KR20240099766A - Integrity monitoring method based on time differenced carrier phase measurement for satellite navigation system - Google Patents

Abstract

A satellite navigation system integrity monitoring method based on a carrier phase time difference measurement value comprises the steps of: determining an initial position and a reliability level; determining a relative position and a reliability level; monitoring a failure of a satellite; determining an absolute position reliability level; and calculating a protection level.

Description

{INTEGRITY MONITORING METHOD BASED ON TIME DIFFERENCED CARRIER PHASE MEASUREMENT FOR SATELLITE NAVIGATION SYSTEM}

The present invention relates to integrity monitoring of a satellite navigation system, and more specifically, to a technology for performing navigation based on a relative position determined using measurements obtained by time-differentiating carrier phase measurements of GNSS and monitoring the integrity of the determined position solution. This is about.

A general positioning algorithm using a GPS receiver receives only direct signals without distortion in an open area environment and has a positioning accuracy of about 2 to 10 m when the receiver is used alone. In addition, if the error elements of the signal calculated from the reference station can be received, accuracy can be achieved even at 1m or less.

However, as in an urban forest environment, accuracy decreases up to 100m or more due to signal blocking by buildings and distortion of signals. In addition to accuracy, integrity is an important factor in determining the performance of a satellite navigation system.

Integrity includes the ability to accurately remove a faulty satellite when a satellite failure occurs, and is closely related to user safety. When integrity information is received from an external source, an external reference station monitors satellites for failure and then delivers correction information to the user.

A representative system is SBAS, which utilizes geostationary satellites. However, in urban areas, geostationary satellite signals are blocked by high-rise buildings, which may cause SBAS unavailability. Even in this case, users must monitor the integrity themselves to perform safe navigation.

Measurements that can be collected from a GPS receiver include pseudorange and carrier phase measurements. First, the pseudorange measurement is a measurement that simply indicates the distance between the satellite and the user and has the advantage of being simpler to use than the carrier phase measurement. However, it has large noise of the m level and causes extreme performance deterioration in situations where signals are blocked and distorted, such as in urban areas. There are drawbacks.

The carrier phase measurement consists of the distance between the satellite and the user and an arbitrary integer, and the arbitrary integer is generally called an undetermined number. At this time, the carrier phase measurement has mm-level noise and is robust to signal distortion in urban areas, but there is a problem of determining an unknown constant.

Because carrier phase measurements have better signal characteristics than pseudoranges, expensive surveying equipment calculates the position by determining an unknown number. There are various methods for determining an unknown number, but they require considerable time and complex calculations. Moreover, in the case of dynamic users, determining the unknown number is known to be more difficult. Therefore, in the case of low-cost (general) receivers, carrier phase measurements can be collected, but it is difficult to use them directly for navigation, so pseudorange-based navigation is performed.

KR 10-2263393 B1 KR 10-2288771 B1

This research and development describes an algorithm that enables precise and safe navigation while maintaining initial performance for a certain period of time when external integrity correction information is lost.

The present invention relates to integrity monitoring of a satellite navigation system. More specifically, it is a technique in which a receiver independently monitors the integrity of a satellite navigation system using the time change rate of a carrier phase measurement. Most existing pseudorange-based integrity monitoring methods show poor performance in urban areas where multipath errors are severe. On the other hand, using carrier phase measurements provides the advantage of low noise level and robustness to multipath, but as mentioned earlier, there is the difficulty of determining unknown parameters. However, since the unknown number has the characteristic of remaining in integer form even when time changes, if the time change rate is used, the precise position can be estimated through the carrier phase measurement without the need to determine the unknown number.

At this time, because the carrier phase measurement has robust characteristics against multipath signal distortion, when used, it is possible to perform precise navigation compared to pseudorange in situations where multipath error is extreme, such as in urban areas. When using time differential carrier phase measurements, the relative position (moving distance) is estimated from the reference position. Therefore, in order to obtain the absolute position, the relative position must be added to the reference position. At this time, the initial position is assumed to be one whose integrity is guaranteed from an external integrity monitoring system such as SBAS (Satellite Based Augmentation System). Since the integrity of the initial location is assumed to be guaranteed, integrity monitoring is performed only for the relative location.

The time differential carrier phase measurement used to determine the relative position has a small noise level compared to the pseudorange and leaves only a time change rate that is relatively small in size compared to the satellite error element, so the performance obtained through the initial external correction information can be maintained for a certain period of time. In addition, the present invention has great utility in that it can be used not only in expensive GPS receivers but also in equipment based on low-cost GPS receivers.

1 is an operation flowchart of a satellite navigation system based on carrier phase time difference measurements according to an embodiment of the present invention.
Figure 2 is a flowchart of a satellite navigation system integrity monitoring method based on carrier phase time difference measurements according to an embodiment of the present invention.
Figure 3 is a graph comparing the present invention with pseudorange-based RAIM and multi-frequency-based RRAIM.

