Precision Navigation Achieved by ASTRO-H Space-Borne GPS Receiver

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

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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.

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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. 3. Appearance of the 5th-generation GPS receiver. RF and the navigation software determines which signals are Table 2. Features of the four navigation filters. to be used by considering the received signal strength of L1C/A. The bias among two antennas was estimated through Number of Position Measurement the filter and compensated prior to each observation update, Filter State Accuracy so as not to adversely affect the navigation solutions. The new L1 C/A L2C Variables (RSS 95%) GPSR has four kinds of navigation filters to achieve high Single pseudo PR, DR N/A 15 10 [m] precision navigation, depending on the availability of signals range NAV and the demand for filter convergence. Navigation accuracy is GRAPHIC PR, CP N/A 23 6 [m] generally degraded by ionospheric delay, pseudo-range noise, NAV and SIS-URE. The four filters proposed are intended to Dual pseudo PR, DR PR, DR 13 8 [m] suppress the effects of those error sources. Table 2 lists the range NAV features of these four navigation filters. PR stands for Pseudo Dual carrier Range, DR stands for Delta Range, and CP stands for Carrier CP CP 23 3 [m] phase NAV Phase. PR is the distance from a GPS satellite to the receiver estimated through a travel time measured by a modulated code 2. The GPSR for ASTRO-H on the signals. CP is also a distance from a GPS satellite to the receiver, but is measured by the carrier. Since the frequency JAXA and NEC Corporation recently developed the of the carrier is higher than that of the code, the resolution of 5th-generation GPS receiver in order to meet the demands of CP is higher than that of PR, but it contains carrier phase upcoming high-performance land observation or scientific ambiguity, and thus cannot obtain the absolute distance by a observation satellites. The receiver requires onboard high single observation. DR is a derivative of the relative distance precision and robust navigation, while minimizing its obtained from the increment in CP per unit of time. resources. The hardware and software of this receiver have There are more state variables than those of observations as been upgraded so as to achieve those goals. Table 1 lists the the hardware accepts at most 12 signals from GPS satellites. major specifications. Thus, it is impossible to obtain sufficient observability by measurements at a moment. 2.1. Hardware design The Kalman filter eventually solves the recursive weighted For the hardware, the receiver has been reduced in size and least squares method at every time step, thus allowing the mass due to the following measures: 1) introduction of direct observability of the filter to be evaluated by checking the rank sampling method to remove the down conversion electrical of the observability matrix, similar to a batch-processed circuits, and 2) integration of six RF front-ends, correlator weighted least squares approach. ASIC, and MPU into a single package called the Multi-Chip Some of the state variables are predictable by propagating Module (MCM). The MCM integrates both analogue and the past states using a system model. With this feature, by digital circuits in a single package; however, there were inputting successive observation data to the Kalman filter at difficulties in isolating electrical noise, in the thermal design, every time step, the number of measurements increases as and in maintaining a good yield ratio. time passes. Eventually, the number of observed The correlator in this receiver can receive both L1 and L2 measurements exceeds the number of estimated states, so that bands and possesses 88 channel inputs from a maximum of the observability matrix has full rank that provides sufficient three antennas, which provide broad coverage to satellites. observability. The broad field of view allows satellites to change attitude 2.2.1. Single pseudo range NAV dynamically without losing GPS acquisition. Although the This filter uses only L1 signals and estimates ionospheric delay constant Cion. Ionospheric delay is estimated through the latest GPSR possesses as much as 88 channels, ASTRO-H has Lear model12) as: only two antennas and thus not all the channels were assigned. 40.3VTEC For L1C/A and L2C signals, 12 channels are assigned to each  2 (1) I M (Es ) 2 Cion , antenna, thus 24 channels are spared for both L1C/A and L2C fL1 signals. For the L2P signal, the signal source is selected where VTEC stands for Vertical Total Electron Content, and depending on the received signal strength of L1C/A signals.

