Aircraft radome and radar assembly

Abstract

Aircraft Radome and Radar Assembly The invention relates to an aircraft radome and radar assembly (EAR) comprising: a radome (Rd) disposed on the nose of the aircraft (NA); a radar (Ra), disposed inside the radome (Rd), the radar (Ra) being configured to operate in the millimeter range and comprising a transmitting antenna (Tx) configured to emit a TM polarized wave and a receiving antenna (Rx); the radome (Ra) comprising a window (F) integrated in said radome, the window (F) being in a material transparent to millimeter waves, the window (F) being opposite the radar (Ra); the radar (Ra) being disposed such that the angle (θi) between the maximum gain direction (DGM) of the transmitting antenna (Tx) and the normal (N) to the window (F) is within an angular range [θB - θ1;θB + θ2], with θB being the Brewster angle for an air / window interface, θ1 and θ2 determined to obtain a reflectivity at said air / window interface below a predefined threshold; and an isolation partition (IC) disposed between the transmitting antenna (Tx) and the receiving antenna (Rx), the isolation partition (IC) being substantially in contact with the window (F) and with the radar (Ra). Figure for the abstract: Fig. 1;

Description

Title of the invention: Aircraft radome and radar assembly technical field

[0001] The invention relates to an aircraft radome and radar assembly, and in particular to an aircraft radome and radar assembly operating in the millimeter wave range and enabling the imaging of a runway in fog. PRIOR TECHNOLOGY

[0002] Today, aircraft approach and landing operations in fog are difficult for pilots to perform manually due to poor visibility. Without visibility, landing must be carried out using autopilot with instrument landing systems (ILS). Indeed, current equipment installed in aircraft to improve pilot visibility during approach or landing operates in the infrared spectrum and cannot capture images of the runway in fog or reduce the decision altitude when fog is present. Weather radars installed in the radome operate in the X-band (8-12 GHz) and do not offer sufficient resolution for pilot use during approach or landing.

[0003] Millimeter-wave radars can be used to capture images or videos in fog. However, to obtain a usable image, the millimeter-wave radar must have good RF performance without degrading the aircraft's characteristics. For example, it must have a negligible impact on aerodynamics and a low impact on the aircraft's mass.

[0004] Furthermore, in order to obtain an image of sufficient quality to allow the pilot to visualize the runway, the radar must point towards the runway with a sufficiently directional beam so as not to disperse the emitted energy in directions unnecessary for image generation. This therefore implies constraints on the radar's aiming angle relative to the approach slope so that it correctly targets the runway.

[0005] The radar's viewing angle and its associated useful cone must not intersect the aircraft's metallic structure so as not to interfere with the signals emitted and received by the radar. Therefore, this constraint necessitates installation at the front of the aircraft, within the aircraft's radome.

[0006] However, millimeter waves do not penetrate conventional radomes (often made of quartz-epoxy honeycomb). Therefore, it is not possible to install a radar emitting waves in the millimeter range in the radomes of current commercial aircraft.

[0007] Finally, some current radars have contiguous transmitting and receiving antenna areas. This can induce spurious reflections between these transmitting and receiving antenna areas and thus distort the images obtained.

[0008] There is therefore a need to develop a solution enabling the real-time generation of images or videos of the approach scene including the runway in fog, the quality of which is sufficient to allow the improvement of the pilot's vision during an approach or landing, while respecting the constraints of current commercial aircraft, in terms of standards of material resistance, aerodynamics and mass. Summary of the invention

[0009] In order to overcome the aforementioned drawbacks, the invention proposes an aircraft radome and radar assembly comprising: - a radome positioned on the nose of the aircraft; - a radar, located inside the radome, the radar being configured to operating in the millimeter range and comprising a transmitting antenna configured to emit a TM polarized wave and a receiving antenna; the radome comprising a window integrated into said radome, the window being in a material transparent to millimeter waves, the window being facing the radar;

[0010] the radar being arranged so that the angle between the direction of maximum gain (DGM) of the transmitting antenna and the normal to the window is within an angular range [0B - 0i ; 0B + 02], with 0B being the Brewster angle for an air / window interface, and 0i and 02 determined to obtain a reflectivity at said air / window interface below a predefined threshold; and - an insulation partition placed between the transmitting antenna and the receiving antenna, the insulation partition being substantially in contact with the window and with the radar.

