FREQUENTLY ASKED QUESTIONS

GENERAL FAQ

What is InSAR?

InSAR stands for Interferometric Synthetic Aperture Radar and is a proven technique for measuring ground movements. Satellites record images of the earth, and these images can be combined to measure movements of the ground surface.

How precise can you measure deformation?

The precision of each individual measurement is in the order of millimeters. This depends on the kind of satellite we use. By combining multiple measurements over time, we can derive the deformation velocity with a precision of around 1 mm/yr.

How is it possible that from 800 km altitude, such precision is achieved?

The distance is not relevant, the basis of calculating deformation is the use of the interferometric phase. Because the phase of the wave can be measured with sufficient precision, it does not matter how long the distance is.

The measurements originate from hard objects such as rocks, buildings, bridges or other infrastructure. How reliable are these points?

Very reliable. We only provide deformation measurements from reflections which do not change during the observed time interval. Major changes to the surface – like maintenance or resurfacing – will result in a change of the radar signal, and no reliable estimate is possible. The radar points are not selected manually by identifying hard objects, but by the criterion of an undisturbed radar signature.

What is the validity of an individual measurement?

InSAR relies on radar scattering to calculate the deformation data. The basic principles and the technology has been validated in hundreds of scientific papers and in thousands of cases. Nevertheless, we caution strongly against overly deterministic evaluation and interpretation of individual measurements points.

Can you influence the locations of the data points?

There are no sensors to be installed and we cannot influence the exact locations of the measurements.

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

We provide subsidence measurements of single buildings, bridges, mines, streets, whole neighborhoods and areas up to 10.000 km2. 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 measurement 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 frequent can you measure my area?

We can measure semi-daily, depending on your requirements.

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.

Aerial photography images are formed by reflected sunlight which is collected by a sensor, the camera. Radar images differ from these, because they are acquired in a much longer wavelength signal. The long wavelength enables the signal to travel through clouds. Furthermore, the radar does not rely on reflected sunlight, but acts itself as a source of radiation by transmitting pulses, which reflect on the surface. Finally, the radar measures the time between transmission and reception of the pulse, making it a distance measurement. Aerial photos cannot determine distance and are sensitive to directions under the influence of the lens. This is the essential capability enabling radar images to provide information about distance or distance changes (deformations), while optical instruments cannot.

How large is the deformation that you can measure?

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 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 subsidence 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.

TECHNOLOGY FAQ

In what direction does the satellite measure subsidence?

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. Based on this the absolute direction of the deformation vector can be determined, and we can distinguish subsidence from e.g. shear along a slope or horizontal deformation.

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 horizontal resolution of InSAR?

This varies per satellite and depends on the look angle towards the surface. ERS and Envisat have a ground resolution of approximately 5 to 23 meters, and a sampling of approximately 4 to 20 meters. TerraSAR-X has a variable resolution, usually we use 3 by 3 meters. Radar images resolution should be interpreted differently compared to optical images. Within a radar resolution cell of 4 x 20 meters, an object with dimensions of less than 1 meter can dominate the reflection. For example, a lamp post covers a much smaller area than 4 by 20 meters, but can provide a useful reflection.

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.

How are measurement points distributed in time?

We rely on the revisit frequency of the satellite.

What is the absolute geolocalization?

The absolute location has a standard deviation on the order of 1-2 meter, our algorithm is set to make sure we are measuring the same object every time and measures the motion of that object with millimeter precision over time.

PLATFORM FAQ

What is SkyGeo Maps?

SkyGeo Maps is a secure, online platform with your personalized maps and deformation data. SkyGeo Maps has a variety of tools to help you detect patterns in dynamic deformation. View the time series to understand the deformation 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 deformation 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 deformation 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 deformation 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.

What’s the difference between PS and DS data? 

PS stands for Persistent Scatterer, typically originating from a dominant scatterer within a resolution cell of a radar image. DS stands for Distributed Scatterer: in the absence of a single dominant scatterer within a resolution cell, we sample the signal of neighboring cells as well and average the signal.

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 deformation 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.

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