XPNAV-1 (X-ray Pulsar Navigation Satellite-1)
In October 2016, CAST (China Academy of Space Technology) announced plans to launch the world's first X-ray pulsar navigation satellite (XPNAV-1) in November, according to the Xinhua News Agency. The move brings autonomous spacecraft navigation and a more precise deep-space GPS one step closer to reality. 1)
X-ray pulsar navigation is an innovative navigation technique whereby the periodic X-ray signals emitted from pulsars are used to determine the location of a spacecraft in deep space. Once the technology comes true, the ground intervention for satellite orbit control could be minimized.
According to Shuai Ping, chief scientist behind the research of XPNAV-1 satellite, the key aim of this satellite is to detect the details of X-ray signals of 26 nearby pulsars, and to create a "pulsar navigation database." This target could be achieved within five to 10 years, Shuai estimates.
Pulsars, 2) which are recognized as rotating neutron stars, can emit regular signals from the radio to the high energy band . Navigation using X-ray pulsars is regarded as a revolutionary technology providing autonomous spacecraft navigation capability in the whole solar system. 3)
To demonstrate this technology, CAST (China Academy of Space Technology) has brought forward a three-step space demonstration scheme. The first step is the X-ray pulsar navigation-I (XPNAV-1) satellite, the core goal of which is to validate the capability of observing X-ray pulsars. The following step is to launch a medium-sized satellite in about 2~3 years to accumulate more X-ray data for pulsar parameter database construction and to test the pulsar navigation algorithm onboard after 3~5 pulsars are timed enough accurately. The third step is to build a constellation system to demonstrate navigation application and time service using X-ray pulsars.
The purpose of the XPNAV-1 mission is to test the technology of pulsar observation in the soft X-ray band through the X-ray instruments developed by CAST. Three objectives are outlined: (1) test function and performance for the X-ray instruments in the outer space; (2) detect typical X-ray pulsars' radiation photons and acquire the pulse profiles to verify the ability of X-ray pulsar observation; (3) accumulate X-ray data for a long time to measure the pulsars' parameters via X-ray timing.
The core objective of the XPNAV-1 mission is to validate the ability of observing X-ray pulsars. To achieve this, eight X-ray sources are chosen as default targets. Not only 4 IRPs (Isolated Rotation-powered Pulsars), but also 4 XB (X-ray Binaries) are included because of their brightness. The eight sources' angular positions and pulse periods are shown in Table 1 and Figure 1. 4) 5) 6)
Table 1: Default targets' angular positions and pulse periods
Figure 1: Default targets' angular positions plotted in the J2000 frame (image credit: Laboratory of Space Technology, Beijing)
The XPNAV-1 minisatellite was designed and developed by CAST (China Academy of Space Technology) with a mass of 270 kg. 7) The minisatellite features a three-axis stabilization attitude mode with the ability to quickly point to any inertial position according to demand as accurate as 2 arcmin and to provide up to 90 minutes' sustained observation limited by the power supply.
XPNAV-1 provides four observation modes: the self-test mode, the scanning mode, the default target observation mode, and the arbitrary target pointing mode. The self-test mode works when the lens is covered with the hood in order that the instrument noise can be measured. The scanning mode is to scan a belt of the space via rotating the satellite in order to evaluate the background noise. The default target observation mode makes the X-ray instrument point at one of the 8 default targets. The arbitrary target pointing mode is similar to the former mode except that the angular position parameters should be uploaded from ground (Ref. 3).
Figure 2: XPNAV-1 satellite structure overview (image credit: Laboratory of Space Technology, Beijing)
Figure 2 provides an overview of the XPNAV-1 satellite structure. The satellite applies integrated electronics design, with which the functions of OBDH (Onboard Data Handling), TTC (Tracking Telemetry & Command), ADCS (Attitude Determination and Control Subsystem), GNSS navigation, and power control are integrated in one electronic component. The zero momentum three-axis ADCS is exploited with high-precision inertial pointing capability.
Orbit: Sun-synchronous near-circular orbit, altitude of 500 km, inclination = 97.4º, LTDN (Local Time on Descending Node) is 6:00 hr.
• Xiaoxiang-1, a 6U CubeSat (8 kg) developed at the Changsha Gaoxinqu Tianyi Research Institute, Hunan, China.
• CAS 2T & KS 1Q, a student-built amateur radio satellite that uses the 2U CubeSat form factor; it is mounted on the upper stage of a CZ-11 launch vehicle (as an attached payload, not separating from the CZ-11 upper stage).
Orbital data shows the Long March 11 launch left six objects in orbit – four are located in a 490 x 510 km orbit inclined at 97.4º, while the other two entered an elliptical orbit of 500 x 1,030 km at an inclination of 98.8°.
Sensor complement: (TSXS, HTPC)
The satellite operates two X-ray devices. One is the TSXS (Time-resolved Soft X-ray Spectrometer), and the other is the HTPC (High Time-resolution Photon Counter).The TSXS uses a Wolter- type lens of 4 nested mirror shells with the collecting area of 30 cm2 to focus X-ray photons within 15 arcmin FOV (Field of View) onto a SDD (Silicon Drift Detector). A GPS calibrated Rb clock is included to provide accurate time and a quasi-parallel optical star is used to assist the inertial pointing. The TSXS device provides the 1.5 µs time resolution and the 180 eV@5.9 keV energy resolution in the 0.5~10 keV energy band. The HTPC device uses the collimator to confine the FOV to 2º and the MCP (Microchannel Plate) X-ray detector to count the X-ray photons in the 1~10 keV energy band from the pulsar. Compared to the TSXS, the MCP of the HTPC has a higher time resolution of 100 ns and a bigger collecting area of 1200 cm2 (Ref. 3).
In order to examine the spaceborne X-ray instruments, the project chose the Crab pulsar (Ref. 13) as the calibration target because this pulsar is believed to be one of the best studied objects in the sky and one of the brightest X-ray sources regularly studied. Current data show that TSXS exhibits a good and steady performance in X-ray photon collection.
Two instruments, TSXS and HTPC, are mounted along different directions. During an observation cycle, the satellite spins to make the X-ray instrument inertially pointing at the target, and when the observation is finished, the satellite spins to make the solar array point to the sun. The two instruments cannot work simultaneously because of the insufficient capability of the power supply.
TSXS (Time-resolved Soft X-ray Spectrometer)
The grazing incidence focusing X-ray pulsar detector , TSXS, was developed by the Beijing Institute of Control Engineering. The following principles were considered during the design of the grazing incidence focusing X-ray pulsar detector: 10)
1) Scientific exploration mission is the primary factor in the design of the payload, which determines the function, performance and composition of the instrument. First of all, the physical characteristics of the observed object, the impact of the signal to noise ratio background noise and other factors shall be focused on. For example, the X-ray optics key parameters and detection device type should be selected according to the pulsar radiation flow, radiation energy range, and suitable background rejection measures should be selected according to the space radiation background.
2) The instrument needs to endure a complex space environment, adapting to the space heat exchange environment and the rocket launch shock is the key to success. In addition, we must also consider the mechanics, high and low temperature, vacuum, ultraviolet radiation, Galaxy cosmic rays and solar radiation and other factors.
3) The instrument needs to accurately track the observed pulsars, i.e., precise pointing. In addition, it is necessary to establish the relation matrix of the payload reference and the satellite benchmark, and the control subsystem should study the occultation of the objects such as the Earth.
4) Consider the matching between the payload power and the satellite energy, as well as whether the effective observation time in the observation arc, the payload working time and the satellite energy supply are optimal.
The grazing incidence focusing X-ray pulsar detector mainly consists of Wolter-I X-ray optics, silicon drift detector, magnetic diverter, electronics, high-energy particle shield, optical bench and a star tracker.
The TSXS adopts a multi-layer nested optical system to collect pulsar radiation X photons. The star tracker is used to realize the rough search and precise positioning of the target pulsar in orbit. Realtime tracking of the pulsar is realized based on the active control technology of the satellite spin and the control subsystem. The radiation flux and the radiation of the target pulsar energy spectrum, time of arrival, contour, radiation cycle and so on are observed. The optical system and star tracker of the grazing incidence type pulsar detector are installed outside the cabin, and the outside of the optical system is subjected to thermal control by covering and heating, and the rest is placed in the cabin, and the circuit is cooled by means of semiconductor cooling control, reducing the impact of thermal noise on the device to ensure that the silicon drift detector works in a good state.
Figure 3: Principle diagram of the TSXS (image credit: Beijing Institute of Control Engineering)
Working principle and function.
The working principles of all components of the grazing incidence focusing detector are as follows:
• Wolter-I optical system: The optical constants of X-rays determine that it is difficult to achieve focusing by refraction. Based on the Wolter-I grazing incidence optical system, the X-ray photons of pulsar radiation are focused on the detector based on the principle of grazing incidence and total reflection. The optics has the characteristics of high inherent optical gain and high signal to noise ratio. Multi-layer nesting technology is adopted to increase the effective area of the optical system, thus improving the detection sensitivity.
• Silicon drift detector: The X-ray detectors are mainly based on the photoelectric effect, they indirectly measure the X-ray photons by measuring the secondary electrons released by collision between the incident photons and the detector material. However, under the action of an external electric field, Silicon drift detector directly detects X-ray by collecting the electron hole pairs generated by the X-ray photon, with high count rate and high energy resolution.
• Pulse signal processing circuit: it consists of the pre-amplification, analog board, digital board, and power board. X-ray photons stimulate charges inside in the detector, and convert into current signal. The number of charges and their corresponding transient currents are proportional to the incident photon energy. These current pulse sequence intervals vary randomly, and the amplitude and duration are constantly changing. Main parameters characterizing the transient current sequence are: counting rate and energy spectrum. The time-varying properties of X-ray photons are calculated by accumulating them over a certain period of time. In order to obtain the time of arrival of X-ray photons, the fast-changing analog pulse is converted into a trigger signal through the comparator, and the time of arrival of X-ray photons is marked by the onboard clock.
• Magnetic deflector: it applies the Lorentz force on the electrons entering into the payload, thus making them deviate from the original movement trajectory and reducing the impact on the detector.
• High-energy particle shield: it is closed when the payload enters into the SAA area and strong radiation belt, thus avoiding damages on the detector by high-energy particles.
• High-stability structure: providing mechanical support and space particle protection for the payload.
• Star tracker: characterizing the optical axis of the pulsar detector optical system, thus achieving the inertia pointing of target pulsar.
Function of the payload:
• Pulsar detection function: achieving high-precision marking of time of arrival of X-ray photons, with high energy resolution.
• High-precision time scale function: receiving GPS second pulse, UTC whole second broadcast data, combining with rubidium clock generated high-precision clock signal, generating high-precision photon time scale.
• Scientific data formatting function: the detector inserts part of the satellite parameters into the payload data format according to the received broadcast data, thus formatting the payload data.
• Scientific data management function: in the satellite service system scheduling, with telemetry data acquisition function and sent to the astronaut computer telemetry parameters, from the astronaut computer to receive remote control commands and broadcast data functions.
The grazing incidence focusing pulsar has high efficiency, high energy resolution and high sensitivity. In particular, the product has the ability to simultaneously record X-photon time of arrival and energy, including:
• High-efficiency grazing incidence focusing optical system. Single-reflection optical system with high optical gain is adopted, realizing inherent low background noise and high detection sensitivity.
• SDD (Silicon Drift Detector) with high response rate. The SDD has high quantum efficiency, high response rate at low energy, high counting rate, and high energy resolution.
• Efficient space background suppression efficiency. Through the optical, circuit, active and passive shielding and other comprehensive strategy, the product achieves a very low noise, greatly improving the detection sensitivity.
Figure 4: Working principle of the grazing incidence focusing X-ray pulsar detector (image credit: Beijing Institute of Control Engineering)
Main Performance Indexes:
Table 2: Main indexes of grazing incidence focusing X-ray pulsar detector
• On November 17, 2016, XPNAV-1 fulfilled the satellite test phase and entered the observation phase (Ref. 3).
- Until February 2017, XPNAV-1 has observed three sources, that is, PSR B0531+21 (Crab), PSR B0540-69, and PSR B1509-58. During each observation, the X-ray devices are shut down in the region of SAA (South Atlantic Anomaly) to avoid the potentially harmful impact on the working detector. During the Earth occultation, no observation is planned. Besides, when the angle between the target and the sun or the moon is less than 45º, observation is ceased to avoid the interference signal.
- The preliminary data show that TSXS has steady performance in the signal-to-noise ratio. Thus, the project implemented most observations on the Crab pulsar using TSXS. In total, 162 TSXS observations of Crab were performed, each of which lasted 10 to 90 minutes.
In-orbit test results (Ref. 10):
In-orbit test process: At 7:42 on November 10, 2016, X-ray pulsar test satellite was successfully launched. 10 minutes later, the satellite was separated and successfully entered the intended orbit. The payload, grazing incidence focusing pulsar detector, performed in-orbit test, and the work performance of the payload was confirmed by the scientific detection data initially obtained during the orbit test. Some results are selected as follows.
In-orbit observation restriction condition: The grazing incidence focusing detector adopts the inertial space pointing method, which should meet the following conditions during in-orbit observation:
• Angle between the vector from the satellite to pulsar and the Earth center vector ≥80°
• Angle between the vector from the satellite to pulsar and sun vector ≥45°
• Angle between the vector from the satellite to pulsar and the moon vector ≥5°
• The detector does not work in the SAA (South Atlantic Anomaly) area.
Preliminary test results:
After the satellite status checking is completed, the grazing incidence focusing pulsar detector is powered on and the status and function check is successfully completed. During the main task of the detector, it entered the PSR B0531+21 (Crab Pulsar) observation state, and the PSR B0531+21 observation data at different times was obtained. The following data are for the observation data of MJD57727-57741 during the main task, with the observation target of PSR B0531+21.
1) Flux characteristics:
The detector observed the PSR B0531+21, and the average flow rate of the photon count rate observed by the detector in each observation period is shown in Figure 5. During the main task observation period, the photon count rate observed by the detector was stable with an average count rate of 14.7 photon/s.
Figure 5: Time-varying characteristics of the PSR B0531+21 pulsar flux (image credit: Beijing Institute of Control Engineering)
The probability of PSR B0531+21 photon counting rate detected in each observation period is calculated as shown in Figure 6. After fitting, it can be seen that the observed data in the pulsar photon counting rate statistics correspond to the Poisson distribution. 12)
Figure 6: Probability characteristics of the PSR B0531+21 pulsar flux (image credit: Beijing Institute of Control Engineering)
2) Energy spectrum:
The photon energy spectrum information of the PSR B0531+21 is obtained by the photon energy information obtained by the detector. The photon energy spectrum information in 0.5-5 keV is shown in Figure 7, and the photon energy of pulsar can be obtained to be the power law spectrum distribution. 13)
Figure 7: Spectra of the PSR B0531+21 pulsar flux (image credit: Beijing Institute of Control Engineering)
3) TOA interval of photons
The observed TOA (Top-of-Atmosphere) interval of photons of PSR B0531+21 is shown in Figure 8, the TOA interval is fitted and the result is a negative exponential model distribution, which is consistent with the theoretical model of pulsar photon TOA (Ref. 13).
Figure 8: Distribution of TOA intervals (image credit: Beijing Institute of Control Engineering)
4) PSR B0531+21 profile recovery
The profile of PSR B0531+21 is recovered after the photon TOA is corrected to the SSB coordinate system. The recovered profile and phases are consistent with the profile (2-10 keV) of RXTE (Rossi X-Ray Timing Explorer), as shown in Figure 9.
Figure 9: Profile recovery of the PSR B0531+21 pulsar (image credit: Beijing Institute of Control Engineering)
5) TOA of the PSR B0531+21 pulse
According to the measured in-orbit data, the measured profile and standard profile of the PSR B0531+21 are compared to obtain the initial residual information of TOA of PSR B0531+21 pulse, as shown in Figure 10.
Figure 10: TOA of the PSR B0531+21 (image credit: Beijing Institute of Control Engineering)
The above information is fitted to eliminate the timing residuals caused by the model error. The information obtained after fitting is shown in Figure 11, and it can be known that the minimum fitting residual of the pulse arrival time (TOA) reaches 52 µs in the above observation time.
Figure 11: Fitted TOA of the PSR B0531+21 (image credit: Beijing Institute of Control Engineering)
As shown in Figure 11, the observed results of PSR B0531+21 show that the pulsar photons flux is stable and the photon flux is consistent with the Poisson distribution. The observed spectrum of PSR B0531+21 shows a power-law distribution. The observed TOA interval of photons of PSR B0531+21 shows a negative exponential distribution, which is consistent with the theoretical model. The recovered profile of PSR B0531+21 is consistent with that obtained by RXTE, with correlation coefficient of 0.968. In the selected main task observation period, the residual of the obtained TOA pulse information of the PSR B0531+21 after fitting is about 52 µs.
In summary, the grazing incidence focusing pulsar detector is the main payload of the pulsar test satellite 01. The payload is equipped with a multi-layer nested Wolter-I X-ray optical system, a silicon drift detector, a magnetic deflector, the pulse signal processing circuit, the magnetic deflector, the high-energy particle shield, the high-stability structure and the star tracker for a number of scientific missions. Among them, the use of multilayer nested Wolter-I X-ray optical system for in-orbit X-ray pulsar observation is the first time in China. The flux characteristics, energy spectrum distribution, TOA of photons, profile and phase information of PSR B0531+21 are obtained by preliminary analysis of the in-orbit observation data. The results show that the working conditions of all components are normal, meeting the task requirements, and the payload can support the follow-up scientific exploration mission.
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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).