[0011] In one embodiment, the radar is a frequency modulation continuous wave radar.

[0012] In one embodiment, the radar emits in a frequency band between 95 GHz and 100 GHz.

[0013] In one embodiment, the window has a dimension such that an emitted wave contained within an angular field of emission and a received wave contained within the angular field of reception of the radar pass through the window.

[0014] In one embodiment, the window material comprises cross-linked polystyrene plastic, or polyetherimide resin, or polytetrafluoroethylene.

[0015] In one embodiment, the window has a radius of curvature greater than 1.2 m.

[0016] In one embodiment, the window material has a permittivity such that a value of the associated Brewster angle is between 50° and 65°.

[0017] In one embodiment, a window material has a permittivity such that a value of the associated Brewster angle is compatible with a positioning of the radar inside the radome and / or with the inclination of the radome.

[0018] In one embodiment, the insulation partition has a metallic core covered with an absorbent material.

[0019] In one embodiment, the window is integrated into the radome with a connecting material.

[0020] The following description presents several embodiments of the aircraft radome and radar assembly of the invention: these examples are not limiting to the scope of the invention. These embodiments demonstrate both the essential features of the invention and additional features related to the embodiments considered. Brief description of the drawings

[0021] The invention will be better understood and other advantages will become apparent upon reading the following description, given by way of non-limiting example, and from the figures, among which:

[0022] [Fig-1] [Fig.1] illustrates an aircraft radome and radar assembly according to the invention side view;

[0023] [Fig.2a] [Fig.2a] represents a top view of the aircraft, illustrating the azimuth lobe at the receiving antenna;

[0024] [Fig.2b] [Fig.2b] represents a side view, illustrating the elevation lobe at the receiving antenna and the elevation lobe at the transmitting antenna;

[0025] [Fig.3] [Fig.3] illustrates the reflectivity according to the angle of incidence when the window is made of rexolite™ type cross-linked polystyrene;

[0026] [Fig. 4] [Fig. 4] illustrates a simplified model of optical paths of rays originating from elementary antennas and propagating through the window, according to an example, the view being an azimuth cross-section; and

[0027] [Fig.5] [Fig.5] illustrates the optical path difference of a wave emitted by the transmitting antenna. DETAILED DESCRIPTION

[0028] The invention relates to an assembly comprising an aircraft radome and a radar. [Fig. 1] illustrates an aircraft radome and EAR radar assembly according to the invention.

[0029] The aircraft radome and radar assembly EAR includes a radome Rd located on the nose of the aircraft NA. The aircraft is, for example, an airplane or a drone.

[0030] Furthermore, the aircraft radome and EAR radar assembly includes a radar Ra. The radar Ra is located inside the radome Rd. The radar Ra is configured to operate in the millimeter wave range. Advantageously, the millimeter wave range allows for image or video capture in fog while using small radars, unlike the weather radars commonly used for weather imagery, or in clouds or rain. Indeed, these weather radars require, for the same resolution, radar sizes approximately ten times larger to capture images in fog than millimeter wave radars, and therefore could not be carried on board an aircraft. The radar Ra includes a transmitting antenna Tx configured to emit a polarized TM wave and a receiving antenna Rx.Thus, the transmitting antenna (Tx) emits a millimeter wave that propagates towards the runway within the radar's field of view as the aircraft, such as a plane, prepares to land. The wave is then backscattered by the runway, even in adverse weather conditions such as fog. The reflected wave is then captured by the receiving antenna (Rx) and subsequently transformed into an image or video by the radar (Ra). Furthermore, millimeter-wave radars can be compacted, which is particularly advantageous on board aircraft where space and weight constraints are significant.

[0031] The waves emitted by the transmitting antenna Tx and received by the receiving antenna Rx form lobes. Figures 2a and 2b show a top view and a side view of the aircraft, illustrating on [Fig.2b] the elevation lobe at the receiving antenna LRE and the elevation lobe at the transmitting antenna LTE, and on [Fig.2a] the azimuth lobe at the receiving antenna LRA.

