In recent years, the need for public transportation has risen dramatically. Rail transportation alone has increased by over in the last 16 years in Switzerland . However, current infrastructure is reaching its capacity limits. To keep up with this growth, there is a need to improve the system efficiency.
In train applications, a crucial part of the current infrastructure is the traffic control system. Most of current rail control systems divide the railroad tracks into so-called blocks . The block size is determined by the worst case braking distance of every vehicle that is likely to operate on this track. Vehicle localization and interlocking of the blocks is performed using infrastructure-side beacons. Such a fixed block strategy results in very conservative interlocking and thus, decreases the overall efficiency of the system.
The new European Train Control System (ETCS) Level 3 aims to replace the fixed blocks with moving blocks centered around the vehicle. This concept has the potential of increasing the capacity of train networks by a factor of to . Furthermore, fixed track-side sensing infrastructure (e.g. axle-counters, Balises) may be replaced with on-board sensors, leading to a more cost-effective solution in the long-run. Even with the vast amount of research in related applications (e.g. autonomous cars), the success of ETCS Level 3 is subject to the development of new algorithms that are able to precisely and reliably estimate both the position and velocity of all rail vehicles [4, 5].
In rail applications only few restrictions exist in regard to weight and power consumption of the localization solution. Therefore, one is pretty open in choosing suitable sensor modalities and estimation algorithms. Current research in train localization mainly focuses on the fusion of global navigation satellite system (GNSS) with inertial measurements coupled with infrastructure-side beacons. In safety critical application such as train localization, a high level of reliability can only be achieved using redundant and complementary sensors.
Recently, the robotics and computer vision communities have reported visual motion estimation and localization systems with an impressive accuracy and robustness[6, 7, 8, 9, 10]. We believe that synchronized visual and inertial sensors have the right properties to be an ideal extension to the currently used sensor modalities. A continuous global localization is often not feasible using vision sensors due to ambiguous environments or drastic appearance changes. However, combining incremental odometry information with localization to reduce drift accumulated by the odomertry method can provide a continuous and high-accuracy pose estimation. For this reason, as a first step towards such a system, we want to investigate current state-of-the-art visual(-inertial) motion estimation frameworks for their applicability on train applications. The main challenges include high speeds, constrained motion leading to potential observability issues of IMU biases , challenging lighting conditions and highly repetitive patterns on the ground.
Our contribution consists of the first evaluation of popular generic visual-aided odometry frameworks for rail applications. We use a real time kinematics (RTK) Global Position System (GPS) device as ground truth, in order to evaluate the pose estimates on datasets recorded on two trajectories in an industrial, sub-urban and forest environment. Furthermore, we identify, investigate, and discuss specific challenges of current methods.
Ii Related Work
Motion estimation is the backbone of many established small autonomous ground robots and has had an increasing presence due to the rise of autonomous driving. Current solutions in commercial products rely on the use of sensor fusion of GNSS (in outdoor scenarios), wheel odometry, and additional sensors such as Light Detection And Ranging devices (LiDARs) or cameras, e.g. [12, 13, 14].
In contrast to generic ground robots, trains have a distinct feature: their motion is constrained to the rail network. This paper studies advantages and disadvantages this constraint implies to the motion estimation performance compared to more generic approaches.
Ii-a Rail vehicle odometry and localization
The current research goal is to increase the accuracy and robustness of motion estimation and localization. The approaches are split into improving the in-place infrastructure (e.g. track-side Balises, odometer and speed sensors) or investigating new sensor modalities.
describe measures on how to increase the robustness and reliability of the currently used on-board odometry measurements using wheel encoders and ground speed radars. Recent works also include using machine learning approaches such asleast squares support vector machines to improve the localization accuracy .
To decrease the dependency on track-side infrastructure, new sensor modality research is highly focused on the use of an inertial measurement unit (IMU) together with a tachometer  or GNSS. Otegui et al.  summarizes many works fusing IMU and GPS signals employing an extended Kalman filter (EKF) or a particle filter (PF).
To further improve accuracy, Hasberg et al.  and Heirich et al.  introduce Simultaneous Localization and Mapping (SLAM) for path-constrained motion. Fusing GNSS with IMU measurements and a path constraint, high accuracies in position retrieval () and map building with a precision of around are presented .
