1. Introduction
Fixed-wing unmanned aerial vehicles (UAVs) have been widely used in area surveillance, convoy protection, coordinate combat, and forest firefighting. One of the main applications is cooperative standoff “targets” tracking, usually a vehicle on the ground, static or moving. To render the problem feasible, there is a need to divide it into three subproblems. The first subproblem is to generate the optimal flyable path for a single UAV where UAVs should follow in order to achieve stable standoff tracking of static targets. In the standoff tracking approach, UAVs should stably keep an appropriate distance from the target. The next component is concerned with standoff tracking of moving targets, mainly the states (location and velocity) estimation of the target, which could help modify the guidance law for a moving target. However, it is usually ignored in many researches. The last subproblem is the coordination of a team of UAVs, mainly keeping a fixed intervehicle phase. The three subproblems are interrelated and they should be solved simultaneously.
Tremendous research efforts have been made in order to generate the optimal path for a single UAV. The guidance laws are usually based on several approaches: Helmsman behavior control [
1], model prediction control [
2,
3], reference point control [
4,
5], and vector field guidance. As a classic guidance law, Lyapunov vector field guidance (LVFG) has been improved up to now. It is in a decoupled structure in which the rate of heading and speed change are separately controlled for standoff distance and phase angle keeping. Dale L et al. [
6] introduced the Lyapunov vector field into the loitering algorithm for standoff tracking. Frew E.W et al. [
7] extended single UAV tracking to cooperative tracking. To shorten the convergence time, Chen H et al. [
8] combined the tangent with the Lyapunov vector field guidance and Oh H et al. [
9] used the tangent vector field in conjunction with the sliding mode control that is modified by introducing additional adaptive terms. Besides integrating control methods into vector field, Lim S et al. [
10] added a new non-dimensional parameter
into original algorithm, adjusting the convergence time to standoff circle by changing
and in this way, arrival position and time are controlled. Aiming to limit the UAV turn rate within the capability imposed by the angle limits, Pothen A et al. [
11] transformed
to a simple function of
, orbit radius. Shun Sun et al. [
12] analyzed the criteria guidance term
should fulfill, proposed a series of guidance functions satisfying the conditions, and an offline parameter search algorithm was designed for selecting the optimal function, while flexibility is limited. The methods mentioned above mainly concern themselves with converging to the standoff distance at the highest speed and keeping it steady, but they cannot address the fast convergence and stable tracking simultaneously.
When tracking a ground moving target, the UAVs need to modify the guidance laws using the state of the target. The access to the location and velocity of the target is of vital importance to standoff tracking. In the past research of guidance laws in standoff tracking, some researchers [
5,
7,
13,
14] simplify the problem by assuming that the state of the target is available. They assumed that the state of the target can be obtained via direct communication with a cooperative ground vehicle, or continuous data for the target state are used in experiments. It is obviously not realistic when tracking an uncooperative target. Other researchers take tighter experimental conditions into consideration. In Seungkeun Kim’s research [
2], the state-vector fusion based on the extend Kalman filter (EKF) was utilized to estimate the state of the moving target for two UAVs. Summers T.H et al. [
15] took unknown wind and target motions into one variable and proposed an adaptive estimator for a more accurate estimation of the unknown factor. A jerk model for tracking highly maneuvering targets was utilized in Qian Z’s work [
16]. In cooperative tracking of the moving target, Hu C. et al. [
17] applied squared-root cubature information filtering for data processing from four quadrotor UAVs, and the consensus algorithm is used in order to improve estimation accuracy. The methods mentioned above mostly cannot estimate the state of a maneuvering target satisfactorily or are not suitable for fixed-wing UAVs.
