CanX-4 and 5
CanX-4&5 (Canadian Advanced Nanospace eXperiment-4&5)
CanX-4&5 is a dual-nanosatellite formation flying demonstration mission of UTIAS/SFL (University of Toronto, Institute for Aerospace Studies/Space Flight Laboratory), Toronto, Canada. The overall mission objective is to prove that satellite formation flying can be accomplished with sub-meter tracking error accuracy for low ΔV requirements. The formation flying maneuvers for this mission require the development of control algorithms for autonomous formation maintenance and reconfiguration in the presence of orbital perturbations. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)
The CanX program was established to develop new state-of-the-art nanosatellite technology and train graduate students through exposure to real nanosatellite missions. The CanX program follows the philosophy of low cost, rapid development and aggressive experimentation. The low cost is maintained by basing each nanosatellite around the GNB (Generic Nanosatellite Bus) developed at SFL. The GNB has a 20 cm cubic form factor with nearly 30% of its mass and volume dedicated to mission specific payloads making it ideal for rapidly developing a range of cheap, yet sophisticated, scientific and technology demonstration missions.
The UTIAS/SFL partner in the CanX-4&5 mission is the University of Calgary. Sponsor organizations are: DRDC (Defense R&D Canada) Ottawa, NSERC (Natural Sciences and Engineering Research Council) of Canada, CSA (Canadian Space Agency), OCE (Ontario Centers of Excellence), MDA (MacDonald Dettwiler and Associates Ltd) of Richmond, BC, Routes Astro Engineering of Kanata, Ontario, and Sinclair Interplanetary of Toronto.
The specific CanX-4&5 mission objectives are as follows:
1) Demonstrate the autonomous achievement and maintenance of several dual satellite formations
2) Demonstrate carrier-phase differential GPS techniques to perform relative position determination measurements with accuracies of 10 cm or less
3) Demonstrate sub-meter position control
4) Develop and validate fuel efficient formation flying algorithms
5) Demonstrate enabling technologies, such as CNAPS (Canadian Advanced Nanosatellite Propulsion System) and the S-band ISL (Intersatellite Link).
The mission will demonstrate two formation types, each at two separations—an along-track orbit (ATO) at distances of 1000 m and 500 m, and a projected circular orbit (PCO) at 100 m and 50 m. In each formation, the deputy spacecraft performs active orbit maintenance and control to achieve the desired relative motion with respect to the chief spacecraft. If desired, the spacecraft may swap roles throughout the mission, though all mission objectives are achievable with a single chief/deputy assignment. The spacecraft will maintain each formation for ten orbits with one orbit of reconfiguration maneuvers between formations. The nominal mission life is thus less than seven days once the spacecraft are fully commissioned. 12)
To maintain a satellite formation requires precise relative position determination and accurate thrusting. The total ΔV and relative position determination requirements are driven by the desired degree of precision for the satellite formation control. The CanX-4&5 thrusters are located on one face only. Therefore, attitude pointing requirements are placed on the attitude determination and control system for accurate thruster pointing. In addition, the two satellites must communicate with each other to relay position, velocity and attitude information. The intersatellite communication system must accommodate the desired relative distance of the satellites in each formation as well as the required data rates. 13)
Table 1: CanX-4&5 formation flying performance requirements
Figure 1: Illustration of the CanX-4 & -5 nanosatellites - shown in opposing views (image credit: UTIAS/SFL)
Spacecraft produced within the CanX program are built upon the heritage of their predecessors. This modular, step-wise design approach results in progressively more reliable and sophisticated nano-spacecraft.
CanX-4&5 are identical nanosatellites based on the modular GNB (Generic Nanosatellite Bus) design of UTIAS/SFL (GNB is of BRITE/CanX-3 heritage). The GNB structure features a cubic form factor of 20 cm side length and consists of two trays and six external panels. A dual tray aluminum structure was selected to maximize the payload bay and provide ease of integration. The two trays contain all the necessary components for a basic satellite mission, including communications, attitude determination and control, power and thermal/structural components.
Figure 2: The GNB configured for CanX-4&5 (image credit: UTIAS/SFL)
Power is generated with 36 body mounted triple junction GaInP2/GaAs/Ge solar cells with EOL efficiency of 26.8%. Energy is stored in two 5.3Ah Li-ion batteries allowing the satellite to operate in extended eclipse periods. The power system architecture is a peak power tracking system which provides switched power to the loads and power regulation where required. - A BCDR (Battery Charge and Discharge Regulator) is connected in series with each battery. The BCDRs provide the peak power tracking for the solar array while regulating the charge and discharge of the batteries. The solar array is connected to the main power bus in a DET (Direct Energy Transfer) configuration. Therefore, by regulating the main bus voltage the BCDRs can maximize the power produced by the solar array when required for optimal battery charging.
