1. Introduction
The strapdown inertial navigation system (SINS) is a completely independent navigation system that does not depend on the transmission of external signals [
1], and it is therefore the most widely applied positioning and attitude determination navigation system [
2]. The initial alignment is one of the key technologies of SINS and provides the initial attitude information for SINS. The accuracy of the initial alignment directly affects the navigation accuracy, and the duration time of the initial alignment impacts the system’s rapid response ability [
3]. Since most of the current methods of self-alignment of SINS estimate the attitude matrix by both coarse and precise alignment [
4,
5,
6], the accuracy and speed of coarse alignment directly influence the accuracy and speed of alignment.
Inertial navigation coarse alignment is divided into static and swing alignment according to the motion state of the carrier. Static coarse alignment uses gravity and the Earth’s rotation to determine the attitude matrix. At present, the problem of static coarse alignment has essentially been solved [
3,
7]. In the case of coarse alignment on the swing base, the carrier itself swings; for example, ships in mooring state swing because of engine speed, unit movement, cargo loading, wave impact, etc., and road vehicles swing because of engine speed, cargo loading, etc. Since the swing of the carrier itself makes it impossible to distinguish the angular velocity of the carrier from the angular velocity of the earth, the static alignment method cannot be used to determine the initial attitude matrix, and the noise will increase when the carrier shakes. Thus, it is necessary to design a new coarse alignment method that relies solely on gravity for observation and to minimize the impact of noise on the estimation results.
Currently, the coarse alignment methods for SINS on a swing base can be divided into two categories: one category is to directly determine the initial attitude matrix through the gravity vectors of two or more moments by the TRIAD algorithm [
8,
9,
10,
11,
12]. In [
8], the initial rotation matrix is decomposed by attitude matrix decomposition to isolate the influence of the swing base and estimate the rotation matrix by two noncollinear gravity vectors. On this basis, [
9,
10,
11] one can reduce the impact of the inertial measurement unit (IMU) output noise on the estimation results by adding a low-pass filter. Different from other studies [
9,
10,
11], the authors of [
12] used an LMS adaptive filter to reduce sensor noise, and the authors of [
13] computed two initial rotation matrices using the gravity vectors from multiple moments and weighted averaged according to the standard deviation observed at each time, which improves the alignment accuracy. In an earlier study [
14], to extract the effective observation vectors from the inertial sensors’ outputs, a method for parameter recognition and vector reconstruction was designed, where an adaptive Kalman filter was employed to estimate the unknown parameters. However, these alignment methods require two noncollinear gravity vectors as observations, which would require an interval of at least 50 s between each observation. Therefore, these methods require a long alignment time to ensure alignment accuracy. In addition, the insufficient utilization of observation information is another factor leading to low alignment accuracy. To solve this problem, researchers have proposed a second kind of method, which transforms the coarse alignment problem into Wahba’s problem [
15,
16,
17,
18,
19,
20]. In [
15], the Q-method is suggested to resolve the Wahba problem to achieve coarse alignment. A method of extracting the attitude quaternion from the constructed K-matrix by the Newton iterative algorithm is proposed in [
16]. In [
17], the observation vector is recursively processed by using the filtered quaternion estimation (QUEST) method. An Optimal-REQUEST is proposed [
18], and compared to the traditional attitude determination method, the filtering gain of the proposed method is tuned autonomously; thus, the convergence rate of the attitude determination is faster than in the traditional method. In contrast to the Q-method, a new method of resolving Wahba’s problem based on eigenvalues is recommended in [
19], and symbolic solutions to the corresponding characteristic polynomial are derived. In an earlier study [
20], a method for solving Wahba’s problem based on SVD decomposition is proposed. Although the above Wahba method does not require noncollinear vectors as observations, nonconvex issues and those regarding the fact that it is not guaranteed to be globally optimal occur in resolving Wahba’s problem.
In contrast to the quaternion, the special orthogonal group
SO(3) conveniently represents the rotation in the serial movement of the carrier based on the mapping relationship between
SO(3) and Lie algebra. Therefore, attitude estimation methods directly based on
SO(3) have been developed instead of resolving Wahba’s problem in recent years. According to the properties and advantages of
SO(3), an
SO(3) optimization method is proposed in [
21] by adopting the vector product of the measurement and prediction vectors as error Lie algebra and updating the rotation matrix based on the Lie group differential equation. Several cost functions are discussed in [
22], which guarantee the properties of
SO(3) in terms of the definition of error rotation, and the cost function itself is defined by the difference between the observation and prediction vectors. In [
23,
24,
25], the Euclidean distance between the measurement and estimation vectors is employed as the attitude error function, and a system renewal equation is established with special orthogonal groups to realize attitude estimation. Although the abovementioned methods avoid the nonconvex problem of the Wahba methods and do not require noncollinear vectors as observation values, these methods all rely on the mapping relationship between the
SO(3) group and Lie algebra as the update equation, and the innovation term applied in these estimation methods on
SO(3) is not the true error of Lie algebra. Therefore, these existing estimation methods based on the special orthogonal group are not sufficiently accurate.
