NEOSSat (Near-Earth Object Surveillance Satellite)
NEOSSat is a Canadian microsatellite mission, jointly funded by CSA (Canadian Space Agency) and by DND/DRDC (Department of National Defence/Defence Research and Development Canada). A 'Supporting Arrangement' between CSA and DND was signed on February 24, 2005. In addition, a JPO (Joint Project Office) was set up by DRDC and CSA to manage the NEOSSat design, construction and launch phases. The NEOSSat mission builds upon the demonstrated effectiveness and success of CSA’s highly successful MOST (Microvariability and Oscillation of Stars) astronomy microsatellite mission, launched on June 30, 2003 and still in operation in 2011 after > 8 years on orbit, providing world-class science. 1)
1) To discover and determine the orbits of NEOs (Near-Earth Objects) that cannot be efficiently detected from the ground. After discovery of asteroids and/or comets, this involves in particular the monitoring of their trajectories.
2) To demonstrate the ability of a microsatellite to produce useful metric (position/time) data on man-made, Earth-orbiting objects in the altitude range of 15,000 and 40,000 km
3) To carry out a flight demonstration of the CSA’s first MMMB (Multi-Mission Microsatellite Bus). The development of an affordable multi-mission bus for Canada is a stated goal of CSA.
The NEOSSat project was started in 2005 and has passed through a competitive bidding process that resulted in the NEOSSat Phase B/C/D development contract award in July 2007 to an industrial team led by MSCI (Microsat Systems Canada Inc), Mississauga, Ontario, a private company formed from the former Space Division of Dynacon. The CDR (Critical Design Review) was held in April 2009.
The NEOSSat spacecraft will deploy an optical telescope of 15 cm aperture, to detect objects down to 20th V magnitude.
The DND's basic interest is the surveillance of space mission debris in the MEO (Medium Earth Orbit) and GEO (Geostationary Earth Orbit) regions; this is referred to as HEOSS (High Earth Orbit Space Surveillance), while the CSA’s asteroid tracking mission is referred to as NESS (Near Earth Space Surveillance). The NEOSSat mission will be used for 50% of its operating time to observe the inner portion of the solar system to discover, track and study asteroids. The other 50% of its operating time will be dedicated to tracking satellites and debris in high Earth orbit to update their orbit parameters.
Earth orbits in a transient “cloud” of asteroids that have been perturbed from the Main Asteroid Belt. This population is estimated to be comprised of ~100,000 asteroids >140 m in diameter orbit, many of them on Earth-crossing orbits. Although most will impact the Sun or be ejected from the Solar System, a few percent of these objects will eventually impact the Earth.
The NESS project will use NEOSSat to complement the international effort to discover this near-earth population by searching the sky as close to the Sun as its microsatellite custom baffle design allows (with minimal alterations to the Maksutov telescope design validated by the MOST spacecraft of CSA). The technological limit was judged to be a 45º solar elongation, and meeting this specification for stray light rejection has proven challenging for a microsatellite compatible baffle design. Deriving from this capability, NEOSSat will search an area from 45º to 55º solar elongation along the ecliptic plane and tentatively ± 40º in ecliptic latitude. The observation strategy will be optimized based upon recent models of the NEA (Near Earth Asteroid) population.
This solar elongation range is challenging for ground-based telescopes as observing capability decreases as air mass increases at low altitudes and only brief windows of observing opportunity exist near the Sun as the Earth rotates through 15º every hour. However, ground-based telescopes will be used when possible to do astrometric follow-up for orbit determination of the NEOSSat discovered objects (to free the NESS spacecraft observing time for discovery).
The near-Sun observing strategy is efficient at discovering the Aten orbital class (orbital semimajor axis < 1 AU (Astronomic Unit) and perihelion distance > 0.983 AU) of NEA’s and is also efficient at discovering Apollo class (orbital semimajor axis >1 AU and perihelion distance < 1.017 AU) objects with long synodic periods. This observing strategy will also allow the NEOSSat spacecraft to characterize the population of asteroids that orbit entirely within earth’s orbit, known as the Atira orbital class (orbital semimajor axis < 1 AU and aphelion distance < 0.983 AU).
