1. Introduction FADS (Focused Air Data System) uses an array of pressure sensors distributed at different locations along the nose circumference of an aircraft (or on both sides of the wing) to measure pressure. It indirectly obtains dynamic and static pressure through calculation, thereby acquiring atmospheric data such as vacuum velocity, Mach number, and pressure altitude. NASA's Dryden Flight Research Center began research on embedded atmospheric data systems in the 1960s. This sensing system has been applied to many aircraft such as the F-14, F/A-18, X-31, X-33, X-34, and X-38. However, it uses traditional pressure sensors, requiring long cables, which is not conducive to its use on smaller weapons and munitions. Wireless surface acoustic wave (SAW) pressure sensors have the advantages of small size and wireless measurement capabilities. Therefore, embedded atmospheric data systems using SAW pressure sensors can be applied to smaller weapons and munitions. Combined with small, inexpensive strapdown inertial navigation systems (SINS), they can form inexpensive yet highly accurate integrated navigation systems, which can be conveniently used to improve the accuracy of small munitions. 2. Basic Principles FADS (Flat Air Detection and Ranging System) is generally installed at the front of the aircraft. However, to avoid interfering with the installation of radar and fire control devices, FADS is sometimes installed at the front of the wing. The F-14's FADS consists of 23 pressure sensors installed at the front of the fuselage. The X-33's FADS system consists of 6 pressure sensors installed at the front of the fuselage. There is no fixed rule for the number of pressure sensors. The pressure sensor layout of the F-14's FADS is shown in Figure 1. A higher number of pressure sensors in the FADS system results in better fault tolerance, but also more complex system calculations and higher performance requirements. However, since measuring the angle of attack and sideslip angle requires a corresponding pressure difference, there must be symmetrical pressure measurement points around the center point. The aerodynamic model of the FADS system combines a potential flow model with a modified Newtonian flow model (the former mainly applicable to subsonic conditions, the latter mainly applicable to supersonic conditions) and a correction coefficient ε to form a compensated aerodynamic model for different Mach conditions. The value of ε is selected by comprehensively considering factors such as compression effects, aerodynamic shape, and system influence. During flight, pressure can be considered a function of angle of attack, sideslip angle, and Mach number, and this functional relationship can be determined before flight. The aerodynamic derivation process is omitted here; the complete aerodynamic model of the FADS system is given below: pi is the pressure measured by the i-th (i=1, 2…23) pressure sensor (referred to as point i); qc is the dynamic pressure; p∞ is the static pressure; M∞ is the Mach number; ε is the pressure coefficient; α is the angle of attack; β is the sideslip angle; φi is the circumferential angle at point i; λi is the conical angle at point i; θi is the angle of incidence at point i (the angle between the surface normal at that point and the incoming flow velocity vector); g is a definite monotonic function. By measuring the pressure at pressure points and using corresponding algorithms, the values ​​of dynamic and static pressure, Mach number, angle of attack, and sideslip angle can be obtained. These values ​​can then be used to calculate atmospheric parameters such as pressure altitude and vacuum velocity. 3 Wireless SAW Pressure Sensor Installing FADS at the front of the aircraft requires a very small pressure sensor for point measurements. The wireless SAW pressure sensors reported in references [4] and [5] are all implemented using delay lines. The insertion loss and propagation loss of the delay lines are relatively large, which affects the telemetry distance of the SAW pressure sensor. In addition, the SAW delay line is flat and long, which is not suitable for installation at the front of the aircraft to measure the pressure at a single point. The structure of the SAW pressure sensor shown in Figure 2 can change the shape of the SAW pressure sensor and increase the wireless measurement distance of the SAW pressure sensor. Unlike the SAW pressure sensor with a surface acoustic wave delay line structure, the structure shown in Figure 2 uses two single-ended resonators connected in parallel. When the SAW resonator resonates, the Rayleigh wave is superimposed multiple times through the reflection grating, and its energy is also superimposed multiple times, thus relatively reducing the propagation loss and insertion loss, and having a high Q value. At the same time, the SAW resonator has high sensitivity, high accuracy, and can remain stable for a long time. Using this structural design to wirelessly measure pressure can reduce the propagation loss and insertion loss of the SAW pressure sensor, so this principle structure has good application potential. When pressure is applied to the sensor surface, the piezoelectric film of the wireless SAW pressure sensor deforms. The strain in the film material changes the propagation speed of the surface acoustic wave (SAW), thus altering its center resonant frequency. By wirelessly detecting this change in the center resonant frequency of the SAW pressure sensor, pressure changes can be obtained. Assuming that temperature has little effect on the center resonant frequencies of the two single-ended resonators, the relationship between the change in the center frequency difference between the two resonators and the measured pressure can be expressed as: [Equation omitted for brevity]. Here, S1 and S2 represent the pressure sensitivity coefficients of single-ended resonators 1 and 2, respectively, and are related to the parameters of the single-ended resonators and the diaphragm. The pressure sensitivity coefficients can be expressed as: [Equation omitted for brevity]. Here, R is the radius of the diaphragm; h is the thickness of the diaphragm; E is Young's modulus; μs is Poisson's ratio; and r1 and r2 are the linearity coefficients of the substrate materials of the two single-ended resonators to mechanical disturbances. By wirelessly measuring the change in the center frequency difference between the two SAW resonators, the pressure value can be obtained, greatly improving the application flexibility of embedded atmospheric data systems. 4. Wireless Measurement Structure In embedded atmospheric data systems, the advantages of wireless measurement using SAW pressure sensors enhance application flexibility. Figure 3 shows the wireless measurement structure of an SAW pressure sensor. The entire wireless measurement system consists of two circuits: signal interrogation and signal reception. The signal transmitting device comprises a reference oscillator, an RF pulse generator, and a transmit/receive (T/R) switch; the signal receiving device comprises an integral down-converter and two A/D converters. The signal processor module processes the data obtained after A/D conversion, obtaining orthogonal I and Q signals. Processing the I and Q signals yields the frequency and phase change information of the SAW sensor due to physical changes. Simultaneously, the signal processor module generates pulse control signals and transmit/receive conversion signals to coordinate the transmission and reception conversion. Since the pulse interrogation signal requires time to propagate, be processed in the SAW sensor, and return after being transmitted by the transceiver circuit, independent transmission and reception intervals must be defined and converted using the transmit/receive switch. The reference oscillator generates an interrogation signal during pulse signal transmission and provides a local oscillator reference signal for the integral down-conversion during signal reception. After down-conversion, the signal is digitized by a dual-channel A/D converter, and the signal transmission and reception are controlled by a programmable logic device. After signal processing, it is connected to the inertial navigation system to form a small integrated navigation system. 5. Simulation Results The overall performance of the SAW pressure sensor can be analyzed using the equivalent circuit of the SAW single-ended resonator. Figure 4 shows the equivalent circuit of the SAW single-ended resonator. In the figure: C1 represents the dynamic capacitance caused by substrate elasticity; L1 is the dynamic inductance caused by substrate inertia; R1 is the dynamic resistance caused by damping; C0 is the static capacitance of the interdigital transducer. The SAW pressure sensor is obtained by connecting two single-ended resonators in parallel. Simulation using HP Eesoft software yields the relationship between the return admittance amplitude and frequency of the two resonators. The simulation results are shown in Figure 5. Figure 5 shows that the two SAW resonators have two center resonant frequencies, 434 MHz and 434.4 MHz, respectively. Both frequencies fall within the ISM standard wireless frequency measurement range, allowing for effective wireless measurement.