VELOX-CI is a minisatellite of NTU (Nanyang Technological University) of Singapore in collaboration with DSO (Defence Science Organization) National Laboratories, Singapore. The objective is to develop a follow-on microsatellite with a tropical environmental monitoring mission. The aim of the project is: 1) 2) 3) 4) 5)
1) To further enhance Singapore's capability in the development of space hardware using the mission as a vehicle to develop technology and manpower.
2) To focus the effort at NTU on a mission which will have high scientific impact both locally and internationally. The satellite will carry a GRO (GPS Radio Occultation) instrument to provide high accuracy vertical resolution atmospheric sounding parameters for tropical weather monitoring.
The scope of VELOX-CI program is to develop a 3D atmospheric measurement payload comprised of a GPS occultation module and radio frequency probe, hybrid attitude determination system with GPS measurement, and a real-time star tracking algorithm for attitude determination.
The VELOX-CI program started at the end of 2012, and the NTU research team has completed the design and implementation of radio occultation payload EQM (Engineering Qualification Model) by the end of 2013. In August 2014, the VELOX-CI project has completed the satellite EQM integration and test. The VELOX-CI project is funded by EDB (Economic Development Board) of Singapore.
Nanyang Technological University has 33,500 undergraduate and postgraduate students in the colleges of Engineering, Business, Science, Humanities, Arts, & Social Sciences, and its Interdisciplinary Graduate School.
Figure 1: Overview of NTU/SaRC satellite missions (image credit: NTU/SaRC)
The VELOX-CI is a minisatellite with a mass of 123 kg and a size of 61.5 x 60.8 x 84.8 cm. It features 3 solar panels, one body mounted panel on one of its larger side panels and two deployable solar panels that are hinged on the side panel with the body-mounted solar cells. In total, the satellite hosts 24 solar cells connected in 10 strings on the three panels feeding power to a 20Amp-hour (End of Life) battery. Thermal control uses heaters and external radiators. 6) 7)
ADCS (Attitude Determination and Control Subsystem): A hybrid ADCS is employed by the satellite comprised of a conventional system that employs a pair of star trackers, sun sensors and an IMU (Inertial Measurement System). The experimental attitude determination system is based on a GPS payload that hosts three antennas. Pairs of antennas are used to record the carrier signal from a GPS satellite and calculations can then be made taking into account the known path difference and baseline vectors of the two antennas in an equation including the unit vector to the GPS satellite and the attitude matrix from reference to body frame.
Based on pseudorange and double difference measurements, this system can achieve an attitude determination accuracy of around one degree, however, common visible satellites within the field of view of both antennas rapidly change and lead to frequent disruptions of the system. Also, the typical 1Hz update rate of the GPS receivers may not be sufficient for operational use. Coupled to a three-axis fiber optic gyro, the GPS attitude determination system can overcome these challenges and achieve a usable accuracy better than 0.4º. The ADCS has an accuracy better than 0.2º and attitude control is achieved with reaction wheels and magnetic torquers pointing the satellite to within 2 º of the commanded orientation.
Figure 2: Schematic view of GPS-based attitude determination (image credit: NTU)
RF communications are accomplished in S-band for telemetry downlink and command uplink at a data rate of 32 kbit/s while payload data is downlinked via a high speed X-band link at 160 Mbit/s.
Figure 3: Illustration of the deployed VELOX-CI minisatellite (image credit: NUS)
Table 1: Overview of spacecraft parameters
Launch: The VELOX-CI minisatellite 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. 8) 9)
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. 10)
The secondary payloads on this flight were:
• VELOX-C1, a minisatellite of NTU (Nanyang Technological University), Singapore.
• VELOX-2, 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 4: 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.
The satellite will pass by NTU mission control center about 15 times per day and will be controlled by the NTU ground station. Data collection will mainly be downloaded to NTU ground station or via the CRISP (Center for Remote Imaging, Sensing and Processing) ground station at NUS (National University of Singapore), Singapore.
