13.2 GPS and positioning techniques

GPS Principles and Applications

GPS and positioning techniques provide the spatial framework for all geophysical fieldwork. Without accurate coordinates and elevations tied to each measurement, even the best gravity, magnetic, or seismic data loses much of its value. These methods range from standard GPS (meter-level accuracy) to carrier-phase techniques like RTK that deliver centimeter-level precision.

GPS Fundamentals

GPS (Global Positioning System) is a satellite-based navigation system providing positioning, navigation, and timing services worldwide. The constellation consists of satellites orbiting at approximately 20,200 km altitude, each transmitting radio signals that encode the satellite's position and the precise time of transmission.

A GPS receiver determines its location through trilateration:

1. The receiver picks up signals from multiple satellites and measures the time delay between transmission and reception.
2. It multiplies each time delay by the speed of light to calculate the pseudorange (distance) to each satellite.
3. With distances from at least four satellites (three for position, one additional to solve for receiver clock error), the receiver computes its 3D position.

Note the distinction: trilateration uses distance measurements, while triangulation uses angle measurements. GPS relies on trilateration.

Differential GPS and Applications

Differential GPS (DGPS) improves on standard GPS by using fixed, ground-based reference stations at known coordinates. Each reference station compares its GPS-derived position to its true position and calculates correction values. These corrections are then broadcast to nearby DGPS receivers, which apply them to reduce systematic errors in the satellite signals.

GPS serves several roles in geophysical surveys:

• Spatial referencing: Every gravity, magnetic, or seismic measurement gets tagged with precise coordinates, enabling the creation of spatially accurate maps and subsurface models.
• Real-time navigation: Surveyors can follow pre-planned survey lines or grids with GPS guidance, ensuring consistent data coverage and minimizing gaps or overlaps.
• Efficiency: GPS-guided surveys are significantly faster than traditional optical surveying methods that require manual angle and distance measurements at each station.

Positioning Techniques for Data Acquisition

Static and Kinematic GPS Positioning

Static GPS positioning places a receiver at a fixed location for an extended observation period (hours to days). The long occupation time allows errors to be averaged out, producing high-accuracy coordinates. This technique is commonly used to establish control points or benchmarks that anchor an entire survey network.

Kinematic GPS positioning tracks a moving receiver in real time, enabling continuous data acquisition along survey lines or profiles. The receiver is typically mounted on a vehicle, boat, or aircraft, or carried by a surveyor on foot. This approach suits surveys requiring dense, continuous measurements over large areas.

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

RTK GPS achieves centimeter-level accuracy in real time through the following setup:

1. A base station is established at a known location and continuously tracks satellite signals.
2. The base station computes corrections by comparing its known position to its GPS-derived position.
3. These corrections are transmitted via radio link (or cellular network) to the mobile rover receiver.
4. The rover applies the corrections on the fly, resolving carrier-phase ambiguities to achieve centimeter-level positioning.

PPK GPS follows a similar principle but without the real-time radio link. Both the base station and rover record raw satellite observations independently. After the survey, the data are combined and processed in software to resolve carrier-phase ambiguities and compute high-accuracy positions.

Both RTK and PPK use carrier-phase measurements, which track the phase of the GPS carrier signal itself rather than just the modulated code. Carrier-phase measurements are far more precise than code-based pseudoranges, but they introduce integer ambiguities that must be resolved to achieve full accuracy.

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

Precise Point Positioning (PPP) achieves high accuracy using a single receiver, with no base station required. Instead, it relies on precise satellite orbit and clock corrections provided by organizations like the International GNSS Service (IGS). PPP typically delivers decimeter-level accuracy with short observations and can approach centimeter-level with longer sessions (30+ minutes). The convergence time needed to reach full accuracy is one of PPP's main drawbacks compared to RTK.

Inertial Navigation Systems (INS) use accelerometers and gyroscopes to track changes in position, velocity, and orientation. On their own, INS solutions drift over time because small measurement errors accumulate. However, when integrated with GPS (GPS/INS integration), the two systems complement each other:

• GPS provides absolute position fixes that correct INS drift.
• INS provides continuous, high-rate positioning that bridges GPS outages (e.g., in tunnels, dense forest canopy, or urban canyons).

