# Review of Satellite Interferometry for Landslide Detection in Italy
## 3 Temporal Evolution of the Scientific Production  
A total of 253 documents was collected for this review. The first documents date back to early 2000, and the most recent ones were published in September 2019 (the end of the articles' collection).  
Figure 1 shows the temporal distribution of the scientific papers, with respect to the acquisition band of the sensor. Due to the unavailability of other satellite platforms, all the papers published before 2009 focus on C-band and before 2003 to ERS 1/2 images only.  
The first example of a multi-temporal InSAR application for landslide detection was published in 1999. [PERSON] et al. [25] introduced the concept behind the Permanent Scatterer approach, by presenting a case study of the Ancona landslide (Marche region, central Italy), repeatedly reactivated following intense rainfall events [26].  
In the following 10 years, PSI products derived from ERS 1/2 C-band images were exploited to monitor landslides in 45 different scientific works and 14 different Italian regions. The first application of a multi-band approach was proposed by [PERSON] et al. [27]. These authors analyzed Envisat and COSMO-SkyMed data by means of the Stable Point Interferometry over Urbanized Areas (SPINUA) algorithm [28]. The goal was to map and characterize landslides in some municipalities of the Lombardy region (Northern Italy).  
In the decade 1999-2009, the average scientific production was 4.5 articles/year (with a peak of 13 in 2006). In the following 10 years (2009-2019), this number was significantly raised to 19.7 papers/year (with a peak of 34 in 2013 and 37 in 2015), an increment of over 300%. This is clearly due to the increased awareness, within the scientific community and among end users, of InSAR capabilities. The spreading of InSAR applications was fostered by the technical evolution of algorithms, the launch of new C- and X-band satellites, and, in recent years, the increased computational capability offered by cloud computing services [29]. The distribution of the ERS 1/2, Envisat, and COSMO-SkyMed interferometric products derived in the framework of the PST-A project (_Piano Straordinario di Telerilevamento_--Special Plan of Remote Sensing of the Environment [30]) certainly ensured a better and wider downstream of InSAR applications.  
Figure 1: Scientific papers published per year from 1999 to 2019 subdivided for satellite acquisition band. Multi-band refers to different combinations between C-, X-, and L-band SAR images.  
C-band images are the most widely adopted for InSAR-related landslide applications in Italy. This is due to different factors, such as (i) the longer temporal coverage, (ii) the wider area capability, (iii) the better performances in peri-urban areas, (iv) the recent launch of the Sentinel-1 constellation, and (v) the free access to the PST-A products. X-band sensors, mainly COSMO-SkyMed, are starting to be used more often, as shown by the growth of applications in the last five years. This is related to the high value offered by X-band data in urban areas, and for single infrastructures monitoring. L-band images are less frequently used; this is probably due to, e.g., the long revisit time of the first L-band constellations and the image availability policy.