Radar imaging [1
] represents a powerful tool in several applications, such as security, surveillance, and environmental monitoring, with particular emphasis on disasters and crisis management [2
]. In this frame, radar systems mounted onboard aerial platforms, such as airplanes [5
], helicopters and drones [9
], are gaining increasing interest due to their features allowing to overcome several limitations of radar systems mounted onboard terrestrial vehicles (carts, cars, ground track rails) [10
] or spaceborne platforms [12
More specifically, if compared to the terrestrial imaging radar systems, such as the conventional Ground Penetrating Radar (GPR) [10
] or the Ground Based (GB) Synthetic Aperture Radar (SAR) [11
], the aerial imaging radar systems are flexible and able to ensure a wider spatial coverage. Furthermore, in critical scenarios, as the ones induced for instance by natural disasters (Earthquakes, volcanoes’ eruptions, flooding, rapid landslides), ungentle climatic conditions (cryosphere monitoring) or anthropic actions (ordnance deployment), they allow safe monitoring of areas that would be difficult to reach (if not unreachable) through terrestrial systems.
On the other side, in contrast to the satellite platforms [12
], the aerial ones allow to timely reach the area of interest, to fly practically along any direction, and to keep very short the so called revisiting time, that is, the time interval elapsing between subsequent observations of the same area.
In addition, the aerial radar systems are relatively cheap, thus offering the appealing opportunity of assessing the potentialities of novel technologies and/or measurement modalities, before these are made operative on expensive spaceborne missions.
In this regard, the increasing interest registered in the last years toward aerial radar systems operating at low frequencies has been in some extent driven by the ongoing or future spaceborne missions. Indeed, the exploitation of GPR systems for the ongoing planetary exploration missions [15
] has certainly taken benefit from the results obtained through the several experimental campaigns carried out in the last years with aerial penetrating radar (usually named Sounder) systems operating in the HF, UHF and VHF bands [20
]. For the same reason, in anticipation of the forthcoming spaceborne ESA-Biomass mission [34
], increasing of the number of aerial P-Band SAR missions is registered in the last years [35
In this context, the Italian Space Agency (ASI) has recently funded the development of an aerial multi-mode pulsed imaging radar system operating in a multi-frequency modality in the UHF and VHF bands. In particular, the system is able to work either as Sounder or as full polarimetric SAR. The development of this system is aimed at making available to the Italian community of researchers and end-users an aerial imaging radar system with several attractive features. First, it grows the relatively small family of available aerial SAR [35
] and Sounder [23
] systems operating in the UHF and VHF bands. Secondly, the system allows to easily collect over the same area radar data characterized by different scattering mechanisms (since it may work either as SAR or as Sounder), at different carrier frequencies and (for the SAR modes) with a diversity in the polarization.
The overall system has been originally developed by CO.RI.S.T.A. [47
] according to an ASI funding set up in 2010. After, the first version of the system has been upgraded, again by CO.RI.S.T.A., in the frame of a contract signed in 2015 between ASI, CO.RI.S.T.A. and different public Italian Research Institutes and Universities, namely IREA-CNR, Politecnico di Milano and University of Trento, which have been entrusted with the processing of the data acquired by the radar. In particular, they have designed ad-hoc strategies to process the both Sounder and SAR data. These strategies exploit the navigation information acquired during the flight and adopt model-based procedures, which are the microwave tomographic imaging for the Sounder and the back-projection procedure for the SAR.
With the aim of obtaining a first assessment of the performances of this radar system, a helicopter-borne campaign has been conducted in 2018 over a desert area in Erfoud, Morocco. During the campaign, several tracks have been flown exploiting all the three different operational modes of the system. In this work, we present first results relevant to a small subset picked up from the huge dataset collected by the system during this campaign.
The work is organized as follows. In Section 2
we provide a brief description of the system. The acquisition campaign is described in Section 3
. The processing chain applied to the Sounder data is illustrated in Section 4
along with some first results. Similarly, the SAR data processing chain is described in Section 5
along with the corresponding first results. Section 6
is devoted to the concluding remarks.
2. System Description
The radar system exploits the pulsed radar technology and can operate at different carrier frequencies as Sounder and Synthetic Aperture Radar (SAR). More specifically, the Sounder operates at 165 MHz, whereas the SAR may operate either at 450 MHz (SAR-Low mode) or at 860 MHz (SAR-High mode). Summing up, three operational modes are enabled, namely, Sounder, SAR-Low and SAR-High, each working at a different frequency.
The bandwidth of the transmitted (chirp) pulses is 40 MHz, leading to a (slant) range resolution of 3.75 m (in air). Moreover, in the SAR-High mode, through the stepped chirp technology [60
], an overall bandwidth of 80 MHz can be reached, that is, a (slant) range resolution of 1.87 m. The SAR system is full polarimetric thanks to two separate receiving channels.
The overall system basically consists of a radar module along with three different antennas, one for each operational mode.