All these existing methods rely heavily on GPS, IMU and wheel encoders / ground speed radars, each of which have their own failure cases. For instance GPS has denied areas, suffers from multi-path effects near structures and is easy to jam, IMU bias may become unobservable [11, 22], encoders suffer from wheel slippage or mis-calibration  and radars have problems with reflectance off the ground [15, 23]. To improve the robustness and achieve high safety levels through redundancies, additional sensing modalities such as cameras will be critical.
Wohlfeil  performs visual track recognition based on edge extraction with a known track gauge width. Failure cases were observed where switches were missed or confused, especially in challenging lighting conditions (e.g. bright sky). Furthermore, a continuous position estimate is not provided, only the direction of travel after a switch. Bah et al.  present a pure vision-based odometry method for trains by transforming a front facing camera image to a birds-eye view of the track and finding correspondences on two consecutive frames. This method might fail with low-texture or repetitive grounds which often occur in train environments.
The mentioned visual-aided odometry and localization algorithms are not directly suitable for continuous motion estimation as they only provide information near switches  or require manual data association . Furthermore, by not considering specific constraints, the visual odometry can later be fused with this information to get even more reliable and accurate pose estimation, to detect when a method is failing or to detect changes in the expected environment . The goal of this paper is to study the performance of generic visual-aided ego-motion estimation for the rail application.
Ii-B Generic visual-aided ego-motion estimation
State-of-the-art approaches in odometry estimation and SLAM
using vision sensors can be classified into filter approaches (mostlyEKF), where typically all the past robot poses are marginalized and only the most recent pose is kept in the state [6, 27], and sliding-window approaches, where multiple poses are kept in an optimization [7, 8].
Sliding-window based approaches are studied in detail by Strasdat et al. , proving to outperform filter-based approaches when employing the same computational resources. Furthermore, sliding-window based approaches are very flexible for incorporating measurements from different sensing modalities with different propagation models. To keep the computational costs within hardware limits, these schemes typically limit the state to within a sliding window. Efforts to unify both approaches are presented by Bloesch et al. .
The most prominent visual odometry methods are probablyORB-SLAM  and Direct Sparse Odometry (DSO) . ORB-SLAM extracts and tracks keypoint features in an image stream while DSO uses a direct approach based on image gradients. These methods cannot recover the metric scale of the map, which is critical in the given application.
One prominent method to recover the metric scale is adding an IMU. Extending the previous works in  and  respectively, the scale can be observed by incorporating inertial measurements, often referred to as visual-inertial odometry (VIO). However, depending on the performed motion, the IMU biases are not fully observable [30, 22, 11] resulting in errors in scale estimation.
Another method to recover the scale is stereo vision shown in  and . Leutenegger et al.  proposes the combination of stereo vision and IMU measurements, resulting in a reliable feature-based sparse map and accurate camera pose retrieval.
In automotive applications, many of the challenges such as high velocities and constraint motion are similar to the rail domain. There, odometry is often solved by using wheel encoders [35, 36] as they do not suffer from high slip as in rail applications. Furthermore, stereo vision  and monocular visual odometry (VO) with learned depth  have also been used successfully for ego motion estimation. A multitude of state-of-the-art stereo-visual odometry frameworks are tested for automotive applications in the visual odometry part of the KITTI Vision Benchmark Suite . One popular and well performing open-source pipeline is ORB-SLAM2 . Unfortunately, the KITTI dataset does not include synchronized IMU measurements and therefore does not allow in-depth insights into the benefits inertial data could provide. Finally, the scale of the motion can also be retrieved by exploiting non-holonomic constraints  which, however, relies on frequent turns.
In contrast to the mentioned approaches for automotive applications, this paper aims to investigate the benefit inertial data can provide for motion estimation in the rail domain and compares it to already successfully deployed stereo visual odometry.
Iii Visual-aided odometry pipelines
|Estimator type||Sensor measurements||Comment|
|ROVIO ||Light-weight EKF based VIO using patch tracking.|
|VINS-Mono ||Tightly coupled indirect monocular VI fusion.|
|Batch optimization ||Offline global batch VI bundle-adjustment.|
|ORB-SLAM2 ||Indirect stereo visual SLAM framework.|
|OKVIS ||Keyframe based tight coupling of stereo VI fusion.|
|Stereo-SWE ||Tightly coupled VI fusion using depth as independent measurement.|
In order to evaluate the performance of visual-aided ego-motion estimation for rail applications, we made a selection of the most promising available pipelines summarized in Table I.
