Precision Navigation Achieved by ASTRO-H Space-Borne GPS Receiver
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Trans. JSASS Aerospace Tech. Japan Vol. 16, No. 5, pp. 454-463, 2018 DOI: 10.2322/tastj.16.454 Precision Navigation Achieved by ASTRO-H Space-borne GPS Receiver By Yu NAKAJIMA,1) Toru YAMAMOTO,1) Yoshinori KONDOH,2) Koji YAMANAKA,1) Kyohei AKIYAMA,3) Mina OGAWA,4) Susumu KUMAGAI,5) Satoko KAWAKAMI,5) and Masaru KASAHARA5) 1) Research and Development Directorate, JAXA, Tsukuba, Japan 2) Human Spaceflight Technology Directorate, JAXA, Tsukuba, Japan 3) Space Tracking and Communications Center, JAXA, Tsukuba, Japan 4) Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan 5)Space Engineering Division, NEC Space Technologies, Ltd., Fuchu, Japan (Received June 13th, 2017) JAXA and NEC Corporation developed a state of the art space-borne GPS receiver in 2013. This latest receiver has been reduced in terms of the size and mass of its hardware, along with improved software to achieve high accuracy onboard navigation. In 2016, ASTRO-H, a new X-ray astronomy satellite, was launched with this GPS receiver. The navigation performance of ASTRO-H was evaluated by comparing its onboard GPS receiver data and offline precise orbit determination data. The position error RSS was < 1.7 m (95%) and velocity error RSS was < 11 mm/s (95%). And given this receiver’s specifications of 6 m for position and 30 mm/s for its velocity, it has achieved its design goals. This paper describes how the new GPSR improves the ionosphere-free pseudo-range and carrier phase noise, thereby enabling improved offline precise orbit determination of a satellite. The orbit of ASTRO-H was estimated to be within the precision of a few centimeters, which is among the most accurate for the satellites developed by JAXA to date. Key Words: GPS Receiver, GNSS, Performance Evaluation, ASTRO-H Nomenclature received data was downlinked and the OREX vehicle’s orbit was determined offline on the ground. Even though the a : perturbation modeling error vector receiver lacked the onboard capability to determine the C : clock error vehicle’s position, the maximum error of position was around c : light speed 300 m for each axis.1) Since this was our first experience in I : ionospheric delay GPSR design, the receiver weighed up to 12 kg and consumed R : geometric distance a maximum of 50 W, which seem quite huge as compared to r : position vector recent receivers. The experience gained in the OREX project v : velocity vector thus contributed to future advances in Japanese GPSRs : noise through improved performance and a reduction of resources. : pseudo-range A major modification to the GPSR was applied to Advanced : clock error Earth Observing Satellite-II (ADEOS–II). ADEOS-II : carrier phase achieved accurate position determination onboard. The Subscripts absolute position was determined within 26.0 m and absolute b : bias speed was determined within 8.2 cm/s. 2) d : drift 100 1 Pos 95% 1. Introduction Vel 95% Japan’s first space-borne GPS receiver (GPSR) was developed more than 20 years ago. Since then, many GPSRs 10 0.1 have been developed by reducing unit size, mass, and power Position[m] consumption, while upgrading GPSR functions and [m/s] Velocity performance. Figure 1 shows the improvement of onboard navigation performance. 1 0.01 ADEOS-2 ALOS GOSAT ALOS-2 ASTRO-H The Orbit Reentry EXperiment (OREX) flight experiment 2002 2006 2009 2013 2014 2016 launched in 1994 was the first spacecraft equipped with a GPSR in the history of Japanese space development.1) The Fig. 1. Improvement of onboard navigation precision of Japanese purpose of the GPSR on the OREX vehicle was to collect GPSRs. GPS signals during its reentry stage back to the earth. The Copyright© 2018 by the Japan Society for Aeronautical and Space Sciences and1 ISTS. All rights reserved. 454 Trans. JSASS Aerospace Tech. Japan Vol. 16, No. 5 (2018) The next progress of the GPSR was applied to the The basic onboard navigation performance of ASNARO Advanced Land Observing Satellite (ALOS) launched in was evaluated in the previous work,6) although details of 2006.3) ALOS attempted to achieve accurate geographical GPSR performance were not evaluated given the limited data position determination and geometric correction of the obtained by ASNARO due to the telemetry design and observed image, thus making positioning with high accuracy communication bandwidth constraints. GPSR data was essential. In order to meet these demands, a dual frequency obtained every 8 seconds, which was insufficient to evaluate GPS receiver was developed that allows the ALOS ground the navigation filter working at 1 Hz. The details of the GPSR are thus evaluated by using the system to determine its position within sub-meter accuracy flight data of ASTRO-H,10) the second opportunity to use the using dual frequency pseudo-range and carrier phase data. latest 5th-generation GPSR. ASTRO-H