Hardware-in-the-loop Validation of GPS/GNSS Based ...

International Symposium on GNSS 2015 International Symposium on GNSS 2015 Kyoto, Japan. Kyoto, 16-19, Japan.2015. November September 16-19, 2015.

Hardware-in-the-loop Validation of GPS/GNSS Based Mission Planning for LEO Satellites Yung-Fu Tsai*, Shi-Tong Chin, Guo-Xiong Lee, Shu-Ting Goh and Kay-Soon Low ** Abstract GPS satellite navigation system is an all-weather and continuous coverage system with high precision. It has gradually become an important subsystem in the orbit determination for low earth orbit (LEO) satellites. In addition to orbit determination, some LEO satellites carry GPS receivers for scientific research. Owing to the success of X-SAT and VELOX programs (VELOX-I, VELOX-PII, VELOX-PIII), NTU is building two new satellites named VELOX-CI and VELOX-II, which will be readied for launch in the fourth quarter of 2015. VELOX-CI is a 130-kilograme microsatellite that carries a GPS receiver payload to perform radio occultation for atmospheric sounding purpose. It is to be launched into a near equatorial orbit leading to a high revisit rate over Singapore providing significantly more data in the tropical region for long term weather studies. The other satellite, VELOX-II, is a 12 kg nanosatellite for conducting satellite-based communication experiments. In addition to communication mission, VELOX-II also carries a GPS receiver payload for precise orbit determination experiment. Since both the satellites carry out GPS based missions, a mission planning software has been developed based on the satellite configuration and requirement for the hardware-in-the-loop validation. Some unique features of the design would be highlighted in this presentation. Keyword GPS/GNSS, Hardware-in-the-loop, LEO mission planning

1.

Introduction

In the beginning, the GPS system was designed for near earth positioning and navigation. The primary role of GPS is to provide 24-hour, all weather, worldwide coverage PVT (position, velocity, time) information. Since the first space-borne GPS receiver was testified on LANDSAT5 satellite in 1984, it has gradually become an important subsystem in the orbit determination for low earth orbit (LEO) satellites [1]. In addition to orbit determination, some LEO satellites carry space-borne GPS receivers as a payload for scientific research and technology demonstration, such as radio occultation (RO) and attitude determination. Since 2011, satellite research centre (SaRC) of NTU has completed and launched a number of micro/nano/pico-satellites. Those satellite missions that have been completed by SaRC include X-SAT (a 106kg remote sensing micro-satellite in collaboration with DSO), VELOX-PI and VELOX-PII (technology demonstration 1.3kg pico-satellite built by students), VELOX-PIII (a pico-satellite piggyback inside VELOX-I) and VELOX-I (a 4.5kg technology demonstration nanosatellite). Due to the success of X-SAT and VELOX programs, NTU is presently building two satellites that will be launched into space in the last quarter of 2015 – Singapore’s first climate research satellite and experimental communication satellite: a. VELOX-CI: a 123kg microsatellite for climate study in collaboration with DSO. b. VELOX-II: a 10kg nanosatellite demonstrating satellite based communication system.

*

Satellite Research Centre (SaRC), School of Electrical and

Electronic

Engineering

(EEE),

Nanyang

Technological

University (NTU), Singapore, [email protected] Associate Professor and SaRC Director, School of EEE, NTU Singapore, [email protected] **

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In addition, a third nanosatellite named AOBA/VELOX-III in collaboration with Kyushu Institute of Technology (Kyutech) which carries a pulse plasma thruster will be launched by Japan's national agency, the Japan Aerospace Exploration Agency (JAXA) in 2016. Since both VELOX-CI and VELOX-II will carry out GPS based missions, the self-developed mission planning software for LEO GPS mission hardware-in-the-loop validation will be described in this paper.

2.

NTU Velox-CI & Velox-II satellites

2.1 VEXLO-CI X-SAT, the first Singapore-built satellite, has marked four years in space on 20 April 2015. Owing to the success of the XSAT program, NTU is collaborating with DSO National Laboratories, Singapore to develop a follow-on microsatellite with a tropical environmental monitoring mission, VELOX-CI. The aim of VELOX-CI project is two-fold: firstly it is to further enhance Singapore’s capability in the development of space hardware using the mission as a vehicle to develop technology and manpower. The second is to focus the effort at NTU on a mission set which will have high scientific impact both locally and internationally. For VELOX-CI project, a 135kg microsatellite would be developed for technology demonstration of space-borne tropical environmental monitoring. The scope of VEXLOX-CI mission is to develop a 3D atmospheric measurement payload comprises of a GPS radio occultation receiver module and radio frequency probe, hybrid attitude determination system with GPS measurement, and real-time star tracker algorithm for attitude determination. The overall mission operations concept is illustrated in Fig. 1 [2]. VELOX-CI is scheduled to be launched together with TeLEOS-1 microsatellite built by ST Electronics (Satellite Systems) Pte Ltd, as well as other four CubeSats, into an orbit of around 550 km in the end of 2015. Since all six satellites are from Singapore, this satellite launch mission will be optimized for near equatorial orbit (15° inclinations) to have high revisit

