Exam2 Review2

Exam 2 Review: UAS, GNSS, and Photogrammetry

1. Introduction to UAS

  • Overview of Unmanned Aerial Systems (UAS) and their importance in various fields including surveying and mapping.

2. Global Navigation Satellite Systems (GNSS)

  • Key GNSS Systems:

    • GPS (U.S.A.)

    • GLONASS (Russia)

    • GALILEO (EU)

      • Provides low and high precision services.

    • BEIDOU/COMPASS (China)

    • QUASI-ZENITH SATELLITE SYSTEM (QZSS - Japan)

    • IRNSS (India)

3. NAVSTAR GPS

  • Development History:

    • 1970s: Early GPS experiments; civilian use started in the 1980s.

    • Initially for military purposes; now globally used.

  • System Characteristics:

    • Passive system; no limits on the number of receivers.

    • Composed of three segments: space, control, user.

    • Operates in a large bandwidth for transmitting vast amounts of data.

4. GPS Satellite Constellation

  • Consists of 24 satellites plus spares positioned across six orbital planes.

  • Inclinations provide maximum global coverage.

5. Dilution of Precision (DOP)

  • Concept of DOP:

    • Indicates the geometrical strength of satellite configuration above the observer.

    • Types include:

      • Horizontal DOP (HDOP)

      • Vertical DOP (VDOP)

      • Position DOP (PDOP) = HDOP + VDOP

    • A PDOP value of 6 or less is optimal.

    • Outages occur when DOP exceeds acceptable limits.

6. Good vs. Bad DOP

  • Bad DOP:

    • Satellites clustered in one area leads to high DOP values.

  • Good DOP:

    • Satellites spread across the sky resulting in low DOP values.

7. Broadcast Ephemeris

  • Provides satellite positions relative to Earth.

  • While pre-calculated, it lacks precision compared to precise ephemeris data.

8. Error Budget

  • Key factors contributing to GPS accuracy:

    • Satellite clock bias

    • Ionospheric effects

    • Tropospheric effects

    • Orbital bias

    • Receiver noise

    • Multipath interference

9. Multipath Errors

  • Definition:

    • Reception of GPS signals via multiple indirect paths.

    • Reflected signals are weaker due to their diffused nature.

10. Antenna Design for Multipath Reduction

  • Key Designs:

    • Ground Plane: Eliminates low-angle signals.

    • Choke Ring Design: Reduces low elevation gain.

    • Recommended cutoff angle of 15°.

11. Real-Time Kinematic (RTK)

  • Accuracy of +/- 2 cm achieved through:

    • Using a carrier phase radio connection from base station(s) to rovers.

    • Developed in 1980s for real-time positional accuracy over distances of 10-20 km.

12. CORS (Continuously Operating Reference Stations)

  • Offer relative positioning without the need for a personal base station.

  • Support DGPS and RTK using dual-frequency GPS data formatted in RINEX.

13. RTK and DGPS

  • Corrections from base stations are applied to rover data, relying on correlated errors over shorter baselines.

14. GNSS Applications

  • Key Sectors:

    • Agriculture: Field mapping, soil sampling.

    • Aviation: Navigation and tracking.

    • Construction: Machine control.

    • Environment: Wildlife studies, forest management.

    • Marine: Vessel control, waterway management.

    • Public Safety: Monitoring and emergency services.

    • Transportation: Fleet tracking, intelligent systems.

    • Recreation: Hikers, cyclists.

    • Surveying and mapping.

15. Stereo Perception

  • Humans perceive depth based on the parallactic angle formed by the intersection of lines of sight.

16. Stereo Aerial Photos

  • Vertical images from two locations (A and B).

  • Height of buildings can be deduced from depth cues and parallactic angles.

17. Multi-ray Photogrammetry

  • Involves two or more cameras to reduce error.

18. Pix4D Approach

  • Software utilizing automation to enhance image processing through high redundancy.

19. Automatic Tie Points

  • Unique features detected and matched across images to establish reference points for alignment.

20. Image Content and Quality

  • Ideal Conditions:

    • Heterogeneous, non-reflective, static scenes with simple geometry.

  • Not Recommended:

    • Homogeneous, reflective, dynamic scenes with complex geometry.

21. Stereoscopic Parallax

  • Apparent displacement caused by a change in observation position; demonstrated in hand-held visual observations.

22. Flight-line Axes for Stereoscopic Measurement

  • Measure coordinates from stereo images to determine feature positions.

23. Overlapping Aerial Photographs

  • Importance of side and end lap in overlapping images for accurate measurements.

24. Image Acquisition Techniques

  • Recommended flight paths include 75% end lap and 70% side lap with nadir camera orientation.

25. UAS Critical Camera Features

  • Features include:

    • Computer controllable, high resolution, stability in flight, easy file extraction.

26. UAS Overview

  • Flexibility in image acquisition with higher overlaps compared to conventional methods.

27. Multirotor vs. Fixed Wing UAS

  • Multirotor Advantages:

    • Maneuverability, lower price, compact, higher payload capacity.

  • Fixed Wing Advantages:

    • Stability in wind, longer range, safer recovery.

28. Summary Comparison Table

  • Comparison of UAS types based on maneuverability, price, size, and other key features.

29. 2D, 3D, & 4D from UAS

  • Visual Outputs:

    • 2D: Orthophoto

    • 3D: Digital Surface Model

    • 4D: Change detection over time.

30. Quality – Project Dependency

  • Factors affecting quality include terrain, weather, equipment, and planning strategies.

31. Quality – Accuracy

  • Relative vs. Absolute Accuracy:

    • Relative: Based on GPS.

    • Absolute: Incorporates Ground Control Points (GCPs) for verification.

32. Ground Control Point (GCP)

  • Characteristics of GCPs:

    • Static, flat, matte finish, and contrasting appearance.

33. GCP Placement Recommendations

  • Recommended number of GCPs for modeling and verification. Spacing considerations.

34. UAS Applications

  • Extensive applications including:

    • Search and rescue, surveying, environmental studies, marketing, and more.