VELOX-II is the second nanosatellite built by the NTU/SaRC (Nanyang Technological University/Satellite Research Center), Singapore. The primary mission of VELOX-II is to demonstrate the IDRS (Inter- satellite Data Relay System) and the secondary mission objective is to perform radio occultation experiments providing more data to assist the VELOX-CI mission, an equatorial orbit climate research microsatellite. 1) 2) 3)
The mission objectives are:
• To design, build and launch the first 6U CubeSat (a nanosatellite) in the world with an intersatellite communication capability between a LEO (Low Earth Orbit) and a GEO (Geostationary Earth Orbit) satellite.
• To demonstrate data downlink anywhere anytime in orbit without passing the ground station.
• To demonstrate precision navigation and fault tolerant electronics.
The project is part of NTU's Undergraduate Satellite Program, which provides an opportunity for engineering students to participate in a multidisciplinary hands-on space project. VELOX is a Latin word meaning swift (or quick, speedy).
Figure 1: Illustration of the deployed VELOX-II nanosatellite (image credit: NTU/SaRC)
VELOX-II is an advanced experimental nanosatellite with 6U CubeSat dimensions. It carries an advanced intersatellite communication payload that allows VELOX-II to perform data downlink anytime anywhere in the orbit even when it is not in line of sight of the ground station. This project is a collaboration of NTU/SaRC with Addvalue Innovation Pte Ltd.
The satellite structure consists of an aluminum chassis and stainless steel load bearing structures, spring loaded deployment systems and an extendable optical mechanism. Two 6U solar panels are stowed against the satellite side panels during launch and deploy in orbit to deliver power to the satellite along with two fixed 3U solar panels. The six arrays with a 2 series, 5 parallel cell layout can deliver a peak power of 40.8 W, while the smaller panels generate up to 6 W of electrical power. Power is stored in a 11.6 Ah battery delivering a nominal bus voltage of 7.2 V. 4) 5)
VELOX-II is a 3-axis stabilized satellite. It uses reaction wheels for attitude stabilization, sun tracking and target pointing purposes. For a three axes stabilized satellite, the most commonly used attitude control actuators for LEO satellites are reaction wheels and magnetorquers. The reaction wheels are able to provide a high pointing accuracy performance, but suffer from the drawback of possibly experiencing a gradual increase of their spinning speed (i.e. momentum saturation), due to external disturbances. Momentum saturation occurs when the stored momentum of reaction wheel reaches its maximum limit. The saturation of momentum storage results in the satellite losing its three-axis stabilization capability.
Table 1: Summary of spacecraft parameters
PSS (Power Supply Subsystem): The PPS in VELOX-II was built in-house to meet the mission requirements and is based on the PSS of VELOX-I nanosatellite launched and operated successfully in 2014. Figure 2 shows the VELOX-II PSS flight hardware stacked up together with different subsystems. The PSS consists of a solar array module, a battery module and 2 power conversion modules operating in parallel to support the primary communication payload with a peak power consumption of 25 W. 6)
The solar array module consists of two deployable 6U solar panels with a peak power of 40.8 W and two fixed 3U solar panels with a peak power of 12 W. The solar array module power is optimized by a peak power tracker and is used to charge the battery module and supply to the satellite bus.
The battery module consists of 4 lithium ion batteries connected in 2 series and 2 parallel (2s2p) configurations to achieve a nominal voltage of 7.2 V and a capacity of 11.6 Ah. The battery module is one of the critical components that determine the mission lifespan. It is recommended that the battery depth of discharge should not be more than 25% in order to prolong the battery life. The high depth-of-discharge accelerates irreversibly ageing factor. Since the lithium-ion battery is sensitive towards overcharging and over discharging, an overcharging protection system and an inrush protection circuit have been incorporated to protect the batteries.
To optimize battery charging in VELOX-II, a modified peak power point tracking algorithm with controlled battery current has been implemented. In the modified algorithm, the solar panel is operated at a point that gives sufficient energy to the system while maintaining the desired charging current instead of the solar panel maximum power point. In the event when the satellite bus requires additional power, the algorithm will adjust the operating point to meet the additional power requirement but the battery current remains the same.
