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InSAR Frequently Asked Questions (FAQs)

What is InSAR?

InSAR stands for Interferometric Synthetic Aperture Radar and is a proven technique for estimating ground movements with millimeter precision.

How precisely can you measure displacements with InSAR?

The precision of each individual measurement is in the order of millimeters. It is dependent on several factors, including the InSAR satellite resolution and the number of images in a time series. With a long time series, we can derive the displacement velocity with a precision of around 1 mm/yr.

What is the validity of an individual measurement?

InSAR technology has been validated in hundreds of scientific papers and thousands of cases over the last 30 years. On a project basis, there are several ways to validate InSAR data, including:

  1. Ground Truth Data: Compare with ground-based measurements like GPS or total station data.
  2. Independent Sources: Cross-validate with other remote sensing techniques or field measurements.
  3. Consistency Checks: Ensure temporal and spatial consistency in the data.
  4. Statistical Analysis: Apply statistical tests to assess reliability and identify biases.
  5. QA/QC Procedures: Implement rigorous quality control during data processing.
  6. Field Verification: Confirm deformation patterns with on-site inspections.
  7. Cross-Validation with Known Events: Match observed deformations with documented events like earthquakes or landslides.

Can you influence the locations of InSAR data points?

InSAR is an opportunistic monitoring technique and we cannot influence the exact locations of scatterers. However, if points are required at specific locations, such as co-located with ground instrumentation, corner reflectors or artificial reflectors can be installed.

What’s the typical scale of the area of interest?

We provide displacement measurements of single buildings, bridges, mines, streets, whole neighborhoods up to countrywide maps. The smaller the area of interest, the higher the satellite resolution is required. The larger the area, the more favorable the economics become.

What is the InSAR point density for my area of interest?

The data point density depends on many factors. We can assess the point density of your area of interest and optimize it for your business requirements. Contact us, we probably already have measurements of your area of interest.

How frequently do InSAR satellites collect images over my area?

The frequency of image acquisition depends on the area of interest and the satellites used. The frequency can be as high as semi-daily, but more typically we acquire images weekly.

How does InSAR differ from conventional measuring methods?

InSAR is based on radar reflections from infrastructure, the measurements are not related to pre-determined physical points on the ground. This contrasts with leveling, tachometry, or GNSS (GPS) measurements, all of which rely on physical markers such as measuring bolts in infrastructure, underground markers, etc.

How large are the displacements that you can measure with InSAR?

It depends on which satellite is used but it is possible to measure changes of several meters with great precision provided that the radar signature of the earth’s surface has not changed. This is the case for large instantaneous movements like earthquakes and landslides, or large but slow movements in dry, undisturbed areas.

You claim to look back in time. Does that apply everywhere, and how far back can you look?

Radar satellites started recording surface movements in 1992. Since then the number of satellites and the resolution of the images have increased significantly. We currently have access to historical data that covers 70% of the earth’s land. We can use that data to calculate and visualise surface movements in your area of interest.

Can the InSAR measurements be made ‘absolute’ to link them to a National Height Reference System (NHRS)?

This is possible through the use of radar reflectors, where the precise location of the radar reflection point is known. Two variables must be distinguished: (i) the instantaneous absolute height of the point, for example in the NHRS, and (ii) the instantaneous absolute displacement rate (ie, absolute altitude change of the point relative to the NHRS).

The value of a link to the NHRS should be critically considered. Just like GPS measurements are made in a reference system (‘datum’) compared to leveling measurements, the radar measurements are referenced – only in a different system. In most cases our measurements are about changes of the points over time

In what direction does the InSAR satellite measure displacement?

We measure motion in the line of sight from the satellite; in an East-West vertical plane, looking down in a 45-75 degree angle. If we do an analysis from different satellite viewing directions, multiple projections are measured.

Can you measure horizontal motions?

Yes, we can measure horizontal motions. SAR measures the projection of ground motion onto the Line of Sight (LOS) direction. The horizontal motions also contributes to the projection and can thus be measured by InSAR. However, if we want to calculate the contribution of horizontal motions based on the LOS measurement from either one satellite or the decomposition of two satellites, there has to be certain assumptions. In general, it is true to say that the SAR is less sensitive to north-south movements; and if we neglect the contribution of ground motion from north-south direction, we can estimated the vertical & east-west motion with two satellites given the above assumption.

What is the spatial resolution of InSAR?

The spatial resolution varies per satellite and per sensor mode. TerraSAR-X StripMap Mode has a resolution cell of 3 by 3 meters. Within that cell, individual points can be geolocalised with approximately 1 m precision in x, y and z directions.

