Abstract
In recent years, teams of robot and Unmanned Aerial Vehicles (UAVs) have been commissioned by researchers to enable accurate, online wildfire coverage and tracking. While the majority of prior work focuses on the coordination and control of such multi-robot systems, to date, these UAV teams have not been given the ability to reason about a fire’s track (i.e., location and propagation dynamics) to provide performance guarantee over a time horizon. Motivated by the problem of aerial wildfire monitoring, we propose a predictive framework which enables cooperation in multi-UAV teams towards collaborative field coverage and fire tracking with probabilistic performance guarantee. Our approach enables UAVs to infer the latent fire propagation dynamics for time-extended coordination in safety-critical conditions. We derive a set of novel, analytical temporal, and tracking-error bounds to enable the UAV-team to distribute their limited resources and cover the entire fire area according to the case-specific estimated states and provide a probabilistic performance guarantee. Our results are not limited to the aerial wildfire monitoring case-study and are generally applicable to problems, such as search-and-rescue, target tracking and border patrol. We evaluate our approach in simulation and provide demonstrations of the proposed framework on a physical multi-robot testbed to account for real robot dynamics and restrictions. Our quantitative evaluations validate the performance of our method accumulating \(7.5\times\) and \(9.0\times\) smaller tracking-error than state-of-the-art model-based and reinforcement learning benchmarks, respectively.
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Note that the state variables in \(\mathbf {\Theta }_t\) and \(\mathbf {\Phi }_t\) include firespot and UAV positions, \(q_t\) and \(p_t\), as well as \(R_t, U_t\) and \(\theta _t\) which are the FARSITE wildfire propagation model parameters. While these variables are generally application-dependent, we emphasis that \(R_t, U_t\) and \(\theta _t\) are specific to FARSITE (see Sect. 3) model and can be replaced with other measurable model parameters, in case FARSITE is replaced with other fire propagation models such as the correctable fire simulation model in [88].
Note that, this step might not be required in applications other than aerial wildfire monitoring, in which instead of \(N_h\) fire areas, \(N_h\) specific moving points/targets need to be monitored. Accordingly, the CE-TSP step in our framework can be replaced with a regular TSP [93].
The scenario designs are motivated such that they expand the applicability of our framework to domains other than wildfire monitoring, and thus, here we use the term target points instead of firespots.
We explicitly estimate the latent fire dynamics in our AEKF model.
Here, we used the FARSITE model which can be replaced with any other parameterized fire propagation model, such as the correctable fire simulation model introduced in [88].
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This work was sponsored by ONR under Grant N00014-18-S-B001, MIT Lincoln Laboratory Grant 7000437192, Lockheed Martin Corporation under Grant GR00000509, and GaTech institute funding.
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Appendix
1.1 A.1 Time independency of the EKF’s measurement residual
Our analytical URR bound in Eq. 12 depends on the state-estimation measurement residual computed at different time-steps. To maintain control over the measurement uncertainty, we posit that the UAV observers would want the measurement uncertainty residual with respect to a target on the ground not to increase from \(t=t_0\) to \(t=t_0+kT_{UB}\) for any positive integer constant k if the UAV observes the target from the same relative position. Therefore, we examine the time-dependency of the propagated error through our EKF formulation. To this end, we follow the mathematical proof and discussions provided in [16] and [9, 100]. We state that the measurement uncertainty about the states of a dynamic point \(q_t\), observed by a flying UAV is independent of time and is only a function of distance between the observer and the point. In the following, we mathematically proof this point.
First, we present how the uncertainty residual is quantified by an EKF. The total uncertainty residual propagated by EKF is composed of a model and an observation measurement uncertainties, both of which follow the general nonlinear uncertainty propagation law, shown in Eqs. 29 and 30, where \(\Sigma _{t|t-1}\) is the predicted covariance estimate, \(\Lambda _{t|t}\) is the innovation (or residual) covariance, \(F_t\) and \(H_t\) are the process and observation Jacobian matrices, and \(Q_t\) and \(\Gamma _t\) are the process and observation noise covariances, respectively.
