remote sensing

Review Ground-Based Radar Interferometry: A Bibliographic Review

Massimiliano Pieraccini * and Lapo Miccinesi

Department of Information Engineering (DINFO), University of Florence, via Santa Marta 3, 50139 Firenze, Italy; lapo.miccinesi@unifi.it * Correspondence: massimiliano.pieraccini@unifi.it



Received: 9 April 2019; Accepted: 27 April 2019; Published: 30 April 2019


Abstract: Ground-based/terrestrial radar interferometry (GBRI) is a scientific topic of increasing interest in recent years. This article is a bibliographic review, as much complete as possible, of the scientific papers/articles published in the last 20 years, since the pioneering works in the nineties. Some statistics are reported here about the number of publications in the years, popularity of applications, operative modalities, operative bands. The aim of this review is also to identify directions and perspectives. In the opinion of authors, this type of radar systems will move forward faster modulations, wider view angle, MIMO (Multiple Input Multiple Output) systems and radar with capability to detect the vector of displacement and not only a single component.

Keywords: radar; interferometry; ground-based synthetic aperture radar; terrestrial radar interferometer

1. Introduction Radar interferometry started as space technology. In the nineties, the satellites ERS-1, JERS-1, RADARSAT-1, and ERS-2 [1,2], made use of the phase information of radar images for detecting ground changes. Spaceborne Synthetic Aperture Radar (SAR) systems, operating from an orbit at 800 km altitude, provide impressive interferograms due to the ground displacement after an earthquake [3,4]. These extraordinary spaceborne developments had an early follow-up in analogue ground-based radar systems. Pioneering works anticipated some ideas. In 1997 [5], Tarchi et al. detected by radar the structural change of a beam in a large anechoic chamber. In 1999 [6], the same bulk laboratory equipment was used to detect the changes of a dam. In 2000, Pieraccini et al. [7] measured the deformations of a pedestrian bridge during its static test with a portable radar. In 2003 [8], a landslide was monitored by an interferometric Ground-based Synthetic Aperture Radar (GBSAR) for the first time. In February 2000, a specifically modified radar system with one transmitter and two receivers (the second at the end of a 60 m mast that extended from the payload) flew on board the Space Shuttle Endeavour during the 11 day STS-99 mission [9]. This equipment was able to generate a high-resolution digital topographic database. Inspired by this exciting idea, a GBSAR has been modified for acquiring data with different baselines to obtain the Digital Elevation Model (DEM) of its field of view [10]. Currently, ground-based/terrestrial radar interferometry (GBRI) is a popular remote sensing technique for monitoring landslides, mines, bridges, towers, dams, and other civil infrastructures. The strength of GBRI is its complementarity with spaceborne SAR [11–13], while the latter is a powerful and effective remote sensing technique for surveying large areas (many km2) at long term (return time in the order of several weeks or months), GBRI is suitable for monitoring small areas (from a single building to a slope) at short term (sampling time up to a few of minutes) as sketched in Figure1.

Remote Sens. 2019, 11, 1029; doi:10.3390/rs11091029 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, x FOR PEER REVIEW 2 of 22

(fromRemote aSens. single 2019 ,building 11, x FOR PEERto a slope)REVIEW at short term (sampling time up to a few of minutes) as sketched2 of 22 in Figure 1. Remote(from Sens.a single2019, building11, 1029 to a slope) at short term (sampling time up to a few of minutes) as sketched2 of 22 in Figure 1.

Figure 1. Complementarity of GBRI with spaceborne SAR.

After about twenty Figureyears 1.of Complementarity research and development of GBRI with in spaceborne this field, SAR. the aim of this article is to review the vast literature and possibly identify further developments. This paper is focused on a After about twenty years of research and development in this field, the aim of this article is to bibliographicAfter about review; twenty for yearsmore technicalof research reviews and development on GBRI the inreader this field,can refer the aimto [14–16]. of this article is to review the vast literature and possibly identify further developments. This paper is focused on a reviewThis the review vast literatureis structured and aspossibly follows. identify First, thefurther authors developments. recall briefly This the paperworking is focused principle on of a bibliographic review; for more technical reviews onon GBRIGBRI thethe readerreader cancan referrefer toto [[14–16].14–16]. GBRI. This kind of radar can operate without any Direction‐of‐Arrival (DOA) capability [17], or with This reviewreview is is structured structured as as follows. follows. First, First, the the authors authors recall recall briefly briefly the working the working principle principle of GBRI. of a rotary system able to rotate the antenna [18]. More advanced DOA techniques include Synthetic ThisGBRI. kind This of kind radar of can radar operate can operate without without any Direction-of-Arrival any Direction‐of‐Arrival (DOA) capability(DOA) capability [17], or with[17], aor rotary with Aperture Radar (SAR) [19] Multiple Input Multiple Output (MIMO) [20], and moving slot antennas asystem rotary able system to rotate able to the rotate antenna the [antenna18]. More [18]. advanced More advanced DOA techniques DOA techniques include Synthetic include Synthetic Aperture [21]. These different designs are discussed, as well as the possible operative bands. Since GBRI is ApertureRadar (SAR) Radar [19 ](SAR) Multiple [19] Input Multiple Multiple Input Output Multiple (MIMO) Output [20 (MIMO)], and moving [20], and slot moving antennas slot [21 antennas]. These essentially a monitoring/testing technique, its applications are classified on the basis of items under different designs are discussed, as well as the possible operative bands. Since GBRI is essentially a test[21]. (or These to be different monitored) designs that can are be discussed, landslides as [8], well bridges as the [7], possible buildings operative [22], mines bands. [18], Since towers GBRI [23], is monitoring/testing technique, its applications are classified on the basis of items under test (or to be glaciersessentially [24], a monitoring/testingmonuments [25], technique,or volcanoes its applications[26]. Finally, are the classified authors on discuss the basis the of mostitems recentunder testmonitored) (or to be that monitored) can be landslides that can be [8 ],landslides bridges [ 7[8],], buildings bridges [7], [22 buildings], mines [18 [22],], towers mines [ [18],23], glaciers towers [23], [24], developments: FM modulation that reduces the sweep time [27], Arc‐SAR modality that enlarges the glaciersmonuments [24], [ 25monuments], or volcanoes [25], [ 26or]. volcanoes Finally, the [26]. authors Finally, discuss the the authors most recentdiscuss developments: the most recent FM view angle [28], MIMO that makes the radar dramatically faster [29], bistatic configurations that developments:modulation that FM reduces modulation the sweep that reduces time [27 the], Arc-SAR sweep time modality [27], Arc that‐SAR enlarges modality the that view enlarges angle [ 28the], allows to detect the displacement vector [30], W‐band that enlarge the bandwidth and reduce the size viewMIMO angle that makes[28], MIMO the radar that dramatically makes the radar faster dramatically [29], bistatic configurationsfaster [29], bistatic that allowsconfigurations to detect that the of the equipment [31], and radar systems with 3D imaging capabilities [32]. allowsdisplacement to detect vector the displacement [30], W-band thatvector enlarge [30], W the‐band bandwidth that enlarge and reduce the bandwidth the size of and the equipmentreduce the [size31], 2.ofand Working the radar equipment systems Principle [31], with of and 3DGBRI radar imaging systems capabilities with 3D [32 imaging]. capabilities [32].