In this paper, a coarse alignment method based on the special orthogonal group optimal estimation method is developed. By integrating the advantages of inertial frame coarse alignment and special orthogonal group representation, a coarse alignment model is directly established based on the differential equation of the special orthogonal group. Because the state in the newly established coarse alignment model is the special orthogonal group of order 3 (), a novel optimal estimation method based on the SO(3) group is developed to estimate the initial attitude. In this estimation method, based on the physical definition of Lie algebra, the solution of error Lie algebra is transformed into the calculation of the rotation axis and angle of the error rotation matrix to acquire the true error Lie algebra. According to the right-hand rule, the rotation axis is determined by the vector product of the prediction and observation vectors. The rotation angle is obtained by calculating the angle between the prediction and observation vectors. Then, the initial SO(3) group is compensated by using the true error Lie algebra as the innovation term based on the mapping relationship between the SO(3) group and Lie algebra. The Lyapunov stability analysis is employed to prove the stability of the novel optimal estimation method. Finally, simulation and experiments are designed to verify the performance with respect to the alignment accuracy and time, and the proposed method is compared to existing coarse alignment methods.
The organization of this paper is as follows: in the
Section 2, the coarse alignment method based on
SO(3) representation and optimal estimation is proposed.
Section 3 analyzes the stability of the method developed in the second section.
Section 4 provides the results of the simulation and contrast experiments, and
Section 5 contains conclusions. The definitions of various reference systems in this paper are shown in
Table 1 2. Coarse Alignment Model Using SO(3) Representation
In this section, to avoid the nonconvex issue when resolving Wahba’s problem and the long alignment time of the TRIAD algorithm, the initial alignment model based on
SO(3) representation is established. The basics of Lie algebra and
are provided in the
Appendix A.
According to the chain rule, the attitude matrix can be decomposed into the products of those attitude matrices corresponding to the motions of Earth and the body and the attitude matrix at the initial time. With
SO(3) as the representation of the attitude, the attitude matrix on the swing base can be decomposed as follows:
where
represents the attitude matrix of the current
b frame relative to the current
n frame,
represents the attitude matrix of the current
b frame relative to the initial
b frame,
represents the attitude matrix of the initial
b frame relative to the initial
n frame, and
represents the attitude matrix of the initial
n frame relative to the current
n frame.
According to the differential equation of
SO(3), the attitude matrix
of the Earth’s motion and the attitude matrix
of the body’s motion can be updated in the following ways:
where
is the angular velocity of
b frame relative to
i frame under
b frame, which can be measured by the gyro;
, where
represents the angular velocity of
e frame relative to
i frame under
n frame. The parameter
is the angular velocity of
n frame relative to
e frame under
n frame, and they can be calculated with the following equation:
where
and
are the radii of the curvatures of the meridian circle and prime unitary circle, respectively, of the carrier;
is the speed along the north direction; and
is the speed along the east direction. In the case of swing base,
and
are approximately equal to 0, so
is approximately equal to 0;
is the rotation angular speed of Earth.
If the attitude matrix
is estimated,
can be obtained by Equation (1). Therefore, the state equation is set as
According to the alignment principles of SINS and the chain rule, the relationship between
and
is as follows:
where
and
are the velocities in
n frame and velocity in
b frame, respectively, and
and
can be obtained by the following equation:
where
is the gravitational acceleration under
n frame,
is the local gravitational acceleration,
,
is the gravitational acceleration relative to
b frame, and
can be measured by the accelerometer.
Therefore, the system model of coarse alignment based on
SO(3) representation can be written as follows:
3. Special Orthogonal Group Optimal Estimation
In this section, to avoid the error caused by using the inaccurate error Lie algebra as the innovation term in traditional SO(3) optimal estimation methods, a novel optimal estimation method on SO(3) is designed.
According to the definition of the rotation matrix and Equation (8),
and
are the same vectors under
n frame, and the vector product of
and
is 0. However, due to the occurrence of errors, there is a deviation between the estimated and real rotation matrices
, which causes the estimated rotation matrix
to be the attitude matrix of
b frame relative to
frame.