The detailed description of the present invention described below refers to the accompanying drawings, which show by way of example specific embodiments in which the present invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It should be understood that the various embodiments of the invention are different from one another but are not necessarily mutually exclusive. For example, specific shapes, structures and characteristics described herein with respect to one embodiment may be implemented in other embodiments without departing from the spirit and scope of the invention. Additionally, it should be understood that the location or arrangement of individual components within each disclosed embodiment may be changed without departing from the spirit and scope of the invention. Accordingly, the detailed description that follows is not intended to be taken in a limiting sense, and the scope of the invention is limited only by the appended claims, together with all equivalents to what those claims assert, if properly described. Similar reference numbers in the drawings refer to identical or similar functions across various aspects.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the drawings.

1 is an operation flowchart of a satellite navigation system based on carrier phase time difference measurements according to an embodiment of the present invention.

Main utilized sensors: GPS receiver module

Main measurements consist of: carrier phase time difference measurements (=time rate of change of carrier phase measurements)

Relative position estimation algorithm

▷ Initial position estimation unit

- Estimation of initial position (reference position) with guaranteed integrity using external integrity correction information

- Estimation of initial position error covariance using integrity correction information

- Integrity correction information provided not only by SBAS but also by other GNSS Integrity Channels can be utilized.

▷ GPS-based relative position estimation unit

- Estimation of relative position (moving distance) using carrier phase time difference measurements alone

- Estimation of error covariance of estimated relative position

- Determination of failure of satellite measurements by calculating judgment variables and threshold values of the estimated relative position

▷ Failure monitoring unit

- When estimating relative position, set decision variables using a combination of time differential carrier phase measurements.

- Monitor satellite failure after setting a threshold using the probabilistic range of the judgment variable

▷ Integrated absolute position estimation unit

- Determine the absolute position by adding the previously estimated integrated relative positions based on the absolute initial position where the navigation system starts.

▷ Integrated absolute position protection level estimation unit

- Calculate the absolute position error covariance consisting of the correlation between the initial position and the relative position using the error covariance of the two.

- Calculate protection level using absolute position error covariance and relative position judgment variable threshold

The effects that can be achieved through the present invention can be summarized into three categories.

1) Development of advantageous integrity monitoring technology in urban forest environments

- Precise location acquisition and integrity monitoring are possible even in urban areas with severe signal distortion errors by estimating precise relative position using carrier phase time difference measurements that are robust to signal distortion and summing them from the initial position with guaranteed integrity.

2) Development of low-cost GPS receiver-based integrity monitoring technology

- Unlike existing algorithms that solved the ionospheric problem based on multiple frequencies, we proposed an algorithm that can be applied not only to multiple frequencies but also to single-frequency-based low-cost GPS receivers by directly considering the time change rate of the ionospheric delay error.

3) It is possible to obtain relative navigation accuracy within m level without reference station correction information data.

- The pseudorange measurements used by general GPS receivers have a signal distortion error of up to 100m or more, but the carrier phase measurements have a size of up to 4~5cm. Therefore, in the case of this navigation system utilizing this, it basically has dm-level relative navigation accuracy in an open environment where there is no change in visible satellites, and m-level relative navigation accuracy even in urban areas where visible satellites rapidly decrease.

4) Maintain existing performance in situations where external integrity correction information is disconnected

- If integrity correction information is initially received from the outside, the information can be used even if the correction information signal is cut off, making it possible to maintain existing performance for a certain period of time.

5) Reduction of navigation solution decision time and cost reduction

- When using carrier phase measurements, there is no need to use the existing complex method of determining unknown constants because it is used by time difference rather than determining unknown constants. Accordingly, even low-cost GPS receivers can determine the location in a short time.

6) Location accuracy and integrity performance can be improved by utilizing multi-satellite constellation measurements.

- Improvements in location accuracy and integrity performance can be expected by combining various satellite navigation systems such as Galileo, GLONASS, and BediDou as well as GPS.

Figure 2 is a flowchart of a satellite navigation system integrity monitoring method based on carrier phase time difference measurements according to an embodiment of the present invention.

The algorithm of the present invention can be divided into a total of 5 steps.

[Step I] Determine initial location and trust level

- Determining the initial location and confidence level through the output using a GPS (Global Positioning System) receiver refers to the location where the integrity of the current satellite navigation receiver is guaranteed, such as SBAS (Satellite Based Augmentation System) and GBAS (Ground Based Augmentation System). Includes all ways to obtain

- The carrier phase measurements, initial position, and initial position confidence level obtained at the initial time are stored and used to determine relative position and relative position confidence level.

[Step II] Determine relative location and trust level

- Obtain carrier phase time difference measurements by time-differentiating the carrier phase measurements output from the GPS receiver and obtain the relative position through this.

- Obtain carrier phase time difference measurements by differentiating the carrier phase measurements saved at the initial time and the carrier phase measurements acquired at the current time.

- Relative position at the centimeter level can be determined using a single frequency, single satellite constellation low-cost satellite navigation receiver without the help of a correction information system.

- When the carrier phase measurements are time-differentiated, errors in the form of bias are eliminated. Therefore, the relative position error based on time differential carrier phase measurements is affected by the amount of change in satellite navigation error elements, and among them, the ionosphere, convection layer, and satellite clock error change are the main error factors. The confidence level is determined through the model for the residual error of the measurements remaining after time differentiation.

