Research on RCS Visual Calculation Method of Aircraft Cockpit

Figure 1 (a) The grid diagram before the cockpit interpolation fitting (b) The grid diagram after the cockpit interpolation fitting

By fitting the CR spline surface to the target model, the filtering of the polygonal rough model composed of the type value points to the smooth real model of the complex target is realized. After the geometric description of the target, the standard interface of graphics software (OpenGL ) [4] and the graphics accelerator card hardware to display and blank the target, so as to realize the electromagnetic calculation of GRECO on the computer. The following is the cockpit model displayed by the image generation program. Figure 2 (a), (b) the former is not interpolated The former graph, the latter is the graph after fitting with CR spline.

Figure 2 (a) Fitting the front cockpit model (b) Fitting the rear cockpit model

For the detailed calculation and implementation method, please refer to reference [5].

3. Scattering analysis of the cabin structure When the radar signal enters the cabin, and then passes out of the cabin after multiple reflections, its propagation direction is unpredictable and its intensity is difficult to estimate. Here, the following methods will be used to solve the problem. First, apply layering Media theory [1] Solve the reflection coefficient and transmission coefficient of the hatch cover.
1. Reflection and transmission coefficients of layered media [1]
Let the number of layers of the multilayer medium perpendicular to the Z axis be L, plus its outer boundary, there are L + 2 kinds of media. The layered medium interface is perpendicular to the Z axis, then the electromagnetic field in the incident medium and the layered medium can be expressed as :

(1)

In the formula Is the unit vector along the axis, the specific expression of [Mm] is

(2)

In the formula

δm = (2πNm / λ) dmcosθm
Nm = εmμm-j (4πσmμm / ω) (3)

Where λ is the incident plane wave wavelength, ω is the angular frequency, ε, μ, and σ are the dielectric constant, permeability, and conductivity of the medium, respectively. For simplicity, the admittance Y of the layered structure is defined as

(4)

Therefore, equation (1) can be expressed as

(5)
(6)

Where [B C] T is defined as the characteristic matrix of the layered structure, and

Y = C / B (7)

In fact, the reflection coefficient, transmission coefficient and absorption coefficient of layered media can be expressed as

(8)
(9)
(10)

2. Energy distribution modulation method [6]
In view of the fact that the radar wave penetrates the cabin cover and enters the cabin, during the process of being scattered by the cabin scatterers into the cabin space, it penetrates the cabin cover structure at least twice. The randomness generated by the physical process introduces random factors into the RCS solution of the backscatter in the cabin. The random factor generation program + application is used to obtain good statistical results.
Assuming that the energy entering the cockpit through the canopy is ε, due to the scattering of objects in the cabin such as human bodies, helmets, seats and instrument frames, the energy is randomly at the azimuth angle (φ0, φ0) and pitch angle (θu, θd) Spread within the range. It is equivalent to modulating and weighting the average energy in the case of uniform diffusion with a certain energy distribution function F (θ, φ), so that the energy distribution is closer to the actual situation. F (θ, φ) can be seen in different models The situation is determined by analysis and testing. In this case, the distribution of electromagnetic energy density in the cabin can be expressed as:

(θ, φ) = εF (θ, φ) / ∫θdθu∫θ0-θ0R2sinθdφdθ
= εF (θ, φ) / [2R2φ0 (cosθu-cosθd)] (11)

Therefore, in a certain direction (θ, φ) by The resulting RCS value is:

σ (θ, φ) = lim ［4πR2 (θ, φ) / | Ei | 2]
= 2πεF (θ, φ) / [φ0 (cosθu-cosθd)] (12)

Considering that the radar wave enters the cabin through the cabin cover and is scattered back by the objects in the cabin back to the space outside the cabin, it will inevitably produce energy loss through penetrating the cabin cover structure twice, so

(θ, φ) = εF (θ, φ) .β / [2R2φ0 (cosθu-cosθd)]
σ (θ, φ) = 2πεF (θ, φ) .β / [φ0 (cosθu-cosθd)] (13)

Where β is the attenuation factor , And β is proportional to the square of the transmission coefficient of the hood. Where F (θ, φ) must satisfy

∫θdθu∫φd-φ0F (θ, φ) sinθdθdφ = 2φ0 (cosθu-cosθd) (14)

due to

∫θdθu∫φd-φ0 (θ, φ) R2sinθdθdφ = ε

make

then

σ (θ, φ) = 4πεβF0 (θ, φ) / ∫∫F0 (θ, φ) sinθdθdφ (15)

The selection of F0 (θ, φ) in equation (15) should be determined according to the statistical results. For example, for a uniform distribution, F0 (θ, φ) = 1, while for Gaussian and logarithmic distributions, FG0 ( θ, φ) and FL0 (θ, φ):

Where ξ and α are distribution parameters. After determining F0 (θ, φ), equation (15) can be used to solve the RCS value σ (θ, φ) of the interior scattering for a given direction. The outer metal surface part constitutes the total surface effect field Esf, and the edge part constitutes the total edge scattering field Esw.

4. The total scattering field of the cockpit The internal structure scattering and the metal surface part of the cabin constitute the total surface effect field Esf, and the edge part constitutes the total edge scattering field Esw. The RCS value

(16)

V. Analysis of numerical results For the organic glass with a coating thickness of 40 to 80Å (its thickness is 10mm, the relative dielectric constant and conductivity are εr = 2.62, μr = 1), the conductivity of the film is taken as 5 × 106 (1 / Ω.m), the relationship between the electromagnetic reflectance and the coating thickness at normal incidence is shown in Figures 3 and 4.

Figure 3 Relationship between reflectivity and coating thickness (re-polarization)

Figure 4 Relationship between reflectivity and coating thickness (horizontal polarization)

Figure 5 (a) is the theoretical calculation curve of the aircraft cockpit at 10.5GHz, VV polarization zone with conductive coating (100Å) and cockpit attitude angle (φ, 0,0), and the corresponding experimental curve is shown in Figure 5 (b).

Figure 5 Target: cockpit model, frequency: 10.5GHz, polarization: VV, with conductive coating

Figure 6 (a) is the theoretical calculation curve of the aircraft cabin at 10.5GHz, HH polarization, with conductive coating (100Å), and cockpit attitude angle (φ, 0, 0), and the corresponding experimental curve (b)

Figure 6 Target: cockpit model, frequency: 10.5GHz, polarization: HH, with conductive coating

It can be seen from the experimental and calculation results that the average RCS of the cockpit with the coating structure is -15dBSM, the average of the two is consistent, and the overall trend is consistent, which proves the effectiveness of the algorithm. In addition, the cockpit with the coating structure penetrates into the cabin The energy is smaller, and the scattered energy returned to the outside of the cabin is smaller, which is much smaller than the uncoated cockpit. Using the above calculation method can give RCS values ​​for different flight states, polarizations, and operating frequencies. It can be used as an aircraft CAD / CAM A basic module in the system, together with other CAD modules, is optimized to provide a basis for aircraft design.