IEEE Geoscience and Remote Sensing Magazine - March 2020 - 9

DEVELOPMENT STAGES OF SYNTHETIC
APERTURE RADAR
SAR and interferometric SAR (InSAR) technologies and
their applications went through several paradigm changes in the last decades, from ivory-tower-like ideas about
SAR interferometry to globally used standard applications in surveying and civil engineering. We see three
main phases with distinctive properties and methodologies. The first, from 1991 to 2007, we call the phase of
data sparsity. SAR remote sensing data were available via
scientific missions, such as the ESA's ERS-1/2 or ENVISAT
ASAR. Although the data were readily available for scientists via announcements of opportunities, it was not easy
to obtain the data, especially in a specific area of interest.
An exemption from this was the Canadian Radarsat program, which offered commercially available data from
1995 onward.
Data availability was very limited for most users until the launch of TerraSAR-X and COSMO SkyMed marked
the beginning of the phase of data availability. We consider
this phase to have lasted from 2008 to around 2016; it
is the beginning of commercially and scientifically available high-resolution SAR data. With the increase in the
number of sensors and their capabilities, data became
more accessible. The high spatial resolution also led to
increased interest in SAR data from researchers in other
domains. We characterize this phase as having a large
increase in overall interest in InSAR research and an increase in SAR data demand.
The third phase, which started in 2016, is the phase of
data ubiquity. This phase is driven by the global acquisition
scheme of Sentinel-1 and the corresponding free data policy
of the European Commission. SAR data will become ubiquitous, given the variety of new and upcoming sensors and
the entry of commercial start-ups into the SAR data market.
Currently, not all wavelengths are well covered; however,
the expected increase in long-wavelength sensors in the
coming years will offer more data availability over a wider
range of wavelengths.
BIBLIOMETRIC ANALYSIS
In research, these phases have driven different approaches. In the first phase, data scarcity hindered the development of our science. It was difficult to get data, and the
approach was, typically, to shift the areas of research interest to fields for which data were available. InSAR remote
MARCH 2020

IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

sensing was a small community in remote sensing; it was
known for its tremendous capabilities but was also considered a kind of "black magic" that only a few people knew
and were able to use. This changed in the second phase,
beginning in 2008, when it became possible and easier
to order data covering specific areas of interest. More research teams joined in, and we saw a large increase in
SAR- and InSAR-related publications.
Figure 1 shows the number of InSAR-related publications from Web of Science in the research area of "remote
sensing." We limited the search to papers in this research
area, as this allows us to include the general term interferometry in the search. This permitted us to normalize it to all
papers published in the area of "remote sensing" that are
in the Web of Science database. There is a clear correspondence between the availability of data, which depends on
the number of active SAR satellites, as shown in Figure 2, and
the number of publications, which follow the data availability with a delay. Table 1 shows past and present civil
SAR satellite systems.
If we look at the percentage
of InSAR-related papers verWE PROVIDE AN OVERVIEW
sus all papers in the research
OF THE METHODOLOGICAL
area of remote sensing (see
DEVELOPMENT OF SURFACEFigure 3), it looks a bit different. We see a sharp increase
MOTION ESTIMATION FROM
after 1994, with a first peak
SAR AND DISCUSS
around the year 2000 and a
COMMERCIAL ADVANCES
high number of papers pubIN THE FIELD.
lished afterward, many dealing with permanent scatterer
interferometry (PSInSAR) and
related techniques, following the first papers from Ferretti
et al. [1], [2]. Afterward, we see a decline until the beginning
of the second phase of data availability, with the launch of
Cosmo-SkyMed and TerraSAR-X in 2007. The third phase of
data ubiquity may lead to a further increase from 2017 onward, but it is too early to say for sure.
This analysis is also interesting if we compare it to the
percentage of papers dealing generally with radar and SAR
in the research area of remote sensing, as shown in Figure 4.

800
700
600
500
400
300
200
100
0

1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
2010
2012
2014
2016
2018

to Dragon-4), each lasting four years. The latest phase,
Dragon-4, focuses on the scientific exploitation of ESA
missions, Copernicus Sentinel satellites, and Chinese Earth
observation data. This latest phase in the cooperation
brings together 28 joint Sino-European teams to investigate these topics. Over the years, more than 1,000 young
scientists have been trained in advanced courses, and over
700 papers have been published in ESA special publications alone.

FIGURE 1. The number of InSAR-related publications in the Web of

Science database.
9

IEEE Geoscience and Remote Sensing Magazine - March 2020

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2020