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2 fL1 denotes the frequency of the L1 signal. VTECC ion is a In addition to those 11 state variables, 12 GRAPHIC biases large number, so that it loses the significant digits when  are to be estimated through this filter. SIS-URE is to be ρ b directly estimated by a digital processor. In order to avoid the partially estimated as GRAPHIC bias and compensated; loss of significant digits, VTEC is defined as a representing therefore, SIS-URE is partially compensated for precision constant parameter and the changes in proportion on navigation. In conclusion, this filter helps to reduce the three ionospheric delay are expressed by Cion. The mapping function major causes of navigation errors by eliminating ionospheric M (Es) is as follows: delay, reducing pseudo-range noise to about half, and 2.037 (2) reducing SIS-URE. M (E s )  , 2 sin E s  0.076  sin E s 2.2.3. Dual pseudo range NAV This filter uses ionosphere free combinations of dual Es denotes a local elevation angle of the observed GPS frequency pseudo-range data and has the fewest state satellite. By using Eqs. (1) and (2), ionospheric delay can be parameters to estimate among the four filters, and thus offers modeled for compensation. The delay cannot be compensated the highest stability. Compared with Single pseudo range completely, given the remaining modeling error of the NAV, Dual pseudo range NAV does not estimate such ionosphere, but this filter compensates for most of the ionospheric characteristics as Cion in state variables, and only ionospheric delay. The Kalman filter is utilized to estimate 15 estimates the 13th order state in total as: variables including the ionospheric delay constant and its rate    T as follows: x  r , c b , v, c d , a, Bant1 , Bant 2  , (8)      T x  r ,c b ,v, c d , a,C ion ,C ion , Bant1 , Bant 2  . (3) however, it perfectly suppresses ionospheric delay using two items of frequency pseudo-range information. This filter The variables to be estimated are position vector  , clock r exhibits the most stable behavior among the four filters as it bias , velocity vector  , clock drift , perturbation c b v c d  has the fewest state parameters to estimate; therefore, it works modeling error vector , ionospheric delay constant Cion and a robustly in cases where only a few satellites are visible in the delay rate C , and bias among antennas Bant1 and Bant2, while ion field of view. And because this filter uses CNAV navigation the GPSR software on GOSAT only estimates 8th order state messages for navigation, SIS-URE is expected to be obtained variables: 3rd order position vector, clock bias, 3rd order in high precision so as to reduce the error. velocity vector, and clock drift. Hence, this filter has an 2.2.4. Dual carrier phase NAV advantage in compensating for perturbation modeling error, This filter uses ionosphere free combinations of dual ionospheric delay, and bias among antennas. frequency carrier phase data. As this filter uses carrier phase 2.2.2. GRAPHIC NAV data, it must estimate ambiguities of the carrier phase on each The GRAPHIC NAV filter uses GRAPHIC  channel  The state parameters are thus defined as follows: measurements,13) which are ionosphere free combinations of b     T L1 code and carrier phase. This filter has slower convergence x  r , c b , v, c d , a, b  . (9) compared to single pseudo-range NAV, but achieves higher precision navigation. Pseudo-range ρ and carrier phase φ are The order of state variables to be estimated is as large as given as: GRAPHIC NAV, which suggests a slower convergence compared with other filters; however, the most precise (4)   R  c b  I   p , navigation could be achieved among the four filters in the presence of L2 signals. This is because the effect of (5)   R  c b  I  N   c , measurement noise for the carrier phase is rather small relative to the pseudo-range. In addition, ionospheric delay where R denotes geometric distance, cτb denotes clock bias, I can be eliminated by using two items of frequency data. denotes ionospheric delay, N denotes the ambiguity of carrier Moreover, thanks to modernized CNAV messages, SIS-URE bias, and εp and εc denote other errors such as pseudo-range can be detected with high accuracy. In conclusion, this filter and carrier phase noise, respectively. Then the GRAPHIC successfully reduces all three major sources of error. measurement is defined as follows: 2.3. Software design * (6)   (  ) / 2  R  c b  N / 2   g . In addition to the new navigation filter explained in 2.2, the new software on the 5th-generation GPSR has two other Equation (6) indicates that ionospheric delay is removed features to improve navigation precision: from the measurement. Moreover, pseudo-range noise εp is 1. The ability to estimate bias among up to three antennas and reduced by about half because εp > εc and εg = (εp +εc)/2 ≈ εp /2. compensate for precision navigation; and By using GRAPHIC measurements as an input, 23 state 2. The ability to reduce errors caused by the antenna phase parameters are estimated and defined as follows: center offset or the satellite’s center of gravity offset during      T attitude pointing, by providing data on the antenna phase x  r , c b , v, c d , a, ρb , (7) center position, the satellite’s center of gravity, and attitude in where the first five elements indicate position, clock bias, advance. velocity, clock drift, and perturbation modeling error, which These features are effective for a spacecraft that frequently are the same as in single pseudo-range NAV mentioned above. changes its attitude.