Iii-a Visual-inertial odometry algorithms
The goal of VIO is to increase robustness and observe the scale of the motion using inertial measurements. Advantages of VIO are gravity aligned maps and complementing sensor modalities. One disadvantage is the dependency on specific motion patterns in order to make the biases observable.
In this paper, the following state-of-the-art algorithms are introduced and further evaluated.
Qin et al.  proposes a VIO algorithm based on indirect tightly coupled non-linear optimization. Compared to a stereo visual-inertial pipeline like OKVIS  (see Section III-C) which can also deal with stereo cameras, VINS-Mono is specifically designed for monocular VIO with main differences in the initialization procedure. Furthermore, VINS-Mono reports slightly better accuracy results in unmanned aerial vehicle (UAV) applications  compared to OKVIS when used in monocular mode.
Iii-A3 Batch optimization
Iii-B Stereo visual odometry algorithms
In addition to using inertial measurements, the metric scale of the motion can also be immediately retrieved using depth measurements of a stereo camera pair. In contrast to VIO, stereo-visual odometry does not require specific motions. However, as the method is purely visual, it will fail whenever the visual system faces challenges in tracking landmarks.
Mur-Artal and Tardos  provide a complete visual SLAM system for monocular, stereo or RGB-D cameras called ORB-SLAM2. The odometry front-end of the system is based on matching ORB features. The optimization is performed on a pose graph only containing the most relevant keyframes. Stereo constraints are incorporated in the cost function by projecting the landmarks with successful stereo matches to an augmented keypoint measurement including the coordinates of both cameras.
Iii-C Stereo visual-inertial algorithms
In order to compensate for failure cases of the previously mentioned approaches, stereo vision and inertial measurements can be combined into a unified framework.
Leutenegger et al.  introduce tight-coupling of inertial and indirect visual measurements in a keyframe based approach optimizing inertial and re-projection errors in a sliding-window. In addition to the previously mentioned algorithms, OKVIS is able to deal with both stereo cameras by fusing landmarks visible in both frames and inertial data by using pre-integrated factors .
Fusing landmarks, such as in OKVIS
, can result in problems if the stereo matches contain wrong matches or outliers. Even if a robust cost function could avoid taking them into account, all additional information this landmark could provide is lost after a wrong merge. Alternatively, stereo matches could also be used as additional independent measurements for each landmark observation instead of fusing the landmarks.
Due to the lack of an available implementation for this approach, we extended the visual-inertial Sliding Window Estimator (SWE) presented by Hinzmann et al. . In addition to the mentioned re-projection error and inertial error, a weighted depth error is introduced for each landmark observation with stereo matches. The depth error is the difference of the measured depth obtained by triangulating the stereo matches and the depth of the corresponding landmark projected to the camera. Inspired by , depth error uncertainties are scaled by their distance to the cameras and only considered up to a certain distance relative to the baseline between the cameras.
Iv Experimental evaluation
In this section, the experimental evaluation of the mentioned algorithms is shown111All evaluations were performed on an Intel Xeon E3v5, RAM laptop but not in real-time ()..
We start with describing the datasets, explain recoding and evaluation procedure, and show the results and some in-depth analysis.
Due to the lack of suitable available datasets, the estimators are tested on data recorded in Potsdam, Germany on a Siemens Combino tram (see Figure 2), which is customized for autonomous driving tests. Trajectory1 is a short low-velocity (up to ) track around the depot in an industrial environment and close-by structures as shown in Figure 3. Trajectory2 is along a public tram-line about long with speeds up to representing a real-life scenario. This trajectory includes rural, sub-urban, urban, and woody environments. The datasets were captured on a sunny day in August 2018 as dealing with extreme conditions for visual sensing (rain, fog, nighttime) is beyond the scope of this paper.
Iv-B Hardware setup
Iv-B1 Vio setup
For the data collections, we deployed a custom-built stereo visual-inertial sensor which is synchronizing all measurements in hardware similar as in . To feature higher accuracy, exposure time compensation is utilized.