[0032] Furthermore, the radome Rd includes a window F integrated into the radome Rd. The window F is made of a material transparent to millimeter waves. The window F faces the radar Ra. Radomes, for example those of commercial aircraft, are typically made of quartz-epoxy honeycomb. Millimeter waves from the radar Ra do not pass through these materials. It is therefore necessary for the radome to include a portion transparent to millimeter waves in order to transmit and receive the waves. Advantageously, the window F thus allows the use of a localized millimeter-wave radar within the radome.

[0033] As illustrated in [Fig.2b], the Ra radar and the F window are configured so that the transmit and emit lobes pass through the F window, thus enabling the radar to image the runway.

[0034] Furthermore, the radar Ra is arranged such that the angle θi between the maximum gain direction DGM of the transmitting antenna Tx and the normal N to the window F is within an angular range [0B - θi°; θi + θ2], with θb being defined for the air / window interface, and θi, θ2 determined to obtain a reflectivity at said air / window interface below a predefined threshold. The predefined threshold is, for example, between -25 dB and -10 dB, preferably equal to -15 dB or -20 dB. As indicated above, the transmitting antenna Tx is configured to transmit a TM-polarized wave. The TE polarization has a reflection coefficient that increases strictly with the angle of incidence. The TM polarization exhibits a minimum reflection at the Brewster angle of incidence, which corresponds to total transmission of the wave through the window F.In the present invention, the reflection coefficient must be minimized since it corresponds to losses at both the transmission (the wave reflected into the radome) and reception (echoes from the environment reflected at the radome's surface). Furthermore, the energy reflected at transmission tends to mismatch antennas and degrade transmission performance (power, pointing accuracy). Advantageously, when the radar Ra is positioned such that the angle θi between the direction of maximum gain of the transmitting antenna Tx and the normal N to the window F is equal to the Brewster angle θB, it is possible to minimize the energy reflected at the air / window interface at transmission. A certain tolerance around this angle θB is acceptable, from -θ1 to +θ2.

[0035] Furthermore, the aircraft radome and EAR radar assembly includes an isolation partition CI positioned between the transmitting antenna Tx and the receiving antenna Rx. The isolation partition CI is configured to prevent reflections of the rays emitted by the transmitting antenna Tx from the window from being detected by the receiving antenna Rx. The partition CI thus forms a lateral barrier to the emitted wave. The partition CI is substantially in contact with the window F and with the radar Ra to prevent the aforementioned reflections. "Substantially in contact" means contact within the limits of mechanical clearance. In one embodiment, this mechanical clearance is ensured by a deformable absorber (of the foam type) between the partition and the aircraft radome, and / or between the partition and the radar. The isolation partition CI therefore creates an isolation boundary between the transmitting antenna Tx and the receiving antenna Rx.This separation between the Rx and Tx antennas helps preserve the transmission lobe of the Tx antenna. Advantageously, the isolation partition (CI) reduces reflections between the Tx and Rx antennas, thus limiting unwanted waves and improving the quality of the images or videos of the runway obtained by the Ra radar.

[0036] Thus, advantageously, the invention makes it possible to assist pilots in landing in difficult weather conditions such as fog. The invention makes it possible to take images of sufficient quality to allow The goal is to improve the pilot's vision during an approach or landing, while respecting the constraints of current commercial aircraft in terms of materials, aerodynamics, and weight. This allows the pilot, for example, to perform approach and landing protocols, even in fog, such as the "EFVS approach" or "EFVS landing" protocols (EFVS stands for enhanced flight vision system).

[0037] In one embodiment, the insulating partition CI is fixed to the radar Ra, and a baffle is disposed in the window F, in which the insulating partition CI is located, thus allowing mechanical clearance between the insulating partition CI and the window F. In an alternative embodiment, a baffle is disposed in the window F in which the insulating partition CI is located, and another baffle is disposed on the radar, between the two transmit and receive antennas Tx, Rx, in which the insulating partition CI is also located. Advantageously, the baffle thus makes it possible to withstand the mechanical stresses to which the radar Ra is subjected in a harsh flight environment.

[0038] In one embodiment, the Ra radar is a frequency-modulated continuous-wave radar. Advantageously, the frequency-modulated continuous-wave radar allows for continuous imaging of the runway during an approach or landing.

