Frequently Asked Questions

Can radar penetrate a vegetation canopy?

 

Yes, of course, but the amount of signal lost through scattering depends on things like the leaf state, the measurement geometry and the vegetation canopy.  Also, shorter wavelengths (e.g. C-band, X-band) are affected more than longer wavelengths (e.g. L-band).

 

Does two-pass InSAR work over vegetation canopies?

 

Yes, but not everywhere and not at the same time.

 

Two-pass InSAR is where two SAR images taken at different dates are combined to form an interferogram.  The quality of the resulting interferogram will be affected by the amount of signal penetration but also any changes that occurred between the two dates and this can be quite subtle (wind, moisture etc.).  Any signal that does not penetrate the canopy is likely to be randomly bounced around the canopy before being reflected back to the sensor.  This is called diffuse scattering and any InSAR observation of diffuse scattering is likely to be of very poor quality, with very low coherence.  Because of all of these influences, it is a very common misconception that InSAR never works over vegetation.  But there are some notable exceptions….

 

Zhong Lu et al. (2005) found a ‘surprising’ capability for C-band InSAR data to be able to maintain good strong fringes over flooded forests in Louisiana, despite the common view that the signal would be scattered by the canopy.  That work initiated a capability to use InSAR to routinely measure the surface water level over the swamp forests of the southern USA that continues to this day.

 

Through our work, we also know that, occasionally, strong fringes can be seen over tropical forests, and below is an example from the North Selangor Peat Swamp Forest, which is an area we have comprehensively surveyed on behalf of academics from the University of Nottingham and John Moores University.

Images generated from C-band Sentinel-1 data of the North Selangor Peat Swamp Forest, Malaysia.

A Google Earth image 

A two-pass differential interferogram 

We see this everywhere.  If we have two-pass interferograms that are taken using pairs that are very close in time we can often see fringes over many different vegetation types.  Below is another example, this time from the Black Forest in Germany.

Images generated from C-band Sentinel-1 data of the Black Forest, Germany

A Google Earth image 

A two-pass differential interferogram 

What causes those InSAR fringes over thick vegetation?

 

Any fringes seen over a dense forest canopy cannot originate from the diffuse-reflecting canopy because the coherence would be too low.  In this case, we speculate that high quality fringes, of high coherence, must originate from specular reflection, most likely from the ground, either directly or, as they presuppose from the Louisiana/Florida swamp forests, from a double bounce from surface to trunk.  This agrees with Polarimetric SAR analyses of scattering over such areas where the assumption of the dominance of diffuse reflection has had to be revised to account for fringes in co- and cross-polarised interferograms.

 

Below is a figure that illustrates this, taken from an FAO report on tropical peat monitoring.

Can we see ground motion through a forest canopy?

 

Yes.  Below is a map of subsidence around the former Clipstone coal mine in the East Midlands, UK which lies next to Sherwood Forest.  A conventional InSAR survey which can only make measurements of hard targets covering the town itself and some of the infrastructure but is only able to identify some elements of the motion.  If we filter the coherence using the APSIS© method and utilise high coherent measurements through the canopy, we can see the full extent of the subsidence, which is oval-shaped extending under agriculture and thick forest canopies.

Images of subsidence in Clipstone, Notts using the same stack of Sentinel-1 data.

Subsidence using conventional InSAR

Subsidence using the APSIS© method

How can I make precise and dense sets of InSAR measurements through vegetation?

 

Terra Motion Limited have a solution, called APSIS©.  It is an enhancement of the Patent Pending Intermittent SBAS technique invented by the University of Nottingham in 2012.  It has been used successfully by the research community since then for monitoring a whole range of phenomena in urban and rural terrains, through forests, agriculture and natural surfaces.  Most notable is the work in the development of peat condition assessment criteria and the monitoring of earthwork dams.

 

Can I use PSInSAR to make measurements of land motion over vegetation?

 

No.  PSInSAR is a technique that is based upon the analysis of high quality InSAR observations that are of persistently high coherence, which is not a characteristic of vegetated surfaces at all.

Contact one of our team for more information +44 (0)1156 712 180

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