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International Symposium on GNSS 2015 Kyoto, Japan. September 16-19, 2015.

rates over tropical regions (including Southeast Asia). Since the high revisit rate will enable the environmental monitoring to occur at a 100 minutes interval, the satellite will pass by NTU mission control centre 15 times per day for satellite control and operation. Data collection mainly would be downloaded to NTU ground station for further processing and analysis. Besides, the CRISP (Centre for Remote Imaging, Sensing and Processing) ground station at NUS, Singapore will be the backup ground station for VELOX-CI frequently operation.

images based on star catalogue are used for existing research, the star images of VELOX-CI onboard star tracker will be download and used by NTU research team for the testing of real time star identification and star tracking algorithm (Fig. 5). The in-orbit experimental results will then be benchmarked with the satellite attitude to confirm its accuracy and performance.

Fig. 3 VELOX-CI onboard GPS antenna configuration

Fig. 1 VELOX-CI mission overview The VELOX-CI project was kicked off at the end of 2012. NTU research team has completed the design and implementation of radio occultation payload engineering qualification model (EQM) and flight mode (FM) in the end of 2013 and 2014, respectively (Fig. 2). In addition, VELOX-CI project has completed the satellite EQM/FM integration and test in August 2014 and February 2015, respectively.

Fig. 4 VELOX-CI onboard star tracker system

Fig. 5 Star pattern recognition and attitude determination for VELOX-CI Fig. 2 VELOX-CI radio occultation payload In addition to radio occultation mission, NTU research team used the same payload receivers with additional GPS antennas to conduct GPS based attitude determination experiment. Because the error of GPS based attitude determination would not accumulate over time, it has been given great attention in the past two decades [3]. Generally speaking, two antennas form a single baseline and the double difference method could be performed for ambiguity resolution and the solution of single baseline vector. As illustrated in (Fig. 3), GPS based attitude determination could be conducted with two baselines vectors formed by three GPS antennas. Since the baseline is very short (26.7 cm), only coarse accuracy of attitude determination could be expected [4][5]. The star tracker is an optical-electronics device for satellite three-axes attitude estimation in space. Since it is the most accurate sensor for attitude estimation, VELOX-CI is designed to carry two star trackers for the requirement of the satellite attitude (Fig. 4). NTU research team has been working on a new novel star tracking algorithm. Since only simulated star Journal of IPNT, Vol.1 No.1 pp.1-8

2.2 VELOX-II VELOX-II is a 6U experimental communication nanosatellite (Fig. 6) that is developed by Nanyang Technological University, with the primary payload is developed by Addvalue Technologies Ltd, Singapore. Since VELOX-II will be launched together with VELOX-CI into near equatorial orbit, there are approximately 14 to 15 ground passes per day for ground communication purpose. The VELOX-II ground control is primarily conducted through VHF (at 145.930Mhz) and UHF (at 437.255Mhz). It has two deployable solar panels that provide approximately 41W of power to satellite system. Additional two 3U fixed solar panels are installed to ensure the satellite is capable of harvesting solar energy during random tumbling scenario. In addition, VELOX-II is a three-axes stabilized satellite. The satellite rotation is maintained using the three-axes reaction wheel. Due to the reason that low earth magnetic field variation in equatorial region results in a lower efficiency in reaction wheel momentum dumping, an improved momentum dumping method in [6] is used to prevent the reaction wheel saturation.

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signal path, which is shown in Fig. 7. The fundamental relation to be solved in RO processing is to deal with the excess phase, which is the residual of the carrier phase measurement corrected for geometrical range between LEO and GPS satellites, receiver and GPS satellite clock offset, and relativistic effect. If a high accuracy excess phase can be obtained, then the Earth’s atmospheric properties can be estimated, such as temperature, pressure, and humidity in the troposphere, and electron density in the ionosphere [7]. From GPS Satellite