[0039] In one embodiment, the Ra radar transmits in a frequency band between 95 GHz and 100 GHz. Advantageously, this frequency band makes it possible to take images through fog, and thus to take images of the runway, enabling pilots to be assisted in landing.

[0040] In one embodiment, the window F has a dimension such that an emitted wave within the angular emission field and a received wave within the angular reception field of the radar pass through the window. Advantageously, the size of the window maximizes the radar's field of view, so that the emitted waves are not blocked by the radome.

[0041] In one embodiment, a material of the window F has a permittivity such that a value of the associated Brewster angle (air / window interface) is compatible with positioning the radar inside the radome and / or with the radome's inclination. In particular, the radar Ra must be positioned opposite the window F so as to aim at the runway. The positioning angle of the radar Ra is therefore limited by the shape of the radome Rd, often defined by the aircraft manufacturer, as well as by the angle required to aim at the runway. Thus, it can be difficult to obtain the Brewster angle at the incidence on the window for the beam emitted from the transmitting antenna Tx (DGM direction) given these constraints. It is possible to adjust the value of the Brewster angle by adjusting the permittivity of the material of the window F. It is therefore possible to choose a material for window F whose permittivity allows obtaining the desired Brewster angle value.

[0042] Thus, in one embodiment, the window material comprises a cross-linked polystyrene plastic, for example of the rexolite™ type, or a polyetherimide resin, for example of the ultem™ type, or polytetrafluoroethylene. These materials have a permittivity such that the associated Brewster angle value is between 50° and 65°. In particular, the permittivity of ultem™ allows a Brewster angle of 63°, that of polytetrafluoroethylene allows a Brewster angle of 54°, and rexolite™ allows a Brewster angle of 58°. Figure 3 shows the reflectivity as a function of the angle of incidence for the two TE and TM polarizations when the window is made of rexolite™ type cross-linked polystyrene. It is well illustrated that rexolite™ allows a Brewster angle of 58 to be obtained.A range of incidence angle values ​​around Brewster's angle is acceptable for minimizing losses and unwanted reflections, as described above, for example, a range [38°; 73°]. Preferably, a reflected wave attenuation of at least -15 dB, or even -20 dB, is sought. As illustrated in [Fig. 3], for rexolite™ and an attenuation of -20 dB, this range of incidence angle values ​​(θi) is preferably between 46° and 65°.

[0043] Rexolite™ is particularly advantageous because it allows the window thickness to be reduced, typically by 6 to 7 mm to attenuate the signal acceptably while maintaining the rigidity required in the harsh environmental conditions of aircraft.

[0044] In one embodiment, the window F has a radius of curvature greater than 1.2 m.

[0045] In order to bring the radar as close as possible to the radome, it is necessary that the window have an acceptable radius of curvature. Indeed, a flat shape would complicate the alignment with the rest of the radome in azimuth.

[0046] A simulation was performed to evaluate the acceptable radius of curvature of the window F. A Rexolite™ window was used for the simulation. To limit the window dimensions, the width of the antenna lobes is considered to be limited by the angle values ​​corresponding to a 3 dB attenuation relative to the maximum transmission direction. This applies to both the transmitting antennas Tx and the receiving antennas Rx, in both azimuth and elevation. The angular width of the LTE cone is +10° upwards and -14° downwards around the DGM direction. The angular width of the transmission cone in LTA azimuth is 15° to the left and to the right.

[0047] In azimuth, a curvature of the window is modeled in three sections, and three rays OMI, OM2, OM3 emitted respectively by u the elementary transmitting antennas Txl, Tx2, Tx3, each pass through one section of the window, as illustrated in [Fig.4].

[0048] The window sections thus modeled have parallel inner and outer surfaces. Consequently (since the dielectric is considered homogeneous), the entry and exit directions of the millimeter waves are parallel. However, the geometry and the laws of refraction (obeying Snell's law) applied locally induce non-rectilinear propagation from the emitting horns to free-space propagation, as illustrated in [Fig. 4]. Thus, there is a difference in optical path length due to the window, compared to free-space propagation.

