THE PERFORMANCE ANALYSIS OF A UAV BASED MOBILE MAPPING SYSTEM PLATFORM

To facilitate applications such as environment detection or disaster monitoring, the development of rapid low cost systems for collecting near real-time spatial information is very critical. Rapid spatial information collection has become an emerging trend for remote sensing and mapping applications. This study develops a Direct Georeferencing (DG) based fixed-wing Unmanned Aerial Vehicle (UAV) photogrammetric platform where an Inertial Navigation System (INS)/Global Positioning System (GPS) integrated Positioning and Orientation System (POS) system is implemented to provide the DG capability of the platform. The performance verification indicates that the proposed platform can capture aerial images successfully. A flight test is performed to verify the positioning accuracy in DG mode without using Ground Control Points (GCP). The preliminary results illustrate that horizontal DG positioning accuracies in the x and y axes are around 5 m with 300 m flight height. The positioning accuracy in the z axis is less than 10 m. Such accuracy is good for near real-time disaster relief. The DG ready function of proposed platform guarantees mapping and positioning capability even in GCP free environments, which is very important for rapid urgent response for disaster relief. Generally speaking, the data processing time for the DG module, including POS solution generalization, interpolation, Exterior Orientation Parameters (EOP) generation, and feature point measurements, is less than one hour. * Corresponding author: M.L.TSAI – taurus.bryant@msa.hinet.net INTRODUCTION With the number of natural disasters increasing due to climate change, the development of a rapidly deployable and low cost system for collecting near real-time spatial information has become very critical. Therefore, rapid spatial information acquisition capability has become an emerging trend for remote sensing and mapping applications. Airborne remote sensing, more specifically aerial photogrammetry, in its classical form of film-based optical sensors (analogue) has been widely used for high accuracy mapping applications at all scales and rapid spatial information collection for decades. Recently, film-based optical sensors (analogue) have been replaced by digital imaging sensors. Generally speaking, conventional photogrammetric methods rely on huge numbers of Ground Control Points (GCP). Although photogrammetry has adopted digital technology, GCPs are generally considered the only source of reliable georeferencing information (Gibson et al, 1992). Recently, Direct Georeferencing (DG) technology has become possible by integrating Global Positioning System (GPS) and Inertial Navigation System (INS), making Exterior Orientation Parameters (EOP) available with sufficient accuracy at any instant of time (Gibson et al, 1992). The integration of INS and GPS improves the georeferencing of photogrammetric data and frees it from operational restrictions. Together with digital data recording and data processing, it allows multi-sensor mapping systems. Numerous studies have been conducted on the application of Unmanned Aerial Vehicle (UAV) for photogrammetry applications. A detailed review of UAV photogrammetric applications can be found in (Eisenbeiss, 2004; Eisenbeiss, 2008). Although most schemes apply low cost INS/GPS integrated systems for flight control, a DG based UAV photogrammetric platform equipped with an INS/GPS integrated Positioning and Orientation System (POS) that can provide EOP of the camera in a GCP free environment has not been proposed until recently. TECHNICAL CONFIGURATIONS OF PROPOSED


INTRODUCTION
As the number of devastating disasters increases due to climate change, a quickly and low-cost system for collecting near realtime spatial information has become very important.Rapid spatial information collection has become a trend in remote sensing and mapping applications.Airborne remote sensing, more specifically aerial photogrammetry, with film-based optical sensors (analog) has been widely used for high-accuracy mapping applications at all scales and for rapid spatial information collection for decades.Recently film-based optical sensors have been replaced by digital imaging sensors.Figure 1 shows the procedures applied in conventional aerial photogrammetry.
In general, the necessity for Ground Control Point (GCP) was so evident that all operation methods relied on it.Only recently has Direct Geo-referencing (DG) become possible by integrating the Global Positioning System (GPS) and an Inertial Navigation System (INS), allowing all exterior orientation information to be available with sufficient accuracy at any instant of time (Schwarz et. al., 1993).The integration of GPS/INS puts the geo-referencing of photogrammetric data on a new level and frees it from operational restrictions.Together with digital data recording and data processing, it has initiated the era of multi-sensor systems.
Operational flexibility is greatly enhanced in all cases where a block structure is not needed.Costs are considerably reduced, especially in areas where little or no ground control is available.The accuracy of existing commercial systems is sufficient for many mapping applications.As shown in Figure 1, the cost and production efficiency are improved significantly with DGbased photogrammetric platforms.However, there are some limitations to existing DG-based photogrammetric platforms.The cost of renting a plane to conduct aerial photogrammetry is high and there are strict regulations and complicated procedures for obtaining a permit to conduct a flight plan in Taiwan.In addition, the flexibility and capability of conducting small-area surveys or rapid spatial information collection are rather limited.Therefore, a DG-ready airborne platform that is relatively free of government regulations and reduces flight cost while maintaining high mobility for small-area surveys and rapid spatial information collection is desired for urgent response applications such as disaster relief and assessment.Table 1 summarizes a comparison of various candidates for photogrammetric platforms in terms of system configuration and applications.Generally speaking, the selection of candidate platform is application-dependent.The primary objective of developing Unmanned Aerial Vehicle (UAV) based photogrammetric platform is to meet requirements such as small operational area, rapid deployment, low cost, high mobility, and acceptable accuracy.Therefore, it is not practical to use these platforms as replacements for conventional photogrammetric applications.

