Simplifying Aerial Surveys

Integrating direct georeferencing with flight management makes aerial surveying faster and more accurate.
By Joe Hutton, Alan Wing Lun Ip, and Andrew Stott

Over the last decade, the development of GNSS-based flight management systems has led to the automation of many in-flight tasks as well as many new and improved methodologies for aerial mapping. Indeed, today's systems can actually replace the navigator/camera operator crewmember. These methodologies have focused in particular on data collection and processing efficiency; high efficiency and productivity are demanded by niche market segments such as emergency response. Hurricane assessment, flood mapping, forest fire damage evaluation, and securities planning are all examples of rapid response application segments that benefit from shorter in-field data collection and processing times to generate map products. These new technologies and methods have reduced overall costs while maintaining the quality of map products.
Digital sensors and direct georeferencing (DG) are leading these technology advancements, producing quality map products previously produced using field-assisted photogrammetric methods. By directly calculating the exterior orientation parameters of each image capture and eliminating the need for aerial triangulation and the use of ground control points with in-direct georeferencing, these emerging technologies have proven cost effective in reducing data post-processing times and operational costs.

The emergence of digital sensor technology has also eliminated the expensive and time-consuming film development and scanning procedures. The resulting exterior-oriented solution provides a means of generating orthophotos and orthomosaics within a seamless workflow environment.
However, these efficiencies only partly fulfil the time reduction requirements, especially in emergency response applications where immediate mission deployment is the main criteria; efficient flight planning and management is also required. Applanix, a mobile mapping company, has developed an integrated direct georeferencing/flight management system for mission planning and execution to address the preparedness burden of rapid response. 

Pulling Several Components Together

Based on the Positioning and Orientation System (POS), POSTrack is an integrated real-time direct georeferencing and flight management system designed for the airborne geospatial community to assist in flight planning and operation for pilots and operators.

Engineered for installation on any aircraft, it has a flight management computer system (FCS) module that communicates directly with the Applanix POS AV Computer System (PCS).  The FCS monitors the real-time position and orientation of the sensors as computed by the PCS, then instructs it when to control the imaging sensor to perform certain tasks.  In addition to the FCS and PCS, the POSTrack system incorporates a pilot touch display, GNSS antenna, and an inertial measuring unit (IMU).

By combining the functionality of GNSS-aided inertial navigation with that of pilot guidance and sensor control, many significant benefits are achieved:

    • precise project flight path monitoring via high-rate
    • real-time position and orientation,
    • accurate sensor positioning and image acquisition via
      real-time precise timing hardware,
    • air time optimization over the target area, and
    • in-flight task automation maximization.

Navigation:  From the GNSS and IMU data, the PCS computes the real-time position and orientation of the imaging sensor at up to 200 times per second. This, along with the status of the raw IMU and GPS data being collected for direct georeferencing, is displayed on the pilot display, giving immediate warnings of any problems.  The real-time orientation data is used for automatic control of 3-axis gimbal mounts. The orientation angles (roll, pitch, and heading) are calculated by the PCS and fed into the mount to ensure it remains level and to steer the azimuth of the mount to follow a desired track along the ground.

This offers a significant advantage when compared to the inclinometer approach, where false readings due to horizontal acceleration can cause the mount to tilt incorrectly during turbulent conditions. In addition to steering, all gimbal mounts receive the mount's motion via the gimbal encoder, which is logged as part of the navigation data for GNSS-inertial post-processing. This ensures lever arm parameters are always calculated with respect to the mount's center of rotation.

Flight Management and Planning: The flight management software running on the FCS module is an OEM version of the XTRACK developed by Track-Air. It is used in all aspects of flying an airborne mission, from flight preparation to image acquisition.  The flight planning components are installed on the navigation laptop, where the operator has better access to flight preparation components such as existing vector and raster backgrounds.  This ensures that generated flight plans can be easily synchronized with the server module installed on the FCS. Flight plans can be transferred directly from the laptop to the FCS using the Ethernet connection, or they can be transferred via a USB flash drive and the USB port equipped on the pilot display.

Mission planning is greatly simplified. The planner, choosing among several planning methods (fixed forward overlap, predetermined photo positions, etc.) can quickly and accurately determine the flight altitude that best meets the job specifications (scale, GSD, etc.).  The system also allows for the interactive optimization of flight lines for altitude. Upon completion of the flight, the planner knows that the flight plan fully covered the area stereoscopically, there were no unexpected gaps due to terrain height, flight altitudes were optimized for easy transition from one level to the next, and altitudes complied with ATC restrictions. 