was designed to ALOS succeeded in offline high precision navigation by investigate the physics of the high-energy universe by introducing a dual frequency receiver, but there was still room performing high-resolution, high-throughput spectroscopy for improving onboard navigation accuracy. with moderate angular resolution. ASTRO-H was launched There was strong motivation for real-time precise onboard into circular orbit with an inclination of 31°. The typical position determination regarding the Greenhouse gases altitude was 575 km, and its attitude was fixed to the inertial Observing SATellite (GOSAT) developed in 2009. Although coordinates. ASTRO-H is equipped with a single GPSR with the GPSR on GOSAT had no ability to use L2 signals, the no redundancy. Two GPS antennas are installed on both the number of channels was increased from six to eight. The upper and lower panels of its body as illustrated in Fig. 2. presence of more visible satellites provided a stable signal that The GPSR data on ASTRO-H was generally decimated was effective in improving the accuracy and stability of the every 30 seconds and then downlinked. However, for navigation solution. checkout purposes, consecutive 1-Hz GPSR data was In 2014, ALOS-2 equipped with a new navigation filter was recorded for 12 hours during its checkout phase, but launched.4) A new filter (called “Single Pseudo-range NAV”) unfortunately represented all the data obtained by the GPSR, 11) was implemented for estimating ionospheric delay to as ASTRO-H lost contact with the ground in March 2016. compensate and achieve precision position determination. This paper therefore analyzes the recorded 12-hour data from Two GPS antennas were installed on top of the satellites to various viewpoints. provide a broad view as ALOS-2 was planned to change its This paper also introduces the main functions and designed attitude during the observation, while GPS observation was performance of the GPSR for ASTRO-H. The flight data required even during the observation. The receiver can receive obtained through the checkout phase is analyzed and L2 signals for offline precision orbit determination and evaluated in the latter sections. The onboard flight data was provide more accurate orbital information for post geometric analyzed to clarify navigation performance, which suggests correlation. that the design goals were satisfied. Following the GPSR aboard ALOS-2 with the latest improvements,5,6) a new GPSR has been developed by drastically reducing its size, while improving its navigation performance by providing an adequate number of channels and sophisticated navigation filters. This brand-new GPSR was initially installed on Advanced Satellite with New system Architecture for Observation (ASNARO) developed by the Ministry of Economy, Trade and Industry (METI) in 2014.7) This GPSR adopts the direct sampling method8,9) and integrates its electrical circuit into a single package, both of which reduced its size, mass, and power consumption. The advances made not only include a reduction in GPSR Fig. 2. GPS antennas installed on the ASTRO-H. size and power consumption but also improved robustness to track signals from GPS. A new correlator can process 88 Table 1. Specifications of the ASTRO-H GPS receiver. channels for signal inputs. With the help of these abundant Item Specification channels, satellites can install up to three GPS antennas and Size/Weight 96×218×155 [mm] / 1.95 [kg] cover a wide range of view. Such a wide view enables a Power 16 [W] (nominal) / 19 [W] (max) 3 antennas at most satellite to change attitude without losing its connection with Number of antennas the GPS satellite. Thus, the receiver is suitable not only for 6 RF inputs (2 frequencies×3 antennas) satellites facing the earth but also for satellites that are fixed Number of channels L1C/A ×36, L2C ×36, L2P(Y) ×16 on inertial frames or which frequently change position for Onboard L1C/A Position 6 [m] RSS for 95% of the time observation. Navigation Accuracy Velocity 3 [cm/s] RSS for 95% of the time Four navigation filters dependent on the available signals Position 3 [m] RSS for 95% of the time have also been improved, thereby realizing precision onboard Onboard L1C/A & L2C Navigation Accuracy Velocity 3 [cm/s] RSS for 95% of the time navigation, depending on the available types of signals. 2 455 Trans. JSASS Aerospace Tech. Japan Vol. 16, No. 5 (2018) The 12 channels are assigned for the L2P signal. Therefore, a total of 60 channels was available for GPSR on ASTRO-H, with the rest of the channels not being used for navigation. Figure 3 shows the appearance of the GPSR. For more details, refer to the reference.5) 2.2. Navigation filter design The signals from three antennas are input into the navigation filter. The signals were not multiplexed to avoid interference among the signals. The signals received from respective antennas are independently processed through each Fig.