LEO

Fig. 6 VELOX-II flight model The primary mission of VELOX-II is to demonstrate the intersatellite (or satellite relay) communication between LEO satellite and GEO satellite while the satellite is out of ground station communication range. The successfulness of the demonstration will allow satellite or mission data to be available for downloading at any time, disregarding the location of the satellite itself. Furthermore, it also allows the satellite to be control from ground at any time period. It does not require the satellite to be within the ground antenna communication range. The VELOX-II also hosts two secondary payloads, which are GPS receiver module and a fault tolerant memory payload. The radio occultation mission will also be conducted through the GPS receiver module. The fault tolerant payload will demonstrate the EEPROM data recovery when a single event has been occurred on the memory and corrupts the data. The specification and onboard payloads of VELOX-II nanosatellite are list in Table 1. Table 1 VELOX-II specification Near equatorial orbit, altitude 550 km 1 year 340.5 mm x 246 mm x 120 mm (stowed) 8.93kg Fine & coarse sun sensors 2 inertial measurement units 3 * magnetic torquers 3 * reaction wheels 2 deployable 6U panels total 40.8W 2 fixed 3U panels total 6W (backup) 11.6 Ah @ 7.2 V nominal VHF - 145.930Mhz UHF - 437.255Mhz Intersatellite communication terminal GPS receiver module payload Fault-tolerant electronics

Solar Panels Battery COMM Payloads

The distribution of RO profiles depends on the observing satellite, the constellation of the signal sources, i.e., GPS/GNSS satellites, as well as observation parameters such as antenna position and beam-width [8]. As VELOX-CI is scheduled to be launched into a near equatorial orbit, a preliminary analysis of VELOX-CI radio occultation mission has been conducted. Based on satellite orbit of 550 km and 10° inclination with GPS constellation, average 600 RO events per day can be obtained (Fig. 8). Those RO soundings are distributed uniformly around tropical region [9]. As both VELOX-CI and VELOX-II carry out GPS radio occultation missions, a mission planning software (MPS) has been developed for the hardware-in-the-loop validation. Moreover, the MPS will also be utilized to efficiently plan both satellites’ GPS radio occultation experiments during the satellite operations. Fig. 9 shows the control panel of the mission planning software where user can input the parameters, such as satellite operation mode, simulation/planning time and period. The output includes RO events table (RO event approximate location and impact height), RO event chart (RO event approximate time), 2D world map display, and 3D visualization. It is mainly coded by Java version 7 software.

80 60 40 Latitude (deg)

Orbit Mission Life Dimension Mass ADCS

Fig. 7 Radio occultation observation scheme

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3.

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Mission Planning Software

Radio occultation technique is that the quantities of atmosphere can be derived from the refractivity along the radio

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-100

-50

0 Longitude (deg)

50

100

150

Fig. 8 Preliminary Analysis of RO events distribution based on VELOX-CI orbit

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International Symposium on GNSS 2015 Kyoto, Japan. September 16-19, 2015.

Fig. 9 VELOX-CI and VELOX-II GPS/GNSS based mission planning software Since the radio occultation events depend the observing satellite and the signal source satellites, the basic input is propagation predications, LEO satellite orbit information, and GPS/GNSS orbit information. The MPS accepts the NORAD two line element (TLE) sets of LEO as well as GPS/GNSS constellation. In addition to the relative geometry of LEO and GPS/GNSS satellites, the field-of-view (FOV) and configuration of the GPS/GNSS antennas are also the inputs. Those parameters will affect the numbers of the line-of-sight GPS/GNSS satellites. In addition to the RO events predication, the prediction of VELOX-CI attitude determination experiment could be conducted at the same time. The MPS also provides the predication of LEO satellite ground contact for scheduling the experiment data downlink. After the selecting of the planned GPS/GNSS mission, the MPS will generate the desired parameters for payload initialization data (PID). In order to facilitate the operation of VELOX-CI and VELOX-II GPS payload, the operation flowchart is shown in the Fig. 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. After the planned mission is completed, the executed mission data will be stored as payload collected data (PCD), such as the planned GPS experiment data and GPS payload hardware status, etc. The setup of the hardware-in-the-loop experiment is show in the Fig. 11. The control PC will base on LEO satellite operation mode to create a user motion file (*.umt) for the Spirent GSS8000 GPS simulator to generate a GPS mission scenario and the generated GPS RF signals will then be injected to VELOX-CI/VELOX-II GPS payload. The control PC will also install MPS to generate planned GPS mission to create the PID file of VELOXCI/VELOX-II GPS payload. In addition to the generation of the planned mission, the PC will simulate the satellite bus communication for transferring the PID file and collecting the PCD data. Then the collected GPS experiment data could be analyzed and validated.

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Fig. 10 VELOX-CI and VELOX-II GPS payload operation

Fig. 11 Hardware-in-the-loop experiment setup

4.