According to the chain rule, to compensate for the estimated rotation matrix, we need to calculate the rotation relationship between the
and
frames. According to the definition of coordinate system transformation, the above frame rotation relationship is equivalent to the rotation relationship between measurement vector
and prediction vector
, which has been previously described in a large number of papers [
1,
2,
26]. According to the above conclusion, the existing
SO(3) optimal estimation methods define the innovation as
or
, as shown in
Figure 1.
Because
SO(3) optimization methods employ the mapping relationship between the SO(3) group and Lie algebra as the update equation, the innovation term should be the Lie algebra corresponding to the error rotation matrix. However,
Figure 2 shows that these two vectors are not related to rotation, and although
is oriented exactly in the same direction as the rotation axis, its length sum is obviously not equal to the error Lie algebra, while
is completely irrelevant to the error rotation matrix. Therefore, the innovation term used in the traditional
SO(3) optimal estimation methods does not reflect the error of the attitude matrix, which inevitably affects the estimation accuracy.
According to the definition of Lie algebra (the rotation vector), it should be calculated as follows:
where
and
are the rotation axis and angle, respectively, of the rotation relationship between
and
. Based on the right-hand rule, the rotation axis has the same direction as
, and the rotation angle
is the angle between
and
, as shown in
Figure 2.
Figure 2 reveals that
exhibits precisely the same direction as the rotation axis
, and it can be calculated by unitizing
:
Obviously, the rotation angle
between
and
is the angle between vectors
and
. Therefore,
can be obtained by the following equation:
According to Equation (10), the accurate innovation term can be defined as:
Then, the update equation of
SO(3) group optimal estimation can be established as follows:
4. Stability Analysis
The stability is one of the important properties of the control system, and stability analysis of the mathematical model based on the controlled object is an indispensable task in system design. For the proposed optimal estimation method, the purpose of stability analysis is to prove that the estimated value can approach the real value within a certain period of time. Therefore, Lyapunov stability analysis is conducted to prove that the proposed optimal estimation method is asymptotically stable.
The task of the optimal estimation method on
SO(3) is to ensure that
at time
. Therefore, the system state error
is defined as follows:
According to the unit orthogonal property of the
SO(3) group, when the estimated rotation matrix is equal to the real rotation matrix, the following applies:
Therefore, to prove the stability of the method proposed in this paper involves proving that at , which can be accomplished using Lyapunov’s second law.
Because
is a time-invariant matrix, the derivative of the state error to time is as follows:
Substituting Equation (14) into Equation (17), we obtain
The norm of the
SO(3) matrix can be defined as follows [
27]:
For
where
is the Lie bracket operation, and
is the Lie algebra corresponding to the
SO(3) group
. According to Equations (19) and (20), the candidate Lyapunov function is constructed as follows:
It can be proven that the above candidate Lyapunov function has the following properties:
Property 1. If and only if; for anyand. In other words, the selected candidate Lyapunov function contains unique zeros and is positive definite.
Proof. For any skew-symmetric matrix
, the following requirements are met:
Substituting Equation (23) into Equation (22), we obtain
When , the corresponding Lie algebra , for , and when , the corresponding Lie algebra , for . □
Property 2. For any, functionis differentiable at time, andholds for. In other words, the selected candidate Lyapunov function is differentiable, and the derivative is negative.
Proof. For any matrix
holds, and then
To further prove that
holds for
, the following theorems are required, which are proven in detail in [
18]. □
Theorem 1. The derivative of Lie algebrawith respect to time has the following relation with its correspondingSO(3) group:
Theorem 2. For anyand, the following equation holds: Applying Theorem 1 to
:
Substituting Equation (18) into Equation (29) yields
Applying Theorem 2 to Equation (30):
The positive and negative parts of Equation (33) are determined by the scalar product of the two vectors. Therefore, the positive and negative parts of Equation (31) can be determined by determining the angle between and .
Because
is the Lie algebra corresponding to the error rotation matrix, the direction is the same as that of the rotation axis described by the error rotation matrix.
According to the definition of Lie algebra, the direction of
is the same as the direction of the rotation axis determined by
, as shown in
Figure 3.
Equation (13) indicates that the direction of is determined by , and according to the right-hand rule, and the rotation axis determined by are oriented along the opposite direction. Therefore, and exhibit the opposite direction, and holds. Due to the observation error, the direction of may not be exactly opposite to that of , but as the observation error is not too large, can still be guaranteed.
In summary, the selected Lyapunov candidate function is positive definite and contains a unique zero point, and the derivative of the candidate function exists and is seminegative. Hence, the system is gradually stable.
