13.2 GPS and positioning techniques

GPS and positioning techniques are crucial tools in geophysical field methods. They enable precise location tracking for data collection, enhancing the accuracy of surveys and measurements. These technologies have revolutionized geophysical fieldwork, allowing for more efficient and reliable data acquisition.

From satellite-based GPS to advanced techniques like RTK and PPK, positioning methods offer varying levels of accuracy. Understanding their principles, applications, and limitations is essential for geophysicists to choose the right approach for their specific survey needs and interpret results accurately.

GPS Principles and Applications

GPS Fundamentals

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• GPS (Global Positioning System) is a satellite-based navigation system that provides accurate positioning, navigation, and timing services worldwide
• GPS satellites orbit the Earth at an altitude of approximately 20,200 km and transmit radio signals containing information about their position and the precise time the signal was sent
• GPS receivers on Earth use the time difference between when the signal was sent and received to calculate the distance to each satellite, and then use trilateration to determine the receiver's precise location
  • Trilateration involves measuring distances from at least three known points (satellites) to determine the receiver's position
  • The receiver calculates the time delay between signal transmission and reception, then multiplies this by the speed of light to determine the distance to each satellite
  • With distances from at least three satellites, the receiver can triangulate its position on Earth's surface

Differential GPS and Applications

• Differential GPS (DGPS) uses a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions, allowing for higher positioning accuracy
  • Reference stations calculate the difference between their known fixed position and the position calculated from the GPS signals
  • These differences (corrections) are broadcast to DGPS receivers in the area, which apply the corrections to improve their positioning accuracy
• GPS is used in geophysical surveys to accurately determine the location of data points, such as gravity, magnetic, or seismic measurements, which is crucial for data interpretation and mapping
  • Accurate positioning of data points allows for the creation of detailed maps and models of subsurface structures and anomalies
  • GPS coordinates are combined with geophysical measurements to create spatially referenced datasets for analysis and interpretation
• GPS enables real-time navigation and positioning during geophysical surveys, facilitating efficient data acquisition and reducing the need for traditional surveying methods
  • Real-time GPS positioning allows for precise navigation along survey lines or grids, ensuring consistent data coverage and minimizing gaps or overlaps
  • GPS-guided surveys can be conducted more quickly and efficiently compared to traditional surveying methods, which require manual measurements and positioning

Positioning Techniques for Data Acquisition

Static and Kinematic GPS Positioning

• Static GPS positioning involves setting up a GPS receiver at a fixed location for an extended period to collect data, which is later processed to achieve high-accuracy positions
  • Static positioning is often used for establishing control points or benchmarks for geophysical surveys
  • Longer observation times (several hours to days) allow for more precise positioning, as errors can be averaged out over time
• Kinematic GPS positioning allows for real-time positioning of a moving receiver, enabling continuous data acquisition along survey lines or profiles
  • Kinematic positioning is suitable for geophysical surveys that require continuous measurements, such as gravity, magnetic, or seismic surveys
  • The GPS receiver is mounted on a vehicle or carried by a surveyor, allowing for rapid data acquisition over large areas

Real-Time Kinematic (RTK) and Post-Processed Kinematic (PPK) GPS

• Real-Time Kinematic (RTK) GPS uses a base station receiver that sends real-time corrections to a mobile receiver, providing centimeter-level positioning accuracy
  • The base station is set up at a known location and calculates corrections based on the difference between its known position and the position calculated from GPS signals
  • These corrections are transmitted to the mobile receiver via radio link, allowing for real-time high-accuracy positioning
• Post-processed kinematic (PPK) GPS involves collecting raw GPS data from both a base station and a mobile receiver, which is later processed to achieve high-accuracy positions
  • PPK does not require a real-time radio link between the base station and mobile receiver, as the data is processed after the survey
  • PPK can achieve similar accuracy to RTK, but allows for more flexibility in survey design and execution

Precise Point Positioning (PPP) and Inertial Navigation Systems (INS)

• Precise Point Positioning (PPP) is a technique that uses precise satellite orbit and clock information to achieve high-accuracy positioning without the need for a base station
  • PPP relies on precise ephemeris data and clock corrections provided by organizations such as the International GNSS Service (IGS)
  • PPP can achieve decimeter to centimeter-level accuracy, depending on the quality of the satellite data and the duration of the observation
• Inertial Navigation Systems (INS) use a combination of accelerometers and gyroscopes to provide continuous positioning information, which can be integrated with GPS data for improved accuracy and reliability
  • INS measure the acceleration and rotation of the device, allowing for the calculation of position, velocity, and orientation
  • INS can bridge gaps in GPS coverage, such as in tunnels or urban canyons, by providing continuous positioning information
  • The integration of GPS and INS data, known as GPS/INS integration, can provide a more robust and accurate positioning solution for geophysical surveys