Figure 1: Schematic view of NEOSSAT mission observation regimes (image credit: CSA, DRDC)
Legend to Figure 1: The NESS mission searches the East and West search fields ~45 degrees solar elongation. The HEOSS mission tracks satellites in the shaded anti-solar direction, towards Earth's shadow at larger solar elongations.
DND/DRDC's goal is to use NEOSSat to demonstrate the ability of microsatellites to perform a militarily useful mission. The mission in this case is to obtain satellite position/time data (“metrics”) to assist in keeping the US Satellite catalog current (“catalog maintenance”). NEOSSat’s optical telescope will be used to track RSOs (Resident Space Objects); these are Earth-orbiting satellites, rocket bodies and debris with altitudes > 15,000 km (“deep space”).
Currently there are ~2500 objects in this category, including GNSS (Global Navigation Satellite System) constellations and Geostationary satellites. HEOSS is a two phase mission where the first year after spacecraft commissioning is dedicated to proving microsatellite based SofS (Surveillance of Space) experimentation. The subsequent years of the mission are planned to transition the HEOSS time allocation to an operational Space Surveillance role where taskings are controlled by the Canadian military.
NEOSSat is being designed such that it can detect objects having apparent brightness down to astronomical magnitudes Mv ~13.5, translating to a ~2 m RSO observed at a range of 40,000 km. NEOSSat will be able to track objects moving at angular rates up to 60 arcseconds/second (arcsec/s). This extremely high rate of motion of Earth orbiting objects causes the apparent sensitivity difference between the NESS and HEOSS missions. Asteroid integration dwell times per pixel are two orders of magnitude larger for a given image. During RSO observations, the sensor will be able to obtain full images at a rate of one image every ten seconds, with smaller portions of the CCD being available at faster rates. The metric accuracy goal of HEOSS is to produce measurements accurate to within 3 arcsec (~600 m in the plane of the sky at geostationary satellite ranges).
Figure 2: Artist's rendition of the NEOSSat spacecraft in orbit (image credit: CSA)
The microsatellite is being designed and manufactured by MSCI (Microsat Systems Canada Inc.) of Mississauga, Ontario as prime contractor, with support from Spectral Applied Research and Routes AstroEngineering. NEOSSat is a three-axis stabilized microsatellite, the first implementation of MMMB, with a launch mass of ~ 74 kg and a bus size of 1.4 m x 0.8 m x 0.4 m. The mission requirements call for an operational life of 1 year (after the commissioning phase) with a goal of two years. The spacecraft is designed to have no death modes (i.e. modes where, in the absence of failures, the spacecraft ceases to operate or to respond to commands). 12) 13) 14)
The MMMB program was initially proposed by CSA in the timeframe 2003. The objective of the initiative is to develop a generic microsatellite bus, an enabling technology, to provide low-cost access for science and technology demonstration missions. The basic MMMB design is structured around a stacking tray concept over which exterior panels, solar cells, and deployables are attached. The bus features a core mechanical structure approach, adaptable to a wide variety of payloads, and a suite of optional bus components to meet a variety of mission requirements. The approach is in standardizing the MMMB architecture, rather than the technology, to permit the introduction of the latest advanced components.
Figure 3: Photo of the NEOSSat microsatellite (image credit: MSCI, CSA)
MMMB-1: There are several key subsystems that comprise the MMMB-1 and obviously the payload is mission specific and not part of the MMMB-1. Also, due to different power requirements and some mission specific electronics and ACS sensor interfaces, the MMMB-1 does not include the solar panels, battery, and some mission-specific electronics. The MMMB-1 design is compatible with the following launch vehicles: Cosmos 3M, Delta-IV ESPA, Rockot, Dnepr, Falcon, Taurus, and PSLV.