2. Working WorkingWith the Principle term ground of GBRI‐based radar interferometer we define a ground‐based piece of equipment that uses a microwave beam for remotely detecting small displacements of targets through phase With the term ground ground-based‐based radar interferometer we define define a ground ground-based‐based piece of equipment measurement. Figure 2 shows the working principle of this kind of equipment. The reader can find a that uses a microwave beam for remotely detecting small displacements of targets through phase detailed description of the technique in the seminal papers [8], and [33]. measurement. Figure 22 showsshows thethe workingworking principle principle of of this this kind kind of of equipment. equipment. TheThe readerreader cancan findfind aa detailed description of the technique in the seminal papers [[8],8,33 and]. [33].

Figure 2. Working principle of ground based/terrestrial radar interferometry. Figure 2. Working principle of ground based/terrestrial radar interferometry.

A transceiver installed at ground transmits an electromagnetic wave forward a target. The wave Figure 2. Working principle of ground based/terrestrial radar interferometry. is backscattered and comes back to the transceiver that is able to detect the phase difference between

Remote Sens. 2019, 11, 1029 3 of 22 the transmitted and the received wave. The phase difference (∆ϕ) between two acquisitions is given by [16] ∆ϕ = ∆ϕdisp + ∆ϕatm + ∆ϕnoise (1) where ∆ϕdisp is given by the possible displacement of the target, ∆ϕatm is given by changes of the atmospheric conditions (humidity, temperature, pressure), and ∆ϕnoise is a phase noise contribution that can never be completely eliminated in experimental measurements. The atmospheric phase contribution (usually named “atmospheric phase-screen” [34]) can be neglected for short-range short-term applications (indicatively when the distance radar-target is less than 100–200 m and the time between the acquisitions is shorter than 1–2 h). In all other cases, the atmospheric phase screen has to be removed with suitable processing techniques. The proposed algorithms are many [35–37], but their working principle is always based on the identification of stable targets in the radar image and on the compensation with respect to them using a suitable (always approximated) atmospheric model. This phase-screen removal is a very critical point and limits the final accuracy in long term monitoring. When the phase noise contribution is negligible and the atmospheric phase-screen is removed (or negligible), the possible displacement (∆r) of the target between two acquisitions is detected as phase difference (∆ϕ) through the following basic relationship [38]:

∆ϕ ∆r = λ (2) 4π with λ wavelength of the transmitted signal. In principle, it is possible to detect displacement even by using a non-modulated signal (i.e., transmitting a continuous wave at a single frequency, as for example in [39]), but without capability to discriminate the targets in range it is hard to separate the real displacement of the target of interest by the clutter. Therefore, the vast majority of these sensors transmits a modulated signal. In this case the range resolution (∆R) is given by the bandwidth (B)[40]: c ∆R = (3) 2B with c speed of light. A sensor with only range resolution works properly when the beam is narrow, and the target of interest extends mainly along a single dimension (like a tower or a bridge). For more complex targets (like slopes, buildings, villages) it is required a sensor with also cross-range resolution. This can be obtained in different ways. The simplest one is just with a mechanical system that rotates the antenna [41]. More advanced solutions are based on Synthetic Aperture Radar [42] or MIMO (Multiple Input Multiple Output) [43].

3. GBRI in the Scientific Literature Since the pioneering works in the nineties, the scientific literature of GBRI has greatly grown involving several academic groups, private companies, and government agencies. In order to quantify the bibliographic impact of GBRI we have selected a set of relevant publications using SCOPUS, WEB OF SCIENCE, and Google Scholar with combinations of the following key words: ground-based synthetic aperture radar, interferometric radar, GBSAR, terrestrial radar interferometer, real aperture radar. All selected publications have been checked by the authors to exclude false-positives. The criteria we used for publication selection were: (1) relevance to the topic; (2) published before December 2018 (to avoid to update the record each day); (3) journal articles and conference papers (we have eliminated chapters of books, monographs, Ph.D. dissertations); (4) texts written in English; (5) accessible full-text via Institution (University of Florence). The result of this research was a set of 415 publications. The number of papers we have not been able to download has been less than ten, therefore we can estimate that the selected set of publications covers about 98% of relevant literature. Figure3 shows Remote Sens. 2019, 11, 1029x FOR PEER REVIEW 44 of of 22 relevant literature. Figure 3 shows the distribution of these publications starting in 1997 [5]. The averagethe distributionRemote bibliographic Sens. 2019 of, 11 these, x FORproduction publications PEER REVIEW of the starting last four in 1997 year [5 is]. 42 The publications average bibliographic per year. production4 of 22 of the last four year is 42 publications per year. relevant literature. Figure 3 shows the distribution of these publications starting in 1997 [5]. The average bibliographic production of the last four year is 42 publications per year.

Figure 3. PapersPapers published until December 2018 on GBRI. Figure 3. Papers published until December 2018 on GBRI. Radars can detect a range by measuring measuring the time time-of-flight‐of‐flight between the transmitted and received electromagneticRadars wave.canwave. detect TheThe a directionrange direction by measuring of of arrival arrival the (DOA) (DOA) time cannot‐of ‐cannotflight be between detectedbe detected the (we transmitted (we refer refer to this and to kind receivedthis orkind radar or electromagnetic wave. The direction of arrival (DOA) cannot be detected (we refer to this kind or radarinterferometer interferometer with the with term the “no-DOA”), term “no‐DOA”), [17,23 ,[17,23,39,44]39,44] or can or be can detected be detected by the by rotation the rotation of the of radar the radar interferometer with the term “no‐DOA”), [17,23,39,44] or can be detected by the rotation of the radarhead (“Rotary”)head (“Rotary”) without without any synthetic any synthetic aperture aperture [18,45]. [18,45]. The most The advanced most advanced radar systems radar systems are able are to detectradar the directionhead (“Rotary”) of arrival without through any synthetic synthetic aperture aperture [18,45]. (SAR: The Synthetic most advanced Aperture radar Radar). systems The are main able toable detect to detect the thedirection direction of ofarrival arrival through through syntheticsynthetic apertureaperture (SAR: (SAR: Synthetic Synthetic Aperture Aperture Radar). Radar). advantage of SAR compared with equal aperture (physical or scanned) is that the angular resolution is The mainThe main advantage advantage of SARof SAR compared compared with with equal equal aperture (physical (physical or or scanned) scanned) is that is that the angularthe angular twice good [19]. The simplest Ground-based Synthetic Aperture is based on the movement of a radar resolutionresolution is twice is twice good good [19]. [19]. The The simplest simplest GroundGround‐‐basedbased SyntheticSynthetic Aperture Aperture is basedis based on theon the movementheadmovement along of a lineara ofradar a radar mechanical head head along along guide a lineara linear (“linear mechanical mechanical SAR”) [guide7,8]. Alternatively,(“linear (“linear SAR”) SAR”) [7,8]. it can[7,8]. Alternatively, be Alternatively, based on it a can circular it can bemovement basedbe based on (e.g., a on circular a along circular movement an movement arc) [28 ,(e.g.,46 (e.g.,,47 along]. along We an name an arc)arc) this [28,46,47]. kind of We We radar name name “C-SAR”. this this kind kind of Finally, radar of radar “C the‐SAR”. “C synthetic‐SAR”. Finally,apertureFinally, the can thesynthetic be madesynthetic withaperture aperture multiple can can transmittingbe be made made withwith antennas multiple and transmitting transmitting multiple receiving antennas antennas antennasand andmultiple multiple (MIMO: receivingMultiplereceiving Inputantennas antennas Multiple (MIMO: (MIMO: Output) Multiple Multiple [20, 29Input Input]. A Multiple less Multiple common Output) way [20,29]. [20,29]. to realize A Aless less a common synthetic common way aperture way to realize to realize is a to use a synthetic aperture is to use the movement of a helicoidally slot [21]. Figure 4 shows pictures of some syntheticthe movement aperture of a is helicoidally to use the movement slot [21]. Figure of a helicoidally4 shows pictures slot [21]. of someFigure prototypes 4 shows pictures that exemplify of some prototypes that exemplify these six operative modalities. prototypesthese six operative that exemplify modalities. these six operative modalities.

