Sidelobe suppression
As side lobe suppression is known electronic method in radar technology , signals received via the side lobes are received the antenna to attenuate or hide, otherwise they will impede an accurate determination of the direction to the antenna.
Problem
When using pulse radar to determine the direction, strongly bundling directional antennas are used. In terms of their directional characteristic, these have, in addition to the main lobe , which points in the desired direction, also several side lobes which point in undesired directions. The highly sensitive radar receiver receives these interference signals from the side lobes, but displays them in the wrong direction: in the direction of the main lobe.
In electronic warfare , many disruptive tactics make targeted use of these side lobes in order to influence the effectiveness of the radar by disrupting and deceiving.
Sidelobe suppression in the primary radar
One or more compensation antennas are always used in the primary radar for sidelobe suppression. When transmitting, these remain passive, a real suppression of the side lobes is usually not possible. The diagrams of the compensation antennas are ideally similar to the main antenna, only the main lobe is as small as possible compared to the side lobes. The compensation antennas have their own receiver and generate a threshold value for signal processing. Signals from the side lobes of the main antenna must exceed a higher threshold in signal processing than the signals received via the main lobe of the main antenna and are therefore effectively attenuated.
It is easier to generate a suppression channel for modern radars with digital diagram formation. Each antenna element of the phased array antenna has its own receiving channel for digital diagram formation . Only after the received signal has been digitized are the phase shifts necessary for creating a diagram implemented digitally. Therefore, many different antenna diagrams can be generated simultaneously and in parallel. All radiators of the phased array antenna are used to form a main lobe. In order to generate the same diagram with greatly enlarged side lobes, only 10 to 20 percent of all radiators are used, which are selected according to a statistical method. This simulates a so-called thinned phased array antenna which, with the given phase shifts, generates the same antenna diagram, but with much larger sidelobes. Here, too, the suppression channel is used to generate threshold values for signal processing in the main channel.
Sidelobe suppression in the secondary radar
In secondary radar technology , the side lobes have a particularly unfavorable effect. In this case, targets can also be queried using the side lobes and then respond using them. This fact results from the significantly better transmission and reception conditions for secondary radar devices, since the free space attenuation for the transmission and reception path works independently of one another and the transponder responds with a significantly higher power than would be possible with passive reflection. In the case of primary radar, both attenuations are therefore coupled to an r 4 -dependence, while secondary radar only has an r 2 -dependency.
Responses from the sidelobes of the secondary radar antenna can no longer be clearly assigned to a primary target on the radar screen . Rather, they appear as multiple targets at the same distance but in different directions. In extreme cases, an aircraft can be queried continuously while the antenna is rotating. Such a target then appears on the viewing device as a full circle (so-called "ring around").
The methods for suppressing the effects of the side lobe are generally referred to in secondary radar using the English term for side lobe suppression as SLS ( S ide L obe S uppression). The suppression techniques can on the interrogation path ( I nterrogation-Path SLS) or on the response pathway ( R used Eply-Path SLS):
- RSLS
- works in a similar way to primary radar: two response signals received via different antenna characteristics are compared in terms of their size and only if the signal from the main antenna is significantly larger is it accepted. The user can set by how much the signal needs to be larger.
- ISLS
- however, relies on cooperation between the transponders. Here, too, an antenna with omnidirectional characteristics is used in addition to the directional antenna. The two interrogation pulses (P1 and P3) are emitted via the strongly bundled diagram of the directional antenna, a third pulse, called P2, with the omnidirectional antenna. In the transponder it is now examined how large the amplitude of P2 is in relation to P1 and P3. If P2 is smaller than P1, the transponder receives the signal from the main lobe and must respond. If P2 is greater than or equal to P1, the transponder receives the signal from a side lobe and is not allowed to respond.
- With Large Vertical Aperture Antennas (LVA), the differential signal from the monopulse antenna is also superimposed on the omnidirectional antenna. The two lobes of the difference channel of the monopulse antenna additionally sharpen the diagram of the common main lobe in the sum channel.
- IISLS
- With the Improved ISLS, the P2 and P1 impulses are sent through the suppression channel with omnidirectional characteristics and have the effect that diagram falsifications due to reflections from obstacles close to the secondary radar are recognized as "reception from the side lobes".
In the Mode S procedure, which has been binding for civil aviation since 2008, the principle of main beam antenna and omnidirectional antenna is used at the moment of transmission of the interrogation radar, as with ISLS. Here a short pulse is transmitted via the omnidirectional antenna, which superimposes the starting edge of the internal serial modulation of the pulse in the side lobes, which is necessary for synchronization, and thus prevents the query from being evaluated. Sidelobe suppression on the response path is no longer necessary in this mode, as predominantly selective, individually addressed queries take place, i.e. there is only a targeted response to a specific secondary radar if the aircraft is also in the main lobe of the antenna.
Individual evidence
- ↑ Digital beamforming on radar tutorial
- ↑ Thinned array on radar tutorial
- ↑ Individual Mode S query on the radar tutorial [1]