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Trackbeforedetect in distributed sensor applications
EURASIP Journal on Advances in Signal Processing volume 2011, Article number: 20 (2011)
Abstract
In this article, we propose a new extension to a Dynamic Programming Algorithm (DPA) approach for TrackbeforeDetect challenges. This extension enables the DPA to process timedelayed sensor data directly. Such delay might appear because of delays in communication networks. The extended DPA is identical to the recursive standard DPA in case of all sensor data appear in the timely correct order. Furthermore, an intense evaluation of the Accumulated State Density (ASD) filter is given on simulation data. Last but not least, we apply a combination of DPA and ASD on data of a real radar system and present the resulting tracks. Our experience concerning this combination is a seamless cooperation between the track initialization by DPA and a track maintenance by ASD filter.
1. Introduction
Since many years, security applications employing radar sensors for surveillance objectives are increasingly important. In situations where targets with a low signaltonoise ratio (SNR) appear, it is convenient to apply tests on track existence utilizing raw sensor data instead of using thresholded measurements. This approach is generally called TrackbeforeDetect (TBD). It enables a radar system to search for lowobservable targets (LOTs), i.e., objects with a low SNR. These targets can be invisible to conventional methodologies, as most of the information about them might be cut off by the applied threshold. The gain of a TBD algorithm is often paid by high computational costs. Even today, when computational power is cheap and highly available, most of the techniques for TBD still suffer from being hard to realize for a real time processing of sensor data. First and foremost, this is due to the huge amount of data to be considered in each scan.
Capacity and stability of communication channels such as 3G Networks, WLAN, HF, or WANs are subject to an ever increasing development. For many fusion applications, in particular for surveillance tracking, this enables a user to explore new approaches by exploiting multiple sensor systems. When the link capacity is very low or temporarily unavailable, a common centralized tracking scheme is TracktoTrack Fusion (T2TF) [1]. However, T2TF neglects valuable information on LOTs, as track initialization is performed only on local sensor data. Therefore, we address the challenge of TBD and track maintenance (TM) in distributed sensor applications by processing all information available depending on the available bandwidth.
Applications evolving multiple distributed sensors often suffer from effects of the communication links. The major challenge therein constitute in particular timedelayed sensor data, so called OutofSequence (OoS) measurements, which appear, e.g., by timely misaligned scan rates, varying communication delays, or asynchronous sensors caching their data in a local storage. To overcome this challenge, the Accumulated State Densities (ASDs) filter gives a neat and efficient scheme to process such OoS measurements [2–4]. Therefore, the ASDs give an optimal estimation filter for distributed sensor applications performing the TM part.
1.1. Structure
This article is structured as follows. In Sect. 2, an overview to related work is given. The main contribution of this article is a TBD algorithm which is able to process OoS data sets. This algorithm is subject of Sect. 3 and has been tested intensively on real sensor data. The tracking results are presented in Sect. 4, which also includes a numerical evaluation of an ASD filter. The conclusion of this article is given in Sect. 5.
2. Related work
2.1. Outofsequence processing
Since the development of multisensor systems, the challenge of OoS processing is crucial for further development in tracking research. BarShalom was the first, who picked up the problem and provided an exact solution for lags which are equal or smaller than one update period [5]. He extended his approach in [6] to a multistep lag algorithm called Al 1 by applying the equivalent measurement [7, 8] of recent sensor data. This enabled him to use the derived algorithm on OoS data with an arbitrary big lag, but as the equivalent measurement neglects some cross covariances, the result is not an optimal solution. Further generalizations to MHT and IMM scheme followed by various groups as [9–12].
In [13], the idea of augmenting past states and current states for a neat OoS processing occurred. This approach neglects information of current states about timedelayed measurements. In particular, when high maneuvering targets are observed, this results in a suboptimal routine. An algorithm calculating the crosscovariances for each step in between the occurring lag is given in [14]. An obvious drawback of such an algorithm is the number of measurements to be stored and numerical costs. In [15], past states are considered to provide a more comprehensive treatment of issues in particle filtering. A solution for OoS processing using particle filters is presented in [16].
All filter techniques presented in this work are based on the ASD. In 2009, Koch presented a closed formula for an ASD posterior [2]. His work was continued and investigated more intensively in [4]. Extensions to MHT and IMM filtering are given in [3].
2.2. TBD methods
There exist various methodologies to realize TBD. One can separate four different classes of them: Dynamic Programming Algorithm (DPA), Particle Filters, Hough Space Transform, and Subspace Data Fusion. Due to computational reasons, a practical application of the Hough Transform on TBD is often limited to nonmaneuvering targets [17, 18]. While the numerical costs of particle filters are high in general, their accuracy (in theory) can achieve any degree desired. Therefore, many recent research activities concentrate on this approach for TBD [19]. However, these algorithms still face the problem that it takes a long time for the modes (i.e., the tracks) to appear. The subspace approach to TBD algebraically calculates the posterior of the emitter's position given the sensor data with respect to properties of the antenna [20]. While the results on simulation data seem to exceed other techniques, it has not been tested on real data yet. Furthermore, the computational complexity is very high and therefore it might be difficult to implement for applications with real time requirements.
The DPA approach consists of a sequential LogLikelihoodRatio (LLR) test for existing targets in each sensor cell. Unlike conventional track extraction methodologies on thresholded measurements [21], it calculates the probability of a track existence without using an estimated spatial covariance matrix of the target state [22]. A score which is a function of this probability is calculated for each scan. Given the Markov property, this approach solves the global track search asymptotically in an efficient way. In the recent time, Orlando et al. showed that an application to an underwater sonar system is possible [23].
3. Track initiation using OoSDPA
3.1. DPA algorithm
Assume a time series of sensor observations Z^{k} = {z_{1}, ..., z_{ k }} is given, where is the set of measured amplitudes or SNRs in the corresponding sensor bin θ_{ i }, i = 1, ..., N. For a complete track initialization, we are interested in both, the question of track existence and the associated time series of sensor bins for case of a positive result.
Following the description of Arnold et al. [22], we assume there is a function s(θ_{ k }, ..., θ_{1}) which is maximized by the desired sequence of states. This scoring function respects the observed signal strength and the underlying target motion. Whereas for the general solution an exhaustive search over all possible combinations is necessary, the DPA splits the scoring function into temporary elements
This is possible, if the target motion is modeled as a Markov random walk of first order. Then, the solution is given by
An asymptotic solution to this maximization problem can be calculated stepwise by introducing auxiliary function chain {h_{ i }}_{i = 1, ..., k1}which is defined by the following recursive expression:
For a given initialization , we obtain , by
For the derivation of such a score function, we follow the idea of the conventional track extraction methodology [21] and use a sequential likelihood ratio test. Switching to the logarithmic version of it, we are able to prove the necessary splitting property of (1). To this end, we consider the following hypotheses.