The sensor consists of two global-shutter cameras arranged in a fronto-parallel stereo setup and a compact, precision six degrees of freedomIMU. The camera was selected to provide a high frame-rate to be able to get a reasonable number of frames per displacement, even at higher speeds, and also to feature a high dynamic range to deal with the challenging lighting conditions. The IMU was chosen to feature low noise values and also to support temperature calibration, as direct sunlight can highly change the temperature of the sensors. The sensor specifications are summarized in Table II.
In order to investigate the influence of the baseline distance of the stereo setup, data was collected with three baselines of , and . Those baselines were chosen to have a wide variety from baselines common in automotive applications up to those used in fixed wing UAVs. Figure 1 shows the deployed sensor in a baseline configuration mounted behind the windshield inside the front cabin of the test vehicle.
Sensor intrinsic and extrinsic calibration was performed using the Kalibr toolbox . The transformation of the IMU with respect to the master camera (camera 1) is constant and calibrated in a lab environment. The transformation between the two cameras is then determined separately in-situ using a checkerboard with tile sizes of . Even though a larger calibration target might be beneficial to enable mutual observations, for the larger baselines we needed a board of and , respectively, which are more difficult to manufacture and handle and were not available during data collection.
Iv-B3 Ground truth
The main metrics used in this paper to evaluate the performance of visual-aided odometry pipelines are incremental distance and heading errors.
We use the segment-based approach introduced by Nourani-Vatani and Borges  to deal with unbound errors in odometry . Thereby, the trajectory is divided in segments. Two different segment lengths and are evaluated to test the evolution of errors. Each segment is aligned with the corresponding ground truth trajectory segments using the first of the segment. The distance error then corresponds to the distance between the end-points of the segments. The heading error is the difference in heading estimation between the two segment ends.
Iv-D Results and discussion
|Batch optimization ||Median|
|Stereo visual||ORB-SLAM2 ||Median|
|Stereo visual- inertial||OKVIS ||Median|
All ego-motion estimation pipelines investigated in Section III are tested on both trajectories using the different baselines. To enable a high level of comparability, the state-of-the-art pipelines are used out-of-the-box with only minor tuning. As pure odometry is under investigation here, all loop closing capabilities are disabled.
Iv-D1 Estimator performance
Using the baseline, a good calibration can be ensured. Table III shows the evaluation results comparing the different estimation pipelines.
Both ROVIO and VINS-Mono fail to work properly for rail applications. Due to a very constrained motion and frequent constant velocity scenarios, the IMU biases cannot be estimated correctly locally. This results in significant scale drift, especially visible in trajectory2 with longer sections of constant velocity. In addition, unobservable biases lead to inconsistent estimator state and the need to re-initialize multiple times during the trajectory. While ROVIO can partly recover from such a reset, VINS-Mono cannot, resulting in a somehow unfair comparison to the others. However, ROVIO also shows bad performance as it is highly dependent on a good knowledge of the IMU biases.
Using ROVIOLI to build a map and maplab  to globally batch bundle-adjust the maps, the scale and IMU biases can partially be recovered. This suggests that in both EKF and sliding-window approach, the bias estimation problem suffers significantly from linearization issues if there is not enough axis excitement in the window. A further hint for this is the improved performance of VINS-Mono compared to ROVIO which, among other possible causes, could be due to the difference in window size.
In comparison, when using stereo constraints, metric scale can be estimated correctly during the whole trajectory. On trajectory1, both OKVIS and ORB-SLAM2 show very similar performance. Trajectory2 is more challenging due to faster motion, more challening lighting conditions and more dynamic objects such as cars, pedestrians and other trams in the scene. There, OKVIS is able to outperform ORB-SLAM2 in most cases. This is especially visible in the RMSE in Table III which suggests that OKVIS also has a higher robustness compared to ORB-SLAM2. The complementing sensor modalities show benefits for motion estimation, most prominently in dynamic environments and at higher speeds. For Stereo-SWE, using the depth as an independent part in the optimization problem does not show an increase in accuracy. Also, it has a drawback of increasing in tuning parameter number, which is the weighting factor between depth errors and re-projection and inertial errors. This increases the tuning effort.
The distance errors along trajectory2 for the best four performing estimators are shown in Figure 7. Three distinctive challenging scenarios , and can be identified. They are also indicated in Figure 4. These challenging scenarios give evidence to the difference in estimator performances, and are summarized in Table V.