CanX-4&5 are both equipped with three on-board computers (OBC). Each OBC features an ARM7 microcontroller, 2 MB of EDAC (Error Detection And Correction) protected SRAM and 256 MB of flash memory. The housekeeping computer is responsible for communications with the ground station and collecting satellite telemetry.
The ADCS (Attitude Determination and Control Subsystem) computer interfaces with the attitude sensors and actuators and runs the attitude control algorithms. Finally, the formation flying computer will be responsible for interfacing with the propulsion system and GPS receiver and for running the formation flying algorithms. Each OBC runs a custom made multi-threaded operating system called CANOE (Canadian Advanced Nanospace Operating Environment) allowing it to divide processing time between multiple tasks in parallel.
Figure 3: Schematic view of two structural trays of CanX-4 or CanX-5 (image credit: UTIAS/SFL)
Legend to Figure 3: The left image illustrates the tray housing of the ADCS suite. The RF devices (radios) are mounted on the underside. The right image shows the tray holding the computer stack.
The ADCS is of CanX-2 and CanX-3 (BRITE) heritage providing 3-axis stabilization; the sensors consist of 6 coarse/fine sun sensors, a 3-axis magnetometer, and 3 rate gyros. The combination of these sensor sets yields a pointing accuracy of better than 1º. Attitude actuation is provided with 3 orthogonally-mounted reaction wheels (for fine pointing) and 3 magnetorquer coils (for detumbling and momentum dumping). ADCS must be able to slew each satellite such that their propulsion thrusters can deliver the correct impulse in the direction required and with an accuracy of ~1º. ADCS is being implemented on a dedicated OBC.
With thrusters located only on one side of each spacecraft, it will be necessary to slew the satellites to a new attitude vector for each successive thrust during formation flying. To ensure the same GPS satellites are kept in view, both the deputy and chief S/C will simultaneously track the same attitude targets (although only the deputy will provide the thrust).
RF communications: Three different transmission links are used on CanX-4&5: a UHF-band receiver, an S-band transmitter, and a VHF beacon.
• The UHF-band receiver is used for data uplink from the ground station and operates in the amateur band with a data rate of 4 kbit/s. It uses four quad-canted monopole antennas which provide near omni-directional coverage.
• The S-band transmitter will be used for data downlink and intersatellite communication, allowing the chief and deputy S/C to regularly exchange position, velocity, and attitude data. The S-band link provides data rates between 32 and 256 kbit/s. The S-band transmitter uses two patch antennas mounted on opposite sides of the satellite. The ISL (Intersatellite Link) S-band transceiver is carried on each S/C. An ISL data rate of 10 kbit/s can be achieved at a maximum separation distance of 5 km between the two satellites.
• The VHF beacon will continually transmit the satellites' identification and basic telemetry in Morse code during the early stages of the mission to assist in commissioning the satellites.
ISS (Intersatellite Separation System):
An innovative ISS was designed to facilitate the linking and subsequent separation of the two satellites during launch and commissioning phases. The ISS consists of two nearly identical halves, one mounted on the side of each satellite, with a spring-loaded cup/cone interface between them. This interface is coated in an electrically debonding agent, which, when hardened, acts as a rigid glue holding the satellites together. Once they are ready to separate, a small voltage is applied across the mechanism, which weakens the glue to the point where the springs overcome the adhesive force, breaking the bond and separating the two satellites. To ensure the de-bonding adhesive breaks as anticipated, the ISS springs will have a compressive force of 70 N and deliver a ΔV of ~ 8 cm/s to each satellite. The satellites will separate partially in the orbit normal direction in order to reduce this value to the 2.6 cm/s in the along-track direction, necessary to achieve the initial conditions for the first formation. 14)
Figure 4: Location of the ISS linkage system during lhe launch and commissioning phases (image credit: UTIAS/SFL)
Figure 5: Illustration of the ISS concept (image credit: UTIAS/SFL)
Table 2: Summary of the CanX-4/&5 spacecraft bus specifications
Launch: A launch of the CanX-4&5 formation flying constellation (secondary payloads) is planned for 2014 on the PSLV-23 launcher. The launch site is the Satish Dhawan Space Centre (SDSC) SHAR, Sriharikota, India. Antrix Corporation Ltd. of Bangalore, India, is the launch provider and marketing arm of ISRO (Indian Space Research Organization). The primary payload on this flight is SRE-2 (Space Capsule Recovery Experiment-2) of ISRO. 15)
The secondary payloads on this flight are:
• IMS-1B (Indian Microsatellite-1B)
• CanX-4 and CanX-5, a pair of identical nanosatellites of UTIAS/SFL (University of Toronto, Institute for Aerospace Studies/Space Flight Laboratory), Toronto, Canada
• NEMO-AM (Nanosatellite for Earth Monitoring and Observation-Aerosol Monitoring), a nanosatellite of UTIAS/SFL.
