Definition
A primary Radar is a sensor which illuminates a portion of space with a electromagnetic Onde and which receives in return the waves reflected by the targets being in this space. The term of " radar primaire" indicate a radar system used to detect and locate potentially co-operative targets and is specific to the field of air control where one opposes it to the secondary Radar.
Principle
Its operation is based on the principle of the echo, one emits an impulse of strong power which is converted into a narrow face of Onde which is propagated at the speed of the Lumière (300 000 km/s). Then one listens to the possible echoes resulting from the reflection.
Thus one carries out emissions/listening uninterrupted, which makes it possible to cover space on 360°.
The functions of the primary education radar thus result in detections and measurements using means Radioélectrique S, detection being the decision of presence of a target by the recognition of the useful signal.
One measures with a primary education radar:
outdistance it D based over the duration of propagation of the Onde on its way outward journey/return.
- An angle θ based on the position of a directing antenna in azimuth.
- A radial speed by Doppler effect.
One can thus notice that a radar locates an object flying on a quadrant in the vertical plan, but one cannot know the horizontal geographical coordinates exactly, nor the altitude of a plane. This information is obtained by triangulation of several radars.
Equation of the radar
To establish the equation of the radar consists in making the assessment of power on the way go/return of the emitted signal. It is the subject of a detailed article, to consult before continuing.
Localization
Outdistance
It is the measurement of a time between the moment of the emission and the moment when the signal returns after reflection on the target.
For dT = 1 micro seconds, one obtains R = 150 meters
For R = 1 Nautical, one obtains dT = 12.35 micro seconds
Azimuth
Separation
Separation in the axis
Separation in azimuth
volume of confusion
Fluctuation of target and diversity of frequency
The answer of a target is related to its equivalent surface defined in the equation of the radar which is a composition of elementary surfaces. The target being moving, this equivalent surface evolves/moves at every moment.
An elementary surface sk produces an elementary signal ek received on the level of the radar
ek = ak cos (2π f0 T + Φk)
The total signal will be form: S = Σ ek (vectorial sum)
Whereas different the ak is not null, the sum of the ek can be null because of the differences in Φk phases of each term. The only parameter accessible to the level from the radar tracking station is the frequency. So thus improving detection, one uses a radar with two transmitters fixed on different frequencies.
Deformation of diagram in the presence of the ground
Diversity of cover
The compression of impulse
The technique of the compression of impulse makes it possible to reconcile a great range and a good resolution without having for that to emit an impulse of very short duration and great power. The principle is the following: An extremely short impulse is presented to the entry of a dispersive line. This impulse presents a very wide frequency spectrum, and the dispersive line brings a variable delay according to the frequency. At the exit of the dispersive line, the pulse frequencies will thus be spread out in time. With the reception of an echo, it is enough to make the opposite treatment, i.e., use a complementary dispersive line which shows an opposite characteristic of the dispersive line of emission. The impulse is then compressed in time.
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