosphere from the space shuttle can also form PMCs. By observing both the OH (water vapor) and the PMCs, SHIMMER results will help quantify this contribution to PMCs.
Table 2: Design parameters of the SHIMMER monolithic interferometer
Figure 6: Schematic illustration of the SHS configuration (image credit: NRL)
Figure 7: Optics assembly of SHIMMER (image credit: NRL)
Figure 8: Optical design of SHIMMER on STPSat-1 (image credit: NRL)
Figure 9: Observation geometry of SHIMMER on STPSat-1 (image credit NRL)
CITRIS (Computerized Ionospheric Tomography Receiver In Space):
CITRIS is a tri-frequency receiver of NRL utilizing a multi-band antenna located on STPSat-1. The objective is to reconstruct real-time phase screens for multiple frequency scintillation estimation. The program goals are: 18)
- To detect - when and where radiowave propagation through the ionosphere is adversely affected by scintillation and refraction
- To provide a global map of ionospheric densities and irregularities to improve current models of the ionosphere.
Beacons from the CERTO (Coherent Electromagnetic Radio Tomography) experiment, flown on other satellites [such as DMSP/F15 (launch in 2007), ARGOS (Advanced Research and Global Observation Satellite) with a launch on Feb. 23, 1999, and PICOSat, launch Sept. 30, 2001, NPSat-1 (same launch with STPSat-1), and on the ROCSat-3/COSMIC/FormoSat-3 constellation (launch Apr. 14, 2006)], are being detected by CITRIS to provide satellite-to-satellite measurements of TEC (Total Electron Count) and propagation fluctuations. Occultation of the Earth's ionosphere can be used to derive electron density profiles from these TEC measurements.
CITRIS also receives signals from ground-based radio beacons all around the world. This CITRIS receiver will use radio beacon transmissions from the French DORIS network of ground beacons at 401.25 and 2036.25 MHz and space-based beacons at 150, 400 and 1067 MHz to measure the Earth's ionosphere. On board tracking software will lock onto Doppler shifted frequencies to determine total electron content (TEC) and scintillation parameters. The receiver provides both amplitude and phase measurements to provide scintillation data at VHF (Very High Frequency), UHF (Ultra High Frequency), and at L-band frequencies. CITRIS requires an unobstructed hemisphere around its multifrequency antenna and an electromagnetically pristine environment in its bands of operation.
Figure 10: Illustration of the CITRIS instrument (image credit: NRL)
Figure 11: CERTO radio beacon geometry for TEC and scintillation measurements (image credit: NRL)
Figure 12: Receiver antenna of CITRIS (image credit: NRL)
Figure 13: Block diagram of the CITRIS receiver (image credit: NRL)
Figure 14: Spaceborne CERTO/CITRIS operations of DORIS ground beacons or tandem operations of NPSat-1 and STPSat-1 (image credit: NRL)
CITRIS concept summary:
• Orbiting beacons and ground receivers provide a space sample environment of radio diffraction patterns
- Each pattern represents beacon position and propagation direction to a ground receiver
- Reconstruction of a single phase screen is not possible
• An orbiting receiver fully samples phase and amplitude from a ground beacon
- Single pattern that is uniform along the magnetic meridian
- Reconstruction of phase screen by inverse diffraction
- Scintillations and any frequency determined from the propagation through the reconstructed phase screen
• Scintillation now-casting algorithm to be tested using CITRIS data from DORIS beacons
- Scintillation and Tomography Receiver In Space (CITRIS)
- Ground DORIS beacons at 401.25 and 2036.25 MHz
- Validation with CERTO and GPS beacons
• CITRIS is flown on STPSat-1
Figure 15: Illustration of the CITRIS flight receiver (image credit: NRL)
MEPSI (MEMS-based PicoSat Inspector):
MEPSI of AFRL contains a pair of MEMS-based picosats in a launcher that can eject the picosats from the spacecraft. The picosats are battery-powered able to operate (communicate) for up to 24 hours. Each MEPSI consists of a box of size: 100 mm x 100 mm x 125 mm; the mass of MEPSI is < 1 kg.
Figure 16: MEPSI launcher assembly housing two picosats (image credit: AFRL)
The MEPSI program of AFRL was developed to enable a new, low power, autonomous, on-board space system to be used in support of critical satellite operations. This is done through investigating the functionality of MEMS-based subsystems in the space environment and by demonstrating the capability of deploying an onboard miniature autonomous inspector, tasked to conduct visual inspection of the host satellite.
The MEPSI mission is composed of six precursor flights in order to validate the technology prior to the final flight of the MEPSI experiment.
• The first of these flights took place in February of 2000 when the first picosats were deployed from the JAWSAT/OPAL spacecraft. This mission was a complete success.
• The second MEPSI precursor flight is on board MightySat II.1 (launch July 19, 2000), deployment of the two tethered picosats, referred to as PICO20 and PICO22, occurred in July 2001.