Orbit: Sun-synchronous circular orbit, altitude = 635 km, inclination = 97.64º, period = 97.4 minutes, local time at descending node (LTDN) = 9:30 hours.
Formation flying mission plan:
Both CanX-4 and CanX-5 will be deployed individually from the PSLV upper stage, using separate XPOD ejection systems of UTIAS/SFL. Following commissioning of the both spacecraft, which is anticipated to take less than one month, one of the two satellites will be assigned the role of deputy, and will perform a series of drift recovery thrusts to begin maneuvering back towards the other satellite.
At a range of approximately 10 km, both satellites will be within communication range and can begin exchanging GPS and attitude information over the intersatellite link. At this point, the satellites enter a coarse station-keeping mode, in which a minimum separation is maintained in preparation for precision formation flight. Once ready, the satellites will be commanded to execute the ATO (Along Track Orbit) formations, followed by the PCO (Projected Circular Orbit) formations, with intermediate reconfiguration orbits between them. Once station-keeping has been achieved, the anticipated time required to undertake the entire mission is only days, though the drift recovery itself may require several weeks, depending on the drift rate following deployment. Both satellites are sized to recover from a worst-case separation velocity (magnitude and orientation) of 2.5 m/s.
In the ATO formations, both satellites will essentially occupy the same orbit, but with one satellite leading the other by a particular separation distance. In the PCO formation, the satellites have slightly different inclination and eccentricity values so that, when viewed from Earth over the course of one orbit, the deputy appears to orbit the chief satellite (Figure 6).
Once the separation maneuver is concluded, the deputy will commence station-keeping, reconfiguring its formation after 50 orbits in each of the ATO and PCO formations. Since each reference trajectory describes either a circle or an ellipse for the deputy to track in the Hill frame, it is very important that each reconfiguration maneuver begin at an appropriate relative phase angle.
The overall ΔV requirement for the baseline CanX-4&5 mission is anticipated to be approximately 7.5m/s, well beneath the 14m/s ΔV available onboard the deputy satellite.
Figure 6: CanX-4&5 in a projected circular orbit (image credit: UTIAS/SFL)
Experiment verification: Two criteria will be used to determine the performance of CanX-4&5 over the course of the mission. First, the level of accuracy to which the satellites are able to control their relative position in each formation configuration will be determined. Second, the ability of the deputy satellite to minimize its fuel consumption by correcting for secular perturbations in the orbit while ignoring any periodic changes will be evaluated. Each satellite will determine its absolute position and velocity using an onboard GPS receiver. This data will be logged by the satellite and downloaded by the ground station. This data can then be analyzed to accurately determine the relative distance of each satellite over time. In addition, Two-Line Elements (TLEs) obtained from NORAD will be used as a coarse means of verifying performance early in the mission.
Payload/experiments: (CNAPS, GPS receiver, ISL)
CNAPS (Canadian Nanosatellite Advanced Propulsion System):
Precisely achieving and maintaining a satellite formation requires precise relative position determination and accurate thrusting. The total ΔV and relative position determination requirements are driven by the desired degree of precision for the satellite formation control.
Table 3: Performance requirements for CanX-4&5 satellites
The custom-built CNAPS is of NanoPS (Nanosatellite Propulsion System) heritage flown on CanX-2 (launch April 28, 2008).
Both spacecraft, Can-X-4 and CanX-5 will be outfitted with CNAPS, a gas propulsion system to regulate the orbital distance between the two satellites. CNAPS uses liquefied sulfur hexafluoride (SF6) as a propellant and will be able to achieve a specific impulse of at least 35 s. With a fuel capacity of 300 ml, CNAPS is capable of a total ΔV of approximately 14 m/s. 16)
To maintain and transition between formations, the deputy satellite must thrust at regular intervals. Thrust is produced by 4 independently controlled thrusters in a cruciform configuration on one face of the satellite. Each thruster generates a constant thrust magnitude of 5 mN with a minimum impulse bit of 0.1 N s. Given this low thrust magnitude, it is occasionally necessary for CNAPS to thrust for extended periods of time. Since each thruster can be calibrated independently, the 4 thruster arrangement is used to mitigate any unwanted torques generated during extended thrusts due to thruster misalignment.