Figure 4. Examples of ground-based radar interferometers: (a) no-DOA (after [48]); (b) Rotary (after [49]); (c) linear SAR (after [48]); (d) MIMO (after [50]); (e) ArcSAR (after [48]); (f) moving slot

(after [21]).

Remote Sens. 2019, 11, x FOR PEER REVIEW 5 of 22

Figure 4. Examples of ground‐based radar interferometers: (a) no‐DOA (after [48]); (b) Rotary (after [49]); (c) linear SAR (after [48]); (d) MIMO (after [50]); (e) ArcSAR (after [48]); (f) moving slot (after [21]);. Remote Sens. 2019, 11, 1029 5 of 22 As shown in the histogram in Figure 5, the majority of the papers refer to linear SAR (58%) [7]. About 25% of the papers discuss developments or applications of radar interferometers without angularAs shown resolution in the (“real histogram aperture”). in Figure About5, the 8% majority of the publications of the papers refer refer to radar to linear systems SAR (58%)that achieve [ 7]. Aboutangular 25% resolution of the papers just discuss by rotating developments the antenna. or applications These three of radaroperative interferometers modalities without cover 91% angular of the resolutionpublications. (“real aperture”). About 8% of the publications refer to radar systems that achieve angular resolution just by rotating the antenna. These three operative modalities cover 91% of the publications.

Figure 5. Papers published until December 2018 classified on the basis of the operative modality. Figure 5. Papers published until December 2018 classified on the basis of the operative modality. The operative band is a key parameter of a radar. Generally speaking, by increasing the operative frequency,The theoperative angular band resolution is a key improvesparameter and of a theradar. displacement Generally speaking, resolution by increases. increasing At the the operative same time,frequency, the temporal the angular coherence resolution of the targets improves decreases. and the Therefore, displacement a balance resolution is necessary. increases. Figure At6 showsthe same thetime, histogram the temporal of the publications coherence of divided the targets on the decreases. basis of the Therefore, chosen operative a balance frequency is necessary. of the Figure radar. 6 Aboutshows 58% the of histogram publications of the are publications relative to equipmentdivided on inthe Ku-band basis of the (12–18 chosen GHz). operative Some radarfrequency systems of the operateradar. in About C-band 58% (4–8 of GHz),publications and X-band are relative (8–12 GHz). to equipment The Ku-band in Ku is‐band a good (12–18 trade-o GHz).ff between Some sizeradar ofsystems the equipment operate and in C temporal‐band (4–8 coherence GHz), and of the X‐band targets, (8–12 even GHz). if the The C-band Ku‐band is probably is a good preferable trade‐off forbetween monitoring size of snow the coversequipment [51– 53and]. temporal The components coherence cost of canthe targets, be another even issue if the that C‐band can makeis probably the diffpreferableerence in thefor selectionmonitoring of the snow band. covers Until [34–36]. today, components The components in Ka andcost Wcan were be another more expensive, issue that but can withRemotemake the Sens. the large 2019 difference, di 11ff, usionx FOR inPEER of the automotive REVIEW selection radar of the in band. W-band, Until their today, cost components is rapidly decreasing in Ka and [54 W]. were6 of more 22 expensive, but with the large diffusion of automotive radar in W‐band, their cost is rapidly decreasing [54].

Figure 6. Papers published until December 2018 classified on the basis of the operative band. Figure 6. Papers published until December 2018 classified on the basis of the operative band.

GBRI is an applicative topic. So, we have grouped the 415 publications based on the specific application. Obviously, a single publication can cover more than one application, and there are publications that do not refer to any specific application. The result of this classification in shown in Figure 7.

Figure 7. Papers published until December 2018 classified on the basis of their applications.

Landslide monitoring is doubtless the most popular application of GBRI (see Figure 8a). Sinkhole monitoring [55] has been included in this category. Over 40% of the publications are developments or case‐studies about monitoring systems of landslides. The first in‐field test dates back to 2003 [8]. Landslide monitoring is an application of great social and scientific impact and currently many slopes are routinely monitored by radar interferometers [56,57].

Remote Sens. 2019, 11, x FOR PEER REVIEW 6 of 22

Remote Sens. 2019, 11, 1029 6 of 22 Figure 6. Papers published until December 2018 classified on the basis of the operative band.

GBRIGBRI is an is applicative an applicative topic. topic. So, So, we we have have groupedgrouped the the 415 415 publications publications based based on the on specific the specific application.application. Obviously, Obviously, a singlea single publication publication can cover more more than than one one application, application, and andthere there are are publicationspublications that that do notdo not refer refer to to any any specific specific application. application. The The result result of of this this classification classification in shown in shown in in FigureFigure7. 7.

FigureFigure 7. Papers 7. Papers published published until until December December 20182018 classified classified on on the the basis basis of oftheir their applications. applications.

LandslideLandslide monitoring monitoring is doubtless is doubtless the mostthe most popular popular application application of GBRI of GBRI (see Figure(see Figure8a). Sinkhole 8a). monitoringSinkhole [55 monitoring] has been [55] included has been in thisincluded category. in this Over category. 40% of Over the publications40% of the publications are developments are or case-studiesdevelopments about or case monitoring‐studies about systems monitoring of landslides. systems Theof landslides. first in-field The test first dates in‐field back test to dates 2003 [8]. Landslideback to monitoring 2003 [8]. Landslide is an application monitoring of greatis an socialapplication and scientific of great social impact and and scientific currently impact many and slopes currently many slopes are routinely monitored by radar interferometers [56,57]. are routinelyRemote Sens. monitored 2019, 11, x FOR by PEER radar REVIEW interferometers [56,57]. 7 of 22

FigureFigure 8. Examples 8. Examples of applicationsof applications of of GBRI: GBRI: ( (aa)) landslidelandslide monitoring monitoring (after (after [58]), [58 displacement]), displacement map map and pictureand picture projected projected on the on digital the digital elevation elevation model model of the of slope; the slope; (b) monitoring (b) monitoring of a of viaduct a viaduct in Florence, in Italy;Florence, (c) displacement Italy; (c) displacement map projected map on projected the picture on the of picture the monitored of the monitored building building (after [(after22]); ([22]);d) mines monitoring,(d) mines radar monitoring, deployed radar in deployed Thompson in Thompson Creek Mine, Creek Idaho, Mine, USA Idaho, (after USA [59 (after]). [59]).

In the application of GBSAR to landslide monitoring, the data acquired by ground‐based radar installations are often integrated with geological and geomorphological field surveys, spaceborne SAR acquisitions (when available), orthophotos, and inventory maps. Figure 9 shows a typical data flow‐chart [60].

Remote Sens. 2019, 11, x FOR PEER REVIEW 7 of 22

Figure 8. Examples of applications of GBRI: (a) landslide monitoring (after [58]), displacement map and picture projected on the digital elevation model of the slope; (b) monitoring of a viaduct in RemoteFlorence, Sens. 2019 Italy;, 11 (,c 1029) displacement map projected on the picture of the monitored building (after [22]); 7 of 22 (d) mines monitoring, radar deployed in Thompson Creek Mine, Idaho, USA (after [59]).

