Field of the invention

[001] The present invention refers to obtaining topographic ground data and possibly the height of its vegetation cover, particularly by means of airborne radar surveys and, more particularly, by radars carried by unmanned aerial vehicles, also called drones.

Background of the invention

[002] Interferometry is a technique that, applied to synthetic aperture radars, are capable of providing digitized topographic models. This technique is based on the use of properties related to the coherence of electromagnetic radiation, to compare two or more return signals (echoes) of the same region captured by antennas displaced from each other in an orthogonal direction to the path of the aircraft. By calculating the phase difference of these echoes, the topographic and altimetric data of the investigated region are determined.

[003] One of the ways to obtain this data is to perform several flights over the concerned region, in said flights being parallel to each other and laterally displaced. Another way, called single -pass interferometry is based on a single flight employing two physically separated antennas on the same platform. This is a cheaper and less time-consuming surveying process.

[004] Fig. 1 depicts a known system, called GeoSAR, where two sets of P-band radar antennas, 11 and 12, were installed at the wings ends of an aircraft 10 flying at 10,000 meters. The B-separation between the antenna sets, designated as the baseline, is 20 meters. [005] The transmitting antenna 11 emits a beam of electromagnetic radiation 15 that illuminates an area of ground 13. The first set of return signals 16 (echo) is picked up by antenna 11 and the second set of return signals 17 is picked up by the receiving antenna 12 at the opposite end of the baseline.

[[006] The system shown in Fig. 1 also comprises a second set of antennas, composed of a transmitter 12, which illuminates a ground area 14, wherein the echoes are captured by receiving antennas located at 12 and 11.

[007] A system such as that exemplified in Fig. 1 has a relatively high cost, as it requires a medium-sized aircraft, specialized crew and flight authorization. This cost makes it impossible to use this technique in the case of small organizations and/or the survey of small plots, of the order of a few square kilometers. Considering an area measuring 100 km by 100 km, a 12 kilometer- weide swath at each side of the aircraft, a 10-kilometer separation between the right and left strips, an overlap of 2 kilometers and flight lines 100 kilometers long, the aircraft 10 would need only 5 flight lines of 100 km in length to cover the whole area. In case of one-sided illumination, i.e. only one swath either on the left or on right side, the survey would require ten such flight runs. In case of one-sided illumination but with only one P-band antenna, there would be needed 20 runs 100 kilometers long to cover the entire area. The flight time required to cover this area would almost double for one-sided illumination and almost quadruple for only one P-band antenna. In this latter case, there would be needed an extremely precise flight control system, because separation B should to be maintained within a few meters.

[008] One way to reduce the cost of such a survey is through the use of radars carried by unmanned aerial vehicles (drones) that allow carrying out surveys of areas of the order of a few square kilometers, providing digital surface models of the terrain as well as of the vegetation cover. Brazilian Patent application BR10201701 3226-9 titled “Radar for Cartography and Monitoring” describes a millimetre wave radar of small size and weight, carried by a drone, which uses the Synthetic Aperture Radar technique (SAR) in combination with Doppler processing to compensate for irregularities in aircraft orientation during flight produced by crosswinds of variable intensity, a common occurrence in the drone's operating altitudes, requiring a continuous adjustment of the yaw angle.

[009] The unmanned aerial vehicle described in said patent application is a fixed- wing drone, flying at a typical speed of lOOkm/h, therefore requiring a runway for takeoff and landing, as happens with all fixed-wing aircraft. In the system object of said application, the process of determining the surface model of the vegetation canopy uses two antennas operating in the C band. The determination of the ground surface model employs two antennas in the P band vertically or horizontally spaced. In the latter arrangement, the baseline required by the interferometric technique is brought about by placing the two antennas at the ends of the drone's wings.

[010] Such a fixed wing unmanned aerial vehicle is difficult to transport in vehicles such as cars or even pickup trucks. Moreover, the need for a runway, even if of small length, makes it difficult to use in rough terrains. Moreover, the above mentioned patent application does not disclose any means for determining the profile of the ground in places covered with dense tropical forests, such as the Amazon.

Objects of the invention

[011] In view of the above, the present invention has the object of providing a low-cost system for surveying areas of the order of a few square kilometers or smaller mainly covered by vegetation, where optics are prevented from determining the height of the forest canopy accurately and with low cost.

[11] Another object is to dispense specialized crews to operate the system. [013] Another object is to dispense with the need for official authorization for the surveying flights.

[014] Another aim of the invention is to provide a system that, in addition to being easy to transport, dispenses with the need for runways.

[015] Another goal is to allow the determination of the terrain height when covered with vegetation or industrial forests of small density, by means of a single pass of the drone over the surveyed area.