5. Simulations and Experiments
In this section, simulations and experiments are performed to verify and test the performance and advantages of the special orthogonal group optimal estimation (SOGOE) proposed in this paper. In these simulations and experiments, the SOGOE method is compared to four currently popular coarse alignment methods, namely, the TRIAD method, the QUEST method, the adaptive identifier on special orthogonal group (AISOG) method and the fast-linear attitude estimator (FLAE).
The accuracy of these alignment methods is assessed by the difference between the true value of the attitude angle and that calculated by the estimated attitude matrix, as expressed in the following equation:
5.1. Simulation Results and Analysis
The simulation parameters and environment are as follows:
The equatorial radius is = 6,378,165.0 m, the gravitational acceleration is g = 9.7849 m/s2, the angular velocity of Earth is 0.00007292758 rad/s, the initial position is east longitude 118°, and the north latitude is 40°. The gyro constant drift is 0.1°/h, the random drift is 0.01°/h, the constant deviation of the accelerometer is 1 × 10−3 g, and the random measurement noise is 3 × 10−4 g. The attitude update period is set as T = 0.02 s, and the alignment lasts 200 s.
The posture change trajectory is as follows:
where
,
and
are the yaw, pitch and roll, respectively.
In the simulation experiment, Equation (34) is adopted as the real value of the attitude matrix, and the attitude angle errors are calculated with Equation (33).
The alignment results of the simulation experiments are shown in
Figure 4. The alignment time of the above three methods and the variance and mean analysis results of the four different methods from the beginning to the end of convergence are summarized in
Table 2. In addition, the TRIAD algorithm is executed six times in total, each time requiring 100 s, and the mean value and variance in the six-time alignments with the TRIAD algorithm are also listed in
Table 2. The standard deviations of the four different methods are provided in
Table 3.
Figure 4 and
Table 2 and
Table 3 reveal that the alignment methods based on
SO(3) optimization attain a better performance than the alignment methods based on the TRIAD algorithm and Wahba’s problem. More importantly, because the SOGOE method uses a more accurate error Lie algebra as compensation information, it is significantly better than the AISOG method in terms of the alignment speed and accuracy.
5.2. Experimental Results and Analysis
To verify the performance of the proposed coarse alignment based on SOGOE, we perform experiments on a swing base. The experimental equipment is installed on a surface platform, which is located on an open lake. In this experiment, the XW-ADU7612 Attitude Direction Unit Integrated Navigation System and Crossbow VG700AB are applied to conduct the experiments. XW-ADU7612 has a built-in high-precision IMU that independently determines north and is combined with a dual GPS, which outputs high-precision attitude information. In this experiment, the attitude information output of XW-ADU7612 is adopted as reference information. In addition, the XW-ADU7612 module operates stably, which makes the reference information it provides reliable. The system accuracy of this module is summarized in
Table 4. In the experiment, Crossbow VG700AB is employed to obtain the values of the gyro and accelerometer. The precision of the IMU is provided in
Table 5, and the installation of the experimental equipment is shown in
Figure 5. During the experiment, we change the yaw by artificially pulling the platform and manually swing it to simulate the berthing of a ship on water. The initial position is 116.7536221°E and 40.0437500°N. The baseline length is 2.125 m. The computer used in the experiment has an I5 CPU, 16GB ram and 256GB hard disk to ensure the real-time performance. In addition, the computer can support multiple CPU cores to run real-time programs simultaneously.
The change in the carrier attitude angle during the experiment is shown in
Figure 6.
The alignment results of the experiment are shown in
Figure 7. The alignment time of the above four methods and the variance and mean analysis results of the four different methods from the beginning to the end of convergence are summarized in
Table 6. In addition, the TRIAD algorithm was executed six times in total, each time requiring 100 s, and the mean value and variance in the six-time alignments with the TRIAD algorithm are also provided in
Table 6. The standard deviation of the three different methods is listed in
Table 7.
As shown in
Figure 7, since QUEST and FLAE are based on Wahba’s problem to estimate the initial rotation matrix, the above methods inevitably have the problem of non-convexity in Wahba’s problem, which leads to the poor accuracy and convergence speed of QUEST and FLAE compared with SOGOE and AISOG. More importantly, SOGOE attains higher estimation accuracy than AISOG because it uses a more accurate solution to error Lie algebra.