Conclusion

NTU SaRC team is ready to launch VELOX-CI and VELOXII satellites. Both VELOX-CI and VELOX-II carry the GPS receiver payload for radio occultation. The success of this payload will allow the measurement of atmospheric parameters near the equator for long term climate study. Moreover, the GPS payload would enhance the technology and capability of SaRC in LEO GPS/GNSS related mission. Besides the design and construction of the satellites, satellite operation and in-orbit

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experiment after the satellites are launched also formed a very important part of the life cycle of the entire satellite mission. The mission planning software will not only be used for the hardware-in-the-loop validation of prelaunch but also for the inorbit mission operation.

Acknowledgments Authors gratefully acknowledge the funding support from Singapore Economic Development Board. References [1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

H. Bock, “Efficient Methods for Determining Precise Orbits of Low Earth Orbiters Using the Global Positioning System”, Schweizerische Geodätische Kommission, Vol. 65, 2003. Y. F. Tsai and K. S. Low, “GPS Radio Occultation for Tropical Space Weather Application,” 65th International Astronautical Congress, Toronto, 2014. G. Lu, “Development of a GPS Multi-Antenna System for Attitude Determination,” Ph. D Thesis, Department of Geomatics Engineering, University of Calgary, Canada, Jan. 1995. F. X. Cao, K. S. Low, K. V. Ling, E. K. Poh, C. S. Lim, G. X. Lee, and Y. F. Tsai, “A Novel Filtering Mechanism to Improve the Performance of GPS/MEMS Gyro Attitude Determination for NEO Satellite,” ION GNSS+ 2015, Tampa, Florida, 2015. F. X. Cao, K. S. Low, K. V. Ling, E. K. Poh, C. S. Lim, G. X. Lee, Y. L. Ng and Y. F. Tsai, “GPS and Gyro Integrated Attitude Determination System for Low Earth Orbit Earth Pointing Satellite,” Pacific PNT 2015, Hawaii, 2015. M. S. C. Tissera, Y. T. Xing, K. S. Low and S. T. Goh, “An efficient momentum dumping method through an alternative sun pointing strategy for small Near Equatorial Orbit satellite” 66th International Astronautical Congress, Israel, 2015. T. P. Yunck, C. H. Liu, R. Ware, “A History of GPS Sounding, Journal of Terrestrial,” Atmospheric and Oceanic Sciences (TAO), Vol. 11, No. 1, pp. 1-20, 2000. J. C. Juang, Y. F. Tsai, and C. H. Chu, “On constellation design of multi-GNSS radio occultation mission,” Acta Astronautica, doi:10.1016/j.actaastro.2012.04.031, 2013. Y. F. Tsai and K. S. Low, “VELOX-CI – A Tropical Microsatellite for Radio Occultation Experiment,” Pacific PNT 2015, Hawaii, 2015.

Guo Xiong Lee is a research fellow at SaRC, NTU. He received his BE and Ph. D from NTU. His research interests include GNSS navigation algorithm, GNSS attitude determination and micro/nano experimental satellite. Shu-Ting Goh received both B.S and M.Sc degrees from University at Buffalo, USA, and PhD degrees from Michigan Technological University, USA. Currently, he is currently research fellow at satellite research centre at Nanyang Technological University. His research interests include spacecraft attitude determination and navigation, target tracking, nonlinear model parameter estimation and Kalman Filter based applications. Kay-Soon Low received the B.Eng. degree in electrical engineering from the National University of Singapore, and the Ph.D. degree in electrical engineering from the University of New South Wales, Sydney, Australia. He has worked in the academia as well as in the industry. He joined the School of Electrical and Electronic Engineering, Nanyang Technological University in 1994 first as a lecturer and subsequently became an Associate Professor. He has successfully supervised 40 graduate theses and delivered 42 funded projects. He has served as Consultants to many companies and has a number of granted patents on nonlinear circuits and UWB systems. His present funded projects are in the field of sensor and control system, solar energy and satellite development. He has been the centre director of Satellite Research Centre (SaRC), Nanyang Technological University since April 2009. The centre has successfully completed and launched four satellites named X-SAT, VELOX-I, VELOX-PII and VELOX-PIII.

biography Yung-Fu Tsai received the B. S. degree from National Tsing-Hua University, Hsuin-Chu, Taiwan in 2003 and the M. S. and Ph. D. degrees from National Cheng-Kung University, Tainan, Taiwan in 2005 and 2009, respectively. Currently, he is a senior research fellow in satellite research centre at Nanyang Technological University. His research interests include GPS navigation, GNSS navigation processing algorithm, GNSS radio occultation mission, and micro/nano experimental satellite. Shi-Tong Chin received the B.Eng degree in Aerospace Engineering from Nanyang Technological University in 2013. She is presently a research engineer in the satellite research centre at Nanyang Technological University. Her research interests include satellite attitude determination and GPS navigation.

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