With the controlled charging current in the PSS, the charging is disabled when the battery voltage reaches 4.05 V. This is important as lithium-ion suffers from stress when keeping a cell at a high charge voltage as well. Moreover, the battery operating temperature affects the battery performance. As such, the operation temperature needs to be kept above 0ºC in order to ensure proper performance.
Figure 2: VELOX-II PSS (Power Supply System) hardware (image credit: NTU/SaRC)
For the power distribution module, the VELOX-II PSS employs a centralized converter topology. One high power converter is used to supply power to all the subsystems and payloads through different distribution switches. In addition, each distribution switches are protected with over-current protection. The VELOX-II PSS topology is shown in Figure 3.
Figure 3: VELOX-II PSS centralized topology (image credit: NTU/SaRC)
Figure 4: Internal view of the VELOX-II nanosatellite (image credit: NTU/SaRC)
Figure 5: Photo of the VELOX-II nanosatellite with the solar panels deployed (image credit: NTU/SaRC)
Launch: The VELOX-II nanosatellite was launched as a secondary payload on December 16, 2015 (12:30:00 UTC) on a PSLV-C29 vehicle of ISRO from SDSC (Satish Dhawan Space Center) at Sriharikota on the east coast of India. 7) 8)
The primary payload on this flight was TeLEOS-1, a commercial imaging minisatellite (400 kg) of AgilSpace, Singapore. A contract between ST Electronics AgilSpace of Singapore and Antrix Corporation was signed in Feb. 2014. 9)
The secondary payloads on this flight were:
• VELOX-C1, a minisatellite of NTU (Nanyang Technological University), Singapore.
• VELOX-II, a 13 kg 6U CubeSat of NTU, Singapore. A technology demonstration mission for intersatellite communication.
• Kent Ridge 1, a microsatellite (78 kg) of NUS (National University of Singapore) for Earth observation.
• Galassia, a nanosatellite (2U CubeSat, 2 kg) of NUS, Singapore with the objective to acquire TEC (Total Electron Count) data in the ionosphere.
• Athenoxat-1, a 3U CubeSat (technology demonstration) of NTU, Singapore.
Figure 6: The TeLEOS 1 Earth observation satellite (top) and the PSLV's secondary payloads are pictured during launch preparations (image credit: ISRO)
Orbit: Near-equatorial orbit, altitude of 550 km, inclination of ~15º, period of ~96 minutes.
At the end of the launch phase, ISRO performed several restart and cutoff tests of the fourth stage, at approximately 67.5 minutes after launch. It was the first time the agency conducted these tests. It is hoped that this technique will allow the country to launch several satellites at different orbits. ISRO could use this capability during the space agency's 2016 launch manifest. 10)
• The VELOX-II nanosatellite has been operating daily since its launch in December 2015. — As of April 2017, VELOX-II has been operating for 1 year and 5 months, executing more than 7000 ground passes in the Singapore NTU ground station. The satellite has achieved its intended mission duration of 12 months, and completed all the designed mission goals: Inter-satellite communication, fast GPS tracking algorithm, and fault tolerant electronics have been successfully executed and continue to operate till today. 11)
• Feb. 2, 2017: Addvalue (Addvalue Technologies Ltd.) of Singapore and Inmarsat PLC, the leading provider of global mobile satellite communications, have signed a MOU (Memorandum of Understanding) to offer a data relay service to meet the communications needs of the LEO (Low Earth Orbit) satellite market. 12)
- The new service, designed to address this issue, will be based on Addvalue's space tested IDRS (Inter-Satellite Data Relay System) terminal and Inmarsat's established GEO (Geosynchronous Earth Orbit) I-4 satellite-based BGAN (Broadband Global Area Network) network.
