Visual and IR-Based Target Detection from Unmanned Aerial Vehicle
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In many situations it is important to detect and identify people and vehicles. In this study the purpose was to investigate subject’s performance to detect and estimate number of stationary people on the ground. The unmanned aerial vehicle used visual- and infrared sensor, wide and narrow field of view, and ground speed 8 m/s and 12 m/s. Participants watched synthetic video sequences captured from an unmanned aerial vehicle. The results from this study demonstrated that the ability to detect people was affected by type of sensor and field of view. It took significantly longer time to detect targets with the infrared sensor than with the visual sensor, and it took significantly longer time with wider field of view than with narrow field of view. The ability to assess number of targets was affected by type of sensor and speed, the infrared sensor causing more problems than the visual sensor. Also, performance decreased at higher speed.
KeywordsTarget detection Visual sensor IR sensor UAV Human factors
The information explosion that comes from gathering information with new and better sensors is positive since users can access more information. At the same time it is a challenge to select the vital information in a specific situation. Data overload may be a serious problem, and it is necessary to have an understanding of the whole system. How to help human cognition using a medium (i.e. computer) is fundamental to ensure good user performance in the current situation. To build an effective system in a military setting a number of factors must be taken into account. From an ecological approach  and representation design  there is a cognitive triad between domain or environment, interface and users. There is a reciprocal coupling between the user and the environment which often is mediated by a user interface. The components of the triad are the cognitive demands from the domain or environment, user resources and limitations, and the interface. The interface effectiveness is determined by the mapping between the environment and interface (correspondence) and the mapping between the user and interface (coherence). To develop an effective and user friendly system all these three parts must be taken into account.
In many applications part of the information that reaches the user has been acquired with some type of sensor system, involving one or several sensors and signal processing, acting as a filter between the environment and the interface. In order to be able to understand the complete picture and study sensor-related aspects the model has to be extended.
Even though the whole system must always be taken into account, the research presented here focuses on the ability and limitations of the users to extract the correct information about the environment based on data from sensors observing it. The purpose of this study was to investigate and compare performance of manual visual and IR-based detection of static targets seen from a simulated unmanned aerial vehicle (UAV).
It is not always certain what seeing an object means. One way to analyze observers’ ability to perform visual tasks is to use the Johnson criteria [3, 4], often used by scientists who study the capability of sensors, e.g., infrared systems. A differentiation is made on detection (i.e. whether there is something of potential interest), orientation (which sometimes are excluded), recognition (e.g. the difference between a human and car) and identification (e.g. whether it is a friend or foe). The Johnson criteria proposes in detail how many pixels (originally line-pairs) an object needs to contain to make the classification possible. According to Johnson criteria, the detection distance is calculated based on how many pixels an object must contain. In order to detect static objects it requires 2 × 2 pixels, orientation 8 × 2.8 pixels, 8 × 8 pixels for recognition and identification required 12.8 × 12.8 pixels . These figures should not be interpreted as guidelines but rather as values under the best possible conditions. It is not entirely clear in various descriptions but the described values probably mean that the task can be solved in 50% of the cases, which in most cases is unacceptably low. There is also a variety of factors that must be considered, including the contrast between objects and background, atmospheric disturbances, the number of objects in the picture, light, contextual clues, color and type of optics. Moreover, performance is affected by the type of task, the experience of the participants and their level of training for the specific task, motivation, and the relative importance between quick decisions and correct results . There are several methods that can be regarded as further development of Johnson criteria, e.g. TOD (Triangle Orientation Discrimination), TTP (Targeting Task Performance) and TRM (Thermal Range Model). For further description of these methods see Näsström et al. , Wittenstein , and Vollmerhausen and Jacobs .