Geodeti c grade Yes Di gi tal/ fi l m >50kg Hi gh Yes Yes Low Hi gh <20 cm Yes Low
Helicopter Tacti cal grade

THE TECHNUCAL CONFIGURATIONS OF PROPOSED PLATFORM
The configuration of the proposed DG-based UAV photogrammetric platform is shown in Figure 2. The components of each module are described in detail below.
The proposed UAV platform is shown in Figure 3.The proposed UAV is designed for medium-range applications.The maximum operational range is 50 kilometers and the real time video transmission range is 100 kilometers with extended range communication links.The flexible flight altitude and two-hour endurance time make it suitable for small-area and large-scale photogrammetric missions.
Figure 4 shows the DG module designed in this study for facilitating GCP-free photogrammetry and INS/GPS Positioning and Orientation System (POS) aided bundle adjustment.The data storage module used to record the measurements collected by the Inertial Measurement Unit (IMU) (MMQ-G), GPS (AEK-6T) and the synchronized time mark used to trigger camera (Canon EOS 5D Mark II) is Antilog from Martelec.Due to the limitations of payload and power supply, a PC-or notebook-based data storage module is unsuitable.Therefore, a simple mechanization that can store measurements communicated over serial ports is desired.Antilog was chosen due to its flexibility, low power consumption, and reliability.Similarly, since Canon EOS 5D Mark II has its own storage mechanization, it is not included in this module.(1) The physical meanings of , , and are given in Figures 6 and 7, respectively.A two-step boresight calibration procedure is implemented in this study to acquire the rotation matrix ( ) between the camera and INS by using the rotation matrix ) provided by INS and the rotation matrix provided by conventional bundle adjustment during the calibration procedure (Fraser, 1997): (2) Mapping (m) frame The lever arm between the GPS phase center and the IMU center is determined using a surveying process.The lever arm between the camera center and the IMU center is determined using a two-step procedure by comparing the output of conventional bundle adjustment and INS/GPS integrated POS solutions during the calibration process using: (3)  Post-mission processing, when compared to real-time filtering, has the advantage of having the data of the whole mission available for estimating the trajectory (Shin and El-Sheimy, 2005).This is not possible when using filtering because only part of the data is available at each trajectory point, except the last.When filtering is used in the first step, an optimal smoothing method, such as the Rauch-Tung-Striebel (RTS) backward smoother, can be applied (Chiang et. al., 2004).It uses the filtering results and their covariances as a first approximation.This approximation is improved by using additional data that was not used in the filtering process.Depending on the type of data used, the improvement obtained by optimal smoothing can be considerable (Gelb, 1974).
For the geo-referencing process which puts POS stamps on images and the measurement process that obtains the 3D coordinates of all important features and stores them in GIS database, only post-mission processing can be implemented based on the complexity of those processes (El-Sheimy, 2002).Therefore, most commercially available DG systems operate in real-time only for data acquisition and conduct most of the data processing and analysis in post-mission mode.Figure 9 illustrates the Loosely Coupled (LC) INS/GPS integrated scheme implemented in this study.