- If double or triple frequency measurements are used, ionospheric errors can be estimated and removed without correction information from the reference station, thereby improving relative position accuracy.

- The number of visible satellites can be increased by utilizing not only a single satellite constellation, but also multiple satellite constellations (US GPS, China's BeiDou, Europe's GALILEO, Russia's GLONASS, etc.), thereby reducing DOP (Dilution of Precision) and increasing location accuracy.

- In order to utilize multiple frequencies and multiple satellite constellations, the frequency difference between measurements must be considered.

[Step III] Failure monitoring

- In order to monitor the integrity of the receiver alone, satellite failure monitoring is necessary.

- Measurements where a failure is detected are not used when estimating the relative position later.

- The difference between the relative position calculated using all measurements and the relative position calculated excluding the measurements of one arbitrary satellite is defined as the judgment variable. The reason for excluding the measurements of one satellite at this time is because of which satellite the failure occurred. Because it is unknown (perform failure monitoring for all satellites by calculating decision variables for all possible combinations).

- In a general situation, a threshold is set to detect a failure through the probability distribution that the judgment variable may have. If the judgment variable has a value greater than the threshold, a failure occurs in the satellite that was removed when calculating the judgment variable. I decided I did it

- If the threshold is too large, a Missed Detection situation may occur, putting the system at risk. Therefore, a smaller threshold is more advantageous from a safety perspective, but if the threshold is too small, false alarms may occur frequently and the continuity of the system may deteriorate. has exist. (Setting a threshold that satisfies continuity requirements)

[Step IV] Determination of absolute location confidence level

- The absolute position can be determined by adding the relative position obtained in the previous step to the initial position.

- The confidence level of the absolute position is composed of the confidence level of the initial position, the confidence level of the relative position, and the correlation between the initial position and the relative position.

[Step V] Calculate protection level

- If integrity correction information can be provided from the outside, it is possible to know whether the satellite is broken, so the protection level can be calculated using only the confidence level for the initial location.

- If external integrity correction information is not received, both the normal operation situation and the failure situation of the navigation satellite are assumed when calculating the protection level.

- The protection level for normal operating conditions is calculated in the same way as the protection level in the initial position.

- Assuming a failure situation, the protection level is calculated by using the threshold value, which is the size of the maximum failure that cannot be detected by single receiver failure monitoring, and the confidence level of the absolute position.

The results of a simulation of a drone landing vertically in a city center are summarized as follows.

Figure 3 is a graph comparing the present invention with pseudorange-based RAIM and multi-frequency-based RRAIM.

Simulation environment

- Experimental trajectory: constant velocity vertical descent at 1m/s for 60 seconds

- Experimental environment: Visible satellite reduction due to buildings to simulate urban areas

- Utilization of GPS satellites

- Simulation test results: In a situation where visible satellites are reduced in an urban forest, the final vertical protection level is 34.7m, and the protection level performance is within the requirements of LPV-200 (vertical warning limit 35m).

(RAIM protection level based on pseudo-distance in the same section occurs up to 236m)

(In the case of multi-frequency-based RRAIM, the final vertical protection level is 24.4m, and the protection level of the present invention considering the ionospheric error change rate is calculated to be 30% larger. However, the present invention provides a single-frequency low-cost GPS receiver to which the existing algorithm cannot be applied. (Similar performance to existing can be obtained)

As can be seen from the above results, when using the integrity monitoring technique of the present invention, a level of protection similar to the initially obtained integrity performance is maintained even in situations where external integrity correction information is cut off.

Although the above has been described with reference to the embodiments, those skilled in the art can make various modifications and changes to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below. You will understand.

In order to directly use carrier phase measurements, determination of an unknown number is essential, which requires a lot of time and cost. However, if only the time rate of change is used, the precise position can be estimated through carrier phase measurements without the need to determine an unknown number. This method is significant in that it secures not only precise location but also reliability by applying integrity monitoring technology that monitors satellite failures and calculates the level of protection considering the stochastic error of the estimated location. This study is designed with a single frequency-based algorithm and can be applied to low-cost receivers. It is an advanced technology compared to prior technologies in that it can be applied even in urban areas where there is a lack of visible satellites by utilizing a multi-satellite constellation.

Currently, the present invention is at level 3 of Technology Readiness Level (TRL). The basic design has been completed to a level where basic performance can be verified in a laboratory environment. As a result of the simulation, it was shown that the integrity requirements of the LPV-200 level were satisfied with only a single frequency.

Navigation using a satellite navigation system is used in all fields related to navigation, and is applied to all means of transportation, including airplanes, ships, and automobiles, as well as unmanned vehicles. In addition, the urban air mobility (UAM) field, where this technology can be applied, is currently the most popular field and continues to show high growth rates. Morgan Stanley predicts that the global UAM market size will grow rapidly from $7 billion in 2020 to $1.4739 trillion in 2040.

Claims (1)

determining an initial location and confidence level;
determining relative location and trust level;
Monitoring satellite failures;
determining an absolute location confidence level; and
A method for monitoring the integrity of a satellite navigation system based on carrier phase time difference measurements, comprising: calculating a protection level.