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3. Offline Precise Orbit Determination

Offline Precise Orbit Determination (POD) of ASTRO-H was conducted to provide precise orbit products for the mission equipment. In order to evaluate the onboard navigation accuracy of the GPSR, POD products were referred to as a true orbit of ASTRO-H, and thus navigation error could be derived by considering the difference between POD and onboard navigation data. POD was derived from the GPS signal (pseudo-range, carrier phase) and attitude obtained onboard. This chapter introduces the procedure to obtain ASTRO-H POD, which is used for reference.

3.1. POD analysis conditions Fig. 4. Ionosphere free linear combination of carrier phase residuals for The conditions to obtain POD for ASTRO-H basically the period when high frequency onboard GPSR data was available. conform to the conditions listed in Table 3. And as the 00:00 14:00 onboard GPSR data at 1 Hz was obtained only for a limited 14H 24:00 period (2016/03/08 00: 00: 28 - 2016/03/08 23: 59: 58), POD 10:00 14H was executed for the period when high frequency onboard GPSR data was available. Fig. 5. Concept of the overlap method. 3.2. POD analysis results Figure 4 shows the carrier phase residuals of the L1 & L2 ionosphere free linear combination obtained through the period. The standard deviation of the carrier phase residuals was 7.9 mm. Given the fact that the standard deviation of the carrier phase residuals for 25 days was 7.8 mm (as listed in Table 6), the data obtained here was ordinal. As an approach to evaluate the ASTRO-H POD results, the overlap method was used. Figure 5 shows the concept of this evaluation method. The figure shows an example of the handover period, where two POD arcs overlap for four hours. In other words, two individual POD results could be obtained for this period; therefore, the difference between those two POD arcs were obtained and evaluated. Note that this method only evaluates the consistency of the two orbital arcs, and thus not all errors are detected. For instance, this evaluation method cannot detect errors showing the same trend among Fig. 6. Overlap evaluation of ASTRO-H POD. both arcs. Figure 6 shows the results of the overlap method. As the Table 3. Parameters for precise orbit determination. overlap evaluation showed a result of less than 3 cm with Parameter Contents 3D-RMS, the orbit of ASTRO-H was consistent and estimated within an error of several centimeters (3D-RMS). The figure Date of evaluation 2016/03/01- 2016/03/24 indicates that radial and along-track were clearly overlapped Orbit determination 24 hours and those differences on average were almost zero, whereas a length (a single arc) bias error of no more than 5 cm was confirmed in a Observation interval 30 s cross-track direction. This result implies a systematic bias Ionosphere free linear combination pseudo-range Observation input error in a direction perpendicular to the orbit plane, at the Ionosphere free linear combination carrier beginning and end of each arc, in the POD observation model. phase Although it was difficult to clarify the error source from the Position Velocity data, the problem could be solved by changing the GPS Clock offset antenna phase center offset and its related parameters. Carrier phase bias Estimation state Bias among antennas Solar radiation pressure Atmospheric resistance coefficient Empirical acceleration (radial/along-track/cross-track) Carrier phase bias OFF ambiguity estimation

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L1C/A Carrier Phase L1C/A Pseudo-range 14 12 1212 10 8 88 6 4 44 2 number of signals received 0 Number Signals Number of Number Signals Number of 0 0 06:00 09:00 time[dd Timemmm HH:MM:SS] 15:00 18:00 06:00 09:00 time[dd Timemmm HH:MM:SS] 15:00 18:00

L2C Carrier Phase L2C Pseudo range L2C Carrier Phase 14 L2C Pseudo-range 12 1212 10 8 88 6 4 44 2 number of signals received signals of number