• The first iteration of MEPSI was flown on STS-113 (Nov. 24 to Dec. 7, 2002).
• The third flight is on STS-121 (July 4-17, 2006). The objective is to demonstrate MEMS RF switches in Tx/Rx function for communications.
• MEPSI precursor on STPSat-1 (launch 2007). MEPSI, a miniature free-flyer, will be released from STPSat-1 to demonstrate MEMS utility and related microsystems for proximity operations. MEPSI enables remote command and control operations by providing a rapid feedback capability for decision makers for detection and response to spacecraft anomalies that help maintain continual service to the Warfighter and increased spacecraft longevity.
Figure 17: Inside view of the MEPSI PLA (PicoSat Launcher Assembly), image credit: AFRL
Note: MEPSI was de-manifested in October 2006, only several months before launch. However, the PLA (Picosat Launcher Assembly) is flying with the space vehicle and contains two inert mass simulators instead of Picosats. The mass simulators will remain within the PLA and will not be released any time during the mission (Ref. 2).
Early on in the MEPSI program, the STP office selected STPsat-1 as the host for the final demonstration of MEPSI. STPsat-1 was started with a known finish date, independent of the status of the MEPSI improvement flights on the space shuttle. The MEPSI program maintained both a shuttle development schedule and an STPsat-1 integration schedule. The space shuttle Columbia accident in February 2003 stopped all space shuttle flights until July 2005. The second of the planned four MEPSI development flights using the space shuttle was manifested on STS-116 with a liftoff date in December 2006. Six months later, the flight hardware was due for MEPSI on STPsat-1. This provided no reaction time to fix any flaws or improve the final product based on the STS-116 mission results. In actuality, it probably would have taken at least the two more shuttle flights to adequately finish the MEPSI spacecraft. To maintain the integrity of the STPsat-1 launch schedule, a flight SSPL (Space Shuttle Picosatellite Launcher) with mass-model picosatellites was installed on the host spacecraft. 19)
STPSat-1 ground station:
BPSTCS (Blossom Point Satellite Tracking and Command Station), located in Dahlgren, VA, is a fully automated command and control facility capable of supporting multiple satellites concurrently. The system currently supports 13 spacecraft in a wide variety of orbits 24 hours a day, 7 days a week, taking approximately 186 contacts per day. The BPSTCS is manned eight hours per day, five days a week and operates with a high degree of automation.
STPSat-1 is operated solely from Blossom Point, which supports approximately five STPSat-1 contacts per day. During normal operations, STPSat-1 support is limited to approximately one man hour per week, with engineering staff available to support anomaly resolution as needed. The software system is based on the CGA (Common Ground Architecture), developed by NRL , to support all aspects of the satellite development lifecycle from box level testing through operations. The AGO (Automated Ground Operations) software allows the system to run automatically without any operators required. 20)
The SGCS (StreamLINK Ground Control System) of Tiger Innovations is used. The STPSat-1 ground support system is hybrid between the CGA system and the SGCS. To control STPSat-1, the existing antenna, RF (Radio Frequency), and encryption equipment at the Blossom Point facility was interfaced with the StreamLINK Ground Control System (SGCS) equipment that houses the frame sync, command formatter, and control software. The SGCS telemetry interface accepts a synchronous RS-422 serial link and provides command output in ternary format. The tracking facility provides the RF, bit-sync, and encryption hardware, and passes telemetry clock and data to the SGCS. Inside the SGCS, a Tiger Innovations frame sync module receives the serial stream, identifies telemetry frames, performs a cyclic redundancy check (CRC), and passes valid frames to the backend computer for decommutation.
Figure 18: Block diagram of the Blossom Point equipment (image credit: Tiger Innovations)
Automated real time operations system: For any given pass, Blossom Point’s CGA generates antenna pointing angles and AOS/LOS times from daily ephemeris updates. This information is used to schedule antenna and equipment resources and is transferred to the SGCS for pass planning. Prior to AOS, CGA sets up the BPSTCS ground system components (antennas, receivers, bit syncs, key generators, switch matrices, transmitters, etc) to collect the downlink and generate the uplink signals required. During the pass, StreamLINK generates the uplink bit stream for commanding and collects the downlink data. The SGCS uses control scripts to monitor telemetry, command the vehicle and send out anomaly alerts if necessary. For the STPSat-1 mission, the main script waits for AOS, ensures the uplink and downlink are set up properly, and then begins normal pass operations. This includes scheduling the flight transmitter on-time, running a critical health check, uploading payload and engineering commands, and downloading stored data.
The greatly reduced operations cost allowed the extension of the STPSat-1 mission which contributed significantly to the science accomplished by its two payloads (SHIMMER, CITRIS). For this mission, the increased time on orbit facilitated scientific results that were not possible to achieve with only the nominal mission. The following sections summarize those results.
1) P. Davis, G. Cameron, E. Aamot, N. DeVilbiss, “STPSat-1: A New Approach to DoD Experiment Spaceflight,” Proceedings of AIAA/USU Conference on Small Satellites, Logan, UT, Aug. 12-15, 2002, SSC02-V-8
2) R. Barnisin, M. LaGrassa, P. Remias, “STPSat-1: The First Space Test Program Mission to Capitalize on the New ESPA Secondary Launch Capability,”Proceedings of the 21st Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 13-16, 2007, SSC07-VII-5
5) “STPSat-1,” Comtech AA, URL: http://www.aeroastro.com/index.php/program-heritage/stpsat-1
6) Bob Minelli, Steve Haase, Mark Barton, Jay Borges, Clay A. Hunt, Jon Miller, “Designing for ESPA: The Challenges of Designing a Spacecraft for a Launch Accommodation Still in Development,” Proceedings of the AIAA/USU Small Satellite Conference, Logan, UT, USA, Aug. 11-14, 2003, SSC03-II-7, URL: http://spacecraft.ssl.umd.edu/academics/483F08/483F08.refs/SSC03-II-7.pdf
8) “STPSat-1 Celebrates Two Years in Orbit,” March 9, 2009, URL: http://server2.tigerinnovations.com/homepage/news/stpsat_twoyear.php
9) Richard Barnisin, Patricia Remias, Frank Scalici, “Priority Briefing STPSat-1: Two Years of Successful Operations,” MilSat Magazine, May 2009, URL: http://www.milsatmagazine.com/cgi-bin/display_article.cgi?number=533812756
10) R. Barnisin, P. Remias, F. Scalici, “STPSat-1 - One Year of Successful Operations,” Proceedings of the 22nd Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 11-14, 2008, SSC08-II-3
11) “STPSat-1 Completes One Year in Orbit,” Comtech AA. March 23, 2008, URL: http://www.aeroastro.com/STPSat-1-Completes-One-Year-in-Orbit
12) “AeroAstro-built STPSat-1 Satellite Operating Successfully On-orbit,” Spacedaily April 18, 2007, URL: http://www.spacedaily.com/.../AeroAstro_built_STPSat_1_Satellite_Operating_Successfully...
13) J. M. Harlander, F. L. Roesler, C. R. Englert, J. G. Cardon, R. R. Conway, C. M. Brown, J. Wimperis, “Robust Monolithic Ultraviolet Interferometer for the SHIMMER Instrument on STPSat-1,” Applied Optics, Vol. 42, Issue 15, May 2003, pp. 2829-2834, URL: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA521706
14) Information courtesy of Christoph R. Englert of NRL, Washington D.C.
15) C. R. Englert, J. G. Cardon, M. H. Stevens, J. M. Harlander, F. L. Roesler, “SHIMMER on STPSat-1: UV Spatial Heterodyne Spectroscopy for Space Based Remote Sensing of the Middle Atmosphere,” Proceedings of AGU Fall Meeting, SPA Aeronomy, San Francisco, CA, Dec. 13-17, 2004
16) C. R. Englert, M. H. Stevens, J. M. Harlander, F. L. Roesler, “Spatial Heterodyne Spectroscopy for Atmospheric Remote Sensing,” ASSFTS (Atmospheric Science from Space using Fourier Transform Spectrometry) 12th Workshop Quebec City, Canada, May 18-20, 2005, URL: http://bernath.uwaterloo.ca/ASSFTS/Media/ASSFTS%20Presentations/pdfPres/Englert.pdf
17) C. R. Englert, J. C. Owrutsky, J. M. Harlander, “SHIM-Fire Breadboard Instrument Design, Integration, and First Measurements,” Nov. 23, 2005, NRL/MR/7640-05-8926
18) P. A. Bernhardt, C. L. Siefring, I. J. Galysh, T. F. Rodilosso, D. E. Koch, T. L. MacDonald, M. R. Wilkens, G. P. Landis, “Ionospheric Applications of the Scintillation and Tomography Receiver in Space (CITRIS) used with the DORIS Radio Beacon Network,” IDS (International DORIS Workshop) Workshop, Venice, Italy, March 13-15, 2006, also in Journal of Geodesy, Vol. 80, No 8-11, Nov. 2006, pp. 473-485
19) David Hinkley, “Picosatellites at The Aerospace Corporation,” URL: http://www.colwave.com/innovaeditor/assets/Descargas/20_Picosats_9.pdf
20) Doug Firestone, Robert Atkin, Carl Hooks, Christoph R. Englert, David E. Siskind, Paul A. Bernhardt, Carl L. Siefring, Patricia A Klein, “Low-Cost, Automated Ground Station for LEO Mission Support,” URL: http://server2.tigerinnovations.com/homepage/news/TigerInnovations_STPSat1_Automated_Operations.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.