Since the thrusters on CanX-4&5 can only thrust at a constant 5 mN, it is necessary to use a PWM (Pulse Width Modulation) technique, whereby the thrust is held constant but the on time is varied. As long as the time between thrusts (i.e. the PWM period) is small compared to the rate of the dynamics, the PWM technique accurate approximates a continuous thrust method. The CanX-4&5 controller has a PWM period of 65 seconds.
Figure 7: The CNAPS cold-gas propulsion system (image credit: UTIAS/SFL)
GPS navigation algorithm:
Both nanosatellites are equipped with a NovAtel dual-band GPS receiver and a dual-band GPS antenna of AAT (Aeroantenna Technology Inc.) to obtain accurate absolute position and velocity measurements. The GPS antennas are mounted on a face of each satellite orthogonal to its thrust axis, allowing it to retain some directional control over the antenna while the deputy thrusts in different directions. This control will be used to point the antenna as close to the zenith as possible in an effort to maximize the number of viewable GPS satellites.
The telemetry returned by the GPS receiver is used by the specialized relative navigation software (RelNav) to obtain a sub-cm relative position solution between the two spacecraft. RelNav requires raw GPS data from the receivers on both spacecraft in order to obtain this solution. The ISL provides a wireless link between the spacecraft that enables the autonomous transfer of this data between the spacecraft throughout the mission.
To perform the relative navigation both satellites must see a common set of GPS satellite from which to apply the differential navigation algorithms developed by the University of Calgary. The number of satellites required for fine navigation is four common GPS satellites, although there is a significant improvement once six satellites have been acquired. The formation flying algorithm has been developed to be robust to intermittent blackouts in GPS coverage. However, there is an increased cost in fuel and accuracy as a result of a momentary blackout in GPS coverage. 17)
Single-point GPS processing will allow each satellite to determine its absolute position to 2-5 m (rms) and its absolute velocity to 5-10 cm/s (rms). The geomatics group at the University of Calgary has provided UTIAS/SFL with an algorithm which uses carrier phase and Doppler data with double differencing techniques to achieve relative position estimates to within 2-5 cm (rms) and relative velocity estimates to within 1-3 cm/s (rms).
Figure 8: Photo of the Novatel GPS receiver (image credit: UTIAS/SFL)
Formation flying control algorithm:
The ISL was designed to close a 10 kbit/s communications link at separations of up to 5 km with 6 dB margin, well above the nominal maximum spacecraft separation of 1 km. Once a relative position solution is obtained, the deputy spacecraft employs the cold-gas thrusters of CNAPS to perform orbit control.
Both nanosatellites will be equipped with a dedicated OBC to run the formation flying control algorithm, called FIONA (Formation flying Integrated Onboard Nanosatellite Algorithm). The principle objective of FIONA will be to regularly determine the tracking error of the deputy spacecraft and to compute the optimal thrusts necessary to correct this error.
Figure 9: Photo of the ISL device with the enclosure lid removed (image credit: UTIAS/SFL)
2) Nathan G. Orr, Jesse K. Eyer, Benoit P. Larouche, Robert E. Zee, “Precision Formation Flight: The CanX-4 and CanX-5 Dual Nanosatellite Mission,” Proceedings of the IAA Symposium on Small Satellite Systems and Services (4S), Rhodes, Greece, May 26-30, 2008, ESA SP-660, August 2008
3) J. K. Eyer, C. J. Damaren, R, E. Zee, E. Cannon, “A Formation Flying Control Algorithm for the CanX-4&5 Low Earth Orbit Nanosatellite Mission,” Proceedings of the 58th IAC (International Astronautical Congress), International Space Expo, Hyderabad, India, Sept. 24-28, 2007, IAC-07-B4.6.04, URL: http://www.utias-sfl.net/docs/canx45-iac-2007.pdf
