Time synchronization of consumer cameras on Micro Aerial Vehicles

This article discusses the problem of time registration between navigation and imaging components on Micro Aerial Vehicles (MAVs). Accurate mapping with MAVs is gaining importance in applications such as corridor mapping, road and pipeline inspections or mapping of large areas with homogeneous surface structure, e.g. forests or agricultural fields. Therefore, accurate aerial control plays a major role in efficient reconstruction of the terrain and artifact-free ortophoto generation. A key prerequisite is correct time stamping of images in global time frame as the sensor exterior orientation changes rapidly and its determination by navigation sensors influence the mapping accuracy on the ground. A majority of MAVs is equipped with consumer-grade, non-metric cameras for which the precise time registration with navigation components is not trivial to realize and its performance not easy to assess. In this paper, we study the problematic of synchronization by implementing and evaluating spatio-temporal observation models of aerial control to estimate residual delay of the imaging sensor. Such modeling is possible through inclusion of additional velocity and angular rate observations into the adjustment. This moves the optimization problem from 3D to 4D. The benefit of this approach is verified on real mapping projects using a custom build MAV and an off-the-shelf camera.

Applicability Of New Approaches Of Sensor Orientation To Micro Aerial Vehicles

Martin Rehak, Jan Skaloud

This study highlights the benefits of precise aerial position and attitude control in the context of mapping with Micro Aerial Vehicles (MAVs). Accurate mapping with MAVs is gaining importance in applications such as corridor mapping, road and pipeline inspections or mapping of large areas with homogeneous surface structure, e.g. forests or agricultural fields. There, accurate aerial control plays a major role in successful terrain reconstruction and artifact-free ortophoto generation. The presented experiments focus on new approaches of aerial control. We confirm practically that the relative aerial position and attitude control can improve accuracy in difficult mapping scenarios. Indeed, the relative orientation method represents an attractive alternative in the context of MAVs for two reasons. First, the procedure is somewhat simplified, e.g. the angular misalignment, so called boresight, between the camera and the inertial measurement unit (IMU) does not have to be determined and, second, the effect of possible systematic errors in satellite positioning (e.g. due to multipath and/ or incorrect recovery of differential carrier-phase ambiguities) is mitigated. First, we present a typical mapping project over an agricultural field and second, we perform a corridor road mapping. We evaluate the proposed methods in scenarios with and without automated image observations. We investigate a recently proposed concept where adjustment is performed using image observations limited to ground control and check points, so called fast aerial triangulation (Fast AT). In this context we show that accurate aerial control (absolute or relative) together with a few image observations can deliver accurate results comparable to classical aerial triangulation with thousands of image measurements. This procedure in turns reduces the demands on processing time and the requirements on the existence of surface texture. Finally, we compare the above mentioned procedures with direct sensor orientation (DiSO) to show its potential for rapid mapping.

Copernicus Gesellschaft Mbh2016

Assessment of VDM-based autonomous navigation of a UAV under operational conditions

Mehran Khaghani, Jan Skaloud

This paper presents extensions and practical realization of a previously proposed novel approach to navigation and sensor integration for small unmanned aerial vehicles (UAV). The proposed approach employs vehicle dynamic model (VDM) as process model within navigation system, and treats data from other sensors such as inertial measurement unit (IMU), barometric altimeter, and global navigation satellite system (GNSS) receiver as observations within the system. In comparison to conventional approach that employs inertial navigation system (INS) as process model, employing VDM requires no added hardware, yet significantly improves navigation performance, especially in case of GNSS outages. Experimental results from a real flight on a custom made fixed-wing UAV, as well as Monte Carlo simulation results, reveal improvements of 1 to 2 orders of magnitude in navigation accuracy during GNSS outages of 3 minutes’ duration. This is a prerequisite for safer navigation without exteroceptive sensors. Uncertainty levels are predicted consistently within the filter, and a discussion on observability based on covariance matrix analysis is presented. Computation time is also compared to conventional INS-based approach.

2018

Applicability Of New Approaches Of Sensor Orientation To Micro Aerial Vehicles

Martin Rehak, Jan Skaloud

Copernicus Gesellschaft Mbh2016