Bus structure: The bus primary structure is the tray stack, to which all of the satellite’s components and secondary structures are mounted. The tray stack is comprised of seven tray assemblies, and each tray houses components used by the subsystems. The trays are assembled with their open tops facing the floor of the next tray to create a sealed enclosure for each tray. The entire structure is held together by eight tie rods that pass through holes around the perimeter of each tray. Finally, the payload telescope and baffle are fastened to the bottom of the tray stack via brackets.
Figure 4: Schematic view of the NEOSSAT architecture (image credit: MSCI, CSA, DRDC)
ADCS (Attitude Determination and Control Subsystem): The ADCS is based on a zero-momentum strategy; it is the cornerstone subsystem of NEOSSat and is in charge of performing the high accuracy inertial pointing and tracking of the satellite (along with its optical bore sight). Coarse sun sensors (solar cell current readings), a 3-axis magnetometer, 3 rate sensors and a star tracker form the sensor suite on NEOSSat. The sensor outputs are processed by an EKF (Extended Kalman Filter) which then sends control commands to 3 reaction wheels and 3 magnetorquers.
The ADCS is able to perform inertial pointing to an accuracy of better than 5’’ (arcsec) for the pitch and yaw axes, and better than 20’’ for the roll axis based on flight data obtained from the MOST microsatellite mission, which has the same ADCS sensors, actuators, and attitude determination and control algorithms. As for pointing stability performance, the ADCS achieves the following pointing performance in the modes:
- SSM (Star Stare Mode): a stunning 0.4’’ for yaw, 0.5’’ for pitch, and 4.4’’ for roll in a 100 s exposure
- TRM (Track Rate Mode): 0.8’’ for pitch, 1.2’’ for yaw, and 19’’ for roll in a 30 s exposure.
Other than the de-tumbling mode, occurring at launch separation, the primary modes of tracking are short slews and long slews.
- Short slews and fine pointing are the mode of tracking during observations to stabilize the camera optics aligned with the moving object in space. It relies on the star tracker that is sharing the optics and the read-out electronics of the payload.
- Long slews and coarse pointing occur to move from one object to another. They do not need accuracy and stability and rely only on the coarse rate gyros and sun sensors.
EPS (Electrical Power Subsystem): EPS generates electrical power with a bus voltage of 28 V from solar cells distributed around the 6 faces of the spacecraft. The faces are not evenly populated with solar cells and power generation is maximized on the 3 faces that will be facing towards the sun most of the time. Payload power usage is 9 W average, and the S/C bus usage is 23 W average.
Power storage is provided by Li-ion battery packs to handle peak current demands e.g. when the radio transmitter is turned on and during solar eclipses which occur seasonally for a fraction of a LEO sun-sync orbit.
It is possible to task the satellite with a power-negative balance of power generation for a short period of time when the battery charge state allows it. This case may occur for instance to track objects in opposition to the sun (e.g. anti-solar elongation of 180º).
The EPS is sized to handle an average of 60-80 minutes of downlink per day (equivalent to 6 daily ground passes of 10-12 minutes from a single station located at around 45º latitude).
Figure 5: Block diagram of the NEOSSat microsatellite (image credit: MSCI)
C&DH (Command and Data Handling): This subsystem supports the processing of real-time and time-tagged commands, generating telemetry and handling payload data outputs. There is limited data storage on the spacecraft implemented as a round-robin buffer. Although the buffer read pointer could be reset to any value, the buffer would achieve the full state after around 3 days of continuous observations at the rate of 300 images per day if not emptied during ground passes. In this case, overwriting will occur onboard starting with the oldest data.
TCS (Thermal Control Subsystem): A passive TCS is implemented. Radiative cooling is used to improve the signal-to-noise ratio of the payload imager. No active thermal control system exists except for the battery heater.