H_{1}: θ_{ k }, ..., θ_{1} is associated to a target.

H_{0}: There is no target.
Using the LLR test, we obtain for the cumulative scoring function s:
Applying Bayes' Theorem on the argument, we obtain
Because of the Markov assumption, the following equation holds.
Combining the above equations yields for the cumulative scoring function
This satisfies the required assumption of (1). Therefore, the auxiliary functions h_{ i }(θ_{i+1}) are given by:
Various approaches have been discussed to estimate the signal dependent logterm of h_{k1}(see [24] and literature cited therein). For sensors for which the assumption of a Gaussian distributed SNR with mean and additive noise holds, the expression simplifies to
where represents the measured SNR in sensor bin θ_{ k } rescaled such that the noise covariance is unity.
3.2. Outofsequence DPA
As stated in Sect. 1, low computational costs of a TBD algorithm are crucial for real applications. Therefore, it would be highly inconvenient to reprocess stored data in situations where timedelayed measurements occur, i.e., OoS data. In this section, we propose an extension to the DPA algorithm described in Sect. 3.1 such that it can update its states directly on OoS data sets. In particular, we state how to establish the links between the states in order to obtain the estimated time series of bins .
3.2.1. Update of the score
Because of time limitations, it is generally not intended to retrospectively update the scores and links of the past states of time t_{ l } for t_{ l } < t_{ k }. Therefore, the current score values for each sensor bin only reflects the exact posterior for a given state θ_{ k } at time t_{ k }. Let us now assume a timedelayed sensor data set z_{ m } originating from time t_{ m } < t_{ k } occurs. The goal is now to calculate the score conditioned on the new measurement data set Z^{k, m}: = Z^{k} ∪ {z_{ m }}. As in the above scheme, we have
Again, we might apply Bayes' Theorem on the argument of the logarithm and obtain
The term (*) needs a fully smoothed state time series θ_{ k }, ..., θ_{1} for a precise calculation. However, during the track extraction phase we might assume the target to be not maneuvering very strong. Therefore, an appropriate approximation is given by
which is not covered by the Markov property, because we might have k > m > 1. In such a case, it would be necessary to incorporate the system dynamics from the past and the future to obtain an exact result on the conditional density of θ_{ m }. For the sake of simplicity, we only incorporate the system dynamics from the recent processing step k. Using this approximation, we end up with a straight forward score update by the auxiliary function
Nevertheless, this score references to the states at time t_{ m }, therefore a similar approximation might be necessary, if the following data set is originated at time t_{k+1}.
3.2.2. Obtaining the links
Assume, at time t_{ k } the score for sensor bin exceeds the threshold μ_{ t } for track confirmation. If the fixed length for track initialization is k  n + 1, we additionally need to gather . As we can save the backward links which carry out the maximization in the auxiliary function h_{i1}(·), this is a trivial task, if all sensor data appear in the timely correct order. An example is given in Figure 1 where a timebin diagram is shown. The three paths in this figure overlap in some parts, while their score at the most recent instant of time belongs to distinct sensor bins.
Let us now consider the OoS case. If μ_{ t } is exceeded by the score for some referring to time t_{ m }, we gather the sequence of states by following the links starting at . Using the reversed order of the data appearance, this link is always unique. Therefore, we obtain a unique track sequence with t_{ m } ∈ [t_{ k }, t_{ n }] by ordering the elements of the path accordingly to their instant of time of origination. This procedure is visualized in Figure 2 for the timely ordered case (a) and OoS case (b).
4. Evaluation and application results
4.1. DPA evaluation
The evaluation of the OoSDPA is separated into two parts. The first part considers the runtime duration when OoS sensor data appears in comparison to a reprocessing scheme, which starts at the last instant of time such that the remaining data can be used in the timely correct order. The second part addresses the obtained track accuracy. To this end, the ordered DPA output is taken as a reference. For both parts, the data set provided by DSO National Laboratories from a 2D radar system is applied as input. While for the time measurements the whole set of 400 × 372 sensor bins are taken into account, the accuracy performance test concentrates on a small 10 × 10 bins subset. Processing 15 data scans in the correct order with a standard DPA algorithm yields exactly one target. By means of this result, we evaluate number, states, and processing speed estimated by the extended DPA in the OoS case.
Figure 3 presents the results of processing speed for both algorithms, the reprocessing DPA and the OoSDPA. As the reprocessing takes a lot of time, it is obvious that the speed of such a scheme is much lower than a direct update. Furthermore, the time consumption increases linearly in the mean time delay. This behavior, of course, is as expected.