GPS Accuracy and Limitations

Factors Affecting GPS Accuracy

• GPS accuracy depends on factors such as satellite geometry, atmospheric conditions, receiver quality, and multipath effects (signal reflections from nearby objects)
  • Satellite geometry refers to the arrangement of visible satellites in the sky; a good geometry with satellites spread out across the sky leads to better positioning accuracy
  • Atmospheric conditions, such as ionospheric and tropospheric delays, can affect the speed of GPS signals and introduce errors in positioning
  • Receiver quality, including antenna design and signal processing capabilities, can impact the accuracy of GPS measurements
• Multipath effects can introduce errors in GPS positioning, particularly in urban or heavily forested areas
  • Multipath occurs when GPS signals are reflected off surfaces such as buildings or trees before reaching the receiver
  • These reflected signals can interfere with the direct signals, leading to inaccurate distance measurements and positioning errors

Accuracy Levels of Different GPS Techniques

• Standard GPS positioning typically achieves accuracy within 5-10 meters, while DGPS can improve accuracy to 1-3 meters
  • Standard GPS positioning relies solely on the signals from the satellites and does not use any additional corrections or augmentations
  • DGPS improves accuracy by applying corrections from reference stations to compensate for errors in the satellite signals
• RTK and PPK GPS techniques can provide centimeter-level accuracy, making them suitable for high-precision geophysical surveys
  • RTK and PPK techniques use carrier phase measurements, which are more precise than the code measurements used in standard GPS positioning
  • The use of a base station or precise satellite ephemeris data allows for the resolution of ambiguities in the carrier phase measurements, resulting in high-accuracy positioning
• PPP accuracy is typically in the decimeter to centimeter range, depending on the quality of the satellite orbit and clock information used
  • PPP relies on precise satellite ephemeris data and clock corrections to achieve high accuracy without the need for a base station
  • The accuracy of PPP depends on the quality and availability of the precise satellite data, as well as the duration of the observation session

Environmental Limitations and Challenges

• GPS signals can be affected by obstructions such as trees, buildings, or terrain, leading to reduced accuracy or loss of signal in certain environments
  • Signal blockage occurs when objects obstruct the direct line of sight between the receiver and the satellites
  • In heavily forested or urban areas, GPS signals may be weakened or completely blocked, making positioning difficult or impossible
• Ionospheric and tropospheric delays can affect GPS signal propagation, leading to reduced accuracy if not properly accounted for
  • The ionosphere is a layer of the Earth's atmosphere that contains charged particles, which can delay GPS signals and introduce positioning errors
  • The troposphere is the lowest layer of the Earth's atmosphere, and its varying temperature, pressure, and humidity can also affect GPS signal propagation
  • Advanced GPS techniques, such as dual-frequency receivers or atmospheric modeling, can help mitigate the effects of ionospheric and tropospheric delays

GPS Data Integration for Analysis

Combining GPS and Geophysical Data

• GPS data provides precise location information for each geophysical measurement, allowing for accurate spatial referencing and mapping of the data
  • GPS coordinates are recorded simultaneously with geophysical measurements, creating a spatially referenced dataset
  • Accurate spatial referencing is crucial for creating meaningful maps and models of subsurface structures and anomalies
• Geophysical data, such as gravity, magnetic, or seismic measurements, can be combined with GPS coordinates to create spatially accurate maps and models
  • GPS coordinates are used to assign a precise location to each geophysical measurement, allowing for the creation of 2D or 3D maps and models
  • Spatially accurate maps and models enable the identification of patterns, trends, and anomalies in the geophysical data, which can be related to subsurface structures or features

Terrain Corrections and Sensor Positioning

• GPS elevations can be used to apply terrain corrections to gravity data, improving the accuracy of gravity anomaly maps and interpretations
  • Terrain corrections account for the effect of topography on gravity measurements, as the presence of hills or valleys near the measurement point can influence the observed gravity value
  • By using precise GPS elevations to calculate terrain corrections, the accuracy of gravity anomaly maps can be improved, leading to better interpretations of subsurface density variations
• GPS data can be used to accurately position geophysical sensors, such as geophones in seismic surveys or magnetometers in magnetic surveys, enabling precise data acquisition and processing
  • In seismic surveys, GPS is used to determine the exact location of each geophone, which is crucial for accurate data processing and imaging of subsurface structures
  • In magnetic surveys, GPS is used to position the magnetometer and ensure that measurements are taken at consistent intervals along survey lines, allowing for the creation of accurate magnetic anomaly maps

Data Integration and Interpretation

• Integration of GPS and geophysical data allows for the creation of comprehensive datasets that can be analyzed using various software tools, such as geographic information systems (GIS) or geophysical modeling software
  • GIS software allows for the visualization, analysis, and interpretation of spatially referenced geophysical data, enabling the identification of patterns, trends, and anomalies
  • Geophysical modeling software uses GPS and geophysical data to create 2D or 3D models of subsurface structures, allowing for the estimation of physical properties and the testing of geological hypotheses
• Combined analysis of GPS and geophysical data can provide insights into subsurface structures, resource distribution, and other geological features, aiding in the interpretation and decision-making process
  • The integration of GPS and geophysical data enables a more comprehensive understanding of the subsurface, as the spatial context provided by GPS enhances the interpretation of geophysical anomalies and patterns
  • This integrated approach can assist in various applications, such as mineral exploration, groundwater studies, geotechnical investigations, and environmental monitoring, by providing a more complete picture of the subsurface environment