Due to the poor estimation of target position and velocity by a single UAV, a team of UAVs for cooperative standoff tracking are imperative. Kingston D et al. [
18] introduced an orbit radius change method without velocity control for phase keeping of multiple UAVs. Frew E W et al. [
7] proposed a guidance law that adjusts the speed of vehicle to maintain the desired relative phase on the loiter circle, which is adopted in phase separation of multiple UAVs by many researchers [
2,
15,
16]. Song Z Q et al. [
19] added a second derivative term to the speed controller for phase keeping. Oh H et al. [
9] realized the angular separation control by changing velocity or orbit radius in different information structures. Kokolakis N M T et al. [
20] extended the phase separation problem to tracking a moving target, focusing simultaneously on convergence toward the standoff radius, heading, and angular difference. Lim S et al. [
10] controlled the arrival position on the desired circle by adjust the parameter
, and changed the desired radius and vehicle speed to keep a certain phase between UAVs. He S et al. [
21] adopted a new leader–follower information architecture and an acceleration for space angle control. Xu Z [
5] newly developed the predicted reference point guidance method to reduce errors in standoff distance and phase angle control. Different methods are utilized to keep a fixed phase between neighboring UAVs, but the inter-vehicle phase cannot converge to the desired phase in finite time, which is harmful for cooperative tracking of a moving target.
Although significant and constructive research efforts have been performed, stable standoff tracking of uncooperative moving target by a team of UAVs has not yet been addressed properly. It is also a challenge to balance converging speed and stability of tracking, and the estimation of target states with less prior information still needs to be improved, which is of vital important to tracking moving targets. For cooperative tracking, faster settling down to the desired position helps more stable tracking.
Carrying out this study, we hope to deal with the three subproblems better, which means guiding UAVs to tracking target in a faster converging time, getting a more accurate estimation of target state, and making intervehicle phase converge to the desired value in a shorter time.
This paper’s principal contributions can be summarized as follows: Taking the trajectory curvature into consideration, an optimization algorithm based on the gradient descent method for searching the optimal guidance parameter is developed, which means faster convergence and stable tracking can be guaranteed. When UAVs are tracking a moving target, an interacting multiple model based on unscented Kalman filter (IMM-UKF) estimator is built for predicting the target state. In this way, the stability of the uncooperative target is improved. For cooperative tracking, a new speed control-based phase-keeping controller is designed to achieved faster convergence to the desired intervehicle phase, and the stability of controller is proved.
The remainder of the paper is organized as follows:
Section 2 mainly introduces the problems to be handled with. In
Section 3, the overall framework of the LVFG algorithm is presented, and two components of guidance are analyzed. The proposed solution is introduced in
Section 4, and fast but stable tracking, estimator for target states, and phase-keeping controller are presented. To verify the feasibility and benefits of our methods, numerical simulation and Hardware-In-the-Loop (HIL) simulation are conducted, of which the results are presented in
Section 5, and the conclusions are described in
Section 6.
3. Lyapunov Vector Field Guidance Framework
Lyapunov vector field guidance proposed by Lawrence D [
6] and its modified version are utilized to lead the fixed-wing UAV to approach the desired standoff distance and circle around a target in the local coordinate system. A nondimensional parameter is introduced to improve the convergence performance of LVFG [
10]. The basic framework is
where
is a parameter, which can be used to adjust the speed of the generated field converging to the standoff circle. If
, the vector field is the original version of LVFG.
In the above equation,
represents the speed of radial convergence to desired circle, and can be called a contraction component, while
denotes circulation component, which denotes the tangent speed, as shown in
Figure 3.
To discuss the influence of
on the contraction and circulation,
can be represented as
Substituting Equation (5) into Equation (4), we get
Curves of contraction and circulation components with
as a constant are shown in
Figure 4. Without loss of generality, we set
as a constant at 1.
As shown in curves in
Figure 4a, as
goes from
or
to
, meaning that the UAV approaches the standoff circle, the contraction component keeps decreasing, and the larger
is, the faster
goes down from 1, which means it spends more time converging to the desired circle. The opposite situation is shown in
Figure 4b. The circulation component goes up more steeply as
is less when k is near
, which denotes that as
becomes smaller, the convergence speed in standoff tracking is improved significantly.
In the analysis of Shun Sun [
12], when c becomes larger, the vector field converges slowly but can keep a distance from the prescribed radius stably; on the contrary, if c becomes smaller, the convergence time reduces sharply. However, in this situation, it is time-consuming on stabilization, where the UAV travels in and out of the loiter circle. Shun Sun copes with this problem by designing a series of guidance functions and searching for the optimal one before the UAV performs the mission. Obviously, offline search cannot adapt to the application scenario where the target is uncooperative with UAVs owing to the computing burden. In light of the above work, we will design a guidance function simultaneously taking into account both fast convergence and stable tracking performance in an online manner, which means we can adaptively balance the converging speed and stable tracking in real time.