RF communications: Use of a dual string of S-band transmitters and receivers. Helix antennas are orthogonally opposed on the spacecraft providing good distribution of the radiation patterns. Turning on and off of the transmitter is accomplished either by time-tagged commands loaded on the C&DH scheduler or directly with manual commands sent from the MOC (Mission Operation Center) during a satellite pass over a ground station.
The spacecraft power system is sized to handle an average of 60-80 minutes of downlink/day (equivalent to 6 daily ground passes of 10-12 minutes from a single station located at around 45º latitude).
Launch: The NEOSSat spacecraft was launched as a secondary payload on Feb. 25, 2013. The launch vehicle was PLSV-C20 of ISRO, and the launch site was SDSC (Satish Dhawan Space Center), India. The primary payload on this flight was SARAL (Satellite with ARgos and ALtiKa), a collaborative mission of ISRO and CNES. 15) 16)
The six secondary payloads manifested on this flight were:
• BRITE-Austria (CanX-3b) and UniBRITE (CanX-3a), both of Austria. UniBRITE and BRiTE-Austria are part of the BRITE Constellation, short for "BRIght-star Target Explorer Constellation", a group of 6.5 kg, 20 cm x 20 cm x 20 cm nanosatellites who purpose is to photometrically measure low-level oscillations and temperature variations in the sky's 286 stars brighter than visual magnitude 3.5.
• Sapphire (Space Surveillance Mission of Canada), a minisatellite with a mass of 148 kg.
• NEOSSat (Near-Earth Object Surveillance Satellite), a microsatellite of Canada with a mass of ~74 kg.
• AAUSat-3 (Aalborg University CubeSat-3), a student-developed nanosatellite (1U CubeSat) of AAU, Aalborg, Denmark. The project is sponsored by DaMSA (Danish Maritime Safety Organization).
• STRaND-1 (Surrey Training, Research and Nanosatellite Demonstrator), a 3U CubeSat (nanosatellite) of SSTL (Surrey Satellite Technology Limited) and the USSC (University of Surrey Space Centre), Guildford, UK. STRaND-1 has a mass of ~ 4.3 kg.
Orbit: Sun-synchronous near-circular dawn-dusk orbit, altitude of ~786 km, inclination of 98.55º, orbital period of 100.6 minutes, LTAN (Local Time on Ascending Node) = 6:00 hours.
Figure 6: The NESS (Near-Earth Space Surveillance) project uses NEOSSat’s space telescope to discover asteroids of the inner Solar System (image credit: NEOSSAT)17)
• In mid-May 2013, the NEOSSat mission is still in the commissioning phase. 18)
Sensor complement: (NESSI)
NESSI (Near Earth Space Surveillance Imager):
Note: The name NESSI is preliminary, selected by myself since no instrument name was available at the time of this writing.
The objective of NESSI is to provide data for the first time about potentially hazardous situations in the near-Earth environment and to enhance the state of alert, while extending the response lead times. In addition to conducting a survey to discover and monitor NEOs (Near Earth Objects), NEAs (Near-Earth Asteroids) and comets approaching close to Earth, the NEOSSat spacecraft will also be used to increase awareness of man-made objects in orbit around the Earth, such as spacecraft and large pieces of space debris.
Apart from identifying potentially hazardous NEOs, NEOSSat data will be used by scientists to investigate:
• NEOs containing well-preserved evidence of conditions during the creation of our solar system, these being one of the best sources of information on its formation
• NEO data contributing to a comprehensive model of the physical and dynamical properties of the minor bodies in our solar system and of the specific differences and similarities between asteroid populations, and to an exploration of their relationships to each other
• Data needed to evaluate NEO populations for potential future investigations, sample return or in situ resource extraction missions enabled by their proximity to Earth.