In Inthe the application application of GBSAR of GBSAR to landslide to landslide monitoring, monitoring, the the data data acquired acquired by byground ground-based‐based radar radar installationsinstallations are are often often integrated integrated with with geological geological and and geomorphological geomorphological field field surveys, surveys, spaceborne spaceborne SARSAR acquisitions acquisitions (when (when available), available), orthophotos, orthophotos, and and inventory inventory maps. maps. Figure Figure 9 shows9 shows a typical a typical data data flowflow-chart‐chart [60]. [60 ].

Figure 9. Data flow-chart that integrates geological and geomorphological field survey, space-born SAR acquisitions, orthophotos, and inventory map (after [60]).

Testing or monitoring of bridges is another key application (see Figure8b). The radar is a fast, reliable and accurate equipment for remotely sensing the static or dynamic behavior of a bridge. Pieraccini et al. were the first (2000) that demonstrated [7] this possibility. Overall about 30% of publications refer to bridge monitoring. Building is the third class of applications in order of popularity (see Figure8c). This class is less homogenous: we have considered radar systems that monitor construction works [61], villages built over landslide [62] or on terrain subject to subsidence [63]. Probably, this class of application is overestimated because many papers deal with the tests of equipment rather than their effective applications. On the contrary, the class of the open pit mining is probably underestimated. At the knowledge of the authors, hundreds of radars are currently in operation as safety equipment in mining areas (see Figure8d)., but the number of publications on monitoring activity for worker safety in mining areas is rather modest (about 15%). This is probably because mine safety is a rather confidential issue, and the owners hardly agree to publish their radar data. The first publication about it dates back to 2000 [18]. Monitoring of towers/chimneys/electric poles is another emerging application field of GBRI (see Figure 10a). Generally speaking, all nearly one-dimensional structures, like bridges and towers, are particularly suitable for being monitored/tested by radar, as it does not require cross-range resolution. Therefore, if radar operates in no-DOA modality the structure can be sampled at over 50–100 Hz [17,23], and its dynamic characteristics (natural frequencies, modal shape, transient response) can be detected. Remote Sens. 2019, 11, x FOR PEER REVIEW 8 of 22

Figure 9. Data flow‐chart that integrates geological and geomorphological field survey, space‐born SAR acquisitions, orthophotos, and inventory map (after [60]).

Testing or monitoring of bridges is another key application (see Figure 8b). The radar is a fast, reliable and accurate equipment for remotely sensing the static or dynamic behavior of a bridge. Pieraccini et al. were the first (2000) that demonstrated [7] this possibility. Overall about 30% of publications refer to bridge monitoring. Building is the third class of applications in order of popularity (see Figure 8c). This class is less homogenous: we have considered radar systems that monitor construction works [61], villages built over landslide [62] or on terrain subject to subsidence [63]. Probably, this class of application is overestimated because many papers deal with the tests of equipment rather than their effective applications. On the contrary, the class of the open pit mining is probably underestimated. At the knowledge of the authors, hundreds of radars are currently in operation as safety equipment in mining areas (see Figure 8d)., but the number of publications on monitoring activity for worker safety in mining areas is rather modest (about 15%). This is probably because mine safety is a rather confidential issue, and the owners hardly agree to publish their radar data. The first publication about it dates back to 2000 [18]. Monitoring of towers/chimneys/electric poles is another emerging application field of GBRI (see Figure 10a). Generally speaking, all nearly one‐dimensional structures, like bridges and towers, are particularly suitable for being monitored/tested by radar, as it does not require cross‐range resolution. Therefore, if radar operates in no‐DOA modality the structure can be sampled at over 50– 100 Hz [17,23], and its dynamic characteristics (natural frequencies, modal shape, transient response) canRemote be Sens. detected.2019, 11 , 1029 8 of 22

FigureFigure 10.10. ExamplesExamples of of GBRI GBRI applications: applications: (a) towers (a) towers (after [(after64]); ( b[64]);) glaciers (b) (afterglaciers [65 ]);(after (c) monuments [65]); (c) d monuments(after [25]); ( (after) volcanoes, [25]); (d) permanent volcanoes, installation permanent at installation the Stromboli at volcano,the Stromboli Italy (aftervolcano, [66]). Italy (after [66]). Glaciers are fundamental “proxies” for standing the effects global warming on a local scale [67]. So, scientists have great interest in using GBRI for monitoring the change of glaciers (see Figure 10b). Glaciers are fundamental “proxies” for standing the effects global warming on a local scale [67]. The first case study dates back to 2007 [24], but aftermath many other studies on different glaciers have So, scientists have great interest in using GBRI for monitoring the change of glaciers (see Figure 10b). been performed (see for example [13,68,69]). In our bibliographic survey about 13% of the papers refer

to glacier monitoring. Another field of application of great scientific and social interest is the monitoring of cultural heritage (see Figure 10c). Radar has been used for monitoring the lending Tower of Pisa, Italy [23]; the towers of San Gimignano, Italy [70], the Bell-tower of Giotto in Florence [71], the Michelangelo’s David at Accademia Museum in Florence [25], the Baptistery of Florence, Italy [72], the ruins of Roman Forum of Rome, Italy [73], the retaining walls of the historical town of Volterra, Tuscany [74], and other sites [75,76]. Overall the publications the refer to Cultural Heritage represent about 10% of the total. Volcanoes are both a fascinating phenomenon and an impending risk for people living around their flanks. Therefore, although the number of active volcanoes is rather limited, over 8% of the scientific papers refer to volcanoes. In particular, Stromboli volcano, Italy, (see Figure 10d) is one of the most studied volcanoes in the world [77–81] and it has been continuously monitored by terrestrial radar since 2008 [26]. Also, Soufrière Hills in the Montserrat island [82] and Hakone volcano in Japan [83] have been monitored by terrestrial radar. Digital Elevation Model (DEM) production is another interesting GBRI application (see Figure 11a). Twelve papers (6% of total) have been published on this subject. The main finding is that this technique is effective even by a terrestrial radar, but its accuracy is sensibly worse than traditional equipment (photogrammetry or Terrestrial Laser Scanner) [10]. Remote Sens. 2019, 11, x FOR PEER REVIEW 9 of 22

The first case study dates back to 2007 [24], but aftermath many other studies on different glaciers have been performed (see for example [68,13,69]). In our bibliographic survey about 13% of the papers refer to glacier monitoring. Another field of application of great scientific and social interest is the monitoring of cultural heritage (see Figure 10c). Radar has been used for monitoring the lending Tower of Pisa, Italy [23]; the towers of San Gimignano, Italy [70], the Bell‐tower of Giotto in Florence [71], the Michelangelo’s David at Accademia Museum in Florence [25], the Baptistery of Florence, Italy [72], the ruins of Roman Forum of Rome, Italy [73], the retaining walls of the historical town of Volterra, Tuscany [74], and other sites [75,76]. Overall the publications the refer to Cultural Heritage represent about 10% of the total. Volcanoes are both a fascinating phenomenon and an impending risk for people living around their flanks. Therefore, although the number of active volcanoes is rather limited, over 8% of the scientific papers refer to volcanoes. In particular, Stromboli volcano, Italy, (see Figure 10d) is one of the most studied volcanoes in the world [77–81] and it has been continuously monitored by terrestrial radar since 2008 [26]. Also, Soufrière Hills in the Montserrat island [82] and Hakone volcano in Japan [83] have been monitored by terrestrial radar. Digital Elevation Model (DEM) production is another interesting GBRI application (see Figure 11a). Twelve papers (6% of total) have been published on this subject. The main finding is that this technique is effective even by a terrestrial radar, but its accuracy is sensibly worse than traditional equipmentRemote Sens. 2019 (photogrammetry, 11, 1029 or Terrestrial Laser Scanner) [10]. 9 of 22