[016] Another goal is to determine the precise ground profile when covered with dense and natural forests such as the Amazon rainforest or secondary Amazon forests, also using a single pass.

[017] Another object is reducing in half the number of flight lines, by means of increasing the system complexity.

[018] Another goal is to allow the reduction of the system’s size, compacting the drone itself as well as the radar antennae framework, in order to enable its accommodation in a box of manageable size and easy to transport.

[019] Another goal is to allow the compacted system to be manually assembled and disassembled in the field without the use of tools.

[020] Another goal is to provide a system formed by the drone and radar whose overall weight falls within class 3, whose maximum weight is 25 kg, preferably not exceeding 15 kg.

Summary of the invention

[021] The above objects are achieved by providing a radar system carried by an unmanned aerial vehicle - drone - comprising an articulated supporting framework which is compacted for transportation and unfolded for use, said framework supporting a first set of two P-band antennas mutually spaced so as to form a baseline, where said antennas are associated with an interferometric synthetic aperture radar set - InSAR.

[022] According to another feature of the invention, said system can also comprise radar sets in the C- bands, L-band and other bands.

[023] According to another feature of the invention, the determination of height over ground with a single pass is provided by the use of an interferometric synthetic aperture radar set - InSAR - comprising two P-band antennas mutually spaced so as to form a baseline orthogonal to the drone’s flight direction. The effective baseline is the projection of the distance between said antennas over a plane at a right angle to the radar antennas line-of-sight.

[024] According to another feature of the invention, the determination of the ground profile of the ground covered by low-density vegetation, consisting of man-made forests or low-density forests is made with the P-band antennas operating with horizontal polarization.

[025] According to another feature of the invention, the determination of the ground profile covered by dense vegetation, tropical or secondary forests such as the Amazonians, is made with the P-band antennas operating with both horizontal and vertical polarizations.

[026] According to another feature of the invention, the reduction in half the number of flight lines is provided by a second InSAR radar set comprising two antennas in the P-band, displaced 180 degrees in relation to said first set, so as to allow the simultaneous surveying of two strips of terrain, symmetrically disposed in relation to the flight path of the drone.

[027] According to another feature of the invention, said supporting framework, when compacted by folding its hinged structural elements, allows fitting the system inside an easily transportable box of manageable size and weight. [028] According to another feature of the invention, said structural elements comprise rigid hollow bars incorporating hinges equipped with manual locking devices, avoiding the need for tools in the assembling or disassembling of the system between the transportation condition and the condition of use or vice versa.

[029] According to another feature of the invention, the small weight of both unilateral and bilateral radars, not exceeding, respectively 3.5 kg and 5 kg, is provided by the use of lightweight materials.

Brief description of the drawings

[030] Further features and advantages of the invention will become more evident through the description of exemplary embodiments, in which:

[031] Figure 1 shows a state of the art system.

[032] Figure 2 is a side elevation view of the invention's system.

[033] Figure 3 is a perspective view of the invention’s system.

[034] Figure 4 shows the invention’s system in the compacted condition allowing its transportation in a box of manageable dimensions, generally smaller than 1 meter.

[035] Figure 5 is a plan view showing a second embodiment of the invention, where two InSAR radar sets are provided, each one comprising two antennas in the P-band, and disposed in opposite directions in order to sweep two strips symmetrically located in relation to the drone flight path.

[036] Figure 6 shows the system of the previous figure when compacted for packaging and transport.

[037] Figure 7 is a perspective view of the system shown in Figure 5. [038] Figure 8 is a detail of the manual locking device in the hinges provided in the structural elements of the framework supporting the antennas.

Detailed description of preferred embodiments

[039] Referencing, now, figures 2 and 3, the system 20 comprises an unmanned aerial vehicle — drone — comprising a supporting framework structure formed by rigid bars, which support a first set of antennas 21, 22 of an interferometric synthetic aperture radar - InSAR - operating in the P-band, keeping them oriented at an angle a to the horizontal direction, and separated by a distance B, which constitutes the baseline of the InSAR, said baseline being perpendicular to the flight path of the drone.

[040] As shown in the figures, which show the system in the condition of use, the distance B between the antenna centers is the sum of the lengths of bars 23a, 23b, 23a’ and 23b’, which are interconnected by the hinges 24, 24’ and interlocked (in the position of use) in order to form a rigid structural ensemble supporting antennas 21 and 22. The locking device of said hinges is shown in detail in Fig. 8 and is manually operated thereby dispensing the use of tools, both when setting up or compacting the framework structure.

