JPH0429992B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a synthetic aperture antenna type radar device which is mounted on a flying object such as an aircraft and which observes topography, landforms, etc. on the ground or sea surface.

[Technical background of the invention and its problems]

2. Description of the Related Art There are methods shown in FIGS. 1 to 3, in which a radar is mounted on a flying object such as an aircraft, and the topography, terrain, etc. of the ground surface are observed from the air using this radar. Figures 1a and b are for emitting radio waves F _B in the forward direction of the projectile P _L to perform observation in the forward direction of movement, and Figures 2 a and b are for emitting radio waves in the lateral direction of the projectile P _L. It fires F _B and performs lateral observation. In addition, FIGS. 3a and 3b show that a radio wave F _B is emitted directly below the flying object P _L to perform observation in the direct downward direction. Here, a radar for observing in the lateral direction shown in FIGS. 2a and 2b will be explained.

Generally, synthetic aperture antennas are used in this type of radar. This is achieved by installing a small antenna on a flying object, sequentially receiving reflected signals received by this antenna on the flying object, and coherently combining these signals to synchronize with the movement of the flying object. This creates directivity similar to that of an antenna with an equivalently large aperture. By using this antenna and coherent signal processing device, high-resolution geodesy is possible, and the azimuth resolution δ _AZ in the focused case is expressed by the following equation (1): δ _AZ | _Fpcused = L _a /2 ...(1) (However, L _a is the effective length of the antenna in the azimuth direction.) Now, in this type of radar, as shown in Fig. 4, the antenna A _t with the effective length L _a and the effective width W _r is at the altitude H ,
When moving in the x direction at a speed V and emitting radio waves with the beam declination (off-media angle) as θ ₀ and the beam width in the azimuth direction as θ _a , the radar reflected signal from the observation area will be reflected in the opposite direction P ₀ Then, Doppler frequency D is 0,
At the forward end _P1 and the rear end _P2 in the forward direction, the signal is received with a Doppler frequency deviation of 2V/L _a and -2V/L _a , respectively. In other words, the transmission spectrum...
The frequency sequence of f _-1 , f ₀ , f ₁ ... is developed as shown in Figure 5, but on the other hand, among the signals transmitted and received from the side lobe of the antenna, there is a signal at a distance equivalent to within the distance R±ΔR. The signal from the target is received at the same time as the reflected signal from the target observation area, and in principle, the Doppler frequency deviation is distributed up to ±2V/λ and mixed into the adjacent main spectrum. Become. Therefore, it becomes a so-called Doppler Ambiguity signal, which causes a decrease in the azimuth resolution of the radar. Naturally, this Doppler ambiguity can be alleviated to some extent by increasing the pulse repetition frequency (PRF) of the radar. Furthermore, increasing the PRF increases the number of samples sent and received to the observed area when a flying object passes, which is also a good idea from the standpoint of maintaining the required S/N ratio.

In other words, as shown in Figure 6, if the reflected signal is considered as a frequency spectrum, then every 1/T (Hz)
Signals corresponding to higher-order modes of the Doppler frequency are arranged around a Bang corresponding to PRF. In the receiver, unnecessary higher-order mode signals are removed by a filter having a passband width (required reception band) of B _T shown in the figure, and only signals corresponding to B _T are extracted. Here, the higher-order modes from adjacent spectrum groups in the same figure
When a Doppler signal has a signal within the same range from the side lobe of the antenna, it mixes within the passband of the filter like A, B, A', B', resulting in Doppler ambiguity and an image in the azimuth direction. In order to remove this image, the extent to which adjacent spectra overlap can be reduced by increasing PRF (1/T) as described above.

However, when considering radar transmission and reception signals in time series, as shown in Figure 7, reflected signals from side lobes are sequentially received after a period of no reception from the time of transmission T ₀ until the signal T ₁ directly below the flying object is obtained. while receiving the reflected signal _T2 from the main lobe.
This radar transmission/reception gap has a large antenna off-media angle θ ₀ as shown in Fig. 8, and also has a large antenna off-media angle θ _{0 .}
The higher the altitude H is, the larger it becomes. This can be easily understood from the relationship between the radar antenna pattern and the earth E. Furthermore, in this type of radar, it is desirable to increase the PRF in order to increase the number of pulse hits. However, as is clear from FIG. 9, if PRF (1/T) is increased, the transmitted signal t ₀ ,
The received signals E ₀ , E ₁ . . . by the main beams of t ₁ .

[Purpose of the invention]

This invention was made based on the above circumstances, and its purpose is to provide a radar device that can suppress the harmonic mode of the Doppler frequency caused by the side lobe of the antenna and improve the ranging accuracy. It is something to do.