• On December 16, 2016, VELOX-C1 was 1 year on orbit - operating nominally. The minisatellite takes climate measurements such as atmospheric temperature, humidity and pressure – and evaluates a new, precise navigation system. 11)
• April 23. 2016: The VELOX-CI initial mission demonstrated that radio occultation events can be logged all the way from 500 km to 4 km above sea level. The project has also successfully conducted navigation experiments, attitude determination using GPS signals, star tracker experiments, etc. 12)
• Feb. 04, 2016: The two newest NTU satellites are the VELOX-CI, Singapore's first climate monitoring satellite; and VELOX-II, the world's first small satellite which carries a "communication-on-demand" technology which can send data back to NTU anytime and anywhere in space. Since its launch, VELOX-C1 has flown over Singapore more than 750 times in the last 50 days and successfully tested different NTU-designed technologies, including: 13)
1) Precision GPS (Global Positioning System): NTU's GPS receiver can receive dual GPS frequency signals, giving it an accuracy of 20 cm.
2) Ionosphere measurements: Using the GPS receiver, algorithms have been developed to study the ionosphere (a region of Earth's upper atmosphere) so scientists can predict how much impact space weather can have on ground-to-satellite communication and satellite navigation.
3) Radio Occultation: an advanced technique used for climate study. VELOX-CI can detect GPS signals even without a direct line-of-sight to the GPS satellites. By measuring the bending of GPS signals through the atmosphere, data on atmospheric temperature, humidity and pressure used in long term climate studies can be obtained. Preliminary data shows that it is possible to obtain atmospheric data ranging from a few kilometers up to a few hundred kilometers.
4) Star tracker navigation software: the satellite can determine its orientation just by looking at the stars in space. The NTU team has collected hundreds of star images and constellations and added them into their database, which will be used to validate newer advanced algorithms.
Sensor complement: (GRO, RF probe)
GRO (GPS Radio Occultation) instrument
GRO is the primary sensor of the satellite, making use of GPS occultation measurements to determine temperature, pressure and humidity profiles of Earth's atmosphere for application in operational meteorology and climate research.
The GRO instrument consists of GPS receivers, able to track the signals of several MEO satellites and measure the Doppler shift and bend angle of signals that travel through the atmosphere located in the line of sight of the two spacecraft. These phase delay measurements, due to refraction by the atmosphere, can be made from the satellite altitude to very close to the surface leading to precise information on the properties of the atmosphere at an accurate vertical resolution. It takes about 60 s for the satellite to move sufficiently for the signal to traverse the atmosphere from ground level to ~50 km altitude.
Figure 5: Photo of the VELOX-CI GRO (GPS Radio Occultation) payload (image credit: NTU)
Figure 6: Schematic of occultation measurement configuration (image credit: NTU, Ref 7)
RF (Radio Frequency) probe
VELOX-CI also hosts an experimental RF probe.
1) Yung-Fu Tsai, Kay-Soon Low, "GPS Radio Occultation for Tropical Space Weather Application," Proceedings of the 65th International Astronautical Congress (IAC 2014), Toronto, Canada, Sept. 29-Oct. 3, 2014, paper: IAC-14-B1.3.9
2) Yung-Fu Tsai, Guo Xiong Lee, Kay Soon Low, "VELOX-CI: Advanced Application of GPS for Radio Occultation and Satellite Attitude Determination," Singapore Space Symposium 2015, Sept. 30, 2015, NTU, Singapore, URL: http://www.sarc.eee.ntu.edu.sg/NewsnEvents/Documents/VELOX-CI_20150929.pdf
3) "Singapore Space Symposium 2015," NTU, URL: http://www.sarc.eee.ntu.edu.sg/NewsnEvents/Pages/SSSProgram.aspx
4) "Satellite Research Center- SaRC," NTU, URL: http://www3.ntu.edu.sg/home/ekslow/SaRC%20fact%20sheet.pdf
5) "NTU engineering students to train with real satellites in space," NTU News Release, April 20, 2015, URL: http://media.ntu.edu.sg/NewsReleases/Pages/newsdetail.aspx?news=b89f8b9c-0195-48f3-88e2-54e60acf6ee6
7) "VELOX-CI Tropical Climate Satellite," NTU, Nv. 20, 2015, URL: http://www.sarc.eee.ntu.edu.sg/Research/Projects/Pages/VELOX-CI.aspx?print=1
8) "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
9) "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
10) "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
11) "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
12) Information provided by Dr. Kay-Soon Low, Associate Prof. at NTU (Nanyang Technological University), Singapore.
13) "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
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).