Lecture 8b - GNSS and space-based radar techniques

GNSS (Global Navigation Satellite Systems)

Generic term for satellite navigation systems (e.g., GPS).
GPS (Global Positioning System) is a specific system, but the term is often used casually for all such systems.
All GNSS systems operate on the same principle: satellites orbiting overhead broadcast radio signals, which are received and processed by receivers on the ground to determine position.
GNSS is used in phones to collect location data by receiving signals from multiple satellites and calculating position.
Positioning is based on precise timing measurements of signals from multiple satellites to calculate distances.
If the distance between four satellites and receiver is known then receiver's 3D positional coordinates can be estimated.
Coordinates are derived in a global coordinate system (WGS 84) and then converted to a local projected coordinate system (units in meters).

Consumer grade receivers (phones, car satnavs) have accuracy of ~10 meters.
Survey grade systems are used for geophysical measurements requiring centimeter-level accuracy.
Survey grade systems use pairs of receivers: a static local reference (base) and a mobile rover receiver.

Error source: Radio signal delays in the ionosphere affect timing measurements and distance calculations.
Two receivers mitigate this: assume similar delays for both base and rover.
Differential GNSS calculates the relative position of the rover with respect to base, canceling out most errors.
Base receiver is stationary, allowing accuracy to improve through averaging.
Accuracy of differential GNSS is about 20 millimeters.
Vertical coordinate accuracy is generally lower than horizontal due to satellite distribution.

Measuring instrument/electrode positions for geophysical field surveys.
Modern systems can receive correction signals via mobile networks, eliminating the need for a base receiver.
Creating topographic models by walking with a mobile receiver (increasingly replaced by laser scanning or UAS-based photogrammetry).

Permanent installations make continuous measurements to increase accuracy.
Receivers continuously collect and forward data for processing and analysis.
Averaging over days can improve accuracies to a few millimeters for monitoring tectonics and volcanic deformations.
Equipment must be stable and resilient against wildlife.

Example: Continuous GNSS measurements in the San Francisco area around the San Andreas Fault.
Yearly displacement vectors represent differential measurements relative to a static base.
Data identifies relative crustal motion around fault segments.
Continuous GNSS measurements provide insight into relative crustal velocities and strain accumulation rates.

Ability to make absolute measurements of 3D position globally.
Survey grade receivers are relatively cheap with consumer grade receivers being even cheaper.
Government-maintained systems are free to use.

Space-based radar technique for quantifying topographic change.
Synthetic aperture radar provides detailed data from a given antenna size.
The first global topographic model was made using synthetic aperture radar from the Space Shuttle Radar Topography Mission (SRTM).
Data are now freely available at 30-90 meter spatial resolution and 10 meter vertical accuracy.
SAR satellites generate continuous images of the ground, even through cloud cover.
SAR data is often interpreted in terms of image brightness.

INSAR combines two overpasses of a satellite to measure changes in distance to the surface.
Measures topographic change rather than absolute topography.
Radar wave bouncing off the surface, if there's topographic change, the second return signal is slightly out of phase of the first signal.
INSAR measures this phase shift.

Results presented as interferograms, with color representing phase shift magnitude.
Concentric rainbow-like fringes indicate surface deformation.
Radars used in INSAR have wavelengths of multiple centimeters to a few tens of centimeters.
INSAR can measure surface changes down to millimeters.

Faults associated with earthquakes: example of interferogram from Italy earthquake in 2009.
Each fringe represents a change of distance towards the satellite, like contours of surface deformation.
Counting fringes indicates total deformation (in the example, half a meter).
Links seismic measurements to surface fault rupture.
Understanding how much strain is being stored is an important part of understanding seismic hazard.

GNSS data from San Francisco: rates of surface deformation as yellow arrows.
INSAR measurements complement GNSS by filling in spatial gaps.
Interferogram confirms fault creep in most areas (blue and red colors separated across fault).
Locked fault areas (blue on both sides) indicate increased stress and higher earthquake likelihood.

Monitoring surface deformation in London.
Colors represent plus or minus 2 millimeters per year (red = subsidence, blue = heave).
Subsidence along crossrail tunnel is expected and verified against models.
Broader regions of subsidence/heave indicate groundwater changes.
Combining space-based gravity measurements, INSAR, and GNSS gives greater insight into groundwater changes.
Monitoring effects of hydrocarbon withdrawal and validating subsurface activity models.

INSAR gives relative position change, repeat measurements limited by satellite overpass frequency.
Restricted by signal decorrelation.
GNSS measurements can be made frequently and provide absolute 3D positional measurements.
Limited to receiver locations.

GNSS data are integral to geophysical surveys and make direct surface deformation measurements.
INSAR has greater spatial coverage and provides image-style data of surface deformation.
INSAR values aren't absolute measurements.
Combining the two methods brings the best aspects of both.