Considering Eqs. 29 and 30, the gradients in the process, \(F_t\), and observation, \(H_t\), Jacobian matrices are responsible for alterations in the uncertainty values. To compute these gradients, we calculate the derivatives of fire’s propagation model, \({\mathcal {M}}_t\), and UAV’s observation model, \({\mathcal {O}}_t\), with respect to the state variables. As discussed in Sect. 5.1 and considering the introduced state vectors, we first derive the process and observation Jacobian matrices (\(F_t\) and \(H_t\)) as follows in Eqs. (31) and (32), respectively. In Eqs. (31) and (32), \(t^\prime = t-1\).
In Eqs. (31) and (32), we define the process state vector as \(\mathbf {\Theta }_t = \left[ q_t^x, q_t^y, p_t^x, p_t^y, p_t^z, R_t, U_t, \theta _t \right] ^T\) and \(\mathbf {\Phi }_t = \left[ \varphi _t^x, \varphi _t^y, {\hat{R}}_t, {\hat{U}}_t, {\hat{\theta }}_t\right] ^T\) as the mapping vector. As such, we calculate the partial derivatives in Eq. 31 by using Eq. 1–2 and applying the chain-rule to compute the derivatives of \(q_t^x\) and \(q_t^y\) with respect to parameters \(R_{t-1}\), \(U_{t-1}\), and \(\theta _{t-1}\). The partial derivatives are then derived as in Eqs. (33) and (35), where \({\mathcal {D}}(\theta )\) is \(\sin \theta\) and \(\cos \theta\) for X and Y axis, respectively.
To compute the partial derivatives in the observation Jacobian matrix in Eq. 32, we first need to derive the relation between the angle parameters, \(\varphi _t^x\) and \(\varphi _t^y\), and the UAV pose. The angle parameters contain information regarding both firefront location \([q_t^x, q_t^y]\) and UAV coordinates \([p_t^x, p_t^y, p_t^z]\). According to Fig. 3, by projecting the looking vector of UAV to planar coordinates, the angle parameters are calculated as shown in Eqs. (36) and (37) for X and Y axes respectively, where \(q_t = [q_t^x, q_t^y]\) and \(p_t = [p_t^x, p_t^y]\).
The partial derivatives in the observation Jacobian matrix \(H_t\) for X-axis, presented in Eq. 32, are derived as in Eqs. 38–40 and for Y-axis derivatives, we can derive as in Eqs. 41–43.
Now, considering EKF’s covariance propagation equations in Eqs. (29 and 30) as well as the gradients in process Jacobian matrix \(F_t\) as calculated in Eqs. 33–35, we can see that the gradients in process Jacobian matrix are only functions of fire propagation model parameters (e.g., the FARSITE model in this case) such as fuel coefficient, \(R_t\) and wind velocity and direction, \(U_t\) and \(\theta _t\). Consequently, while these parameters do not vary significantly with time, the uncertainty drop due to process model is time-invariant. We note that FARSITE [38] assumes locality in time (i.e., within seconds or few minutes), making the assumption of time-invariant fire parameters fairly acceptable [101]. Moreover, the gradients in the observation Jacobian matrix, Eqs. 38–43, are only functions of the Euclidean distance between the UAV pose and firespot coordinates. We also know that, since at the time of visiting a firespot the planar displacement between UAV and fire locations are approximately zero and the only distance between the two equals to the UAV altitude. Accordingly, both \(F_t\) and \(H_t\) are locally time-invariant and the total measurement uncertainty residual variations between two different time-steps (e.g., \(t=t_0\) and \(t=t_0+kT_{UB}\)) is not a function of time and is only a function of the UAV observer’s altitude.
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Seraj, E., Silva, A. & Gombolay, M. Multi-UAV planning for cooperative wildfire coverage and tracking with quality-of-service guarantees. Auton Agent Multi-Agent Syst 36, 39 (2022). https://doi.org/10.1007/s10458-022-09566-6
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DOI: https://doi.org/10.1007/s10458-022-09566-6