Due to the maximum turning rate of the fixed-wing UAV, the curvature of the route must be analyzed. Transforming Equation (4) into a Cartesian coordinate system, the control manner of Lyapunov vector field guidance can be rewritten into Equation (8).
Then for a stationary target, the course can be represented as
Referring to work of Shun sun [
12], we calculate the curvature by
When
is a constant, Equation (10) reduces to
It is worth noting that the traditional vector field is a special case where is constant at 2, and the curvature of the classical vector field is always positive, as a monotone decreasing function of radical distance , which means that it spends a long time on the path toward the loiter circle, resulting in an increase in convergence time. All curves intersect at the point , which implies that the vector field with different converge to the standoff circle, and the UAV flies steadily along the circle.
Considering the maximum bank angle of the UAV, the maximum turning rate is limited, so the curvature is subject to a saturation constraint as
As shown in
Figure 5, when
is small enough, an extremum lies on the curve near to the desired radius. This extremum may conflict with the curvature constraint of the UAV, which will be discussed in the subsequent section.
4. Design of Cooperative Tracking Guidance Laws
4.1. Design of Vector Field for Fast Convergence Target Tracking
When the UAV searches for the ground target, the target can be detected via an onboard camera when the UAV is far away from the target. Therefore, in this paper, we focus on the situation where the UAV is out of the standoff circle when it receives the ‘tracking’ command.
In the analysis in the previous section, smaller guidance parameter
makes the vector field more radial toward the desired circle. Although faster arrival at the standoff circle, it may result in greater time wasting on stabilization by making the aircraft repeatedly traverse the circle, as shown in
Figure 6.
Two reasons may explain the event that trajectory intersects with the standoff circle. One reason is that the vector field will generate a non-flyable path for the aircraft with an inappropriate guidance parameter . The other is that the starting position for the ‘tracking mission’ is too close to the standoff circle with the course pointing to the standoff circle.
As for the first reason, the extremum of the curvature near the standoff circle may exceed the curvature constraint of the UAV, despite the UAV turns at the maximum bank angle, and the trajectory intersects with the standoff circle, which leads to crossing over the circle several times. Therefore, in order to reduce the convergence time, and put an end to crossing over the circle, the guidance parameter must be small enough but satisfies the curvature constraint.
The optimization problem can be expressed as:
It is too complex to calculate derivative of Equation (11) with respect to , so an optimization algorithm based on the gradient descent method is developed to find . In this way, the optimal guidance parameter can be obtained based on the search method below (Algorithm 1).
Algorithm 1: Optimal Guidance Parameter Search |
Input: standoff radius ; maximum turning rate ; UAV cruising speed ; grid point of parameter ; |
Output: optimal parameter ; |
Calculate the maximum curvature using Equation (11); |
; |
; |
for to do |
Obtain curvature function using Equation (11) |
Find the extremum of the curvature function using the gradient descent method where |
if then |
continue; |
end if |
; |
if then |
; |
; |
end if |
end for |
To cope with another problem, the inverse Lyapunov vector field (ILVF) is introduced. The ILVF is used to guide the UAV to fly away from the standoff circle. The ILVF can be obtained by changing the negative sign of
in Equation (4) to a positive one.
The control manner of ILVF can be rewritten into Equation (14) by transforming Equation (13) into a cartesian coordinate system
The Lyapunov vector field and inverse Lyapunov vector field can be shown intuitively in
Figure 7. As shown in
Figure 7, the vectors in the ILVF point away from the standoff circle.
When the start point for UAVs is too close for UAVs to avoid intersecting with the circle, the ILVF is enabled to guide UAVs to move away from the loiter circle.
By searching for the optimal guidance parameter
when UAV is far away from the standoff circle and fly away from the circle when it is too close, the proposed guidance law can guide UAV to converge to the standoff circle as fast as possible under the premise that the trajectory of the UAV does not cross the circle. In addition, the computation burden is lower than the method in reference [
12], so that we can update the optimal guidance parameter in real time. In this way, we can adaptively balance the converging speed and stable tracking in real time.