Figure 11. Examples of GBRI applications: (a) terrain mapping, digital elevation model of a slope Figure 11. Examples of GBRI applications: (a) terrain mapping, digital elevation model of a slope obtained through GBSAR (after [84]) (b) dams monitoring (courtesy of IDS georadar [48]); (c) snow obtained through GBSAR (after [84]) (b) dams monitoring (courtesy of IDS georadar [48]); (c) snow cover monitoring, map of snow water equivalent change obtained by GBSAR (after [85]); (d) trees cover monitoring, map of snow water equivalent change obtained by GBSAR (after [85]); (d) trees monitoring by GBSAR (after [86]). monitoring by GBSAR (after [86]). Dam-monitoring has been the first in-field application of GBRI: it dates back to 1999 [6] (see FigureDam 11‐b).monitoring The concrete has been surface the of first a dam in‐field is almost application an ideal of targetGBRI: forit dates radar back interferometry; to 1999 [6] (see it is Figurecoherent, 11b). homogenous The concrete enough surface but notof a specular. dam is almost Nevertheless, an ideal the target papers for related radar to interferometry; this application it are is coherent,only 5% of homogenous total, but dams enough have but already not specular. sophisticated Nevertheless, monitoring the systems papers withrelated diff toerent this sensorsapplication [87], areso there only is5% no of demand total, but for dams novel have sensors; already and sophisticated dam safety, as monitoring well as mines, systems is a rather with confidentialdifferent sensors issue, and the owners hardly agree to publish their radar data. Martinez-Vazquez in 2005 [51] proposed the GBRI for assessing snow cover change (see Figure11c), as it has already been done by satellite [88]. The technique’s results are effective with dry snow, while wet snow can be a problem [52]. Very interesting results have been obtained using the radar for mapping the avalanches on steep slopes [53]. Monitoring of vegetation or trees by GBSAR (see Figure 11d) [86] is again a marginal application (2% of the total). Finally, it is worth noting two very peculiar applications. On January 2012, the Italian cruise ship Costa Concordia ran aground and overturned after striking an underwater rock close to the Isola del Giglio, Tuscany. The wreck came to rest on a rock ledge several months before its translation and decommissioning. In this period, an interferometric radar was installed over a rock of the island for monitoring possible displacements of the wreck [89]. The radar detection of the displacement of a pier pulled by a tugboat is the second noteworthy application [90]. These are the applications that we can find in the scientific literature, but they surely do not exhaust all the possibilities of GBRI. Just as an example of a possible interesting development, we can refer to some preliminary works on the use of radar for monitoring wind turbines [91,92]. The rotation of the blades gives a peculiar doppler signature (named “sinogram”); Li et al. [93] had the idea to apply the ISAR (Inverse Synthetic Aperture Radar) techniques for focusing the sinograms. In this way they obtained high-resolution radar images of the blades. A further step could be processing interferograms between different rotations for detecting the deformation of the blades under stress. Remote Sens. 2019, 11, x FOR PEER REVIEW 10 of 22

[87], so there is no demand for novel sensors; and dam safety, as well as mines, is a rather confidential issue, and the owners hardly agree to publish their radar data. Martinez‐Vazquez in 2005 [51] proposed the GBRI for assessing snow cover change (see Figure 11c), as it has already been done by satellite [88]. The technique’s results are effective with dry snow, while wet snow can be a problem [52]. Very interesting results have been obtained using the radar for mapping the avalanches on steep slopes [53]. Monitoring of vegetation or trees by GBSAR (see Figure 11d) [83] is again a marginal application (2% of the total). Finally, it is worth noting two very peculiar applications. On January 2012, the Italian cruise ship Costa Concordia ran aground and overturned after striking an underwater rock close to the Isola del Giglio, Tuscany. The wreck came to rest on a rock ledge several months before its translation and decommissioning. In this period, an interferometric radar was installed over a rock of the island for monitoring possible displacements of the wreck [89]. The radar detection of the displacement of a pier pulled by a tugboat is the second noteworthy application [90]. These are the applications that we can find in the scientific literature, but they surely do not exhaust all the possibilities of GBRI. Just as an example of a possible interesting development, we can refer to some preliminary works on the use of radar for monitoring wind turbines [91,92]. The rotation of the blades gives a peculiar doppler signature (named “sinogram”); Li et al. [93] had the idea to apply the ISAR (Inverse Synthetic Aperture Radar) techniques for focusing the sinograms. In this wayRemote they Sens. obtained2019, 11, 1029 high‐resolution radar images of the blades. A further step could be processing10 of 22 interferograms between different rotations for detecting the deformation of the blades under stress. The same technique could be used for monitoring any other turbines like steam turbines and turbofansThe same techniqueof airplanes. could be used for monitoring any other turbines like steam turbines and turbofans of airplanes. 4. From Step‐Frequency (SF) to Frequency Modulation (FM) 4. From Step-Frequency (SF) to Frequency Modulation (FM) The first terrestrial interferometric radar systems for monitoring landslides or large structures The first terrestrial interferometric radar systems for monitoring landslides or large structures used a Vector Network Analyzer (VNA) as transceiver [5], so they operated Step Frequency (SF) used a Vector Network Analyzer (VNA) as transceiver [5], so they operated Step Frequency (SF) waveforms (see Figure 12). waveforms (see Figure 12).

FigureFigure 12. 12. StepStep Frequency Frequency modulation. modulation.

TheThe main drawback ofof SFSF modulationmodulation is is that that before before changing changing frequency, frequency, it isit necessaryis necessary to waitto wait for forthe the time-of-flight time‐of‐flight relative relative to to the the signal signal from from the the farthest farthest target. target. This This is is not not a a problemproblem in short range range applications,applications, but but for for an an environment environment of of a a few few of of km (e.g., in landslide or mine monitoring), the the SF SF waveform has a sensible loss of efficiency efficiency (η𝜂) and increases of thethe minimumminimum sweepsweep timetime asas follow:follow:

(min) 2𝑅2Rmax T𝑇 = N𝑁 (4)(4) sweep 𝑐c f 𝑇(min) Remote Sens. 2019, 11, x FOR PEER REVIEW 𝜂1Tsweep 11 (5)of 22 η = 1 𝑇 (5) − Tsweep with Rmax distance of the farthest detectable target, c speed of light, Nf number of frequencies of the with Rmax distance of the farthest detectable target, c speed of light, Nf number of frequencies of the sweep, Tsweep sweep time. As an example, a SF radar with Rmax = 2000 m Nf = 5000, has (amin 𝑇) = 66 sweep, T sweep time. As an example, a SF radar with R = 2000 m N = 5000, has a T = 66 ms. ms. By acceptingsweep an efficiency of 50% (3dB of loss on Signalmax ‐to‐Noise Ratio),f it means asweep sweep time By accepting an efficiency of 50% (3dB of loss on Signal-to-Noise Ratio), it means a sweep time of of 133 ms, which could be too long in some applications. The evident solution is to use a frequency 133 ms, which could be too long in some applications. The evident solution is to use a frequency modulation (FM) transceiver [94]. The radar transmits a continuous ramp in frequency, and it is able modulation (FM) transceiver [94]. The radar transmits a continuous ramp in frequency, and it is able to detect the distance of a target by the frequency difference (∆𝑓) between the transmitted and the to detect the distance of a target by the frequency difference (∆ f ) between the transmitted and the received signal (see Figure 13). received signal (see Figure 13).

