Inside Unmanned Systems

APR-MAY 2018

Inside Unmanned Systems provides actionable business intelligence to decision-makers and influencers operating within the global UAS community. Features include analysis of key technologies, policy/regulatory developments and new product design.

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55 April/May 2018 unmanned systems inside ENGINEERING. PRACTICE. POLICY. Reference System (CSRS)—all of which set the table for a discussion about data gathering tech- niques common to UAS-based photogrammetry and LiDAR missions. DIRECT MAP PRODUCTION Surveyors and UAS-based data gathering experts are likely familiar with methods such as precise point positioning (PPP) and broadcast PPP-derived corrections such as Trimble RTX, as well as real- time kinematic (RTK), post-processed kinematic capabilities (PPK) and direct georeferencing (DG). He pointed out that, while popular, RTK and PPK have inherent problems that reduce their accuracy, productivity and efficiency. One of the major factors for the reduced accuracy of RTK and PPK is the cal- culation of orientation angles of any sensor. Surprising to many at the workshop, Mostafa emphasized, "RTK was not recommended in drones for mapping." "Map accuracy is fundamental to the quality of the map product," he said. "The RTK approach sacrifices accuracy and quality for speed. Further, why do you need real-time data when you're going to spend time post-processing the data to develop the map?" "This was a surprise," said Aaron Handl, survey project manager with Harris Kocher Smith who at- tended the workshop, "because RTK is commonly referred to as the Cadillac approach throughout the UAS mapping industry." Handl, who has more than 15 years of experience in land surveying and is one of the company's two licensed UAS pilots, said the workshop "completely f lip-f lopped" everything he thought he knew about UAS-based LiDAR missions and positioning. "I had somewhat of a photogrammetry mindset go- ing into the workshop from the UAS experience I've acquired over the past 12 months all of which revolved around photogrammetry. Dr. Mostafa ex- plained that relative position of the vehicle could be more accurately calculated by relying on the measurements from the UAS's IMU coupled with GNSS rather than relying on RTK-GPS." Mostafa also discussed the value of PPK and DG as alternatives. One disadvantage of PPK is that it requires a base station. However, in many cases, a public network such as CORS may be dense enough to serve the project's purposes. DG, on the other hand, measures the orientation angles of any imaging sensor with high accuracy and high frequency at 200Hz or more (200 times per second) and surpasses the accuracy of tradition- al methods such as traditional aerial triangulation (AT), RTK and PPK. By measuring the true 3-D co- ordinates and orientation angles of any sensor (with the use of a GNSS receiver and an IMU), DG allows asprs review advantages GNSS Only Advantages Limitations •  High accuracy position & velocity •  Moderate accuracy orientation using multiple antennas •  Low bandwidth •  Satellite shading (dropouts) •  Slow ambiguity resolution GNSS-Aided INS Advantages Limitations •  All inertial and GPS advantages • In-motion alignment •  No signifi cant limitations Inertial Only Advantages Limitations • Full 6 DOF solution •  High dynamic accuracy with broad bandwidth •  Self-contained (no dropouts) •  Solution errors diverge slowly with time Source: Dr. Mohamed Mostafa, Chair of the ASPRS Precision Mapping by UAS Committee and Director of Microdrones mdSolutions Team

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