# Review of Works Combining GNSS and InSAR in Europe
## 3 Temporal and Spatial Distribution of Scientific Production in Europe
### Satellite SAR Sensors  
Considering all the aforementioned scientific contributions, whether they are peer-reviewed articles, book chapters or conference papers, 32% of them exploit DInSAR-based data, while the remaining 68% relied on one of the previously described MTInSAR approaches. In detail, more than 100 publications use PSInSAR (Persistent Scatterers Interferometry SAR) data [194], while a minor portion utilized the SPN (Stable Point Network [195]), SqueeSAR [196], SBAS (Small BASeline Subset [197]), iSBAS (Intermittent SBAS [198]), PSP-DINFSAR (Persistent Scatterer Pairs Differential InSAR [199]), CPT (Coherent Pixel Technique [200]), SPINUA (Stable Point Interferometry over Unurbanized Areas [201]), and TSIA (Two-Scale Interferometric Analysis, processing chain that performs a sequence of low-resolution (small-scale) and full-resolution (large-scale) processing [23], based on the SBAS [197] approach) algorithms. It is impossible to establish a precise number of times each algorithm was used since, in some articles, more than one approach was used for different datasets, and in many contributions, the processing algorithm was not clearly specified.  
Considering the satellite wavelength of the whole database collected, approximately 150 C-band datasets were implemented, as well as 35 X-band and 15 L-band datasets. For 15 papers, the C- and X-band datasets were combined, while only 7 C- and L-band datasets were used in parallel. None of the scientific studied viewed combined X- and L-bands. Finally, applications analyzing all three bands were implemented 5 times. Publicationstaken into consideration were also categorized according to the satellite constellation of the SAR data. Most of the paper shows a massive application of ENVISAT data (31.1%), covering the period 2002-2010, and ERS data (27.2%), covering the period from 1992 to 2000, followed by the data of Sentinel-1 (14.2%), acquiring SAR images since April 2014, which was largely diffused over the last few years due to its short and constant revisiting time, due to its scientific contribution aim and free availability [33]. All these satellite constellations acquire in the C-band (5.6 cm wavelength), confirming its more widespread use in more common applications as a good compromise for urban and nonurbanized areas. The minor frequencies of other satellites are registered, listed from the high to low: TerraSAR-X (X-band, approximately 10%), RadarSAT (X-band, approximately 6%), Alos (L-band, approximately 6%), and COSMO-SkyMed (X-band, approximately 4.7%).  
Satellite data are also used in combination with others to obtain longer timespans of analysis or to compare the performances of different bands. The combination of different constellation datasets is reported in the satellite graphs of Figure 5. It is interesting to note that, in at least one paper, all constellation satellites were combined with the other three constellations, except for RadarSAT, which did not show application when combined with the other 3 datasets. In addition, it is possible to state that the aforementioned constellation satellites are rarely combined with the other two and commonly with other datasets. Exceptions are also detectable for the COSMO-SkyMed, X-band, and Alos, L-band, whose data were more often used combined with two other constellations than standalone or only with one other dataset.  
Figure 5: Satellite constellation usage considering all the applicative scientific contributions (center) and a more detailed analysis of the use of the dataset of a single satellite (Alone) or with another satellite (+1), with other 2 (+2) or three (+3) constellations (satellite graphs).