Next, we have a look at the DPA output. We examine the deviation between the ordered case and OoS case for a single target. To this end, we study a small subset of the radar data and compare the results in terms of bin deviations, nondetections and false tracks. As shown in Figure 4, the mean deviation of a track consisting of 15 states is up to 3 bins in the range axes. At a range bin size of 60 m, this corresponds to 180 m range offset. The main reasons for this offset is most probably the approximation in motion penalties mentioned in the section above. The mean deviation on the bearing axis is below 0.5°, thus all estimated bearing bins are almost the same. Note that the deviation in both, range and bearing, are highly dependent on the observed case. However, this shows that the OoSDPA is able to establish a target track such that it can be maintained by a tracker. Furthermore, Figure 5 shows that the chosen target was detected by the OoSDPA in almost every run. There were only three nondetections at a mean time delay of 5s out of 1000 runs (blue line). As mentioned above, a quite small subset of 10 × 10 bins was considered for this evaluation. However, the red line shows, that there was no run with a second (false) target detection in it.
4.2. Numerical evaluation of the ASD filter
This section analyses the performance of an ASD filter in comparison to other existing techniques. Appropriate candidates for such a comparison are a standard Kalman filter (KF), which has to reprocess some stored sensor data in case of an OoS measurement, and the algorithm called Al 1 from BarShalom et al. [6]. On the one hand, the KF needs a lot of storage for all measurements within a time window and extra time for reprocessing depending on the size of the lag of an OoS measurement. On the other hand, the Al 1 consists of the algorithm A1 from [5] applied to the equivalent measurement [7, 8] of the set of sensor data since the time of the last update before the time of the OoS measurement. Therefore, it is an approximation of the optimal estimate, but it does not require a storage of all sensor data. The degree of approximation depends on the level of process noise. In the evaluation below, we compare the resulting performance for different evolution noise levels.
In the following simulation scenarios, a target moves in a nondeterministic manner through a twodimensional space. A virtual sensor observes this target once a second by measuring its range and bearing. These measurements suffer from an additional zeromean Gaussian distributed noise. In particular, the noise level we use a variance of and millidegree^{2} in range and bearing, respectively. All mentioned filters are initialized with the perfect start values of the targets position and velocity. Furthermore, they use a perfect matching evolution model. For the latter, we chose a Continuous White Noise Acceleration Model [25], i.e., the transition probability density function for a given state x_{ k } at time t_{ k } to x_{k+1}at t_{k+1}is given by the following linear Gaussian model:
where
Here, the parameter q describes the speed variance for an update interval of T = 1 s. We use the abbreviation 1 for an identity matrix in the dimension 2. For each setup of this parameter, 1000 Monte Carlo simulations were run, where we tracked the target for 500 steps. The communication link effect is simulated by an additional Poisson distributed delay μ_{ k } with mean and variance for each measurement transfer from the sensor to the fusion center. This causes a regular appearance of OoS measurements. After every update, we quantify the root mean squared error (RMSE) of the filters estimate according to the real target position. Furthermore, we meter the processing time of the filter for each run. This reveals the efficiency regarding to the numerical complexity of the algorithm.
In Figures 6, 7, 8, 9, 10, and 11, the results of the RMSE over an increasing mean time delay are presented. It can easily be seen that the accuracy of the Al 1 algorithm decreases for stronger maneuvering targets and highly delayed measurements. For almost deterministic targets (q = 0.01), a difference in the performance between the three algorithms cannot be observed. In any case, the ASD filter has an equal RMSE to the reprocessing KF. However, the ASD filter is much more efficient regarding to the numerical complexity of the state smoothing and OoS processing. This can be seen in Figures 12 and 13, where a boxplot of the processing time is given for each algorithm. Here, one can also see that the effect of reprocessing measurements in this simple case (i.e., perfect datatotarget association, no false measurements, perfect detection) is very low. This effect, however, will increase in more realistic scenarios, where those conditions are not given. Furthermore, the advantage of a unified handling of filtering and retrodiction^{a} becomes obvious, as the ASD algorithm handles it in less than half of the time required for a separate retrodiction.
4.3. Implementation setup for real data set
Depending on the given circumstances, either communication channels might be of limited capacity, or high bandwidth links might be available. To cover these possible situations, we consider three different setups:

(i)
Single sensor performing TBD on a local sensor site which is connected to a Fusion Center (FC) which maintains the tracks. OoS measurements appear due to varying delays on this link. This setup is visualized in Figure 14.

(ii)
Multiple sensors with separated local TBD modules connected to a FC performing an ASD (see Figure 15). In this scenario, as well as in the previous one, only new tracks and thresholded measurements are sent via the network.

(iii)
Sensors connected to a FC performing a centralized TBD methodology and maintaining the tracks. In this scenario, the raw sensor data are sent to the FC, depicted in Figure 16.
The data set used was provided by DSO National Laboratories (DSO) and consists of raw sensor data obtained by a twodimensional radar system. It includes 389 scans, which corresponds to about 15 min at a given scan rate of T = 2.2 s. At certain instants of time, up to ten targets can be found. The test for existing targets is done by a DPA algorithm. If the test for target existence turns out with a positive result, a new track is initialized. It consists of the Least Squared Errors (LSE) approximation of the recent 15 states and is passed to the TM. This maintenance also essentially consists of a SequentialLikelihoodRatio test, as proposed in [21]. The result of it is called LRscore and depends on the choice of various parameters. Most of them will be explained below.
4.3.1. Filter parameters
Essentially, there are two thresholds A < B to test for track continuation. A LRscore below A will lead to a track deletion, while a score above B indicates a track confirmation. If the score is in between of both, the track is continued, but not be confirmed, i.e., its tracking results are not displayed. A new track arising from the DPA is initialized with an LRscore LR_{0} = 1/10 · B. Therefore, it is not assumed to be confirmed, unless it can be observed by the TM for at least one additional step, depending on the chosen values for track detection P_{D} and the mean false measurement density ρ_{F}.
Another important parameter for TM is μ_{tm}, which gives a lower bound for the distance of two targets before their corresponding tracks are merged. A similar threshold is given on a lower level, as we use a MultipleHypothesesTracker (MHT) [24] extension for the ASD filter [3]. In order to keep numerical expenses at a decent level, similar hypotheses are merged, if their weighted distance d = (x^{1}  x^{2})^{⊤} (P^{1} + P^{2})^{1} (x^{1}  x^{2}) is lower than the threshold μ_{hm} [26]. Here, x^{i} and P^{i} represent the state and the covariance, respectively, for the i th track. Furthermore, hypotheses with a probability lower than a threshold μ_{ p } are deleted immediately.
An overview of all mentioned parameters is given in Table 1.
4.3.2. Results
In order to give a comparison in the mean tracking error, an exact groundtruth trace for some targets would be necessary. However, such a trace is not available for the real data set. Therefore, we present the gained tracking results as situation pictures at an arbitrarily chosen but fixed instant of time.
4.3.3. Scenario one
At first, we have a look at the results of scenario one. Here, a Poisson distributed delay for both, measurements and DPA output of a single sensor, is inserted with a mean delay of 0,10, and 20 s, respectively. Figure 17(a)  17(c) and 17(d)  17(f) shows all tracks occurring at the 300th scan, i.e., at time t = 660 s, in rangebearing and xy space, respectively. On the left hand, a star indicates a position of track initialization, while a diamond is set at the track deletion. A green line in between shows the trace of a target. It includes all positions obtained by processing the data arrived up to the chosen scan. Due to possible delays for , information contained in scans of later instants of time might be processed already. This explains why some tracks appear advanced further in comparison to the ordered case . A similar effect can be observed in the xy space on the right hand. Here, the red rectangle indicates the sensor position and estimated positions in an ASD are plotted by blue crosses. With increasing mean delay , the accumulated state positions are stretched further apart. This is due to the fact that the ASD contains states corresponding to a wider range of points in time.