Number Signals Number of 0 0 Number of Signals 0 06:00 09:00 time[dd Timemmm HH:MM:SS] 15:00 18:00 06:00 09:00 time[dd Timemmm HH:MM:SS] 15:00 18:00

L2P Carrier Phase L2P Pseudo range L2P(Y) Carrier Phase 14 L2P(Y) Pseudo-range 12 1212 10 8 88 6 4 44 2 number of signals received 0 Number Signals Number of 0 Signals Number of 0 06:00 09:00 Time 15:00 18:00 06:00 09:00 Time 15:00 18:00 (a) Carrier phase (b) Pseudo-range Fig. 7. Number of signals received.

Table 4. Number of received signals. Type of signal Average Minimum 4. GPS Measurements Evaluation

L1C/A (CP) 11.8 9 This chapter presents the flight results of recent satellites in L1C/A (PR) 11.7 8 order to verify the validity of the GPSR on ASTRO-H. L2C (CP) 6.8 4 L2C (PR) 6.8 4 4.1. Received signal status First of all, this section evaluates the signal reception status of L2P(Y) (CP) 11.0 7 the GPSR on ASTRO-H. The 5th-generation GPSR can L2P(Y) (PR) 11.4 8 receive three kinds of signals: L1C/A, L2P(Y), and L2C broadcast by GPS. As of March 2016, GPSR data has been Table 5. Average signal acquisition time. obtained at 1 Hz for 12-hour periods, and the few GPS Type of Specification On-board satellites that broadcast L2C signals have not reached Initial signal average average Operational Capability (IOC). For that reason, onboard L1C/A 35 s 17.0 s navigation only uses L1C/A signals, while offline precise L2C 35 s 18.5 s orbit determination uses both L1 C/A and L2 signals. L2P(Y) 435 s 127.5 s 4.1.1. Number of signals received Figure 7 and Table 4 summarize the number of signals *TTAM was evaluated by analyzing average signal acquisition time received from GPS satellites. In Fig. 7, (a) shows the number of carrier phase signals received from GPS satellites, and (b) 4 shows the number of pseudo-range signals received. The three 3 figures in each column show the difference among signal types: L1C/A, L2C, L2P (Y) in order from the top. Even 2 though ASTRO-H attitude was kept inertially fixed, which GDOP 1 was not ideal for GPS signal reception, stable signal reception could be achieved thanks to two antennas and various 0 channels adopted for the GPSR. Among all, L1C/A had the 6:00 9:00 12:00 15:00 18:00 Time best reception condition and was capable of reception by almost all 12 channels, and even under the worst conditions, Fig. 8. GDOP.