For planning a project, the region is defined in either vector or raster format. In the case where existing planning region data is not available, TerraServer USA can be used to download seamless coverage of the entire United States. Furthermore, digitalization can be used to prepare raster backgrounds in conjunction with publicly available raster pictures.

The remaining flight parameters required are the map scale/ground sample distance and endlap and sidelap percentage. These are automatically calculated if camera sensors are supported in the system database (custom camera definition is also available for supporting other sensor platforms such as small-format digital cameras).  Once these criteria are known, the optimal flight pattern using any user-defined direction of flight is calculated.

Digital Elevation Model Support: To support large area mapping over rapidly changing terrain, especially in remote and coastline areas, the support of existing digital elevation models (DEM) for flight planning is incorporated. Accounting for the terrain variation in the mission planning ensures that the required stereo coverage or the overlapping percentage in orthophoto and orthomosaic generation is maintained.

The system also uses the DEM and real-time orientation from POS to project the true image footprints onto the ground, displaying them to the pilot and operator in real-time. It runs a quality check on the overlap, warning the crew about potential problems and automatically setting up a re-flight if necessary. This always ensures that full planned coverage has been maintained before the aircraft leaves the photo area, saving the time and costs of re-flights.

Mission Execution: Mission execution is made simple with the touch-screen capability of the pilot display. In standalone operation, flight plan selection and even flight line sequence can be pre-selected prior to takeoff, ensuring that minimal pilot interaction is required. The PCS provides all the navigation information to the pilot.

Sensor Automation: The system monitors the real-time position and orientation of the imaging sensor and provides certain control to the sensors based on a pre-planned mission. Such in-flight operation includes automatic triggering of frame cameras, automatic on/off control of lidar and push-broom scanners, and the automatic on/off control of 3-axis mount stabilization.

This precise control of image ground coverage minimizes both the amount of sensor data logged and the amount of time the sensor is turned on. This translates into minimizing the time to fly and post-process the mission while increasing the life-span of the airborne sensors, ensuring the most economical and efficient aerial surveying capability.

What Is a Direct Georeferencing System?

A direct georeferencing system provides the ability to directly relate the data collected by an imaging sensor to the Earth by accurately measuring the geographic position and orientation of the device without the use of traditional ground-based control points. The DG system is widely used in the airborne mapping industry for lidar, synthetic aperture radar (SAR), multispectral and hyperspectral scanners, large-format digital line scanners, film cameras, and large- and medium-format digital frame cameras. A key component of POS AV is the GNSS-aided inertial navigation software that runs in real time on the POS computer system and in post-mission as part of the Applanix POSPac MMS office software. To produce the best possible accuracy of the position and orientation of the imaging sensor (for most accurate map production), the POSPac MMS incorporates Applanix IN-Fusion technology and the Applanix SmartBase post-processed Virtual Reference Station module. 

For emergency response applications in which time constraints do not allow complete post-processing activities, the PCS unit supports all external satellite based augmentation services (SBAS) and on-board embedded OmniSTAR XP.

The DG technology has made a difference in the mapping industry and allowed for producing a variety of mapping products that never existed before. One of the direct benefits of DG is that aerial triangulation is not needed in most mapping applications (except for large scale mapping), and this results in enormous savings in data acquisition and processing time. Other sensor systems such as lidar, SAR, and multispectral and hyperspectral scanners have used DG systems as an enabling technology for years and would not even exist if not for DG systems providing the position and orientation of each sensor measurement.

The use of DG on digital cameras has significantly reduced turnaround time, delivering orthomosaic imagery within hours after data collection. This cannot be achieved with digital cameras alone, as GNSS-assisted aerial triangulation requires multiple strips and extensive image processing on every single image collected to derive the exterior orientation parameters for each image. In addition, because rapid response imagery is mainly collected over remote areas such as coastlines and disaster areas, a fully integrated digital camera with DG technology is really the only viable solution.

Designed for the airborne geospatial community, an integrated, real-time direct georeferencing and flight management system offers significant benefits, from planning to execution. This technology for airborne mapping delivers maximum accuracy, simplified operations, reduced implementation costs, and maximum flexibility.

Joe Hutton is the director for airborne products at Applanix. He obtained his B.A.Sc degree in Engineering Science from the University of Toronto and an M.A.Sc. Degree in Aerospace Engineering from the University of Toronto Institute for Aerospace Studies.

Alan Wing Lun Ip is a geomatics analyst with Applanix. With a Master of Science, Geomatics Engineering from the University of Calgary, his expertise centers on photogrammetry and GPS/INS in airborne applications and airborne sensor integration such as cameras and lidar.

Andrew Stott of is a writer and editor for Applanix. He has a degree in communications from the University of Toronto and many years of experience in the industry.

» Back to our May 2009 Issue