Global Positioning Systems - Segments, GPS measurements, errors, Surveying with GPS, Introduction to Co-ordinate transformation
- 1. Global Positioning Systems Global Positioning Systems- Segments, GPS measurements, errors and biases, Surveying with GPS, Introduction to Co-ordinate transformation, accuracy considerations
- 2. What is GPS? GPS (Global Positioning System) is a satellite-based navigation system that provides location and time information anywhere on or near the Earth.
- 3. How GPS Works 1) Signal Transmission: • Each GPS satellite continuously transmits a signal containing its current position and the exact time the signal was sent. • The signal travels at the speed of light and includes information about the satellite’s orbit and the health of the satellite
- 4. How GPS Works 2) Signal Reception: • A GPS receiver on the ground picks up signals from multiple satellites. • By measuring the time it takes for each signal to reach the receiver, the receiver calculates the distance to each satellite.
- 5. How GPS Works 3) Triangulation • Using the distances from at least four satellites, the receiver uses a process called triangulation to determine its exact position (latitude, longitude, and altitude). • The receiver solves a set of equations to find the intersection point of the spheres centered on each satellite, representing the distances.
- 6. Applications of GPS in Surveying 1. Geodetic Surveys: • Used for establishing precise control points over large areas. • Provides high accuracy for mapping and land boundary determination. 2. Topographic Surveys: • Helps in creating detailed maps of the terrain. • Useful for construction planning, environmental studies, and resource management. 3. Construction Surveys: • Assists in the layout of buildings, roads, and other infrastructure projects. • Ensures that structures are built in the correct location and orientation. 4. Cadastral Surveys: • Used for defining property boundaries. • Essential for land ownership and legal documentation.
- 7. Advantages of GPS in Surveying • High Accuracy: Provides precise location data, often within a few centimeters. • Efficiency: Reduces the time and labor required for traditional surveying methods. • Versatility: Can be used in various environments and weather conditions. • Real-Time Data: Offers real-time positioning, which is useful for dynamic surveying tasks
- 8. Limitations of GPS in Surveying • Signal Obstruction: GPS signals can be blocked by buildings, trees, and other obstacles. • Atmospheric Interference: Signal accuracy can be affected by atmospheric conditions such as ionospheric and tropospheric delays. • Multipath Errors: Reflected signals from surfaces like buildings and water can cause errors in position calculation.
- 9. Components of GPS 1. Space Segment 2. Control Segment 3. User Segment
- 10. Components of GPS 1. Space Segment: • Comprises a constellation of at least 24 satellites orbiting the Earth at an altitude of approximately 20,200 kilometers. • These satellites are arranged in such a way that at least four satellites are visible from any point on the Earth’s surface at any given time. • The nominal GPS satellite constellation consisting of 24 satellites distributed in six orbital planes with an inclination of 55° in relation to the equator. In addition, the constellation has 3 backup satellites. • Orbits are nearly circular, a semi-major axis of 26,560 kilometers, at a height of 20,200 kilometers. THe satellites have a speed of 3.9 km/s and a nominal period of 12 hours sidereal time. The present configuration allows users to have a simultaneous observation of at least 4 satellites in view worldwide, with an elevation making angle of 15°.
- 12. Components of GPS 2. Control Segment: • It is also sometimes referred as Ground Segment or Operational Control Segment. • Consists of network of Monitor Station (MS), a Master Control Station (MCS), a backup of the MCS and the Ground Antennas (GA). • The main control station is located in Colorado Springs, USA, with additional monitoring stations and ground antennas spread across the globe. • These stations track the satellites, update their positions, and ensure the accuracy of their signals.
- 14. Components of GPS 2. Control Segment: • The MCS processes the measurements received by Monitoring Stations (MS) to estimate satellite orbits and clock errors, among other parameters, and generate the navigation message. These corrections and the navigation message are uploaded to the satellites through Ground Antennas, which are co-located in four of the Monitor Station.
- 15. Components of GPS 3. User Segment: • Includes GPS receivers used by surveyors, navigators, and the general public. • These receivers decode the signals sent by the satellites to determine the user’s position, velocity, and time.
- 16. Components of GPS 3. User Segment: • GPS receiver allows you to pinpoint your location, anywhere in the world, based on latitude and longitude coordinates. • GPS can tell you in what direction you are heading • GPS can show you how fast you are goint • GPS can show your altitude • GPS can show you a map to help you arrive at a destination
- 17. GPS Measurement 1. Satellite Signal Transmission • GPS satellites continuously transmit signals that include their current position and the exact time the signal was sent. 2. Signal Reception • A GPS receiver on the ground picks up these signals. To determine its position, the receiver needs signals from at least four satellites. 3. Time Calculation • The receiver measures the time it takes for each signal to travel from the satellite to the receiver. Since the signals travel at the speed of light, the time delay can be used to calculate the distance to each satellite.
- 18. GPS Measurement 4. Distance Calculation • The distance to each satellite is calculated using the formula: Distance = Speed of Light × Time Delay 5. Triangulation • Using the distances from multiple satellites, the receiver uses a process called triangulation to determine its exact position. This involves solving a set of equations to find the intersection point of the spheres centered on each satellite, representing the distances.