[0049] Figure 5 illustrates the optical path difference (expressed as a multiple of X, the radar emission wavelength). This is the optical path difference between the central horn and an end horn of the array, in excess of the theoretical optical path difference associated with each viewing angle, and expressed as a multiple of X. Figure 5 illustrates the iso-value curves of optical path difference as a fraction of X. This additional optical path difference is due to the shape of the window F and is a function of the emission angle of the millimeter wave through the window F and the angle that defines the inclination of the lateral segments of the window F. The acceptable area, according to the emission angle and the inclination of the lateral segments, is that for which the additional optical path difference is less than a fixed fraction of the wavelength.It is estimated that the image formed will remain usable for a maximum optical path difference of 0.1X. Using this value of 0.1X, it is possible to determine a suitable angle with the edges of the radome which, in combination with the total width of the antenna, the dimensions of each panel, the angle between the panels, and the manufacturer's constraints, allows us to determine the minimum permissible radius of curvature for window F. As an indication, a value of 1.2 m for the minimum permissible radius of curvature for window F.

[0050] In one embodiment, the isolation partition has a metallic core covered with an absorbing material. Advantageously, this allows the isolation partition CI to prevent reflections of rays emitted by the transmitting antenna Tx onto the radome Rd from being detected by the receiving antenna Rx. Thus, by preventing these reflections, the quality of the images or videos obtained with the radar is improved, and therefore the pilot can visualize the runway more precisely. In one embodiment, the absorbing material is a dielectrically charged elastomer, such as a silicone resin. These materials allow the absorption of waves in the millimeter range.

[0051] In one embodiment, the window F is integrated into the radome Rd with a connecting material. Advantageously, the connecting material limits the mechanical stresses generated by the different materials used for the window and the rest of the radome. In one embodiment, the connecting material is a fiberglass composite.

[0052] Although the invention has been illustrated and described in detail using a preferred embodiment, the invention is not limited to the disclosed examples. Other variations can be deduced by a person skilled in the art without departing from the scope of protection of the claimed invention.

Claims

Demands

1. Aircraft radome and radar assembly (EAR) comprising: - a radome (Rd) disposed on the nose of the aircraft (NA); - a radar (Ra), disposed inside the radome (Rd), the radar (Ra) being configured to operate in the millimeter range and comprising a transmit antenna (Tx) configured to emit a TM polarized wave and a receive antenna (Rx); the radome (Ra) comprising a window (F) integrated into said radome, the window (F) being in a material transparent to millimeter waves, the window (F) being opposite the radar (Ra); the radar (Ra) being disposed such that the angle (0i) between the direction of maximum gain (DGM) of the transmit antenna (Tx) and the normal (N) to the window (F) is within an angular range [0B - 0i; [0B + 62], with 0B Brewster angle for an air / window interface, 0i and 02 determined to obtain a reflectivity at said air / window interface below a predefined threshold;and - an isolation partition (CI) arranged between the transmitting antenna (Tx) and the receiving antenna (Rx), the isolation partition (CI) being substantially in contact with the window (F) and with the radar (Ra).;

2. Aircraft radome and radar assembly according to claim 1, wherein the radar is a frequency-modulated continuous wave radar.

3. Aircraft radome and radar assembly according to any one of the preceding claims, wherein the radar transmits in a frequency band between 95 GHz and 100 GHz.

4. Aircraft radome and radar assembly according to any one of the preceding claims, wherein the window has a dimension such that an emitted wave contained within an angular emission field and a received wave contained within the angular reception field of the radar pass through the window.

5. Aircraft radome and radar assembly according to any one of the preceding claims, wherein the window material comprises cross-linked polystyrene plastic, or polyetherimide resin, or polytetrafluoroethylene.

6. Aircraft radome and radar assembly according to any one of the preceding claims, wherein the window has a radius of curvature greater than 1.2 m.

7. Aircraft radome and radar assembly according to any one of the preceding claims, wherein the window material has a permittivity such that a value of the associated Brewster angle is between 50° and 65°.

8. Aircraft radome and radar assembly according to any one of the preceding claims, wherein a window material has a permittivity such that a value of the associated Brewster angle is compatible with positioning the radar inside the radome and / or with the inclination of the radome.

9. Aircraft radome and radar assembly according to any one of the preceding claims, wherein the isolation partition has a metallic core covered with an absorbing material.

10. Aircraft radome and radar assembly according to any one of the preceding claims, wherein the window is integrated into the radome with a joining material.

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