Exam2 Review2

Exam 2 Review: UAS, GNSS, and Photogrammetry

Introduction to UAS

Understanding Unmanned Aircraft Systems (UAS) is crucial for various applications, including aerial surveying, search and rescue operations, agriculture, and environmental monitoring. UAS can vary significantly in design and capability, with applications tailored to the specific objectives of a mission.

Global Navigation Satellite Systems (GNSS)

Overview of Major GNSS Systems

  • GPS (U.S.A.)Established in the early 1970s, GPS was initially developed for military use but expanded in the 1980s to civilian applications. It features secure, global, passive systems with a large bandwidth.

  • GLONASS (Russia)Russia’s counterpart to GPS offers global coverage and is operated by the Russian Aerospace Defense Forces.

  • GALILEO (EU)The European Union's GNSS, which aims to provide greater accuracy and reliability than existing systems.

  • BEIDOU/COMPASS (China)Offers regional and global services with a focus on increased precision.

  • QUASI-ZENITH SATELLITE SYSTEM (Japan)Designed to enhance GPS signals in urban environments with building obstructions.

  • IRNSS (India)A regional satellite navigation system with comparable capabilities to GPS aimed at improving positional accuracy in the Indian subcontinent.

NAVSTAR GPS

Features three segments: Space segment (24 satellites and spares), Control segment (monitoring stations), and User segment (receivers). The satellites orbit in six planes inclined at 55° and 60° to one another, ensuring global coverage.

GPS Constellation

The range and distribution of satellites in orbit provide a robust network with redundancy to enhance positioning accuracy.

Dilution of Precision (DOP)

DOP is critical in evaluating GNSS accuracy; lower values indicate better positioning accuracy.

  • Horizontal DOP (HDOP) and Vertical DOP (VDOP) influence overall accuracy; ideally, the Position DOP (PDOP) should be six or less.

Good vs. Bad DOP

  • Bad DOP: Results from satellites being clustered, leading to a high DOP factor.

  • Good DOP: Indicates satellites are well-distributed, resulting in a low DOP factor conducive to accurate positioning.

Broadcast Ephemeris

Broadcast Ephemeris contains satellite position data relative to Earth, but its accuracy can vary based on factors like atmospheric conditions and ionospheric delays.

Error Budget

Components affecting GNSS accuracy include:

  • Satellite clock bias

  • Ionospheric effect

  • Tropospheric effect

  • Orbital bias

  • Receiver noise

  • Multipath effects

Multipath Effects

These are caused by signal reflection from surfaces, reducing overall signal integrity; careful consideration of placement and environment can mitigate these effects.

Antenna Design and Multipath Reduction

Effective antenna designs include ground planes to reduce low-angle signals and choke ring designs that minimize interference from multipath signals.

Real-Time Kinematic (RTK)

Provides high positional accuracy, typically within +/- 2 cm, by establishing a carrier phase link between stationary base stations and mobile rovers. It is particularly beneficial for surveying and precision agricultural applications, having been developed in the mid-1980s.

Continuously Operating Reference Stations (CORS)

A network of reference stations enabling precise relative positioning without the need for dedicated base stations, supporting methods such as Differential GPS (DGPS) and RTK using dual-frequency data to improve accuracy.

GNSS Applications

Common sectors utilizing GNSS technology:

  • Agriculture: for precision farming techniques and crop monitoring.

  • Aviation: enhancing navigation safety.

  • Construction: aiding in site surveying and material transport.

  • Environment: monitoring and managing natural resources and disaster response.

  • Marine: improving navigation and safety in maritime operations.

  • Public Safety: guiding emergency services.

  • Transport: enhancing logistic operations and fleet management.

  • Recreation: for outdoor activities like hiking and geocaching.

  • Surveying: crucial for accurate land measurement and mapping.

Stereo Perception and Aerial Photos

Depth perception arises from the angle between two images; vertical photographs taken from multiple locations enhance depth perception, enabling accurate 3D reconstruction of surfaces.

Multi-ray Photogrammetry

Utilizes multiple cameras to reduce positional errors in imagery and improve overall mapping accuracy. Incorporating multiple angles allows for more data acquisition and error correction during processing.

Pix4D Approach

Involves automated processing and robust image capture methodologies that significantly enhance aerial mapping quality and results in high redundancy and accuracy of the data collected.

Image Quality and Content

  • Ideal Scenes: Heterogeneous, non-reflective, and static scenes enhance image clarity.

  • Not Recommended: Homogeneous, reflective, or dynamic scenes may lead to inconsistent data and poor quality imagery.

Stereoscopic Parallax

Describes the perception shift in position when viewed from different observation points; parallax effects are more significant for closer objects, enhancing depth information in imagery.

Overlapping of Aerial Photographs

To create stereopairs, photographs should include an end lap of 60% and a side lap of 30% to ensure sufficient overlapping for accurate reconstruction.

Image Acquisition Grid Protocol

Emphasizes the importance of high overlap in capturing images to ensure precise measurements and mapping effectiveness.

Critical UAS Camera Features

Key performance metrics include:

  • Control mechanisms for stability

  • High-resolution capabilities

  • Quality of images captured

  • Ease of data extraction for analysis post-capture

UAS Types: Multirotor vs. Fixed Wing

  • Multirotor: Highly maneuverable, compact, and typically lower cost; suitable for small area operations.

  • Fixed Wing: Offers extended range and stability but requires larger open spaces for effective operation due to its flight dynamics.

Summary Comparison of UAS Types

Evaluates criteria such as price, operational stability, and efficiency in mapping areas to inform decision-making for project implementation.

Quality Considerations in UAS Projects

Quality is influenced by various factors, including terrain variabilities, weather conditions, camera specifications, and pre-project mission planning efficacy.

Ground Control Points (GCP)

Essential for enhancing the accuracy of measurements; GCPs must be static, flat, and evenly distributed across the project area to minimize errors.

Applications of UAS

UAS have diverse applications that range from innovative search and rescue operations to marketing campaigns, precision agriculture practices, and environmental monitoring studies, showcasing the versatility of unmanned systems in modern technology.