Optics: NESSI utilizes a Maksutov Cassegrain telescope of 15 cm aperture which shares design lineage with the MOST mission. The NEOSSat telescope is simplified from the MOST design and optimized for imaging by addition of field flattening optics. A sun safety shutter is also added to reduce risk of damage to the CCD array should the boresight be pointed toward the sun. The focal length of the optical system is 893 mm (f/5.7). The instrument has been designed and developed at Spectral Applied Research Inc., Richmond Hill, Ontario. 19)
Table 1: Overview of some NESSI instrument parameters
Figure 7: Optical layout of the 15 cm diameter Rumak-Maksutov telescope (image credit: CSA, DRDC, Ref. 9)
Figure 8: Mechanical assembly cross-section of the NESSI instrument (image credit: Spectral Applied Research, CSA)
The detector of the imager consists of two, side abutted, E2V 47-20 mid-band coated frame transfer CCDs (Charge Coupled Devices) serving both the fine guidance and science functions of the NEOSSat microsatellite. The detector arrays are comprised of 1024 x 1024 pixels with a 13 µm pitch (Figure 9). The frame transfer area is covered so light from the optical system is not absorbed during CCD readout.
When the detector is receiving light from the telescope, a point source creates a PSF (Point Spread Function) with a non-Gaussian shape. Current estimates for the width of the PSF are a FWHM of ~1.1 pixel. This PSF and pixel scale corresponds to a ~62% ensquared energy deposition on the best pixel of the CCD array. This approach undersamples the point spread function on the imaging plane, but provides increased sensitivity to fainter objects, at the cost of reduced centroid accuracy for astrometry (Figure 10).
ROE (Read-Out Electronics): The ROE is a custom fabricated board to read out the CCD arrays after frame transfer has occurred. Noise control during readout must be maintained in order to produce images of high scientific quality. The ROE shifts and counts the charge collected on the CCD array in order to create images used by the science team and the ADCS star tracker.
Figure 9: Illustration of a single detector (image credit: E2V, CSA)
Figure 10: NESSI instrument response over the wavelength range (image credit: CSA)
The light suppression baffle of the telescope is designed to suppress stray light from entering the optical system and impinging onto the detector plane which diminishes signal to noise for faint object detection. The illuminated portion of the Earth, sunlight and the moon are some of the primary sources of light which can impede the science missions. For HEOSS observations the light suppression baffle primarily suppresses light from the illuminated sector of the Earth. During NESS observations the baffle suppresses bright sunlight from entering the optical system that would prevent the detection of faint asteroids.
The NEOSSat ground segment is comprised of separate functional components:
• SOC/POC (Science/Payload Operations Center). The NESS SOC/POC is located at the University of Calgary, Alberta, while the HESS SOC/POC is located in Ottawa, Ontario.
• MOC (Mission Operations Center). The MOC is based on an existing facility responsible for operating all satellites operated by the CSA, located in St.-Hubert, Quebec. The MOC is responsible for commanding and controlling the satellite on-orbit operations by transmitting the payload tasking schedules, originating from the MPS, to the satellite and receiving the payload data products after execution. The MOC coordinates the different resources such as ground station access times, spacecraft pass schedules and spacecraft maintenance operations.
• MPS (Mission Planning System) located in Ottawa. The MPS is the interface between the SOC/POC and the MOC. The MPS evaluates the various payload task requests to produce the payload activity schedule. The MPS also acts as a repository of payload data products and will be made available to the users.
Two CSA ground stations, each a 10 m diameter antenna, are currently baselined as the primary system used throughout the life of the mission. The prime ground station is located in St-Hubert (Québec) and the co-prime is in Saskatoon (Saskatchewan). The use of these two facilities will allow for, on average, approximately 50 minutes of communications with NEOSSat per day. The DRDC ground station in Ottawa represents a third ground station that may be used during the life of the NEOSSat spacecraft. Due to its proximity to the St-Hubert ground station, the addition of the DRDC facility will not add contact times to what is already achievable with the CSA resources.