In order to better compare the results of the algorithm, the statistical results in
Table 6 and
Table 7 are used to further analyze the effect of the algorithm. It can be seen from
Table 6 that TRIAD needs to use a pair of non-collinear vectors as observation, so its alignment time, variance and accuracy are poor. QUEST and FLAE can avoid the problem of triad by transforming the alignment problem into Wahba’s problem. However, the estimation accuracy and speed are limited due to the non-convex problem of Wahba’s problem. The optimization method based on
SO(3) avoids the non-convex problem in Wahba’s problem and keeps the advantage of alignment speed based on optimization. It can be seen from
Table 6 that the alignment accuracy and speed of AISOG are better than those of QUEST and FLAE. The most important thing is that SOGOE exerts better effect than AISOG by using a more accurate solution to error Lie algebra. Compared with the traditional QUEST and FLAE method, the alignment speed of SOGOE is improved by about 20 s, and the accuracy is improved by about 20′. In addition, since AISOG and SOGOE depend more on the accuracy of the observation information, the standard deviation increases with the increase in the observation noise under experimental conditions. However, it can be seen from
Table 7 that the standard deviations of the two methods are still similar to those of QUEST and FLAE.
5.3. Position Estimation Experiment
In order to reflect the influence of alignment results on the position estimation process, experiments are carried out on the actual road. This experiment also uses the XW-ADU7612 Attitude Direction Unit Integrated Navigation System and Crossbow VG700AB and the same computer to conduct the experiments. In this experiment, the attitude information output of XW-ADU7612 is adopted as reference information. The installation of the experimental equipment is shown in
Figure 8. The antenna is installed on the roof of the car, with two antennas in the front and rear. The baseline length is 2.315 m. The inertial unit and the integrated navigation system are fixed in the car and coincide with the car body coordinate system.
In order to ensure that the GPS receives good strength of signals during the experiment and that the experimental environment is sufficiently complex, the Fifth Ring Road in Beijing is selected as the test route, and the vehicle trajectory is shown in
Figure 9. The alignment results of QUEST, FLAE, AISOG and SOGOE are shown in
Figure 10 and
Table 8.
To illustrate the impact of alignment accuracy on position estimation, position calculations are performed based on the initial attitude matrices calculated by four methods. In order to highlight the influence of the initial rotation matrix on the position calculation, the position calculation only uses a simple method of integrating the IMU single sensor output to calculate the position. Since the location is solved in the above way, the result of long-term estimation is bound to diverge, so the position estimation results from the initial position to about 2000 m, where there is a significant deviation between the four methods and the real trajectories are intercepted, as shown in
Figure 11.
As shown in
Figure 11, due to alignment result errors, there is a deviation between the direction of speed and the actual course in subsequent position estimates. For a vehicle moving in a 2-D plane, this bias is mainly reflected in the heading bias in the position estimation process. Although the effect of alignment accuracy is not obvious when the driving distance is closer, the effect of initial alignment accuracy on position estimation increases as the driving distance increases. As shown in
Figure 11, SOGOE has a significantly better alignment accuracy than other methods, so it can be seen that SOGOE is significantly better than traditional QUEST, FLAE, and AISOG methods when driving at around 1 km.
By comparing the simulation and experimental results of the five methods, it is found that the alignment method represented by SO(3) attains a higher alignment accuracy and shorter alignment time than the alignment method using the unit quaternion. Compared to AISOG, the SOGOE method proposed in this paper provides a more accurate update model to make the attitude estimation more accurate, which is especially evident in the experimental results.
6. Conclusions
In this paper, a novel coarse alignment method using special orthogonal group optimal estimation is proposed for SINS on a swing base. The coarse alignment model is based on the Lie group differential equation of the special orthogonal group, which avoids the nonconvex problem of Wahba’s model in the traditional coarse alignment method. The proposed optimal estimation method for SO(3) adopts the precise error Lie algebra as the innovation term to compensate for the initial SO(3) group in the estimation process, which avoids the error caused by the inaccurate innovation term in traditional SO(3) optimization methods. Therefore, in theory, the proposed coarse alignment method has clear advantages in the alignment time and accuracy. The simulation and experimental results all prove that the proposed novel coarse alignment method demonstrates a clear improvement in the alignment time and accuracy over the existing methods. Therefore, Since investigations of SO(3) in inertial navigation are still relatively limited, further studies of the applications of SO(3) are planned.
Thus, the proposed method maintains the advantages of the attitude estimation method based on SO(3) optimization, and avoids the nonconvex issue of the coarse alignment problem based on Wahba’s problem which needs a non-collinear vector as observation. Moreover, the proposed optimal estimation method for SO(3) adopts the precise error Lie algebra as the innovation term to compensate for the initial SO(3) group in the estimation process. This avoids the error caused by the inaccurate innovation term in traditional SO(3) optimization methods. Although the proposed alignment method is more effective and exhibits excellent application prospects for SINS on the swing base, similar to most SO(3) optimization methods, the estimation results are more dependent on the accuracy of the observation information, and the variance of the estimation results will be greatly affected by the observation error.