The radar module consists of three main blocks, namely, the Radar Digital Unit (RDU), the Radio Frequency Unit (RFU) and the Power Supply Unit (PSU). In particular, the RDU is fully programmable and allows the parameters setting, the timing generation and the data handling. It also includes the Analog to Digital Converter (ADC) and the data storage unit. The RFU embeds the frequency generation unit (which generates all the synchronization and radio frequency signals) and the chirp generator unit (which generates the low frequency modulated chirp signal by means of the digital direct synthesis technology). RFU also includes the high power amplification unit, which is based on the solid state technology, and an antenna front-end that allows the correct switching among the transmitted and received signals and among the different polarizations of the SAR mode. The PSU provides the power supply to whole system by an external 28 V DC voltage.
The system is completely stand-alone: the power supply connector is the only electrical interface. Most radar modules are shared by the three different operational modes of the system. In particular, base band signal generation, base band data sampling and data handling are common to the Sounder and the SAR modes.
As noted above, different antennas are used for the three different operational modes of the system. In particular, for the Sounder mode, a low-gain log-periodic antenna is deployed; the antenna points to the nadir and radiates the main beam with Half Power Beam Width (HPBW) [61
] of 68° and 50° in the range and azimuth directions, respectively, and with a (maximum) gain [61
] of 7dBi. Exploitation of the overall azimuth aperture thus can lead to an azimuth resolution of about 1m [62
]. Two different side-looking arrays of micro-strip antennas are instead employed for the two different SAR modes [64
]. In particular, these two SAR antennas have the same shape and size, to allow an easier swap when they cannot be installed simultaneously on the airplane/helicopter. More specifically, the array used for the SAR-Low mode consists of 4 patches deployed along the azimuth direction: its (maximum) gain is 11 dBi and its HPBWs are 75° and 20° in the range and azimuth directions, respectively. Exploitation of the overall azimuth aperture thus leads to an azimuth resolution of about 1m [62
]. For the SAR-High mode (which operates at a carrier wavelength scaled about of a factor two respect to the SAR-Low mode), a planar array antenna of 8 × 2 patches is deployed, with the same overall dimensions of the array used for the SAR-Low mode. The corresponding HPBWs are thus reduced of a factor two (in both directions) with respect to the SAR-Low mode. This allows increasing the (maximum) antenna gain of 6 dBi, retaining practically the same achievable azimuth resolution obtainable with the SAR-Low mode. Both SAR antennas are dual-polarized, to enable the full polarimetric capability of the SAR modes.
Currently, during the flight the user can select manually one of the three operational modes of the system. At present, the different operational modes cannot be used simultaneously; development of a synchronization solution aimed at circumventing this limitation is however straightforward and is matter of current activities.
In order to achieve accurate flight information, necessary for the reliable processing of both Sounder and SAR data, the radar encompasses a navigation unit consisting of an Inertial Navigation System (INS) that embeds a Global Positioning System (GPS). In particular, the INS is a MTi model of Xsense Technologies B.V., which is an inertial measurement unit with integrated 3D magnetometers (3D compass), with an embedded processor capable of calculating roll, pitch and yaw in real time, as well as outputting calibrated 3D linear acceleration, rate of turn (gyro) and (Earth) magnetic field data. The INS device integrates a GPS receiver and the related information, position, velocity and time (PVT). The navigation unit is directly connected to the radar central unit by means of an USB interface; all the navigation data are synchronized with the radar pulses and embedded in its output data. Furthermore, two additional GPS devices have been added to allow differential processing of the positioning data. The two GPS receivers are based on M8T chip series of U-blox, one installed onboard, close to the SAR antenna, and the other on the ground in the range of some kilometers from the flying radar. During the calibration campaigns of the radar, differential processing of the positioning data, carried out with the RTKLIB library, allowed a centimetric positioning accuracy.
The overall system is easy to be transported and installed onboard relatively small airplanes or helicopters. Indeed, the radar module is stowed in a rack, which is quite compact and with dimensions of 50 cm × 50 cm × 65 cm, for a weight of about 30 kg.
During the campaign, the radar system was installed on an Eurocopter AS-350 series helicopter (see Figure 1
a). In particular, the radar electronics rack was accommodated in the helicopter cabin, in place of the two rear passenger seats (see Figure 1
b). The sounder antenna is installed on the helicopter nose (see Figure 1
c) through the mechanical framework normally used for handling the rear-view mirror; moreover, it is fixed to the front glass and to the fuselage by means of a standard adapter. The SAR antennas are installed through a certified framework normally used for side-looking cameras and by using a honeycomb panel and some brackets (see Figure 1
d, where just one SAR antenna is shown).
reports the main electrical, mechanical and geometrical parameters of the system.