1) Siamak Tafazoli, Pascal Tremblay, Alan Hildebrand, “NEOSSat and M3MSat - Two Canadian Microsat Missions,” Proceedings of IAC 2011 (62nd International Astronautical Congress), Cape Town, South Africa, Oct. 3-7, 2011, paper: IAC-11-B4.2.9
2) Siamak Tafazoli, William Harvey, Alan Hildebrand, Robert Cardinal, Brad Wallace, Robert (Lauchie) Scott, “NEOSSat - World's First Dedicated Near Earth Object Surveillance Satellite,” Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010
3) David Kendall, “The Canadian Near Earth Object Satellite Mission: NEOSSat,” UNOOSA STSC2009 (Scientific and Technical Subcommittee), Feb. 9-20, 2009, Vienna, Austria, URL: http://www.oosa.unvienna.org/pdf/pres/stsc2009/tech-26.pdf
4) Brad Wallace, Frank Pinkney, Robert Scott, Donald Bedard, Jim Rody, Aaron Spaans, Martin Levesque, Sylvie Buteau, Tom Racey, Doug Burrell, Alan Hildebrand, “The Near Earth Orbit Surveillance Satellite (NEOSSat),” 55th IAC (International Astronautical Congress) 2004, Vancouver, Canada, Oct. 4-8, 2004, IAC-04-IAA.5.12.1.02
5) Donald Bédard, Aaron Spaans, “Responsive Space for the Canadian Forces,” 5th Responsive Space Conference, April 23–26, 2007, Los Angeles, CA, USA, URL: http://www.responsivespace.com/.../SESSION%203/3004_BEDARD/3004P.pdf
6) William Harvey, Tony Morris, “NEOSSat : A Collaborative Microsatellite Project for Space Based Object Detection,” Proceedings of the 22nd Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 11-14, 2008, SSC08-III-5, URL: http://www.google.de/url?sa=t&rct=j&q=the%20near%20earth%20orbit%20surveillance%20satellite&source=web&cd=39
7) H. Ngo, “NEOSSat’s new NEO orbital model,” Vol. 6, EPSC-DPS2011-284-1, 2011, URL: http://meetingorganizer.copernicus.org/EPSC-DPS2011/EPSC-DPS2011-284-1.pdf
8) Donald Bédard, Lauchie Scott, Brad Wallace, Stefan Thorsteinson, William Harvey, Siamak Tafazoli, Michel Fortin, Jaymie Matthews, Rainer Kuschnig, Jason Rowe, “Risk Reduction Activities for the Near-Earth Object Surveillance Satellite Project,” 2006, URL: http://www.amostech.com/TechnicalPapers/2006/Satellite_Metrics/Bedard.pdf
9) Denis Laurin, Alan Hildebrand, Rob Cardinal, William Harvey, Siamak Tafazoli, “NEOSSat – A Canadian small space telescope for near Earth asteroid detection,” Proceedings of SPIE, Vol. 7010, 701013-2, 2008,'Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter,' edited by Jacobus M. Oschmann, Jr., Mattheus W. M. de Graauw, Howard A. MacEwen, URL: http://126.96.36.199/ft/CONF/16416993/16417026.pdf
10) Denis Laurin, William Harvey, Siamak Tafazoli, James Doherty, Alan Hildebrand, Rob Cardinal , Brad Wallace, Robert Scott, Michael Sale, “NEOSSat – the Canadian Near Earth Object Surveillance Satellite,” ASTRO’12 16th ACSI Astronautics Conference, Québec City, Canada, April 23-26, 2012, URL: ftp://astroconference.ca/.../60_NEOSSat_TheCanadianNearEarthObject_Laurin_Paper.pdf
11) William Harvey, “NEOSSat - Near Earth Objects Surveillance Satellite,” URL: http://www.google.de/url?sa=t&rct=j&q=neossat%20mission%2C%20csa&source=web&cd=14&ved=0CE4QFjADOAo&
18) Information provided by Siamak Tafazoli of CSA (Canadian Space Agency).
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.