- 26. GPS Surveying Methods 1. Static GPS Surveying • Involves placing the GPS receiver at a fixed location for a long period (minimum 20 minutes) to collect data. • This method is highly accurate and is used for establishing control points. • Accuracy up to 5 mm can be achieved using Static GPS Surveying
- 27. GPS Surveying Methods 2. Fast-Static GPS Surveying • Similar to static surveying but with a shorter observation period. • It is used for shorter baselines and provides quick results with good accuracy.
- 28. GPS Surveying Methods 3. Kinematic GPS Surveying • The receiver is in motion, and data is collected continuously. • This method is used for applications like mapping and construction where real-time positioning is required.
- 29. GPS Surveying Methods 4. Real-Time Kinematic (RTK) GPS Surveying: • Provides real-time corrections to the GPS data, allowing for centimeter-level accuracy. • It is widely used in precision agriculture, construction, and other applications requiring high accuracy
- 30. Errors in GPS Satellite-Related Errors • Ephemeris Errors: • These are errors in the satellite’s reported position. Even small inaccuracies in the satellite’s orbit can lead to significant errors in the calculated position on the ground. • Satellite Clock Errors: • GPS satellites have highly accurate atomic clocks, but slight deviations can still occur. These clock errors can cause inaccuracies in the timing of the signals, leading to errors in distance calculations. • Selective Availability: • This was an intentional degradation of the GPS signal by the U.S. Department of Defense for security reasons. It was turned off in May 2000, but it used to be a significant source of error.
- 31. Errors in GPS
- 32. Errors in GPS Receiver-Related Errors • Receiver Clock Errors: • Unlike the atomic clocks on satellites, GPS receivers use less accurate clocks. These clock errors can introduce inaccuracies in the timing measurements. • Multipath Errors: • These occur when GPS signals reflect off surfaces such as buildings or the ground before reaching the receiver. The reflected signals take longer to arrive, causing errors in the calculated position. • Receiver Noise: • Electronic noise within the GPS receiver can affect the accuracy of the signal processing, leading to small errors in the position calculation. • Antenna Phase Center Variations: • Variations in the position of the antenna’s phase center can introduce errors, especially in higprecision applications.
- 33. Errors in GPS Signal Propagation Errors • Ionospheric Delays: • The ionosphere, a layer of the Earth’s atmosphere, can cause delays in the GPS signals as they pass through it. These delays vary with the time of day, solar activity, and the satellite’s position. • Tropospheric Delays: • The troposphere, the lower part of the Earth’s atmosphere, can also cause delays in the GPS signals. These delays are influenced by temperature, pressure, and humidity.
- 34. How to reduce Errors in GPS 1. Differential GPS (DGPS) • Uses a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the GPS satellites and the known fixed positions. This helps correct errors in real-time. 2. Real-Time Kinematic (RTK) GPS • Provides real-time corrections to the GPS data, allowing for centimeter-level accuracy. It is widely used in precision agriculture, construction, and other applications requiring high accuracy. 3. Satellite-Based Augmentation Systems (SBAS) • Systems like WAAS (Wide Area Augmentation System) and EGNOS (European Geostationary Navigation Overlay Service) provide additional correction signals to improve GPS accuracy.
- 35. Coordinate Transformation Why Coordinate Transformation is Needed • Different Systems: The Earth is represented using different coordinate systems, such as geographic coordinates (latitude, longitude, and altitude) and Cartesian coordinates (X, Y, Z). • Accuracy: Transformations ensure that data from different sources align correctly on maps and models. • Applications: Necessary for integrating GPS data with other geospatial data, such as maps and satellite imagery
- 36. Coordinate Transformation Common Coordinate Systems • Geographic Coordinate System (GCS): • Uses latitude, longitude, and altitude to define locations on the Earth’s surface. • Example: WGS84 (World Geodetic System 1984). • Projected Coordinate System (PCS): • Projects the 3D Earth onto a 2D plane. • Example: UTM (Universal Transverse Mercator). • Cartesian Coordinate System: • Uses X, Y, Z coordinates to define positions in a 3D space. • Example: Earth-Centered, Earth-Fixed (ECEF) coordinates.
- 37. Coordinate Transformation Transformation Methods • Datum Transformation: • Converts coordinates from one datum to another. • Example: Transforming coordinates from NAD83 to WGS84. • Map Projection: • Converts geographic coordinates to projected coordinates. • Example: Converting latitude and longitude to UTM coordinates. • Helmert Transformation: • A seven-parameter transformation that includes translation, rotation, and scaling. • Used for high-precision applications.
- 38. Coordinate Transformation Steps in Coordinate Transformation • Identify the Source and Target Coordinate Systems: • Determine the coordinate systems you are converting from and to. • Apply Transformation Parameters: • Use mathematical formulas or transformation parameters to convert the coordinates. • Parameters may include translation, rotation, and scaling factors. • Validate the Transformation: • Check the accuracy of the transformed coordinates by comparing them with known reference points.
- 39. Coordinate Transformation Tools for Coordinate Transformation • Software: GIS software like ArcGIS, QGIS, and specialized tools like the NGS Coordinate Conversion and Transformation Tool (NCAT). • Online Converters: Websites like EPSG.io provide online tools for coordinate transformation.