[041] Fig. 4 shows the compacted system in the transporting condition, wherein it can fit into a box with a volume of about 0.5 cubic meters (about 17 cubic feet). In the first exemplary embodiment shon, this box measures 980mm in length, 852mm wide and 675mm high (3.2 feet by 2.8 feet by 2.2 feet).

[042] As shown in Fig. 4, in the transporting condition the bars that support the P-band antennas, viz. 23a’, 23b’, 23b and 23a (not visible in this drawing), are folded by means of hinges 24’ and 24 (not visible in this drawing), in order to allow the fitting the system inside the box. Therefore, the arrangement that becomes baseline B only takes shape after the framework structure is set up in the field for the condition of use, in order to perform the survey. [043] In addition to the antennas in the P-band, the radar can also comprise antennas in the C and L-bands, said antennas 28 and 29 being supported by the framework structure, by means of hinged bars (not referenced) that provide their correct positioning in the condition of use (shown in Fig. 3).

[044] In the exemplary embodiment herein described, the P-band antennas are log-periodic dipole ones, measuring 63cm (24.8”) long and 34cm (13.4”) wide, operating with a wavelength Z of about 70 cm (27”) and pointed at an angle below the horizontal close to -30°, where the length B of the baseline is close to 1 m (39”), although other antennas with equivalent gain and radiation diagram can be used. Should a greater precision of the terrain height profile be required, the baseline length can be increased up to 2 meters (80”). In Fig. 3, arrows 31 and 32 indicate the direction of the main radiation lobe.

[045] The survey is carried out with the drone flying at a height under 150 meters (500 feet), preferably 120 meters (400 feet), simplifying the flight permit process of ANAC or other airspace regulatory agencies, such as the US FAA, to carry out the overflights over the surveyed area..

[046] In a second embodiment of the invention, the system is provided with two InSAR radars, as depicted in the plan view of Fig. 5, where the first set of antennas corresponding to the first radar comprises antennas 21 and 22 and the second set comprises antennas 33 and 34, wherein said first, and second antenna sets point in opposite directions, both orthogonal to the drone's flight path. This figure also shows the L-band antennas, 35 and 36 that can operate together with the P-band antennas. The proposed system can work with up to 3 bands, comprising two P-band antennas, two C-band antennas and one polarimetric L- band antenna. In said second embodiment each radar with two P-band antennas can still be provided with C-band antennas in housings 37 and 38, and also the L-band antennas 35 and 36, wherein antennas 37 and 38 are not depicted in Fig. 5, however their housings can be seen in figures 6 and 7.

[047] As in the first embodiment, the system of the second embodiment can be compacted for transportation by folding the hinged bars of the framework structure, whereupon it will look as shown in Fig. 6. In this case, the transporting box will measure 980mm (38.5”) long, 920mm (36”) wide and 675mm (26.5”) high.

[048] Fig. 7 is a perspective view of the system shown in Fig. 6, when set up in the field.

[049] The drawings of Fig. 8 show in detail the hinge provided in the bars that make up the framework structure. In Fig. 8-a bar 23 is shown partially folded by means of hinge 41 which connects segments 23a and 23b. This drawing also shows the power and coaxial cables 44 that connect the antennas to the electronic radar circuitry in a printed circuit board (not shown), said cables being inserted in the hollow tubular elements of the framework structure, thereby being protected against environmental or mechanical agents, there being no cables or connections outside the structural elements. The mutual locking between segments 23a and 23b when the structure is set up is depicted in Fig. 8-b and Fig. 8-c, which shows hook 43 being retained by the claw 42.

[050] The locking device of Fig. 8 is only exemplary, there being many other possibilities for locking segments 23a and 23b in a straight line, such as a box clasp or other tool-dispensing device.

[051] Both in the first and second embodiments, each radar set works autonomously and is powered by the drone's power system. As a rule, the power consumption of each radar is approximately 100 times lower than that for propulsion of the drone itself. In addition, each radar set has an internal power supply for generating the required power for a GNSS receiver, an inertial platform, a radio frequency transceiver, a radar pulse generation and return signal processing system, including scanning and a control system including a real-time processor and radar and navigation data recording infrastructure.

[052] The GNSS receiver and the inertial platform form the navigation system and are responsible for calculating the position and absolute orientation of all radar antennas with the proper accuracy. Navigation and radar data can be processed in real time during flight or on the ground. For ground processing, data acquired in flight are transferred to a computer, which performs the standard processes of interferometric processing of radar data and generates the ground terrain model without or with vegetation and eventually several other products, depending on the operator and also on the optional use of the C-band and L-band.

[053] Although the invention has been described based on specific exemplary embodiments, it is understood that changes may be introduced by persons skilled in the art, while remaining within the conceptual limits of the invention, which is defined and delimited by the following set of claims.