• December 20, 2016: Subsequent to the successful launch of the VELOX-II satellite on 16 December 2015, Addvalue Innovation Pte Ltd (Addvalue), a wholly-owned subsidiary of Singapore's Mainboard-listed company, Addvalue Technologies Ltd, announced that its IDRS (Inter-Satellite Data Relay System) terminal, designed, built and tested in Singapore, has successfully completed 1 year of on-orbit testing with all primary objectives met. These tests clearly demonstrate that IDRS based communications can significantly improve the operation of LEO (Low Earth Orbit) satellites. The Addvalue IDRS terminal communicated over Inmarsat's proven, high reliability BGAN (Broadband Global Area Network), which operates exclusively from geostationary orbits, and operated in space aboard the VELOX-II satellite platform built under contract by NTU(Nanyang Technological University), Singapore. 13)
- Addvalue's IDRS is an innovative new service that addresses a long standing constraint on the operation of LEO satellites. Currently, communications with LEO satellites is only available when the satellite is within sight of an earth station. Further, this limited connectivity is available only on a rigid time schedule based on the particular LEO satellite orbit and the geographic placement of the earth stations. Thus LEO satellite operators must contend with communication links that are not available on a 24/7 basis. Since the launch of its IDRS terminal aboard the VELOX II satellite on 16 December 2015, Addvalue has demonstrated the technical feasibility of IDRS, its new LEO satellite link, to provide high capacity on-demand 24/7 two-way IP-based data services for LEO satellite missions.
- Tan Khai Pang, Chief Operating and Technology Officer of Addvalue noted that "It is great news that real-time bi-directional data sessions have been repeatedly demonstrated in the first-ever LEO-GEO-Ground data relay link operated over the Inmarsat I-4 GEO satellite constellation. It is equally significant that the hardware, designed without using any export-control components, has stood the test of the space environment for over a year and is still operating well." With the experience and insights gathered from this space heritage, our team is ready to further improve and "space-harden" our IDRS design to support space missions in a commercial LEO satellite operation in the foreseeable future. "
- Inmarsat is similarly excited about the IDRS development. Peter Dingley, Vice President Future Government Technologies, Inmarsat Global Government business unit, commented: "The flexibility and reliability of our satellite networks makes them ideal to support LEO missions. Our networks are already trusted by Governments and commerce for safety of life and mission critical communication across the globe. Enabling real time command, control and data links to LEO satellites from our GEO constellation provides them a unique opportunity to increase their value proposition".
- "The success of our IDRS experiment is a huge tribute not only to our team but also to our partners especially Inmarsat and NTU. What we must now do is to bring the design to commercial readiness as it will also open up many uncharted market opportunities for LEO satellite operators and the like," added Dr Colin, Chairman and CEO of Addvalue.
Figure 7: Illustration of Addvalue IDRS in VELOX-II orbiting around the earth (image credit: Addvalue)
• On December 16, 2016, VELOX-II was 1 year on orbit - operating nominally and carrying a commercial payload. 14)
• May-June 2016: The in-flight data of the initial 5 orbits was used to evaluate the PSS performance in the LEOP stage. Figure 8 shows the battery cell voltage and current, tracked solar power and power consumption by the satellite in the first 5 orbits respectively. In these orbits, the satellite is operating in NOP (Normal Operation Mode). The PSS is required to recover the battery charge and maintain the battery temperature above 0°C (Ref. 6).
Figure 8: (a) Battery cell voltage; (b) battery current in first 5 orbits (image credit: NTU/SaRC)
- Communication payload (PAYL) mission: Similarly, the in-flight data of PAYL (Payload Mission Mode) was downloaded to analyze the performance of PSS during the payload mission. In the payload mission, the intersatellite data relay experiment was performed. In conventional satellite operation, the radio communication between a low earth orbit satellite and the ground station is only available if they are within the line of sight. The data relay experiment allows VELOX-II to maintain communications anytime by relaying the signal via a second satellite in a higher orbit that is visible by both VELOX-II and the ground station. In such a mission, the VELOX-II satellite performs satellite pointing instead of sun pointing. Thus, the available solar power of the deployable solar panel is affected. There are two different mission pointing mode: target pointing and nadir pointing. For the payload mission, the target pointing is performed by pointing the VELOX–II to the higher orbit satellite. Figure 9 shows the difference of sun pointing and target pointing mode.