In order to assess and evaluate a system, experiments with users should be conducted. Theoretically calculated values could be of some value to get an indication of what objects that can be detected or identified (e.g. Johnson criteria) but is never enough. Identification of friend or foe is different from being able to identify who the person is and it is therefore important to be clear about exactly what you mean by different concepts. There is an obvious risk for confusion regarding the interpretation of concepts, since the concepts are used by researcher in different context without a standardized definition. In many situations one must be absolute sure about the identity of a person to make a decision whether to use military force. Friendly fire, where a soldier accidentally opens fire on his own troops, is a well-known phenomenon. In other cases, such as intelligence, is it important to describe what is seen according to a predetermined classification scheme and not just described what they think they see.
In this study the purpose was to investigate subjects’ performance for visual and IR-based detection of targets seen from a simulated unmanned aerial vehicle (UAV).
Participants watched synthetic video sequences captured from an UAV. All video sequences were generated by a sensor simulation system. The task was to detect and count stationary humans (referred to as targets in this paper). The participants also estimated how confident they were in their answers on a scale from 0 to 100%. A within-group design with two visualizations (visual and IR) × two field of view × two UAV speeds (12 m/s and 8 m/s) was used. The field of view (FOV)) used were 2.3 × 1.7° and 1.4 × 1.1°.
Eight subjects (four women and four men) participated in the experiment. All had adequate vision with or without correction.
The video sequences were presented on a 21.5 inch full HD widescreen display with a resolution of 1920 × 1080 pixels. A PC with Windows 7, Intel® Core™ 2 Duo processor with 3 GHz and 4 GB of RAM memory was used. The software Matlab  was used on a separate computer to register response time when the participants clicked with the left mouse button. The response tool was designed so participants could always look at the screen where the stimuli material (video sequences) were presented.
According to the mission, four scenarios were generated with the visual- and IR-sensor respectively. Each scenario had eighteen areas with different target positions. The same areas and positions were used for the visual- and IR scenarios. A total of eight videos were generated according to the aforementioned design. The visual and IR scenarios were presented in a balanced order between subjects’, and within each sensor the four scenarios were presented in a randomized order.
The results include statistical analysis of time to detect targets and estimation of number of targets with confidence estimations. The data were analyzed with a three-way ANOVA  with type of visualization (visual and IR), speed (12 m/s and 8 m/s), and field of view (1.4 × 1.1°). Tukey HSD were used for post hoc testing .
3.2 Estimation of Number of Objects with Confidence Estimation
4 Discussion and Summary
The results from this study demonstrated that the ability to detect targets was affected by type of sensor and FOV. It took significantly longer time to detect targets with the IR sensor than with the visual sensor and it took significantly longer time with wider FOV than narrow FOV. The ability to assess the number targets was affected by type of sensor and speed, IR sensor causing more problems than the visual sensor. The ability to assess the number of targets was overall very high, at 8 m/s (93–99%) and relatively high at 12 m/s (79–91%).
This study also shows a method that can be used to investigate the users’ performance related to detection of ground targets from an UAV. The task was to detect and count number of humans on the ground and make confidence estimations. The task can be modified to identify different type of targets, e.g. to investigate possible differences between target types or classify objects as friend or foe.
One limitation of this study is that although the visual and IR sensor data are realistic, no scientific verification has been made to confirm the similarity between the used stimuli material and real data from sensors. However, one researcher compared the simulated videos with real sensor information and confirmed that the material looked similar. In the future, this procedure need to be improved with objective measures.
This experiment was the first experiment in a series of planned experiments, where different platform speeds and field of view angles were studied. Another possibility for further studies is to use other missions, e.g. manually control the UGV instead of using predefined paths. In this experiment it was daytime in strong sunshine, which gave clear shadows of people and objects. It would be interesting to compare the results achieved in this study with results from a daytime scenario with cloudy weather without clear and sharp shadows visible. Also, night time scenarios’ would be interesting to investigate. Another possibility is not to only examine human performance of detection, but also examine recognition and identification. The follow-up experiment (now in progress) investigates recognition of various vehicles in a similar setting as the experiment presented here.
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