3. Campaign Description
The flight campaign was carried out in May 2018 over an arid region located in southeastern Morocco, around the city of Erfoud at the northern edge of the Sahara Desert. Several tracks, covering different test sites, have been flown by exploiting all the three available radar acquisition modes. As observed above (see again Figure 1
), the radar system was installed onboard an Eurocopter AS-350 series helicopter, and it was possible to embark onboard simultaneously the Sounder antenna and only one SAR antenna. Exploitation of the two different SAR modes has thus required during the mission the swap of the two different SAR antennas.
In order to minimize the effects of structural vibration noise, during the campaign the INS has been placed inside the helicopter cabin (as close as possible to the SAR antenna) on the floor, by using vibration absorbers.
shows all the tracks flown during the campaign, superimposed to an optical image of the overall test area. In particular, the red, green and blue tracks are relevant to the acquisitions carried out with the Sounder, SAR-Low and SAR-High modes, respectively. As can be seen, the flight tracks were flown basically over four different test sites that in the following are named TS1, TS2, TS3 and TS4, respectively. More specifically, the test site TS1, which is located in proximity of the cities of Errachidia and Erfoud, covers an aquifer located in cretaceous rocks at a depth ranging approximatively between 100 m and 200 m. The test sites TS2 and TS3 are both located in proximity of the city of Rissani. In particular, TS2, which belongs to the great Basin Ziz-Rheris, is an almost flat region characterized by alluvial sediments with the presence of aquifers at depth varying approximatively between 20 m and 40 m, whereas TS3 includes the archeological area of Sijilmasa. The test site TS4 is located in proximity of the Erg Chebbi and is characterized by the presence of several dunes and covers an aquifer located at a depth ranging approximatively between 150 m and 200 m.
6. Conclusions and Further Developments
In this work, we have presented a recently developed aerial imaging radar system operating in the UHF and VHF bands as Sounder and as full polarimetric Synthetic Aperture Radar (SAR). More specifically, three operational modes are possible: Sounder, SAR-Low and SAR-High, each working at a different frequency.
To obtain a first evaluation of the potentialities of the system, a helicopter-borne campaign has been conducted in May 2018 over an arid region located in southeastern Morocco, around the city of Erfoud at the northern edge of the Sahara Desert. Several tracks, covering four different test sites (denoted as TS1, TS2, TS3 and TS4), have been flown by exploiting all the three available radar acquisition modes. From the huge dataset collected during the campaign, we have processed a small subset and presented first results.
In particular, to show the potentialities of the Sounder system, we have presented some representative results relevant to the TS1, TS2 and TS4 sites. To focus the data, we have applied a tomographic reconstruction approach capable to deal with the actually flown flight tracks by exploiting the information provided by the INS-GPS system mounted onboard the helicopter. To check the accuracy of the obtained results, the focused images have been compared to the Sounder-to-ground distance evaluated by exploiting the INS-GPS system and the external SRTM DEM. A good agreement between expected and obtained results has been achieved.
To show the potentialities of the SAR system, we have shown some representative results relevant to the SAR-Low mode data acquired over the TS2 and TS4 sites. More specifically, to focus the data, we have applied a Back Projection approach operating in time domain and capable of exploiting the information provided by the INS-GPS system mounted onboard the helicopter and the external SRTM DEM of the observed area. To show the potentialities of the system related to its full-polarimetric capability, we have considered one radar acquisition carried out over the TS2 site and shown the correlation coefficients between the polarimetric channels as well as the obtained Pauli decomposition. It has been shown that these polarimetric products well match the scattering behavior expected for the observed area (which can substantially be considered as a rough bare soil in the presence of a limited number of well confined man-made targets). To show the interferometric potentialities of the system, we have considered three repeat pass acquisitions carried out over the TS4 site and shown the obtained interferograms along with the corresponding coherence maps. In particular, it has been shown that the obtained interferometric coherence is quite good, but for the near range areas and some well confined azimuth strips, due to the spatial decorrelation effects induced by the large spatial baselines generated by the severe track deviations of the helicopter during the radar acquisitions. By the way, in the high coherence regions, which represent the large part of the observed area, interferometric fringes are well visible.
Summing up, first results relevant to both the Sounder and the SAR modes are promising. Of course, further activities aimed at assessing the full capabilities of the system are planned for the next months.
In particular, full exploitation of the available SAR polarimetric channels in order to obtain added value products such as soil moisture as well as surface roughness maps [80
] is matter of current investigation. Beside, acquisition of massive repeat pass interferometric data-sets is planned for the very near future, in order to fully exploit the capabilities of the UHF and VHF bands for the retrieval of the terrain topography below dense forests [81
] or the subsurface structure over areas covered by snow/ice [42
] through advanced tomographic SAR processing techniques. Moreover, deep interpretation of the nature of subsurface returns visible in the focused Sounder images is subject of investigation and interpretation also aided by electromagnetic simulations of surface clutter. In the meantime, processing of the entire dataset acquired during the Morocco campaign is matter of current study and future work.