- VELOX-II is among the first few 6U nanosatellites successfully operating in the orbit. Using the in-flight data downloaded from VELOX-II, the performance of PSS has been evaluated. The results show that the PSS of VELOX-II is performing well according to the design and meet the mission objectives. The heater in the PSS has maintained the temperature of the battery module above 7ºC (Ref. 6).
Figure 9: Satellite pointing mode: (a) sun pointing (b) target pointing (image credit: NTU/SaRC)
• April 23, 2016: The VELOX-II mission has demonstrated IDRS (Inter-satellite Data Relay System) operations and detected "fault tolerant payloads" a number of times. The project is conducting more tests with the mission collaborator, Addvalue Innovation Pte Ltd., to study the IDRS characteristics. 15)
• Feb. 4, 2016: The VELOX-II is designed and built by NTU to test three unique technologies for small satellite systems, including: 16)
1) Fast GPS tracking algorithm: it can determine VELOX-II's position accurately within a minute.
2) On-demand-communication: the main mission of VELOX-II is to send data back to NTU, relayed via a higher orbiting satellite while flying on the other part of Earth. This proprietary technology, known as the IDRS (Inter-satellite Data Relay System), is owned by Addvalue Innovation Pte Ltd and was integrated into the satellite by NTU.
3) Radiation resistant chip: the VELOX-II has a chip specially designed to resist the impact of radiation in space. This chip manages and protects the critical data stored in the satellite's memory.
In addition, VELOX-II has been broadcasting a beacon signal every minute while orbiting in space. This unique beacon contains the satellite name and its status, such as the operating mode, battery levels, temperature, etc. This signal can be received by any amateur radio operator within the line of sight of the satellite.
Sensor complement: (Data Relay Terminal, GPS Receiver, Fast GPS Tracking Algorithm)
Data Relay Terminal
The VELOX-II nanosatellite carries the proprietary data relay terminal of Addvalue Technologies Ltd. as its primary payload. Addvalue's data relay terminal is an essential component of its planned IDRS (Intersatellite Data Relay System). Addvalue's IDRS is an innovative new service that addresses a long standing constraint in the operation of LEO (Low Earth Orbit) satellites. Currently, communication with LEO satellites is only available when the satellite is within sight of an earth station. Further, this limited connectivity is available only on a rigid time schedule based on the particular LEO satellite orbit and the geographic placement of the earth stations. Thus LEO satellite operators must contend with communication links that are not available on a 24/7 basis. With the launch of the VELOX-II satellite, Addvalue will be able to demonstrate the technical feasibility of IDRS, its new LEO satellite link, to provide high capacity on-demand 24/7 two-way IP-based data services for LEO satellite missions. 17)
The primary objective of the VELOX-II nanosatellite is the demonstration of an intersatellite communications system for the transmission of data to satellites in GEO (Geostationary Orbit) so that smaller LEO spacecraft can deliver data in real time.
GPS Receiver Module
The fault-tolerant GPS system can determine accurately the position of VELOX-II. The objective is to demonstrate the EEPROM data recovery when a single event has been occurred on the memory and corrupts the data. In order to facilitate the operation of the VELOX-II GPS payload, the operation flowchart is shown in the Figure 10. All the setting parameters and commands for the GPS payload mission are included in the PID binary file so that NTU in-house developed GPS payload can be operated independently to match different satellite bus system. 18)
Figure 10: VELOX-CI and VELOX-II GPS payload operation (image credit: NTU/SaRC)
Figure 11: Photo of the GPS payload (image credit: NTU/SaRC)
Fast GPS Tracking Algorithm
A secondary objective of the mission is to demonstrate precision navigation and fault tolerant electronics.