4) N. G. Orr, J. K. Eyer, B. P. Larouche, R. E. Zee, ”Precision Formation Flight: The CanX-4 and CanX-5 Dual Nanosatellite Mission,” ASTRO 2008 - 14th CASI Canadian Astronautics Conference, Montreal, Canada, April 29 - May 1, 2008, URL: http://www.utias-sfl.net/docs/canx45-astro-2008.pdf
5) J. Eyer, C. Damaren, R. E. Zee, “The Guidance and Control Algorithms for the CanX-4 &5 Formation Flying Demonstration Mission,” Proceedings of the 3rd International Symposium on Formation Flying, Missions and Technology, ESA/ESTEC, Noordwijk, The Netherlands, April 23-25, 2008, ESA SP-654, June 2008
6) S. Eagleson, K. Sarda, S. Mauthe, T. Tuli, R. E. Zee, “Adaptable Multi-Mission Design of CanX Nanosatellites,” Proceedings of the 20th Annual AIAA/USU Conference on Small Satellites, Logan, UT, Aug. 14-17, 2006, URL: http://www.utias-sfl.net/docs/gnb-ssc-2006.pdf
7) K. Sarda, S. Eagleson, S. Mauthe, T. Tuli, R. E. Zee, C. C. Grant, D. G. Foisy, E. Cannon, C. J. Damaren, ”CanX-4 & CanX-5: Precision Formation Flight Demonstrated by Low Cost Nanosatellites,” .Proceedings of ASTRO 2006 - 13th CASI (Canadian Aeronautics and Space Institute) Canadian Astronautics Conference, Montreal, Quebec, Canada, April 25-27, 2006, URL: http://www.utias-sfl.net/docs/canx45-astro-2006.pdf
8) E. P. Caillibot, C. C. Grant, D. D. Kekez, and R. E. Zee, ”Formation Flying Demonstration Missions Enabled by CanX Nanosatellite Technology,” Proceedings of the 19th Annual AIAA/USU Conference on Small Satellites, Logan, Utah, Aug. 8-11, 2005, URL: http://www.utias-sfl.net/docs/canx245-ssc-2005.pdf
9) Karan Sarda, Stuart Eagleson, Eric Caillibot, Cordell Grant, Daniel Kekez, Freddy Pranajaya, Robert E. Zee, “Canadian Advanced Nanospace eXperiment 2: Scientific and Technological Innovation on a Three-Kilogram Satellite,” Proceedings of the 56th IAC (International Astronautical Congress), Fukooda, Japan, Oct. 17-20, 2005, IAC-05-B5.6.A.15; - also in Acta Astronautica, Vol. 59, 2006, pp. 236-245, URL: http://www.utias-sfl.net/docs/canx2-acta-2006.pdf
10) Erica H. Peterson, Robert E. Zee, Georgia Fotopoulos, “InSAR Microsatellite Constellations Enabled by Formation Flying and Onboard Processing Capabilities, “Proceedings of the 25th Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 8-11, 2011, paper: SSC11-IX-1
11) Grant Bonin, Nathan Orr, Scott Armitage, Niels Roth, Ben Risi, Robert E. Zee, “The CanX-4&5 Mission: Achieving Precise Formation Flight at the Nanosatellite Scale,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B4.7B.5
12) Niels Henrik Roth, “Navigation and Control Design for the CanX-4/-5 Satellite Formation Flying Mission,” Master’s thesis, University of Toronto, Toronto ON, Canada, 2010, URL: https://tspace.library.utoronto.ca/bitstream/1807/25908/3/Roth_Niels_H_201011_MASc_thesis.pdf
13) Niels H. Roth, Grant Bonin, Robert E. Zee, “Hardware-in-the-Loop Simulation for the CanX-4/-5 Nanosatellite Formation Flying Mission,” Proceedings of the 7th International Workshop on Satellite Constellation and Formation Flying (IWSCFF-2013), Lisbon, Portugal, March 13-15, 2013, paper: IWSCFF-2013 -05-07
14) Benoit P. Larouche, Grant Bonin, Cordell Grant, Robert E. Zee, “The Intersatellite Separation System: A Lightweight, Low-Power Deployment Mechanism for Formation Flying Nanosatellites,” Proceedings of the 59th IAC (International Astronautical Congress), Glasgow, Scotland, UK, Sept. 29 to Oct. 3, 2008, IAC-08.B4.6.B8
15) Information provided by Freddy M. Pranajaya of UTIAS/SFL, Toronto, Canada.
16) Mohamed Ali, Stephen Mauthe, Freddy Pranajaya, “CNAPS: A Novel Propulsion System for the CanX-4 & CanX-5 Nanosatellite Mission,” 27 ISTS (International Symposium on Space Technology and Science), Tsukuba, Japan, July 5-12, 2009, paper: 2009-t-02
17) B. Johnston-Lemke, R. E. Zee, “Attitude Maneuvering Under Dynamic Path and Time Constraints for Formation Flying Nanosatellites,” Proceedings of the 24th Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 9-12, 2010, SSC10-XI-6
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.
The CanX series: