CN103033188A - Navigation satellite autonomous time synchronization method based on synthetic aperture observation - Google Patents

Abstract

The invention provides an autonomous time synchronization method for navigation satellites based on comprehensive aperture observation. All cooperative observation satellites and reference satellites carry out timing observations on the same millisecond pulsar at the same time. The reference satellites use the relative position measurement data between satellites at different times to determine Complete the time delay correction from each coordinated observation satellite to the reference satellite, and superpose and process each group of observation data that has completed the time delay correction to obtain the observation waveform of the comprehensive aperture timing observation system; measure the clock error of the reference satellite satellite clock, Calculate the clock difference between each satellite in the navigation constellation and the standard time TCB and broadcast the clock difference correction value in the broadcast ephemeris of each satellite, that is, the autonomous time synchronization of the navigation satellite is completed. The invention achieves the technical effects of reducing the X-ray detector load of the navigation satellite, shortening the observation time, and improving the autonomous time synchronization accuracy and real-time performance of the navigation satellite.

Description

Autonomous Time Synchronization Method for Navigation Satellites Based on Synthetic Aperture Observation

technical field

The invention belongs to the technical field of autonomous navigation of spacecraft, and relates to a method for performing high-precision time synchronization for navigation satellites by using X-ray signals radiated by pulsars.

Background technique

The satellite navigation system is a space-based radio navigation positioning and time transfer system. It is a space infrastructure for real-time acquisition of high-precision measurement information. It can provide all-weather, all-time, high-precision Navigation information services such as position, speed and time. A complete satellite navigation system includes three major components, namely the space constellation part (navigation constellation), the ground control part and the user terminal part. Among them, the ground control part consists of multiple monitoring stations and injection stations distributed around the world, as well as a main control center station, which is responsible for tracking and observing navigation satellites, precisely determining satellite orbits and on-board clock parameters, compiling navigation messages and controlling Commands are injected into the navigation satellites via the uplink data link. The navigation satellite extracts the uplink injection information, and broadcasts the navigation message to the user terminal receiver for use. That is to say, the operation, management and maintenance of the current navigation constellation are all completed by the ground control system, and uplink information needs to be injected once a day (such as the GPS system).

If the navigation satellite can realize autonomous navigation and operation management for a long time, it will effectively reduce the number of ground measurement and control stations, reduce the workload of the ground control system, reduce the operation management and long-term maintenance costs of the navigation constellation; it can monitor the integrity of navigation information in real time , greatly shorten the fault alarm time, improve the continuity and availability of navigation information; reduce the number of uplink information injections from the ground station to the satellite, reduce the dependence of the navigation constellation on the ground measurement and control system, and enhance the autonomous survivability of the satellite navigation system in special periods . It can be seen that the autonomous navigation of the navigation constellation has extremely important practical engineering application value.

For this reason, the United States has modernized its GPS system, adding inter-satellite communication/ranging links to GPS Block II R/II R-M, Block II F and other series of satellites, through inter-satellite two-way ranging, data exchange and satellite Carrying computer filtering processing, constantly correcting satellite long-term forecast ephemeris and clock parameters, and independently generating navigation messages and control commands, maintaining the stability of the basic configuration of the constellation to meet the needs of users for continuous high-precision navigation and positioning applications.

However, this autonomous navigation method of navigation constellation based on inter-satellite link ranging information lacks external space-time reference data, and it is difficult to solve the problem of accumulation of two types of unobservable errors over time. One is the overall rotation error of the constellation, which will cause the overall constellation to drift relative to the inertial coordinate system; the other is the non-uniform rotation error of the earth, which will cause the earth-centered fixed coordinate system to drift relative to the inertial coordinate system. In particular, the accumulation of the overall rotation error of the constellation will lead to a gradual increase in the error of satellite ephemeris and clock parameters, and a serious decline in the accuracy of user navigation and positioning.

At present, there are two basic ways to solve the above problems: (1) By establishing a long-term forecast model for the overall rotation of the constellation and the earth's rotation, to suppress the accumulation of errors (such as the GPS satellite autonomous navigation method). However, from the perspective of practical application effect, this method does not completely solve this technical problem, and cannot meet the technical index requirements of long-term autonomous navigation of the constellation; Signal and modulate the information of the Earth's rotation parameters, and the information is processed on the star to suppress the accumulation of constellation unobservable errors over time. However, this solution by establishing a satellite-ground link violates the principle of long-term autonomous operation of navigation satellites.

In recent years, with the extensive research on spacecraft autonomous navigation technology based on X-ray millisecond pulsar timing observation at home and abroad, it provides a new idea and feasible way for navigation satellite autonomous navigation, especially for navigation constellations. An independent external absolute space-time reference can solve the problem of long-term autonomous operation of constellations. The autonomous navigation of spacecraft based on X-ray millisecond pulsar timing observation is an effective mode to realize the high-precision autonomous navigation of spacecraft in the true sense. Applying pulsar autonomous navigation technology to the Global Navigation Satellite System (GNSS) can realize autonomous time synchronization of navigation satellites and autonomous measurement and update of satellite ephemeris. This kind of autonomous navigation is based on the pulsar inertial space-time reference frame. The pulsar space-time reference frame has no rotation and time drift relative to the inertial reference frame, so it can suppress or eliminate the rotation error of the constellation itself and the long-term drift of the navigation satellite time reference. error. The application of pulsar navigation technology can also accurately measure the constellation average time error and constellation overall rotation error existing in GNSS autonomous navigation.

In navigation applications, use the X-ray timing observation equipment equipped on the navigation satellite to measure the pulse arrival time of a group of pre-selected millisecond pulsars, and measure the time when the pulse of each pulsar arrives at the navigation satellite. The star clock model can predict the time when the same pulse arrives at the center of mass of the solar system. By comparing the time when the same pulse arrives at the navigation satellite and the time of the center of mass of the solar system, combined with the spatial position parameters of this group of pulsars, the relative time of the navigation satellite at the observation time can be calculated. The position vector (three-dimensional coordinates) at the barycenter of the solar system and the clock difference of the on-board clock relative to the standard time (coordinate time at the barycenter of the solar system, TCB). Its basic principle is shown in Fig. 1. In Figure 1, the navigation satellite uses the X-ray pulsar autonomous navigation system to measure the position of the navigation satellite relative to the center of mass of the solar system and the clock difference of the on-board clock relative to the standard time to achieve autonomous positioning and timing. The X-ray pulsar autonomous navigation system mainly includes the following parts: 4 X-ray detectors, used to detect the X-ray pulse signal from the pulsar, and obtain the integrated pulse profile with a satisfactory signal-to-noise ratio within a certain integration time; The space-borne atomic clock provides reference time for TOA measurement; the X-ray pulsar and solar system ephemeris database provides the basic data needed for pulsar space-time reference frames and navigation algorithms; the TOA measurement and processing module adopts appropriate algorithms and uses integral pulses contour, to obtain the observed pulse arrival time; the navigation satellite position, velocity and time algorithm module, use the observed TOA to establish the pulsar navigation observation equation, and use the appropriate algorithm to realize the measurement and calculation of the spacecraft state parameters (position, velocity, time) forecast.

X-ray pulsar timing observation is the basis of pulsar autonomous navigation technology. The basic structure of the X-ray timing observation system is shown in Figure 2. The spacecraft aligns the X-ray detector mounted on the target source, observes and records the arrival time (Time of Arrival, TOA), the measurement of the arrival time is based on the time signal provided by the satellite-borne atomic clock, and then the original observation data is recorded by the data recording system. Due to the high-speed motion of the spacecraft, the observation signal will produce obvious Doppler effect; due to the influence of the special relativity effect, the actual vibration frequency of the space-borne atomic clock will also change relative to its nominal frequency; Most of the millisecond pulsars autonomously navigated by the sensor are in the binary star system, so the orbital movement of the observed pulsar around the center of mass of the binary star system will also have an additional impact on the observed signal; in addition, the Shapiro delay and other multiple error sources need to be considered Influence. Therefore, the original observation data must be further processed by the observation data preprocessing system to eliminate the influence of various error sources on the observation data. After completing the correction of various error sources, the observation sequence is superimposed according to the rotation period of the observed pulsar to obtain an integrated pulse profile with a satisfactory signal-to-noise ratio. The TOA of this observation is obtained by comparing the integrated pulse profile with the standard pulse profile recorded in the database and adding the starting time of this observation.

A large number of observational data show that: for the Crab (J0534+2200) pulsar with the strongest radiation in the X-ray band, an X-ray detector with an effective area of ^1m2 can receive about 7800 X-ray photons from the pulsar per second (signal ) and about 78,000 photons (noise) from the nebula, that is, the number of signal photons in one pulse period is about 260, and the number of noise photons is about 2,600. The X-ray flux intensity of other X-ray pulsars is only one-thousandth or even lower than that of Crab pulsars, which means that every ^1m2 detector can only detect about 10 signal photons or even less per second, The number of background photons is around 100. To obtain a clear pulse profile, at least ten hours of connected observation time is required.

The observation signal with such a low signal-to-noise ratio has become one of the main reasons that limit the practical application of pulsar autonomous navigation technology. If it is used for autonomous navigation of navigation satellites, measures must be taken to improve the signal-to-noise ratio. At present, there are two solutions: one is to extend the observation time; the other is to use a large-area detector. However, both schemes have their limitations. For the first scheme, the prolongation of the observation time will reduce the real-time performance of the navigation solution, which cannot realize the quasi-real-time monitoring of the integrity of the navigation information, and prolong the fault alarm time; for the second scheme However, due to the limited payload of navigation satellites, it is difficult to carry large-area X-ray detectors.

For the autonomous measurement of the position coordinates of navigation satellites, there is currently a more feasible technical solution, that is, to use two navigation satellites equipped with X-ray detectors to observe the same X-ray variable source at the same time, and the connection between the two satellites constitutes a dynamic The inter-satellite measurement baseline, the time difference between the same signal arriving at two satellites reflects the direction of the inter-satellite baseline relative to the pulsar, and the inter-satellite baseline can be determined in the inertial space-time reference by synchronous observation of multiple X-ray variable source signals. orientation in the shelf. Simultaneously measure the directions of multiple inter-satellite baselines in the inertial space-time reference frame, and combine the measurement results of inter-satellite ranging/communication links with orbital dynamics equations to realize the autonomous determination of the position coordinates of navigation satellites. The flux of X-ray variable sources is usually 5-6 orders of magnitude higher than that of X-ray millisecond pulsars, so high measurement accuracy can be obtained without using a large-area detector or a long observation time. However, autonomous time synchronization of navigation satellites cannot be achieved by using X-ray variable sources, and autonomous time synchronization of navigation satellites can only be achieved by using X-ray millisecond pulsars.

To sum up, if X-ray pulsar autonomous navigation technology is to be applied to navigation satellites to realize autonomous time synchronization of navigation constellations, the problem of low signal-to-noise ratio of observation signals must be solved. There are two main methods for improving the signal-to-noise ratio of the observation signal: increasing the effective area of the detector or prolonging the observation time. The two methods have the following problems respectively:

For navigation satellites, a large number of equipment for generating, maintaining and transmitting navigation signals, such as atomic clocks, time-frequency signal generation and maintenance systems, antennas, and solar cells, must be mounted on them. The payload that can be assigned to the X-ray detector system is very limited, and the large-area X-ray detector has a large volume, high mass, and high power consumption, which will definitely occupy more satellite payloads and is difficult to implement. Moreover, the existing pulsar autonomous navigation technology usually requires four X-ray detectors to be carried on the satellite, which makes it even more unfeasible to carry a large-area X-ray detector on the navigation satellite.

In the case of limited detector size, the method of extending the observation time can also be used to improve the signal-to-noise ratio. But except for Crab pulsars, the X-ray radiation flux of millisecond pulsars usually used for navigation is very low, even for a large-area X-ray detector with an effective area of 1m ² , within a time interval of 1 second. The received photon signals are only about 10 or even less. To obtain a clear pulse profile, at least ten hours of continuous observation time is required. During such a long observation period, the navigation satellite has already flown a long arc or even a week along the orbit, but the pulsar autonomous navigation system still gives the satellite's motion state parameters and clock error parameters at the beginning of the observation. Therefore, the prolongation of the observation time will greatly reduce the real-time performance of the navigation solution, making it impossible to realize the quasi-real-time monitoring of the state of the on-board clock, and prolonging the fault alarm time.

Invention patent 200710005043.9 proposes a system and method for autonomous orbit determination, time synchronization and attitude measurement of navigation satellites based on X-ray pulsars. This patent still uses the traditional X-ray pulsar autonomous navigation method to realize the autonomous time synchronization of navigation satellites, which cannot completely solve the problem of weak signal observation in X-ray pulsar navigation.

Contents of the invention

In order to overcome the shortcomings of low signal-to-noise ratio and difficult identification of observation signals in the existing pulsar autonomous navigation technology, the present invention proposes a comprehensive aperture observation scheme in the X-ray band, without increasing the detector area and extending the In the case of observation time, it solves the problem of weak signal detection in the autonomous time synchronization of pulsar-based navigation constellations, and has high measurement accuracy and is easy to implement.

The technical solution adopted by the present invention to solve its technical problems comprises the following steps:

Step 1: According to the ephemeris of each navigation satellite and the position parameters of the millisecond pulsar, select a group of navigation satellites that can observe the same millisecond pulsar at the same time, and select one of them as the reference satellite, and the remaining navigation satellites As a coordinated observation satellite, the observation data of the coordinated observation satellite should be attributed to the reference satellite;

Step 2: All the coordinated observation satellites and reference satellites conduct timing observations on the same millisecond pulsar at the same time, and save their original observation data; at the same time, the coordinated observation satellites and reference satellites complete the original observations according to their respective position and speed parameters Signal Doppler effect, special relativity effect and Shapiro delay correction;

Step 3, after each timing observation is completed, each coordinated observation satellite transmits its own observation data to the reference satellite;

Step 4: The reference satellite uses the inter-satellite relative position measurement data at different times to complete the time delay correction from each coordinated observation satellite to the reference satellite. The specific method is:

所观测毫秒脉冲星的方向矢量为则t时刻协同观测卫星至基准卫星的TOA测量数据时延改正值为

其中c为光速；协同观测卫星i所观测到的光子到达时间序列toa_ij进行逐一改正，得到一组新的到达时间观测序列，即等效于将协同卫星i的探测器放置在基准卫星上所得到的TOA观测序列；At time t, the position vector of the i-th cooperative observation satellite relative to the reference satellite in inertial space is

The direction vector of the observed millisecond pulsar is Then the time delay correction value of the TOA measurement data from the coordinated observation satellite to the reference satellite at time t is

where c is the speed of light; the photon arrival time sequence toa _ij observed by the coordinated observation satellite i is corrected one by one to obtain a new set of arrival time observation sequences, which is equivalent to the result of placing the detector of the coordinated satellite i on the reference satellite Obtained TOA observation sequence;

Step 5, performing superposition processing on each group of observation data that has completed the time delay correction to obtain the observation waveform of the comprehensive aperture timing observation system;

Step 6, measure the clock error of the reference satellite on-board clock, the specific method is:

其中

分别为所观测毫秒脉冲星的单位方向矢量在太阳系质心坐标系中的三个坐标分量，基准卫星在观测历元时的太阳系质心坐标系位置矢量为

其中

分别为基准卫星的位置矢量在太阳系质心坐标系中的三个坐标分量，综合孔径计时观测系统观测得到的脉冲到达时间为toa_sat，由该颗毫秒脉冲星的钟模型预报得到的同一脉冲到达太阳系质心的时刻为toa_SSB，则观测方程为Suppose the unit direction vector of the observed millisecond pulsar in the barycentric coordinate system of the solar system is

in

are the three coordinate components of the unit direction vector of the observed millisecond pulsar in the barycentric coordinate system of the solar system, and the position vector of the barycentric coordinate system of the reference satellite in the observation epoch is

in

are the three coordinate components of the position vector of the reference satellite in the barycentric coordinate system of the solar system, the arrival time of the pulse observed by the synthetic aperture timing observation system is toa _sat , and the same pulse predicted by the clock model of the millisecond pulsar reaches the solar system The moment of the centroid is toa _SSB , then the observation equation is

$<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; toa</span>}}_{\mathrm{<span\; class="notranslate">\; sat</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; toa</span>}}_{\mathrm{<span\; class="notranslate">\; SSB</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; =</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; \&CenterDot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; +</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; \&CenterDot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; +</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; \&CenterDot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; c</span>}$

In the formula, Δt _c is the deviation between the reference satellite on-board clock and the standard time TCB, and δ is the sum of other timing observation errors;

Step 7: Calculate the clock difference between each satellite of the navigation constellation and the standard time TCB, convert the clock difference of each navigation satellite satellite clock relative to the TCB into a clock difference relative to the earth time, and record it in the respective broadcast ephemeris of each satellite By broadcasting the clock correction value, the user's navigation and timing service accuracy can be guaranteed, that is, the autonomous time synchronization of navigation satellites is completed.

The beneficial effect of the invention is that the observation results of multiple small detectors are combined after time-delay correction to form the observation effect of one large detector. The technical effect of reducing the X-ray detector load of the navigation satellite, shortening the observation time, and improving the autonomous time synchronization accuracy and real-time performance of the navigation satellite is achieved. The invention is also applicable to the application field of high-precision time synchronization of low-earth orbit, deep space and interplanetary flying spacecraft formations, as well as non-dense atmosphere celestial body lander formations and surface roving formations.

Description of drawings

Figure 1 is a schematic diagram of pulsar autonomous positioning and timing;

Figure 2 is a diagram of the X-ray pulsar timing observation system;

Figure 3 is a schematic diagram of time delay correction from the coordinated observation satellite to the reference satellite;

Figure 4 is a flow chart of the comprehensive aperture observation scheme.

Detailed ways

The present invention comprises the following steps:

First, a group of millisecond pulsars suitable for time synchronization of navigation satellites is selected according to long-term ground timing observations, and a database of millisecond pulsars for time synchronization (including clock model parameters and astrometric parameters of each millisecond pulsar) is established, and It is stored in the onboard computer of each navigation satellite. In practical application, according to the ephemeris of each navigation satellite and the position parameters of the millisecond pulsar, a group of navigation satellites that can observe the same millisecond pulsar at the same time is selected, and one navigation satellite is selected as the reference satellite. The rest of the navigation satellites are used as coordinated observation satellites, and the observation data of the coordinated observation satellites should be attributed to the reference satellites. All coordinated observation satellites and reference satellites conduct timing observations on the same millisecond pulsar at the same time, and save their original observation data. At the same time, the cooperative observation satellite and the reference satellite complete the Doppler effect, special relativity effect and Shapiro delay correction of the original observation signal according to their respective position and velocity parameters. After each timing observation is completed, each cooperative observation satellite transmits its own observation data to the reference satellite through the inter-satellite ranging/communication link. The reference satellite uses the inter-satellite relative position measurement data at different times given by the inter-satellite link autonomous navigation system to complete the time delay correction from each coordinated observation satellite to the reference satellite. The process of delay correction is shown in Figure 3:

所观测脉冲星的方向矢量为则t时刻协同卫星至基准卫星的TOA测量数据时延改正值为其中c为光速。由于第i颗协同观测卫星相对于基准卫星的动态位置矢量是连续变化的，所以协同观测卫星i所观测到的光子到达时间序列（toa_ij）要一一进行相应的改正。得到一组新的到达时间观测序列，即等效于将协同卫星i的探测器放置在基准卫星上所得到的TOA观测序列。At time t, the position vector of the i-th cooperative observation satellite relative to the reference satellite in inertial space is

The direction vector of the observed pulsar is Then the time delay correction value of the TOA measurement data from the coordinated satellite to the reference satellite at time t is where c is the speed of light. Since the dynamic position vector of the i-th cooperative observation satellite relative to the reference satellite is continuously changing, the photon arrival time series (toa _ij ) observed by the cooperative observation satellite i must be corrected accordingly. A new set of time-of-arrival observation sequences is obtained, which is equivalent to the TOA observation sequence obtained by placing the detector of cooperative satellite i on the reference satellite.

Secondly, each group of observation data that has been corrected for time delay is superimposed to obtain the observation waveform of the synthetic aperture timing observation system.

After obtaining the observation waveform of the synthetic aperture timing observation system, it is first necessary to measure the clock error of the reference satellite on-board clock. The measurement principle is as follows:

（其中

分别为所观测毫秒脉冲星的单位方向矢量在太阳系质心坐标系中的三个坐标分量），基准卫星在观测历元时的太阳系质心坐标系位置矢量为（其中

分别为基准卫星的位置矢量在太阳系质心坐标系中的三个坐标分量），综合孔径计时观测系统观测得到的脉冲到达时间为toa_sat，由该颗毫秒脉冲星的钟模型预报得到的同一脉冲到达太阳系质心的时刻为toa_SSB，我们可建立如下观测方程。Suppose the unit direction vector of the observed millisecond pulsar in the barycentric coordinate system of the solar system is

(in

are the three coordinate components of the unit direction vector of the observed millisecond pulsar in the barycentric coordinate system of the solar system), and the position vector of the barycentric coordinate system of the solar system when the reference satellite observes the epoch is (in

are the three coordinate components of the position vector of the reference satellite in the barycentric coordinate system of the solar system), the pulse arrival time observed by the synthetic aperture timing observation system is toa _sat , and the same pulse arrival time predicted by the clock model of the millisecond pulsar The moment of the barycenter of the solar system is toa _SSB , we can establish the following observation equation.

$<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; toa</span>}}_{\mathrm{<span\; class="notranslate">\; sat</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; toa</span>}}_{\mathrm{<span\; class="notranslate">\; SSB</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; =</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; \&CenterDot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; +</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; +</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; c</span>}$

In the formula, c is the speed of light, Δt _c is the deviation between the reference satellite on-board clock and the standard time TCB, and δ is the sum of other timing observation errors.

After the measurement of the on-board clock error of the reference satellite is completed, since the relative time synchronization of each satellite in the navigation constellation has been completed, the clock error of each satellite in the navigation constellation and the standard time TCB can be calculated. Convert the clock difference of each navigation satellite satellite clock relative to TCB to the clock difference relative to the earth time, and broadcast the clock difference correction value in the broadcast ephemeris of each satellite, so as to ensure the user's navigation and timing Service accuracy, that is, the autonomous time synchronization of navigation satellites is completed.

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

The invention provides a navigation constellation time synchronization method based on an X-ray comprehensive aperture observation scheme, which has the advantages of short observation time, high observation accuracy and easy implementation.

The invention uses X-ray millisecond pulsar as the external measurement reference, uses the X-ray signal radiated by it as the input information of the autonomous navigation system, combines the solar system planetary ephemeris database, X-ray pulsar clock model and characteristic parameter database, strapdown inertial navigation System (SINS), autonomous navigation algorithm module, inter-satellite communication/ranging link, etc., solve the problem of high-precision autonomous time synchronization of navigation constellations.

Key points 1. Synthetic aperture observation in the X-ray band. That is, each satellite is equipped with an X-ray detector, and multiple satellites conduct timing observations on the same pulsar at the same time through the coordination of the reference satellite, and record their respective observation data. After the observation is completed, each satellite transmits its observation data to the reference satellite through the inter-satellite communication/ranging link. After the reference satellite completes the time delay correction of the observation data from different satellites, it integrates all the observation data , to obtain the synthetic aperture timing observation data of the reference satellite. The technical effect of reducing the X-ray detector load of the navigation satellite, shortening the observation time, and improving the autonomous time synchronization accuracy and real-time performance of the navigation satellite is achieved.

Key point 2. Calculation of reference satellite clock error parameters. On the basis of obtaining the synthetic aperture observation data of the reference satellite, using a suitable algorithm, the clock error of the reference satellite on-board clock relative to the standard time is calculated. And use the inter-satellite ranging/communication link to realize the synchronization between other satellite on-board clocks and the reference satellite on-board clock, and then realize the autonomous time synchronization of the navigation constellation. The technical effect of high-precision autonomous time synchronization of the navigation satellite constellation has been achieved.

Once the navigation satellites are finalized, the equipment carried on each satellite is the same. According to this feature, the implementation of the technical solution of the present invention first needs to carry an X-ray detector on each satellite. After using other technologies to complete the autonomous measurement of the position coordinates of the navigation satellite, the reference satellite uses the inter-satellite ranging/communication link to coordinate other satellites (coordinated satellites) to perform timing observations on the pre-selected X-ray millisecond pulsar, that is Both the reference satellite and the coordinated satellite point the X-ray detector to the measured pulsar, and record the arrival time of the X-ray photons they receive. At the same time, all satellites participating in the observation use their respective position, velocity, and attitude parameters to complete the correction of the Doppler effect and the special relativity effect. If the observed millisecond pulsar is in a binary star system, it is also necessary to consider that the pulsar revolves around the center of mass of the binary star system The resulting change in the apparent period. After the observation is completed, each cooperative satellite transmits the observation data to the reference satellite through the inter-satellite ranging/communication link.

The reference satellite first completes the time delay correction of the observation data from different cooperative satellites, and the process of time delay correction is shown in Figure 3:

所观测脉冲星的方向矢量为

则t时刻协同卫星至基准卫星的TOA测量数据时延改正值为

由于第i颗协同观测卫星相对于基准卫星的动态位置矢量是连线变化的，所以协同观测卫星i所观测到的光子到达时间序列（toa_ij）要一一进行相应的改正。得到一组新的到达时间观测序列，即等效于将协同卫星i的探测器放置在基准卫星上所得到的TOA观测序列。At time t, the position vector of the i-th coordinated satellite relative to the reference satellite in inertial space is

The direction vector of the observed pulsar is

Then the time delay correction value of the TOA measurement data from the coordinated satellite to the reference satellite at time t is

Since the dynamic position vector of the i-th cooperative observation satellite relative to the reference satellite changes linearly, the photon arrival time series (toa _ij ) observed by the cooperative observation satellite i must be corrected accordingly. A new set of time-of-arrival observation sequences is obtained, which is equivalent to the TOA observation sequence obtained by placing the detector of cooperative satellite i on the reference satellite.

When the TOA observation sequence of all n cooperative observation satellites has completed the delay correction from the cooperative satellite to the reference satellite, it is equivalent to placing the n detectors at the reference satellite and obtaining their respective TOA observation sequences . Adding the observation sequence of the reference satellite detector, the TOA observation sequence of n+1 independent detectors at the reference satellite can be obtained, and the observation sequence of n+1 detectors can be superimposed together to obtain the equivalent observation of the synthetic aperture detector sequence.

倍。即对于同一颗脉冲星，为获得同样信噪比的积分脉冲轮廓,利用综合孔径观测技术，可将观测时间缩短至原来的

Compared with the single detector, the synthetic aperture detector can improve the signal-to-noise ratio of the observed signal to the signal-to-noise ratio of the original signal by superimposing the observed signals of n+1 sub-detectors.

times. That is to say, for the same pulsar, in order to obtain the integrated pulse profile with the same signal-to-noise ratio, the observation time can be shortened to the original

Fig. 4 is the flow chart of above-mentioned comprehensive aperture observation scheme:

First of all, under the coordination of the reference satellite, all the cooperative observation satellites carry out timing observations on the same millisecond pulsar at the same time, and each satellite saves its own original observation data, and completes the Doppler effect, Correction of multiple error sources such as special relativity effects. After the observation is completed, each cooperative observation satellite will transmit its own observation data to the reference satellite through the inter-satellite ranging/communication link. The reference satellite uses the inter-satellite relative position measurement data at different times given by the inter-satellite link autonomous navigation system to complete the time delay correction from each coordinated satellite to the reference satellite. The observation waveform of the comprehensive aperture timing observation system can be obtained by superimposing each group of observation data that has completed time-delay correction.

After obtaining a clear pulse profile, it is first necessary to measure the clock error of the reference satellite on-board clock. The measurement principle is as follows:

导航卫星在观测历元时的太阳系质心坐标系位置矢量为观测得到的脉冲到达卫星的时刻为toa_sat，由钟模型预报得到的同一脉冲到达太阳系质心的时刻为toa_SSB，我们可建立如下观测方程。Let the unit direction vector of the observed pulsar be

The position vector of the barycentric coordinate system of the solar system when the navigation satellite observes the epoch is The time when the observed pulse arrives at the satellite is toa _sat , and the time when the same pulse arrives at the barycenter of the solar system predicted by the clock model is toa _SSB , we can establish the following observation equation.

$<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; toa</span>}}_{\mathrm{<span\; class="notranslate">\; sat</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; toa</span>}}_{\mathrm{<span\; class="notranslate">\; SSB</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; =</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; +</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; \&CenterDot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; +</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; k</span>}}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; c</span>}$

In the formula, c is the speed of light, Δt _c is the deviation between the reference satellite on-board clock and the standard time TCB, and δ is the sum of other timing observation errors. Since the navigation satellite has already used other autonomous navigation technologies to complete the measurement of the position coordinates of the barycentric coordinate system of the solar system, there is only one unknown quantity Δt _c on the right side of the above formula when the measurement error is ignored. Theoretically, one timing observation of a millisecond pulsar can complete the measurement of the reference satellite clock error, but in order to better eliminate observation errors, multiple observations of multiple millisecond pulsars are needed to improve Observation accuracy. After the measurement of the on-board clock error of the reference satellite is completed, since the relative time synchronization of each satellite in the navigation constellation has been completed, the clock error of each satellite in the navigation constellation and the standard time TCB can be calculated. Convert the clock error of each navigation satellite satellite clock relative to TCB into clock error relative to TT (Earth Time), and broadcast the clock error correction value in the broadcast ephemeris of each satellite, which can ensure the user's Navigation and timing service accuracy, that is, the autonomous time synchronization of navigation satellites is completed.

Claims (1)

1. the autonomous method for synchronizing time of Navsat based on synthetic aperture observation is characterized in that comprising the steps:

Step 1, according to the ephemeris of each Navsat and the location parameter of millisecond pulsar, selected one group of Navsat that can observe simultaneously same millisecond pulsar, and therefrom choose one wantonly as reference satellite, remaining Navsat is as collaborative observation satellite, and the observation data of collaborative observation satellite wants reduction to the reference satellite place;

Step 2, all collaborative observation satellites and reference satellite are carried out timing observation to same millisecond pulsar simultaneously, and original observed data is separately preserved; Simultaneously collaborative observation satellite and reference satellite according to separately the Position And Velocity parameter finish Doppler effect, the special relativity effect and Shapiro delay correction of original observation signal;

Step 3, after each timing observation was finished, each collaborative observation satellite was sent to reference satellite with observation data separately;

Step 4, reference satellite are utilized different constantly relative position measurement data between star, finish each collaborative observation satellite to the time delay correction at reference satellite place, and concrete grammar is:

At t constantly, i collaborative observation satellite position vector with respect to reference satellite in inertial space is

The direction vector of the millisecond pulsar of observing is

Then t constantly collaborative observation satellite to the TOA measurement data time delay corrected value of reference satellite be

Wherein c is the light velocity; The photon sequence time of arrival toa that collaborative observation satellite i observes _IjCorrect one by one, obtain one group of new observation sequence time of arrival, the detector that namely is equivalent to collaborative satellite i is placed on resulting TOA observation sequence on the reference satellite;

Step 5 is carried out overlap-add procedure to obtain the observation waveform of synthetic aperture timing recording geometry to the observation data of respectively organizing of finishing the time delay correction;

Step 6 is measured the clock correction of the spaceborne clock of reference satellite, and concrete grammar is:

If the unit direction vector of the millisecond pulsar of observing in solar system geocentric coordinate system be

Wherein

Be respectively three coordinate components of unit direction vector in solar system geocentric coordinate system of the millisecond pulsar of observing, the solar system geocentric coordinate system position vector of reference satellite when epoch of observation is

Wherein

Be respectively three coordinate components of position vector in solar system geocentric coordinate system of reference satellite, the pulse arrival time that the observation of synthetic aperture timing recording geometry obtains is toa _Sat, the moment that the same pulse that is obtained by the clock model prediction of this millisecond pulsar arrives solar system barycenter is toa _SSB, then observation equation is

$({\mathrm{toa}}_{\mathrm{sat}}-{\mathrm{toa}}_{\mathrm{SSB}})\&CenterDot;c={\stackrel{\&RightArrow;}{k}}_x\&CenterDot;{\stackrel{\&RightArrow;}{r}}_x+{\stackrel{\&RightArrow;}{k}}_y\&CenterDot;{\stackrel{\&RightArrow;}{r}}_y+{\stackrel{\&RightArrow;}{k}}_z\&CenterDot;{\stackrel{\&RightArrow;}{r}}_z+\mathrm{\&Delta;}{t}_c\&CenterDot;c+\mathrm{\&delta;}\&CenterDot;c$

In the formula, Δ t _cBe the deviation of the spaceborne clock of reference satellite and standard time TCB, δ is the summation of all the other timing observational errors;

Step 7, calculate the clock correction of each satellite of navigation constellation and standard time TCB, the spaceborne clock of each Navsat is converted to clock correction with respect to terrestrial time with respect to the clock correction of TCB, and in every satellite broadcast ephemeris separately, the clock error correction value is broadcast away, just can guarantee user's navigation and time service service precision, namely finish the autonomous time synchronized of Navsat.

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