459 Trans. JSASS Aerospace Tech. Japan Vol. 16, No. 5 (2018) up to 9 signals were observed. Conversely, the number of L2C measurement accuracy (measurement noise) is defined for signal received was quite small and 8 signals at most were each code (L1C/A, L2P (Y), L2C), Eq. (10) is used to derive observed, but this is considered a reasonable result because the reference specification as follows: 2 2 not all GPS satellites had the capability to broadcast L2C 2 2  f   f     L1     L2   (10) signals in 2016. Similar to L1C/A, the L2P (Y) signal was IF  2 2 L1   2 2 L1  .  f L1  f L 2   f L1  f L2  received by up to 12 channels, but unlike L1C/A, it was difficult to acquire L2P(Y) code, because the acquisition time The comparison results in Table 6 indicate that the was longer than other types of the signal. This why the measurement residual (1σ) of the carrier phase obtained by the average number of L2P(Y) signals received by satellites is on-orbit evaluation of ASTRO-H is approximately the average smaller than that of L1C/A. of the specification for the L1C/A & L2P and L1C A & L2C ionosphere free linear combinations. In March 2016, not all 4.1.2. Satellite acquisition time the GPS satellites were broadcasting L2C signals, and thus the Table 5 lists the signal acquisition times, the so-called Time POD used both L2P and L2C signals depending on their To Available Measurement (TTAM) of each signal. The L1 availability. The results were integrated and cannot be C/A, L2C, and L2P(Y) signals satisfied the product distinguished by signal type. In March 2016, 18 out of 30 specifications. L1C/A and L2C were roughly equivalent to the GPS satellites were broadcasting L2C signals. Considering qualification test typical results. For L2P (Y), TTAM that the orbit evaluation result of ASTRO-H contains both confirmed onboard was shorter than in the ground test results. L1C/A & L2P and L1C/A & L2C ionosphere free linear This was probably because the signal level on orbit was combinations, the measurement noise of the 5th-generation higher than the conservative premise used for testing on the GPS receiver was roughly equivalent to the specification. The ground. The rich signal observed on-orbit enabled the measurement residuals of the carrier phase shown in Table 6 correlator to lock the code or carrier in a shorter time. were so small that modeling errors in the precise orbit 4.1.3. GDOP determination process cannot be ignored. In other words, the This subsection introduces the evaluation result of actual measurement of carrier phase noise is expected to be Geometric Dilution of Precision (GDOP) shown in Fig. 8. better than the figures given in Table 6. As for the Periodic jump of GDOP was observed at two-hour intervals. pseudo-range residual (1σ), it was confirmed from the This is because GDOP could not be calculated right after GPS comparison result of Table 6 that the residual was sufficiently smaller than the specification defined for the GPSR on ephemeris was updated in the GPSR software. Therefore, it ASTRO-H. was an expected behavior and had no impact on the solution. GDOP was suppressed to 3 or less for the entire observation Table 6. Satisfying the requirement for range measurement accuracy. time, and the GPSR was expected to obtain accurate Measurement ASTRO-H Specification (reference) navigation results. accuracy residual 4.2. GPS measurement accuracy 9.9 mm (L1C/A&L2P) Carrier phase (1σ) 7.8 mm This section evaluates the measurement noise that appears 5.3 mm (L1C/A&L2C) in residuals of the pseudo-range and carrier phase. At first, the 5.4 m (L1C/A&L2P) Pseudo-range (1σ) 1.0 m orbit of ASTRO-H was derived by offline Precise Orbit 2.3 m (L1C/A&L2C) Determination (POD). Then the residuals of each observation were calculated by subtracting the calculated range data (C) from the observation range data (O). 4.2.1. Analytical conditions Table 3 lists the parameters and values for orbit determination using the pseudo-range and carrier phase. 4.2.2. Measurement noise evaluation The carrier phase residuals (O-C) of the L1 & L2 ionosphere free linear combination are plotted in Fig. 9. The figure suggests that the residual data was confirmed about once every 12 hours. This is because the ASTRO-H telemetry data could only be obtained when ASTRO-H has a communication link to the Japanese ground facilities. Fig. 9. Carrier phase residuals of L1 and L2 Ionosphere free linear Otherwise, GPS telemetry data was collected, but could not be combination observed in March 2016. downlinked. There were also cases where ASTRO-H Table 7. Range measurement accuracy comparison with other GPSR. performed attitude maneuvers, resulting in a jump in the Measurement ALOS-2 ASTRO-H position determination result. Thus, subsequent evaluation accuracy used the measurement of residuals excluding this jumped value. Carrier phase (1σ) 17.3 mm 7.8 mm Table 6 summarizes the standard deviation (lσ) evaluation Pseudo-range (1σ) 1.6 m 1.0 m results of the residuals. As the specification value of GPSR

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(a) ALOS-2 (b) ASTRO-H Fig. 10. Carrier phase residuals of each GPSR vs elevation angle.

(a) ALOS-2 (b) ASTRO-H Fig. 11. Pseudo-range residuals of each GPSR vs elevation angle.