Iv-D2 Challenging scenarios
|Stereo-SWE||High speed||Optimize feature tracking.|
|ORB-SLAM2, Stereo-SWE||High speed & Aliasing||Detect and neglect affected region; use IMU fusion.|
|all||Lighting conditions||Shadow compensation ; improve auto exposure .|
Scenario is visible approximately of the way through the trajectory. There, the tram is moving with high velocity, which increases the complexity of feature tracking. While the optimized feature tracking of ORB-SLAM2 and OKVIS are able to deal with this, Stereo-SWE and ROVIOLI (used to build the map for batch optimization), which share the same feature-matching algorithm, have trouble finding enough feature matches.
Around scenario , the tram is also moving at high speeds as shown in Figure 7. However, in contrast to scenario , there is no curve and the optical flow of all nearby structure is in the same direction as the repetitive patterns on the ground such as railings or railroad ties. This leads to aliasing and wrong feature matches. More evidence can also be found at the beginning of trajectory2 before entering the curve. Using the IMU as a complementing sensor modality, as in OKVIS, seems to be beneficial in this case. Stereo-SWE still has troubles with feature tracking as in scenario . By masking out the area in-front of the vehicle where most visual aliasing is happening, different behaviours for the estimators can be observed as shown in Figure 8. While OKVIS behaves almost the same, the increase in error in scenario for ORB-SLAM2 can be reduced. However, by removing some of the visual information in slow sections and no other available measurement source, the level of robustness is decreased. This can be observed after about of the trajectory where short heavy reflections in the upper part of the image can cause huge errors. In this scenario, one could use Nyquist theory to detect and neglect affected regions instead of neglecting static parts of the image. The SWE shows higher errors in scenario due to the reduced visual information in an already challenging feature-matching scenario.
Iv-D3 Baseline comparison
In general, a larger baseline should provide more reliable depth information as the quantization error in the disparity is less prominent. However, the quality of depth calculation is highly sensitive to a good calibration. Using conventional methods, calibration is much more challenging using a higher baseline as it is harder to guarantee mutual observations of the calibration target for both cameras. This results in a degrading calibration quality with higher baselines. Table IV summarizes evaluation results using the stereo visual(-inertial) pipelines with different baselines.
For the stereo algorithms shown in the scenarios of trajectory1 and trajectory2, it seems more important to have an accurate calibration using a small baseline than to be able to reliably incorporate further away landmarks. Using IMU measurements, motion estimation is still possible up to a fixed scale error, visible in Figure 5 and Figure 6, while ORB-SLAM2 loses track at higher speeds. In order to benefit from the advantages of higher baselines, improved calibration procedures such as online calibration  could have a high benefit.
Being able to accurately localize a rail vehicle has a high potential to improve infrastructure efficiency. In a real-world application, high safety levels are typically achieved using redundant systems. This paper studies the contribution visual systems can provide to getting closer to robust high accuracy odometry. We did an in-depth experimental comparison using real-world rail datasets of several state-of-the-art visual-aided odometry pipelines.
It was observed that the monocular visual-inertial odometry methods ROVIO and VINS-Mono experience severe scale drift and are not able to keep a consistent estimator state due to locally unobservable IMU biases. Using stereo vision, accurate motion estimation is achievable, especially using the stereo visual-inertial pipeline OKVIS. Specific challenging scenarios for the individual pipelines can result from high speeds, aliasing with repetitive patterns on the ground, and challenging lighting conditions.
In conclusion, even without enforcing specific motions, visual-aided odometry can achieve high accuracies for rail vehicles, but is not reliable enough for use in isolation for safety critical applications. However, in combination with other odometry and localization methods such as GNSS, wheel odometry or ground radars, vision can complement for failure cases of other sensors. Motion constraints can be incorporated either as a separate part in the estimation pipeline or directly into the investigated algorithms using a motion model in the propagation state for EKF based algorithms or additional motion model error term in optimization based algorithms. High-speed scenarios will require higher frame-rates to ensure feature tracking and a larger baseline for reliable depth estimation of unblurred landmarks implying calibration challenges. Furthermore, all tested datasets are recorded during nice weather. Like most visual pipelines, the investigated approaches will suffer from limited visibility. However, using cameras with extended spectral band sensitivity such as long-wave infrared (LWIR) shows potential to enable also good performance in bad weather conditions .
This work was generously supported by Siemens Mobility, Germany. The authors would like to thank Andreas Pfrunder for his help in initial data collections and evaluations.
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