FigureFigure 13.13. FM modulation.

In this case the sweep time and the efficiency is given by:

2𝑅 𝑇 ≫ (6) 𝑐

𝜂1 (7) Therefore, the time sweep can be much shorter than SF and the efficiency is always close to 100%. Aguasca in 2004 [27] was the first to introduce FM modulation in GBRI, followed by Wadge in 2005 [82]. The current commercial radar interferometers of IDS geo‐radar [95] and meta‐sensing [96] are both FM systems.

5. Wider View Angle The key advantage of synthetic aperture is to reduce of a factor of two the size of the radar in comparison with the more conventional rotary radar [53]. This is probably the reason why these kinds of systems are the most popular operative modality. On the other hand, the drawback of a linear GBSAR is that its angular resolution (∆𝜑) degrades with the azimuth angle (𝜑). Indeed [97]: 𝜆 ∆𝜑 cos 𝜑 (8) _ 2𝐿 with 𝜆 wavelength and L length of the linear scan (see Figure 14a). This can be a problem in some applications, as an example, in the monitoring of open pits of circular shape when the radar is positioned close to the center and it is required to monitor the whole slope. The ArcSAR, recently developed [28,46,47], can be a solution for this kind of environment. The radar‐head of a ArcSAR is moving along an arc for synthesizing an aperture. A notable feature of the ArcSAR is that angular resolution (∆𝜑) is constant with azimuth is equal to [47] 𝐷 ∆𝜑 (9) 2𝑟

with D physical dimension of the rotating antenna and r0 radius of the ArcSAR (see Figure 14 b). This expression has to be compared with the standard formula for a rotary radar: 𝜆 ∆𝜑 (10) 𝐷

Remote Sens. 2019, 11, 1029 11 of 22

In this case the sweep time and the efficiency is given by:

2Rmax Tsweep (6)  c 2R 1 η = 1 max (7) − c Tsweep Therefore, the time sweep can be much shorter than SF and the efficiency is always close to 100%. Aguasca in 2004 [27] was the first to introduce FM modulation in GBRI, followed by Wadge in 2005 [82]. The current commercial radar interferometers of IDS geo-radar [95] and meta-sensing [96] are both FM systems.

5. Wider View Angle The key advantage of synthetic aperture is to reduce of a factor of two the size of the radar in comparison with the more conventional rotary radar [19]. This is probably the reason why these kinds of systems are the most popular operative modality. On the other hand, the drawback of a linear Remote Sens. 2019, 11, x FOR PEER REVIEW 12 of 22 GBSAR is that its angular resolution (∆ϕlinear_GBSAR) degrades with the azimuth angle (ϕ). Indeed [97]:

with 𝜆 wavelength (see Figure 14c). In the practiceλ ArcSAR allows to obtain better resolution ∆ϕ cos ϕ (8) with smaller antenna. Furthermore, its resolutionlinear_GBSAR does≈ 2L not depend on operative band. Both aspects are notable. with λ wavelength and L length of the linear scan (see Figure 14a).

Figure 14. Angular resolution of Linear GBSAR (a), ArcSAR (b), and rotary radar (c). Figure 14. Angular resolution of Linear GBSAR (a), ArcSAR (b), and rotary radar (c). This can be a problem in some applications, as an example, in the monitoring of open pits of 6. Multiple Input Multiple Output (MIMO) circular shape when the radar is positioned close to the center and it is required to monitor the whole slope.The The linear ArcSAR, GBSAR recently as well developed as any rotary [28,46 radar,47], can and be C a‐SAR solution need for the this physical kind ofmovement environment. of a radar The radar-headhead. The mechanical of an ArcSAR moving is moving parts along are the an most arc for prone synthesizing to failures, an aperture.especially A in notable systems feature that operate of the ArcSARoutdoors is for that long its angular periods. resolution This is the ( ∆caseϕArcSAR with) the is constant monitoring with of azimuth mines and and slopes. it is equal The phased to [47] array radar [98] is a possible solution. This technology is well‐known and widely used in the military field. D Its problem is its cost, which is too high for∆ ϕcivil applications. (9) ArcSAR 2r An emerging paradigm in the radar field is the≈ MIMO0 (multiple input multiple output). The idea is to use a set of NTX transmitting antennas and a separate set of NRX receiving antennas suitably positioned in space (see Figure 15).

Figure 15. MIMO working principle, NTX transmitting antennas with NRX receiving antennas are

equivalent to NTXNRX “virtual” antennas.

Remote Sens. 2019, 11, x FOR PEER REVIEW 12 of 22

with 𝜆 wavelength (see Figure 14c). In the practice ArcSAR allows to obtain better resolution with smaller antenna. Furthermore, its resolution does not depend on operative band. Both aspects are notable.

Remote Sens. 2019, 11, 1029 12 of 22

with D physical dimension of the rotating antenna and r0 radius of the ArcSAR (see Figure 14b). This expression has to be compared with the standard formula for a rotary radar:

λ ∆ϕrot (10) ≈ D with λ wavelength (see Figure 14c). In the practice ArcSAR allows to obtain better resolution with smaller antenna.Figure 14. Furthermore, Angular resolution its resolution of Linear doesGBSAR not (a), depend ArcSAR on(b), operativeand rotary radar band. (c). Both aspects are notable. 6. Multiple Input Multiple Output (MIMO) 6. Multiple Input Multiple Output (MIMO) The linear GBSAR as well as any rotary radar and C‐SAR need the physical movement of a radar head.The The linear mechanical GBSAR moving as well parts as any are rotary the most radar prone and C-SAR to failures, need especially the physical in systems movement that of operate a radar outdoorshead. The for mechanical long periods. moving This partsis the arecase the with most the prone monitoring to failures, of mines especially and slopes. in systems The phased that operate array radaroutdoors [98] for is a long possible periods. solution. This isThis the technology case with the is well monitoring‐known of and mines widely and used slopes. in the The military phased field. array Itsradar problem [98] is is a its possible cost, which solution. is too This high technology for civil applications. is well-known and widely used in the military field. Its problemAn emerging is its cost, paradigm which isin too the high radar for field civil is applications. the MIMO (multiple input multiple output). The idea isAn to emerging use a set of paradigm NTX transmitting in the radar antennas field is and the MIMO a separate (multiple set of inputNRX receiving multiple antennas output). Thesuitably idea positionedis to use a in set space of N TX(seetransmitting Figure 15). antennas and a separate set of NRX receiving antennas suitably positioned in space (see Figure 15).