This integration is particularly valuable for airborne geophysical surveys where continuous, smooth positioning is critical.

GPS Accuracy and Limitations

Factors Affecting GPS Accuracy

Several factors determine how accurate a GPS position will be:

• Satellite geometry: Satellites spread widely across the sky yield better accuracy. When visible satellites cluster in one part of the sky, the solution degrades. This is quantified by Dilution of Precision (DOP) values; lower DOP means better geometry.
• Atmospheric delays: The ionosphere (charged particles at ~80–1,000 km altitude) and troposphere (lowest ~12 km, where weather occurs) both slow GPS signals, introducing range errors.
• Multipath: GPS signals reflecting off buildings, rock faces, or vehicles before reaching the receiver create interference with the direct signal, distorting range measurements. This is especially problematic in urban areas and near steep terrain.
• Receiver quality: Antenna design, number of tracking channels, and signal processing algorithms all affect measurement precision.

Accuracy Levels of Different GPS Techniques

RTK and PPK achieve their high accuracy through carrier-phase measurements, which resolve the wavelength of the GPS signal (~19 cm for L1) down to a fraction of a cycle. Standard GPS and DGPS rely on code-based pseudoranges, which are inherently less precise.

Environmental Limitations and Challenges

• Signal blockage: Dense forest canopy, buildings, or steep terrain can obstruct the line of sight to satellites, reducing the number of usable signals or causing complete loss of lock.
• Ionospheric delays: Vary with solar activity, time of day, and geographic location. Dual-frequency receivers (tracking both L1 and L2 signals) can largely eliminate ionospheric errors because the delay is frequency-dependent.
• Tropospheric delays: Caused by variations in temperature, pressure, and humidity. These are modeled using empirical atmospheric models, though residual errors remain, especially for the wet (humidity) component.
• Baseline length: For RTK and PPK, accuracy degrades as the distance between base and rover increases, because the two receivers experience increasingly different atmospheric conditions. Baselines beyond ~20 km typically require network RTK solutions or PPP.

GPS Data Integration for Analysis

Combining GPS and Geophysical Data

GPS coordinates are recorded simultaneously with geophysical measurements, creating spatially referenced datasets. This spatial referencing is what transforms raw measurements into interpretable maps and models of the subsurface.

For example, a gravity survey produces a list of gravity values that are meaningless without knowing exactly where each reading was taken. GPS provides the latitude, longitude, and (critically) the elevation for each station, enabling the computation of gravity anomalies and the construction of contour maps.

Terrain Corrections and Sensor Positioning

GPS elevations play a direct role in several geophysical corrections:

• Gravity surveys: Free-air and Bouguer corrections both require accurate station elevations. Terrain corrections account for the gravitational effect of topography near each station. Even small elevation errors (a few centimeters) can introduce measurable errors in high-precision gravity work, since the free-air gradient is approximately $0.3086 \text{ mGal/m}$.
• Seismic surveys: GPS determines the exact position of each geophone and source point. Accurate positions are essential for proper stacking, migration, and imaging of subsurface reflectors.
• Magnetic surveys: GPS ensures measurements are taken at consistent intervals along survey lines and provides the coordinates needed to produce accurate magnetic anomaly maps and remove regional field trends.

Data Integration and Interpretation

Integrated GPS and geophysical datasets are analyzed using tools such as:

• GIS software for visualizing, querying, and overlaying spatially referenced data layers (e.g., gravity anomalies draped on topography).
• Geophysical modeling software for building 2D or 3D models of subsurface structures, constrained by the spatial coordinates GPS provides.

This integration supports applications ranging from mineral exploration and hydrocarbon prospecting to groundwater studies, geotechnical site characterization, and environmental monitoring. The spatial context GPS provides is what ties geophysical anomalies to real-world locations and geological features, making interpretation and decision-making possible.