To objectively evaluate the performance of the GPSR on compared to those found in the GPSR equipped on ALSO-2, ASTRO-H, the measurement residuals were compared with a thereby showing significantly improved performance. conventional GPSR onboard ALOS-2.4) GPS antennas It was also confirmed from the comparison in Table 7 that installed on ALOS-2 and ASTRO-H were the same, and thus pseudo-range measurement noise was also significantly the difference was caused by GPSR performance and the improved from the GPSR used by ALOS-2. The GPSR on environment surrounding the antennas. Compared to GPSR on ALOS-2 had quite large residuals, even when the satellite was ALOS-2, the GPSR on ASTRO-H showed better performance observed through a high elevation angle, which generally because both the GPS processor and RF have been improved. shows low noise. However, noise was observed regardless of In addition, ALOS-2 installed other mission equipment the GPS antenna’s elevation angle. In contrast, ASTRO-H around the GPS antennas, and thus the residuals from a certain exhibited behavior where the signals from a high elevation direction deteriorated due to the multi-path effect. angle contained less noise and the pseudo-range was observed Figures 10 and 11 show the measurement residuals of the with high precision. GPSR installed in each satellite vs. the elevation angle of the GPS antenna. Table 7 lists the results of comparing the 5. Onboard Navigation Performance standard deviation (1σ value) of measurement residuals observed by each GPSR. Based on the comparison results in This section evaluates the onboard navigation accuracy. Table 7, both pseudo-range and carrier phase residuals have The ASTRO-H GPSR software has two options: either Single been reduced by the new GPSR thanks to its new correlator Pseudo Range NAV or GRAPHIC NAV under the condition and powerful processor. Note that there were obstacles such where only insufficient L2 signals are available. GRAPHIC as antennas around the GPS antenna on ALOS-2, while fewer NAV needs a longer period of time to converge the filter, but obstacles were positioned around the antenna on ASTRO-H; can achieve higher accuracy navigation. Thus, in this paper, consequently, multi-pass was likely to occur for ALOS-2. The the navigation performance of GRAPHIC NAV filter mode carrier phase measurement noise of the GPSR implemented has been evaluated. on ASTRO-H recorded less than half the residuals as

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5.1. Evaluating conditions 6. Expected Performance of Dual Frequency Navigation In order to evaluate navigation accuracy, onboard navigation data downlinked through telemetry was used. The The previous chapter evaluated onboard navigation onboard data for evaluation was generated by performing accuracy. This chapter evaluates the GRAPHIC NAV filter, clock bias time correction. For reference as the true orbit, the which is the most precise filter in the absence of L2 signals. offline POD data introduced in Chapter 4 was used. As modernization of GPS proceeds, more GPS satellites are 5.2. Evaluation results broadcasting L2C signals, but Initial Operation Capability Onboard navigation accuracy was evaluated by comparing (IOC) has yet to be declared. It was not acceptable to use the POD data and onboard navigation data. Given the onboard dual frequency navigation on an operating satellite 30-second period for POD output and the 1-second period for under an insufficient number of L2C signals for GPS. Thus, in onboard navigation data output, POD data is interpolated by order to evaluate dual frequency filter performance, the filter 8th order Lagrangian interpolation and the epoch is matched, is emulated on the ground. ASTRO-H did not use dual with both being compared. Figure 12 shows the evaluation results of onboard frequency filter mode, but it observed L2C signals for POD; navigation accuracy. The upper figure shows the navigation thus, L2C observation data was available. As a result, L2C accuracy of position; the lower figure shows the navigation signal data was entered in a computer with the onboard result of velocity. The light blue line in the figure represents navigation software to emulate onboard processing on the 3-D error, and the blue, green and red lines represent error in ground. And because high accuracy navigation using L2C is the radial, along-track, and cross-track directions, respectively. important for future space missions, this evaluation is The analysis period was 12 hours, during which it was particularly significant in terms of demonstrating future confirmed that both position and velocity could be estimated technology. stably. Tables 8 and 9 summarize quantitative navigation Table 10 lists the analyzed conditions for emulation. The accuracy. The onboard position navigation accuracy was rest of the parameters were chosen from among those used for within 1.7 m for 95% of the evaluated period, while the design ASTRO-H. specification was 6 m. The navigation accuracy of velocity Tables 11 and 12 list the ground simulation results. The was 11 mm per second (95%), while the design specification navigation accuracy results in GRAPHIC NAV filter mode was 30 mm per second. Thus, the specifications were were 1.7 m (95%) for position and 10 mm/s (95%) for satisfied. velocity, which are similar to the onboard navigation accuracy

5 radial Cross-track Radialtangential evaluated in the previous chapter. These results suggest that Along-track 3Dnormal 3d ground simulation to emulate the onboard navigation process 0 succeeded with high precision. Dual frequency navigation