Figure 15. MIMO working principle, N transmitting antennas with N receiving antennas are Figure 15. MIMO working principle, NTX transmitting antennas with NRX receiving antennas are equivalent to N N “virtual” antennas. equivalent to NTX×NRXRX “virtual” antennas. For each transmitting antenna, all the receiving antennas are working (simultaneously or sequentially), so the number of independent measurements is given by N = N N . The resulting TX × RX system is a radar equivalent to a phased array that needs much less transmitting/receiving channels. The first to propose this approach for a GBSAR was Hong in 2010 [99]. Tarchi et al. published in 2013 [20] a notable implementation of MIMO-GBSAR named MELISSA; 16 transmitting antennas operating with 16 receiving antennas, an equivalent aperture of 2.56 m and an acquisition time of 3.6 ms. This very high speed of acquisition opens new interesting perspectives to this kind of radar, especially in the dynamic monitoring of buildings, towers and bridges. The MIMO approach, a radar like MELISSA, is again an equipment rather complex and expensive, that hardly can find an industrial justification in the market of monitoring equipment. The working principle of any GBSAR (MIMO or not-MIMO) is based on dense spatial sampling (steps smaller than a quarter of wavelength) along the synthetic aperture. Compressive sensing (CS) is a recent sampling paradigm [100,101] which asserts that a certain signal can be recovered from far fewer samples or measurements than a traditional methods. Its basic idea relies on the ‘sparsity’ of Remote Sens. 2019, 11, x FOR PEER REVIEW 13 of 22

For each transmitting antenna, all the receiving antennas are working (simultaneously or sequentially), so the number of independent measurements is given by N = NTXNRX. The resulting system is a radar equivalent to a phased array that needs much less transmitting/receiving channels. The first to propose this approach for a GBSAR was Hong in 2010 [99]. Tarchi et al. published in 2013 [20] a notable implementation of MIMO‐GBSAR named MELISSA; 16 transmitting antennas operating with 16 receiving antennas, an equivalent aperture of 2.56 m and an acquisition time of 3.6 ms. This very high speed of acquisition opens new interesting perspectives to this kind of radar, especially in the dynamic monitoring of buildings, towers and bridges. The MIMO approach, a radar like MELISSA, is again an equipment rather complex and expensive, that hardly can find an industrial justification in the market of monitoring equipment. The working principle of any GBSAR (MIMO or not‐MIMO) is based on dense spatial sampling (steps smaller than a quarter of wavelength) along the synthetic aperture. Compressive sensing (CS) Remoteis a recent Sens. 2019 sampling, 11, 1029 paradigm [100,101] which asserts that a certain signal can be recovered from13 of 22 far fewer samples or measurements than a traditional methods. Its basic idea relies on the ‘sparsity’ of thethe signals signals of of interest interest (the (the radar radar signalssignals typicallytypically have this this property property [102]), [102]), and and the the incoherence incoherence of of the thesensing sensing modality. modality. The The latter latter property property is is obtained throughthrough randomrandom sampling. sampling. The The combination combination of of MIMOMIMO and and Compressive Compressive Sensing Sensing appear appear very very attractive, attractive, i.e., i.e., the the Compressive Compressive Sensing Sensing can reducescan reduces of aboutof about 40% 40% the numberthe number of channels of channels of a of MIMO a MIMO used used in GBSARin GBSAR applications applications [103 [103].]. In In 2018, 2018, Pieraccini Pieraccini andand Miccinesi Miccinesi [50 [50]] designed designed a Compressivea Compressive Sensing Sensing MIMO MIMO with with four four transmitting transmitting antennas antennas and and four four receivingreceiving antennas antennas that that have have been been successfully successfully used used for for testing testing a pedestriana pedestrian bridge. bridge.

7.7. Detection Detection of of the the Displacement Displacement Vector Vector TheThe interferometric interferometric radar radar detects detects only only the the displacement displacement component component along along the the view view direction. direction. TheThe real real displacement displacement (of (of slope, slope, building, building, bridge, bridge, tower, tower,... …)) is is retrieved retrieved with with some some hypothesis hypothesis about about motion.motion. For For example, example, in in a a slope, slope, the the displacement displacement can can be be supposed supposed along along the the gradient, gradient, in in the the lower lower deckdeck of of a bridgea bridge along along the the vertical vertical axis axis and and in in a towera tower along along the the horizontal. horizontal. TheseThese suppositions suppositions can can be be reasonable reasonable inin mostmost cases, but but there there are are important important exceptions. exceptions. As As Dei Deiet etal. al. in 2013 in 2013 [104] [104 demonstrated,] demonstrated, some some targets, targets, that are that not are at the not center at the of centerthe span, of have the span,a component have aof component horizontal of displacement horizontal displacementduring static monitoring during static of bridges. monitoring If this of horizontal bridges. displacement If this horizontal is not displacementconsidered, the is not results considered, are absurd the (a results deck arethat absurd rise when (a deck it is loaded). that rise Figure when it16 is shows loaded). how Figure this effect 16 showsis in the how relationship this effect with is in the the thickness relationship of the with deck the and thickness the position of the of deck its neutral and the axes. position of its neutral axes.

Figure 16. Effective displacement of a point P at an edge and close to the central span during the static test of a bridge (after [104]).

TheFigure case 16. of slopeEffective monitoring displacement can be of morea point complex. P at an edge For and example, close to for the detecting central span the during real displacement the static test of a bridge (after [104]). pattern of Lamosano village in Italian Alps, built on a landslide, it has been necessary to combine data from both interferometric radar and lidar [62]. Unfortunately, this complex behavior of landslides is The case of slope monitoring can be more complex. For example, for detecting the real rather common. displacement pattern of Lamosano village in Italian Alps, built on a landslide, it has been necessary In order to obtain more than one component of the displacement vector, in 2014, Severin et al. [105] to combine data from both interferometric radar and lidar [62]. Unfortunately, this complex behavior used two GBSAR systems in different positions that monitor the same slope. In 2015, Zeng et al. [106] of landslides is rather common. proposed the same principle using two MIMO. Two radar systems operating independently can be a solution, but surely is expensive and not practical. Pieraccini et al. [30], in 2017, proposed a bistatic configuration [107] that uses the bounce of the signal on a transponder for obtaining an independent component of the displacement (see Figure 17). Remote Sens. 2019, 11, x FOR PEER REVIEW 14 of 22

In order to obtain more than one component of the displacement vector, in 2014, Severin et al. [105] used two GBSAR systems in different positions that monitor the same slope. In 2015, Zeng et al. [106] proposed the same principle using two MIMO. Two radar systems operating independently can be a solution, but surely is expensive and not practical. Pieraccini et al. [30], in 2017, proposed a bistatic configuration [107] that uses the bounce of the signal on a transponder for obtaining an independentRemote Sens. 2019 component, 11, 1029 of the displacement (see Figure 17). 14 of 22

Figure 17. Bistatic GBSAR with transponder (after [30]). Figure 17. Bistatic GBSAR with transponder (after [30]). Other solutions are effective only in short range; Hu et al. [108] proposed to divide the scan in two sub-aperturesOther solutions for obtaining are effective two points only in of short view range; with the Hu drawback et al. [108] to proposed reduce the to angular divide the resolution; scan in twoPieraccini sub‐apertures et al. [109 ]for proposed obtaining to use two the points rotation of of view the antenna with the in adrawback vertical plane to reduce for obtaining the angular two or resolution;three components. Pieraccini They et al. tested [109] the proposed technique to duringuse the the rotation static testof the of aantenna bridge inin Florence,a vertical Italy plane [110 for]. obtaining two or three components. They tested the technique during the static test of a bridge in Florence,8. Higher Italy Operative [110]. Frequency