[Summary of the invention]

This invention disperses the harmonic mode of the Doppler frequency adjacent to the required reception band within the required reception band by emitting a transmission pulse signal whose pulse repetition period is sequentially changed, and transmits the dispersed signal with the pulse repetition period. By performing coherent processing on each signal, the harmonic mode of the Doppler frequency is suppressed.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

First, the principle of this invention will be explained. 6th
In the spectrum of the radar reflected signal shown in the figure,
Pulse repetition period (1/PRF) is T, T+ΔT,
Suppose that it is changed to T - ΔT. In this case, the spectrum of the radar reflected signal is fd
Considering (Doppler frequency) = 0 as a reference, the fundamental component of the Doppler frequency does not change on the frequency axis regardless of changes in the pulse repetition period, while the higher-order mode components of the Doppler frequency adjacent to the passband BT is dispersed within the passband BT compared to FIG. 6 (indicated by a broken line and a dashed-dotted line in FIG. 10). Therefore, the known pulse repetition period T
By generating transmission signals with known pulse repetition periods of T + ΔT and T - ΔT, and integrating the received signals obtained by coherent processing for each of these pulse repetition periods, the desired Doppler signal is generated within the passband BT. The frequency components are emphasized and the harmonic modes of the Doppler frequency corresponding to the antenna side lobes are suppressed as shown in FIGS. 11-12.

Next, embodiments of the present invention based on the above principle will be described.

In FIG. 13, the output signal of a coherent oscillator (COHO) 31 is supplied to a transmitter 33 via a stabilizing oscillator 32. This transmitter 33
are sequentially supplied with signals having pulse repetition periods T, T+ΔT, and T−ΔT outputted from the trigger oscillator 34, and a transmission pulse signal is outputted from the transmitter 33 corresponding to the pulse repetition period. The transmission pulse signal whose pulse repetition period has been changed is supplied to the antenna 36 via the transmission/reception switch 35,
It is fired towards the observation area. On the other hand, the reflected signal from the observation area is received by the antenna 36,
It is supplied to the mixer 37 via the transmission/reception switching device 35. This mixer 37 includes the stabilizing oscillator 32.
an output signal is provided, and the received reflected signal is converted to an intermediate frequency signal. The output signal of this mixer 37 is branched and supplied to first and second phase detectors 38 and 39. The first phase detector 38 is supplied with the output signal of the COHO 31, and the second phase detector 39 is supplied with a 90° phase shifter 40.
The output signal of the COHO 31 is supplied with the phase shifted by. Therefore, the first and second phase detectors 38 and 39 output Doppler frequency components converted into so-called orthogonal videos (I, Q components) having a phase difference of 90 degrees from each other. The output signals of these first and second phase detectors 38 and 39 are supplied to a coherent signal processor 42, and this processor 42 is supplied with signals corresponding to pulse repetition periods T, T+ΔT, and T−ΔT from a mode controller 43. A mode indication signal is provided. That is, this mode controller 43
A signal corresponding to the pulse repetition period T, T+ΔT, and T−ΔT is supplied from the trigger oscillator 34, and a predetermined mode instruction signal is output in response to this signal. Therefore, in the coherent signal processor 42, predetermined coherent signal processing is performed for each group of pulse repetition periods T, T+ΔT, and T−ΔT. This processor 4
The output signal of No. 2 is supplied to an integrator 44 together with the output signal of the mode controller 43. Integrator 44
integrates the output signal of the processor 42 obtained for each group corresponding to the pulse repetition period within the passband BT. However, this integrator 44
A Doppler signal in which adjacent harmonic modes have been suppressed is output from the controller 45, and this signal is supplied to the indicator 45.

〔Effect of the invention〕

As detailed above, according to the present invention, by emitting a transmission pulse signal whose pulse repetition period is sequentially changed and coherently processing a reflected signal for each pulse repetition period, the Doppler frequency due to the side lobe of the antenna is It is possible to provide a radar device that can suppress the harmonic modes of , and improve distance measurement accuracy.

[Brief explanation of drawings]

Figures 1a and b, Figures 2a and b, and Figure 3a and b are diagrams showing different types of synthetic aperture antenna type radar equipment, and Figures 4 to 9 are respectively diagrams showing conventional synthetic aperture antennas. Figures 10 to 1 are diagrams shown to explain the received signals of the radar device of this type.
FIG. 2 is a diagram for explaining the principle of the present invention, and FIG. 13 is a configuration diagram showing an embodiment of the radar device according to the present invention. 31... Coherent oscillator, 32... Stabilizing oscillator, 33... Transmitter, 34... Trigger oscillator, 36...
Antenna, 37... mixer, 38, 39... first and second phase detectors, 42... coherent signal processor, 44... integrator.

Claims (1)

[Claims] 1. In a radar device using a synthetic aperture antenna system, in which a radar is mounted on a flying object and the topography, topography, etc. are observed using radio waves emitted from the radar, a means for emitting a transmission pulse signal with a changed pulse repetition period, and a means for emitting a transmission pulse signal with a changed pulse repetition period, means for converting the reflected signal into an orthogonal video signal; means for processing the orthogonal video signal into a coherent signal corresponding to the changed pulse repetition period; and integrating the processed signal for each corresponding pulse repetition period. A radar device characterized by comprising means for.