Flight planning
The flight altitudes set for aerial photography is 600 meters above ground.Owing to the limit of the payload and the impact of side wind affecting the attitude of UAV, the endlap and sidelap were increased to 80% and 40% respectively to insure that the coverage of the stereo pair can overlap completely during the test flight.Although more images have to be processed, it can be guaranteed that the completed coverage of the stereo pair.Figure 11 illustrates the flight path.
Figure 11 Trajectories of test flight

Calibration results
In this study, the camera calibration process was implemented first to obtain the Interior Orientation Parameters (IOP) of camera.Then the lever arm and boresight angle were calibrated after installing the cameras on the UAV.Consequently, the performance analysis of DG accuracy was performed by comparing DG results with checking points with precisely known coordinates.Table 2 shows the preliminary IOP result.The error of the camera calibration is accepted at this stage.This number may be improved in the future work.
Table 2 The IOP of EOS 5D Mark II Figure 12 shows the Exterior Orientation Parameters (EOP) result.The estimated accuracy of image referencing is 0.33 pixels.The influence of the EOP is around 0.06 meters in terms of the three-dimensional positioning accuracy.Figure 13 illustrates the trajectory of INS/GPS integrated POS solutions during the test.The INS/GPS integrated POS solutions processed with Extended Kalman Filter (EKF) trajectory is shown in red line and RTS smoother is shown in green color, respectively.Because of the kinematic alignment process applied due to the use of low cost INS/GPS integrated POS module, the beginning (around 100 seconds) of EKF trajectory is not smooth.However, only smoothed trajectory provided by RTS smoother is applied for further processing.
Figure 12 The EOP result Figure 13 The trajectory of integrated POS A two-step approach is implemented in this research to acquire the lever arm and boresight angle of each camera.Table 3 and 4 shows the lever arm and boresight angle results.
Table 3 The result of lever arm Table 4 The result of boresight angle The Verification of DG Capability of Proposed UAV Photogrammetric Platform The reference coordinates of the check points are obtained through the precise control survey with GPS RTK (Real Time Kinematic) technology and network adjustment.Therefore, the DG coordinates of those check points are then compared with their reference coordinates.The results are illustrated in Table 5.This study uses L1 carrier phase raw measurements that can be applied for differential GPS processing with single frequency carrier phase measurements and provides sufficient positioning accuracy in civil purpose such as mapping and disaster monitoring.When the flight height is 600 meters, the estimated positional error is around 10 meters if the angle offset is 1 degree.In other words, should the flight of UAV be reduced to 300 meters, the anticipated DG positioning accuracy of proposed system can be improved to be less than 10 meters level horizontally and 15 meters vertically, which is ideal for most of near real time disaster mapping and rescue applications.

CONCLUSIONS
This study develops a DG based UAV photogrammetric platform where an INS/GPS integrated POS system is implemented to provide DG capability of the platform.
The performance verification process indicates the proposed platform can capture the aerial images successfully.With the proposed low cost but high mobility DG based UAV based platform, the preliminary result illustrates horizontal DG positioning accuracies in x and y axes are both less than 20 meters, respectively.On the contrary, the positioning accuracy of z axis is less than 30 meters with 600 meters flight height above ground.Such accuracy is good for near real time disaster relief.Therefore, the DG ready function of proposed platform guarantees the mapping and positioning capability even in GCP-free environments, which is very important for rapid urgent response for disaster relief.
In addition, the future study will be conducted to implement a static ground calibration procedure to improve the DG positioning accuracy of proposed UAV based photogrammetric platform.The one-step approach will be developed to guarantee the accurate lever arms and boresight angles calibration and a cluster based tightly coupled integrated scheme will be investigated to guarantee the stability of POS solutions for practical GCP-free applications.

Figure 5
Figure 5 illustrates the set up of the DG module within the UAV platform.As shown in this figure, the MMQ-G is set right on top of the camera.

Figure 2
Figure 2 Configuration of proposed DG-ready UAV photogrammetric platform

Figure 6
Figure 6 Concept of airborne DG

Figure 8
Figure 8 illustrates the INS/GPS POS-assisted Aerial Triangulation (AT) based photogrammetric process and DGbased photogrammetric process implemented in this study, respectively.

Figure 9
Figure 9 Loosely coupled INS/GPS integrated scheme RESULTS AND DISCUSSIONS To validate the performance of proposed platform, a field test was conducted in the fall of 2011.The area of the test zone is 3 kilometers * 3 kilometers, which is covered by the red square shown in Figure 10.The blue region illustrates the fly zone approved for this test.

Figure 10
Figure 10 Proposed DG-ready photogrammetric procedure The primary error sources contributing to DG positioning error including 1~2 meters limited by the positioning accuracy of onboard INS/GPS integrated POS module and 10~15 meters limited by limited by the orientation accuracy of onboard INS/GPS integrated POS module, respectively.Generally speaking, the orientation accuracy provide by onboard INS/GPS integrated POS module is around 0.5 to 1 degree.

Table 5
The result of the DG test International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B1, 2012 XXII ISPRS Congress, 25 August -01 September 2012, Melbourne, Australia