4.2. Modification of the Guidance Law for Moving Targets Tracking
When a team of UAVs are tracking a ground moving target, the motion state of the target must be considered. As a correction item, the state of the target is used to modify the desired course of the UAV. If the UAV moves in the designed course in the local coordinate system of the target, the relationship between the modified velocity and target velocity is:
where
is the desired velocity,
is the velocity estimation of the target,
is the corrected desired speed in the global coordinate system, and
is the correction factor.
Every UAV in the team could position the ground moving target using onboard sensors, while the result is not accurate enough for subsequent operations, such as guiding shells to strike or UAVs to track it stably. In this section, the state of the target is estimated based on the data fusion technology. A classic interactive multi-model is integrated with the unscented Kalman filter (IMM-UKF) to improve the geolocation precision. UKF is an improvement of EKF, which is important in integrating information [
22]. Furthermore, a classic federated filter is adopted to reduce estimation errors by integrating independent sensor information from UAVs.
The framework of IMM-UKF with federated filter is shown in
Figure 8. When tracking a moving target, every UAV localizes the target with a relatively low accuracy. The localization result will be processed using the IMM-UKF, which works well in estimating the state of the moving target. Then, the main filter collects the results of the local filter and distributes estimate results after time update and data fusion. The data fusion process with federated filter is introduced in
Appendix A.
In order to depict the state in a more accurate manner, three motion models are adopted in the IMM-UKF algorithm. Suppose state variables are
where
,
are the position variables,
,
are the velocity variables,
and
are the acceleration variables.
Motion models adopted in the paper will be introduced. The constant velocity model is used to model the target state in the constant velocity. All process noise mentioned below are zero-mean white Gaussian noise.
where
is the state transition matrix, and is the process noise.
The acceleration model can be expressed as
where
is the state transition matrix, and is the process noise.
Similarly, the turning model can be rewritten as
where
is the state transition matrix, and is the process noise.
The estimation process is introduced in
Appendix B. After the state estimation, we get the estimation of the current state
The position vector is used for the basic tracking of the static target while the velocity vector is utilized to modify the guidance law of UAVs when tracking a moving target.
4.3. Phase Keeping for Cooperative Tracking
Besides standoff tracking of a ground target, it is of vital importance to control the intervehicle phase. On the one hand, the unpredicted maneuver of a vehicle can be observed when UAVs are distributed evenly on the circle. On the other hand, the precision of target localization can be enhanced by fusing independent sensor information.
We develop a new controller based on the speed controller from Frew E W [
7], which can achieve faster convergence. Different from the original method, we set an ideal seat for each UAV. These ideal seats are evenly spaced around the circle. Then, the speed controller is utilized to guide each UAV to its ideal seat by controlling the speed. As a result, the team of UAVs will be evenly distributed on the circle. Without loss of generality, we adopt three UAVs to perform the tracking mission, and UAV1, UAV2, and UAV3 are arranged counterclockwise, as shown in
Figure 9.
Figure 9 shows a team of three-UAVs, with corresponding phase angles
,
, and
defined relative to the instantaneous tracking circle. Consider a phasing Lyapunov function
where
denotes the unwrapped difference between the phase angle and the desired angle. The method for calculating the desired angle will be introduced later. The time derivation of this function is
Then, we choose the angular speed commands
where
is a positive proportional gain. Since the desired seat circles around the UAV cruising at
, the desired angle speed is
Then, Equation (24) results in
Note that is negative semi-definite, which ensures that converges exponentially to zero, hence the relative angle converges exponentially to zero, which means the UAV phase angle converges to the desired angle.
The corresponding speed control commands are then
The term
is introduced to restrain the phase keeping operation when the UAVs are far away from the target [
7].
In the above algorithm, the choice of the desired seat is of vital importance. The optimal choice of the desired seats allow UAVs to reduce their speed, as well as the burden on the flight controller. As shown in
Figure 9, the total angle difference between the desired seat and the corresponding UAV is
where
,
. When
,
reaches the minimize value. Thus, the desired phase for each UAV in any instantaneous frame can be calculated.