Position [m] Position performance was also obtained through emulation, and dual -5 6:00 9:00 12:00 15:00 18:00 pseudo-range NAV offered a more accurate navigation Time solution than dual carrier phase NAV, even though dual carrier 0.05 radial Cross-track Radialtangential normal Along-track 3D3d phase NAV was designed to achieve higher precision. In particular, the position error of dual carrier phase NAV 0 exceeded the specification. The reasons for this are the still

Velocity [m/s] Velocity limited number of GPS satellites that broadcast L2C signals, -0.05 6:00 9:00 12:00 15:00 18:00 and insufficient input to converge the filter with a high order Time state. This behavior is expected to be solved when there are Fig. 12. Onboard navigation results. more GPS satellites that broadcast L2C signals in the near Table 8. Stochastic evaluation of onboard position error. future after GPS declares IOC, and will be improved after the Position 3D RSS 3D RSS Radial Along Cross error (m) (Spec.) (Onboard) declaration of Full Operation Capability (FOC). Average 0.098 -0.201 -0.064 N/A 1.1 Maximum 1.6 1.6 1.8 N/A 2.4 Table 10. Conditions for ground emulation. Std. 0.72 0.62 0.38 N/A 0.38 Parameter value 2016/03/08 06:37:07 ～ 19:23:07 95% N/A N/A N/A 6 1.7 Duration (GPST) Sampling period 1 s Table 9. Stochastic evaluation of onboard velocity error. GPSR for ASTRO-H onboard Input Velocity 3D RSS 3D RSS observation data Radial Along Cross error (mm/s) (Spec.) (Onboard) Attitude Fixed to inertial frame Average L1 only: 30 -0.07 -0.01 -0.06 N/A 4.0 Number of GPS L1&L2: 18 (correspond to the Maximum 38 19 33 N/A 48 satellites condition on 2016/03/08) Std. 4.1 2.1 2.7 N/A 3.5 95% N/A N/A N/A 30 11