4.3.4. Scenario two
For an evaluation of the second scenario, the data set was split up. Two sensors were simulated, the first using all even scans, the second using all odd scans. Furthermore, in order to obtain a gain of the sensor fusion on common tracks, each sensors Field of View (FoV) was restricted to a fixed part of a full circle. This is shown in Figure 18, where the filled area corresponds to the FoV.
The tracking results are given in Figure 19. As one can see, the all tracks of the optimal case, where , can be recovered in the OoS cases. To this end, the fusion center benefits from both sensors without the need of storing and reprocessing data. The thresholded measurements from sensor 1 can be seen in Figure 19(d)19(f) as thin black dots. The effects described for scenario one apply in this situation, too.
5. Conclusion
In this article, we proposed a new extension to the DPA approach for TBD challenges. This extension enables the DPA to process timedelayed sensor data directly. These might appear because of delays in communication networks. The extended DPA is identical to the standard DPA for case all sensor data appears in the timely correct order. Therefore, one might speak of a natural extension. However, some approximations to the exact solution are necessary to prevent dramatically increasing costs on numerical power. In an evaluation on a data set of a real radar system, this approximation was shown to have at most marginal effect on the results. This is supposed to hold whenever the observed targets are in a non or moderatemaneuvering state during the track extraction phase.
Then we numerically proved that the intensity of the process noise can have severe impact on the estimation error when using approximations. The results clearly show that an ASD filter processes OoS measurements optimally in terms of a mean squared error, i.e., equivalent to a KF reprocessing outdated data.
Last but not least, we applied the combination of a DPA and an ASD on data of a real radar sensor system. As no groundtruth is available, we cannot derive an estimation error. However, the situation pictures give an impression of how seamless this combination worked out. The experienced process time might be higher than a KF using Global Nearest Neighbor (GNN) [24], but it is still applicable for real time processing. Furthermore, it is clear that tracking results of an MHT is superior to the GNN scheme, because it involves all measurements within the selected gate [27]. A more profound analysis of this consideration about tracking performance and processing time is left as future work at this point.
Endnote
^{a}We prefer the term retrodiction to smoothing, as it combines the process of prediction and a retrospective view.
Abbreviations
 ASD:

Accumulated State Density
 DPA:

Dynamic Programming Algorithm
 FC:

Fusion Center
 FoV:

Field of View
 GNN:

Global Nearest Neighbor
 KL:

Kalman filter
 LSE:

Least Squared Errors
 LLR:

LogLikelihoodRatio
 LOTs:

lowobservable targets
 MHT:

MultipleHypothesesTracker
 OoS:

OutofSequence
 RMSE:

root mean squared error
 SNR:

signaltonoise ratio
 TBD:

TrackbeforeDetect
 TM:

track maintenance
 T2TF:

TracktoTrack Fusion.
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Govaers, F., Rong, Y., Chee, L.H. et al. Trackbeforedetect in distributed sensor applications. EURASIP J. Adv. Signal Process. 2011, 20 (2011). https://doi.org/10.1186/16876180201120
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Keywords
 Trackbeforedetect
 Outofsequence
 Real data application
 Dynamic programming approach
 Accumulated state density
 TBD
 OOSM
 DPA
 ASD