1) M.S.C. Tissera, Y.T. Xing, K.S. Lowt, S.T. Goh, "An efficient momentum dumping method through an alternative sun pointing strategy for small Near Equatorial Orbit satellite," Proceedings of the 66th International Astronautical Congress (IAC 2015), Jerusalem, Israel, Oct.12-16, 2015, paper: IAC-15- C1.5.3
2) "NTU gearing up for two new satellites," NTU, Nov. 25, 2014, URL: http://media.ntu.edu.sg/NewsReleases/Pages/newsdetail.aspx?news=782a2526-36d3-4222-ad93-2d43b75ecc46
3) "Satellite Research Center- SaRC," NTU, URL: http://www3.ntu.edu.sg/home/ekslow/SaRC%20fact%20sheet.pdf
5) "VELOX-II Nanosatellite," NTU, URL: http://www.sarc.eee.ntu.edu.sg/Research/Projects/Pages/VELOX-II.aspx?print=1
6) Jia Min Lew, Htet Aung, Jing Jun Soon, Kay-Soon Low, "In-flight Data Studies of VELOX-II Power Supply System," Proceedings of the 4S (Small Satellites, System & Services) Symposium, Valletta, Malta, May 30-June 3, 2016, URL: http://congrexprojects.com/docs/default-source/16a02_docs/4s2016_final_proceedings.zip?sfvrsn=2
7) "NTU successfully launches its fifth and sixth satellites," NTU, Dec. 17, 2015, URL: http://media.ntu.edu.sg/NewsReleases/Pages/newsdetail.aspx?news=3284b499-56bd-45f8-a3e1-339d90e30540
8) "India to launch 6 Singaporean satellites," Space Daily, Dec. 14, 2015, URL: http://www.spacedaily.com/reports/India_to_launch_6_Singaporean_satellites_999.html
9) "ST Electronics, Antrix, ATK, Satrec Initiative + SPOT Asia—The TeLEOS-1 Adventure Is Underway (Satellite—Launch Preparations)," Satnews, Feb. 6, 2014, URL: http://www.satnews.com/story.php?number=1002365176
10) Tomasz, Nowakowski, "India's PSLV rocket puts six Singapore satellites into orbit," Spaceflight Insider, Dec. 16, 2015, URL: http://www.spaceflightinsider.com/organizations/isro/indias-pslv-rocket-puts-six-singapore-satellites-orbit/
11) Information provided by Lim Wee Seng, Director of the Satellite Research Center, NTU (Nanyang Technological University), Singapore, and by Lester Kok of NTU.
12) "Addvalue and Inmarsat sign MoU to provide on-demand communications links to enhance LEO satellite operations," Addvalue News Release, Feb. 2, 2017, URL: http://www.addvaluetech.com/misc/AVT-PR_Addvalue_and_Inmarsat_signed_MOU_020217.pdf
13) "Addvalue's Inter-Satellite Data Relay Terminal completes one year of in-orbit testing," Addvalue Technologies Ltd, Press Release, Dec. 20, 2016, URL: http://www.addvaluetech.com/misc/AVT_PR-One_Year_of_IDRS_Testing_20Dec16.pdf
14) "Two NTU satellites pass first year with flying colors," NTU, Dec. 25, 2016, URL: http://www.sarc.eee.ntu.edu.sg/NewsnEvents/Pages/News-Detail.aspx?news=8cf2e83b-c3db-4296-87aa-044473654e65
15) Information provided by Dr. Kay-Soon Low, Associate Prof. at NTU (Nanyang Technological University), Singapore.
16) "New made-in-NTU satellite technologies pass space tests," NTU, Feb. 4, 2016, URL: http://phys.org/news/2016-02-made-in-ntu-satellite-technologies-space.html
17) "Proprietary LEO satellite Data relay terminal developed by Addvalue successfully launched into the orbit on the VELOX-II satellite," Addvalue Technologies Ltd. News Release, Dec. 2015, URL: http://www.addvaluetech.com/misc/AVT-Press_Release_proprietary_data_relay_terminal_on_Velox-II_launched_successfully_171215.pdf
18) Yung-Fu Tsai, Shi-Tong Chin, Guo-Xiong Lee, Shu-Ting Goh, Kay-Soon Low, "Hardware-in-the-loop Validation of GPS/GNSS Based Mission Planning for LEO Satellites," International Symposium on GNSS 2015, Kyoto, Japan, Nov. 16-19, 2015, URL: http://tinyurl.com/j8v5rzb
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 (firstname.lastname@example.org).