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Table 11. Stochastic evaluation of ground emulated position error. dual frequency navigation was still not as good as that of Position error RSS Ground Spec. Onboard single frequency signal navigation. However, dual frequency 95% (m) simulation navigation is expected to be improved after sufficient signals Single 10 N/A 3.1 pseudo-range NAV are broadcast as the modernization of GPS proceeds. As part of further development, JAXA plans to develop this GRAPHIC NAV 6 1.7 1.7 receiver on HEO and GEO satellites with partial modification. Dual pseudo-range 8 N/A 5.0* NAV Dual carrier phase References 3 N/A 3.2* NAV 1) Tomita, H. and Miyano, T.: Flight Data Analysis of OREX Onboard GPS Receiver, The institute of Navigation, Proceedings Table 12. Stochastic evaluation of ground emulated velocity error. of ION GPS-94, 1994, pp. 1211-1217. Velocity error RSS Ground Spec. Onboard 2) Ito, T., Kojima, Y., Kawauchi, H., Ishijima, Y., Kumagai, S., and 95% (mm/s) simulation Tanamachi, S.: Flight Data Analysis of GPS Receivers Boarded on Single 50 N/A 14 ADEOS-II, Proceedings of the 47th Space Science & Technology pseudo-range NAV Conference, 1003, pp. 1211-1216 (in Japanese). Iwata, T., Toda, K., Kondoh, Y., Yamamoto, T., Kakinuma, M., GRAPHIC NAV 30 11 10 3) and Kumagai, S.: Dual-Frequency Spaceborne GPS Receiver for Dual pseudo-range the Advanced Land Observing Satellite (ALOS): Design and Flight 50 N/A 7.3* NAV Results, 22nd International Meeting of the Satellite Division of The Dual carrier phase Institute of Navigation, Savannah, GA, Sep. 22-25, 2009. 30 N/A 12* NAV 4) Yamamoto, T., Kawano, I., Iwata, T., Arikawa, Y., Itoh, H., *An insufficient number of satellites is available for dual frequency Yamamoto, M., and Nakajima, K.: Autonomous Precision Orbit navigation prior to IOC; therefore, the results would improve by Control of ALOS-2 for Repeat-pass SAR Interferometry, International Conference on Synthetic Aperture Radar (APSAR), increasing the number of GPS satellites with L2C signals. Asia-Pacific Conference on Tsukuba, Japan, Sep. 23-27, 2013. 5) Kondoh, Y., Yamamoto, T., Matsumoto, S., Yamanaka, K., 7. Conclusion Kawakami, S., Eda, T., Kasahara, M., Takeda, Y., Kumagai, S., and Ishige, Y.: Japanese Next Generation Spaceborne GPS Receiver for LEO Satellites, 9th International ESA Conference on This paper presented the evolution of space-borne GPS Guidance, Navigation, & Control Systems, Porto, Portugal, 2014. receivers developed by JAXA. The receiver has been reduced 6) Nakajima, Y., Yamamoto, T., Kondoh, Y., Yamanaka, K., Harada, in size, mass, and power consumption by each evaluation, R., Kasahara, M., Kawakami, S., Kumagai, S., and Ishige, Y.: while its navigation performance and robustness have been On-board Precision Navigation Achieved by ASNARO Spaceborne GPS Receiver, International Symposium on GNSS 2015, Kyoto, improved. The latest Japanese 5th-generation space-borne Japan, Sep. 16-19, 2015. GPS receiver was recently developed, with aspects of both 7) Ogawa, T., Saito, K., Miyazaki, K., and Ito, O.: System Outline of hardware and software being significantly upgraded to realize Small Standard Bus and ASNARO Spacecraft, Proceedings of the overwhelmingly precise navigation and reduced size as 4S (Small Satellites Systems and Services) Symposium, Portoroz, Slovenia, June 4-8, 2012. compared with conventional receivers. This receiver was 8) Psiaki, M. L., Powell, S. P., Jung, H., and Kinter Jr., P. M.: Design installed on ASTRO-H, and onboard GPSR data was and Practical Implementation of Multi-Frequency RF Front Ends consequently downlinked and evaluated from various aspects. Using Direct RF Sampling, The institute of Navigation, Proceedings of ION-GPS-09, 2009, pp. 1404-1412. GPS measurement noise was also reduced compared to the 9) Akos, D. M. and Ene, A.: A Prototyping Platform for previous-generation GPSR, thereby improving the accuracy of Multi-Frequency GNSS Receivers, The institute of Navigation, precise orbit determination. Assuming that ASTRO-H had a Proceedings of ION-GPS-03, 2003, pp. 117-128. desirable surrounding condition to reduce the multi-path effect, 10) Takahashi, T., Mitsuda, K., Kelley, R., Aharonian F., Akamatsu, H., Akimoto, F., et al.: The ASTRO-H X-ray Astronomy Satellite, improvement of the receiver also contributed to reducing Proceedings of Space Telescopes and Instrumentation 2014: noise. The onboard navigation performance was also Ultraviolet to Gamma Ray, 9144(2014), No. 25. significantly improved to achieve position and velocity 11) Takahashi, T., Kokubun, M., Mitsuda, K., Kelley, R., Ohashi, T., accuracy of 1.7 m and 11 mm/s for 95% of the time, Aharonian, F., et al.: The ASTRO-H (Hitomi) X-ray Astronomy Satellite, Proceedings of Space Telescopes and Instrumentation respectively. 2016: Ultraviolet to Gamma Ray, 9905(2016), No.27. Given the insufficient number of GPS satellites that 12) Leung, S., and Montenbruck, O.: Real-time Navigation of broadcast L2C signals, not all of the filters implemented on Formation-Flying Spacecraft Using Global Positioning System Measurements, J. Guidance, Control, and Dynamics, 28(2005), pp. this GPSR were used in space. However, the L2C signals 226-235. observed onboard at 1 Hz were downlinked to the ground in 13) Montenbruck, O.: Kinematic GPS positioning of LEO satellites order to estimate the performance of dual frequency using Ionospheric-free Single Frequency Measurements, Aerospace navigation. The onboard environment was emulated by a Science and Technology, 7(2003), pp. 396-405. ground simulation computer using the same software. The ground emulation results indicated that the performance of

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