What is the horizontal resolution of InSAR?

Yes, we can measure horizontal motions. SAR measures the projection of ground motion onto the Line of Sight (LOS) direction. The horizontal motions also contribute to the projection and can thus be measured by InSAR. However, if we want to calculate the contribution of horizontal motions based on the LOS measurement from either one satellite or the decomposition of two satellites, there has to be certain assumptions. In general, it is true to say that the SAR is less sensitive to north-south movements; and if we neglect the contribution of ground motion from north-south direction, we can estimate the vertical & east-west motion with two satellites given the above assumption.

What is the absolute geolocalization?

Sentinel-1 is a valuable source for SkyGeo but the satellite cannot be controlled for a specific monitoring task. We are satellite agnostic and currently have active projects using: Sentinel-1, TerraSAR-X, Cosmo SkyMed, ALOS-2, SAOCOM and Radarsat.

Do you use ESA’s Sentinel-1 satellite?

Sentinel-1 is a valuable source for SkyGeo but the satellite cannot be controlled for a specific monitoring task.

Platform FAQ

What is SkyGeo Maps?

SkyGeo Maps is a secure, online platform with your personalized maps and displacement data. SkyGeo Maps has a variety of tools to help you detect patterns in dynamic displacement. View the time series to understand the displacement dynamics over different distance scales: from 10m2 all the way up to 10.000 of km2.  Contact us for a quick demo of SkyGeo Maps.

What are these colored dots in the displacement map?

The colored points are radar reflections that are consistent over time. The point density depends on the characteristics of the terrain. A very ‘smooth’ terrain (flat relative to the radar wavelength) provides little or no reflection to the satellite. Another area may be “rough” and reflects in all directions. It is important that the characteristics of an object do not change significantly between consecutive satellite recordings. We call this type of scatterers coherent. If a reflection remains consistent over time, we call it a persistent scatterer.

What does the color of the measurement points mean?

The color quantifies the extent of motion of the measurement point over a given interval – it can be either the total displacement or the slope of a fit over time, typically in mm/year.

What is the significance of the size of the dots?

The size of the dot is uniform for the map. The size is only adjusted for the sake of visualizing and pattern detection.

The measurements are ‘relative’. What does a strain rate of 0 mm / year mean?

All measurements must indeed be interpreted relative to one another, as from space is not possible to determine if any point can be considered stable. The interpretation is always “how does point A move relative to point B.”

However, the results need to be shown numerically in the displacement map. For this reason, a reference point is chosen. At this reference point, the strain rate is by definition 0 mm/year. The movement of all other points within the analysis is relative to the reference point.

How is the reference point chosen, and what is the physical interpretation of this point?

The exact location of the reference point is not relevant for displacement rate.

For the physical interpretation, however, this choice is relevant. All measurements are calculated on the assumption that the reference point is stable. Without prior knowledge of the physical characteristics of  the area of interest, it is impossible to select an absolute stable reference point. This is a very important factor when interpreting the data, as it easily could lead to misunderstanding.

Note that this assumption is identical to leveling, where stable reference points are assumed. Although in case of leveling usually more physical information about the reference point is available, all estimates are based on this assumption.

Why do you separate high and low points? What is the threshold for high/low division of points?

You can filter measurements that originate at street level. We can adjust the cutoff height for the included vs. excluded measurements in light of the standard deviation of the X,Y,Z.

Can I use my own aerial photos as a background?

Yes, if you supply your own aerial photos, preferably in .tiff format, we can put them in the viewer and overlay these photos with the displacement data.

Can I supply my own map-layers, such as building outlines, location of pipes, etc.?

Yes you can add your map layers into SkyGeo Maps and then query the data based on their position relative to the objects in this layer.

How precise is the location and height of the measurements?

The standard deviation of location and height of the measurement points is 1 meter for high resolution date and 2.5 meter for standard resolution data. So InSAR is particularly useful for monitoring dynamic position changes.

Why is there a measurement point on this building/street/tree here, but not on a similar building/street/tree next to it?

Which objects we measure depends on:

  • The way the objects reflect the radar signal to the satellite. The reflections from buildings and other solid infrastructure are generally good.
  • The consistency of the reflections over time. We measure with mm-precision, so to be sure we are measuring the same object every time the reflections have to be consistent in time. Vegetated surfaces are therefore hard to measure; a tree moving because of the wind is not constant enough.
  • The satellite we use.

Can I use SkyGeo data for my own GIS system?

Yes, you can download the data for your own system.