8. HigherAs mentioned Operative above, Frequency C-band (4–8 GHz), X-band (8–12 GHz), amd Ku-band (12–18 GHz) all together cover about 89% of the publications. Radars at higher operative frequency are only a few.As Nevertheless, mentioned above, the interest C‐band in these(4‐8 GHz), high frequencyX‐band (8‐ (especially12 GHz), amd W-band) Ku‐band is increasing. (12‐18 GHz) Many all togethersemiconductor cover about producers 89% areof the developing publications. a low-cost Radars W-band at higher radar operative for automotive frequency applications. are only a These few. Nevertheless,small radar-heads the couldinterest have in a these great impacthigh frequency on radar GBRI. (especially The high W frequency‐band) is allowsincreasing. us to designMany semiconductorsmall size radar producers systems with are the developing same angular a low resolution,‐cost W‐ butband with radar much for better automotive range resolution applications. with Theserespect small to the radar current‐heads interferometric could have a radargreat systemsimpact on [31 radar,32,111 GBRI.,112]. The Nevertheless, high frequency radars allows operating us to designat these small high frequenciessize radar systems can have with problems the same of electronic angular and resolution, mechanical but stability with much which better are hard range to resolutionaddress. Just with as anrespect example, to the a Ku-band current radarinterferometric is sensible radar to displacements systems [31,32,111,112]. of about 0.1 mm, Nevertheless, so it needs radarsa mechanical operating moving at these system high with frequencies a positioning can have accuracy problems of this of order electronic of magnitude. and mechanical A W-band stability radar whichis 4.5 times are hard more to sensible,address. soJust it as needs an example, a mechanical a Ku‐ systemband radar able is to sensible warranty to 20–25displacementsµm, that of is aabout very 0.1hard mm, specification so it needs for aa systemmechanical that has moving to operate system in the with field. a positioning Another important accuracy point of this about order the use of magnitude.of W-band radar A W is‐band the highradar attenuation is 4.5 times of themore atmosphere sensible, so at theseit needs frequencies, a mechanical which system confines able these to warrantyradars to short-range20–25 𝜇m, that applications. is a very hard specification for a system that has to operate in the field. Another important point about the use of W‐band radar is the high attenuation of the atmosphere at these9. Three-Dimensional frequencies, which Imaging confines these radars to short‐range applications. The idea of obtaining three-dimensional radar images using a GBSAR started with this radar 9. Three‐Dimensional Imaging itself. Already in 2000 Lopez-Sanchez [113] described a SAR migration algorithm for focusing three-dimensionalThe idea of obtaining images inthree an‐ anechoicdimensional chamber. radar Inimages the same using year, a GBSAR Tarchi etstarted al. [114 with] developed this radar a itself.GBSAR Already that obtains in 2000 a 3DLopez image‐Sanchez of the [113] reproduction described (in a SAR scale migration 1:2) of an ancientalgorithm façade. for focusing three‐ dimensionalGenerally images speaking, in an anechoic complex chamber. targets like In the buildings, same year, tunnels, Tarchi and et al. construction [114] developed works a GBSAR would thatrequire obtains 3D imaging a 3D image capability, of the reproduction but there are (in two scale practical 1:2) of problems. an ancient The façade. first is the long acquisition time. With reference to Figure 18, in order to image in the three-dimensional space using a liner SAR, the radar head must scan step-by-step and line-by-line a planar surface. As an example, a fast GBSAR able to scan a line of 100 points in 5 min [115], will scan a plane of 100 lines in more than 8 h. It means that the radar is able to detect only very slow movements. The second problem is the size and bulk of the mechanical system, which makes its deployment hard. Remote Sens. 2019, 11, x FOR PEER REVIEW 15 of 22

Generally speaking, complex targets like buildings, tunnels, and construction works would require 3D imaging capability, but there are two practical problems. The first is the long acquisition time. With reference to Figure 18, in order to image in the three‐dimensional space using a liner SAR, the radar head must scan step‐by‐step and line‐by‐line a planar surface. As an example, a fast GBSAR able to scan a line of 100 points in 5 minutes [115], will scan a plane of 100 lines in more than 8 hours. It means that the radar is able to detect only very slow movements. The second problem is the size Remoteand bulk Sens. 2019of the, 11 mechanical, 1029 system, which makes its deployment hard. 15 of 22

Figure 18. GBSAR with 3D imaging capability. Figure 18. GBSAR with 3D imaging capability. These are the main reasons why GBSARs with 3D imaging capability are not very popular. TheThese use are of the higher main frequency reasons why (e.g., GBSARs W-band) with mitigates 3D imaging the problem capability of the are bulky not very equipment, popular. and partiallyThe even use of the higher problem frequency of acquisition (e.g., W‐ time,band) as mitigates the movements the problem are smaller of the bulky and, therefore, equipment, faster. and Thepartially radar even HYDRA the problem [32] is a step of acquisition in this direction. time, as the movements are smaller and, therefore, faster. The Aradar more HYDRA radical [32] solution is a step is a two-dimensionalin this direction. MIMO [116], preferably based on the compressive sensingA more approach radical [117 solution]. These is are a two exciting‐dimensional developments, MIMO but [116], they preferably are again prototypesbased on the in compressive a laboratory andsensing not suitableapproach to be[117]. used These in the field.are exciting developments, but they are again prototypes in a laboratory and not suitable to be used in the field. 10. Conclusions 10. Conclusions In this paper the authors reviewed the vast bibliography related to ground-based/terrestrial radar interferometryIn this paper by considering the authors 415 reviewed papers fromthe vast 1993 bibliography to 2018. This researchrelated to shows ground that‐based/terrestrial the linear SAR isradar the most interferometry common system, by considering followed 415 by papers the no-DOA from 1993 systems. to 2018. The This majority research of shows radar systems that the worklinear inSAR Ku-band. is the most More common than 40% system, of case followed studies are by relativethe no‐DOA to landslides systems. followed The majority by bridges of radar (30%) systems and buildingswork in Ku (20%).‐band. More than 40% of case studies are relative to landslides followed by bridges (30%) and Whatbuildings we can (20%). now ask ourselves is, where is GBRI going? We can affirm that the main development directionsWhat are we forward can now faster ask radar ourselves (from SFis, modulationwhere is GBRI to FM going? modulation), We can radar affirm with that wider the viewmain angledevelopment (using ArcSAR directions configurations), are forward MIMO,faster radar radar (from with SF capability modulation to detect to FM the modulation), vector of displacement radar with andwider not view only angle a single (using component, ArcSAR and configurations), small size radars MIMO, operating radar with at higher capability band (into detect Ka or Wthe band). vector of displacementCurrently FM and modulation not only a single and ArcSAR component, appear and already small size mature radars for operating the market at higher of monitoring band (in equipment.Ka or W band). Some manufacturers have products with these characteristics. MIMO GBSAR is at a good levelCurrently of industrialization FM modulation and probably, and ArcSAR in a short appear time, already we will mature see its for di fftheusion. market The of detection monitoring of theequipment. displacement Some vector manufacturers is again athave the products level of prototypes,with these characteristics. but its usefulness MIMO is evident, GBSAR is and at a it good will probablylevel of industrialization soon be established and as probably, a standard in a technique. short time, Finally, we will interferometric see its diffusion. radar The in detection Ka or W bandof the aredisplacement again laboratory vector prototype, is again at but the we level think of they prototypes, will have but a role its in usefulness the future, is as evident, well as radars and it with will 3Dprobably imaging soon capability. be established as a standard technique. Finally, interferometric radar in Ka or W band A final question we can address is about the long-term developments of radar interferometry. As mentioned in the introduction, the strength of GBRI is its complementarity with spaceborne SAR. Nevertheless, there is again a gap between GBRI and spaceborne SAR, that is only partially filled by airborne radars. An exciting new field of application is the use of small UAVs (unmanned aerial vehicles) as radar platforms. Some preliminary results appear very promising [118–120], but the real challenge for doing interferometry from a UAV is the accuracy of the flight trajectory that is again too poor for these kinds of applications. Remote Sens. 2019, 11, 1029 16 of 22

Author Contributions: Methodology, M.P. and L.M.; Writing—Original Draft Preparation, M.P.; Writing—Review & Editing, L.M. Funding: This research received no external funding. Acknowledgments: We would like to thank the anonymous reviewers whose comments have greatly improved this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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